Kinetics of the Sulfur-Methane Reaction


(42) Schlesman, C. H., Denton, W. I., and Bishop, R. B., Ibid.,. 2,456,597 (1948). .... The packed bed section was 35 mm. in diameter and 10 em. in le...
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LITERATURE CITED

Ai, H. C., M.S. thesis in chemical engineering, Polytechnic Institute of Brooklyn, 1953. Amiel, G., C o m p t . rend., 196, 1122, 1896 (1933); A h . ehim., 7, 70 (1937). Avramenko, L. I., Ioffe, I. I., and Lorentso, R. V., D o k l a d y A k a d . Wauk S.S.S.R., 66, 1111 (1949). Bibb, C. H., U. S. Patent 1,547,725 (1925). Bibb, C. H., and Lucas, H. J., IND. ENQ.CHEW,21, 635 (1929). Bone, W.A., and Newitt, D. &’I., Can. Patent 384,682 (1939). Bremner, J. G. M., Brit. Patent 628,503 (1949). Brewer, A. K., and Kueck, W.,J . P h y s . Chem., 35, 1293 (1931). Charlot, G., Ann. chim., 4, 415 (1934). Cotton, W. J., T r a n s . Electrochem. Soc., 91, 407 ff. (1947); U. S . Patents 2,485,476-81 (1949). Coulson, C. A , and Baldock, G. R., Discussions F a r a d a y Soc., No. 8,27 (1950). Denton, W.I., Doherty, H. G., and Krieble, R. H., IND.EXG. CHEW,42, 77 (1950). Duyokaerts, G., B u l l . soc. roy. sei. Liege, 1 8 , 152 (1949). Eisenstedt, E . , J . Org. Chem., 3,153 (1938). Eley, D. D., Discussions Faraday Soc., No. 8, 34 (1950). Faith, W. L., Keyes, D. B.,and Clark, R. L., “Industrial Chemicals,” New York, John Wiley & Sons, 1950. Friedel, R, A., and Pierce, L., A n a l . Chenb., 22, 418 (1950). Geib, K. H., and Harteck, P., Be?., 66, 1824 (1933). Geib, K. H., and Harteck, P., T r a n s . F a r a d a y Soc., 30, 131 (1934). Gill, D., Mardles, E. W.J., and Tett, H. C., Ibid., 24, 574 (1928). Glockler, G., and Lind, S. C., “Elcctrochemistry of Gases and Other Dielectrics,” New York, John Wiley & Sons, 1939. Griffith, R. H., and Hill, S. G., T r a n s . F a r a d a y Soc., 32, 829 (1936). Harkins, W. D., and Jackson, W. F., J . Chem. P h y s . , 1, 37 (1933). Harmon, R.A,, C.S.Patent 2,382,148 (1945). Harteck. P.. T r a n s . F a r a d a v Soc.. 30. 134 (1934) Hinshelwood, C. K,,and Fort, P: J.,’Proc. R o y . Soc. ( L o n d o n ) , A127, 218 (1930). Jost, W., Meuffling, V., and Rohrmann, 2. Elektrochem., 42, 46 (1936). Kleinschmidt, R. T7., C.S. Patent 2,023,637 (1935). Krieble. R. H.. and Denton. W. I.. Ibid.. 2.415.100 (1947). 2,439,812, 2,440,233, 2,440,234 (1948). Lavin, G. I., and Stewart, F. B.,Proc. A‘atl. A c a d . Sei., 15, 829 (1929). Leminger, O., C h e m . Obzor., 24, 88, 100 (1949). Lien, A. P., U. S. Patent 2,499,515 (1950).

Kinetics of the 0 . W.

(33) Mardles, E. W. J., T r a n s . F a r a d a y Sac., 27, 681 (1931). (34) Marek, L. F., “Catalytic Oxidation of Organic Compounds in the Vapor Phase,” New York, Chemical Catalog Go., 1932. (35) Martin, R. W., A n a l . Chem., 21, 1419 (1949). (36) Milas, N. A., J . Am. Chem. SOC.,59, 2342 (1937); U. S. Patent 2,395,638 (1946). (37) Moyer, W.R7.,Ibid., 2,328,920 (1943). (38) Moyer, W. W.,and Klingelhoeffer, W. C., I b i d . , 2,223,383 (1940). (39) Sewitt, D. >I., and Burgoyne, J. H., Proc. R o y . Soc. ( L o n d o n ) , A153. 448 11936). (40) Oshima; K., and Imoto, E., Chem. H i g h P o l y m e r s , 3, 57 (1946). (41) Porter, F., U. S. Patent 2,392,875 (1946). (42) Schlesman, C. H., Denton, W. I., and Bishop, R. B., Ibid., 2,456,597 (1948). (43) Simmonds, J. H., and McArthur, R. E., IKD.ESG. CHEM.,39, 364 (1947). (44) Tomicek, O., and Dolezal, J., Chem. L i s & , 43, 193 (1949). (45) Weiss, J. AI., and Downs, C. R., IND.EM. C H m r . , 12,229 (1920). (46) Zethelius, S., Rea. colombiana quim., 3, 5 (1949). GENERAL REFERENCES

Bone, W. A,, T r a n s . C h e m . Soc., 61, 871; 71, 26, 46; 79, 1042; 81, 535; 85, 693, 1637; 87, 910, 1232, 89, 652. 660. 939: 93. 1198: J . Chem. Soc.. 83. 1074: 89. 660. C h e m . Eng., 58, h o . 10; 232 (1951). Dryer, W. P., I b i d . , 54, KO. 11, 127, 132 (1927). Dushman, S., IND. ENG.CHEM.,40, 778 (1948). Elektrochemische Werke, U. S.Patent 2,015,040 (1935). Gilmont, R., IXD. EKG. CHEM.,ANAL. ED., 18, 633 (1946); A n a l . Chem., 20, 89 (1948); 23, 157 (1951). Hougen, 0. A,, and Watson, K. M., “Chemical Process Principles,” New York, John Wiley & Sons, 1947. I. G. Farbenindustrie, A.-G., Ger. Patent 501,467 (1930). hones, G. W., Chem. Reas., 22, 1 (1938). Jost, W.,“Explosion and Combustion Processes in Gases,” New York, McGraw-Hill Book Co., 1946. Normand, C. E., IND.ENG.CHEM.,40, 783 (1948). O’Connor, J. A., C h e m . Eng., 58, S o . 10, 215 (1951). Perry, J. H., “Chemical Engineers’ Handbook,” 3rd ed., New York, hIcGraw-Hill Book Co., 1950. Schlesman, C. H., U. S. Patent 2,553,944 (1951). Schoch, E. P., Bur. Ind. Chem., Univ. of Texas, B u l l . 5011 (1950). Warren, R. F., Chem. Eng., 58, No.8,279 (1951). Kedge, C., Brit. Patent 2,010 (1901). Weingartner, H. C., IND.ENG.CHEM.,40, 780 (1948). RECEIVED for review J a n u a r y 8, 1953.

ACCEPTED April 3. 1953.

ethane Reaction

N A B O R ’ A N D J. M. S M I T H

P U R D U E UNIVERSITY,. LAFAYETTE, IND.

