Carbon Disulfide f rorn Natural Gas and Sulfur REACTION OF METHANE AND SULFUR OVER A SILICA GEL CATALYST HILLIS 0. FOLKINS, ELMER MILLER, AND HARVEY HENNIG The Pure Oil Company, Chicago, Ill. T h e formation of carbon disulfide by the reaction of methane and sulfur vapor over a silica gel catalyst has been investigated in the temperature range 500" to 650" C. Conversion data are given for a wide range of space velocity. The methane-sulfur system is discussed briefly as well as the thermodynamics of the reaction, the effect of varying reactant ratio, the life of the catalyst, and the
quality of the carbon disulfide produced. The reaction is approximately second order with an activation energy of 38,300 calories per gram mole and proceeds without the occurrence of side reactions. The results of this research have furnished the basis for the development of a commercial method, now in operation, for the manufacture of carbon disulfide.
T
and sulfur a t temperaturos above 1oOO" C. Bodenstein ( 4 ) disclosed a process for producing carbon disulfide and hydrogen PUIfide by contact of gaseous, liquid, or solid hydrocarbons with SUIfur in the presence of an iron-nickel-chromium catalyst, The present article deals with one phase of the process for making carbon disulfide from methane, or natural gas, and sulfur arid presents the results of B detailed investigation of the rcaction of methane and sulfur vapor over a silica gel catalyst.
HE effects of various catalysts upon the reaction of methane and sulfur vapor for the formation of carbon disulfide have been discussed previously (IS, 14). These earlier publications dc.alt with preliminary research work on a process that has been developed for the commercial production of carbon disulfide by the reaction of gaseous hydrocarbons, particularly natural gas, with sulfur vapor in the presence of an active catalyst. In this process, pure carbon disulfide is obtained in conversions of over 90% in a once-through operation at temperatures of BOO0 C. or below. A commercial plant for the production of cltrbon disulfide by this method is now in operation. Several references may be found in the literature dealing with the reaction of hydrocarbons and sulfur or sulfur-containing compounds for the production of carbon disulfide. A patent, issued to d e s i o (6),describes a process for reaction of methane and sulfur vapor to form carbon disulfide and hydrogen over metallic sulfide catalysts in the temperature range 800"to 1000O C. A more recent paper by Bacon and Boe ( I ) confirms the earlier publication from this laboratory (14) and discussed the catalytic reaction of methane and sulfur at temperatures below 700" C. The reaction of methane and hydrogen sulfide to form carbon disulfide has been mentioned in a number of instances. Waterman and van Vlodrop (16)reported conversions of methane to carbon disulfide as high as 70% when mcthane and hydrogen sulfide reacted thermally a t 1125" C. A British patent (7) claims a catalytic process for producing carbon disulfide from methane, or other hydrocarbons, and hydrogen sulfide a t temperatures around 1OOO" C. A similar process is claimed by Pier and Winkler (10). Although the reaction of methane and sulfur to form carbon disulfide is thermodynamically favorable over a wide temperature range, the thermal conversion rate is slow at moderate temperatures and an active catalyst is necessary to obtain high conversions. High conversions to carbon disulfide from methane and hydrogen sulfide are thermodynamically possible only at high temperatures and, for satisfactory yields, temperatures of loo0 C. or above are required. Unsaturated hydrocarbons and paraffins of higher molecular weight will react with sulfur, also, to form carbon disulfide. A British patent (9) deals with the production of carbon disulfide from acetylene and sulfur. Wheeler and Francis (17) obtained carbon disulfide and benzene by the reaction of hydrocarbons O
METHANE-SULFUR SYSTEM The thermodynamics of the reactions of various hydrocar1)oriu with sulfur vapor to form carbon disulfide have becii c:dculatetl during this investigation. As this paper deals primarily with tho reactions of methane and natural gas with sulfur vapor, the thermodynamics of sulfur vapor and of the methane-sulfur system are considered briefly. These subjects have been discussed previously (12, 24, l a ) . Sulfur vapor, a t temperatures of '700"C. and lower and a t atmosphcric pressure, is composed of the thrce molecular species Sz, So, and Sa. The monatomic form is present in negligible quantities a t temperatures as high as 850" C. The relative concentrations of the individual species vary as conditions are altered. Thus the concentrationa of the heavier molecules decrease as the temperature is raised. On the other hand, the proportion of heavier molecules increases with increase in pressure at a given temperature. In considering the reaction of methane and sulfur vapor to form carbon disulfide, free energy calculations were made to determine equilibrium conversions. The following reactions were considered: CHa 2Sz = CSz f 2H1S CHa 2/3& = CSp 2HzS CHI 1/2Ss = CSa 2H2S Sz = CSz 2H2 CH4 1/3Se CSz 2Hz CH4 1/4Ss = CS2 2He CHa
