NIST

ilBLICATIONS

A11102

imBSIR 88-3093

DEVELOPMENT OF STANDARD

MEASUREMENT TECHNIQUES AND

STANDARD REFERENCE MATERIALS

FOR HEAT CAPACITY AND HEAT OF

VAPORIZATION OF JET FUELS

Jane E. Callanan

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303-3328

August 1988

^C"

lOO

056

88-3093

1988

C.2

imulatmg America's Progress

1913-1988

)

NBSIR 88-3093

DEVELOPMENT OF STANDARD

MEASUREMENT TECHNIQUES AND

STANDARD REFERENCE MATERIALS

FOR HEAT CAPACITY AND HEAT OF

VAPORIZATION OF JET FUELS

Jane E. Callanan

Chemical Engineering Science Division

Center for Chemical Engineering

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303-3328

August 1988

Prepared for Aero Propulsion Laboratory

Air Force Wright Aeronautical Laboratories

Wright-Patterson AFB, Ohio 45433-6563

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secreta

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

CONTENTS

Page

LIST OF TABLES iv

LIST OF FIGURES iv

EXECUTIVE SUMMARY v

ABSTRACT vii

INTRODUCTION 1

APPROACH - HEAT CAPACITY 2 General 2 Use of standards 2 Heat capacity reference materials 2 Measurement procedures 3 Corrections and data reduction 4

RESULTS - HEAT CAPACITY 4

Pure materials 4

Jet fuels 5 JP-4 (5); JP-7 (5); JP-8x (5); All fuels (6)

TECHNICAL DIFFICULTIES - HEAT CAPACITY 6

APPROACH - ENTHALPY OF VAPORIZATION 20

RESULTS - ENTHALPY OF VAPORIZATION 22

TECHNICAL DIFFICULTIES - ENTHALPY OF VAPORIZATION 24

ACKNOWLEDGEMENTS 24

REFERENCES 2

iii
LIST OF TABLES

Table Page

1. Coefficients of fitted heat capacity equation 10

2. Heat capacity of jet fuels 10

3. Enthalpy of vaporization for pure materials 23

4. Comparison with literature data - enthalpy of vaporization 23

LIST OF FIGURES

Figure

1. Deviation plot showing fit of heptane literature data. 7

2. Deviation plot for duplicate measurements on diphenyl ether, 8

310-355 K.

3. Plot of replicate measurements on diphenyl ether, 225-355 K. 9

4. Specific heat of a single specimen of JP-4 showing transition 11

at 320 K.

5. Fitted specific heat of JP-4. 12

6. Deviation plot for replicate measurements on JP-4. 13

7. Specific heat of a single specimen of JP-7 showing initial 14

transition

8. Fitted specific heat of JP-7. 15

9. Deviation plot for replicate measurements on JP-7. 16

10. Specific heat of a single specimen of JP-8x showing transition 17

from 220-240 K.

11. Fitted specific heat of JP-8x. 18

12. Deviation plot for replicate measurements on JP-8x. 19

13. Plot for determination of heat of vaporization. Curve a, 21

vaporization curve; curve b, mass loss curve.

IV
EXECUTIVE SUMMARY

GOALS

° Develop measurement techniques for the heat capacity of jet fuels from 220

to 360 K (-53 to 87 °C)

° Develop measurement procedures for heat of vaporization using a commercial

calorimeter

° Identify a suitable reference liquid for heat capacity to replace the

customary sapphire standard.

APPROACH

° Identify suitable reference liquids through a literature search and obtain

them in requisite purity.

° Apply measurement procedures developed in this laboratory to these

reference materials to permit 1. their evaluation and 2. comparison with

results with a sapphire standard.

° Measure the heat capacity of JP-4, JP-7, and JP-8x.

° Apply one of several possible methods for determining heat of vaporization

with commercial calorimetric equipment.

° Measure the heat of vaporization of pure materials with varying boiling

points to permit evaluation of the suitability of the procedures, their

accuracy and precision.

° Make preliminary measurements of the heat of vaporization of fuels.

FINDINGS

° Heptane is a suitable reference liquid for heat capacity measurements from

220 to 360 K (-53 to 87 °C)

o The heat capacity of a second reference fluid (diphenyl ether) was measured over a limited temperature range with an accuracy of better than

1.4 percent and a precision of + 0.4 percent.

° The heat capacities of three jet fuels were determined with the precision

indicated (fuel, precision). The data are given in tabular form and by

representative equations.