T

HE kinetics of the homogeneous reaction between sulfur vapor and methane for the production of carbon disulfide were studied by Fisher and Smith ( 3 ) . Forney and Smith (6) reported a specific reaction velocity constant for the silica gelcatalyzed reaction at 600’ C., assuming that equilibrium existed between the sulfur species SZ, SS, and Ss present a t that temperature. The present paper reports data taken over a temperature range 550” t o 700” C. and conclusions regarding the kinetics of the reaction based upon an analysis of these data. It is of particular interest t o know which of the sulfur species plays the prominent part in the reaction with methane. The possibility of reaction of methane and sulfur in a vaporphase process for the production of carbon disulfide, as an alternative t o the high temperature, conventional electrothermal process, was first mentioned by de Simo ( 1 ) . I n two pioneer papers, Thacker and Miller ( 1 0 ) and Folkins, Miller, and Hennig ( 4 ) presented data establishing silica gel as an active and stable catalyst and showing the effects of such operating variables as ratio of temperature and reactants on the conversion to carbon 1

Present address. Magnolia Petroleum Co., P.O. Box 600, Dallas 1, Tex.

1272

disulfide. Folkins, Miller, and Hennig also carried out thermodynamic calculations indicating that the equilibrium conversion of the reaction

CH,

+ U S , = CSZ+ 2HzS

(UZ =

4)

(1)

is essentially 100% for SZ, Sg,or Sa a t temperatures above 500” C. The monatomic species, SI, is not present in appreciable quantities a t temperatures below 800’ C. The results of Forney and Smith (6) indicated that the rates of diffusion of reactants and products to and from the catalyst surface were large in comparison with the rates of the processes of reaction, adsorption, and desorption on the catalyst. Hence, in the present investigation no attempt was made to obtain data under conditions of constant space velocity and varying values of catalyst bed depth and reactants flow rate. Comparison of the results of Fisher ( 3 ) and Forney (6) showed that the rate of the homogeneous reaction, while small, was of sufficient magnitude to necessitate applying a correction procedure in evaluating the catalytic data. An integral-type reactor was used because of the difficulty that

I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

Vol. 45, No. 6

-

i

ASa substitute for t h e high

temperature, electrothermal process, t h e vapor phase reaction of sulfur and methane t o produce carbon disulfide has been of growing interest, especially i n areas where natural gas is available. A knowledge of t h e kinetics of t h e reaction is desirable i n order t o develop t h e o p t i m u m process design. Rates of reaction were measured over a range of space velocities and temperatures (550" t o 700" C.) using t h e preferred silica gel catalyst. Interpretation of t h e data was complicated by t h e existence of a finite rate for t h e homogeneous reaction. A method was proposed for separating t h e catalytic and homogeneous reactions. It was found t h a t a simple, second-order rate equation, based upon SZ as t h e reactive species of sulfur, correlated t h e data a t a l l temperatures. T h e reaction rate constant i s given by t h e expression: log k,,,

= 10.9

- 2.303(RT)' 31'400 ~

While t h e proposed rate

equation supplies no information as t o t h e interaction of reactants and products w i t h t h e catalyst, and hence t h e true mechanism, t h e results do provide a method of predicting reaction rates over t h e range of operating conditions investigated, This information should be of value i n designing commercial reactors for this process.

P

c

would be involved in attempting t o separate the conversions due to the homogeneous and catalytic reactions in a differential reactor. I n the latter case, the mass of catalyst is necessarily small. Hence the extent of the homogeneous reaction occurring in the unavoidable volume upstream and downstream from the catalyst bed and in the void space of the bed would be relatively large compared to the total conversion. As a result of the deeper catalyst bed in the integral reactor, the conversion due to the homogeneous reaction was small with respect to the total conversion. PROPERTIES OF MATERIALS

The methane employed was of 99% purity or better. Forney ( 5 ) ,whose 600" C. data were included in the analysis, used 96%

methane. Both types were supplied by the Matheson Go., Inc. The major known impurity was ethane. The sulfur used was the roll type, twice distilled to remove residual dirt. The catalyst was Davison commercial grade silica Gel, 4008-08-226. The rock salt used as a pre- and afterpacking material in the bed to reduce void volume was taken from a water softener supply. Fisher ( 8 ) has shown that such material has no appreciable activity as a catalyst. Both catalyst and salt were sized t o 6- to 8-mesh and the off-color pieces were removed before use.

SULFUR CONDE

EQUIPMENT AND OPERATION

The major portion of the flow system used is shown in Figure 1. The methane was metered by means of a capillary tube enclosed in a constant temperature bath. The sulfur was metered in the vapor state at about 540" C. by means of a quartz capillary. These capillary meters were satisfactory for the aboratory scale flow rates desired (a few hundredths to somewhat over 1 grammole per hour). After the methane and sulfur vapor streams were combined, the gas.mixture entered the reactor lead-in and passed on to the packed bed section. The reactor was made of uartz because metallic sulfides possibly would form on the sur?ace o! a metal reactor and catalyze the reaction (1). The packed bed section was 35 mm. in diameter and 10 em. in length. Rock salt, inert to the reaction ( d ) , was used in the lower portion of the packed bed section, a bed of catalyst filled the middle portion, and salt served to complete the filling of this section. The reading of the thermocouple placed at the middle of the packed bed was taken as the reaction temperature. To assure a reproducible temperature at this point, a calibrated platinumplatinum-rhodium thermocouple was used. The thermocouples shown on the outside of the packed bed section in Figure 1 were more sensitive t o temperature changes in the system and were used in this wa for control purposes. Radial temperature gradients across tEe catalyst bed were indicated by the difference of the readings of the two thermocouples at the level of the bed. These gradients were from 3" to 5" C. After leaving the packed bed, the gases passed to the condenser which removed unreacted sulfur, and from thence to the product (carbon disulfide) condenser kept at -60" C. in a dry ice-acetone bath. The hydrogen sulfide and unreacted methane passed on to the caustic absorber. Fisher made some complete analyses and material balancea for this system (d). H e decided that the absence of side reactions made necessary only measurement of the amount of carbon disulfide produced. The work of Folkins, Miller, and Hennig ( 4 ) also led to this conclusion. The product was first stabilized t o remove dissolved hydrogen sulfide and then diluted with benzene. The specific gravity of the resulting solution furnished a measure of the concentration of carbon disulfide, TABULATED DATA

SULFUR METER

MET'ERED METHANE

&SULFUR

*IR X THERMOCOUPLE -I- GROUND JOINT

Figure 1.

June 1953

RESERVOIR

JUNCTION

Assembly for Product Collection

The experimental results are tabulated in Tables I through IV. The packed bed section of the reactor may be thought of &a divided into three sections numbered a t four points 0, 1, 2, and 3 from the entrance t o the exit. When the reactor is packed, section 0-1 contains rock salt, section 1-2 contains catalyst, and section 2-3 contains rock salt. All the reaction was assumed t o occur between points 0 and 3. For each series, there are tabulated the reactants ratio, m (gram-moles of methane per diatomic weight of sulfur), the total conversion, 2 3 , the total feed rate, F, the total time that the catalyst had been exposed to methane at the middle of the run (used where corrections had t o be made for

INDUSTRIAL AND ENGINEERING CHEMISTRY

1273

Tabulated Data for 550" C.

