NBSIR 76-1000

The Construction, Operation

and Performance of a Reactive

Gas Generator; with Specific

Application to HCHO,

CH3CHO, CH2CHCHO, SO2,

HCN and HCI Production

James A. Walker and W. Tsang

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

January 1976

Final

U S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 76-1000

THE CONSTRUCTION, OPERATION

AND PERFORMANCE OF A REACTIVE

GAS GENERATOR: WITH SPECIFIC

APPLICATION TO HCHO,

CH3CHO, CH2CHCHO, SO2,

HCN AND HCI PRODUCTION

James A. Walker and W. Tsang

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

January 1976

Final

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

The Construction, Operation and Performance of a Reactive

Gas Generator; with Specific Application to HCHO, CH^CHO,

CH^CHCHO, S0„, HCN and HCl Production

James A. Walker and W. Tsang

An Instrument capable of generating a variety of reactive gases

in trace levels has been constructed and tested. Operational details,

with specific application to HCHO, CH^CHO, CH^CHCHO, SO^, HCN and

HCl production are discussed.

Supported in part by the Office of Air and Water Measurement and

the Center for Fire Research of the National Bureau of Standards.

Table of Contents

Page

I. Introduction 1

II. Apparatus and General Principles 2

III. Identification of controls and connections 12

A. Front panel 12

B. Left side panel 14

C. Right side panel 14

D. Rear panel 14

IV. Operation 15

A. Requirements 15

B. Procedures 15

V. Service Procedure 16

A. Replacement of furnace heater 16

B. Replacement of cell heater 17

C. Cleaning diffusion cell 18

D. Cleaning buffer cell 19

E. Maintenance 19

VI. Shut -Down 20

VII. Data on Specific Reactive Gases 20

A. HCHO 20

B. CH^CHO 27

C. CH^CHCHO 31

D. HCN 35

E. SO2 39

F. HCL . 44

VIII. Parts List 48

IX. References 50

Introduction

This report describes the construction, operation and performance

of an instrument capable of producing accurate and precise

quantities of reactive gases over wide concentration ranges. Possible

uses include the calibration of analytical instruments, the evaluation

of analytical methodology and the carrying out of toxicity investigations.

More specifically, test results on the generation of SO^, HCHO, HCl,

CH^CHO, HCN and CH^CHCHO (acrolein) will be summarized.

The reactive nature of the gases which are of concern in pollution,

occupational health or toxicological contexts render unsatisfactory

standard static methods (1) of sample preparation. Obviously, a

dynamic method where the required samples can be prepared immediately

prior to use can be expected to circumvent some of these problems.

The present instrument is designed to serve this function. It involves,

first, the generation of a dilute mixture of a large organic "parent"

molecule in an inert diluent through the use of the well established

diffusion cell technique (1) and then flowing this mixture through a hot

tube where complete stochiometric decomposition occurs and the

reactive gas of interest is formed. The "parent" molecule of choice

is one that undergoes pyrolytic decomposition exclusively via the

reaction

parent molecules -> hydrocarbon + reactive compound.

Thus the quantity of the reactive gas delivered to the detector can

be deduced from the amount of the hydrocarbon that is present. Calibration

of the latter is straightforward. The presence of this internal

standard is an added feature since it minimizes the possibility of

1
operator error. Furthermore by varying the quantity of the "parent"

molecule diffusing Into the Inert diluent this method has for practical

purposes Infinite dynamic range. In contrast, the concentration of

static samples Is always fixed.

Apparatus and Principle of Operation

Front and top views (with Instrument open) of the reactive gas

generator can be found In Figures I and II. A more detailed photograph

of the diffusion cell can be found In Figure III.

Fundamentally, the generator can be considered to be made

up of two constant temperature compartments. The first, containing

the metal diffusion and buffer cells can be maintained at temperatures

from ambient to 250°C. Heat Is supplied by a 4 In X 10 In (1 In = 2.54 cm)

flexible heater fixed to the bottom of the aluminum box holding the cells.

Temperature control of + O.l'C Is achieved using a RFL-70 controller (2).