JP-4, ± 2%

JP-7, ± 1.5%

JP-8x, + 2%

° The heats of vaporization of cyclohexane, cis-decalin and heptane were measured with a precision of better than + 1.0 percent.

° Agreement with literature values for heats of vaporization was satisfactory, particularly in view of the differences between 1. our measurement temperature and the listed boiling point and 2. measuring

techniques (calorimetry versus vapor pressure)

RECOMMENDATIONS

° Extend measurements to higher temperature ranges (340 to 520 K, 67 to 2-

°C; 500 to 680 K, 227 to 407 °C)

° Identify a second reference liquid in order to extend this work to hip.h- ‘ temperatures; heptane is useful only to about 450 K (177 °C)

V
o Determine the best method of applying the correction for vaporization

within sealed cells. This correction must be applied at higher temperatures . Measure the heats of vaporization of mixtures of pure materials , which

have known heats of vaporization; this work must precede further

measurements of the heat of vaporization of the fuels to allow for needed

modifications of procedures.

Measure the heat of vaporization of jet fuels with the newly- developed

procedures

Investigate the accuracy and precision associated with summation of heats

of vaporization over broad temperature regions or to the application of

scanning methods.

Evaluate techniques on a broader range of fuels , including endothermic

fuels

Prepare for one of the ultimate goals of the work: the introduction of the

measurement techniques to the American Society of Testing and Materials

(ASTM) and the development of a standard reference material for

measurements on jet fuels to be distributed by the National Bureau of

Standards

VI
ABSTRACT

Development of standard measurement techniques and standard

reference materials for heat capacity and heat of vaporization

of jet fuels

Jane E. Callanan

Chemical Engineering Science Division

National Bureau of Standards

Boulder, Colorado

Procedures developed in the NBS -Boulder Laboratory for heat capacity

measurements of solids have been adapted successfully for the determination of

liquid heat capacities. Heptane was used as a standard in the measurement of

the heat capacity of three jet fuels, JP-4, JP-7, and JP-8x, from 220 to 360 K. Transitions were observed in these fuels as follows (fuel, temperature): JP-4,

320 K; JP-7, 220-230 K; JP-8x, 220-240 K. Combined thermogravimetric and

scanning calorimetric techniques were used successfully to measure heats of

vaporization of cyclohexane, cis-decalin, and heptane. The precision of these

measurements was better than 0.9 percent; agreement with literature values was satisfactory in view of small differences between the measuring temperature and

the literature values for the boiling points.

Key words: differential scanning calorimetry; heat capacity; heat of

vaporization; jet fuels; organic liquids; thermogravimetry.

Vll

INTRODUCTION

The two major concerns in the work reported here were the development of

reliable measurement techniques, which could be applied to jet fuels, both

for heat capacity and for heat of vaporization. These techniques were to be

developed for commercially available calorimeters, most probably differential

scanning calorimeters.

For the first portion of the work, the temperature range of the heat capacity

measurements was 220 to 360 K (-53 to 87 °C)

heats of vaporization were to

be determined at 300 and 350 K (27 and 77 °C)

It was hoped that a suitable

liquid reference material could be found and substituted for the sapphire

reference, which is so commonly used for heat capacity measurements. A

liquid reference would exhibit behavior more like the jet fuels than would a solid reference material.

The enthalpy of vaporization is by definition the enthalpy required to

vaporize a quantity of liquid without change in temperature. This quantity

may be determined at the normal boiling point or at temperatures removed

from it.

Enthalpies of vaporization are often determined from vapor pressure

measurements through use of the Le Chatelier principle and the Clausius

Clapeyron equation. This procedure involves the assumption that the

enthalpy of vaporization is independent of temperature. This assumption is

valid only over narrow temperature ranges.

Classical calorimetric methods for the determination of the enthalpy of

vaporization are discussed in detail by Partington [1]. These methods may be

difficult to apply or involve complex apparatus. More recently, several

techniques involving commercial calorimeters have been developed. [2-6]

The original plan for this portion of the study involved use of 1. Hsu's

procedures and equipment (a modified power- compensated scanning

calorimeter [4]) and 2. a commercial Calvet calorimeter. In the course of

the work, however, Pratt and Whitney requested that we delay measurements on

the first system and proceed directly to the Calvet calorimeter work.

A long range goal of the program is establishment of a standard measurement

method and a standard reference material that 1) would permit comparisons

among fuels, 2) might be submitted to the American Society for Testing and

Materials for approval, and 3) would facilitate the certification of a standard reference liquid, distributed by the Office of Standard Reference

Materials of the National Bureau of Standards, to be used by industry.

