A111DD ^4071 NBS

PUBLICATIONS

NBSIR 80-2178 (DoE)

Vaporization and Chemical

Transport Under Coal Gasification

Conditions

J. W. Hastie, D. W. Bonnell, E. R. Plante, W. S. Horton

High Temperature Processes Group

Chemical Stability and Corrosion Division

Center for Materials Science

National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

| December 1980

•red for 100 Department of Energy

on cn Kington, DC 20585

80-2178 r Contract EA-77-A-01 -6010

1980

NBSIR 80-2178 (DoE)

VAPORIZATION AND CHEMICAL

TRANSPORT UNDER COAL GASIFICATION

CONDITIONS

ATIOJfAL BtniUEAD W BTAVDARDS

UB&AST

w- - SEP 1 6 mi

no i ClCC - Cxj\Xs

0-UOT)

/y oD

C<

^

J. W. Hastie, D. W. Bonnell, E. R. Plante, W. S. Horton

High Temperature Processes Group

Chemical Stability and Corrosion Division

Center for Materials Science

National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

December 1980

Prepared for U.S. Department of Energy

Washington, DC 20585

Under Contract EA-77-A-01-6010

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Jordan J. Baruch, Assistant Secretary for Productivity. Technology, and Innovation

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director
. i

I *A -!>• •

.t ? ! ‘3?
Abstract

The present studies have resulted in the development of a new experimental technique, transpiration mass spectrometry, for the quantitative

analysis of high temperature vapors and gases. This technique has been

applied to vapor transport and thermodynamic activity determinations for

real and synthetic coal slag samples in reactive coal gas components at

pressures up to atmospheric. The results indicate a highly non- ideal and

non-monotonic (with respect to temperature and composition) behavior for

alkali metal vapor transport. Thus a priori predictions of alkali metal

transport in coal gasifiers without actual activity data are virtually

impossible at the present time: Surface segregation and diffusion limitations of alkali species in slags are also possible complicating effects

in relating laboratory data to plant experience.
'

St,
FOREWORD

Since 1974, the National Bureau of Standards has been engaged in

research designed to address materials problems and needs pertinent to coal

conversion and utilization technologies. This work has been sponsored in

part, at first by the Office of Coal Research, then by the Energy Research

and Development Administration, and most recently by the Department of Energy

The main emphasis has been on test method development and particularly on accelerated procedures for evaluating materials behavior and durability.

Complementary efforts have been made on the determination of the mechanisms

of materials degradation and on the development and operation of materials

data centers to provide evaluated information on properties and performance

including failures occurring in operating plants.

During the FY77-79 period the NBS program, entitled "Materials Research

for the Clean Utilization of Coal" consisted of a number of interrelated

tasks including:

1. Metal Corrosion

a. Constant Strain Rate Test

b. Pre-cracked Fracture Test

2. Ceramic Deformation, Fracture, and Erosion

3. Chemical Degradation of Ceramics

a. Reactions and Transformations

b. Slag Characterization (viscosity)

c. Vaporization and Chemical Transport

4. Failure Prevention

a. Failure Information Center

b. Materials Properties Data Center

The results of the research have been disseminated through quarterly reports

of progress (available from NTIS; report designation EA-6010; Dist. Category

UC-90C) as well as numerous scientific publications in technical journals.

Further, as individual tasks are completed, an overall report is prepared

detailing the results and accomplishments of the project. This present

publication is the final report for the project on Vaporization and Chemical

Transport.

SAMUEL J. SCHNEIDER

Program Manager

Center for Materials Science

National Bureau of Standards

TABLE OF CONTENTS

Paae

1. Introduction 1

1.1 Background 1

1.2 Objectives 3

2. Technical Approach 3

3. Results and Discussion 4

3.1 Reference Materials Analyses 4

3.2 Vapor Transport Over Illinois No. 6 Coal Slag (K-^) .... 5

3.3 Vapor Transport Over Synthetic Slag (Kg) 5

3.4 l<2 Slag + Hg System 7

3.5 Kg Slag Systems—Time Dependent Observations 8

3.6 Modeling of the Kg Slag + Hg Systems 9

4. Summary and Future Directions 9

5. Publications and Talks 10

6. References 11

7. Appendix A: Transpiration Mass Spectrometry of High Temperature Vapors

8. Appendix B: Slag and Metal Oxide Vaporization in Reactive Atmospheres

VAPORIZATION AND CHEMICAL TRANSPORT UNDER

COAL GASIFICATION CONDITIONS

1. Introduction

1 . 1 Background

In recent years the economic and strategic advantages of coal as an energy source have gained widespread attention. As a result,

numerous processes for converting coal to low and high Btu gas are under development. The conversion reactions involve

extremes of temperature (700 to 1500 K range) and pressure

(1 to 70 atm range) as well as very reactive atmospheres, ranging

from oxidizing to highly reducing. Reactor wall liner materials

must therefore be constructed from refractory ceramic materials.

However, the durability of even established refractory oxides

under the unique conditions of coal gasification is open to

question (Sadler, et al., 1979, Raymon and Sadler, 1976).

Chemical degradation of ceramics in reducing atmospheres (H^) and

H^O-containing atmospheres is relatively well known. Vapor transport of reduced species such as Si 0 , A^O, and Mg, and of hydrated

species such as Si(0H)^ is a significant factor in process atmosphere corrosion. We have summarized the laboratory evidence for

vapor transport processes of this type elsewhere (Hastie, 1975,

p. 37, 91, 212). This evidence is based on systems with relatively

few components, and quantitative extrapolation to coal gasification

conditions is not possible. That is, there is a need to examine

refractory-coal gas interactions under conditions more closely

approximating those proposed for gasification as was stressed by

Raymon and Sadler (1976).

1
In addition to the reactive nature of the coal gas itself, the

coal mineral impurities, even in minor amounts, can be highly

corrosive to ceramics. Volatile alkali metal (Na and K), iron,

sulfur, and halogen-containing species are the prime candidates

for ceramic degradation. Vapor transport (to ceramic surfaces)

of these impurities will be primarily controlled by their thermodynamic activity in the coal slag or ash. This activity will, in

turn, be highly dependent on the slag-coal gas reactivity. For

instance, release of Na will be controlled by the vapor-phase

processes:

Na

2

0 (in slag) = 2Na + hO

2 (1)

H

2 + h02 = H

2

0 • , (2)

where a reducing atmosphere will drive reaction (1) in the

forward direction. Other impurities and gas components will

also affect this vapor transport process., e.g. ,

2HC1 + Na

2

0 (in slag) = 2NaCl + H

2

0 (3)

H

2

0 + Na

£

0 (in slag) = 2NaOH . (4)

Clearly, thermodynamic prediction of the vapor concentration of,

say, Na requires consideration of many competing equilibria.

Another complication arises from the possible presence of new species and reactions. For instance, in high pressure steam an enhanced vapor transport of NaCl has been attributed to the

hydrate, NaCl *21^0 (see Hastie, 1975 p.82). Existing thermochemical tables, which may be used to select likely vapor transport

reactions are inadequate as the species selection is based largely

on results for simple low pressure systems. Thus, under the

higher pressure and complex mixture conditions of coal gasification,

novel species and vapor transport reactions are likely.

2
The identification of species and reactions under the combined

conditions of high pressure (> 1 atm), high temperature (T ~ 1700 K),

and complex chemistry, is a challenging measurement problem. At

the outset of this research project, no single measurement tool

existed for the quantitative molecular characterization of high

temperature processes representative of coal gasification. The

best available methods of Knudsen effusion mass spectrometry (KMS)

and transpiration have limited applicability. The KMS technique

is molecular specific but is limited by molecular flow requirements

-4

to an upper limit total gas pressure of about 10 atm. Transpiration, on the other hand, is well suited to pressures of 1 atm

and higher but does not provide molecular species identification

in complex gas mixtures. A coupling of these two classical

techniques would clearly resolve the individual limitations.

1.2 Objectives

A two-fold experimental objective was required' for this project.

First, a novel measurement technique was to be developed, cased

on a coupling of the Knudsen effusion mass spectrometric and

transpiration methods. This technique was then to be applied to

a molecular-level thermodynamic characterization of synthetic

coal-gas-slag-ceramic vapor transport processes. As a secondary

objective, multicomponent, heterogeneous , thermodynamic modeling

methods were to be developed to complement the experimental

analysis and extend the range of applicability of the data.

2. Technical Approach

An NBS-developed High Pressure Sampling Mass Spectrometer System

(HPMS) (Hastie, 1973) was used as the basic vacuum and mass analysis

component of the Transpiration Mass Spectrometer (TMS). The transpiration component of this apparatus consisted primarily of a platinum

reactor with radiative heating provided by a tantalum tube furnace.

Slag or ceramic samples were contained in a platinum boat within the

3
hot zone of the reactor. Reactive coal gas components (e.g. , H

2

, 1^0)

were introduced, together with the carrier or transport gas (N^ or

Ar), and allowed to come to thermal equilibrium with the boat-sample.

The gaseous reaction products were then sampled in real time through a small orifice and a molecular beam produced for mass analysis. Thus,

species partial pressure data could be obtained as a function of inlet

gas composition, pressure, temperature, and slag composition.

Figure 1 shows a photograph of key reactor components. Additional

details of the apparatus and experimental procedure are given in

Appendix A (see Sections 2 and 3).

i Several approaches were devised for theoretical modeling of coal-gasslag-ceramic interactions. First, a generally available computer code

(Gordon and McBride, 1971) was adapted for this purpose. This code

allows for the thermodynamic analysis of multicomponent gas mixtures

reacting with condensed ideal solutions. Hence, in dealing with the

highly non-ideal case of slag, it was necessary to postulate the

formation and ideal mixing of various silicate compounds to simulate

the slag thermochemistry. This approach was found to be less than

satisfactory and a more comprehensive and less generally available code,

SOLGASMIX (Eriksson, 1975) was obtained for explicit treatment of

non-ideal solutions.

3. Results and Discussion

3.1 Reference Materials Analyses

The design, calibration, validation, and implementation of the

TMS system required detailed studies on several wel 1 -characterized

reference materials; namely, liquid NaCl and Na2S0^. Appendix A,

Sections 4 and 5, contains a summary of the TMS system-performance

and new thermodynamic data for these molten salts. In summary,

the results indicate that the reactor can faithfully sample

(i.e., without chemical perturbation) a complex gas-vapor mixture

4
in a heterogeneous system at thermodynamic equilibrium. Also, the

temperature and pressure range attainable was, found to be within

that required for slag-coal gas analyses.

3.2 Vapor Transport Over Illinois Number 6 Coal Slag (K^) 1

In order to interpret the effects of coal gas components on coal-slag vapor transport, one needs, as base data, the

vaporization behavior under vacuum and inert gas conditions.

These data, obtained using the KMS and TMS (N^ carrier gas)

methods, respectively, are summarized in Appendix B, Section

2.2.2. In these studies, the thermodynamic activities of Na

and K in the slag were extremely low and samples with additional

alkali were needed to obtain accurate data on alkali transport

under inert gaseous atmospheres. Vapor pressure data were

then obtained as a function of alkali content and temperature,

as shown, for example, in figures 6 and 7, Appendix B. Addition

of water vapor to the transport atmosphere enhanced the release

of K to the vapor' phase, as shown in figure 9, Appendix B. This enhanced transport is attributed to the processes:

H

2

0 = H

2 + h02 (5)

H 2 + K

2

0 (in slag) = 2K + H

2

0 . (6)

To determine the effect of added H

2

, in amounts typical of

coal gasification conditions, a synthetic slag with greater

purity and homogeneity was selected.

3.3 Vapor Transport Over Synthetic Slag («

2

)

A lower melting, more basic, (as compared with the K-j sample)

synthetic slag was prepared for studies analogous to those

performed for the K-j system.

x The terminology (and K2 in later discussion) is used to designate

potassium-containing slags.

5
Under free vaporization (KMS) or ^-atmosphere (TMS) conditions

the predominant vapor species in this system are K and C^, as shown in figure 2. The initial excess of present in this

slag is believed to result from the preparation procedure

where melting and pouring in room air led to oxygen absorption

by the sample. Pressure bursts of K and were noted in the

initial phase of the TMS experiments, e.g. , at 1500 K in

figure 2, suggesting that trapped gas was released from the

slag.

The appreciable effect of H^O vapor on the release of K from

the l<2 slag is demonstrated in figure 3. The data are consistent

with the reactions (5) and (6) as the primary process leading

to the enhanced release of alkali. Also, as the data agree

with the thermodynamic pressure dependence (slope 0.5 for the

curve of figure 3), the system appears to be at equilibrium.

In addition, a process involving KOH formation was also observed,

but to a much lesser degree than reaction (6), as shown in figure 4.

Previous attempts at modeling complex multicomponent systems,

including slags, have either assumed ideal solution behavior

or an ideal mixing of hypothetical complex species. However,

it is well known, and our experimental data confirm this, that

slags form highly non-ideal solutions. Accordingly, we have

applied, for the first time, a non-ideal solution multicomponent

equilibrium computer program to a slag system. The program,

known as SOLGASMIX is based on the development by Eriksson

[Eriksson (1975)]. As a test case we have taken the slag

system with 1 atm H

2 present. Typical data are given in table 1,

where the effect of non-ideal solution behavior is clearly demonstrated. Also, at 1500 K, the atomic potassium species is much

more significant than KOH; 87 percent of the alkali is released

as K under these conditions.

6
Table 1

EXAMPLE OF THERMODYNAMIC MODEL OUTPUT

3

Species Ideal Solution Non- ideal Solution0

Pressure (atm) Pressure (atm)

(1000 K) (1500 K) (1000 K) (1500 K)

K 6.7 -3 C 1.8 -2 3.3 -8 4.6 -6

KOH 1.4 -2 3.3 -3 8.2 -8 6.7 -7

H

2

0.96 0.95 0.97 0.97

h

2o 2.3 -2 3.4 -2 2.7 -2 2.7 -2

0, 4.2 -24 4.5 -15 5.7 -24 2.7 -15

aSystera K

2 slag wt %; Si02 (47.3), A12

0

3 (11.1), Fe

2

0

3 (12), CaO (13.9),

MgO (7.0), K

2

0 (8.7).

^Activity K

2

0 experimental input (K

2

0 + Si0

2

data used).

c - 3

Computer notation; e.g. , 6.7 -3 = 6.7 x 10 .

3.4 K

2 Slag + H 2 System

In order to verify the above reaction scheme [reactions (5) and

(6)] and to extend the vapor transport conditions to a reducing

environment closer to that present in coal gas, a series of

measurements were made using H

2 as the initial reactant gas.

Compositions of H

2

~N

2

-H

2

0-0

2

gas up to 10 volume percent H

2

were

attained prior to hydrogen- i nduced corrosive loss of the transpiration reactor.

As H

2

is introduced to the slag system, the 0

2

concentration is

decreased and H

2

0 increases in accord with reaction (2). Typical

data are given in figure 5. Note that at relatively low H

? -3 ^ concentration (partial pressure < 5 x 10 atm) the 0

2

scavenging

7
process provides a high concentration of f^O. As there are three

interdependent reactive gases simultaneously present (h^, H^O,

and O

2

), the dependence of K pressure on reactive gas pressure

can be complex, as shown in figure 6. For the decreasing partial

pressure region of the run chronology, the one-half power dependence of K pressure on h^O pressure [resulting from reaction (5)]

is consistent with reaction (6) as the main process.

3.5 l<2 Slag System—Time Dependent Observations

During the initial heating-vaporization phase, using a non-reactive

N

2

carrier gas, a periodic pressure burst phenomenon was noted

for both the K and O

2 vapor species. As shown in figure 7, the

rise time for such a burst is almost instantaneous once the

critical temperature is reached. After about 20 minutes at

constant temperature, the K and O

2 pressures return to the expected

level. This type of phenomenon has also recently been observed

in solar heating experiments of liquid oxides. The explanation

for this effect in our system is unclear' but may be indicative of

a non-homogeneous sample, perhaps containing residual I^CO^ used

in the slag-synthesis. Alternatively, phase separation of an initially homogeneous sample may be possible.

A second type of time dependent phenomenon was observed for this

slag, as shown in figure 8. Once an isothermal condition has

been achieved, the K pressure falls off with time. We believe

that this results from surface depletion of K (and O 2

) from the

sample, due to the bulk diffusion rate being too small relative

to the surface vaporization rate. This effect was found to be

much less pronounced at higher temperatures where the diffusion

rates are probably higher.

3
3.6 Modeling of the 1^ Slag + System

Typical output data obtained from the SOLGASMIX program for the

slag in the presence of H 0 are given in table 2. Note that K

4 Lm is the predominant alkali vapor species, in agreement with experimental observation. However, the partial pressures are much

less than those observed experimentally. It is also noteworthy

that the K partial pressure is relatively temperature insensitive

in the presence of H£ and H

2

O. This agrees qualitatively with

the data presented for the K-j slag system.

Table 2

SPECIES PARTIAL PRESSURES FROM NON-IDEAL SOLUTION

3

MULTICOMPONENT EQUILIBRIUM CALCULATIONS FOR K

£

SLAG + H

Species Pressure (atm) Pressure (atm)

(1600 K) (1700 K)

H

2

0.11 -3 b

. 0.63 -4

H

2

0 0.26 "3 0.27 -3

°2 0.23 -9 ‘ 0.77 -8

K 0.57 -3 0.60 -3

KOH 0.51 -4 0.55 -4

SiO 0.54 -8 0. 31 -7

3 * 3

1^0 activity coefficient specified as 10 and independent

of temperature.

Denotes e. g. , 0. 1 1 x 1 0 .

4. Summary and Future Directions

The present studies have resulted in the development of a new experimental technique, transpiration mass spectrometry , for the quantitative

analysis of high temperature gases and vapors 2

. This technique has

2 This development recently received external recognition in the form of

an IR-100 award (1980).

9
been applied to vapor transport and thermodynamic activity determinations for real and synthetic coal slag samples in reactive coal gas

components at atmospheric pressure. The results indicate a highly

non-ideal and non-monotonic (with respect to temperature and composition) behavior for alkali metal vapor transport. Thus a priori predictions of alkali metal transport in coal gasifiers without actual activity

data are virtually impossible at the present time. Surface segregation

and diffusion limitations of alkali species in slags are also possible

complicating effects.

Future studies should be made on a variety of typical coal slags in

individual coal gas component atmospheres and then in the mixed gases.

In view of possible non-equilibrium or transport-limited effects,

studies should be made as a function of gas residence time and total

pressure. Extension of transport measurements to pressures greater

than one atmosphere is also desirable, particularly when non-equilibrium

effects are rate controlling. It is unlikely that either experimental

or modeling studies can stand alone for reliable prediction of coal

'gasifier materials durability problems; therefore, combined studies

are needed.

5. Publications and Talks

5.1 Publications

a. Transpiration Mass Spectrometry of High Temperature Vapors ,

D. W. Bonnell and J. W. Hastie; Proceedings of the 10th

Materials Research Symposium on Characterization of High

Temperature Vapors and Gases; NBS, Gaithersburg, Maryland;

September 18-22, 1978. NBS SP 561 (1979).

b. Slag and Metal Oxide Vaporization in Reactive Atmospheres ,

J. W. Hastie, D. W. Bonnell, and E. R. Plante, High Temp. Sci.,

in press (1980).

10
5.2 Talks

a. "Transpiration Mass Spectrometry Studies of the Thermal Decomposition and Sublimation of Sodium Sulfate," D. W. Bonne!! and

J. W. Hastie, 25th ASMS Conference, July 1977.

b. "Slag and Metal Oxide Vaporization in Reactive Atmospheres,"

J. W. Hastie , D. W. Bonnell, and E. R. Plante, Int. Colloq. on Use of Refractory Oxides for High Temperature Energy Sources,

Toronto, Canada, July 1979.

c. "Vapor Transport of Coal Minerals and Slags," J. W. Hastie ,

D. W. Bonnell, and E. R. Plante, Symp. Coal Minerals Properties

and Conversion, Materials Research Society, Annual Meeting,

Cambridge, Mass., November 26-30, 1979.

d. "Transpiration Mass Spectrometry of High Temperature Vapors,"

D. W. Bonnell and J. W. Hastie, 10th Materials Research Symp.

on Characterization of High Temperature Vapors and Gases,

NBS, Gaithersburg, Maryland, September 18-22, 1978.

6 . References

Eriksson, G. (1975). Chemica Scripta 8, 100.

Gordon, S., and McBride, B. J. (1971). Computer Program for Calculation of

Complex Chemical Equilibrium Compositions, Rocket Performance, Incident

and Reflected Shocks, and Chapman-Jonquet Detonations, NASA SP-273.

Hastie, J. W. (1973). Combust. Flame 21_, 49.

Hastie, J. W. (1975). High Temperature Vapors: Science and Technology,

Academic Press, NY.

Raymon, N. S., and Sadler, L. Y., III. (1976). Refractory Lining Materials

for Coal Gasifiers, Information Circular 8721, U.S. Department of Interior.

Sadler, L. Y., Ill, Raymon, N. S., Ivey, K. H., and Heystek, H. (1979).

Ceramic Bulletin 58, 705.