The second compartment holds the 1/8 In OD gold tubing In which pyro lysis

is carried out. A cartridge heater provides the thermal input. The

temperature range is from 0 to 760°C and can be held to + 12°C using

a Burling A-IS thermostat. The upper portion or head of the metal

diffusion cell contains the diffusion column. A variety of heads,

with ID up to 0.25 in maybe used. The length of the diffusion column

is of the order of 2.5 in. The substance to be pyrolyzed is placed in

a glass cup which sits on the lower portion or base of the diffusion

cell. The total volume of sample that can be accommodated is of the

order of 15 cc. This is sufficient for practically all conceivable

applications. A 0.125 in hole at the bottom of the base of the diffusion

2
3
4

1

cell provides a means of purging the cell of air so that oxidation

will not make any contribution to the pyrolytic process. The role of

the buffer cell is to take out pressure surges which arises from the

switching of valves or the connection of fittings. The entire system is

operated at about 1/2 atmosphere above ambient pressure. Control is

effected by means of a needle valve at the exit of the pyrolyzer. This

has the advantage of decoupling the system from downstream events. It

also makes it easy to check for leaks in the system. Experience from

these studies suggests that this is the most likely source of error

using this device. Detailed drawings of the instrument can be found in

Figures IV to VII.

The relationship between the diffusion rate and the physical

parameters of the material and the diffusion cell is

^d - r ^p:^"^

V

where Q, = diffusion rate, volume/time, D = diffusion coefficient, A = a

diffusion tube cross-section, L = diffusion tube length, P = pressure

in diffusion cell and P^ = partial vapor pressure of diffusing vapor.

It is important to note that the diffusion rate can be varied by changing

the diffusion tube cross-section A and/or by changing the cell

temperature. The latter arises from the three-halves power dependence

of the diffusion coefficient on temperature (approximate) and the

exponential relationship between vapor pressure and temperature

(Clapeyron Equation). For vapor pressures sufficiently lower than the

cell pressure this is equivalent to an exponential dependence between

diffusion rate and temperature. Thus by suitable adjustment of the

physical parameters an enormous range of diffusion rates can be
7
8
9
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to3

10
generated. Inversely, this means that for a given range of diffusion

rates the present apparatus can acconnnodate a wide variety of substance

through the proper selection of diffusion cell cross-section and

temperature. The sensitivity of the diffusion rate to the latter

makes necessary satisfactory controls ('x- 0.1*C) on diffusion cell

temperature. On the other hand it also means that diffusion rate

can be set easily and continuously over a wide range. This is in

contrast to the situation for generation from static methods.

Finally, it will be noted that the concentration of reactive gas

in related to the diffusion rate by the equation

C=Q^/Q

where Q is the flow rate across the diffusion cell. Thus the concentration

can also be controlled by the flow rate.

The compounds of choice for the present application are those

which decompose unimolecularly into molecular fragments and which

do not have significant side reactions. The avoidance of surface or

chain induced decomposition processes is of prime importance. These

processes are frequently irreproducible and thus completely unsuitable

for the present purposes. This is the reason for the use of the

gold reactor. An excellent source for possible "parent" molecules

is the review by Benson and O'Neal (3). The unimolecular nature

of these reactions dictate that the extent of conversion is

dependent only on the reaction time (t), temperature (T) and the

thermal stability of the appropriate compound. The relation is

In C^/C^ = t A exp (-E/RT)

11
where A and E are Arrhenlus parameters for unimolecular decomposition

reaction (3) and characteristic of the compound of interest and and

are the initial and final concentrations. In the present context interest

is focused in the region of high conversions or where C^/C^ is greater

than 50. Thus - concentration of reactive gas. Under these

conditions the concentration of products is relatively insensitive

to changes in reaction variables. Thus these variables need not be

strictly controlled and the operational procedure to locate the

minimum necessary temperature is to increase the pyrolysis

temperature until the product yield reaches a maximum and/or when

the parent molecule disappears.

Identification of Controls and Connections

A. Front panel

There are three convenience outlets inside the cabinet.

Each outlet is wired independently and is controlled by the appropriate

switch on the front panel. A wiring diagram can be found in Figure VIII.

1. Main power switch

This switch is in series with and ahead of the other

switches.

2. Cell heater switch

The cell temperature controller, which is mounted under

the cabinet lid, has a convenience outlet for the cell heater and

a jack for the cell temperature sensor. The controller is

plugged into the outlet controlled by the cell heater switch.