Table I. Series H2

c3

Run 19 20 21

22 23 24 25 26 27 28 29 30 31

nl

'/e '/Z 1/Z

So-s '/2 1

=

'l // az I/a l/a

1 1

'/Z 1/Z

Sa-I = CI-e = Sa-a =

C4

32 33 34 35 36 37 38

'/2 1

1 '/a '/a 1/Z '/a

-

So-1 = CI-z = 83-3

c10

63 64 65 66

'/e 1 1/Z

1 = CI-I = Se-a = 80-1

Table I I.

hlid-CHi F, Gram-Mole/ R1.:; csz Time, Hour Mole F Min. 0.900 (Very small) 79 0.375 0.0048 269 0.225 0,0089 428 No catalyst 7 1 . 5 ml. Vo-a = 33.9 ml. 0.375 o 1203 94 0 300 0 1550 221 0 300 0 0943 329 0 150 0 1987 4 84 0 800 0 0751 6 04 0 200 0 1120 719 0 150 0 1760 826 0 800 0 1109 980 0 750 0 0945 1099 0 375 0 1357 1230 W = 10.2851 grams Vo-r = 1 4 . 0 ml. 3 0 . 0 ml. 1 4 . 5 ml. VI-z = 6 . 7 9 ml. Vz-a = 13.1 ml. 2 8 . 0 ml. 0.0448 0.0524 0.1630 0.0343 0.1975 0.0667 0.0324 W = 4.0031 grams Vo-1 = 1 4 . 5 ml. 2 9 . 5 ml. VI-Z = 2 . 9 5 ml. 6 . 0 ml. 3 3 . 5 ml. Va-8 .= 1 6 . 5 ml. 0.2105 86 0.150 0.2268 263 0.150 0.1983 400 0.150 0,2171 542 0.150 W = 10.2762 grams Vo-0-1 = 1 4 . 0 ml. 2 9 . 5 ml. VILZ = 6 . 8 7 ml. 1 4 . 5 ml. Va-a = 1 3 . 0 ml. 2 7 . 5 ml.

Series

0

B

khNCE4N81

(2)

D

80 81 82 83 84

F

93 94 95 96 97 98 99 100 101

H

108 109 11oa 111 112a

I

118 119 120 121 122 123

J

126 127 128 129 130 131

K

132 133 134 135 136 137

L

138 139

The basic reactor design equation may be w i t t e n in the following way for the homogeneous and catalytic reactions: (3)

At low total pressures and high temperatures, the partial presure of a component may be expressed as the product of the total pressure and the mole fraction. The present study was carried out a t a total pressure of 1 atmosphere, so that mole fractions, X, are equal to partial pressures. Hence the rate equation may be written T

1274

= Icf(Ni, N j ,

. . .)

140

141 142

53,

Moles Csz Mole F 0.0241 0.0673 0.0498 0.0254

Mid-CHd Time, Min.

...

... ... ... 75 170 275 360 436 535 63 5 745 842.5 940 1040 1140 1239.5 1327.5 1420

'/Z '/Z 1/2 '/l

'/2

1 1

1

1

'12

'/a

'/a '/a '/a '/a

W

OF CALCULATION

P dx = r c d W (catalytic reaction) P dx = rh dV (homogeneous reaction)

F, Gram-Mole/ Hour 1/2 0.300 '/z 0,150 1 0.200 '/2 0.300 N o catalyst SO-3 = 5 6 . 0 ml. m

=

18.0 ml. CI-a = 11.0 ml. SL-P= 2 3 . 0 ml. '/a 0.300 '/2 0.150 '/z 0.450 '/a 0.900 1 /2 0.300 80-1 =

Fisher's k h values were not used because they were not available over the entire temperature range desired and it was found experimentally that the kh value is t o some degree a specific function of the conditions of operation, particularly the temperature of the line between the packed bed and the sulfur condenser. METHOD

52 53 54 55 56 57 58 59 60 61 62 63 64 65 66

decreased conversion due to catalyst fouling), and the catalyst weight, W . The bulk volumes of the rock salt and the catalyst used in packing the reactor are tabulated as S and C. The corresponding void volumes for the three eections, denoted by Ti, were calculated from the bulk volumes S and C by assuming a constant total void volume for all packings and assigning values to the sections in proportion to the bulk volumes, S and C. (Several measurements revealed the void volume to be constant within l%.) During the courqe of the project, several reactors were used; this accounts for the slight variation in total void volume. The fir-t series of runs in each table consists of homogeneous reaction data taken a t operating conditions comparable to those used for the catalytic data obtained later. These data were used to evaluate the homogeneous phase reaction rate constant according to Fisher's ( 3 )reaction mechanism rh

Run 33 34 35 36

Tabulated Data for 600" C.

6 . 9 5 0 grams Vo-1 = 1 3 . 2 ml. 171-2 = 8 . 0 5 ml. 112-3 = 1 6 . 9 ml.

0.1640 70 0 2167 176 0.1387 285 0.0937 372.5 0.1553 460 TV = 2.610 &&ms Sn-1 = 1 9 . 0 ml. Vo-I = 1 2 . 4 ml. GI--2 = 4 . 5 ml. VI-z = 2 . 9 4 ml. Sz-a = 36.0 ml. Vi-a = 2 3 . 5 ml. '/2 0 450 0 2556 65 '/2 0 900 0 2300 120 0 400 0 2280 175 ' 0 450 '/ZIa 0 2688 235 1/3 0 800 0 2063 297 5 1 0 400 0 2495 355 '/Z 0 450 0 2711 420 1 0 800 0 2313 475 '/z 0 450 0 2622 530 W = 14.744 grams 80-1= 1 6 . 5 ml. VO-1 = 10.8 ml. Ci-2 = 2 2 . 0 ml. V I L ~= 14.35 ml. 5 2 - 3 = 2 1 . 0 ml. Ve-a = 1 3 . 7 ml. '13 0.267 0.1663 65 '/a 0.133 0.1417 166 '/a 0.400 0,1403 290 111 0.800 0.0950 370 '/a 0.267 0,1551 540 W = 3.566 grams So-1 = 24 5 ml. V8-1 = 1 6 . 7 ml. C1-Y= 5 . 5 ml. VI-e = 3 74 ml. Sa-a = 2 7 . 0 ml. V2-s = 18 4 ml. '/z 0.300 0.1733 65 1/a 0,267 0.1559 160 0.1887 250 '13 0.133 113 0.400 0.1233 332 '/3 0.800 0.0820 407. 0.1603 495 '/2 0.300 W = 2.615 grams VOW = 1 6 . 7 ml. SO-I= 2 6 . 0 ml. Vl-2 = 2 . 8 9 ml. CI-2 = 4 . 5 ml. VZ-s = 1 9 . 2 ml. 52-3 = 3 0 . 0 ml. 0.1983 64.5 1 0 ROO 1 0 I50 0.2233 170 1 0 400 0.1768 260 0 1927 350 1 0 .ROO 425 1 0.800 0.1263 0 1933 500 1 0.300 W' = 3.533 grams S - I = 2 5 . 0 ml. VO-1 = 1 6 , s ml. C I - Y = 5 5 ml. VI-2 = 3 . 6 2 ml. Va-a = 1 8 . 7 ml. S2-9 = 2 8 . 5 ml. 1 0.300 0 1773 60 1 0.150 0.2133 160 1 0.400 0.1578 250 1 0.300 0,1700 340 1 0,800 0.1048 420 1 0.300 0.1703 495 W = 2.592 grams So-' = 2 6 . 0 ml. VI-% V c - I = 126..981 ml. ml. Ci-2 = 4 . 5 mi. Vz-a = 1 9 . 1 ml. S2-8 = 2 9 . 5 ml. '13 0.267 0.1228 60 '/3 0.133 0.1699 155 0 . 4 0 0 0.0928 245 '/3 333 '/8 0.800 0.0490 >/a 0.267 0.1079 425 W = 1.299 grams Vo-1 = 17.0 ml. Sc-I = 2 6 . 0 ml. C1-s = 2 . 5 ml. VI-2 Vz-a = 2 1.63 0 . 2 ml. SZ-3 = 31.0 ml. I

(4)

I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

Vol. 45, No. 6

Table I I I . Series

Run

I

1

VI

24 25 26

2 3 4

1

VI1

VI11

IX

X

XI

XI1

27 28 29

30 31 32

33 34 35

36 37 38

40 41

42 43 44

Tabulated D a t a for 650" C.