1
APPROACH - HEAT CAPACITY

General

General procedures developed previously in our laboratory were applied to

the heat capacity measurements of the liquids with both sapphire and a potential reference liquid as standards. The procedures used in our laboratory, described briefly here, are described more fully elsewhere. [7]

Measurements in our laboratory are made with a commercially available power

compensated scanning calorimeter, which is adjusted for optimal performance

before the beginning of each new study. These adjustments vary not only with

the type of instrument but also with the individual instrument; the frequency

with which adjustments and calibration are required depends on the behavior

characteristics of the individual instrument being used and the accuracy

required for the results. Some general guidelines are provided.

Before beginning a new study, the general condition of the instrument should

be checked and any defects remedied. Attention should be paid to dirty

sample holders or gas lines, poor fit of moving parts, worn o-rings,

deformed or darkened specimen cells for reference, empty, and standards, and

gas flow balance. Following these gross adjustments, the calorimeter

baseline and temperature calibration, over the range of temperature to be

used for the measurements, are to be checked and adjusted according to the

manufacturer's instructions. It is essential that these adjustments be made

at the scan rate and with the gas flow that will be used for the

measurements

When these adjustments are satisfactory, three measurements of the heat

capacity of a standard substance are made over the temperature range and ^ under the conditions to be used in the study. The first and third of these

are used as standards and provide correction factors for the second, which

is treated as a sample. Thus, the accuracy associated with the measuring

conditions can be assessed.

Use of standards

Standards are essential to heat capacity measurements made with 4^ scanning

calorimeter since these instruments are not^ absolute measuring qpjstruments

true values can be obtained only by use of calibration standards. Even

instruments with an electrical calibration capability (Joule calibration)

require verification of the correctness of the values determined for the

standard. However, this latter group of instruments may require less

frequent use of standards than instruments lacking this capability.

^ , li'tJ.? Heat capacity reference materials

oc-alumina oxide (sapphire) is the most widely used reference material for

heat capacity in the temperature range used in this study because of its

stability and the availability of excellent reference data. [ 8 ] However,

since measurements made with scanning calorimeters are affected by the nature

2
of the sample, particularly its thermal conductivity, it was desirable to

investigate the use of a reference liquid to be used in fuel studies. Also,

a liquid reference would show vaporization properties more like those of the

fuels than sapphire. The liquid selected must be available in pure form and

have reliable reference data. Ideally, the reference data would refer to

the actual standard substance used.

Standard reference heptane was developed in 1952 under the auspices of The

Calorimetry Conference; reference values are available for this material. [9]

A limited quantity was obtained and used as a reference in this work, in

addition to sapphire.

A sample of pure diphenyl ether, distributed by NBS as a fusion standard,

was used as a second reference liquid to allow for comparison of results

with the sapphire and heptane standards. Accurate heat capacity

measurements exist for diphenyl ether, though they were not made on the

actual material used. [9]

Measurement procedures

Standard reference mercury (SRM 2225 NBS) [10] and certified naphthalene (CRM

16-03 NPL) [11] were used for the temperature calibration of the calorimeter.

The specimen cells found to be satisfactory for this work were large volume

(c. 45 /il) stainless steel cells sealed with a Viton^ o-ring. Microliter

syringes were used to inject 40 to 50 /iL into the cells. Specimen masses,

which were determined on a balance calibrated to 0.01 mg, varied from 23.53

to 57.07 mg. The cells were reweighed after each use; measurements on

cells which showed a mass loss of 0.2 percent or greater were discarded as invalid.

Measurements were made at 2.5 K'min'^ over the temperature range 220 to 360

K. A constant flow of argon was maintained through the head during the

measurements; a dry box enclosure over the head itself was maintained under

a flow of dry nitrogen. The measurement sequence used was empty, standard,

sample, standard, sample, , standard. Sapphire and heptane were used as standards with both diphenyl ether and the fuel samples provided. Specimens

were placed in the calorimeter at 360 K; this procedure provided conditioning

of the cells so that any volatile material adhering to the outer cell surface

would be removed before measurements were begun.

1. Certain commercial equipment, instruments, or materials are identified in this paper in order to adequately specify the

experimental procedure. Such identification does not imply

recommendation or endorsement by the National Bureau of Standards,

nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

3
Corrections and data reduction

The data reduction procedures used in our work incorporate a pan weight

correction in the initial measurement program. The data for heat capacity

as a function of temperature for standards and samples are transmitted to a mainframe computer for further processing. Correction factors are calculated

for the standards run before and after each sample; these are time averaged

with reference to the sample run and are applied to the heat capacity

determined by the instrument. Data for replicate^ runs are fitted and the

fitted curve for the heat capacity is derived; the equation for this curve and deviations from it are also determined.