11
.
II X}

05 - .r: a» u

i— CO GL p 3 P 8 * O t/» E 05

i~ r—

a*

m

x

m

p oc

a> X 05 P c e a. o

t/5 E

*0 t- <u O X pU XCO P 0) "I—* c <u O X •<- ZJ p p CO

S- c

•<- o

Q. •>— tO p c CO

CO ts_ -r- P CL

to •O c

a. t: X +-> E

a» *o

to c

to CO

CO

to «

•<-

p "O 03O P XI O

05

O r— P CL O E X CO a. to o

s-3

05 * «

PARTIAL

PRESSURE

(ATM)

TEMPERATURE (K)

1818 1538 1333

104H'(K)

Figure 2 . Vaporization of K and from the slag, expressed as log P vs 1/T

plots: open circles—O^ (KMS), closed circles--0

2 (.TMS), open

triangles K (KMS), closed triangles K (TMS). Chronological order of

data taken with increasing temperature except for arrowed data points

where the temperature was decreasing.
PRESSURE

K

(ATM)

10 -5

k2 slag

1610 K

0.21 ATM N2

10 -6

- 6

10 -7 J I I I L

10 -3 10~ 2

PRESSURE H20 (ATM)

10

Figure 3. Dependence of K-pressure on H

2

0-pressure at constant temperature

for the K

2 slag (TMS data). The curve of slope 0.5 represents the

theoretical pressure dependence for reaction (6) (see text).
CO

CO

UJ

oc

a.

i—Z

LU

cr<

a.

a.<

<u cn O

<T3

C

r— 0) CO oc

CM <vQ.

a)

<d -a -c

4-> a>s_

s_ 3 o to

to

a) o» j- S_ c. 3+J

rt3n3

S- CJ

O) •r— Q. +-> s: aj

cv s_ +j o

CD

-r-> !

X

ra -M aj

t/1 ( c 4-> oo 00

4-> +J S3

(13 a;

to

CJ CD s_ s_ 3 c.

00 O) to s_ at uo c. • o 4_> C~ CD

CD CL • s_ O -—«. ra r— <^r c. to Q

<c L— < o o a

""CM

a> o

s_ (U c 3 a o CJ — a CD c

S- -*-> 3 H—

t/J 00 to 3

1) • O ^

30 a. 03 O i -M t— ~*~

(T3 03 o c~ dl 3

O') U- 21 to o DL to >

1 CD a CJ

CJ

— o

aJ «=3- c. CO ' 1

<o

CD r— '

CL) L-J o Q (Q

5T

<us_3

co (s/Miv)3ynss3b3d
I

Figure 5. Reponse of slag system to added H^, showing conversion of slag

°2 t0 H 2°- Con ^itions: temperature, 1655K; N

2

carrier gas Dressure,

0.18 atm, capillary nozzle.

.

'

i

*1

1 *

i 0

H

2

ION

INTENSITY

(mV)
(wiv) 3anss3ad z o

<O h-

• L

to

io

Io

lO

io

<3“

Io

in

i o

CD

io

( wiv

3anss3ad »

PRESSURE

H

2

0

(ATM)

Figure

6.

K-pressure

(open

symbols)

and

(^-pressure

(closed

symbols)

with

H

2

0-pressure

for

l<2

slag

in

the

presence

of

added

H2

.

Conditions:

temperature,

1655

K;N

?

carrier

gas

pressure,

0.18

atm.,

capillary

nozzle.
K

ION

INTENSITY

(jiV)
ISOTHERMAL

o

A1ISN31NI NOI *

Figure

8.

Typical

K

+

ion

intensity

signal

decay

with

time

for

l<2

slag.

Conditions:

temperature,

1570

K,

N

9

carrier

gas

pressure,

0.21

atm,

capillary

nozzle.
V
Appendix A

National Bureau of Standards Special Publication 561, Proceedings of the 10th Materials

Research Symposium on Characterization of High Temperature Vapors and Gases held at NBS,

Gaithersburg, Maryland, September 18-22, 1978. Issued October 1979.

TRANSPIRATION MASS SPECTROMETRY OF HIGH TEMPERATURE VAPORS

0. W. Bonnell and J. W. Hastie

National Bureau of Standards

Washington, DC 20234

Material transport via vapor phase species at high temperature

and pressure is an important phenomenon in materials science and technology. The preparation of high purity semiconductor and ceramic materials utilizes vapor transport to advantage, whereas the corrosion of

power plant boilers, gas turbines and other advanced energy devices

results from reactive vapor transport. To understand and control these

transport phenomena, it is essential to be able to identify and measure

the key chemical species present in the vapor phase systems. However, in

the past, classical characterization methods such as transpiration and

Knudsen or Langmuir effusion have been limited because they do not establish the molecular identity of transport species or because low pressures

are necessary to make effusion measurements.

We have developed a new technique - Transpiration Mass Spectrometry

(TMS) - that overcomes both of these limitations by combining the basic

features of transpiration and mass spectrometry. With this technique,

it is possible to sample reactive gases directly from high-temperature,

high-pressure atmospheres for qualitative and quantitative characterization with a mass spectrometer. To demonstrate the method, the thermodynamic equilibrium vaporization of the salts and NaCl was studied

at total pressures up to an atmosphere using ^

or Ar as carrier gases. The

molecular composition of the vapors was determined over a wide range of

temperature and pressure and the results indicated no measurable perturbation of the vaporization equilibria due to the presence of a molecular

beam sampling probe. In fact the accuracy of data obtained by the TMS

method is competitive with that of established, lower dynamic range,

techniques.

357
1. Introduction

1.1 Technological background

Many high temperature processes of technological importance rely upon, or are adversely

affected by, chemically active heterogeneous subsystems. One such case involves chemical

interaction of a multicomponent high temperature (e.g., ~500-2000 K) high pressure (e.g.

^0.1-100 atm) gas mixture with a solid or liquid substrate. This interaction frequently

leads to the formation of intermediate species containing elements of both the gaseous and

substrate materials. These reaction intermediates can be transported in the presence of

temperature, concentration, or momentum (e.g., forced convection) gradients. Transport to another regime of temperature, pressure, or concentration can lead to a reaction-reversal or the introduction of secondary processes. In many instances, this changing chemistry along

gradients results in a deposition of intermediate species with the overall result that the

initial substrate has been physically transported to another part of the system.

Such material transport can have either an undesirable or beneficial effect on the

system of interest. Examples in modern technology include, respectively:

(1) Hot corrosion of gas turbines, jet engines, rockets, coal gasifiers, magnetohydrodynamic channels, coal fired boilers, and numerous pyrometallurgical systems.

(2) Controlled material transport for production of crystals and films, extractive

metallurgy, flame inhibition and fire extinguishment, combustion modification such

as smoke and antiknock control with additives, and regenerative lamp cycles.

A detailed recent discussion of these and other examples of high temperature materials

transport has been given elsewhere [l] 1

. In order to understand these heterogeneous processes for development of new or improved

control strategies, it is necessary to define at a molecular level the transport mechanisms

and particularly the reaction intermediates. To date, extensive reliance has been placed on speculation and empirical observations based solely on macroscopic variables defining the

initial and final state of the system. For instance, current understanding of hot corrosion

derives from metallurgical examination of the clean and corroded material under ambient

conditions. Extrapolation of these observations to the actual conditions of usage requires

many assumptions about the corrosion mechanisms. However, detailed mechanistic information

is practically nonexistent. A further inaccuracy occurs because standard test procedures

(e.g., use of laboratory burner rigs) simulate only part of the actual system. To properly

relate test results to practical usage would require a knowledge of scale-up factors derived

from a detailed mechanistic description of both the test and the end-use systems.

Information on reaction mechanisms, including the intermediate transport species, has

been practically nonexistent heretofore because of the measurement problems associated with

these extreme conditions. Very limited molecular-specific information can be obtained using

optical spectroscopic methods at high temperature and pressure. In principle the broad

spectrum of gaseous and vaporized species present could be identified and their spatial and

temporal concentrations determined by the high pressure sampling mass spectrometric (HPMS)

figures in brackets indicate the literature references at the end of this paper.

358
technique. This molecular beam technique, which uses a sonic nozzle for sample extraction,

has been amply demonstrated by us [2] and others for homogeneous systems, including flames.

A summary of recent work in this area has been given by Stearns, et al. elsewhere in this

volume [3]. For heterogeneous systems, the gas is saturated by condensible species. In this

case, the nozzle probes used to interface the hot gas and the mass spectrometer become part

of the reaction system and severe limitations arise. Conventional use of the HPMS technique

(homogeneous systems) has required probe temperatures to be considerably less than for the

sample system. With saturated gases, this leads to condensation of inorganic species at the

probe tip and results in physical blockage of the small entrance orifice or in corrosive loss

of the probe.

We have developed a procedure for avoiding this difficulty, in addition to providing

other advantages, as described in Section 2. Basically, the sample, probe and skimmer (to a lesser degree) are maintained isothermal by an external heat source. Maintenance of a steady

state (and constant pressure) between gas and substrate requires a continuous replenishment

of the gas extracted by the sampling process. This leads naturally to a transpiration procedure, where the input flow rate matches the gas extraction rate through the small -orifice

probe. The technique therefore combines the basic features of both the transpiration [4] and

the HPMS methods.

1.2 Scope of application

In the following sections we describe this technique of transpiration mass spectrometry

(TMS) and its application to the characterization of high temperature vapors. We have demonstrated that it is a nonperturbing method2

, which is always an important concern with probe

methods. The method extends the dynamic range of classical vaporization and vapor transport

techniques by many orders of magnitude; at least four orders with respect to Knudsen effusion

mass spectrometry. We believe that TMS should be applicable as a quantitative measurement

tool for laboratory simulations of most of the technological transport systems mentioned

above and eventually may be applicable to plant-scale systems.

With regard to systems of initial academic interest, one can most likely expect this

technique to be a useful means of producing novel high temperature species in a spectroscopically cool form, resulting from free jet expansion cooling. This would greatly

extend the utility of thermally sensitive molecular beam spectroscopic methods. In particular, electron diffraction, microwave, laser fluorescence, Raman, photoelectron, and photoionization spectroscopy, as well as matrix isolation methods, would benefit from a coupling

with the TMS technique. The extended high pressure range should also allow for the study of

basic thermodynamic properties of novel complexes and adducts of the type suggested indirectly

from gas-solid solubility studies (e.g., see Hastie, pp. 126-148; pp. 73-87 [1]). Future

extension to even greater pressures should permit definitive molecular characterization of

systems in their critical state and greatly supplement equation of state studies and the

fundamental understanding of fluids.

2 At least for the equilibrium systems studied, and this is most likely the case for

many nonequilibrium systems.

359
2. Apparatus

2. 1 The mass spectrometer system

Figure 1 shows a schematic of the mass spectrometer (stage II) and vacuum system layout

with the transpiration apparatus attached (stage I). The aluminum walled vacuum system is

partitioned into two differentially pumped stages. The stage labeled I utilizes a 6 in diffusion pump (Varian/NRC VHS-6) 3 with a cold trap (Granvi lle-Phillips Model 270-6) and gate

valve (NRC 1279-6) close-coupled to the base of the vacuum chamber. Net pumping speed at the

bottom of the chamber is calculated to be ~800 1/s. The backing system for this pump-stack

uses a Roots blower (Leybold-Hereaus WA250) and a rotary two stage vane pump (Sargent-Welch

1375). Use of a blower in series maintains, even for high gas loads, a pressure at the

“4

diffusion pump outlet well below the ^10 atm4 critical backing pressure. This assures

-5

that the diffusion pump will continue to pump near its rated speed into the 10 atm

pressure range. A high sensitivity thermocouple gage controller (Hastings-Raydist model

SL-1R) and tube (type DV-8M) was used to monitor this pressure range.

Figure 1. Detailed schematic of assembled transpiration reactor-mass

spectrometer system.

Commercial sources are identified only for purposes of accuracy and do not imply endorse

ment by the U. S. Government nor that they are the most suitable product for the work

performed.

2 4

1 atm = 1.01325 x 10 kpascal ; see introduction to this volume for SI unit conversions.

360
A mechanical shutter separates stages I and II, acting as a valve to isolate the stages

during removal of the transpiration reactor assembly. In addition, there are a variety of

differential pump apertures (three hole sizes) in the shutter plate for control of the molecular beam diameter and stage I to II pressure difference. The aperture used for transpiration experiments is 0.05 cm in diameter and is located 4 cm from the skimmer orifice.

Stage II, the high vacuum mass spectrometer section, is pumped by a stack consisting of

a 4 in diffusion pump (Varian/NRC VHS-4), a 4 in cold trap (Granvi 11 e-Phillips Model 270-4)

and a butterfly valve (Vacoa BFV-4). This stage has an approximate pumping speed of 400 1/s

at the vacuum chamber base. No-load vacuum in both stages is <5x10 ^ atm. The background

mass spectrum at this pressure shows the 18 amu ^0*) ion as the major peak (~50 percent of

total ions) which arises from h^O desorption from the aluminum-walled vacuum system. At

maximum load (^50 seem [standard cubic centimeters per minute]) the stage I and II pressures

•6 *9 are ^5x10 atm and ^5x10 atm, respectively. A titanium sublimation pump (GranvillePhillips TSP series 287 in a 214-669 chamber) and an ion pump (Ultek Model 10-250), not

shown in figure 1, are used as holding pumps on the stage II section when the diffusion

pump is valved off to cycle the cold trap. This periodic trap cleaning procedure minimizes

release of trapped reactive gas back into the mass spectrometer chamber.

The beam modulation chopper system, located in the stage II region, is a toothed wheel

(3 or 24 teeth) driven by an AC synchronous motor (Globe 75-121-2) which has been cleaned for vacuum service. A reference signal, needed for phase sensitive detection, is generated by

the wheel interrupting light from a small incandescent bulb (GE number 40) impinging on a photo transistor (GE-L14B). A sine wave oscillator (Donner mode] 1202) supplies the drive

frequency to a chopper drive system [Extranuclear Laboratories (EL)] which produces the

capacitively shifted two phase 115 V power to drive -the chopper motor. The three tooth

chopping wheel yields beam modulation frequencies from ~40 to 270 Hz. Higher frequencies

can be obtained with the twenty-four tooth wheel. The actual chopping frequency is normally

selected by observing a very weak modulated beam mass spectral peak and adjusting for minimum

noise interference from the 60 Hz line frequency. For these studies a chopping frequency of

214 ± 1 Hz was used.

The mass spectrometer itself is a quadrupole mass filter (EL-Model 270-9) with a crossedbeam configuration ion source (EL Model 041-2). We have modified the commercial ion source

by mounting a stainless steel plate at the ion source entrance for additional neutral beam

collimation. Standard quadrupole power supply (EL model 011-1) and ionizer control electronics (EL type II) are used. The mass analyzed ion beam is deflected to a channeltron

electron multiplier (Galileo Electro-Optics, model 4700) mounted off axis and operated at a nominal 1700 volts dc. Since the final amplifier is a frequency and phase sensitive lock-in

detector (Ithaco Model 393), the multiplier gain is set for optimum signal to noise ratio at the lock-in, rather than for maximum gain. Total system gain using a fast electrometer amplifier (EL Model 031-2) is ~4xl0^; signal to noise, rather than total signal being the sen- sitivity limiting factor.

This mass filter system is supported by a 6 in flange (conflat) mounted on a large

welded bellows assembly which allows three-axis positioning of the ion source entrance

361

r
aperture. In practice, a pinhole and collimator alinement assembly is mounted on the beam

axis with a He-Ne laser directed along the same axis. The mass filter assembly is then

positioned so that the ion source entrance and exit apertures are centered on the laser beam.

Final molecular beam alinement is achieved by adjusting both the mass spectrometer and the

molecular beam source angle position to obtain maximum ion intensity of a non-scatterable

beam component. This alinement procedure proved to be a critical phase of the experiment.

2.2 The transpiration reactor

The transpiration reactor is located in stage I of the overall assembly (see fig. 1).

Essential features of this reactor include: a sample container or boat, a boat carrier, a thermocouple for temperature measurements, a carrier gas inlet system, and a gas extraction

system or probe. The boat carrier allows for boat removal from the reactor without need for

a complete disassembling of the transpiration system. Molecular beam sonic probes are typically conical nozzles with design details determined by reasonably well established gas

dynamic criteria, as outlined in Section 3. However, for highly reactive systems, we also

found it desirable to develop a relatively more robust capillary probe, at the possible

expense of sampling fidelity. Pertinent gas dynamic considerations for this type of probe

are given in Section 2.3. Below we describe the construction details of these probes and the

transpiration assembly in general.

2.2.1 Reactor internal details

Figure 2 shows a schematic view of a typical transpiration reactor assembly, including

details of the assembled boat carrier and boat. A platinum tube, with outside diameter

1.23 cm, wall thickness 0.075 cm, and length 5 cm is used as a boat carrier. This tube has

machined ridges at each end to allow a snug fit with the main chamber walls for prevention

of diffusion losses. Welded to the rear of the carrier is a section of 0.64 cm diameter

platinum tube through which the carrier gas passes. This carrier gas tube is sealed to an SS304 1/4 in stainless steel tube via a SWAGELOK joint machined to fit inside the 1.27 cm i.d.

f of the main chamber. The seal is 21 cm from the front of the boat carrier. This long path

length allows the gas to reach thermal equilibrium with the walls before interacting with

the sample. The platinum section of the carrier gas tube also serves^as a guide for introduction of an alumina sheathed Pt-Pt/10 percent Rh thermocouple to the sample area. The boat, which is fabricated from welded platinum sheet, has a slanted base (~5° to the front) to allow liquid samples to be preferentially retained in the front of the boat

(fig. 3). The gas baffle shown in figure 2 is located -'-0.75 cm behind the boat, and serves

to interrupt the fast gas flow into the boat region, thereby allowing the gas maximum time

to equilibrate thermally before passage at a relatively slow rate over the sample. The

baffle also supports the tip of the alumina thermocouple sheath so that only bare wires are brought into the boat region, thereby minimizing exposure of potentially reactive ceramic

to the system.

362
GAS BAFFLE BOAT CARRIER

MAIN CHAMBER SKIMMER

SAMPLE BOAT

WITH THERMOCOUPLE BEAM

Figure 2. Transpiration reactor showing internal details.

2.2.2 Conical sampler

Details of the nozzle-skimmer arrangement are shown in figure 3. Nozzle cones are con- structed by alternate pressing and heat annealing of thick platinum disks (2 cm x 0. 1 cm) in

steel dies until a 45° half angle cone with a relatively sharp tip (~0. 1 cm radius) is obtained. Minor surface machining is followed by ELOX drilling of a hole in the tip to achieve

an orifice 0.006-0.009 cm in diameter with a maximum hole channel length of 0.004 cm. The

tip exposed to the source gas normally has a flat face ~0. 1 cm wide. We discontinued use of

knife edge machined nozzles since they occasionally failed in service due to cracking in the

wall close to the orifice. Experience showed that gas leaking through such cracks created

serious disturbances in the expanding gas flow and sufficiently disrupted the trajectory of

the initial high intensity jet to lower its density, after passage through the skimmer, by

three to four orders of magnitude. We believe that micro cracks originate during the cone- forming and machining process and that these cracks propagate as a result of the combined

effects of thermal cycling under pressure and reactive gas mixtures (nonoxidizing). The

design of a sturdier probe (snubnosed), such as that depicted in figure 3, avoided this

problem which was particularly severe for slag and strongly basic samples.

The skimmer (shown in fig. 3) was fabricated in a manner similar to the nozzle, using

30° half angle dies. Finish machining specified a sharp-edged hole since cracking would not

be a problem and minimal flow disturbance is mandatory for skimmers. The skimmer hole was drilled with a high speed twist drill (number 69-0.074 cm). Clogging of the skimmer was a primary concern in the use of supersonic beam sampling, since at high temperatures and source pressures a single experiment could be expected to deposit condensabl e -material on a cool

skimmer at a rate of several hundred mg/hr. Such deposits were avoided by physically

attaching the skimmer to the nozzle assembly with thin platinum support struts (not shown

in fig. 3). Skimmer temperatures were then within 50 K of the nozzle temperature. This

arrangement also provided an essentially permanent alinement of skimmer and nozzle.

363

T
1.1 CM

Figure 3. Schematic of conical sampler construction details and relative

location of internal components. Forward edge of skimmer is attached to outer edge of chamber exterior with 3 thin (0.2 cm thick) platinum struts

(not shown in fig.). For clarity, not all dimensions are drawn to scale.

2.2.3 Capillary sampler

A capillary sampler, shown in figure 4, was fabricated to be interchangeable with the

conical nozzle-skimmer assembly. Note the absence of a skimmer with the capillary method.

This omission was possible since beam velocity distribution measurements [5] with short

channels (length to orifice diameter ratio of 2) and no skimmer have indicated formation of

a well developed isentropic expansion leading to a high Mach number supersonic beam. Our

longer channel (corresponding ratio of 30) capillary also produces a good quality supersonic

beam (see Section 3.4.2).

Capillary fabrication involved first welding a 0.32 cm diameter platinum rod to the

center of a 2 cm x 0.1 cm thick platinum disk. Then, using a 0.013 cm wire, a hole was drilled from the disk side by the EL0X process to a depth of ~0.5 cm. Overcut from the EL0X

process produced a converging capillary, as shown in figure 4. The rod was then cut at the

bottom of the hole and crushed in steel dies to reduce the hole to the approximate dimensions

shown (see fig. 4). ELOX machining smoothed any irregularities from the postforming process.