Mk

F, Mid-CHa Time, Gram-Moles/ Mole F Min. 112 Hour '/Z 0.600 0.0777 69 1.200 0.0339 171 '/Z '/Z 1.800 0.0211 264 I/P 0.810 0.0605 358 No catalyst So-s = 57.5 ml. 7 0 - 8 = 29.6 ml. 0.1688 38.5 '/Z 0.600 0.1168 129.5 '/a 0.900 0.1399 204.5 1.~ /a 0.600 W = 2.0179 grams V L W= 17.9 ml. 270-1= 3 7 . 0 ml. VI-z = 1.46 ml. CI-a = 3 . 0 ml. VZ-8 = 1 4 . 8 ml. &-a = 30.5 ml. 34 0.600 0.2385 83 0.900 0.2018 128 0.2330 0.600 W = 4.0170 grama 7 0 - 1 = 1 6 . 0 ml. So-1 = 3 2 . 5 ml. VIA = 2 . 7 1 ml. c1-2 = 5 . 5 p1. V2.g = 1 5 . 5 ml. 5 2 - 8 = 3 1 . 5 ml.

Run

XI11

46 47 48

XV

50 51 52

XVI

53 54 55

'/Z

'/a

'/l

0 2773 44 0.600 0.2920 95.5 0.300 0.2751 172.5 0.600 W = 6 0020 grams Vo-1 = 14.4 ml. So-1 = 2 9 . 5 ml. VLZ = 3 . 9 1 ml. Cl-a = 8 . 0 ml. VZ-8 = 15.9 ml. 82-8 = 3 2 . 5 ml. '/a '/Z '/Z

0.2145 43 0.600 0.800 0.1754 99.5 0.1672 165 0.600 W = 4.0141 grams Vo-I = 1 5 . 4 ml. So-1 = 3 0 . 0 ml. VI-z = 2 . 8 3 ml. C1-2 = 5 . 5 ml. Vz-a = 1 6 . 0 ml. Sz-s = 3 1 . 0 ml.

XVII

56 57 58

XVIII

59 60 61

XIX

62 68 64

XX

65 66 67

'/8

'/a

'/a

0.1590 37.5 0.600 0.1274 106.5 0.800 0.1408 166.5 0.600 W = 2.0065 grams Vo-I = 17 0 ml. So-' = 3 3 . 0 ml. VI-z = 1 . 2 5 ml. CLZ = 2 . 5 ml. Va-s = 17.0 ml. Sz-s = 3 3 . 0 m1. '/8 '/8

'/a

0.1707 118.5 0 800 179.5 0.800 0.1591 W = 3.0195 grams Vo-1 = 1 6 . 6 ml. 80-1 = 3 1 . 0 ml. VI-z = 2.07 ml. CI-z= 4 . 0 ml. VZ-8 = 16 1 ml. Sz-a = 31 .O ml. 1 0.800 0.1813 43 0.1529 91.5 1 1.100 0.1314 130 1 ' 0.800 1/a 1/8

Mid-CH, F. Gram-Moles/ ~Mol%% Time, Hour Mole F Min. W = 4.0219 grams Vo-I = 1 5 . 4 ml. Sa-1 = 3 1 . 0 ml. VI-z = 2 . 7 3 ml. CI-z = 5 . 5 ml. Vz--8 = 1 6 . 1 ml. Sa-s = 3 2 . 5 ml. 1 1.000 0.2001 78 139.5 1 0.600 0.2136 1 1,000 0.1956 183 W = 6.0029 grams VLI = 1 6 . 9 ml. 80-1 = 3 4 . 0 ml. VI-Z = 3 . 9 6 ml. Ci-e = 8 . 0 ml. VZ-s= 1 3 . 4 ml. &-a = 2 7 . 0 ml. 42.5 1 1.000 0.2130 1 0.600 0.2276 99 0.2001 152 1 1,000 W = 8.0113 grams VO-z = 1 5 . 3 ml. 80-1 = 29.0 ml. VI-a = 5 . 2 6 ml. CI-z= 1 0 . 0 ml. Sa-8 = 2 6 . 0 ml. VZ-s= 13.7 ml. 1/Z 0.750 0,2525 52 '/Z 0.600 0.2655 105 '/Z 0.750 0.2512 170 W 10,0305 grams 80-1 = 3 2 . 0 ml. Vo-I = 1 5 . 9 ml. CI-9 = 1 3 . 0 ml. VLP Vz-s = 161..494 ml. SZ-8 = 2 4 . 0 ml. 0.1466 42.5 1 1.000 1 0,600 0.1778 107 0.1288 157.5 1 1,000 W = 3.0050 grams 80-1 = 35,O ml. VQ-I = 1 7 . 2 ml. CI-a = 4 . 5 ml. VI-a = 2 . 2 2 ml. 81-8 = 3 0 . 5 ml. VZ-B= 1 5 . 0 ml. 37 1 0.700 0.2233 97 1 0.500 0.2363 143 1 0.700 0.2061 W = 6.0074 grams So-1 = 29.0 ml. VO-I = 1 4 . 7 ml. CI-2= 8 0 ml. VI-* = 4 . 0 5 ml. 8 2 - 8 = 3 0 . 5 ml. VZ-s= 1 5 . 5 ml. '/a 0.600 0.2227 40 !/a 0.800 0.2237 104 1/8 0.600 0.2227 165 W = 10 0084 grams V(-I = 15.8 ml. 80-1= 3 2 . 0 ml. VI-z = 6 . 6 4 ml. CI-z = 1 3 . 5 ml. V8-s = 1 1 . 8 ml. S z - a = 2 4 . 0 ml. '/Z 0.750 0.2723 61 '/a 0.600 0.2835 123 '/Z 0.750 0.2651 196.5 W = 10.0080 gram8 Vo-I = 1 5 . 1 ml. So-I = 3 3 . 0 ml. CI-z= 1 3 . 5 ml. VI-a = 6 . 1 5 ml. Sz-a = 2 8 . 5 ml. Vz-a = 13.0 ml. m

-

surface reaction, and desorption processes that constitute the actual mechanism.