RESULTS - HEAT CAPACITY

Pure materials

Preliminary studies with heptane and sapphire as standards and diphenyl

ether and JP-7 as test fluids showed that heptane was an excellent reference

material. Diphenyl ether was a useful test liquid for this work despite the

existence of a fusion transition at room temperature. The heat capacity data

above and below the transition were uniform and reproducible, as was the

transition. No problems were experienced in measuring the heat capacities of

the JP-7.

Because of instrumental difficulties experienced in the preliminary work,

the only quantitative data cited in this report are from later studies, made

after these difficulties had been solved.

Evaluation of the accuracy of heptane and sapphire data under the conditions

of the study showed heptane accuracy to vary from 0.7 to 1.4 percent from 220

to 360 K. The accuracy of the sapphire data varied from 0.9 to 2.5 percent.

Therefore, Calorimetry Conference heptane was used as the reference material

(standard) for all data reported here.

Literature values for the heat capacity of the Calorimetry Conference

heptane^ were fitted to derive an equation,

Csat = 0.65983238 x 10^ - 0.69114058 x lO’^ T

0.39371944 x lO'^ T^ - 0.10822115 x lO'^ T^ (1) + 0.15057257 x 10’^ T^ - 0.8336760 x lO'^^ ,

where Cg^^ is the saturated heat capacity and T the Kelvin temperature. This

2. In this report, the term replicate refers to multiple specimens of

the sample; the term repeat, to a single specimen that is rerun

after its first run.

4
equation was incorporated into the correction program. The deviation plot

for this fit is shown in figure 1.

Measurements on diphenyl ether from 310 to 360 K with heptane as a reference

liquid showed an accuracy of 0.8 to 1.4 percent and a precision of + 0.4

percent over the temperature range of the study. The deviation plot for

diphenyl is given in figure 2. Figure 3 shows the results of four

measurements on the diphenyl over the entire 220-360 K range.

Jet fuels

The heat capacity measurements, as depicted on the calorimeter screen,

showed variations among the fuels but reproducible behavior for each fuel.

Each fuel is discussed individually here, but data are tabulated for all

three fuels in tables 1 and 2.

JP-4

The heat capacity scan depicted on the calorimeter screen always showed a narrow, sharp transition around 315 - 320 K. The temperature of this event

varied slightly from specimen to specimen. The corrected data used for

fitting sometimes showed elevated values for these temperatures and

sometimes did not (figure 4). Data for fitting were spaced at 5 K

intervals; as a result, small variations in the temperature of the maximum

were not observed in the corrected data. In order to derive an equation

representing the heat capacity of JP-4, the data through the transition

region were smoothed if necessary. The energy of this transition was 0.39 J

g'-*-. The plot of the fitted heat capacities for replicate runs (figure 5)

shows the fit of the data; a deviation plot for the fit is given in figure 6.

Replicate runs showed a variability of + 2 percent; repeat runs on a single

specimen agreed within + 1 percent.

JP-7

The heat capacity values for JP-7 at 225 K were always high. Its behavior

during the equilibration period and information from Pratt and Whitney

indicate that a transition has just been completed at this temperature

(figure 7). Once that event is over, the heat capacity curve is uneventful.

The fitted curve for JP-7, representing data from 230 to 360 K, is given in

figure 8; the deviation plot is shown in figure 9. Replicate runs varied by

+ 1.5 percent; repeat runs by + 2 percent.

JP-8x

The heat capacity of JP-8x on the calorimeter monitor showed a broad, illdefined transition beginning at 220 K. On fitting these data, a rather

sharp transition is apparent from 220-240 K (figure 10) . Replicate runs varied by about + 2 percent; repeat runs, by + 0.6 percent. Figures 11 and

12 show the fitted data and the deviation plot.

5
All fuels

The coefficients of the equation,

Csat = + a2 X T + a3 X + a4 X , (2)

are given for all three fuels in table 1. Heat capacity data for the three

fuels are presented in table 2.^

TECHNICAL DIFFICULTIES - HEAT CAPACITY

The sapphire measurements from just prior to the beginning of the

measurements for this contract were of satisfactory accuracy. In the course

of the preliminary work, it was noted that sapphire scans had deteriorated; a check of the accuracy of the sapphire and heptane showed that instrument

performance had deteriorated severely and it was necessary to install a new

head and recalibrate the calorimeter.