The entire assembly was then welded to the main chamber tube, using a jig to maintain proper

alinement. System alinement procedure was essentially identical to that used for the conical

sampler system.

364
Figure 4. Schematic of capillary sampler showing construction details

and relative location of internal components. Channel not drawn to scale for clarity. A typical temperature profile, obtained along the

central axis of the chamber, is also shown.

2.2.4 Reactor external details

The complete transpiration assembly is shown in figure 5. Attachment to the support

flange is made with a 1.59 cm (commercial 5/8 in) CAJON 0-ring fitting. Initially a large

stainless steel SWAGELOK fitting was used to make this connection, but differential expansion

on heating caused unacceptable leaks. To protect the VITON 0-ring from thermal degradation,

the CAJON nut was water jacketed. A rigid connection was provided by compressing a machined

ridge (an integral part of the main chamber) between the nut and 0-ring. With this protection, the system could operate even at the ^1700 K maximum temperature for many hours.

However, the large temperature gradient resulting from the location of this water cooled

connection only 15 cm from the sample area, required careful optimization of the position of

the Ta foil furnace and its radiation shields. By careful positioning of a partial shield

(see fig. 5) at the rear of the furnace, it was possible to control the temperature gradient

to <10 K over the length of the boat (3 cm) at the highest operating temperature. Gradients

between the thermocouple location and the nozzle were much less, as shown in figure 4, and

constituted a negligible source of experimental error.

365
To Mass

Spectrometer

Figure 5. Transpiration inlet assembly showing details of furnace,

radiation shield and assembly proportions.

The main body of the furnace is supported by water cooled bus bars (not shown in fig. 5)

which are sealed to the support flange via SWAGELOK compression of TEFLON sleeves against

the bus bars. Power to the furnace is supplied by a 208 V, 20 amp variable autotransformer

driving a 20:1 stepdown transformer. Typically, 250-300 amperes is required to attain 1700 K. Precise temperature adjustment with this arrangement is only marginally acceptable. However,

alternative use of phase angle fired controllers creates unacceptable noise in the mass spectrometer signal amplifier circuit, and zero crossing controllers cannot fire into the

inductance presented by the stepdown transformer. Several remedies to improve temperature

control are under investigation.

A water cooled copper jacket surrounding the furnace assembly condenses high temperature

species scattered by the skimmer. The front plate of this jacket is mounted on copper standoff legs to improve pumping access to the furnace region (see fig. 5). The entire transpiration assembly is flange mounted and attached to the vacuum .system as a unit, as shown in

figure 1.

366
2.3 Reactor alinement

Alinement of the skimmer to the nozzle is performed by mounting the sampler (i.e.,

transpiration reactor) assembly in a jig with the same type CAJON fitting used for the vacuum

seal at the support flange. This jig is carefully machined to have a rear face perpendicular

to the sampler axis. A small He-Ne laser is then alined by reflection to project its beam

parallel to the sampler axis. The laser is then translated, maintaining parallelism, until

the beam is centered on the orifice. It is then a simple matter to deform the skimmer struts

slightly to obtain a visually perfect circular diffraction pattern through the skimmer. This

adjustment is very sensitive and can detect alinement errors of a few hundredths of a millimeter.

Approximate centering of the sampler -on its support flange is achieved by translating

the entire sampler on its mount until the nozzle-skimmer-line is centered in the aperture of

the front plate of the water cooled shield. This initial alinement is facilitated by shining

a light down the sampler and positioning the light viewed through the nozzle and skimmer.

With the entire assembly mounted and evacuated, a final tuning of the initial alinement is

made by telescopic observation of the light passing through the nozzle, skimmer and mass spectrometer ion source. The sampler is then centered on the differential pump aperture

(separating stage I and II). Final alinement is achieved by moving the quadrupole mass

filter assembly and adjusting for maximum observed beam intensity. This is a very minor

adjustment, requiring movement of the mass filter by less than 1 mm.

2.4 Carrier gas system

Mass flow meters (Tylan Corporation model FM 360) and mass flow controllers (Tylan model

FC 260) are used to provide carrier gas and mixed gas flow to the sampler. The flow con- trollers are slaved electronically to the carrier gas output to ensure constant and reproducible (±0.5 percent) gas mixture compositions. These mixtures can contain from 40 to 2 x 10^ ppm of each of as many as three solute gases in the carrier gas stream. Flow controller

ranges can be readily changed from 0.2-10 seem to 20-1000 seem to achieve these dilutions.

During a transpiration run, gas flows of typically 3-30 seem are used. The flow meter con- trollers are calibrated against a mercury piston volume gage to better than 2 percent

absolute seem. The mixed gases are metered to the sampler through a rotameter-type flowmeter

(Matheson series 600; 602 flow tube) and valve (NRS number 1). Before passage through this

flow meter, excess carrier gas is exhausted through a metering valve arid a fixed differential

pressure check valve to a hood system. The total gas pressure at the sampler inlet is

measured by a capacitance pressure transducer (Rosemount model 1332A4, 0-30 psia) calibrated

to read directly in atmospheres. Gage sensitivity is 0.002 atm and long term accuracy is ±0.003 atm (0.15 percent).

A

367
3. Gas Dynamic Aspects of High Pressure Sampling

The design of an effectively non-perturbing high pressure molecular beam sampling

device, and subsequent data analysis, relies heavily oh fundamental gas dynamic principles.

In deciding on an optimum nozzle (probe)--skimmer geometry, consideration must be given to system pumping capacity, source pressure and temperature, desired beam intensity, and the

characteristic thermodynamic and dynamic properties of the sample gas. Stearns, et al . , elsewhere in this volume [3], have discussed this subject in some detail and cited the

pertinent literature. The following treatment deals primarily with those aspects necessary

to system design and data analysis considerations..

Following the landmark paper of Kantrowitz and Grey [6] which suggested that intense

molecular beams could be obtained from small supersonic nozzles, a number of theoretical and

experimental verifications [7-11], have laid the groundwork for extending this technique to studies of high pressure-high temperature systems. The primary advantages over conventional

effusive techniques provided by the use of a supersonic beam source are, enhanced beam intensity resulting from the use of higher source pressures, and the narrower angular distribution

(better beam collimation) obtained as a consequence of the conversion of random thermal

motion to ordered motion in the expansion process. A possible disadvantage results from

changes in the distribution of translational velocity, rotational and vibrational populations

during the initial free jet expansion stage.

3.1 Free-jet relaxation effects

Factors affecting the final beam characteristics include: the nature of the free-jet

expansion process (e.g., time to reach a collision free condition), down-stream effects

(shock boundaries, background pressure, geometry, etc.), and the gas heat capacity(s).

Inferences made concerning pre-expansion source conditions (i.e., composition of the sample

gas of interest) from observations on the final molecular beam are highly dependent on an awareness of these factors and their relationship to source conditions and system design.

3.1.1 Relaxation of internal modes

In the supersonic sampling process, isentropic conditions generally govern the gross

mass flow characteristics of the beam (pressure, temperature and density). In an excellent

review article, Morris [12] discusses relaxation phenomena under isentropically expanding

hypersonic conditions. Using the Navier-Stokes equations, which have been shown [13] to satisfactorily describe the flow properties, Morris [12] estimated the translational relaxation time T t for rigid non-attracting spheres to be

368
where x c is the mean collision time based on the number of gas collisions. By assuming that

molecular collisions are non-adiabatic with respect to rotation, it can be shown that

rotational relaxation should readily occur. Experimental data for simple molecules such

as N

2

, NH

3

, and CH^ indicate the ratio

i /t <50, where t = rotational relaxation time,

r c r

Even H, relaxes rotationally in 400 collisions. As the collision controlled expansion time

^ -7

is of the order of 10 s, for the rapid expansions typical of the supersonic sampling process,

both the translational and rotational degrees of freedom are essentially equilibrated [3].

On the other hand, vibrational relaxation is a relatively much slower process with times

2 7 ranging from 10 to 10 x [12]. Higher temperatures should increase the vibrational

relaxation rates. Only binary coll isions -appear to be important and polyatomic species

typically have a single characteristic vibrational relaxation time. For the very fast

expansions characteristic of supersonic sampling, the degree of vibrational relaxation

depends mainly on the gas mixture used and may vary for different constituents. Bray [14]

points out that the falling temperature and pressure as the gas passes through the nozzle

both tend to increase the vibrational relaxation time, and the vibrational temperature can be

represented by a sudden freezing approximation.

3.1.2 Chemical relaxation

Chemical relaxation is of paramount concern in the application of free-jet expansion

methods to the analysis of species identity and concentration in high pressure sources. Bray's [14] one-dimensional sudden freezing model of chemically reacting expanding gases,

where the reaction is at equilibrium upstream and frozen downstream provides a reasonable

approximation to finite rate non-equilibrium systems. This model is also supported by the

experimental results of Wegener [15] who used a low expansion ratio? one-dimensional nozzle

system. The reaction studied

N

2 + N

20

4 j

N 2 + 2N0

2

is quite fast, with a half life of ~10 ^s [15]. In spite of the fact that the expansion

3 time for Wegener's nozzle was about 3 x 10 s, i.e., relatively long, he observed major

departures from the expansion flow equilibrium—expected for an infinitely fast relaxation

4 rate. Hence, even for reactions of the order of 10 times faster than the expansion process,

much of the chemical information may be frozen with respect to the source composition--which

is the desired condition for high pressure sampling.

5 Number density ratio for pre and post expansion regimes (see Section 3.2.2).

369
A more critical test for chemical relaxation effects is given by the very fast ionmolecule reactions studied by Hayhurst and Telford [16]. Their nozzle system geometry is

similar to ours. Using a 1 atm flame source they monitored the reaction

H

30+

+ H

2

0 + M * H

502

+

+ M

-9

which has an estimated forward rate relaxation time of 2 x 10 s. The concentration ratio

observed was much higher than the equilibrium concentration expected in the

sampled flame. Using a three-dimensional method-of-characteristics solution to the expansion

flow field, they obtained results which were in good agreement with one dimensional solutions.

Although they found significant evidence for boundary layer perturbations, caused by the

water cooled nozzle, they concluded that flow equilibrium had prevailed during the early

phase of expansion. They also suggested as a general criterion for ideal unperturbed sam- pling that the reactions of interest have a relaxation time significantly longer than con- tinuum flow time.

For normal gas-gas and condensed-gas reactions involving uncharged species, reaction

times are typically greater than 10 ^s. As continuum flow times of 10 ^s are typical for

modern sampling systems, including ours, the criterion for minimal sampling perturbation

should apply for most molecular systems of interest. In the absence of chemical reaction

kinetic data, however, it is desirable to provide experimental verification of sampling

fidelity. This has been done for the TMS system, as described in Section 4.

3.2 Beam flux and sensitivity-conical probes

3.2.1 Beam flux-source pressure dependence

Greene, et al. [17] showed that the flux through the skimmer of an isentropically ex- panding gas from a supersonic nozzle (i.e., conical probe) has the functional form of effusive flow multiplied by a function dependent only on the gas specific heat and the Mach

number at the skimmer. Experimentally it has also been shown [18] that, to a good approximation, the dilute components of a gas mixture undergoing supersonic expansion are accelerated with the characteristics of the solvent carrier gas. In the absence of mass separation

effects (see Section 3.3) these results suggest that the usual mass spectrometer relationship, based on effusive flow, can be applied to the supersonic jet sampling process. That

is,

= UTn 0 0

where I is the observed ion intensity,

T

q the source temperature,

k the effective mass spectrometer sensitivity constant, and

P Qthe source pressure.

370
As will be shown by our data, the functional dependence of beam flux on specific heat

and Mach number can be empirically absorbed into the k factor when this "constant" is obtained

from the observed ion intensity for the carrier gas at known conditions of P Q

and T . In practice, k is measured at every temperature and pressure of an experimental run. The k's

obtained show a minor variation with temperature and pressure, reflecting small changes in

the expansion process including mass separation effects.

3.2.2 Nozzle characteristics

The size of main chamber tube (see fig. 2) and sample which could be accommodated in the

current vacuum system design, and the need to assure saturation of the carrier gas, determined the transpiration gas flow range (less than 40 seem). In the physical design of the

sampling system, available pumping speed and desired expansion ratio governed the choice of

nozzle orifice size. Experimentally, it was determined for the chosen nozzle geometry and

available pumping speed, that an orifice diameter of 0.008 cm, required a nitrogen carrier

gas flow rate of <10 seem to maintain a stage I pressure of ^5x10

7 atm. The Knudsen number (Kn

Q

) can be determined from its definition,

Kn

o

*0

1/2 2 -1 where \ = [(2) na n Q

] = mean free path,

d^ is the nozzle orifice diameter,

oa is the collision diameter, and

nQis the carrier gas number density.

Using a ~3.7 x 10® cm [19], Kn

Q for this system is 0.003 at 1 atm and 1000 K. This value is well within the limit for continuum flow which is generally taken as Kn

Q <0.1. To insure

that the sample beam represents the bulk source gas and not a stagnant boundary layer near

the orifice, the Knudsen number should be small enough that boundary layer effects can be

ignored. For Kn

Q

<.0.01 these effects are minimal [20]. In the present system, where the

nozzle is maintained at T , this Knudsen number constraint is probably of lesser importance

than the continuum flow requirement.

Pumping speed in the nozzle-skimmer region can be calculated from

F o

n

I SI

where is the net flow rate in molecules/s from the source,

0

3

n

T is the stage I number density in molecules/cm , and

1 3 Sj is the pumping speed in cm / s. For the above conditions, the pumping speed in the nozzle region is calculated as '330 L/s.

Conductance estimates, based on the stage I geometry, indicated the net pumping speed in the

nozzle region should be '-300-400 L/s in good agreement with the above flow rate determination.

371
The nozzle expansion ratio for the sonic conical nozzle can be given by the ratio of

source number density to stage I number density, i.e.,

at typical conditions of P o = 0.8 atm and T

q = 1000 K. This ratio is essentially constant as long as Sj and the nozzle-skimmer geometry are fixed. It was not feasible to construct a real-time variable distance nozzle-skimmer system for the following reason. The high flux of

condensibles from the transpiration zone at high temperatures precluded the more conventional

use of a room temperature or cooled skimmer [21] since plugging of the skimmer orifice would

be quite rapid (see Section 2.1). This problem was avoided by mounting the skimmer directly

onto the heated nozzle, just outside the furnace zone. The high available pumping speed of

800 L/s in stage I, outside the nozzle-skimmer region, produces an operating mean free path of

~15 cm. A simple plate, with an orifice for line of sight transmission (see fig. 1, region

I) allows the skimmer itself to behave as a differential pumping barrier with the stage I pump performing double duty. The disadvantage of a higher post-skimmer pressure is more than

offset by the system simplification provided, and since the skimmer orifice would be located

about 4 cm from the differential pump aperture, this nozzle-skimmer arrangement seemed to be

a reasonable compromise.

3.2.3 Skimmer characteristics

Location of the skimmer should be well upstream of the Mach disc to avoid beam scat- tering through a detached shock. Ashkenas and Sherman [9] have experimentally confirmed the

relation determining the Mach disc location, in nozzle diameters, as

for distance 2 from the nozzle orifice, at room temperature expansion ratios to ^17,000 and

T

Q = Tj. This expression places the Mach disc in our system at ^475 nozzle diameters or ~4 cm from the nozzle. Greene, et al. [17] have successfully utilized a system with similar nozzle

size, geometry, and expansion ratio parameters. Their determination of beam intensity dependence on nozzle to skimmer distance indicated a maximum intensity at 120 nozzle diameters for

1 atm pressure. At smaller Z/d

Q the intensity dropped sharply but at greater distances the

decline was gradual. In order to avoid rapid changes in instrument sensitivity with pressure,

the skimmer-nozzle spacing was somewhat larger than for the maximum intensity condition,

i.e., 1.1 cm (Z/d

Q

~140). An estimated 30 percent of the maximum beam intensity was sacrificed with this arrangement. This spacing also placed us well within the expected Mach

disc location, thus providing a safety margin against a possible over-estimate of the expansion ratio. It is also desirable to locate the skimmer downstream of the "freeze-in" point,

372
where essentially collisionless flow begins, to minimize shock induced scattering in the

downstream regions of the skimmer.

Anderson and Fenn [8] have developed an expression for estimating the location at which

the terminal Mach number occurs. At this location the Mach number has a maximum rate of

change and the collision dominated region of the expansion ends. Stearns, et al. [20] have

calculated, using the derivations of Anderson and Fenn [8], expressions for the terminal Mach

number for typical ratios of the gas heat capacity y,

M

t (Y=5/3) = 2.03

400

M

t (y=7/5) = 2.48

M

t (y=9/7) = 2.85 (^)'0 ' 222

where e is a collision effectiveness parameter, measured by Anderson and Fenn [8] for

monatomic gases to be '-0.25. This terminal Mach number can be applied in the general semiempirical expression for center- line Mach number

M = A(y) ) r \ (M>4)

o

where A(y) is 3.26 for y=5/3, 3.65 for y=7/5, and 3.96 for y=9/7. Using the y=7/5 expression

for our nozzle, a value of M. -^10. 7 is calculated. Z + /d is then about 15. Thus our choice

t to

of Z/d

Q ''140 is well downstream of the point of translational freezing.

Using the sudden-freeze approximation of Stearns, et al. [20], the number density at the

end of the isentropic expansion process can be calculated from

"t * no V * ^

19 3 15 which for typical conditions (n

Q = 2.2 x 10 molecules/cm ) in our system gives n t ^3.6x10

molecules/cm2

. The number density (n

s

) at the skimmer is derived from molecular flow con- siderations as,

1 3 3 giving n

g MxlO molecules/cm . With a skimmer orifice size of 0.08 cm, the skimmer Knudsen

number is ^56, indicating that flow into the skimmer is well into the effusive range.

The choice of skimmer aperture size was based on an expected core beam with a half angle

spread of '4°. Since the skimmer could not be readily adjusted during an experiment, additional beam attenuation due to downstream scattering for this relatively large skimmer

373
orifice would be a lesser problem than loss of the beam due to shifting of the skimmer during

thermal cycling. This has proved to be the case since we routinely monitor the low abundance

isotopes ^Ar and which are 0.3 and 0.7 percent of the carrier gases, respectively,

and find these signals to be 3 or 4 orders of magnitude above the minimum detectable signal

for high background regions of the spectrum. That is, minimum sensitivity is about 1 ppm.

3.3 Capillary probe sampling

For the converging nozzle geometry of our capillary, two descriptions of the flow con- ditions can be used. The standard isentropic relations, assuming adiabatic conditions, yield

expressions identical to those for the sonic nozzle, since the isentropic solution is independent of channel length when the exit pressure is'lower than the critical pressure, P c

6

. For this case, the pressure drop across the capillary channel is given by,

ro

c = ,2,*/(y-i)

v‘y+l J

and the exit velocity is sonic, i.e. , the Mach number is 1. A simple graphical treatment of

the velocity function along the capillary suggested that in the early phases of expansion,

the isentropic temperature drop might occur in a time scale where vibrational relaxation

could play a role. This could manifest itself, for example, as an apparent temperature error

in Clausius-Clapeyron plots of pressure versus temperature data.

Another possible treatment is to consider that for a narrow channel wall collisions

should be important. Thus, energy transfer should occur and since the capillary is essentially at T , the flow description should be isothermal. The solution for the isothermal case is a straightforward problem, very similar to the isentropic case, and yields a pressure

drop along the nozzle channel of,

0.606.

- 1/2 The flow velocity yields an exit Mach number of My . These conditions are very similar to the isentropic case and thus one would expect expansion in the stage I vacuum chamber to be

indistinguishable from isentropic expansion within the channel. In practice both the conical

and capillary samplers showed similar, though not identical, gas dynamic character.

6 This critical pressure in the minimum pressure which can exist within the

channel for a given initial pressure at the channel entrance.

374
3.4 System mass discrimination

3.4.1 Contributing factors

In order to make quantitative measurements of species partial pressures, it was necessary

to determine the magnitude of possible mass discrimination effects in our system. One such

effect is inherent in the free-jet expansion process and results from differences in the

perpendicular velocity component for various beam species, leading to radial diffusion and

depletion of low mass species in the beam. In practice, this effect is superimposed on the

normal mass discrimination effects of the quadrupole mass analyzer and electron multiplier.

The radial diffusion effect, often referred to as "Mach number focusing," has been

explained by Stern, et al. [22] as follows. As the expansion progresses, the translational

temperature falls and random velocities in the beam decrease. However, lighter gases near

the axis (and elsewhere) will have a higher net velocity and thus a larger perpendicular

velocity component with respect to the beam axis. Qualitatively, then, one would expect to observe in the regions where the flow is collision dominated, a depletion in the beam of the

lighter mass species. Past the collision dominated region, reverse diffusion would be

expected to gradually reduce this process.

Reis and Fenn [23] observed mass separation effects in nitrogen-hydrogen mixtures which

could be attributed to formation of a detached shock. A theoretical analysis by Sherman [24]

indicated agreement with the results of Reis and Fenn [23] where the bow shock was swallowed

(by the skimmer); but where a detached shock was formed, a much higher separation of light

and heavy species was predicted. He concluded that large separation effects result from a probe perturbation and vary proportionally to the inverse of the Reynolds number.