and the design equation

(5) As the mole fractions of the system components are related to the amount of conversion, the right side of this equation depends only upon x. By assuming a reaction mechanism and thus the form of the mole fraction function, the right side of the equation when integrated may be plotted versus ( W / F ) or ( ' V / F ) ; the correct assumption will give a straight line of slope equal to the rate constant. The use of a simple form (Equation 4) of the rate equation for the catalytic case is worthy of discussion. The actual mechanism by which the over-all reaction is accomplished, involving the interaction of one or more of the reactants and the products with the catalyst, is both complex and unknown. Furthermore, the data obtained do not permit the elucidation of the several steps making up the actual mechanism. For this reason, the data were correlated by using a simple kinetic equation, analogous in form to those used for homogeneous reactions. The reaction velocity constants determined in this manner do not refer to a single chemical process, but rather represent effective k values which give the same result as the combination of adsorption, June 1953

Series

COMPOSITION O F REACTANT SYSTEM

Equation 5 represents the general case for a single reaction. I n the present instance, the situation is more complex. In the reactant system, there exist a t any partial conversion the products carbon disulfide and hydrogen sulfide, and the methane and sulfur still unreacted. The sulfur, however, exists in the forms SZ,Sa, and Sa in the range of temperatures investigated (9). The question arises as t o which one or more of these species participate in the reaction which produces carbon disulfide. The rigorous approach to this problem requires that the rates of interaction among the sulfur species be known; the concentration of all the sulfur forms can then be determined as the reaction proceeds. This information is not available. I n their analysis of the homogeneous reaction, Fisher ( 3 ) and Forney ( 6 ) assumed that the rates of the reactions among the sulfur species were i s s t as compared to the over-all rates; hence the concentrations of t sulfur forms were those existing a t equilibrium among the species. The chief reason for extending the present work to 700" C. was to test this assumption. As shown in Figure 2, a t 700" C. the concentrations of S g and SSin the feed t o the reactor are very small with respect to that of Sz.

INDUSTRIAL AND ENGINEERING CHEMISTRY

1275

The equation Table IV. Series H3

Run 39 40 41

c5

42 43 44 45

C6

46 47 48 49

c7

50 51 52 53

C8

54 55 56 57

C9

58 59 60 61 62

Tabulated Data for 700" C.

F % Gram-ilkoles/ m Hour 1.800 1/2 '/a 1.350 1/2 0.900 K O catalyst Sa-a = 70.0 ml. 1.800

hIole F 0.0402 0.0613 0.0826

VO- = 3 3 . 9 ml. 0.1598 1/8 1.600 0 . 1301 1 2.500 0.1482 '/z 1.800 0.1512 TV = 2.0102 grams 1'0-1 = 15.8 ml. 80-1 = 3 2 . 5 ml. V1-z = 1 . 4 6 ml. CI-a = 3 . 0 ml. VZ--3 = 16.6 ml. Sn-a = 3 4 . 0 ml. 1/z 0.900 0.2462 1/3 1.333 0.1825 1 2.000 0.17813 1/2 0.900 0.2465 W = 3 . 0 5 2 1 grams T'o-1 = 1 5 . 0 ml. So-I = 3 0 . 0 mi. 1'1-2 = 2.24 ml. (21-2 = 4 . 5 ml. VZ-3 = 1 6 . 7 ml. Sz-a = 3 3 . 6 ml. 1/2 0.750 0,2740 ]/a 1,200 0 2219 1 2.000 0 2250 1/2 0.750 0.2863 . W = 5.1001 grams VO-I = 1 6 . 4 ml. So-1 = 3 4 . 5 mi. VI-% = 3 . 0 8 ml. CI-z = 6 . 5 ml. VE-3 = 14.4 ml. Sr-a = 3 0 . 5 ml. 1/2 0.750 0.2975 1.200 0.2287 1/3 1 2.300 0.2313 1/2 0.760 0,2875 W = 7.0317 mams 170..1 = 14. 1 ml. So-1 = 2 7 . 5 ml. V L Z= 4 . 6 2 ml. Ci-z = 9 . 0 mi. 772-3 = 1 5 . 2 ml. Sz-a = 29.5 ml. 0.750 0.3011 l/z* 0.2438 l/a 1.200 0 2306 1 2.500 0.3081 1/z 0.750 1 2.500 0.2411 W = 10.0819 grams V0-i = 1 3 . 1 ml. So-1 = 2 7 . 0 ml. V I M = 6 . 5 4 ml. CI-2 = 1 3 . 5 ml. VZ-s= 1 4 . 3 ml. 5 2 4 = 2 9 . 5 ml.

.

-

~~

~

=

hlld-CHd Time, hlin. 156.5 252.5 365.5

23,

+

[ m / ( m 1) - 21 (1

+ 2 X S , + 3111\3

('3)

also results from this derivation. The expressions for -1c, and N E p s are not given, as thermodynamic equilibrium considerations for the methane-sulfur reaction ( 1 ) show the equilibrium constant to be large for all species of sulfur over the temperature range considered; hence the rate expression involving the product mole fractions need not be considered in the kinetic analysis. As the amount of methane in the feed is increased, it effectively dilutes the sulfur and causes a greater fraction of the sulfur to be present in the Sa form. As carbon disulfide is produced-Le., as conversion increases-the same effect is noted. Referencc to Table V shows that SZ becomes increasingly important at higher temperatures while SSbecomes less important. Figure 2 shows the effect of temperature on the feed composition (zero conversion) for one value of m. It is evident from this figure that study of the reaction a t higher temperatures is especially desirable, as reactions mechanisms involving S g or SSwould CYhibit unusual rate behavior due to the large reduction of these concentrations with temperature. West ( 1 1 ) has clearly shown the effect of temperatuie on the equilibrium between the sulfur species as well as for other reactors involving sulfur.

32 73 118 156

41 86 138 182

32 72 123 170

25.5 54.5

KINETIC T R E A T M EN+

111.5

There are two general methods of treating integral conversion data for catalytic reactions. The conversion values may be plotted versus ( ? V I P )and curves drawn through the data. They may then be graphically differentiated to give the reaction ratej, as reference to Equation 3 will show. The rates found are plotted against the mole fraction function for the assumed mechanism; the correct assumption gives a straight line of slope k,. The other method is that of assuming a mechanism and a rate constant, and then comparing the calculated conversions with the experimental values. The latter approach was used a t 550 ', 600", and 650" C., while for purposes of comparison, the data a t 700' C. \yere analyzed by both methods.

80.5

31.5 66.5 101.5 140.5 163.5

~

The equilibrium constants for the sulfur interactions have been determined by Preuner and Schupp (9). From their results equations can be derived relating the equilibrium mole fractions of two of the species to the third ; they are 1/3

N s , = ZNs,

(6)

N s , = YATL{3

(7)

The Y and Z values and the corresponding logarithms found for the temperatures investigated are tabulated in Table V.

Table V. Sulfur Equilibrium Constant Values Temp.,

c.

550

600 650 700

log Y 0.58423 0.42860 0.28983 0.16633 -

Y

-1 -1

1 1

0.3839 0.2683 0,1949 0.1463

log z 0.49503 - 1 0.83067 - 1 0.12977 0.39820

Z

0.3126 0.6770 1.3482 2.5015

As the molar ratio of methane to sulfur in the feed was one of the variables to be included in the investigation, a convention was adopted that made this ratio the same a t all temperatures. The ratio m was defined as the moles of methane per diatomic weight of sulfur-Le., all sulfur present in the fresh feed was &lculated as Sz for this purpose. A material balance for a mole of 'reactant mixture when treated on this basis, combined with Equations 6 and 7 , gives, after some manipulation, the equation TEMPERATURE,

Figure 2.