Hermetically sealed specimen cells, which were used originally, could not be

sealed reliably with the liquids in this study. The configuration of these

cells is such that they are difficult to use with liquids. If any liquid

makes its way to the lip of the cell, it does not seal properly. Attempts to

seal the liquid in a large -volume , stainless steel, cell were successful and

these cells were used for the remainder of the measurements. It was necessary to heat at 2.5 K'rnin"^, however, because of the lower thermal

conductivity of the system. The accuracy and precision estimates were reevaluated using the stainless steel cells.

Some difficulty was experienced in sealing the stainless steel cells. It is

essential that the sealed cell have an absolutely flat bottom surface for

contact with the heater/thermometer unit. Excess pressure in sealing causes

the bottom surface of the pan to become concave.

The pure material, diphenyl, showed excellent precision and accuracy in the

test runs. The precision of the fuels is less good. Two factors may

contribute to this behavior. First, the time required to sample and to seal

the cells varied from specimen to specimen; it is believed now that some volatilization may have occurred as the cells were being sealed, even though

the fluids were cold. Thus, the actual content of the cells differed

slightly. Second, there were variations in the specimen volume to cell

volume ratio in the specimens used. The sample placed in the cell varied

from 40 to 50 /iL. Since the amount of material was determined from its mass,

the precise voliame added was thought to be immaterial. In addition, the

sealing process was such that there could be slight variations in the total

cell volume. It is now recognized that slight differences in the ratio of

specimen volume to cell volume may affect the heat capacity results. It is

3. For correct determination of the heat capacity at

various temperatures, it is essential that all digits

given for the coefficients of the equation be retained.

6
o

REFERENCE

o

oTT

O

O

in

m

o

in

m

in

rvj m LJ

Q:; o

om

ccacLJ

Q_

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in

(M

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(M

m

o

CMo o

o —

o o

noo

roo

d o o d d1 d1 CD

(SGo:Id3/(iOnuO)d3-(S90)cd3]1001

- 7 -

Figure

1.

Deviation

plot

showing

fit

of

heptane

literature

data.
O

D3117

A

03217

Ln

tn

ro

o

inro

m<rro

o

ro

inm

m LJ

ct: i—

CO

cn O Lij K 0- m 21LJ

I—

in

(Mm

or\3 m

in

m

o

m

o 01 01 01 (S90)d0/( ony3)d0-(sgo)d0)00i

- 8 -

Figure

2.

Deviation

plot

for

duplicate

measurements

on

diphenyl

ether,

310-355

K.
O

DUG

A

D125

0123

V

0124

( (>l-0)/r) iy3H 3IJI33dS

9

Figure

3.

Plot

of

replicate

measurements

on

diphenyl

ether,

225-355

K.
Table 1.

Coefficients of fitted heat capacity equation

ra JP-4

1 2.7814903e+00

2 -1.6220220e-02

3 7.1702244e-05

4 -8.4395979e-08

JP-7

3.88956330e+00

-2.85037580e-02

1.15508970e-04

-1.37130860e-07

JP-8x

3.02391470e+00

-2.20516230e-02

9.44146410e-05

-1.13743400e-07

Table 2.

Heat capacity of jet fuels

T(K) Heat Capacity (J'

JP-4 JP-7 JP-8x

225 1.801

230 1.817 1.776

235 1.834 1.790

240 1.852 1.806 1.597

245 1.870 1.823 1.616

250 1.889 1.840 1.635

255 1.908 1.858 1.654

260 1.928 1.877 1.674

265 1.948 1.896 1.694

270 1.968 1.915 1.714

275 1.988 1.935 1.734

280 2.009 1.954 1.755

285 2.029 1.974 1.775

290 2.049 1.993 1.795

295 2.070 2.013 1.815

300 2.090 2.032 1.835

305 2.110 2.050 1.854

310 2.130 2.069 1.873

315 2.149 2.086 1.891

320 2.168 2.103 1.908

325 2.186 2.119 1.925

330 2.204 2.134 1.941

335 2.222 2.148 1.956

340 2.238 2.161 1.970

345 2.254 2.173 1.983

350 2.269 2.184 1.995

355 2.284 2.193 2.005

The heat capacities given in this table represent smoothed data for all

of the experimental measurements on the fuels.

10
O

J416R

o

CDm

o

ro

or\j ro

o >_ o

q:

az

DZUJ o 0-

cD z; (M LJ

o

CD

rsi

o'ir r\j

oCM

f\J CDOO

CD

rsj CM

CDCM

CD

CM

tTCM

CO

CM

CM

CM

CM D CD CM

( (>l-9)/r) 1U3H 0IJI03dS

11

Figure

4.