12
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13
3. Furnace heater switch

The furnace temperature controller and heater are plugged

into the outlet controlled by the furnace heater switch.

A . Fan switch

Operates cooling fan.

5 . Pressure gauge

Indicates system pressure.

6. Micrometer

The handle of the mlcrometering valve for flow control.

B. Left Side Panel

1. Purge valve

Allows diffusion cell to be purge by directing diluent

gas through the cell.

2. Purge outlet

Cell vent during purge.

3. Inlet

Connection for diluent gas.

C. Right side panel

1. Outlet

Exit for pyrolysls products.

D. Rear panel

1. Cell T.C.

Binding post for monitoring the diffusion cell temperature

with chromel-alumel thermocouple.

2. Furnace T.C.

Binding post for measuring temperature of furnace with

chromel-alumel thermocouple.

14
Operation

A. Requirements

1. 120V AC power source.

2. Diluent gas cylinder with two stage regulator.

3. Exhaust vent.

4. Detector G.C., MS. etc.

5. Precursor material.

B. Procedure

Raise the lid of the cabinet to gain access to the cell compartment.

Remove the top of the compartment and the Insulating panel to reach

the cell. To charge the cell with a precursor material loosen the

two "Swagelok" nuts on the "tee" attached to the cell. Next remove

(4) // 10-32 Allen head cap screws. The cell cover can now be raised

with the "tee" acting as a hinge. Exercise care In handling the

teflon gasket. Add 10-15 cc of liquid or about 15 grams of solid

material to the pyrex container In the cell. Lower the cell cover,

replace and tighten the screws, then tighten the "Swagelok" nuts

securely.

Connect the cylinder of diluent gas to the Inlet, on the left

side panel, with 1/8 In CD copper tubing. Connect the purge outlet to an

exhaust vent with copper or plastic tubing. The sample outlet on the

lower right side panel Is connected to the test facility with 1/8 In OD

stainless steel tubing.

Open the valve on the diluent gas cylinder and set the pressure

regulator to 25 pslg. Open the micrometer one complete turn and the

15
purge valve 1/4 turn ccw. After about 15 minutes close the purge

valve. Adjust the micrometer and cylinder pressure to give a

flow of 40 cc/mln. and a system pressure of about 7 pslg. A flow

calibration curve Is provided which gives approximate control

settings for flow rates from 10 to 200 cc/mln (He) . Use of a bubble

flowmeter Is recommended for a more accurate determination of flow

rates

Check all connections for leaks with soap bubbles or a

commercial liquid leak detector. Tighten any connections that

leak. When assured of no leaks replace the Insulating panel and

the cell compartment cover. Plug In power cord.

Turn on the main power, cell heater and furnace heater switches.

Set cell and furnace temperature controllers to the desired

temperatures, allowing 2 hours for the system to equilibrate.

Since the cell takes time to equilibrate the cell heater should be

left on continuously. The diluent gas should also be flowing through

the system. On the other hand, the furnace heater can be turned off each

evening and turned on each morning because it requires less time to reach

operating temperature. If, however, the temperature of the cell is such

that the material used as a precursor is exiting the cell with a vapor pressure

higher than that at room temperature it is advisable to keep the furnace heater

on continuously also. This is necessary to avoid material collecting in J

the cool furnace and exit lines. ^

Service Procedures

A. Replacement of furnace heater

1. Turn off furnace heater switch.

2. Unplug heater cable from outlet in the rear of cabinet.

16
3. Gently pull cartridge heater from furnace.

4. Remove fiberglass insulation carefully.

5. Cut the two heater leads at a length equal to the

leads on the new heater. I

6. Splice the new heater leads and tape securely.

7. Rewrap heater leads with fibergla,ss insulation.

8. Push heater into furnace as far as it will go.

9. Plug heater cable into heater outlet.

B. Replacement of cell heater

1. Turn all power off and unplug power cord,

2. Unplug furnace heater cable.

3. Disconnect furnace from buffer cell. '

4. Disconnect s.s. exit tube from furnace.

5. Remove furnace from insulators mounted on the side of the

outer box of the cell compartment by removing four screws on square

flange.

6. Next remove diffusion cell, buffer cell and s.s. tubing

from the box.