++ +++ +
=i
++ +++
lil
Figure 1 shows a plot of theorctical equilibrium conversions as calculated for these reactions. (The equilibrium conversions for CH,
+ 2H2S
E
CSZ
+ 4H3
(7)
were calculated also and are plotted in this figure.) T h e reac-
2202
INDUSTRIAL A N D ENGINEERING CHEMISTRY
November 1950
tions of methane with sulfur vapor thus resolve themselves into the following forms:
+
CH, US. = CS1 CH' f bSz = CSz
++ 2Hs 2H1S
2203
+
system CH, US, may be calculated. Table I1 gives values for this system for different temperatures and preasures.
(4)
(a)
where x equals the average number of sulfur atoms in the molecule under the oonditions of temperature and pressure employed. Under conditions where the reactants are present in stoichiometric proportions it is obvious that ax = 4 and bx = 2.
MATERIALS AND METHODS Commercial &rade silica gel (8- to 14mesh) was obtained from the Davison C emical Com any and was used without further purification. Methane was.o&ained from the Carbide and Carbon Chemicals Corporation and was stated to conform to the following volume specifications: methane not less than 92%; impurities (air, ethane, and nitrogen) not over 4.5, 2, and 2%, respectively. This material was passed over freshly reduced copper turnings at about 300" C. to remove an oxygen present before the as entered the reaction zone. In Lter work this recaution was tispensed with. An small amounts of oxygen &at may have been present had no Ltectable effect upon the reaction. Sublimed sulfur was used throughout the work covered by this paper, in order that the least ossible amount of impurities would be present to foul the sma% equipment used. The apparatus and procedure employed in this investigation were similar to those described previously (14). The essential differences may be described best by reference to the flowsheet shown in Figure 2.
-'"p
Figure 1. Theoretical Equilibrium Conversions
It is seen that, thermodynamically, Reaction a is more fitvorable than Reaction b at temperatures below 700" C. Experimental results have shown that the former reaction occurs almost exclusively at lower temperatures. Because sulfur vapor is composed of different molecular species, it is necessary to know the relative proportions of &, So, and Sa molecules present under different conditions in order to calculate enthalpies, partial pressures, exact equilibrium conversions, and residence times in certain zones. Table I gives relative mole fractions of E&, Se,and SI in sulfur vapor at different temperatures and pressures. These values were calculated from the data of Preuner and Schupp (11) and from the equilibrium constants of Kelley (8) as derived from those data. Extrapolation has been necessary for the values at higher pressures, because Preuner and Schupp's data are limited to 24 pounds per square inch absolute, but, as the carbon disulfide reaction proceeds readily over a wide temperature and pressure range and commercial application of the process utilims pressures higher than atmospheric, a knowledge of the relative partial pressures of &, Se,and 91 in the reactant gases is desirable and these extrapolations were required. Thcrefore the assumption haa been made that the equilibrium constants for the dissociation of sulfur vapor hold over the pressure range given. From the data calculated for the mole fractions or partial pressures of the ditrerent sulfur species in sulfur vapor, the partial pressures in the system CH, US, may be calculated. Inasmuch as CH, as. = CSz 2H9S may be represented by the equations
+
CHI CHI CH,
-
+
+
-
= CS, + 2HtS where PCH4 = 1/2P& +++ 2S2 2/3& C& + 2HsS where PCH, 3/2PSs 1/2S~= CS, + 2HtS where PCHI = 2P&
the reactants CH4 and sulfur vapor may be related as PCH4 = 0.5PSt
+ 1.5PSs + 2%
For an assumed total pressure of US, the relative partial pressures of the species are known. Using these values, the equivalent pressure of methane may be calculated by the above equation, which when added to the pressure of the sulfur will give the total pressure of the system. Repeating this a t various assumed pressures for the sulfur vapor, total and partial pressures for the
L
",
F
8
G
II I L---A
1 1 L
O 0
1:
Figure 2. Flow Diagram of e Laboratory Unit A.