A first power dependence of mass separation with molecular weight was observed by Green,

et al. [17] which suggested that the lesser dependence observed by Stern, et al. [22] may be

due to skimming downstream of the Mach disc location. However, this is not the case in the

work of Reis and Fenn [23], and the large separations observed by Greene, et al. [17] might

have a similar explanation, even though the conditions are quite different. Greene, et al. [17] used rather small pumps and the skimmer region pressures were such that the mean free

path was of the order of 0.5 cm. A calculated Reynolds number at their skimmer, based on their reported background gas pressure, is ~0.08. Even though collisional interaction in

their beam presumably ceased well upstream of the skimmer, shock production and detachment at

the skimmer were still possible, and the above arguments could then apply at least qualitatively.

3.4.2 Experimental observations

In combination, the various mass discrimination effects are taken into account by calibration with a commercially prepared (Matheson) standard gas mixture analyzed (Matheson)

against gravimetric standards. This gas mixture contains ~1 percent of each of the rare

gases, helium, neon, argon, krypton, and xenon in nitrogen and has been analyzed to 0.01

percent of the gas composition. The overall mass discrimination function, as obtained in the

temperature and pressure region of operation and normalized to the carrier gas 1^), is used

375
to correct the ion intensity and, hence, pressure data (see Section 4.2). Table 1 gives the

gas mixture analysis (as supplied by Matheson). This analysis was independently confirmed by

us using GC analysis.

Table 1. Calibration gas mixture characteristics.

Gas Fraction of mixa Ionization cross section^ Isotope Isotope

mass (amu) abundance

N

? 0.9489 1.18 ± 0.1 28 0.9892

L 29 0.0072

He 0.0102 0.08 ± 0.03 4 1.00

Ne 0.0100 0.11 ± 0.03 20 0.905

22 0.092

Ar 0.0104 2.6 ± 0.3 36 0.0034

40 0.9959

Kr 0.0104 3.8 ± 0.6 84 0.570

86 0.173

Xe 0.0105 6.2 ± 2 129 0.264

132 0.270

134 0.105

aGas chromatographic analysis (Matheson) calibrated against gravimetric standards.

^Kieffer and Dunn [39]; cr taken from their graphs at 30 eV, errors estimated

from their data spread.

A typical plot of normalized ion intensities versus mass number, at several temperatures

and pressures, is given in figure 6. The normalization of ion intensities, I , for species m, 29

m

to the nitrogen carrier gas (using the ^isotopic species) is made using the expression

I m (norm) = »

m

A

m am

f 29' A29' a

29

! 29

where f is the mole fraction,

m

A the isotopic abundance, and

m

r a the ionization cross section at 30 eV (see table 1). m The mass spectrometer resolution used was sufficient to separate (>10% valley) each xenon

36

isotope. It should be noted that the Ar isotope concentration in the mixture is only

376
34 ppm. This mass peak is relatively noisy due to background argon and other unidentified

residual gas components. However, with reasonable integration times (4s), a signal-to-noise

ratio significantly better than 10 could be obtained, indicating a conservative detection

limit of <10 ppm. The reference discrimination curve (see effusive curve of fig. 6) was

-5

obtained under effusive conditions at a pressure of less than 10 atm using a large orifice

cell. This curve may be used as a measure of the mass discrimination effects present in the

quadrupole filter and detector.

Figure 6. Mass discrimination behavior obtained using a standard gas

mixture of: He(1.02%), Ne(l.OO), Ar(1.04), Kr(1.04), Xe(1.05), and

^(94.85): open circles - conical sonic orifice sampler at 1065 K and 0.6 atm, closed circles - capillary sampler at 800 K and 0.3 atm,

and half closed circles - effusive source at 295 K and <10 ^ atm. Error bar represents worst case for variations in ion signal; most

abundant isotope of each species normally exhibits variations less

20 than half this amount. An exception is Ne, where contributions

40 2+

from Ar and the narrow peak width limit the signal to noise

ratio. Ion intensities measured at 30 eV ionizing electron energy

29 + and normalized with respect to ^

.

Figure 6 shows a small enhancement (M0%) of higher mass species in supersonic beams

which agrees qualitatively with the arguments outlined in Section 3.4.1, considering an early

onset of collisional "freezing" and the relatively large skimmer-orifice separation used.

The rapid signal fall-off at He is mainly a mass spectrometer resolution effect. Instrument

tuning conditions produce very narrow peaks below 12 amu and consequent difficulties in

377
dwelling on the peak maximum. The effective coincidence of the mass separation curves for

the conical and capillary probes (see fig. 6) suggests that both probe types are effectively

isentropic in their expansion characteristics.

To account for the combined mass discrimination effects of mass separation in the

expanding jet and quadrupole mass filter discrimination, a sensitivity factor,

^Na+ (NaCl)’

for example, was obtained from

SNa

+ (NaCl

) (“5^j

58 amu

+ ( V23 amu

where h and e refer to the high pressure and effusion curves of figure 6, respectively.

It should be noted that the experimental mass discrimination function is essentially

fixed for a given set of quadrupole mass filter operating parameters. However, when the mass spectrometer is re-tuned by varying the ion source and resolution settings a new discrimination function is obtained.

3.5 Aggregation effects

Besides mass discrimination effects, other perturbations are possible in supersonic free

jet expansion. Condensation or species aggregation is a particularly well known case, e.g.

see [10,25] and cited references. A particularly useful test of species aggregation is to monitor the formation of Ar^ in the expansion process. However, the single pump differential

pumping scheme employed by us in the nozzle-skimmer region did not allow operation in a low

scattering condition at pressures where the Ar^ intensity would be above our detection limit.

Bier and Hagena [10] give limiting criteria for minimizing condensation, based on the source pressure-nozzle diameter product P Q *d

o

. At a source temperature of 295 K, P Q *d

o should be

less than 60 Torr-mm for 85 Torr-mm for argon, and 500 Torr-mm for nitrogen. At atmospheric pressure and room temperature our system operates at P o *d

o <60 Torr-mm and, conse- quently, condensation should be minimal or non-existant, particularly at higher temperatures.

3.6 Pressure operating characteristics

Figure 7 shows the behavior of the detected ion signal as a function of source pressure.

For this instrument, the expansion ratio is essentially constant, set by the available pumping

speed, and the turnover of the unit slope portion of the curves, marked by arrows on figure 7, occurs consistently at an observed pressure of Pj ~2 x 10 ^ atm. The fact that this turnover

occurs for both the capillary (no skimmer) and conical sampling system at the same stage I pressure, independent of temperature, suggests the effect is a post-expansion scattering

phenomenon dependent primarily on the mean free path. This argument is also consistent with

the sensitivity of the turnover point onset to the skimmer alinement relative to the nozzle.

It appears possible, as Green, et al. [17] showed, to operate at pressures where scattering is significant and still maintain sampling fidelity; our data also indicate this (see

Sections 4 and 5). However, the majority of our experimental runs were made in a pressure

378
range where scattering was relatively low, i.e., within the linear region of the intensitypressure curves such as those shown in figure 7.

Figure 7. Dependence of carrier gas ion intensity on applied pressure and

temperature. The 1200 K data have been offset vertically +0.1 div for

clarity. Solid curves are lines of unit slope and zero intercept in accord

with the theoretical relationship. Arrows indicate where stage I pressure

exceeds 2 x 10 ® atm. The capillary curve deviates from linearity at lower

pressure because the total flow rate is higher through the relatively large

capillary orifice.

4. The NaCl System: Results and Discussion

This system was chosen to test the validity and accuracy of the transpiration mass spectrometric technique, since the literature data for NaCl is extensive, reasonably selfconsistent, and has been critically evaluated and formatted as thermochemical tables by

JANAF [26].

379
Between about 800-1600 K the significant reactions in the equilibrium NaCl vaporization

process are

NaCl (s) = NaCl (g) R1

NaCl (1) = NaCl (g) R2

2NaCl(s) = (NaCl)

2 (g) R3

2NaCl(l) = (NaCl)

2 (g) R4

3NaCl (s) = (NaCl)

3 (g) R5

3NaCl(l ) = (NaCl)

3 (g). R6

Reactions R1 - R4 can be expressed in homogeneous form as

2NaCl(g) = (NaCl

)

2 (g), R7

and similarly

3NaCl(g) = (NaCl)

3 (g). R8

This variety of reactions, involving both homogeneous and heterogeneous equilibria should

provide a stringent test of the TMS technique, and particularly where the second law method

of data analysis is used.

Barton and Bloom [27] measured the total pressure of NaCl(l) by a boiling point method

and reported excellent agreement with the results of earlier workers. Vaporization of NaCl(s)

from a copper Knudsen cell was reported by Miller and Kusch [28] who used a velocity profile

analysis on effusive vaporization and determined the presence of substantial amounts of dimer

(NaCl)

2 (g). More recently, Milne and Klein [29] measured heats of sublimation and dimerization for the alkali chlorides, including NaCl, using the mass spectrometric Knudsen effusion

method. By incorporating selected literature data with their results they were able to make

an extrapolation to the liquidus region. Second law heats of vaporization for the monomer

(reaction R2) differ between laboratories by no more than 3.5 k cal/mol. This uncertainty is

sufficiently low that the reaction may be used to test the temperature dependent characteristics of the transpiration reactor sampling system.

Certified reagent grade (A.C.S.) sodium chloride (Fisher lot number 757984) was used

without further treatment. Lot analysis indicated contaminants of less than 0.1 percent and

no impurity ion signals were observed in the mass spectra.

4.1 NaCl mass spectral fragmentation data

The characteristic mass spectral fragmentation pattern for NaCl vapor has been reasonably

well established by others using Knudsen effusion mass spectrometry [29,30] over solid

k 380
samples. A potential problem with this system arises from the presence of both NaCl and

(NaCl)~ in the vapor, where either of these species could be a precursor for the ions Na

+ and

NaCl . Feather and Searcy [31] have considered the possible contribution to NaCl from

(NaC1) ?. They showed that if one assumes the dimer as the NaCl

+ progenitor, a 35 percent

change in the ratio R = Na /NaCl should result over the temperature range 870 to 1020 K. However, within a 15 percent uncertainty, they observed no temperature dependence in the

observed R value of about two. Our Knudsen effusion data (see Section 4.5) support this

observation; R values of about 1.5 were practically invariant- over the same temperature

range (see fig. 10, in Section 4.5). Hence, NaCl

+ can be used as a measure of the monomer

partial pressure7. Typical mass spectral fragmentation data are summarized in table 2.

Table 2. Typical mass spectral ion intensities

3 for the NaCl system.

Na

+ NaCl +

Na

2 Cl +

R

b Cell T(K) eV Source

1.00 0.49 0.88 2.0 Cu-Knudsen 968 70 [31]

1.00 0.73 0.91 1.4 Cu- Knudsen •v-960 20 [29]

1.00

c 0.66 0.66 1.5 Pt-Knudsen 957 30 Section 4.4

1.00

d

0.04 0.08 25.0 Pt-transp.

e 1312 30 Section 4.3

1.00 9 0.03 0.14 33.3 Pt-transp.1360 30

Normalized to +, and corrected for Cl-isotope fractions.

= I Na+/I

NaCl

+

* 7 Equivalent to a signal intensity of 920 pV(lO'fi).

d 4 Equivalent to a signal intensity of 1.8x10 pV.

e Carrier gas N., at 0.54 atm; I 9Q =1.00 (=1.8x10^ pV) and k

w =

L ^N„ N

2

Transpiration weight loss run. Note S factors differ to e. k

pV/atm K (0.65 atm).

N,

1.66xl0"

10 pV/atm K.

= 3.7x1

0~

1

Equivalent to a signal intensity of 1.9x10^ pV.

The transpiration mass spectrometric data for the vapors over solid NaCl ( i . e .

at temperatures more compatible with Knudsen effusion data) showed no measurable NaCl

+ ion.

However, while the 23 amu Na

+ ion is at a low noise position, NaCl

+ at 58 and 60 amu is relatively noisy (an order of magnitude greater than for 23 amu) and R values of about 2 cannot be ruled out on this basis. But, well into the liquid range, where the ion signals at both 23 and 58 amu are strong and relatively noise-free, we observe R ~26 ± 4

8

. This ratio

7 Recent angular distribution mass spectrometric measurements (M0-70 eV) also indicate NaCl

as the predominant (>99 %) precursor for Na

+ and NaCl+

, as well as (NaCl)

2 for Na

2 Cl +

.

(R. Grimley, Purdue Univ. , personal communication (Nov. 1978)).

8 Error is standard deviation for data above 1170 K.

381
varies slightly between experimental runs. The average ratio above 1200 K is quite constant

within a variable temperature run and shows no significant variation from -v-1200 - 1500 K, even with carrier gas pressures varying over the range 0.35-0.8 atm. That the relatively high Na

+ signal intensities are not an artifact of the external

geometry of the sampling system, such as NaCl-furnace interaction, is evidenced from the

following observations. If the furnace power is suddenly turned off, the transpiration

sampler cools much more slowly than the furnace and the Na , NaCl

+ and signals respond

to the sampler temperature, not the furnace, thus precluding a decomposition mechanism

involving the hotter furnace. Salt does not deposit on the exterior of the sampler elements

since, with the sampler cool, a sudden heating of the furnace does not produce significant

signals of sodium-containing species until the lagging sampler has reached an internal temperature where these species are normally observed, ks a final test of the absence of external

deposits, the sampler can be cycled down in temperature, the sample boat and boat-carrier

removed, and the sampler immediately returned to a high temperature. The small amount of

residual salt in the sampler produces ion signals of less than 4 percent of those obtained at the same temperature with a normal sample level present, and the ion signal decays rapidly to barely detectable levels. Ion intensity ratios can also depend on mass discrimination differences between individual species, as was discussed in Section 3.4. However, for the ions in question the mass discrimination curves of figure 6 indicate only small differences between the 23 and 58 amu

species.

It appears, then, that the high Na /NaCl ion intensity ratio, with respect to those

characteristic of Knudsen effusion measurements, are not an artifact of the sampling process.

The remaining explanation must be associated with the electron impact ionization process and

the non-thermal nature of the molecular beam. A possible pertinent observation in this

regard was made by Berkowitz, et al. [32] for the related case of Li F , using Knudsen effusion

mass spectrometry, where a factor of three decrease in Li /Li F was observed for a 300 K temperature increase. It was suggested that the tendency for Li F to yield Li +

in a state of excess energy increased with temperature and that the ion source discriminated against these

excess energy ions thereby yielding lower Li +/LiF

+ ratios at higher temperatures. In sharp

contrast to effusive beams, the supersonic beam used for our TMS studies had an effective

temperature of the order of 10 percent of the transpiration source temperature. We speculate

that this large difference in temperature between effusive and supersonic beams might well

produce a similar ion source discrimination effect for NaCl. That is, the effectively much

cooler (translational ly and probably vibrational ly) NaCl species produced by high pressure

sampling would ionize to form Na with relatively less excess energy, and the ion source would produce higher relative amounts (with respect to NaCl

+ and Na

2

Cl +

) of Na

+ for mass analysis and detection, leading to high R values. On the other hand, the effusive molecular

beams obtained using Knudsen cells remain at high temperature during passage to the ionizer

and low R values would result from the effect of excess energy discrimination. That R is

essentially temperature independent for a TMS experiment may be explained from the known

382
properties of supersonic beams where the final beam temperature does not vary to nearly the same degree as the source temperature.

This explanation of apparently anomalous mass spectral fragmentation warrants further

investigation with the TMS technique, possibly including other alkali halide species such as

Li F. However, as will become evident from the discussion to follow, a definitive explanation

of the apparent high R values obtained by TMS is not necessary to successful application of

the technique to thermodynamic studies.

4.2 NaCl vaporization and pressure calibration

The effective temperature insensitivity of the Na

+/NaCl+ ratio, R, together with aux- iliary appearance potential data (not presented here as it basically agrees with earlier

work), leads to the conclusion that both Na - and NaCl may be used as a measure of the NaCl

partial pressure without significant interference from electron impact fragmentation of the

(NaCl )2 dimer species 9

. Typical partial pressure data for NaCl derived on this basis are shown in figure 8. This plot represents a number of sets of data obtained using either Ar or

N£ as carrier gases, with total pressures varying from 0.35-0.75 atm and 0.4-0. 8 atm, respectively. Two different conical orifice samplers of similar, but not identical, construction

were used. Also, the data plotted in the liquid range include pressures calculated from both

Na

+ and NaCl

+ ion intensities. The essential concordance between these various data sets

support the ion-precursor assignments and all other aspects of the procedure adopted for

converting ion intensities to partial pressures. Note also in figure 8 the good agreement

between the experimental and literature melting points (T^), indicative of accurate temperature measurement.

4.2.1 Partial pressure-ion intensity relationships

There are three methods by which the mass spectral ion intensity data were converted to absolute pressures in the present study:

(a) classical transpiration,

(b) integrated ion intensity- time method, and

(c) reference gas ion intensity comparison.

With method (a) the salt mass loss is measured (gravimetrically) for a known volume of

transpiration carrier gas at a fixed temperature and total pressure. Typical data are given

in table 3. To convert these data to a partial pressure for the NaCl species, a small cor- rection was made for the dimer species (NaCl)^ using the JANAF [26] thermochemical data.

Subsequent analysis of our own data for the dimer-monomer equilibrium reaction (Section 4.5)

will indicate that this dimer correction may be too high but is still within the quoted

uncertainty for P Na(^given in table 3.

9 The similar T dependence of Na

+ and NaCl

+

, but not Na

2

Cl +

, shown in figure 10 (Section 4.5)

also supports this claim, as do similar data reported in the literature.

383
TEMPERATURE K

1400 1300 1200 1100 1000 900

Figure 8. Vapor pressure curves for NaCl (s and 1) obtained by TMS. Open squares - P^aCl^3 ) in ^

carrier gas, total pressure 0.4-0. 8 atm,

at 30 eV. Closed squares - (NaCl

+

) in ^

carrier gas total

pressure 0.4-0. 8 atm, at 30 eV. Open circles -

Pfj a c](Na

+

) in Ar carrier gas, total pressure 0.35-0.75 atm, at 70 eV. The data points

A and • were obtained using the integrated ion intensity (b) and classical transpiration (a) methods, respectively (see Section 4.2.1). All

other points were obtained using the ion intensity comparison method (c).

Solid curve represents data from the JANAF compilation [26]. The broken

line labeled Knudsen limit was calculated assuming a typical orifice

diameter of 0.1 cm.

Method (b) is an adaptation of the integrated ion intensity method often used with Knudsen

effusion cells as described in Section 4.4. In the present case use is made of equation 4.1

and the relationship

kNaCl = R( V

) At/IlAt

384
where R is the Universal gas constant, n the number of moles of salt transported as NaCl

species, and V the volume of carrier gas transported for time t and constant temperature T.

Table 3. NaCl Absolute Pressure Calibration

transpiration mass loss data:

time 172 min

N

2 flow rate 7.9 seem

T = 1360 K

N

2 pressure 0.7 atm

NaCl mass loss 0.193 gm

auxiliary input data :

average mol wt 72

a

P Na

2

Cl 2

/

(

P Na

2

Cl 2

+ P NaCl^ = 0,233

derived pressures:

^

>NaC1

total

~ ^NaCl /(n

NaCl

+ n N

2 ^P total " 2,96 (±0,6) x 10 atm

P NaCl = 2,28 (±0 * 4) x 10

" 2 atm

aDerived from JANAF [26] data and also self consistent with independent

Knudsen and TMS data from the present study.

^n refers to moles of gas transported.

Method (c) utilizes the known pressure and ion intensity of the carrier gas (Ar or N

2

),

but relative mass discrimination factors and ionization cross sections are needed. For the

relatively straightforward case of Na as a measure of NaCl, ion intensities were converted

to partial pressures as follows. The partial pressure of NaCl is given by,

P NaCl (Na ^ kNaCl‘

I Na

+ ‘ T ( 4 . 1 )

where k,NaClis a constant which takes into account mass spectrometer sensitivity (k 1 ), mass discrimination (S) and ionization cross section (a) effects; I^ a

+ is the measured Na ion

intensity at the sample temperature, T. The constant k^.^ can be derived from a knowledge

of the carrier gas pressure and ion intensity, i.e.,

5 = k Ar

KAr VT (4.2)

385
and

then

kAr

= P Ar

/

kNaCl = kA eafexr

(4.3)

(4.4)

The term S (^1) is an empirical correction factor discussed in Section 4.2.3.

For the case of NaCl

+ as a measure of NaCl , an average R value, denoted as R, was used

as a scaling factor (necessary for the total ionization cross section argument to be valid)

to avoid overwhelming the NaCl

+ variations by the large Na+ contribution. The

^Na

Q-| (NaCl

+

) data were obtained from the expression

WNaC1 >-«! *>-w:iWT (4.5)

Figure 8 shows the pressures derived from this expression to be equivalent to those using the

Na

+ ion intensity data [and eq. (4.1)].

4.2.2 Ionization cross sections

Values of o for the reference gases ^and Ar are known from the literature [39]. However, no such data exist for the high temperature species NaCl and (NaCl^. Expressions of

the type 4.4 can be used to obtain ratios of ionization cross sections. Values of o, relative

to Ar, were measured for N^, 0

^, S0

2 and NaCl using the TMS technique and the results are summarized in table 4. The data for N

2 and 02 agree with the literature results within the

combined experimental errors. Also the measured a's for S0

2 and NaCl seem reasonable in

comparison with data for electronically similar species. To derive a

f

Sj a

Q-j , ^N a ci was obtained

using expression 4.1 and the value of P^-j obtained by transpiration [method (a)].