7. Remove four 8-32 screws holding the cell support plate.

8. Remove the heater cable clamp where the cable passes through

the outer box.

9. Carefully lift the inner box making sure there is enough

slack in the heater cable and the sensor cable.

10. Remove the flexible heater which is attached to the bottom

of the inner box by a thin aluminum cover held by four screws.

17
11. Slide the fiberglass insulating sleeves away from

heater connections and cut the leads of the old heater.

12. Splice the new heater wires to the power lead.

13. Solder and tape the splices. '

14. Slide the fiberglass insulating sleeve over the splices.

15. Reverse the above steps to reassemble.

C. Cleaning Diffusion Cell

CAUTION!!

Before attempting to remove the cell for cleaning or

changing the precursor material, all switches must be off and the

cell and furnace must be at room temperature. The diluent gas

supply should be turned off at the cylinder valve.

1. Completely remove the "Swagelok" nuts on the "tee" attached

to the cell.

2. Remove the brass "Swagelok" nut from the union on the

lower left side of the cell. (purge line)

3. Loosen brass screws on cell hold-down clamps and rotate

the clamps 90°.

4. Carefully rotate cell, to free tube connections, and

lift out.

5. Place cell in hood and remove 4 # 10-32 Allen head cap

screws from the cell cover being careful not to damage the "teflon"

gasket

6. Pour out any remaining material if liquid and rinse with

solvent

7. Apply suction to dry.

18
8. If cell contains a solid material it may be necessary

to gently warm the cell with an electric heat gun in order to

remove it

.

9. Rinse with a solvent then wash in soap and hot water.

10. Dry with high pressure air.

11. Rinse with acetone, drain and dry with suction.

D. Cleaning Buffer Cell

Note: The buffer cell normally requires no special cleaning,

merely apply suction at the system outlet for 10-15

minutes after the diffusion cell has been disconnected.

Under extreme conditions removal may be necessary.

Proceed as follows:

1. Disconnect cell from tube to the furnace.

2. Loosen hold-down clamps and lift out.

3. Run hot water through the cell for several minutes then

drain.

4. Rinse with acetone, drain and dry with suction or dry air.

5. Reverse the procedure to reassemble.

6. Always use two wrenches in opposition when tightening or

loosening "Swagelok" connections.

E. Maintenance

The system requires no regular maintenance except to replace

the diluent gas cylinder occasionally and to recharge the

diffusion cell.

Replacement of burned-out heaters is described in the

service procedures section above.

19
Shut -Down

When shutting the system down for a week or less turn the main

power switch off but allow the diluent gas to continue to flow

through the system. Make sure the outlet is vented. For a

longer period the diluent gas should be turned off at the cylinder

valve and the sample removed from the cell.

Data on Specific Reactive Gases

A) HCHO ; The "parent" compound for formaldehyde generation is

3-methyl-3-buten-l-ol. The decomposition reaction is

CH_ = C(CH-)CH-CH_OH ^ iC,H„ + HCHO

The validity of the postulated stochiometry is established by gas

chromatographic analysis of the reaction products with Poropak P-S

columns. Since formaldehyde is insensitive to flame ionization

detection, conversion to CH^ (after separation) is effected by

passage over nickel catalyst in the presence of hydrogen following

the prescription of Stevens, O'Keefe and Ortman (3). The resulting

area ratio over the applicable range is 4.22 + .12 to 1. The

departure from exact 4 to 1 dependence is attributed to

substantial tailing in the formaldehyde peak making complete

integration difficult. A plot of the results can be found in Figure

Maximum thru-put (100% conversion) in terms of formaldehyde output

per minute as a function of cell temperature can be found in Figure X

Least squares analysis of the data yields at 7.4 psig and over the

temperature range of 30-90°C

20
0 5 10 15

Peak Area HCHO (Arb. units)

FIGURE 9 Relative yields of isobutene and formaldehyde from 3-methyl-3-but

21

Log^Q[yg HCHO/mln] = -2704.8 [ ^ 1 + 9.09 + .09

The thru-put range is 2-50 yg/min. The standard deviation or

"settability" is 4%. With flow rates of 20-200 cc/min (He)

the pyrolyzer temperatures necessary to Insure close to 100%

conversion range from 600 to 680°C. Note that at the lowest thru-put

and flow rates small conversions 5% of HCHO to CO have been observed.