Methane oylinder
C. D. E. F.
Flowmeter Manometer Q~meter Sultur melter
B. Copper purifier
a.
Sulfur pump H. Methane heater I. Sulfur boiler J. Reeotor K. Sulfur condenser L. To recovery system
The reactors and preheaters were constructed of stainlese steel tubing of 16% chromium-13% nickel-3% molybdenum composition. The remainder of the unit wm of carbon steel. Reactors of various sizes have been used in the investigation of this reaction. The one used in this work contained 200 cc. of 8- to 1 4 mesh catalyst. Essentially it consisted of a cylindrical tube 2 inchesin outside diameter, 1.78inchesin inside diameter, and about 6 inches long. An internal thermocouple well ran the length of the vertical reactor. The annular space between the thermocouple well and the inside wall of the reector served as a catalyst bed. Temperatures of the catalyst bed were mertsured by means of Chromel-Alumel thermocouples located at the top, middle, and bottom of the bed. The temperature gradient, in most cases, was maintained well within 4' C. The furnace temperature wag held constant to within ~ 3 C."by means of Transtats and a Micromax controller. The methane was preheated and the sulfur vaporized and preheated in separate chambers, after which the gaseous mixture p m e d to the reactor through a short transfer line. Considerable difficulty was experienced in pumping molten sulfur in small quantities. Liquid sulfur exhibits peculiar viscosity characteristics as the temperature is increased from its melting point to its boiling point (8, 6 ) . As the temperature is raised above the melting point to about 160' C., the viscosity decreases slowly from about 10 to 7 cp. At about 160" C. the viscosity increases rapidly with temperature and reaches a maximum
INDUSTRIAL AND ENGINEERING CHEMISTRY
2204
Table I. Molecular Distribution of Sulfur Vapor Lb./Sq. Inch Abs. 5 LO
450' C.
8s 0.083 SS 0.600 Sa 0.317
SI 0.060 Sa 0.572 SI
20
30
E+
Ss Sa 82
Ss SS
50
SI
70
SI
100
8n 8s Ss
Sa 5% Sa 8s
0.378 0.031 0.533 0.436 0.024 0.508 0.468 0.016 0.475 0.609 0.013 0.453 0.534 0.010 0.430 0 560
550" C. 0.495 0.420
500' C. 0.240 0,572 0.188 0.145 0.600 0.255 0.092 0,590 0.318 0.068 0.583 0.349 0.048 0.557 0.395 0.038 0.541 0.421 0.030 0.520 0.450
0.085
0.336 0.521 0.143 0.219 0.573 0.208 0.171 0,583 0.246 0.121 0.586 0.293 0.092 0.584 0.324 0.074 0.571 0.355
800" C. 0.795 0.187 0.018 0.638 0.317 0.045 0.440 0.448 0.112 0.347 0.501 0.152 0.255 0.544 0.201 0.206 0.562 0.232 0.164 0.568 0.268
650' C. 0.964 0.034 0.002 0.881 0.110 0.009 0.733 0.235 0.032 0.618 0.325 0.057 0.478 0.426 0.096 0.396 0.479 0.125 0.321 0.521 0.158
700° C. 0.992 0.008 0.000 0.979 0.020 0.001 0.922 0.074 0.004 0.859 0.130 0.011 0 729 0.231 0.040 0.635 0.303 0.062 0.540 0.370 0.081
+ OS,
Table 11. Partial Pressures in System CHI (az
Lb./Sq. InehGsge CHa 0 0.585 0.598 10 0.608 25 40 0.613 0.614 50 0.615 pi0
At 5OO0 C. Sr So 0.082 0.245 0.062 0.232 0.045 0.228 0.034 0.227 0.031 0.226 0.030 0.226
-
4)
Sa 0.088
0.107 0.120 0.125 0.128 0.131
CHd 0.533 0.556 0.575 0.585 0.590 0.695
At 550' C. Sa 8s 0.193 0.221 0,143 0.233 0,103 0.237 0.083 0.237 0.074 0.238 0,067 0.238
0.009 0,011 0.024 0.032 0.034 0.035
0.377 0.411 0.446 0.469 0.483 0.496
0.560 0,470 0.390 0.335 0.305 0.276
0.446 0.480 0.512 0.530 0.539 0.546
0.390 0.298 0.228 0.188 0.170 0.155