Specific

heat

ofa

single

specimen

of

JP-4

showing

transition

at

320

K.
o

J1131

A

J437

J4130

( (>l -Sl/n 1U3H OIJIOGdS

12

Figure

5.

Fitted

specific

heat

of

JP-4.
O

J4131

A

J437

J4430

OUDro

• • I • I o o1 I

oTT' ro

or\] ro

o

o

ro LlJ or

ZD

h-> CEq;

LlJ o Q_

00 z:

r\] LlJ

o

UDCM

o

'T

r\j

o

(M

Od

(sao)dO/( onu3)d0-(sao)d0)00i

13

Figure

6,

Deviation

plot

for

replicate

measurements

on

JP-4,
O

J7630

oinm

o

ro

orsj to

o

to

oCD

rsj

o

CD

oo

OTT

r\]

oCNJ CM

tiJ q;ZD

f—ccLJ

Q_

z:UJ

( (>l-9)/r) 1U3H 3IJI33dS

14

Figure

7.

Specific

heat

ofa

single

specimen

of

JP-7

showing

initial

transition.
O

J7130

A

J753R

J7630

( (>l-9)/n 1U3H 3IJI03dS

15

220

2^0

260

280

300

320

340

360

TEMPERATURE

(K)
O

J7130

A

J753R

J7630

UJ

ct:

CE

ct^ UJ

Q_

21LJ

(—

( S90 ) dO/ ( ( oiyo ) dO- ( S90 ) dO ) 00

16

Figure

9.

Deviation

plot

for

replicate

measurements

on

JP-7.
O

J0131 o

tD

ro

CD

»r' roor\j roo

r)oOD

CM

LJ

0::

cc

q:UJ

CL

21LJ

fO

S-c0

sz

COX

CO1 Cl_

•-0

cz

<u0

a;Q.

CO

<u01

cz

t— ro 4-o

-M

03 o o QJ UD uz C\J

fNJ o 1 o •r— CO

4- C\J

•rTMU E

o; o

D. i_

O 1/3 4-

rsj oi"H

OJ

i~ o cn

r\) • r— oo U.

CD

r\j

CD cn oo (D tD UD ( (>l-0)/r) iy3H 3IJI03dS - 17 -
O

J8131

A

J8226

J8327

LJ

a:

az

ocUJ

Q_

21UJ

( (>l -Sl/D 1U3H 0IJI33dS

18

Figure

11.

Fitted

specific

heat

of

JP-8x.
O

J8131

A

J8226

J8327

(sgo)cJ3/( (0ny3)d3-(S90]d0)00i

19

TEMPERRTURE

(K)
recommended that in the future, precise determination of specimen and cell

volumes be made.

APPROACH - ENTHALPY OF VAPORIZATION

The Calvet calorimeter used was a Setaram combination TG/DSC 111, located at

ASTRA Scientific, Inc., San Jose, CA.^ The variety of configurations of this

equipment allowed for five different modes of measurement. In the time

available for this study, only one of these modes was pursued in detail; an isothermal technique that used the balance capability of the equipment in

addition to the calorimeter.

The method was devised and tested using pure materials in order to provide

an estimate of its accuracy and precision. Reagent grade cyclohexane and

cis-decalin, with normal boiling points of 371.58 K and 468.9 K respectively, were used for the experiments in which the method was devised. The accuracy

and precision of the method were evaluated with the heptane reference

material used in the heat capacity studies. The technique was then applied

to the fuels.

For the vaporization studies with the calorimeter-balance assembly, the

heating unit was maintained in vertical orientation. The sample cups, which

were located outside of (above) the heater-detector zone for loading, were suspended from the balance beam. After introduction of the sample, the unit was lowered to introduce the sample to the heated zone. A short period of

time was required for the fluctuations, caused by the motion of the system

and the temperature equilibration of the sample cells, to cease.

A description of the experimental procedure follows. The calorimeter was equilibrated at the desired temperature; a flow of dry nitrogen was maintained through the measuring system. The liquid to be tested was placed

in both the reference and the sample cells of the calorimeter, which were

then located outside the heated zone of the calorimeter. The purpose of this

technique was to prevent the vaporization signal from beginning until the

system had recovered from the disturbance associated with sample pans being

lowered into the heated zone and equilibrating. During this time, the sample

was evaporating from both cells at the same rate and the signal, once it had

recovered from the initial disturbance, was flat. When all the material had

evaporated from the reference cell, the differential measuring system

detected the vaporization enthalpy and responded. When the sample had

evaporated completely, the signal returned to baseline. Figure 13

illustrates the signals obtained from both the calorimeter and balance.