The long term behavior of the generator, and the dependence of

concentration on flow rate and pressure can be found in Table I.

The results of studies carried out using a commercial formalin

solution is summarized in Figure XI. Long term behavior and dependence

of concentration on flow rate is summarized in Table II. Note that

in this system substantial quantities of methanol and water are also

generated.

r

23
3.0 3.2

1000/T

FIGURE 11 Yields of HCHO/min from commercial formalin solution as a

of temperature.

24
Table I

Long term behavior of formaldehyde generation. Cell temperature

AO'C. Flow rate = 30 cc/min. Pyrolyzer at 650°C. System pressure

7.2 psig.

hours concentration

0 75 ppm

2 75 ppm

6 74 ppm

72 76.5 ppm

Variation of HCHO concentration as a function of flow rate at

88°C. 7.5 psig cell pressure, pyrolyzer temperature 675°C.

flow rate (cc/min) (flow rate) ^ (min/cc) ppm HCHO

33.2 .0301 881

81.0 .0124 397

210.0 .0048 148

Variation of HCHO concentration as a function of pressure at

88 "C; 210 cc flow rate, pyrolyzer temperature at 655 "C

pressure (psia) ppm HCHO

26.4 122

22.1 . 148

18.3 172

25
i Table II

Formaldehyde output from Reactive Gas Generator Using

Formalin Solution as 60.5°C.

time (hrs.) flow rate concentration (ppm) mass thru-put

(yg/min)

0 25 cc/min 678 20.9

66 25 cc/min 671 20.7

68 100 cc/min 167 20.5

70 ' 200 cc/min 83.5 20.6

26
B) CH^CHO: The parent compound for acet aldehyde generation Is

4-penten-2-ol. The decomposition reaction is

H2C=CHCH2CH(OH)CH2 C^H^ + CH^CHO

From gas chromatographic analysis with a Poropak P-S column and flame

ionization detection the ratio of the areas of the propylene to

acetaldehyde peaks is 2.89 + .05 to 1. A plot of the results over

a concentration range of a factor of 25 can be seen in Figure XII. When both

substances are methanated the new ratio is 1.485 to 1. These ratios

are sufficiently close to the expected 3 to 1 and 3 to 2 values so

that the stochiometry can be considered to be established. Maximum

thru-put (100% conversion) in terms of acetaldehyde output /min as

a function of cell temperature can be found in Figure XIII. Least

squares analysis of the data gives at 7.0 psig and over the temperature

range of 30-90°C

Log^Q[yg CH^CHO/min] = 2814 + 30 [ ^ ] + 9.86 + .09

The thru-put range is thus 4-100 ug/min. The standard deviation

is of the order of + 4%. Note that through the use of the internal

standard a gain in accuracy of a factor of 2 is attainable.

For flow rates of 20 to 100 cc/min (He) the pyrolyzer temperature setting

for complete conversion must range from 650 to 700°C. The long

term behavior of this system is summarized in Table III. In order

to study the possibility of obtaining lower thru-put a hypodermic

needle (0.025 in ID) has been epoxied into the horizontal portion of

the 1/8 in diffusion column. The results as represented by the dotted

line can be seen in Figure XIII. It is seen that thru-put has been

lowered by a factor of 20. We did note however a substantial longer

time period is required for equilibration.

27

29
Table III

Long term behavior of acetaldehyde generator at 30.2°C.

Flow rate = 40 cc/min. Pyrolyzer at 680°C. System Pressure

7.0 psig.

hours concentration

0 56 ppm

96 58 ppm

120 58.5 ppm

168 56.5 ppm

30
C) CH2CHCHO: The parent compound for acrolein generation is

5-methyl-l,5-hexadlen-3-ol. The decomposition reaction is

CH^ = CHCHOH CH^C (CH^) = CH^ CH^CHCHO + iC^Hg

From gas chromatographic analysis with flame ionization detection

the isobutene to acrolein ratio is 1.90 + .02 to 1. In view of

the earlier results with acetaldehyde this is sufficiently close to

the expected 2 to 1 ratio and thus establishes the stochiometry

A plot of the results covering a concentration range of a factor of

30 can be found in Figure XIV. Maximum thru-put (100% conversion)

in terms of acrolein output /min as a function of cell temperature

is plotted in Figure XV. Least squares analysis of the data yields

at 7.2 psig and over the temperature range of 30-90°C

Log^Q[yg CH^CHCHO/min] = 2901 + 27 [ ^ ] + 9 . 49 + . 08

The thru-put range is 1-30 yg/min. The standard deviation is

+3.5% and it should be noted that with the internal standard a gain

of over a factor of two in accuracy is attainable. For flow rates of

20-200 cc/min (He) the pyrolyzer temperature setting for complete conversi

must range from 600-675°C. Long term behavior is summarized in Table IV.