reciprocating piston displacement pump was not too satisfactory, largely because of sticking of eheck valva. The problem w w solved by designing and constructing a special pump to handle molten sulfur. This pump operated on the principle of displacing liquid sulfur from a cylinder, maintained at a constant temperature (140' C.), by advancing, at a constant predetermined rate, one or more of a series of pistons of known diameter into the sulfurfilled cylinder. By this method sulfur could be pumped at a large number of constant charge rates within the boundaries of about 25 to 300 grams per hour. On frequent calibration the reproducibility of the charge rate was within * 1%. The product gases leaving the reactor were cooled to around 140' C. in the sulfur condenser, where the unreacted sulfur was condensed and removed from the system. The overhead g w s from the sulfur condenser were passed through a trap cooled with dry ice and methanol where the carbon disulfide and part of the hydrogen sulfide were condensed. Hydrogen sulfide remaining in the gas wa.s removed by scrubbing the gas with a dilute solution of sodium carbonate. The residual gas, consisting mainly of methane, was metered and ttnalyzed. The condensed material in the dry ice trap was stabilized in a low temperature column to remove the hydrogen sulfide, and the thus stabilized carbon disulfide wm diluted in thiophene-free benzene and analyzed by the method of Bell and Agruss ( 9 )for carbon disulfide, mercaptans (thiols), sulfides, disulfides, and residual sulfur. In the commercial process the carbon disulfide is removed from the product gas by oil absorption, The hydrogen sulfide is converted to elemental sulfur, which is recycled to the reaction system.
At 650° C.
At 600" C . 0 10 25 40 50 60
9a 0.053 0.069 0,080 0.090 0.096 0,102
0.155 0,202 0.240 0.257 0.261 0.264
0.058 0.106 0 147 0,170 0.180 0 189
Vol. 42, No. 11
80
0,005 0,012 0.022 0.027 0.032 0.037
z B 60
5
9
40
.9 20
of 93,200 cp. a t 188' C., after which the viscosity decreases rapidly to 2300 cp. at 306" C. and then gradually as the temperature approaches the boiling point. It has been found imperative therefore to keep the sulfur reservoir, pump, and transfer lines jacketed at a temperature between the melting point and 160" C. The temperature chosen was around 140' C. In order to study the reaction rates over a wide range of space velocities it was necessary to charge liquid sulfur at reproducible rates. The size of the equipment called for the uniform charging of from about 30 to 300 grams of sulfur per hour. Early work with a jacketed 100,
I
0 RATIO
SzCH4
Figure 4. Effect of Sulfur-Methane Ratio upon Conversion at 525°C. The per cent conversion to carbon disulfide was obtained from an average of (1) the percentage methane reacted, (2) the per cent sulfur reacted with the methane according to the reaction CH,
+ US=
CS,
+ 2H2S
and (3) the amount of carbon disulfide recovered.
EXPERIMENTAL RESULTS EFWCT OF TEMPERATURE AND SPACE VELOCITY
80
GO
z
P
2
+
w
40
z
-
:
2
Table I11 presents results of experiments which show the effect of space velocity, at different temperatures, upon the methanesulfur reaction over silica gel catalyst. ,411 these experiments were carried out at atmospheric pressure, and the sulfur-methane charge rates were maintained at stoichiometric proportions necessary for the reaction CHI US, = CSZ 2HzS. I n calculating the total gas volume of charged reactants and hence space velocities, sulfur has been assumed to be present altogether a~ the diatomic modification. Hence space velocity haa been defined arbitrarily as the total volume of reactants at 0' C. and 760 111111. of mercury passing over a unit volume of catalyst space per hour. Figure 3 shows these data in graphical form. Conversion values are averages of the three methods listed above. The last column of Table I11 shows the per cent deviation of the individual values from the average and is thus a measure of the quality of the run.