Because of the detection of the mass change associated with the sample cell,

it was possible to determine the enthalpy of vaporization per gram of sample

vaporized. This method can be applied both at the normal boiling point and

at temperatures removed from it.

The tasks outlined in the original proposal called for determination of the

heat of vaporization at specific temperatures, rather than only at the

normal boiling point. Accordingly, measurements were made at a series of

temperatures. To allow for comparison with literature data, measurements

20
37.(H

MASS (mg )

21

Figure

13.

Plot

for

determination

of

heat

of

vaporization.

Curve

a,

vaporization

curve;

curve

b,

mass

loss

curve.
were made also at the boiling points of cyclohexane, cis-decalin, and

heptane ; heats of vaporization as a function of temperature are not readily

available for organic liquids. For comparison with literature data, the

data near the normal boiling point have been converted to a molar basis.

Following the development of suitable measuring procedures, the technique

was applied to the jet fuels.

RESULTS - ENTHALPY OF VAPORIZATION

The results of measurements on pure materials by the method outlined above

are shown in table 3. Comparison with literature data is made in table 4.

The initial vaporization measurements on the fuels were made on JP-7, since

it was the best characterized of the materials. Three /iL were used in the

sample cell and 1 jiL in the reference cell. Despite the use of these small

amounts of material, the desired flat initial region was almost nonexistent

and vaporization from the sample cell was not completed after 75 minutes. A

second trial with a 7:4 yuL distribution, used to try to get a suitable

initial flat portion of the curve, also was unsuccessful.

It was decided to continue measurements on JP-4, which has the highest vapor

pressure, thus presumably the lowest vaporization temperature, of the three

fuels. Measurements at temperatures lower than 450 K were not successful.

Even at 450 K, a flat region could not be definitively established because of

the vaporization of the very light components. GC/MS analysis at NBS -Boulder

showed this fuel to have a great many components that boil below or near 373

K. Attempts to measure the disturbance incurred in the system as the pans were lowered into the heated zone, for use as a blank correction, were

inconclusive. Since the fuels did not arrive until the last day of work at

ASTRA, time for experimentation was very limited. It is very possible that an average blank correction could be determined and allow measurements of the

early vaporization. As the final masses determined indicated that some residue remained in the system, it is surmised that some waxy materials

never vaporized at 450 K.

Two values were obtained for JP-4 and JP-8x. JP-4 showed a heat of

vaporization of 347 J*g'^; for JP-8x, the value was 364 J*g‘^. The value

for JP-4 does not include heat effects associated with the very early

vaporization. For a broad spectrum fuel like JP-4, it would be necessary

either to make measurements at various temperatures and sum them to get an estimate of the cumulative heat of vaporization or to use a pressure cell

that would prevent vaporization until the system had equilibrated.

22
Table 3.

Enthalpy of vaporization for pure materials

Substance & Temperature (K) Number of Enthalpy

proportions

/iL sam:ref

Nominal Actual determinations J-g-1

Cyclohexane

24:6.2 323 323.3 2 381.7 ± 0.9

40:20 350 348.9 2 353.8 ± 0.8

Cis -decalin

40:20 470 463.4 3 288.7 ± 1.6

Heptane

3:1 300 302.6 5 381.0 ± 1.4

22:10 350 344.5 5 349.4 ± 1.1

50:30 371 363.2 5 328.9 ± 2.9

Table 4.

Comparison with literature data - enthalpy of vaporization

Literature Experimental

Boiling point

K

Enthalphy Temperature

kJ'mol'^ K

Enthalpy

kJ • mo 1

- ^

Cyclohexane [12]

353.88 30.08 348.87 29.77

Cis-decalin [13]

468.90 39.33 463.45 39.91

Heptane [12]

371.58 31.69 363.18 32.96

23
TECHNICAL DIFFICULTIES - ENTHALPY OF VAPORIZATION

The proportions of sample to be placed in the reference and sample cells

varied with the properties of the liquid tested. The amount necessary had to

be determined empirically for each liquid at each temperature.

JP-4, in particular, has some very low boiling components despite a majority

concentration of high boiling components. Even at 450 K, the higher boiling

materials did not vaporize at a reasonable rate; the low boiling materials

could not be contained at this temperature without application of pressure.