2 a Table 4. Ionization Cross Sections (7ia Q

units [39])

Ar

n

2 °2

so

2 Na

NaCl

(NaCl).

30 eV - (2.6)

b

1.48 (1.2)

1.26 (1.0)

1.3 — (4.6 ± 0.3)

1.00 ( ± 0. 3)

C

1.5

d

70 eV

3.0

2.8

(4.0)

( 2 . 8 )

( 2 . 8 )

(4.0 ± 0.2)

Experimental values determined from expressions of the type 4.4 using Ar as reference ct.. b

J Literature values of Kieffer and Dunn [39] given in parentheses. Typical uncertainty ± 0.2.

c Based on S = 0.6, as determined in Section 5.2.

^Determined from literature empiricism for dimer to monomer cross section ratio of ^1.5

(Meyer and Lynch [40]); probable uncertainty ± 0.2.

386
4.2.3 Pressure correction due to gas scattering

The empirical factor S contained in expression 4.4 corrects for a differential gas

scattering effect. This effect, which is characteristic of our TMS system, arises as follows.

In the working pressure range of our system, the beam number density at the skimmer is

only four times higher than for the background. Since the differential pump aperture does

not completely limit the solid angle viewed by the ion source to beam species, some of the

carrier gas or Ar) admitted arises from the scattered background (in region I) and not the beam itself. This would not be the case for the condensible salt species. Hence, since

the ion intensities of salt species are normalized against the carrier gas signal [by expressions (4.3) and (4.4)], as part of the pressure calibration procedure, they would be underestimated by the degree that scattered carrier gas competes with beam gas. Thus, in treating

ion signals of condensible species relative to the carrier gas signal, we need to include an empirically determined multiplying factor of about 1.67 (S = 0.6) as a calibration constant,

e.g. , in expression 4.4. This factor is essentially invariant with total pressure since the

ratio of background to beam pressure remains constant for our unthrottled pumping system (see

Section 3.2). We have applied this correction factor to all condensible species partial

pressures included in this study. An independent test and verification of this correction procedure will be demonstrated

for the system in Section 5. We should emphasize, however, that even without this

correction the agreement between our data and the JANAF [26] values is well within that

usually expected for vapor pressures of complex high temperature systems.

m

4.2.4 Data comparison with JANAF [26]

Below the melting point, the sublimation data (fig. 8) fit the least squares expression:

!og P NaC1 (atm) = 7. 8(±1 . 6)-11970(±l 500)/T.

It follows that the enthalpy and entropy of sublimation (reaction Rl) are, respectively,

AH

g = 54.8 ± 7 kcal/mol and AS

g =35.6+8 cal/deg mol at 1000 K.

For the liquid region, the corresponding data-fit is given by:

log P NaC] (atm) = 4. 85(±0. 3)-8820(±200)/T,

and for reaction R2,

AH ( = 40.4 ± 0.9 kcal/mol and AS = 22.2 ± 1.4 cal/deg mol at 1290 K. v v

3

The enthalpy of sublimation compares favorably with the JANAF [26] selection of 51.6

±2.4 kcal/mol where the uncertainty represents the standard deviation in the second law

data analyzed by JANAF [26]. For the liquid region, the literature data are based on total

387
pressure measurements (i.e., not molecular specific) which have been corrected for dimer

contribution by extrapolating mass spectrometric data obtained over solid NaCl. JANAF [26]

selects AH

v = 42.7 ± 3.5 kcal/mol and AS

v = 24.2 ± 0.5 cal/deg mol at 1290 K for reaction R2. Both sets of data are in very good agreement.

The key uncertainty in the present data arises from the cross section and S terms in

expression 4.4. However, the agreement between NaCl pressures obtained by the methods (a) -

(c) at 1360 K and a subsequent check on S using Na2S0

4 strongly support the pressure calibration procedure and hence the entropy data.

In view of the sparsity of molecular specific measurements in the liquid range by other

investigators, the general agreement between JANAF [26] and our data is excellent. Further,

this appears to be the first report of measurements in the temperature range from the melting

point to 1250 K. This result demonstrates an additional advantage of TMS over the classical

effusion (e.g. , Knudsen effusion mass spectrometry) and total pressure (e.g., conventional

-4 -3

transpiration) methods in its ability to bridge the 10 - 10 atm gap in existing partial

pressure measurement capability at high temperature. The temperature range of >450 K covered

by our measurements is exceptionally large, and measurements over an even wider range are feasible with this technique.

4.3 NaCl dimerization and test for homogeneous equilibrium

The NaCl sublimation and vaporization reactions represented by the data in figure 8 provide a convincing test of the attainment of heterogeneous equilibrium, for condensible

vapor species, in the transpiration reactor and their faithful sampling into the mass spectrometer. However, in the absence of additional experimental insight, it is conceivable that

the sampling process could perturb homogeneous vapor phase chemical equilibria. A sensitive

test of the system fidelity in this regard can be made using the known equilibrium represented by reaction R7.

It is well established from the literature, and our results support this, that the

Na2Cl

+ ion may be used as a measure of the dimer species (NaCl^. Our appearance potential

and temperature dependence data indicate that Na

2

Cl +is a fragment ion of (NaCl)

2 and that

-f +

^ ^ any Na or NaCl arising from dimer electron impact fragmentation is negligible (for present

purposes). The equilibrium constant for the dimerization process (R7) is then given by:

K

d =

I Na

2

Cl +

(I Na+ or I NaCl +)2k

Na

2

Cl 2

' T

(4.6)

where kNa ^includes the relative ionization cross section, mass spectrometer sensitivity,

differential gas scattering and beam segregation factors for the monomer and dimer species, as

discussed previously. The term Ij!j a ^i + refers to the scaled NaCl

+ ion intensity, i.e., as in

expression 4.5. In practice, where I Na

+ and I Na £-j+ are used as measures of monomer and

dimer, respectively, kNa ^

= 4.0 k^ . 388
TEMPERATURE K

Figure 9. TMS data, at 30 eV with ^

carrier gas for NaCl monomer - dimer equilibrium.

Open circles and squares using capillary sampler at 0.3-0. 5 atm with Na

+ and NaCl+

(scaled--see text) as measures of P NaCl respectively. Closed circle and open

triangle obtained with conical sampler, 0.4-0. 6 atm, using Na and NaCl see text), respectively [see eq. (4.6) in main text].(seal ed--

Typical data sets for K

d as a function of temperature are given in figure 9. These data

were obtained using both the conical and the capillary- type samplers. Note the satisfactory

agreement between the various data sets and the JANAF [26] literature curve, indicating

good control over the various effects discussed earlier as contributing errors in converting

ion intensity to partial pressure, or equivalent equilibrium constant, data. Both the I M +

* iNa [as per eq. (4.1)] and the scaled [as per eq. (4.5)] signals were used as measures of

^NaCV ^ east s Q uare s line shown in figure 9 is an average of individual least squares

fits to each data set, giving

log K d = 10640 (± 400)/T -6.97 (± 0.4).

The calculated enthalpy of dimerization (reaction R7) at 1265 K is then AH

d = -48.7 (± 1.8)

keal/mol, which is in good agreement with the selected JANAF [26] value of -47.1 keal/mol , particularly since the corresponding literature data span a difference of more than

7 keal/mol (see JANAF [26]). Our data for the conical probe were obtained at carrier gas
pressures varying over the range 0.4-0. 6 atm and gas flow rates in the range 3-10 seem. The capillary probe data, were obtained at pressures of 0.3-0. 5 atm and gas flow rates in

the range 15-40 seem. That there is agreement between these data sets over a range of flow

rates suggests saturated flow conditions in the transpiration reactor.

The isentropic expansion model discussed earlier (see Section 3.3) for the capillary

sampler suggested that a temperature decrease of a few percent might occur in the early gas

expansion phase along the capillary, while the velocity in the channel was still low enough

that a re-adjustment of simple gas equilibria might be kinetically feasible. However, the

generally good agreement between the capillary and conical samplers, and between the JANAF

[26] literature data, suggests that during the early expansion period the isothermal model is

a better description of capillary flow. That is, isentropic expansion dominates only when

the remaining residence time in the capillary is a few microseconds and the flow velocity

approaches unit Mach number. Direct computer modeling is required to predict more definitively the early flow conditions for this type of sampling orifice. However, the empirical

approach adopted seems to satisfy present requirements.

4.4 NaCl vaporization at low pressures

using Knudsen effusion mass spectrometry

At the outset of the transpiration mass spectrometric studies we considered the accuracy

of the literature data for NaCl vaporization, as compiled by JANAF [26], to be more than

adequate for demonstrating and calibrating the TMS technique. However, small, but systematic,

differences persisted between our measured dimerization reaction enthalpies and entropies and

those recommended by JANAF [26]. That this difference occurred for different reactor and

furnace geometries, different probe types, carrier gas flow rates and pressures, temperatures, thermocouples, mass spectrometers, ion sources, and sample loadings, seemed to rule

out systematic error as an explanation. Closer inspection of the original literature data

concerning the NaCl dimerization process revealed the thermodynamic data to be less definitive than suggested by the JANAF [26] compilation. The data for the liquidus NaCl region

relied heavily on an extrapolation of solid NaCl data through the melting phase transition.

We therefore considered it desirable to check the original dimerization data, as obtained

over solid NaCl by Knudsen effusion mass spectrometry, using a similar technique ourselves.

Such an experiment also served the useful purpose of providing an independent check on the

Na

+

/NaCl (=R) characteristics for a thermal effusion beam of NaCl, as distinct from the

non-thermal cool supersonic beams generated by the TMS method.

For these Knudsen effusion studies a lightweight platinum Knudsen cell with a leak free

welded lid-seal was used in a vertical orientation. A Pt-Pt +10 %Rh thermocouple was used

to measure the cell temperature at the base and an optical pyrometer (calibration traced

to NBS standards) used to measure the effusion orifice temperature. These two independent

temperature measurements agreed to within the pyrometer reading uncertainty of 5 K. As the

thermocouple readings were of higher precision they were used as the primary source of

390
3 temperature data. A 33 mg sample was completely vaporized from a cell volume of 0.7 cm through an orifice 0.051 cm diameter drilled in a lid of 0.01 cm thickness.

Figure 10 shows the ion intensity-temperature dependence plots for the major ion

species--Na

+

, NaCl , Na

2

Cl and Na^C^* 10. Near the end of the experiment unsaturation

effects were noted; see the high temperature region in figure 10. The sequence of loss-ofsaturation followed the expected order of species loss, i.e., trimer > dimer > monomer. Note

also in figure 10 that during the unsaturation period, both Na

+ and NaCl

+ continue to increase

with temperature even though the dimer Na

2 Cl + and trimer Na^C^* ion intensities fall off

dramatically. This observation further supports the earlier assignments of ions to their

neutral precursors. These data also show an essentially constant R ratio, even during

unsaturation. As was mentioned earlier (Section 4.1) the magnitude of this ratio is con- siderably less than that obtained by TMS. • If the higher ratio found by TMS was related

solely to sampl

temperature we should have expected the Knudsen effusion data to show a trend towards larger R values with increasing temperature. Thus our earlier interpretation

of the corresponding TMS ratios seems consistent with these Knudsen data.

Conversion of the ion intensities to partial pressures was made using relationships of

the same form as (4.1). The proportionality constant k. , for species i, was determined using

the established integrated signal-weight loss method [33] which can be expressed in the form:

G.A.(2nR)

1/2W

^i ~ t/2 Y72 (^-7)1 aC(M

i ) l/ ^[Ii (T)

l/ ^At]

where

G is a gravimetric factor,

A an isotopic abundance factor,

R the universal gas constant,

W the weight loss,

a the orifice area,

C the orifice Clausing factor,

M the molecular weight,

I the ion intensity,

T the temperature, and

At the elapsed time.

The contribution of the trimer to observed weight loss was neglected (<1 percent error). To calculate the individual k.

1 s for NaCl(g) and (NaCl^g) it was necessary to estimate the

relative concentrations for each species. Based on relative cross section estimates and the

quadrupole mass discrimination for monomer and dimer species and their ion intensities (given

in fig. 10), we partitioned the weight loss as ~20( ± 5) percent dimer and ~80 percent monomer.

10 Note that the Na^C^* ion was not routinely monitored during TMS experiments owing to its

relatively low intensity and the preferential instrument tuning for low amu species.

391
This partitioning was considered acceptable as it produced NaCl monomer partial pressures in

good agreement with JANAF [26]. From this partitioning estimate the above k^ expression

(4.7) leads to

-11

(Na+

+ NaCl +

) = l- 00 x 10 (±30 %)pV/atm K, and

k(Na qi +

) = 4.15 x 10 ^ (± 30 %)pV/atm K.

1100 TEMPERATURE K1000 9 00 800

Figure 10. Kundsen effusion mass spectrometric data, expressed in the form of

Clausius-Clapeyron plots, for vaporization of NaCl(s). Open circles - Na

+ (NaCl),

closed circles - NaCl

+ (NaCl), open squares - Na 9 Cl + (Na 9 Cl 9 ) , closed squares - +

£ £ u 25 Na^C^ (Na^Cl^). The Cl-containing species refer only to the Cl isotopic species.

Vertical arrows denote the end of saturated vaporization and the highest temperature

included in the least squares fit. Experimental chronology was in the direction

of increasing temperature.

392
These constants were used to convert the ion intensity data (fig. 10) to partial

pressures, as shown in figure 11. The least squares lines have the algebraic form,

log P NaC1 (atm) = 5.75 (± 0.14) - 10170 (± 120)/T

log P (NaCl ) 2 (atm) = 5.81 (± 0.11) - 11050 (± 100)/T

and yield, at ^900 K, in comparison with corresponding literature data (in parentheses)

AH

S (Rl) = 46.5 ± 0.6 (52.3 [26]) kcal/mol

AS

g (Rl) = 26.3 ± 0.6 cal/deg mol

AH

$

(R3) =50.6+0.5 (57.0 [26]) kcal/mol, and

AS

s (R3) = 26.6 ± 0.5 cal/deg mol.

For the trimer (fig. 10 data),

AH

S (R5) = 62 ± 5 (62.2 ± 2.3 [31]) kcal/mol.

The monomer data are in good agreement with the JANAF [26] results (see fig. 11) but the

corresponding dimer data show a significant, though not excessive, difference.

TEMPERATURE K

1000 900 800

104

/ T K

Figure 11. Vapor pressure of NaCl monomer and dimer obtained via Knudsen effusion

mass spectrometry. Heavy curves represent the JANAF [26] data. Open squares -

P NaCl^a^

+

+ Na+ )’ c ^ oseci squares - P^

a ^

(Na^Cl*).

393
4.5 Comparison of Knudsen effusion and TMS-NaCl

dimerization data

The various Knudsen effusion and TMS data sets have been replotted in the form of

K^ (R7), as shown in figure 12. Table 4 summarizes the essential thermodynamic parameters

represented by the various individual and combined data sets given in figure 12.

TEMPERATURE K

1400 1200 1000 900

Figure 12. Comparison of

various data sets for NaCl

dimerization (reaction R7)

(see also table 4). Horizontal and vertical bars

indicate (to a good approximation) the slope uncer- tainties for the JANAF [26]

and experimental curves,

respectively (see text).

The closed circle data point

was obtained using calibration method (a) (see Section

4.2.1).

Table 4. Comparison of dimerization data (reaction R7).

log K^ -AH^ -AS^ T Range Data source

atm

"l kcal/mol cal/deg mol K

9300 (t 200 ) _ 5 ?0

10640 (t 400 ) _ 6 9?

10312 (t 150 ) _ 6 4g

150 > -6.60

(± 0.2) 42.5 ± 0.8

(± 0.4) 48.7 ± 1.8

(± 0.1) 47.1 ± 3.5

(± 0.13) 46.4 ± 0.7

26.1 ±0.9 820-

31.9 ± 1.8 11 DO29. 7 ± 1.0 800-

30.2 ±0.6 820-

980 Knudsen MS

(fig. 11)

1480 TMS

(fig. 9)

1600 JANAF [26]

1480 Knudsen + TMS

(fig. 12)

394
Note (in fig. 12) that our Knudsen effusion data extrapolate almost perfectly to the liquidus

data curve obtained by TMS. Thus both the Knudsen effusion and TMS data sets appear to be more consistent with each other than with the JANAF [26] data, though the difference is not

disturbingly great and is almost within the combined data uncertainties (see fig. 12). Such

agreement between widely differing measurement techniques clearly establishes the TMS method

as reliable and accurate, and capable of extending the pressure range of conventional effusion

methods by four orders of magnitude or greater. The temperature range is also naturally ex- tended by such a high pressure sampling capability.

5. The Na2S0

4 System: Results and Discussion

Most practical systems adaptable to characterization by the TMS technique are heterogeneous, involving equilibria between reactive gases and volatile substrates. Experience

with Knudsen effusion mass spectrometric studies of gas-solid (or liquid) systems has indicated a need to exercise great care in achieving, and then demonstrating, thermodynamic equilibrium11

. On the other hand, considerable experience has shown that equilibria involving

only condensible vapor species and their substrates are generally attainable in Knudsen cells.

With the TMS technique, the results for the NaCl system clearly demonstrated the

presence of a thermodynamic equilibrium between condensible vapor species and the corre- sponding substrate. However, these data showed the need for a small, but noticeable, correction factor in relating partial pressures of condensible NaCl species to the carrier gas

pressure (see fig. 8). This effect was attributed to mass spectral contributions of

non-beam scattered carrier gas. Such an effect should also be present in heterogeneous

systems where the scatterable gas is one of the reaction components. To further test our interpretation and data correction procedure for this effect, and to validate the TMS technique for the more general, but more difficult, case of heterogeneous gas-solid (or liquid)

equilibria, we selected the Na

2 S0

4 system as a test case. This system is known [34-38] to vaporize according to one or more reactions:

Na

2 S04 (s,l) = 2Na + S02 + 02 R9

Na2S0

4 (s,l) = Na2 S0

4 (g) RIO

Na

2

S0

4 (s,l) = Na

2

0 (soln or gas) + S0

2 + 1/2 02 Rll S0

3 = S0

2 + 1/2 0

2 R12

Historically, there has been considerable disagreement among researchers concerning the

relative contribution of each reaction to the overall vaporization process (see table 5 in

Section 5.5). It now appears that container reactions (catalytic or otherwise), temperature,

sample treatment, and gas composition (e.g. , external 0

2 or S02 present), all contribute to the overall process defined by reactions R9-R12. For the high temperature (>1360 K) conditions of interest to the present study, we can readily eliminate reaction R12 as a contributing process. Previous mass spectrometric observations of Kohl, et al . [37] indicate that

above 1300 K, SO^ dissociation is favored and this species forms less than 10 percent of the

xl For example, see the chapter by Hildenbrand elsewhere in this volume - “Attainment of

Chemical Equilibrium in Effusive Beam Sources of the Heterogeneous Reaction Type."

395
vapor. Our own observations indicate SO^ to be barely detectable at ~1 1 00 K and virtually

absent at higher temperatures.

As will be shown later (Sections 5.4 and 5.5), for most conditions reaction R9 is usu- ally predominant. This is a very convenient reaction for a critical test of TMS sampling of

gas-condensed phase equilibria. In addition to carrying out temperature dependence studies

for the isolated system, it is possible to externally control the concentration of product

species SO^ and 0

2 and provide an isothermal mass action (or Le Chatelier effect) test for

equilibrium. With our particular design of external gas supply multiple mass flow meter/

controller system we were able to introduce either S0

2

, 0^ or their mixtures at concentrations, with respect to the N

2 or Ar carrier gas, in the range of ‘'-0.02-4 percent. Usually

we maintained a constant 0

2 pressure and varied the SO^ concentration. The capability for

varying the SO^ partial pressure over a range of two orders of magnitude was more than

adequate for a mass action test of equilibrium sampling.

5.1 Sample preparation and side reactions

Certified ACS grade anhydrous Na

2 S0

4

, supplied by Fisher Scientific Company (lot number

770832), was used as a starting material. Initial preparation techniques for our transpiration runs involved melting Na

? S0. into the transpiration boat at 1200 K using a muffle furnace

under ambient conditions. Observations of Na and C0

2 in the initial mass spectra at relatively low temperatures of 900-1100 K lead us to the conclusion that a small quantity of the

salt reacted with atmospheric C0

2 to form Na

2 C0

3

. Under vacuum vaporization conditions,

Na^O^ decomposes according to the reaction:

Na

2C0

3 (s,l) = 2Na + C0

2 + 1/2 0£ ,

leading to much higher Na pressures at low temperatures than for reaction R9. These impurity

signals disappeared during the pre-run bake out period.

Another impurity reaction noted by Fryxell , et al. [34] is

2NaHS0

4 (s,l) = Na

2 S0

4 (s,l) + H 2 S04 ,

followed at elevated temperatures by

H

2 S0

4 = H

2

0 + S0

2 + 1/2 02

.