2w
SPACE 4VELOCITY 00
6W
800
Figum 3. Effect of Temperature and Space Velocity upon Conversion of Methane to Carbon Disulfide
+
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1950
2205
Table 111. Effect of Space Velocity (Catalyst, silica gel)
8 aae
Av.
Run
Temp.,
Veracity
496 494 498 498 500 501 498 501 4Y8 500 500 525 525 622 523 524 525 525 525 524 525 525 b26 525 528 527 527 525
140 147 156 223 279 292 304 376 442 809 622 70 70 140 158 227 277 281 281 284 289 289 297 297 313 383 450 510
c.
No.
11 2 10 9 55 8 4 7 5 67 68 20 27 19 17 16 82 56 83 85 87 89 15 18-B 14 13 12 69
S.T.P.
Mole Ratio &:CHa 1.96 1.98 1.98 2.00 1.98 1.98 2.01 2.00 2.00 2.00 2.02 2.01 2.01 2.00 1.97 2.00 1.97 2.01 2.00 2.00 1.99 2.00 1.96 2.00 2.01 1.98 1.98 2.00
% (av.) 56.8 56.5 86.6 49.3 43.5 42 3 36.1 37.4 31.3 30.7 27.6 88.8 87.4 75.1 72.1 66.2 59.0 66.9 59.4 58.6 60.3 60.2 60.6 60.3 62.8 56.4 55.0 47.0
Converted % deviation 2.2 0.3 1.4 0.9 0.9 0.1 0.4 0.4 1.2 0.6 1.2 0.2 1.4 1.0 1.1 1.3 1.8 1.0 1.2 2.1 1.4 1.4 1.1 1 0 3.3 1.8 4.1 1.0
Av.
=Y?*
70 65 66 60 57 68 64 71 76 79 61 72 62 63 75 73 74 77 78
527 525 b2b 649 549 550 551 549 550 552 550 549 549 550 573 575 676 600 600
627 64 1 983 220 30 1 371 379 513 517 518 612 628 73 1 820 457 516 A32 516 834
Mole Ratio %:CH4 1.99 1.99 1.99 1.99 1.98 1.99 1.99 2.00 2.00 2.00 1.99 2.00 1.98 2.02 2.00 2.00 1.98 2.02 1.99
80 81
661 651
518 636
2.01 2.01
92.9 89.8
1.2 1 .o
114 115 116 117
525 525 525 525
2.58 394 444 545
1.99 1.99 2.01 2.00
65.4 56.6 52.9 44.6
1.3 0.2 0.3 0.7
Table IV. Effect of Varying Sulfur-Methane Ratio Run
No.
Av. T y p
Bpaqe Velocity
521 524 52 1 523 524 523 525 522 522 521 527 526 526 625 526 525 524 524 525
165 165 164 164 246 244 247 250 250 249 332 335 334 334 330 334 333 334 317
21 22 23 24 25 26 28 29 30 31 32 a3 34 35 36
37 38 39 86
8.T P.
Mole Ratio &: CH4 CHI:& 12.45 0.080 1.79 0.56 0.75 1.33 0.40 2.50 6.08 0.164 4.31 0.23 1.64 0.61 0.74 1.35 0.61 1.64 0.24 4.17 12.57 0.080 3.54 0.28 1.77 0.56 1.53 0.65 0.88 1.14 0.47 2.13 0.40 2.50 0.17 5.88 0.17 5.88
Sulfur Converted CHa Converted (Calcd. from % (av.) % deviation Curve) 99.7 0.4 16 0 71.1 0.5 79.7 89.7 59.7 0.0 100 8 20.1 0.8 79.1 5.4 26.0 78.1 2.4 36.8 59.7 0.6 72.9 98.0 36.3 1.5 101.8 31.1 1.5 92.9 18.9 1.1 12.1 76.0 2.6 37.0 65.5 0.3 60.2 53.5 0.6 49.4 0.6 64.7 37.8 0.2 85.5 22.4 0.5 95.0 19.0 1.0 94.8 93.6 7.8 0.6 93 5 7.9 0.1 I
Table V. Convemion Vereue Temperature at Constant Space Velocity (S.T.P.)