31
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33
Table IV

Long term behavior of acrolein generator at 31° C. Flow

rate = 40 cc/min. Pyrolyzer at 620°C. System pressure = 7.0 psig.

hours concentration

0 9.7 ppm

24 9.6 ppm

48 9.7 ppm

72 9.8 ppm

96 10.0 ppm

34
D) HCN ; The parent compound for hyd'rogen cyanide generation

is ethylcyano formate. The decomposition reaction is

C^H^OCOCN ^ C^H^ + HCN + CO^

Gas chromatographic analysis with a Poropak P-S column and flame

ionization detection yielded over the entire range (a factor of 30)

an ethylene to hydrogen cyanide area ratio of 6.29 + .09. A plot

of the results can be found in Figure XVI. Although the constancy

of the area ratio is suggestive, the lack of any literature values on

the relative sensitivity of flame ionization detections of hydrogen

cyanide to hydrocarbons prevents the use of this number to establish

the stochiometry of the reaction. Accordingly, absolute determinations

of HCN concentration have been carried out by collecting HCN in NaOH

solution and measuring CN concentration using a specific ion electrode.

The results are summarized in Table IV and it is seen that over the

entire range one to one production of ethylene and HCN is Indeed the

case. Maximum thru-put (100% conversion) in terms of HCN output /mln

as a function of cell temperature is plotted in Figure XVI. Least

squares analysis of the data yields at 8.8 psig and over the

temperature range of 30-95 °C

Log^Q[yg HCN/min] = -2316 + 29 [ ^ ^ + 8. 24 + . 09

This covers a thru-put range of 4-100 yg/min and the standard

deviation or "settablllty" over this range is 4%. As is the case

for the two larger aldehydes the use of the internal standard will

Improve the accuracy to + 1.5%. For flow rates of 20-200 cc/min (He)

the temperature range of the pyrolyzer is 600-680°C. The long term

behavior of the system is summarized on Table V.

35
Peak Area HCN (Arb. Units)

FIGURE 16 Relative yields of ethylene and hydrogen cyanide from ethyl cyanoformate
Table V

a) Comparison of ethylene and hydrogen cyanide yields from ethyl

cyanoformate using G.C. and CN specific ion electrode.

Temp. G.C. Analyses for Ethylene Specific ion electrode

analysis of CN

Se^C 1440 ppm 1460 ppm

30°C 75 ppm 80 ppm

b) Long term behavior of HCN generator cell temperature 29.6°C

Flow rate 40 cc/min Pyrolyzer temperature = 630°C System Pressure

= 8.8 psig.

hours thru-put yg/min

0 3.91

24 3.82

48 3.86

72 3.86

96 3.86

168 3.82

37

E) SO : The parent compound for sulfur dioxide generation is

trimethylene sulfone. The decomposition reaction is

In an earlier report Cornell and Tsang (5), using an evaporative

method (1) have demonstrated the utility of trimethylene sulfone

as a source for SO^. In particular, they show that in the range

covered (10-100 ppm and 20-AO cc/min, flow rate) equivalent

amounts of hydrocarbons and sulfur dioxide are produced. The

present results will demonstrate that this compound is compatible

with the new generator. Indeed, it is shown that the use of

the diffusion technique offers a considerable increase in flexibility

so that the range covered can easily be extended.