Several possibilities exist for overcoming this problem but the time and

funds available did not permit their application. Further discussion of

those methods will be pursued with Pratt and Whitney at a later time.

In the early experiments, the calorimeter signal appeared to be swamped if

measurements were repeated at frequent intervals. This was believed to

result from residual vapors in the detection zone. A flow of dry nitrogen,

maintained through the detection zone, solved this problem completely.

ACKNOWLEDGEMENTS

We thank M. Ducros of the Ecole Nationale Superieure des Techniques Avancees,

Palaiseau, France for helpful suggestions with regard to the heat of

vaporization measurements

The work reported here was conducted under subcontract to the Materials

Engineering Laboratory of Pratt and Whitney. The work was sponsored by the

Fuels Branch of the Aeropropulsion Laboratory, Air Force Wright Aeronautical

Laboratories, under Contract No. F33615-85-C-2500

24
REFERENCES

[1] Partington, J.R. An Advanced Treatise on Physical Chemistry New York:

Longmans ; 1951.

[2] Farritor, R. E. ; Tao, L. C. Thermochim. Acta 1: 297-304; 1970.

[3] Mita, I.; Imai, I.; Kambe, H. Thermochim. Acta 2: 337-344; 1971.

[4] Hsu, S.M.; Cummings, A. L. ; Clark, D. B. Base Oils for Automotive

Lubricants, SP-526: 127-139. Society for Automotive Engineers, Inc. 1982.

[5]

[ 6 ]

[7]

[ 8 ]

[9]

Barone, G.; Della Gatta, G.; Elia, V. J. Therm. Anal. 29: 763-772; 1984.

Brostow, W. ; Iniguiz, J. C.; Shmorhun, M. A. Materials Chemistry and

Physics 12: 557-569; 1985.

Callanan, J. E.; Sullivan, S. A.; Rev. Sci. Instr. 56: 2584-2592; 1986.

Ditmars, D. A.; Ishihara, S.; Chang, S. S.; Bernstein, G.;West, E. D. Enthalpy and heat-capacity standard reference material: synthetic

sapphire ( 0C-AI 2 O 3 ) from 10-2250 K. Journal of Research (N. B. S.)

87: 159-163; 1982.

Ginnings, D. C.; Furukawa, G. T. Heat capacity standards for the

range 14-1200 K. J. Am. Chem. Soc. 75: 522-527; 1953.

[10] Office of Standard Reference Materials, National Bureau of Standards,

Gaithersburg, MD. 20899

[11] Office of Reference Materials, National Physical Laboratory,

Teddington, Middlesex TWll OLW, UK.

[12]

Stull, D. R. ; Westrum, E. F. Jr.; Sinke, G. C. The

Chemical Thermodynamics of Organic Compounds. New York: Wiley; 1969.

13] Reid, R. C.; Prausnitz, J. M. ; Sherwood, T. K. The

Properties of Gases and Liquids. 3rd ed. New York: McGraw-Hill; 1977.

25
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. 3. PubI ication Date

NBSIR 88-3093 August 1988

DEVELOPMENT OF STANDARD MEASUREMENT TECHNIQUES AND STANDARD REFERENCE MATERIALS FOR

HEAT CAPACITY AND HEAT OF VAPORIZATION OF JET FUELS

5. AUTHOR(S)

Jane E. Callanan

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s) 7. Contract/Grant No. national bureau of STANDARDS F33615-85-C-2508

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234 Final

August 1987-May 1988

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Aero Propulsion Laboratory

Air Force Wiright Aeronautical Laboratories

Wrlght-Patterson AFB, Ohio 45433-6563

10. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT ( A 200-word or less factual summary of most significant information. If document includes a significant

bi bl iography or literature survey, mention it here)

Procedures developed in the NBS-Boulder Laboratory for heat capacity measurements

of solids have been adapted successfully for the determination of liquid heat

capacities. Heptane was used as a standard in the measurement of the heat

capacity of three jet fuels, JP-4 , JP-7, and JP-8x, from 220 to 360 K. Transitions were observed in these fuels as follows (fuel, temperature): JP-4, 320 K; JP-7,

220-230 K; JP-8x, 220-240 K. Combined thermogravimetric and scanning calorimetric

techniques were used successfully to measure heats of vaporization of cyclohexane,

cis-decalin, and heptane. The precision of these measurements was better than 0.9

percent; agreement with literature values was satisfactory in view of small differences between the measuring temperature and the literature values for the

boiling points.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) differential scanning calorimetry; heat capacity; heat of vaporization; jet fuels;

organic liquids; thermogravimetry.

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