Consistent with this process we observed, during the initial measurement phase with fresh

samples, impurity signals of S0

2 and H2 0 at temperatures of about 1300 K. However, these

impurities could be readily removed by a period of outgassing prior to commencement of a measurement run. The presence of trace amounts of water in the system, either in the sample itself or as an impurity in the carrier gas, could alter the Na

2 S0

4 vaporization process according to

396
the reaction

Na

2 S0

4 (s,l) + H20 = 2NaOH (soln or gas) + S02 + 1/2 02 R13

To remove water from the sample, the salt was heated for a 6-12 hour period at 400 °C in the

assembled transpiration system prior to beginning an experimental run. This in situ drying

process was monitored mass spectrometrically by following the decay of the H

2 0 + ion signal.

Background H

20 from the aluminum walls of the stage I vacuum region limited ultimate

H

20 detection sensitivity to ~25 ppm. The zero grade nitrogen and pre-puri fied argon con- tained <3 ppm H

2 0 (Matheson supplied specification). This specification was verified by us where a dew point of less than -80 °C was found. Even using our limited detection sensitivity as an upper limit for P

H g, and allowing reaction R13 to set the values for P^g

and Pg , we can show from the JANAF [26] values of Kp(R13) that P^

g^

is negligible.

During experimental runs with nitrogen as a carrier gas we periodically monitored the ions

40 (NaOH

+

), 39 (Na0

+

), 80[(Na0H)

2

] and 79 (Na

2 02 H +

) amu but no evidence for these species

was found, in agreement with the thermodynamic prediction.

The following arguments can be made concerning the possible competition of reactions

RIO and Rll with reaction R9. Cubicciotti and Keneshea [35] have calculated reaction Rll

with Na

2 0(g) as a product, to be MO Orders of magnitude less significant than reaction R9. We can also show the corresponding reaction leading to Na

2 0 (soln) to be insignificant in

this system. From JANAF [26], the equilibrium constant for the reaction

Na

2

0 (soln) = 2Na + 1/2 0

2 R14

at 1500 K has a calculated value of

log Kp ( R 1 4 ) = log

similarly for reaction R9 at 1500 K,

log Kp(R9) = log

p 2p 1/2

K NaK 0

2 aNa

2 0

p2 ,p .p ^Na

K S0

2

r 0

2 ‘Na

2 S0

4

= -5.9;

= -16.4.

With the reasonable assumption that the activity a^

a ^g M, and congruent vaporization,

reaction R9 requires a materials balance

1/2 PNa

= P S0

2 = V

397
Hence, the sodium pressure can be given as

P Na = [4 K

p (R9)]1/4 * 1 x 10

" 4 atm (at 1500 K) -

Substitution of this value into the equilibrium constant expression for reaction R14 with

the approximation that 1/2 P^

a = Pq (valid if reaction R9 is dominant) predicts

aNa

20 <4 x 10

-5

This predicted low activity of the basic oxide is supported by the observations of

Fryxell , et al. [34] that their post-run salt exhibited a neutral pH. Likewise, our post-run

samples, even for experiments as short as 2-3 hr, did not exhibit any basic pH character.

From these considerations we can eliminate reactions R11-R14 as possible contributing

factors to Na^SO^ vaporization.

5.2 Closed system temperature dependence

Under closed system conditions, i.e., no external reactive gas lead applied, we observed

reaction R9 as the predominant process in the temperature range ca 1400-1700 K. The measured

temperature dependence of the equilibrium constant,

vR9 > = PL- VS (5.1)

is shown in figure 13, where an excellent agreement with the JANAF [26] curve is indicated.

The right-hand side axis represents an equivalent partial pressure of sodium calculated from

1 /4

the observed Kp(R9) using P^

a = [4Kp ( R9 ) ]

. This is a useful representation of Kp(R9) for

making comparisons with the literature data, as will be shown in Section 5.5. The partial pressures were obtained using the established relation,

P, = k

( Ii T

for conversion of the ion intensities for Na

+ (23 amu), S02

+ (64 amu), and 02

+ (32 amu),

k.j was derived from k^

r or k^ at each temperature T where

P o'f

i _ , P o’

f

i kAr

= and k Mr N I„e -T "2 I 36Ar

29.

398
P Q

is the source pressure of carrier gas ( Ar

I oc the ion intensity of the argon isotope at 36 amu,

36

Ar

I 9Q the ion intensity of the nitrogen isotope at 29 amu,

N

2 36

f. = 0.0034 is the Ar isotope fraction of argon gas, and

29

f.j = 0.0072 is the ^

isotope fraction of gas.

TEMPERATURE (K) 1700 1600 1500 1400

E

is2

5o

C/3 UJ

<rD

C/5

C/3 UJ

GC

Q.O

UJ

h-2

3

O-i

<4Oo

o

Figure 13. Temperature dependence of the equilibrium constant for reaction

^SO^O) = 2Na + SO

2 + O2 from TMS observations of Na

+

, S02

+ and 02+

. Data

taken at 30 eV ionizing electron energy using Ar carrier gas at pressures of

0.4 atm (closed circles) and 0.5 - 0.8 atm (open circles). The solid line is taken from JANAF [26]. The Knudsen limit was estimated for a cell with ~0.1 mm orifice diameter. The right-hand side axis has been calculated, assuming con1/4 gruent vaporization, as = [4Kp( R9 )

A typical data point for figure 13 is given by the experimental parameters:

T = 1563 K, P Ar = 0.81 atm, I 36 = 26 x 1

3

, I Na = 2100, I $G = 500 and I Q = 2300 pV. Ar ~ 2 2 These data agree within experimental error with the JANAF data if a scattering

399
factor of S = 0.6 (±0.06) is assumed. That this factor has the same magnitude as for the

NaCl study tends to establish its use as a universal correction factor for our particular

TMS system.

2

t-<

o

CAO

LU>

ccLUCA

ffi O

0 1 2 3 4 5 APPLIED PRESSURE S02 (X 103 ATM)

Figure 14. Dependence of "observed" SO

2 on externally applied partial pressure;

where P SO,(obs) = k^ +*T. Data taken at 30 eV ionizing electron energy and

1485 K using carrier at ~0.6 atm in the presence of Na^SO^. The least squares

fit yields the relationship P^q (appl) = 1.55 (±0.5) k^ I<.q +*T + 2.5 (±0.2)

-5 ^ 2 2 x 10

3 atm.

The constants k^ are then derived as follows. For the case of k^Q , use can be made

of the system response to a known external pressure of SC^. Figure 14 indicates the

relationship between calculated and "observed" pressures for SO

2

, where the observed data

refer to kw as the proportionality constant. The figure shows the excellent linear

n

2 response of the mass spectrometer system to known concentrations of reactive gas species and

gives a direct relation between observed intensity and true pressure. These data may be

used to derive cross sections, such as those given in table 4. From the slope of the line

in figure 14 we obtain,

p so

2 (obs) cn

2

’ i so

2

+ ' t

f>S0 (appl ) kS0-*

I S0o

+ * T

1

1 . 55

hence, k SQ = 1.55 kw (at 30 eV). With the mass discrimination function of figure 6, the

N

400
cross section data of table 4, and the scattering correction (multiplication factor of 1.67)

obtained from the NaCl TMS data, the equilibrium constant is then

K

p (R9) = 4.8(kAr

T)

4

(I Na

-)

2

-I s02-Io

2

+ • (5.2)

The data represented in figure 13 were obtained at various argon carrier gas pressures

P

Q

in the range of 0.4 - 0.8 atm. Separate least squares fits to the low (0.4 atm) and

high (0.5 - 0.8 atm) P Q

data poin +

s yield, respectively (in terms of Na):

AH

V = 69 ± 1 and 71 ± 4 kcal/mol

AS

V = 27.5 ± 1.5 and 28.1 ±4 cal/deg mol (for reaction R9).

The good fits within and between various total pressure-ranges is indicative of saturated

sampling conditions since the carrier gas flow rate is proportional to P

Q at constant T. Also, the excellent agreement with the JANAF [26] data for both AH^ and AS^ (evident in

fig. 13) is consistent with a non-perturbing sampling process.

5.3 Open system, isothermal, variable SO

2 conditions

The effect of variable SO

2 pressure on the equilibrium constant for reaction R9 at near

isothermal conditions (1475 ± 7 K) is shown in figure 15. Note that the least squares curve

has zero slope [required by definition of Kp(R9)] and is located very close to the JANAF [26]

curve. There are two contributing factors to the data scatter observed in figure 15. First,

during the runs, the VARIAC driven furnace system allowed a temperature variation of ±7 K, due partly to fluctuations in line power and to interaction of variable background SO

2 with

the thin-foil tantalum furnace. The second factor resulted from a slow response of I^g +

to changes in applied P SO,

believed to result from SO,

from high to low SO

2 concentrations. This slow response is

dissolving, and being retained, in the liquid Na

2

S 0 ^.

5.4 Closed-system Knudsen effusion mass spectrometric results

There appears to be considerable discrepancy in the data obtained by various methods,

and in different laboratories, concerning the relative contribution of reactions R9 and R10

to the overall Na2S0^ vaporization process (e.g., see later in table 5). For the TMS studies

considered so far, emphasis has been on reaction R9. No attempt was made to study

reaction R10 in this work; in fact the mass spectrometer tuning was optimized for the low mass range needed for Na, O

2 and SO2 rather than for the species Na2S0^. In order to be sure that reaction R10 does not compete appreciably with reaction R9 for our TMS conditions, we undertook a Knudsen effusion mass spectrometric study of Na2S0^. The apparatus and experimental procedures used were very similar to those indicated for the NaCl study (Section 4.4). An all-platinum Knudsen cell was used to avoid the sample-cell interaction problems reported

by others (e.g., see table 5).

401
10 16 z

<z

o

o

S

3

g 10-17 D

a

z

LU

cc<

CL

CL<

- A

10

•18 -

10

'

6

- LITERATURE (JANAF) ^

^

^ AVERAGE

A A A

? ?

10-3 SO 2 PRESSURE (ATM)

i i i 10 •

Figure 15. Isothermal equilibrium behavior of Kp(R9) with variable SC^ partial

pressure at 1475 (±7) K, using ^

carrier gas at 0.5 atm and 30 eV ionizing

electron energy. Error bar on JANAF [26] value (broken line) represents the

change in Kp(R9) for tne experimental temperature variation of ±7 K. The solid

line is the average of all measurements; a two parameter fit yields a similar

result with a statistically insignificant pressure dependent term. Arrows on data symbols indicate the direction of change of applied SO

2 pressure. Three

symbols with double arrows denote the turning points of the two cycles of

variation in SC^.

Our mass spectrometric results, plotted as log (I*T) vs 1/T for the principal ions Na

+

and NapSO^ are given in figure 16. The Na

+ curve gives an enthalpy of vaporization (R9)

as sodium of 72.3 ± 4 kcal/mol at 1165 K, in excellent agreement with the JANAF [26] data.

From the Na2S0^

+ curve we obtain a second law enthalpy for reaction RIO of

AH

Na

2 S0

4 = 65.1 ± 3.5 kcal/mol

which is in very good agreement with the result of Kohl, et al. [37] (shown in table 5).

The location of their least squares curves for NapS0.

+ (relative to Na

+

) are indicated for

comparison in figure 16, using the same relationship between Na2S0

4 and Na as given in

their reported mass spectrum. The agreement for the relative importance of Na2S0

4 and Na between the laboratories of Kohl, et al. [37] and ours is quite satisfactory. Also within

the errors quoted, and considering the differences in mean temperature, both mass spectrometric studies are in good agreement with the transpiration measurements of Cubicciotti

and Keneshea [35].

402
TEMPERATURE K

1200 1100

ItyVT K

Figure 16. Knudsen effusion mass spectrometric data for I Na+-T and I^

a

<-q +*T

from ^SO^l). Open circles are I Na +T and closed circles are I^

a ^g

+T data,

corrected by effusive mass discrimination plot of figure 6. Due to a substantial

time difference, and instrumental changes between obtaining the figure 6 data and

these results, the relative locations of the two lines could be uncertain by a

factor of 2. Broken lines represent two sets of I^

a ^g + , T data from Kohl, et al.

[37] located relative to our fitted I Na +*T line using their reported mass spectral

relative intensities [their fig. 2(b)].

5.5 Data summary and literature comparison

Table 5 summarizes the most recent literature for the Na2S0^ vaporization process(es).

As total vaporization techniques do not monitor individual species, they usually do not identify the separate contributions from reactions R9 and RIO. The data reviewed, including

molecular specific results, are therefore expressed in terms of P^

a for comparison with

the literature total vaporization data. The decomposition data (reaction R9) have been

converted using the assumption of congruent vaporization, i.e., P Na

2P

S0, = 2Pq as dis-

'2 u

2 cussed above. In the case of the data reported by Ficalora, et al. [36], their cited result

403
Table

5.

Comparison

of

data

among

various

investigators

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Table 5

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for AH^

a (= 78.8 ±3.3 kcal/mol) was not in agreement with the result calculable from their

raw data (their fig. 3), and we have listed the latter in table 5. In the study of Ficalora, et al. [36] no ion attributable to ^SO^Cg) was observed

and they concluded that reaction R9 together with the gas phase equilibrium R12 best

described the vaporization of ^SO^Cs). The work of Kohl, et al. [37] included tests for

sample interaction with tantalum and tungsten cells using ZrO^ liners, and with platinum

cells with or without Al^O-, liners. Their results indicated a significant enhancement of Na and SO^ signals relative to from the ZrC^ lined tantalum and tungsten cells,

and formation of additional species due to sample reduction by the tungsten cell. They

concluded that Ficalora, et al . [36] failed to identify because of a cell-interaction

enhancement of reaction R9. Transpiration data, in an all-platinum apparatus., were obtained by Cubicciotti and

Keneshea [35]. They distinguished between reactions R9 and RIO by using, separately, an atmosphere of pure and of 1/2 SO

2 + 1/2 C^. Uhlig, et al. [38] assert that the only

vapor species present in their studies is ^SO^, as no dependence of vaporization rate

on an applied partial pressure was observed. This result is inconsistent with our data and indeed the bulk of the literature. Their static technique is highly surface layer

diffusion controlled, as evidenced by the fact that the actual weight- loss rate is much

lower than expected from a saturated flow or vacuum evaporation system.

In conclusion, the summary of table 5 clearly indicates the inherent and historical

problems associated with the vaporization system and, in comparison, the quality of

the TMS data must be considered very good. The reliability of the TMS technique for heterogeneous gas-liquid (or solid) systems, as demonstrated for the very demanding system,

appears to outweigh that of alternative approaches.

We wish to acknowledge the able technical assistance provided by Mr. Art Sessoms during

the course of this study. Support by the Department of Energy under contract EA-77-A-01-6010

is also acknowledged.

References

[1] Hastie, J. W. , High Temperature Vapors: Science and Technology (Academic Press,

New York, 1975).

[2] Hastie, J. W. , Inti. J. Mass Spectrom. Ion Phys. 1_6, 89 (1975); Combust. Flame, 21_, 49 (1973).

[3] Stearns, C. A., Kohl, F. J. , and Fryburg, G. C. , this volume (1979).

[4] Merten, U. and Bell , W. E. , in The Characterization of High Temperature Vapors ,

J. L. Margrave, ed. , p. 91 (Wiley, 1967).

[5] Gordon, R. J. , Lee, Y. T. , and Herschbach, D. R. , J. Chem. Phys. 54, 2393 (1971).

[6] Kantrowitz, A. and Grey, J. , Rev. Sci. Instr. 22, 328 (1951).

406
[7] Parker, H. M. , Kuhlthau, A. R. , Zapata, R. , and Scott, J. E. , Jr., Rarefied Gas

Dynamics, Proc. First Int. Symp. , F. M. Devienne, ed. , ]_» 69 (1960).

[8] Anderson, J. B. and Fenn, J. B. , Phys. Fluids, 8, 780 (1965).

[9] Ashkenas, H. and Sherman, F. S. , Rarefied Gas Dynamics, Proc. Fourth Int. Symp.,

J. H. deLeeuw, ed. , 2, 84 (1966).

[10] Bier, K. and Hagena, 0., ibid , 200 (1966).

[11] Knuth, E. L. , Appl. Mech. Rev. 17, 751 (1964).

[12] Morris, J. F. , The Chemistry of Propellants , AGARD Symposium, S. S. Penner and

J. Ducarme, eds. , 410 (Pergamon Press, N. Y. , 1960).

[13] Sherman, F. S. , Second Biennial Gas Dynamics Symposium, Am. Rocket Soc. (Northwestern University, 1957).

[14] Bray, K. C. N. , Ninth Symposium (International ) on Combustion , 770 (Academic Press,

N. Y. , 1963); and J. Fluid Mech. 6, I (1959).

[15] Wegener, P. P. , Combustion and Propulsion , Fourth AGARD Symposium, A.C. Jourmatte,

A. H. Lefebvre and A. M. Rothrock, eds., 261 (Pergamon Press, N. Y. , 1961).

[16] Hayhurst, A. N. and Telford, N. R. , Proc. Roy. Soc. London, A322 , 483 (1971).

[17] Greene, F. T. , Brewer, J. , and Milne, T. A., J. Chem. Phys. 40, 1488 (1964).

[18] Abuaf, N. , Anderson, J. B. , Andres, R. P. , Fenn, J. B. , and Marsden, D. G. H. , Science, 256, 997 (1967).

[19] This collision diameter is from Anderson and Fenn (ref [8]) for monatomic gases

6 = 5/3 = C /C A larger collision diameter for diatomic N 0 makes Kn smaller.

P v _8 2 o The choice of 3.7 x 10 ° cm for this calculation shows Kn

Q

<0.01 for argon which

was used for some studies.

[20] Stearns, C. A., Kohl, F. J. , Fryburg, G. C. , and Miller, R. A.; NASA Technical

Memorandum 73720 (1977) and references cited therein.

[21] Brown, R. F. and Heald, J. H. , Jr., Rarefied Gas Dynamics, Proc. Fifth Int. Symp.,

1407 (1966).

[22] Stern, S. A., Waterman, P. C. , and Sinclair, T. F. , J. Chem. Phys. 33, 805 (1960).

[23] Reis, V. H. and Fenn, J. B. , J. Chem. Phys. 39, 3240 (1963).

[24] Sherman, F. S. , Phys. Fluids, 8, 773 (1965).

[25] Milne, T. A. and Greene, F. T. , J. Chem. Phys. 47, 4095 (1967).

[26] JANAF Thermochemical Tables , 2nd ed. , NSRDS-NBS 37 (1971 ), U. S. Govt. Printing

Office, Washington, D.C. [27] Barton, J. L. and Bloom, H. , J. Phys. Chem. 60, 1413 (1956); ibid . 63, 1785 (1959).

[28] Miller, R. C. and Kusch, P. , J. Chem. Phys. 25, 860 (1956); ibid . 27, 981 (1957).

[29] Milne, T. A. and Klein, H. M. , J. Chem. Phys. 33, 1628 (1960).

[30] Akishin, P. A., Gorokhov, L. N. , and Sidorov, L. N. , Russian J. Phys. Chem. 33,

2822 (1959).

[31] Feather, D. H. and Searcy, A. W. , High Temp. Sci. 3, 155 (1971).

[32] Berkowitz, J. , Tasman, A. A. , and Chupka, W. A. , J. Chem. Phys. 36,

2170 (1962).

[33] Margrave, J. L. , ed. , The Characteri zation of High Temperature Vapors , p. 38 (Wiley,

New York, 1967).

407
[34] Fryxell, R. E. , Trythall, C. A., and Perkins, R. J. , Corrosion-NACE, 29, 423 (1973).

[35] Cubicciotti, D. and Keneshea, F. J. , High Temp. Sci. 4, 32 (1972).

[36] Ficalora, P. J. , Uy, 0. M. , Muenow, D. W. , and Margrave, J. L. , J. Amer. Ceram.

Soc. 51, 574 (1968).

[37] Kohl, F. J. , Stearns, C. A., and Fryburg, G. C. , Meta1-S1 aq-Gas Reactions and Processes ,

Z. A. Foroulis and W. W. Smeltzer, eds. , p. 649 (The Electrochemical Society, 1975);

also NASA TMS 71641.

[38] Uhlig, G. F. , Davis, H. H. , and Graham, H. C. , J. Amer. Ceram. Soc. 58, 201 (1975).

[39] Kieffer, L. J. and Dunn, G. H. , Rev. Mod. Phys. 38, 1 (1966).

[40] Meyer, R. T. and Lynch, A. W. , High Temp. Science, 5, 192 (1973).

Discussion

Moderator (Green): You showed that the partial pressure in the high pressure molecular beam

sampling system was proportional to the intensity times the temperature, which is a little

bit surprising, though I expect it is right. But you actually used this relationship?

Response (Bonnell): Yes, we do. What we actually do is monitor the carrier gas along with

everything that moves with it, i.e., the transported vapor. We use the carrier gas (known

pressure) through P=kIT to back-calcul ate an effective instrument constant, k, for every data

point taken. (Actually when we do this at every temperature we simply use ion intensity

ratios.) The real point is that the beam characteristics outside of the effusive nature of

that function are carrier gas dependent. And one really does need to take a measurement on the carrier gas each time.