1276

DEG. CENT

Composition of Fresh Feed

INDUSTRIAL AND ENGINEERING CHEMISTRY

Vol 45, No. 6

The data must be corrected for the homogeneous reaction which occurs. This reaction accounts for all the conversion in sections 0-1 and 2-3. Both the homogeneous and catalytic reaction occur in section 1-2. F r o m t h e homogeneous reaction data, the following equation can be fitted when the conversion integrals resulting from integration of the right side of Equation 5 are plotted against (V/F):

0.30

4

W

'=

FEED

OR.-MOLES

METHANE

DIATOMIC WT. SULFUR

L

J J

=IOIO 0 0

1. I.

E E

ac9

Zh is here used to denote the

J C

rr)

integral of dx/(NcH,Ns,). The X quantity (V/F), which corresponds to a reactor residence time, is corrected by the 0 20 amount b (the intercept on the plot). T h i s i n t e r c e p t w a s noted by Fisher ( d ) , who attributed it to incomplete preFigure h e a t i n g of t h e r e a c t a n t s . Then, by using the values of ( V / F ) for sections 0-1 and 2-3 in Equation 10, the change in I h can be obtained for section 0-1 (from which XI can be found) and for section 2-3. From the known value of 2 3 , Tho-# can be found. Finally, subtraction of Zhz-t leaves from which z2 can be computed. Handling of section 1-2, wherein both homogeneous phase reaction and catalytic reaction occur, is more difficult. The void volume of section 1-2 and weight of catalyst can be related by a constant factor, v, so that the design equation for this section can be expressed as follows: dW/F = dx/(vrh

+

To)

(11)

With the rates written in the usual form, this may be integrated to give vkh(h'CH,NS,)

+ kc f(Nd,N j , . . . )

av.

BV.

W

W = F(XZ- XI)

(12)

The catalytic and homogeneous effects are now separated and Equation 12 is valid, provided the average values may be properly evaluated. For the case of a single reaction, this average mole fraction function is determined by finding the average of its reciprocal over the range x1 to xz and inverting the result. This procedure may be used here for both averages. The averages found are approximations of the true average values. Subsequent errors due to these approximations are small if one of the quantities averaged is small as compared to the other; this holds true here, as the homogeneous and catalytic rates are the actual quantities which are averaged. I n the event that the homogeneous mechanism and the catalytic mechanism are the same, there is no approximation involved. Rearrangement of Equation 12 and combination with Equations 5 and 10 result in the expression

It is thus apparent that the design variable has been related to the integral corresponding t o a catalytic mechanism minus a contribution due to the homogeneous reaction, properly weighted June 1953

40

60

00

W ' , GR. CATALYST F GR.-MOLE FEED/HR. 3. Test of Rate Equation at 550" C. t o account for the fact that the homogeneous and catalytic reactions may have different mechanisms. This equation could be used directly for testing an assumed catalytic rate equation by plotting the right side against (W/F). The correct assumption will give plotted results that may be fitted with a straight line of slope k, by means of a least squares approach. However, this fit gives a minimum root mean square difference for the integral (experimental minus calculated value), while the "best" value of k , is one that gives a minimum root mean square difference of the experimental and calculated conversions. This latter objective may be achieved by calculating, for an assumed rate equation and k , value, the theoretical integral 'conversion curve, which may then be compared with the experimental catalytic conversions. Calculation of the catalytic integral conversion curves is not possible, however, because the catalytic conversion obtained for a given ( W/F) value will vary with the level of conversion as the gases enter the catalyst bed. (The gas composition a t the catalyst bed entrance depends upon this conversion level, and hence the initial catalytic rate and the average catalytic rate across the bed will vary with this conversion level.) I n other words, the integral conversion curves for comparisons of this sort can be determined only if a common initial conversion level may be used for all runs. Increasing the (W/F) vftlue to a pseudo-value to account for the homogeneous phase conversion occurring before and in the catalyst bed would put all cases on the common basis of zero initial conversion. This idea may be extended to account for the homogeneous phase conversion in section 2-3 also; then

kc(W/F)' =

IC,-,

(14)

Substitution for k,, as determined from Equation 13, gives the equation

If the homogeneous and catalytic rate expressions are the same, this may be reduced to

INDUSTRIAL AND ENGINEERING CHEMISTRY

1277

The factor c was determined by making runs a t identical conditions a t different times and by determining the ratio of the right sides of Equation 13 for the tlyo runs. This ratio is the same as the ratio of the Wt values for the two cases. Wt values were calculated and used in place of It; on the right side of Equation 15.

0.30

MECHANISM CONS1DERATIONS

Rough plots of

23

versus

( W / F ) were made for the

Figure 4.

Test of Rate Equation a t

CORRECTION FOR FOULING OF CATALYST

preliminary analysis of the reaction, since these plots could be prepared directly from the experimental data ivithout correcting for the effect d the homogeneous reaction. These initial graphs had the same general appearance as the final plots, Figures 600" C. 3,'4, 5, and 6, which are based upon values of ( W I F ) ' and hence are corrected for the homogeneous reaction. To determine the rate equation which is applicable, the data for m = 1/3 and m = 1, corresponding to excess sulfur and excess methane, respectively, were examined. The maximum possible conversion is 0.25 for these two cases. If a first-order expression is valid, one of these sets of data should approach this maximum conversion value with a finite slope, since the rat'e would be greater than zero a t the maximum conversion. As this is not the case, such mechanisms may be eliminated from further consideration. These same two mole ratios are useful in examining possible second-order equations. Three were considered:

The question of reduced conversion due to fouling of the catalyst was considered by Forney ( 6 ) and Jackson ( 8 ) . Their data were re-examined closely; the conclusion reached was that the change in conversion, attributed by Forney and Jackson entirely to catalyst fouling, was made up of both fouling effects and inherent variability in the data, bot,h of which were of the same order of magnitude. The only data in which fouling was found t o be significant were those taken a t 650" C. This was the first set taken by Jackson ( 8 ) and proper operating techniques had not been completely worked out a t this time. After this period, fouling was minimized to the point where no definite trend in conversion with time could be noted. Cursory examination of the data indicated that conversion changes of 0.01 could easily be expected; those runs a t 0.30f650" C. in which the change was greater than this were corrected. Data a t the other temperatures were not corrected. Forney (6) reached the conclusion that methane was the reactant responsible for fouling and this conclusion was verified in the present investigation. For this reason, the time of exposure of the catalyst to methane was kept a t a minimum and used as the basis of the corrections a t 650' C. Fouling was considered t o reduce the number of reactive sites, or, equivalently, ro X the weight of catalyst effective in producing carbon disulfide. The effective weight 0 5 was assumed to be relatel to the original weight by

Wt = W(1 1278

+ ct)

(17)

Figure 5.

l'c = k,NCH,Ns, r c = koi$TcE,l\'s:

=

(18) (19) (20)

kolVCR,NS

-I

'c

k IO

INDUSTRIAL AND ENGINEERING CHEMISTRY

n

I-

= Kc NCH, N S e 0.424

15 CATALYST F GR.-MOLE FEED/HR. Test of Rate Equation a t 650" C.

W ' , GR.