All of the present results have been obtained with a diffusion cell

head containing a column with a 1/4 in diameter. This is in contrast

to the 1/8 in diameter columns used for the other compounds. Gas

chromatography with a Poropak T column and helium ionization detection is

used for analysis. Because of an interfering water peak, which

eluded on the tail of the SO^ peak, the Poropak T column was

preceeded by a methyl silicone column. Quantitation of the experimental

results is based on bottled samples of propylene (206 ppm, measured)

and sulfur dioxide (1000 ppm, stated) . It is assummed that in

the range covered the detector is linear. Total yield

of hydrocarbons as a function of SO^ yield is plotted in

Figure XVIII. Over the entire thru-put range (a factor of 100)

the ratio of hydrocarbon to SO^ yield is 1.024 + .04. Together

with the earlier results this is a satisfactory demonstration of the

(^2)3802 -y cyclo-C^H^ + SO

propylene

39
40
postulated stochiometiry . The larger then usual uncertainty is

actually an artifact that arose from the existence of a small water

impurity which impinged on the tail of the sulfur dioxide peak.

This is especially important at low thru-put. Thus there is an

actual drift in the measured concentration ratio ranging from

.97 at thru-put of 100 yg/min to 1.09 at the 1 yg/min level.

It is suspected that the former number is more likely to be the

correct one and that had it been possible to correct for the

water peaks a lower uncertainty limit would have been obtained. We

have also attempted to measure sulfur dioxide concentration using

a sulfur dioxide specific ion electrode following the prescription given

by Orion Research. Unfortunately the results showed wide scatter

(+ 50%) and have not been used. Maximum thru-put (100% conversion)

in terms of output/min is plotted as a function of temperature

in Figure XIX. A least squares analysis of the data yields

Log^QlygSO^/min] = -3145 + 41 [ | ] + 9.16 + .1

The standard deviation is + 5%. These data covers a thru-put range

of 1 to 100 yg/min and extends over the temperature range of 70-170°C.

Long term stability and the dependence of concentration on flow rate

is summarized on Table VI. For flow rates of 20-200 cc/min the pyrolyzer

temperature should range from 600-660"'C.

41
2.0

•gi.o

o

tn

bi

3.O

iH

00O

2.4 2.6 2.8

1000/T

FIGURE 19 Yields of sulfur dioxide/min from trimethylene sulfone as a

function of temperature.

42
1

Table VI

a) Long term behavior of SO^ generator Cell Temperature 100. 5 °C

Pyrolyzer temperature 650°C Flow rate 41 cc/min. System

Pressure =7.0 psig.

hours concentration

0 58.6 ppm

3 57.0 ppm

6 57.2 ppm

24 57.8 ppm

27 56.8 ppm

31 57.0 ppm

98 57.8 ppm

b) Dependence of concentration as a function of flow rate

System Pressure =7.0 psig Pyrolyzer temperature 650'*C Cell

Temperature = 100. 5 °C.

flow rate (cc/min)

20.4

42.7

82.0

163.0

concentration (ppm)

111.0

57.8

27.0

13.1

43
F) HCl ; The parent compound for HCl generation is cyclohexyl

chloride. The decomposition reaction is

C^H,,C1 -> C,H,^ + HCl

b li b iU

\

In the temperature range where these pyrolytic studies are carried

out the conversion to ethylene and butadiene is less than 10%.

Verification of the postulated stochiometry has been carried out using

gas chromatography with a Porapak P-S column and flame ionization

detection for the hydrocarbon and a CI specific ion electrode

for the quantitation of HCl yields. The results are summarized in

Figure XX. The ratio of cyclohexene (including the small quantity

converted to ethylene and butadiene) to HCl is 1.007 + -03 to 1 and

covers a concentration range of a factor of 25. Maximum thru-put in

terms of HCl output per minute as a function of cell temperature can

be found in Figure XXI. Least squares analysis of the data at 10 psig

and over the temperature range 30-90°C yields

Log^Q[yg HCL/min] = -2239 +32 [ ^ ] + 7.81 + .09

In this temperature range the thru-put covers from 2 to 50 yg/min. The

standard deviation is 4%. With flow rates in the 20-200 cc/min (He)

range the pyrolyzer temperature must be set between 580-650°C.

The long term behavior of the generator is summarized in

Table VII.

44
I

1000

500

500 1000

HCl ppm

FIGURE 20 Yields of cyclohexene (Including ethylene decomposition products)

and butadiene, and HCl from cyclohexyl chloride.