We find that the function (i.e., k) varies slowly with T and P. We could, in fact,

process our data with the average of all the instrument constants (k) we calculate. But, it

is rougher. There is actually a dependence on the expansion process that is observable as a change in instrument constant, particularly as a function of temperature in the transpiration

cell

Question (Hildenbrand): I didn't understand the explanation for your sodium chloride total

pressures. You said they were low by about 50 percent relative to the JANAF data.

Response (Bonnell): The current interpretation is that molecular beam collimating system is not particularly a restriction in the beam only. The beam that actually enters the mass spectrometer ion source is of the order of the size of the entrance of the mass spectrometer

itself. This arrangement allows scattered carrier gas, in addition to beam carrier gas, to enter the ion source and give too high a reference gas signal. I would like to make the

argument that 50 percent agreement is pretty good. I mean, that is quite typical of ordinary

Knudsen cell work. This question is discussed more fully in the main text.

Question (Hildenbrand): Do you expect the same kind of disparity over other condensible

species?

408
Response (Bonnell): Yes, as a matter of fact, in the sodium sulphate case, we apply that

correction to the sodium partial pressure. We do not need to apply it to the sulphur

dioxide or the oxygen.

Moderator (Greene): Did you do any diagnostics such as varying the orifice size in your

study of sodium chloride monomer-dimer equilibrium?

i

Response (Bonnell): All right, varying orifice size is a little tricky. These are platinum

cones and we take what we can get when we drill holes. We did use two totally different

samplers with approximately 30 percent difference in orifice size and there seems to be no difference in the data.

We have changed the skimmer to orifice distance slightly. This is not something that is

easily done. We are, in fact, depending upon measurements by a number of other people as to where the maximum expansion ratio occurs, which is typically like 120 nozzle diameters. So we haven't made a systematic change in orifice size. We are really not equipped with the

pumping to do it yet. That is coming. (It should be noted that the nozzle-skimmer geometry

is very similar to that used earlier for flame sampling and other 1 atm pressure studies as reported on by Hastie).

Comment (Hastie): I think the interesting thing about this work is represented by the

figure where we show data over about an 800 degree temperature range for sodium chloride

and about six orders of magnitude in pressure—that is, the data from the conical and the

capillary sampler both fall on the same line (Cl ausi us-Cl apeyron relationship). The

really striking thing was that we were able to get this kind of data with a capillary.

We thought the capillary would be much cruder and we were originally interested in the

capillary just because it would be a more robust type of sampling probe for very extreme

environments. But it does seem to give pretty good thermodynamic data.

409

Appendix B

SLAG AND METAL OXIDE VAPORIZATION IN REACTIVE ATMOSPHERES

J. W. Rastie, D. W. Ronnell and E. R. Plante

Chemical Stability and Corrosion Division

Center for Materials Research

National Bureau of Standards

Washington, D. C. 20234

Abstract

Metal oxides, whether they are present in the form of refractory

ceramics or slags, are key components of high temperature energy systems.

Planned magnetohydrodynamic (MHD) and coal gasifier systems are particularly dependent on the thermochemical properties of ceramic and slag

materials. However, almost no basic thermochemical data exist for

oxides in the presence of high temperature-high pressure reactive gases

such as H^O. The present study utilizes a modification of the now

classical Knudsen effusion mass spectrometric technique and a new

technique, transpiration mass spectrometry, for molecular-level thermochemical analysis of MgO and a potassium enriched MHD coal slag in the

presence of Ho 0 vapor and related gases. New data are presented for the

enthalpy of formation of MgOH (AH^ = -35.8 ± 3.0 kcal/mol) . Species

partial pressures and derived activity coefficients are given for a

potassium enriched Illinois number 6 coal slag over a range of K? 0

content (15.4-8.0 mole%) and temperature (1100-1820 K) . Data on the

effect of H^O vapor on alkali vapor transport are also given.

1. Introduction

One of the key limitations to further technological improvement, or

the new development, of fossil energy systems is the ability of ceramic

components to withstand chemically corrosive environments at high

temperature and pressure [1,2]. Alkali metal impurities in coal and

some oil fired combustors are known to be particularly important in the

degradation of alumino-silicate refractories, e.g., see Rigby and
Hutton [3], and Clews, et al. [4], The thermodynamic activity of alkali

metal species in coal minerals, slags and ceramics is therefore an

important, but usually unknown, parameter in the design or performance

evaluation of coal fired combustors or gasifiers.

Related to this problem is the interaction of gaseous species,

particularly H^O and , with the alkali-containing systems and also

with the ceramic components. These gases can greatly modify the vapor

transport properties of alkali and other metal containing components by

virtue of the formation of volatile oxide, hydroxide and posssibly

hydrated vapor species. Numerous examples of such species have been

cited elsewhere [5]. Recent citations pertinent to the present discussion include:

• vapor transport of silica, possibly in the form of Si(OH)^

species, under coal combustion conditions [6];

o water enhanced vapor transport of Cr^O^ from chromite-based

ceramics, possibly in the form of Cr0 o (OH) species [7,8];

• water enhanced vapor transport of Na? 0 from soda-lime-silica

melts [ 9]

and

• water enhanced vapor transport of MgO ceramics, possibly in

the form of Mg(0H) 9 species [10].

These studies are based either on the classical transpiration (at ^ one

atm) or, in a relatively few instances, the Knuds en effusion mass

-4

spectrometric method (at ^ 10 atm) . Only in a very few cases is there

any assurance of species identity and in each instance the conditions of

pressure, temperature, and reactive gas composition differ greatly from

those present in practical energy systems. These two basic experimental
- 3 -

limitations make it difficult, if not impossible, for one to accurately

interpret or predict, through the use of thermodynamic models, the

nature of ceramic-slag-gas interaction in fossil fueled energy systems.

A further complication in modeling such complex systems is the

possible presence of non- equilibrium or rate limiting processes.

Diffusion effects, for instance, can significantly reduce vaporization

rates from oxide solids [11] and melts [12].

The present study addresses these interrelated problems of species

identity, relation of experimental conditions to practice, and applicability of thermodynamic equilibrium, through the use of a modified gas

inlet Knudsen effusion mass spectrometric method (KMS) and a novel

transpiration mass spectrometric technique (TMS) . The systems of interest

include:

(a) HgO(s) + K

2 0, H

2

, C> 2

, and

(b) A K-seeded Illinois Humber 6 coal slag derived from an

MHD test channel [13].

The first system has practical significance in open cycle coal fired '1HB

systems, where the durability (i.e., enhanced volatility) of MgO components in the presence of reactive gases at temperatures in excess of

2000 C is of concern. The importance of the second system relates to

the retention of potassium seed by slag. This seed loss may reduce

plasma conductivity and will also limit the recovery of seed which, for

economic feasibility, must exceed about 98 percent.

Previous transpiration vaporization studies, e.g., see Maeda et

al. [10], have indicated the probable formation of Mg(OH) 0 and MgOH from

a reaction of MgO(s) with H ? 0 or . The JA1TAF thermochemical tables [14]
- 4 -

have estimated a value for the heat of formation of HgOH as no reliable

experimental data are available for this species. No basic thermochemical -data are available for potassium species vapor pressures and

activities in MHD slag compositions.

2. Experimental

2.1 Apparatus and Procedure

2.1.1 Gas-Inlet Knudsen Effusion Mass Spectrometer System

The Knudsen effusion cell, furnace, and vacuum assembly used for

the present study are similar to those described elsewhere [15] with the

(a) following exceptions. A Bendix time of flight (TOF) mass spectrometer was used as the beam analyzer and the vacuum system consisted of a

differentially pumped configuration using mercury diffusion pumps. The

Knudsen cell was modified by insertion of a gas inlet tube through the

cell base. A mechanical chopper was used for beam modulation to distinguish between beam and scattered gaseous species in the ion source of

the mass spectrometer. The TOF instrument was modified to operate in a

continuous mode giving significantly increased sensitivity.

Knudsen Cells

An iridium Knudsen cell, with dimensions 1.6 cm length, 0.95 cm

O.D. , 0.076 cm lid thickness, and 0.1 cm orifice diameter, was used for

the MgO experiments. Gas injection was carried out through an iridium

reentrant tube 15 cm long, 0.43 cm O.D., which was welded into the

Knudsen cell bottom such that the internal portion extended 0.64 cm into

the cell cavity. Gas injection to the Knudsen cell occurred through two

holes, 0.025 cm diameter, located in the side of the reentrant tube and

(a) Certain commercial items are identified only for completeness and

this does not imply that they are the most suitable available

instrumentation.
- 5

at right angles to the cell orifice. The entire cell was weld-sealed to

prevent leakage. Samples were loaded through the effusion orifice. Cell

heating was achieved by radiation from a tungsten resistance heater 5 cm

long and 1.6 cm diameter.

For the slag vaporization studies weld-sealed platinum Knudsen

cells, having orifice diameters of about 0.025 cm, were used.

Temperatures were obtained from optical pyrometer sightings of the

cell orifice made through a prism,- vacuum chamber window, the ion

source, the chopping wheel and an adjustable aperture separating the

mass spectrometer and Knudsen cell chambers. A correction of 4.38 x

10 ^ deg ^ was used with the apparent absolute temperature, to correct

to the IPTS-68 temperature scale.

Partial Pressure Determination

Conversion of mass spectral ion intensities to species partial

pressures was made through the basic relations

( 1 )

£_.m

.

l

( 2 ) 1 ac/M.II. +

t/T

l l

where

P. is the species partial pressure

l

1^ the corresponding ion intensity

temperature

k_^ instrument and system sensitivity

G gravimetric factor

Em weight loss

t time
c clausing factor

- 6 -

a orifice area and

IL the molecular weight of species i (or an average value

for Knudsen effusion)

,

as described elsewhere [15]. In some cases, for different species,

but arising from the same experimental configuration, can be interrelated through known ionization cross sections, o t and transmission/

detector efficiencies, S. i.e.,

1

k

i “ (ai*

S i*

A

i

)_1 (3)

where is an isotope abundance factor (see Bonnell and Hastie [16]).

These basic parameters are summarized in table 1. The degree of agreement for the k^'s obtained from weight loss [expression (2)] or cross

section data [expression (3)] provides a useful check on the internal

consistency of the partial pressure data.

2.1.2 Transpiration Mass Spectrometric System

The transpiration mass spectrometric system (HIS) has been described

in detail elsewhere [16], and the basic apparatus is shown schematically

in figure 1. Sample transpiration and molecular beam production are

carried out in chamber I, while beam modulation and mass spectral

analysis are achieved in chamber II. In the present study a capillarytype sampler of proven sampling fidelity [16] was used.

Relation (1) can also be applied to '’’MS data. The sensitivity

factors k_^ are obtained by several independent methods, thereby providing

a good test of internal consistency in the data. The basic relationships

are:
- 7 -

and

where

k = R ^

At/ElAt (4)

(5)

R is the gas constant

n number of moles transported

V volume of gas transported

At transport time

I species ion intensity (corrected for isotope abundances)

a species ionization cross section

S beam expansion and quadrupole mass discrimination factor, and

§ an instrument gas scattering correction 0.6, [16]).

The reference or transport gas sensitivity factor is obtained from

equation (1) since the pressure of transport gas is known from an

external manometric determination. Table 1 summarizes the ionization

cross sections and sensitivity factors used to obtain partial pressures

from ion intensity data.

2.2 Pvesults

2.2.1 KMS Data for MgO + H ? System

MgO vaporizes primarily as Mg and 0 o species under free vaporization conditions. From the JANAF thermochemical data [14], we can

expect H0 or H

? 0 to enhance the vaporization of MgO by the formation of

volatile hydroxides such as MgOH and Mg(0H) ?

. However, the extent of

this interaction is uncertain by at least an order of magnitude oiling to

the estimated nature of the thermochemical data for MgOH [14]. In the

present study, the reactions:

MgO(s) = Mg + 1/2 0,

H„0 = H„ + 1/2 0,

( 1 )

( 2 )

Mg + 1/2 H9 0 + 1/4 0

2 MgOH ( 3 )
- 8 -

were monitored, virtually simultaneously, by the KMS technique as a

’ +

function of temperature and pressure using the ion intensities of Mg , MgOH

+

, H

2

0

+

, 0

2+

and H

2+

.

Species Production and Identification

In order to enhance the production of MgOH, molecular H

2 was used

as the reagent gas rather than Ho 0 which would favor Mg(OH)

2 formation.

This served as a convenient source of H

2 0 while keeping 0 o at a reduced

level [via reaction (2)] and maintaining a high level of Mg [through

reaction (1)]. A simultaneous formation of Mg(OH)

2 and MgOH was considered undesirable as the (MgOH) +

ion could not be assigned unambiguously to a single molecular precursor. In the present experiments, with

excess H

2 present, Mg(0H) o +

was below the mass spectral detection limit

of ^ 10

^

atm. The MgO species was also undetectable. An observed 40

amu ion was attributable, from isotope arguments, to a Ca impurity. In

the absence of added H

? no modulated ion signals were detected at 41

amu.

Identification of the 41 amu ion, in the presence of H

2

, as MgOH

+

(with precursor MgOH) was verified by its intensity dependence on the

square root of the H

2 +

intensity for isothermal conditions. This is

consistent with the reaction:

MgO(s) + 1/2 H

2 = MgOH . (4)

Calibration and Pressure Determination

Conversion of ion intensitites to partial pressures for each

species was made as follows. In the absence of H^ , the Mg+ion intensity

can be related to the known partial pressure of Mg over MgO(s) [14] by

relationship (1) , yielding a value of the sensitivity factor k^ for

each data point. Then, from relation (3),
- 9 -

k. « k /a *S *A^

Mg g Mg Mg Mg

the other species present, also from a relation of the type (3).

An independent test of the calibration procedure was provided by

conversion of observed 0^~ and 0

+

(at low eV) ion intensities to partial

pressures and equilibrium constants for the reaction:

0

2 = 2 0 . (5)

The equilibrium constants, obtained under neutral and reducing conditions

(H^ present), and at various temperatures, agreed with JANAF [14] to

within a factor of two.

Mass Action Test for Equilibrium

A.S a further test of data consistency and, more particularly, the

establishment of thermodynamic equilibrium, an attempt was made to

measure the equilibrium constant, K , for reaction (2). At constant

temperature, K^(2*) should be invariant with ion intensity (i.e. ,

partial pressure) for an equilibrium process. Typical data are given in

figure 2. Note that the K ’ s are essentially invariant with H +

ion

P

- intensity but that the absolute magnitude is about a factor of 30 less

than the literature value [14]. This low K (2) value suggests P is

p l 2 too low as Pq was verified through reaction (5) and it is unlikely that

H

? 0, as a reaction product, could be produced in excess of the equilibrium amount. We believe that the low intensity H9 +

signals resulted

from a known discrimination effect of the TOF ion source magnet on light

masses such as H_ +

. Hence the JANAF values of K (2) were used to

2 P

calibrate for this discrimination effect in the derivation of lc

T

.

This correction was not necessary, however, to the measurement of MgOH

equilibria as use of reaction (3) eliminates explicit consideration of

H9 partial pressures.
- 10 -

Thermodynamic Data for MgOH Production

The temperature dependence of K (3) is shown in figure 3. A

P

representative example of data used for the calculation of K (3) is

P

given in table 2. Table 3 summarizes the partial pressure, equilibrium

constant and third law enthalpy data for reaction (3) . The Gibbs (free

energy) function data used for the enthalpy calculations were obtained

from JANAF [14] and should be reliable in view of the simple electronic

and geometric structure for MgOH.

Evidence in support of reaction (3) being at equilibrium is provided

by the invariance, within experimental uncertainty, of K (3) with P , p h

2 u e.g., see the 2054 K data points in table 3.

JANAF* s most recent estimate of the standard heat of formation of

MgOH, based on bond energy considerations, is -39.38±9 kcal/mol. The

average heat of reaction (3) of -42.2 kcal at 298 K can be combined with

the heat of sublimation of Mg(s) (+35.3 kcal/mol) and one half the heat

of formation of H^O (-28.9 kcal/mol) to yield a standard enthalpy of

formation for MgOH of AH^ ?g8 MgOH = -35.8±3 kcal/mol. The error limit

arises from an estimated factor of two uncertainty in the cross sections

used to convert ion intensities to partial pressures. From the data

listed in table 3 and figure 3, a second law evaluation gives

AH9g8 (3) = -52.4 ± 10 kcal/mol,

which is equivalent to

AH^ 9g gMgOH = -46.0 ± 10 kcal/mol.

The uncertainty is based on two standard errors in the second law

enthalpy. Agreement between the second and third law values of AH

2g g(3)

is satisfactory considering the limited experimental temperature range

(e.g., see figure 3).
- 11 -

2.2.2 TMS and KMS Data for MHD slag K

A detailed study was made of vapor transport over a high melting

temperature (y 1700 K) potassium-enriched coal slag (Illinois No. 6).

The initial slag composition, in wt%, was: A1 9 0

3 (12.06), CaO (3.8),

Cr

2 0

3 (1.3), Fe 9 03 (14.25), K

2

0 (19.54), MgO (1.03), Na

2

0 (0.47), S0

3

(0.21), Si0

2 (46.82), Ti0 9 (0.52). This slag sample was obtained from

combustion of Illinois No. 6 coal with additional potassium added to the

combustor [13]. For identification purposes we shall denote this high

potassium-content slag as K^. X-ray diffraction data indicated that the

bulk of the potassium in the slag is present as the compound KAlSiO^.

TMS analysis indicated that about two percent of the potassium was

present in relatively volatile form, mainly K

2

S0^ and K

2

C0

3

* We present here data on vapor species identity and vapor transport

dependence on temperature, N9 carrier gas pressure, H

2 0 pressure, and

slag K9 0 content. The results are interpreted to indicate that the bulk

of the potassium is highly bound in the slag and that its release to the

vapor phase may, in some instances (particularly at reduced temperature),

be controlled by the rate of alkali diffusion to the surface.

Identity of Volatile Species

The as-received potassium-enriched coal slag was subjected to a

series of heating cycles (runs) in nitrogen carrier gas. During the

initial heating cycle mass spectral scans, obtained using the TMS

technique, revealed many volatile species, in addition to the expected K

and Na species. A typical mass spectrum is given in figure 4 where the

following species can be positively assigned: Ho 0, C0 o , S0 o , 0 o , K,

and Na. Some of the other ion signals can be tentatively assigned to

the species (some hypothetical): KO or KOH, KS or KSH, SiS, SiSH, H? S,
- 12

H^SO,,, and KSiO. From JANAF [14], we can expect to see KOH under these

conditions but not H0 S. Some of these more minor species may result

L. from slag occlusions and would not represent an equilibrium release from

the slag. Following this initial heating cycle, the only significant

slag vapor species were K ard 0 ? .

Initial Sp pcies Partial Pressure—Temperature Depend ence

The initial volatiles showed a non-monotonic variation of partial

pressure with temperature, as shown in figure 5. These volatiles

constitute only a few percent of the total slag components and are not

representative of the bulk slag composition. However, they do provide a

sufficiently high flux of/ alkali (Na, K) and SO^ to be a potential

source of corrosion. The high initial partial pressures of S0 9

, C0 o , K,

and Na are indicative of the presence of alkali sulfate and carbonate in

the slag. An additional contribution to low temperature alkali release

could result from the high Ho 0 content leading to the formation of

volatile hydroxide species (KOH) at T < 1250 K. Mote that the T > 1400

K the K pressures fall below those expected from KAIO^, but that the

S0 o , C0 9 , and Ho 0 pressures are still relatively high. Apparently, at

this stage, the K produced by sulfate and carbonate decomposition is

retained in the bulk slag. After further heating at 1620 K for about 30

min., the sample was virtually depleted of Na, S0 ?

, and CO^; Ho 0 also

continued to fall off in pressure to a negligible level.

Potassium Partial Pressure—Temperature Dependenc e

The variation of K-partial pressure was followed over a wide

temperature range (y 1150-1820 K) using both the TMS and KMS techniques.

Usually, the 0 o partial pressure tracked with the K data (but not always

in stoichiometric proportion) indicating the main vaporization process
- 13

to be:

K

2

0(slag) = 2K + 1/2 0

2

, C6)

with possible secondary contributions, as discussed later. At temperatures corresponding to K partial pressures of 10 ^ atm, or less, the

rate of loss of KpO from the bulk was sufficiently low that the da*7 a

represent constant composition conditions. At higher temperatures, and

vaporization rates, the data are greatly modified by the effects of

changing slag composition.

Representative vaporization data are given in figure 6. For a

9 Q 4

typical TMS data point, at 1466 K, PVJ = 0.64 atm, Iv

= 3.2 x 10 yV,

l "9 “9

39 -6

I = 200 yV, yielding P = 7.0 x 10 atm. In order to obtain accurate

K K

data at K-partial pressures below 10 ^ atm, use was made of Knudsen

effusion mass spectrometry (KMS) . The KMS system used for these slag

measurements differed from that used for the MgO studies in that no gas

inlet was used and a quadrupole mass filter was substituted for the TOF

analyzer. A relatively high background signal at 39 amu (K+

position)

limited the sensitivity of the TMS measurements. By combining data from

both the TMS and KMS methods we were able to cover a wide range of

temperature and K9 0 mole fraction. The rate of vapor transport differs

appreciably for the two methods, with KMS losing about three times as

much material as for TMS at equal K pressure. Thus the bulk slag composition changes more rapidly during a KMS experiment.