I,

20

Vol. 45, No. 6

of k,, one was found for which this summation was a minimum; this was the best value of k,. The results so obtained are shown as solid lines in Figures 3, 4, 5, and 6. The final value of k , selected is shown on each plot. The four k , values were transformed to the classical basis-i.e., rate expressions written as the rate constant multiplied by reactant concentration functions rather than by partial pressure functions-by the equation

kc-c = kc(RT)2

I

0

I

I

I

2

4

Figure 6.

I

I

I

I

I

I

I

I

- ,

By comparing the values of the conversion integrals for these rate equations, it may be seen that, for a given value of conversion, the integral value [and, hence, the ( W / F ) ' value] is smaller for m = 1 than for m = 1/3 if Equation 18 applies; the relation of these conversion integrals is reversed for Equations 19 and 20. Equation 18 is thus seen to be compatible with the plotted data except those for 650" C., which scatter too much for a conclusion to be reached. Equation 18 also requires the crossing of the conversion integral values for m = 1 and m = 1/2 (for which the maximum conversion is 0.333). This is apparent in the plotted data for 600' and 700" C. The 550" C. data do not reach this point and the 650" C. data again scatter rather badly. Third-order expressions were not considered, because they are rare, and all that are known involve nitric oxide as one of the reactants. Thus, the only reaction that justifies quantitative treatment is Equation 18.

I

12

6 8 IO W' OR. CATALYST F OR.-MOLE FEEWHR. Test of Rate Equation a t 700" C.

I

(21)

The ko-o values computed in this fashion were plotted in the Arrhenius equation form

1

14

and the values of log ko-s and ( 1 / T ) fitted with a straight line (Figure 7 ) . The equation of the straight line is log kc-c = 10.9

-

31,400 2.303(RT)

The values found by Forney (6) and Jackson (8) are also shown on the plot. A summary of the quantitative results is given in Table VI. The final values of k , and ke-c are given along with the preliminary k , based upon minimum differences of the integrals (Table VI). The 700" C. data were also treated by means of the graphical differentiation method described previously. The rate constant value determined in this way is also included in Table VI. The root mean square errors tabulated are the minimum values of the square root of the average squared difference between experimental and calculated conversion values.

RESULTS

c

n

The experimental values of the total conversion, 2 3 , are plotted against (WIF)',which is ( W / F )plus a quantity to account for the homogeneous contribution to the total Conversion, in Figures 3, 4,5, and 6. To obtain the theoretical curves for comparison purposes, a first approximation to k , was obtained by plotting I,,-, us. (WIF)' (Equation 14) and determining the best k, value for this plot. This was easily done, as the intercept of the plot was the origin, according to theory. k , was then equal to the (summation of the integral values)/ [summation of the ( W I F ) ' values]. This result multiplied by experimental values of ( W/F)'gave a theoretical value of the conversion integrals. In this case these were

A""

dx/NcH,Ns,, since Equation 18 was applicable.

From plots

of this integral prepared previously, 23 was found. A comparison of the calculated x3 values with the experimental results was made by finding the value z3

exptl.

- Z3 calod.

for each run. These quantities were squared and then summed for all runs at a given temperature. By selecting various values June 1953

Table VI.

+

Summary of Conclusions

+

Reaction. CH4 US* = CS? 2HzS. ax = 4 Reaction mechanism. T C = k J v " ~ ~ N 8 ~ R a t e constant equation. log L o= 10.86 (31.4)(10s)/(2.303RT) wherein kc-c = k c ( R T ) 2for R = 0,08206 k, units. (Gram-moles of CSz) (gram of catalyst) -l(atmospheres) - 2 units. (Gram-moles of CS?)(gram of catalyst) -l(gram-moles) -%(liter)2

-

kC-C

550

R.M.S. Error In xa

0.0714b 0.0713d

326c

0.0168

600

0.214b 0.200d

1098C

0.0119

650

0.424a 0.415d

243OC

0.01R6

1.046

665OC

0.0074

....

....

700

1.122 1.08

a Square root of average squared difference between experimental and calculated values of x. k, is selected t o make R.M.S. error a minimum. b Value chosen t o give minimum R.M.S. error. Value used for Arrhenius plot, Figure 7 . d Value obtained from summation of I,. -"- values divided bv summation of ( W / F ) ' values. 6 Value obtained b,y graphical differentiation of freehand curves drawn through 5 3 vs. ( W / F ) values.

INDUSTRIAL AND ENGINEERING CHEMISTRY

6

1279

CONCLUSIONS

The data for all temperatures except 650" C. follow the theoretical relations very well. The 650' C. data were a poor set, owing t o fouling of the catalyst. Furthermore, the correction for fouling is crude, and this may account for some of the scattering. A tendency was observed for any series of runs to be consistent within itself, although the entire series of runs might be displaced from its expected position in Figure 5 . One possibility is thermocouple error. A Chromel-Alumel thermocouple was used for this set of data and it TYas not possible to achieve good reproducibility of calibration Tvith this thermocouple.

4.0

3.5

and Hennig (4). The value of 31,400 cal. per gram mole does not represent the activation energy for a single chemical reaction but, like the specific velocity constants, represents an effective activation energy giving the combined effect of temperature due t o the adsorption, surface reaction, and desorption steps comprising the actual mechanism. The small difference between the activation energies for the catalytic and homogeneous reactions suggests that the value of silica gel as a catalyst is due to the number of reactive sites provided. This might explain the greater effectiveness of silica gel as compared to other materials ( 4 , I O ) , as silica gel is commonly reported to have a much higher unit surEace area. The agreement of the values of k , for the 700" C. data (Table V I ) is good. I t definitely indicates the general equivalence of the graphical differentiation method as compared to the analytical approach. Agreement a t the other temperatures would not be so good, because wider scattering of the data occurs. The root mean square errors in Table T'I give some idea of what precision may be expected when the given k, values are used in the design equation. The high value of this error for the data a t 650" C. is understandable in light of previous comments upon this set of data; the low value a t 700 C. can perhaps be attributed t o the fact that the total time of catalyst use before replacement vias smallest a t this temperature (the time necessary to collect sufficient product for analysis was short) and hence changes which ordinarily occur Kith time and contribute to the variability of the data did not go on to as great an extent in this case. ACKNOWLEDGMENT

3 .O

The authors wish to express their appreciation to the Pure Oil Co. for its interest in this project and for financial support in the form of a research fellowship. NOMENCLATURE

2.5

1.0

-+

Figure 7.

I. I

1.2

( I ~ ) , ( D E G KELVIN)-' .