45
46
Table VII

Long term behavior of HCl generator at AQ'C. Flow rate

40 cc/min. Pyrolyzer at 630°C. System pressure = 10 psig

hours concentration

0 116 ppm

48 113 ppm

120 113 ppm

168 113 ppm

47
Parts List

Cell Compartment Heater

Furnace Heater

4" X 10" flexible heater, 120 v 200

watts, cat. no. HF4-10 Thermal

Corp., Huntsvllle, Alabama.

Firerod cartridge heater 5/8" dia.

X 3" long 120 V, 500 watts. Cat. no.

L3A113 Thermal Corp., Huntsvllle,

Alabama

Model A-IS Thermostat

Burling Instrument Co. , Chatham,

NJ 07928

Cell Compartment Temp. Controller RFL Model 870 Proportional Controller

RFL Industries, Inc. Boonton, NJ 07005,

Furnace Temp. Controller

Valves

s.s. metering valve

Brass Ball Valve

s.s.-2mg NuPro

B41S2 (Whitney)

Swagelok Fittings

s.s. union 1/8" tube s.s. 200-6

s.s. union 1/8" tube to 1/8" pipe s.s. 200-1-2

1/8" tube to 1/8" pipe B20-1-2

s.s. Bulkhead Union 1/8" s.s. 200-61

s.s. Male Branch "Tee" TTM

Brass Bulk head union 1/8" B-200-61

Tubing

s.s. 1/8" CD

copper 1/8" OD

gold .015" wall X .125" OD (.635 troy oz.)

1 each

1 each

1 each

2 each

2 each

1 each

3 each

10 ft.

1.5 ft.

1 ft.

48
SWl Toggle switch

SW2, -3, -A Toggle switches

Chemicals

3-methyl-3-buten-l-ol

4-penten-2-ol

5 -met hy 1- 1 , 5-hexadien- 3-o

ethyl cyano formate

trimethylene sulfone

cyclohexyl chloride

DPST

SPST

Chemical Samples

Chemical Samples

Chemical Samples

Aldrich Chemical Company

NBS

Aldrich Chemical Company

49

1
References

1. G. 0. Nelson, Controlled Test Atmospheres, Principles and Techniques

Ann Arbor Science Publishers, Ann Arbor, Michigan (1971).

2. Certain commercial materials and equipment are identified in this

paper in order to specify adequately the experimental procedure.

In no case dooes such identification imply recommendations or endorsement

by the National Bureau of Standards, nor does it imply that the

material or equipment is necessarily the best available for the purpose.

3. S. W. Benson and H. E. O'Neal, Kinetic Data on Gas Phase Unimolecular

Reactions, Nat. Stand. Ref. Data. Ser. , Nat. Bur. of Stds. (U.S.),

21, 645 (Feb. 1970).

4. R. K. Stevens, A. E. O'Keefe and G. C. Ortman, unpublished results.

5. D. Cornell and W. Tsang, Analytical Chemistry, 933, 46, 1974.

USCOMM-NBS-DC 50
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 76-1000

2* Gov't Accession

No.

3* R.Pf'iniPnt*<5 Appf<;^inn Mn

4. TITLF AND SUBTITI.H

The Construction, Operation and Performance of a Reactive

Gas Generator; with Specific Application to HCHO, CH..CHO,

CH2CHCHO, SO2, HCN and HCl TjroattCtibn

5. Publication Date

January 1976

6. Performing Organization C'ode

7. AUTHOR(S)

James A. Walker and Wing Tsang

8. Performing Organ. Report No,

NBSIR 76-1000

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Projcct/Task/Work Unit No. 3162201

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP) 13. Type of Report & Period

Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual siunmary of most si^ifleant information. If document includes a significant

bibliography or literature survey, mention it here.)

An instrument capable of generating a variety of reactive gases in trace levels

has been constructed and tested. Operational details, with specific application

to HCHO, CH^CHO, CH2CHCHO, SO2, HCN and HCl production are discussed.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) ^etaldehyde ; acrolein; formaldehyde; gas chromatography;

hydrogen chloride; hydrogen cyanide; pyrolysis; pollution; reactive gas generator;

sulfur dioxide

18. AVAILABILITY Unlimited

For Offi cial Distribution. Do Not Release to NTIS

f I Order From Sup. of Doc, U.S. Government Printing Office

Washington, D.C. 20402, SD Cat. No. C13

Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

54

22. Price

$4.50
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