The N0 carrier gas used for the TMS experiments also contained a

small, but significant, amount of 0 o (typically ^ 5 x 10 ^ atm) . This

caused a suppression of the K-vapor pressure by a reversal of reaction

(6). When this effect is taken into account, using the observed thermochemical data for reaction (6)

,

good agreement is found between the TMS
- 14

and KMS data, e.g., see the comparison temperature (1561 K) and composition point in figure 6. That such an agreement is possible is a

strong indication of thermodynamic equilibrium, at least with respect to

K transport, as species residence times in Knudsen cells and transpiration tubes differ appreciably.

Several processes appear to be contributing to the relatively

shallow slopes of the curves presented in figure 6, which under equilibrium conditions represent enthalpies of vaporization. During run 1

(KMS) the ratio of 0

? to K varied from being much lower than allowed by

reaction (6) at low temperatures to somewhat larger at higher temperatures. Also, during the low temperature phase of this run, release of

K? SO^ and Ko C0

2 was still occurring. Each of these factors can lead to

higher than expected K-pressures at low temperatures and hence a low

apparent vaporization enthalpy.

Ko 0 Activity Coefficients

As most of the bulk-slag composition changes result from reaction

(1) , a continuous monitoring of the K-partial pressure (and 0

?

) allows

one to calculate the slag composition at any stage of an experiment.

For instance, with the TMS method, the number of moles (n) of K? 0 lost

by vapor transport (as K + 0 o ) is related to the known mole number and

pressure of N0 by the transpiration relationship:

[n(N

?

) + n(K

2 0)] PR Q

n(Ko 0) =
- 15 -

Hence the change in composition with time can be determined, provided

KMS method, the Knudsen effusion equation [see relations (1) and (2)]

may be used to follow the bulk-slag composition. For both the TMS and

KMS methods, the basic experimental requirement for monitoring the bulk

composition is the measurement of significant species partial pressures

as a function of time during an experimental run. An independent check

on this approach can be provided by chemical analysis of the sample

remaining at the end of a run. Such an analysis has not yet been

performed on the post run slag samples. However, when all the significant species are measured, and the ionization cross sections are known,

the in situ approach to monitoring composition changes provides a good

mass balance at any stage of the experiment, as was shown for the NaCl

and Na 0 SO^ test systems reported elsewhere [16]

.

Since the mole fraction of K? 0 can be defined at any stage of an

experiment, it is possible to convert K-partial pressures to Ko 0 activity coefficients. By varying the amount of Ko 0 present in the slag

during a vaporization run we were able to follow the dependence of the

K? 0 "apparent” thermodynamic activity on temperature and composition.

The term "apparent" is used to emphasize that the slag system may not

always be in a state of complete thermodynamic equilibrium.

Typical data, expressed in activity coefficient form, are given in

figure 7. Activity coefficients ("apparent") were calculated from the

experimental K-partial pressures using the expression:

the initial sample weight and composition are known. Likewise, with the

[slag]

1/2

[pure Ko 0 reference] X(Ko 0)

4
- 16 -

where X(K9 0) is the slag K^O mole fraction, determined from the calculated mass loss using the integrated ion intensity-time method. The K^O

reference state data were obtained from the literature. This expression

assumes congruent vaporization according to reaction (6)

That this is

the appropriate reaction was confirmed by KMS observations of 0 o in the

appropriate ratio to K, to a reasonable approximation. As was indicated

in figure 6, the corresponding TMS data for y(K? 0) need to explicitly

include the 0

? partial pressure as it was considerably higher than the

dissociation pressure of Ko 0. When this is done the TMS yO^O) data

agree satisfactorily with the KMS yCK^O) values, where comparison

temperatures and compositions are available. The largest experimental

uncertainty in comparing activity coefficient data sets from the two

different techniques (KMS and TMS) is the accuracy of X(K9 0) which is

probably uncertain by ten percent in each case.

Note in figure 7, the non-monotonic nature of the log yCK^O) vs T

curves. For normal non- ideal solution behavior, we would expect a

linear monotonic relationship with a negative slope representing a

negative partial molar enthalpy of solution for Kn 0 in the slag. This

type of behavior occurs for segments of each run (see figure 7), e.g.,

for run 1, up to about 1430 K, and for run 2 between 1430 and 1630 K.

The run 1 data were obtained during the initial heating period when

KpSO^ and K9 C0^ decomposition was a significant source of K. The rapid

reduction in y(K9 0) as the temperature is increased beyond 1430 K results

from the almost complete depletion of these relatively volatile forms of

potassium. For runs 2 and 3, the initial reduction of y(K9 0) with

increasing temperature is believed to be due either to diffusion limited

(in solid slag) K-transport to the slag surface or to changes in the
- 17

mode of 0 9 release from the slag, for instance through Fe^O^ dissociation. The onset of increasing y(K9 0) with temperature is believed to

arise from an as yet unspecified physiochemical change in the slag

leading to a less viscous form, thereby reducing the diffusion limitation, or, again, to secondary sources of 0 9

. At higher temperatures the

bulk composition changes rapidly (see mole fractions in figure 7) and

leads to a peaking in yCK^O) . We believe that the data for runs 2 and

3, at temperatures in excess of 1450 K, represent an equilibrium vaporization condition, particularly as the KMS and TMS data are in agreement

for these conditions. Clearly, these unusual trends in the yCK^O) data

indicate the difficulty involved in making a_ priori predictions of slag

vaporization behavior.

slag + H^O System

m The effect of added H? 0 on the release of alkali from the K^ slag

to the gas phase was monitored with the transpiration mass spectrometer

-3 -1

from 1553-1692 K and 10 -10 atm vapor in ^ 0.5 atm N9

. Typical

data are given in figures 8 and 9. Note in figure 8 the relatively

small dependence of K-pressure on temperature at a constant H 9 0 pressure. However, as is shown in figure 9, at constant temperature, the Kpressure is strongly dependent on the H9 0-pressure . The following

reactions are possible:

K9 0 (£) (in slag) + H o 0 = 2 KOH (7)

K9 0 (Z) Cin slag) + H9 = 2K + H9 0 (3)

Hq° = H9 + 1/2 0 o

. (9)

As no KOH was observed for these particular conditions, reaction (7)

does not appear to be significant. From the instrumental detection

limit for K0H

+

, the species KOH is estimated to be at least an order of
- 18

magnitude less abundant than K. We believe, therefore, that reaction

(8) is the predominant mechanism for H^O-induced alkali release under

the conditions pertaining to figure 9- and that the thermal dissociation

of water [reaction (7)] provides the necessary . However, as is shown

in figure 9, the reactio.is do not appear to have attained equilibrium.

Future experiments with added H9 gas are planned to test this

interpretation.

3 . Summary

In the presence of H^, MgO vaporizes primarily as MgOH, Mg and 0

9

.

From the measured equilibrium between these species (and ^0) a value of

AHf 2 qg

MgOII = -35.8±3 kcal/mol was determined.

A detailed study has been made of vapor transport over a high

melting temperature 1700 H) potassium-enriched slag derived from

Illinois Number 6 coal. The rate of release of K, and secondary species

such as H^O, 0 9 , S0 9 , C0

9 , and Na, has been measured over the temperature range 1100-1820 K, a N9 -pressure range of 0.2-0.63 atm, and for

K? 0 slag contents over the range 15.4-8.0 mole%. Apparent activity

-9 -7

coefficients for K9 0 in the range of 10 -10 were determined for this

regime of temperature and composition. In the presence of H9 0 the rate

of K-release increased but apparently not to the equilibrium level.

Ac knowledgment

These studies were supported in part by the Department of Energy,

Fossil Energy—MHD, and Coal Gasification Divisions. Mr. Art Sessoms

provided invaluable technical assistance in the experimental work.
Table 1

Ionization Cross Sections (a) and

Sensitivity (S) Factors

KMS TMS

C

a h

b

a.

i s.1

H

2 — — 1.48 1.0

°2 2.62 0.177 1.26 1.3

h9 o 1.73 0.236 1.5 0.8

Mg 3.44 0.204 — —

MgOH 4.98 0.156 — —

K

e — 7 .

8

d 1.3

Na 4.5

d

1.0

Cross section for Mg given by J. B. Mann, J. Chem. Phys. 46., 1646 (1967).

Values for MO, and MgOH estimated with 30 percent uncertainty,

b

^ Based on the empirical dependence of multiplier efficiency on particle

mass; relative data only.

cCL and S_^ values measured directly as described elsewhere [16].

^Experimental data cited by L. J. Kieffer and G. H. Dunn [Rev. Mod.

Phys. 38_, 1 (1966)].

0

KMS partial pressure data for K and 0

9 obtained via relationships (1)

and (2), i.e., no G, S data were necessary.

r

- 19 -
Table 2

Typical Reaction (3) Data at 2029 K

Ion A.

l

k

a

K

i

I

+

*T(yV«K) P^, (atm)

Mg

+

0.78 3.16 - 12

b 2.19 + 6 6.92 - 6

°2

+

1.0 3.73 - 12 2.86 + 5 1.07 - 6

Ho 0

+

1.0 4.05 - 12 4.02 + 5 1.63 - 6

MgOH

+

0.78 2.85 - 12 1.58 + 3 4.50 - 9

aValues of a.11and S. used to derive k. are given in table 1.

l

^Denotes e.g., 3.16 x 10

^

atm/uV K (10

/ 0.) .

- 20 -
Table 3

Thermodynamic Data for Reaction (3)

Temp

.

P

Mg

P

H? 0

P

°2

P

MgOH

AH

298

3

K atm atm atm atm cal

2077 1.48-5 7.56-6 7.42-7 1.35-8 11.3 -42306

2077 1.66-5 1.01-5 4.84-7 1.53-8 11.0 -42195

2022 6.10-6 1.47-6 9.73-7 3.34- [ 14.4 -42194

2022 5.14-6 2.64-7 1.19-6 1.09- \ 12.5 -41625

2029 6.92-6 1.63-6 1.07-6 4.50-9 15.8 -42710

2054 9.73-6 1.47-6 1.75-6 6.16- [ 14.4 -42841

2054 1.32-5 6.16-6 1.10-6 1.44-8 13.6 -42608

2054 1.81-5 1.10-5 7.12-7 2.40-8 13.8 -42668

2054 2.99-5 2.00-5 3.84-7 4.10-8 12.3 -42198

2054 2.79-5 2.17-5 2.60-7 3.62-8 12.3 -42198

2107 2.26-5 3.72-6 4.18-6 1.72-8 8.73 -41817

2107 2.95-5 1.34-5 2.77-6 4.75-8 10.8 -42708

2098 3.07-5 1.76-5 1.78-6 4.08-8 8.67 -41615

2167 5.33-5 1.77-5 7.74-6 7.43-8 6.28 -41550

2167 5.98-5 3.39-5 5.96-6 1.09-7 6.34 -41591

AVERAGE -42188

Obtained from the expression:

“298 - T E t- (G° - H°

98 )/T - R£nKp ] ,

( p-r ) where the summation refers to the difference between product (p)

and reactant (r) thermodynamic functions.

21
References

1. Raymon, N. S. and Sadler, III, L. Y., "Refractory Lining Materials

for Coal Gasifiers", Information Circular 8721 pp. 1-22 (1976).

2. Crowley, M. S., "Refractory Problems in Coal Gasification Reactors",

Amer. Ceram. Soc . Bull., 54_> 1072 (1975).

3. Rigby, G. R. and Huttoi , R., "Action of Alkali and Alkali-Vanadium

Oxide Slags on Alumina-Silica Refractories”, J. Amer. Ceram. Soc.

45, 68 (1962)

4. Clews, F. R. , Richardson, H. M., and Green, A. T., "The Action of

Alkalies on Refractory Materials: Part V. Further Observations on the Action of Potassium Chloride Vapor on Refractory Materials at

1,000 °C", Trans. British Ceramic Society 39_, 139 (1940).

5. Hastie, J. W., "High Temperature Vapors 1: Science and Technology",

Academic Press, New York (1975).

6. Taylor, R. W., "Silica Vaporization and Condensation: A Potential

Source of Fouling in Coal Combustion", ANL-77-21, pp. 260-264

(1977).

7. Kessler, W., Jantsch, W., Gebhardt, F., and Majdic, A., "Investigations

of Volatilization in the System Cr^0_ and SiO~-Al 0~-Zr0 9 ", Glastechn.

Ber. 51, 289 (1978). “

.

8. Kim, Y. W. and Belton, G. R. , "Thermodynamics of Volatilization of

Chromic Oxide. I. Species Chromium Trioxide and Chromium Dioxy

Hydroxide", Met. Trans. 5_, 1811 (1974) .

9. Sanders, D. M. and Schaefer, H. A., "Reactive Vaporization of SodaLime Silica Glass Melts", J. Amer. Ceram. Soc. 59_, 96

(1976).

10. Maeda, E., "Vaporization from Magnesia in 0 o -H9 0 Atmosphere", Yogyo

Kyokai-Shi 86_, 491 (1978) .

11. Popkov, 0. S. and Semenov, G. A., "Mass-Spectrometric Study of the

Vaporization of Lithium and Sodium Aluminates", Russian J. Phys.

Chem. 45, 266 (1971).

12. Cable, M. and Chaudhry, M. A., "Volatilization from Soda-LimeSilica Melts at One Atmosphere and Reduced Pressures", Glass

Technol . 16, 125 (1975).

13. Coal slag obtained from diffuser section of an MHD test channel at

University of Tennessee Space Institute, Tullahoma, Tenn.

14. JANAF Thermochemical Tables, 2nd ed., NSRDS-NBS-37 , U. S. Govt.

Printing Office, Washington, D. C. (1971) and continuing supplements.
15 . Plante, E. R., "Vapor Pressure Measurements of Potassium over Ko 0-

SiC^ Solutions by a Knudsen Effusion Mass Spectrometric Method", p.

265 in Characterization of High Temperature Vapors and Gases, J. W.

Hastie, Ed., NBS SP-561, TJ. S. Govt Printing Office (1979).

16. Bonnell, D. and Hastie, J. W. , "Transpiration Mass Spectrometry",

p. 357 in Characterization of High Temperature Vapors and Gases, J.

W. Hastie, Ed., NBS SP-561, U. S. Govt. Printing Office (1979).

- 23 -
Figure Legends

1. Schematic of transpiration mass spectrometer system.

2. Mass action test for equilibrium in the system H^O = + 1/2 0^

present over MgO(s) at 2050 K. Data obtained at 20 eV ionizing

electron energy. Dashed line obtained from JANAF [14].

3. Variation of equilibrium constant with temperature for the

reaction Mg + 1/2 H^O + 1/4 0^ = MgOH. Data obtained at 20 eV

ionizing electron energy. Solid curve through the data points was

obtained from the third law method of analysis. Dashed curves

obtained from JANAF [14] estimates of AH^MgOH. The AH values are

enthalpies, in kcal/mol, for the above reaction.

4. Typical mass spectrum (30 eV electron energy) of species evolved

from a K-enriched Illinois Number 6 coal slag, K^, (see text for

composition) during an initial heating phase at 1428 K (run 1)

Data were obtained using the TMS method with a capillary sampling

probe. About one gm of sample was used with a N^ transport gas

pressure of 0.5 atm and a flow rate of ^ 25 SCCM. The ordinate

(right hand side) indicates approximate partial pressures, P ,

obtained from P_^ = I T, where 1^ is species i signal intensity

- 29

and k^ is the instrument sensitivity constant for ; I

~ =

1 z z 170 mV.

5. Ion intensity and partial pressure variation of initial volatiles

(K^O wt% 19.5-19.1) as a function of temperature for the slag

(m.pt. ^ 1700 ± 30K) using the TMS approach (run 1). Conditions

0.5 atm N^, capillary probe. Pressures, P , were obtained from the

ion intensities I. using the relation P. = k.I.T, where k. is

i

b ill i

obtained from k and the appropriate cross section and sensitivity

z factors

- 24 -
The multiplying factors for K and Na refer only to the pressure

scale, i.e. , K x 2.6 = pressure K. Literature Clausius Clapeyron

curves for K-pressures from K^CO^CD [14] and KAlO? (s) are also

shown for comparison. No literature data are available for KAlSiO^,

the main K-containing constituent in this slag, and the KAIO^ data

are considered as an upper limit to the vaporization rate for the

silicate: E. R. Plante, C. D. Olson and T. Negas , Sixth Inti.

Conf. on Magnetohydrodynamic Electrical Power Generation, Washington,

D. C., June, 1975, II, 211.

6. Variation of K-partial pressure with temperature for K^O contents

of 19.1-17.8 wt% (run 1—open circles, EMS) , 18.17-18.07 wt% (run

2 —open squares, TMS) and 18.07-18.0 (run 3, open squares, IMS).

Corresponding pressure curves (literature data) for the phases

Ko C0

3 (1), Ko S0

4 (1), KA10o (s), and Ko 0*9Alo 0

3 ~Alo 0^ (s) are given

for comparison (see Plante et al. , Fig. 5 caption) . The triangular

point at 1561 K is a HIS data point corrected for the presence of

0^ (see main text) . Each curve represents a separate experimental

run. Usually, data were obtained for successive increases in

temperature, except for run 3—TMS where the arrows indicate the

run chronology, i.e, T increasing or decreasing. HIS conditions,

0.5 atm , capillary probe.

7. KMS data for variation of K^O activity coefficient, y(Ko 0) , with

temperature and composition for the slag. Hie numbers, ranging

from 0.154 to 0.08, refer to the mole fraction of K? 0 remaining in

the sample at each measurement point. For comparison with compositions

listed in Fig. 6 , X„ = -0.0041 + 0.827 w + 0.0136 w~ where X is

2 mole fraction and w is weight fraction. Runs 1-3 were carried out

- 25
consecutively on the same sample. K^O reference state data obtained

from [14] and F. Natola and Ph. Touzain, Can. J. Chem., 48^, 1955

(1970); to a good approximation the standard state data are given

by the expression:

log

/ 2 IP •] Ik

23751

T + 11.7723.

8 . Dependence of K-pressure (log-scale) on temperature in the presence

_2

of H? 0 (^ 3 x 10 atm) and (0.5 atm) for the slag. Conditions, TMS method, capillary probe.

9. Dependence of K-pressure (log-scale) on H^O content in (0.35

atm) at 1673K for the slag. Conditions, TMS method, capillary

probe. The dashed curve of slope 0.5 represents the theoretical

thermodynamic equilibrium dependence for reaction (8) (see text) .

- 26
PHASE

SENSITIVE

DETECTOR

Figure 1
JANAF

—

I

1—0-1

1 —O—

1

1 1—0

J_i 1

.

—1

1 1

h*

I

00

CO to CO

I I I (luajeddv) jueisuoQ lunuqnmbg 6o"j

Log

Pressure

H

2

(Apparent),

ATM
Temperature (K)

2170 2080 1923

Figure 3
ION

INTENSITY

(mV)

50

10

0.1

20 30 40 50 60 70 80

MOLECULAR WEIGHT (AMU)

20

2

0.2

Figure 4

PARTIAL

PRESSURE

x

10°(ATM)
ION

INTENSITY

(mV)

6.0 7.0 8.0

RECIPROCAL TEMPERATURE

107 T K

Figure 5

PRESSURE

x

10

(ATM)
PRESSURE

K

(ATM)

TEMPERATURE K

1650 1550 1450 1350 1250

Figure 6
K_0

ACTIVITY

COEFFICIENT

TEMPERATURE K

190018001700 1600 1500 1400 1300 1200

Figure 7
TEMPERATURE

(K)

1695

1639

1537

1538 LO

CO

iwiVgOi- * ^aanssaad

00

Figure
o in o in • ^ • • CO CM 04 twiVsOi. x >i3anss3ad

2

x

10“

4

10~

3

10~

2

10

-

PRESSURE

H

2

0

(ATM)

NBS-114A (REV. 2*8C ) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

4. TITLE AND SUBTITLE

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No.

NBSIR 80-2178

Publication Date

VAPORIZATION AND CHEMICAL TRANSPORT UNDER COAL GASIFICATION CONDITIONS

5. AUTHOR(S)

J. W. Hastie, D. W. Bonne!! , E. R. Plante, W. S. Horton

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. national bureau of standards _

DEPARTMENT OF COMMERCE S WASHINGTON, D.C. 20234

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Contract EA-77-A-01-6Q1

. Type of Report & Period Covered

U.S. Department of Energy

1000 Independence Avenue, S.W.

Washington, D.C. 20585

10.

SUPPLEMENTARY NOTES

j

| Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most si gnificant information. If document includes a significant

bibliography or literature survey, mention it here)

The present studies have resulted in the development of a new experimental technique,

transpiration mass spectrometry, for the quantitative analysis of high vapor temperature gases and vapors. This technique has been applied to vapor transport and thermodynamic activity determinations for real and synthetic coal slag samples in reactive

coal gas components at atmospheric pressure. The results indicate a highly non-ideal

and non-monotonic (with temperature and composition variables) behavior for alkali

metal vapor transport. Thus a priori predictions of alkali metal transport. in coal

gasifiers without actual activity data are virtually impossible at the present time.

Surface segregation and diffusion limitations of alkali species in slags are also

possible complicating effects.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Coal gasification; high temperature; mass spectrometry ; slag; transpi ration

vapor transport

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ptx) Unlimited

1 | For Official Distribution. Do Not Release to NTIS

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