Arrhenius Equation

The kc-c values behave reasonably well, showing no trends with temperature. This fact tends to confirm the choice of the rate equation; values for an incorrect choice would exhibit trends with temperature. The agreement of the 700" C. data with those a t lower temperatures is particularly important, as it tends t o verify the assumption of rapid dissociation rates of sulfur in comparison with the reaction with methane. At lower temperatures the assumption of rapid dissociation rates is necessary in order to explain the Szmechanism, as considerable Se and Sa are present a t equilibrium a t these temperatures. At 700" C., as Figure 2 shows, such an assumption is not necessary because Sg and Sa are present in very small amounts. The fact that the same rate equation applies a t all levels of temperature adds justification for the Szmechanism and the assumption of rapid dissociation. The agreement with Forney's (6) value of at 600" C. is reasonable; Jackson ( 8 )made the corrections for the homogeneous reaction in a n approximate manner which leads to high values for the specific reaction velocity. Fisher ( 3 ) found an activation energy of 34.4 kcal. for the homogeneous phase reaction. In light of this, the value of 31.4 kcal. €or the catalytic reaction is rather high but it is lower than the homogeneous value. as catalytic activation energies should be. It is considerably lower than the value of 38.8 kcal. reported by Folkins, Miller,

1280

A C E F

= constant in Arrhenius equation

I

=

N S

= bulk volume of catalyst, milliliters = activation energy of Arrhenius equation, calories = total feed rate, gram-moles of methane per hour

= =

T = V = W = Y =

2 = c

= = =

k

=

a,

b

VI =

r

=

t

= = =

v z

plus gram-moles of diatomic sulfur per hour (all sulfur considered to be diatomic) conversion integral corresponding t o a reaction mechanism mole fraction, dimensionless bulk volume of salt, milliliters absolute temperature, degrees Kelvin reactor void volume, milliliters weight of catalyst, grams sulfur equilibrium constant, = N s , / N : t sulfur equilibrium constant, = Ns,/h7Lt reactant activity, atmospheres constant correction to ( V / F ) ,units of ( V / F ) factor in equation accounting for catalyst fouling, (minutes)-' reaction rate cons(ant, units to make rate units consistent with reaction mechanism reactants ratio, (gram-moles of methane)/(diatomic weights of sulfur) reaction rate, (gram-moles of carbon disulfide)/(gram of catalyst) (hour) time catalyst was exposed to methane, minutes (void volume)/(catalyst weight) factor, (milliliters)/(gram) conversion, (gram-moles of carbon disulfide)/(gram-mole of feed)

Subscripts c refers to catalyzed reaction c-c refers to catalytic rate equation

h

i j t

0

1 2

written as constant niultiplied by function of concentrations refers to homogeneous reaction refers to generalized component refers to generalized component refers to time, t refers to entrance to lower salt bed of reactor refers to boundary b e b e e n lower salt bed and catalyst bed of reactor refers to boundary between catalyst bed and upper salt bed of reactor

INDUSTRIAL AND ENGINEERING CHEMISTRY

Vol. 45, No. 6

June 1953

INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY

3 = refers t o exit from upper salt bed of reactor Subscripts 0 through 3 are also used to refer t o the level of conversion a t these points Superscripts

’ = refers to pseudo-bed of catalyst of size required to account for catalytic plus all homogeneous reactions REFERENCES

(1) de Simo, M., U. S. Patent 2,187,393 (Jan. 16, 1940). (2) Fisher, R. A., Ph.D. thesis in chemical engineering, Purdue

University, February 1950. (3) Fisher, R. A., and Smith, J. M., IND.ENG.CHEM.,42, 704-9

(1950).

*

1281

(4) Folkins, H. O., Miller, E., and Hennig, H., Ibid., 42, 2202-7

(1950). (5) Forney, R. C., Ph.D. thesis in chemical engineering, Purdue University, August 1950. (6) Forney, R. C., and Smith, J. M., IND. ENG.CHEM.,43, 1841-8 (1951). (7) Hougen, 0. A., and Watson, K. M., “Chemical Process Principles,” Part 111, “Kinetics and Catalysis,” Chap. 19, pp. 902-72, New York, John Wiley & Sons, 1947. (8) Jackson, E. G., M.S. thesis in chemical engineering, Purdue University, August 1951. (9) Preuner, G., and Schupp, W., Z. physik. Chem., 68, 129-58 (1909). (IO) Thacker, C. M., and Miller, E., IND.ENQ. CHEM.,36, 182-4 (1944). (11) West, J. R., Ibid., 42,713 (1950). ACCEPTEDMarch 14. 1953. RECEIVED for review November 24, 1952.

(End of Symposium) Reprints of this symposium may be purchased for 75 cents each from the Reprint Departmlt, American Chemical Society, 1155 Sixteenth St., N.W., Washington 6, D. C.

Finishes for Glass Fabrics for

Reinforcing Polyester Plastics L. P. BIEFELD AND T. E. PHILIPPS Owens-Corning Fiberglas Corp., Newark, Ohio

L

AMINATES of fibrous glass fabrics and heat-convertible, unsaturated polyester plastics are used extensively in the aircraft industry. They have high strength] low weight, and dimensional stability. They resist water, corrosion, and weather (1, 16, 91). Figure 1 illustrates a glass-polyester frame part for a helicopter. Figure 2 shows a comparison of tensile yield strength for glass-polyester combinations, structural aluminum, and steel. Continuous-filament glass fabrics as they come from the loom do not give laminates with these desirable properties. The fabrics contain a n oily, starch size which prevents fast, complete wetting out by the plastic during impregnation, and prevents good adhesion of the plastic to the fibrous glass in the cured laminate (5). I n order to obtain laminates with good properties, the original size must be removed from the fabric and a glass-topolyester coupling or finishing agent must be applied. The desizing and finishing processes convert the fabric from a poor t o an excellent reinforcing agent for the polyester plastics. During the past few years, several finishes for fibrous glass fabrics for reinforcing heabconvertible polyester plastics have been developed and some are being used commercially (1, 7, 16, 17, .94]27, $8, 94). Several of these finishes are considered here with respect to the general requirements of a good finish, the types of finishing agents employed, the desizing and finishing processes used, and the laboratory evaluation of the finished fabrics. FINISHING REQUIREMENTS

FABRICWEWINO BY POLYESTER. Glass fabric-polyester laminates are made up by impregnating a succession of layers of cloth with a n unsaturated] heat-convertible polyester (9.9). The finished fabric should be wet out rapidly and completely during impregnation to eliminate the formation of small air bubbles on the fiber which separate it from the plastic, and to obtain good adhesion between the plastic and the fibrous glass during the curing process. To give good reinforcement, a fibrous glass

filament should be supported a t least every five diameters along its entire length (24). Figure 3 illustrates the effect of fabric finish on the rate and completeness of wetting by a typical, unsaturated polyester laminating plastic. A drop of the polyester was placed on each swatch and photographed after a n interval of 20 seconds. A is a swatch from a desized fabric. B is from a fabric which was desized and finished with methacrylatochromic chloride, a good glass-to-polyester coupling agent. C and D are from desized fabrics t h a t were finished with laurato and stearatochromic chlorides, respectively; they are poor coupling agents. The sizes of the spots show t h a t A and B wet out much better than C and D. FABRIC ADHESIONTO CUREDPOLYESTER. The finished glass fabric should adhere strongly to the cured plastic matrix; the adhesion must not be weakened by the presence of moisture. Because bare glass does not adhere well to heatrcomerted polyesters, especially in the presence of moisture, the finishing agent must be or contain a coupling agent which bonds to the fibrous glass surface and t o the plastic matrix. Thus, even though a desized fabric is well wet out by the polyester, as shown by A in Figure 3, a good laminate is not obtained with this fabric because the cured polyester is not well bonded to the bare fibrous glass. Adhesion between the fabric and the plastic is very important. Without it, the combination is weak; if high external forces are applied, delamination will occur. The stresses must be transferred from fiber to fiber through the plastic matrix to avoid high stress concentrations. Good adhesion is necessary to prevent buckling of the fibers under compression and t o allow the fibers to go into tension if the laminate is put into tension or flexure (34). REMOVAL OF ORIGINAL SIZE. All glass fibers t h a t are to be twisted into yarns and woven into fabrics must have a protective, lubricating size (3, I S , 26). The ingredients of a commonly used