8 ace Verooity
No.
S.T.P.
CH4 Converted % deviation 43.4 1.2 43.9 0.7 32.9 0.6 84.1 1.2 77.4 1.1 74.9 1 .o 71.4 15 62.7 0.6 64.3 0.8 63 7 0.6 60 3 0.5 0.7 59 3 0.1 54.7 0.8 53.3 79,0 1.3 78.1 1.3 72.8 0.5 85.2 1.3 79.3 1.3
% (BV.)
chiometrio proportion, there are included in Table IV conversions based on the amount of charged sulfur These values were calculated from data taken from the smoothed curves representing conversion on the basis of methane charged. From these data it is seen that an excess of either reacting component over the stoichiometric proportion increases conversion to some extent. TEMPERATURE AND CATAbYTIC ACTIVITY
, Table V and Figure 5 portray the influence of temperature upon conversion at constant space velocities. These values have been taken from the smoothed curves of the experimental data of Figure 3. At the higher temperatures extrapolation of the isotherms was necessary for the complete tabulation of conversions
$pace Velocities (8.T.P.) 400 Mu)
625 650 576
800
650
-1MENTS
500
600
700
% CH4 Converted to C&
Temp., C. 35.6 53.9 70.0 83.0 89.8 96.5
31.2 49.0 64.3 77.8 85.2 93.4
27.9 44.9 60.0 73.7 81.1 90.7
25.3 41.3 56.6 70.0 77.6 88.4
WITH VARYINQ SULFUR-METHANE RATIOS
Table IV shows the effect of varying the charge ratio of sulfur to methane. These experiments were camed out at 525 a C. and three series of space velocities were studied. Conversion has been calculated in the usual manner on the basis of the methane charged. Figure 4 shows these conversions graphically. It is wen that an excess of sulfur over the stoichiometric proportion increases the conversion to a considerable extent on the basis of a constant amount of methane charged to the system. With a deficiency of sulfur these curves do not show the efficiency of the reaction, as conversion of methane according to the postulated equation is governed by the amount of sulfur present. In order to demonstrate the effect of a surpius-of methane over the stoi-
I
550
TEMPEMURE,
6W
.c.
1
654
1
Figure 5. Effect of Temperature upon Conversion of Methane to Carbon DhulS.de
In order to demonstrate the catalytic effect of silica gel on the reastion of methane and sulfur vaporl three runs were carried out in the empty reactor at temperatures of 500", 525", and 550" C. The results of thew runs are shown in Table VI. The same charge rates of sulfur vapor and methane were employed aa in the catalytic experiments to which these runs are to be compared.
INDUSTRIAL AND ENGINEERING CHEMISTRY
2206
Vol. 42, No. 11
given set of conditions were made at definite intervals to check any variation Av. Space Mole CHI Converted of activity of the catalyst in relation to Run Temp., Velocity Ratio % % deviation the time it had been used. The multiple OC. S.T.P. &:CHL NO. Catalyst 58 Silica gel 500 279 1.98 43.5 0.9 conversion points, under the same con87 Silica gel gel ,526 289 2.00 ditions of temperature and space veloc57 Silica 549 301 1.98 760.2 7.4 11 .. 41 None 495 259 1.97 0 . 2 (man.) ... ity in Figure 3, represent these check 91 92 None 522 256 2.04 0.5 0.1 runs and show that any loss of activity 550 258 1.99 2.0 0.6 93 None Silica 550 690 2.0 44.8 .. .. .. with length of catalyst use is within esAlfrangel 550 715 2.0 0.8 perimental error. A pilot plant operation at 75% conversion level showed little or no loss in catalyst activity over a period of 2.5 months' continuous operation. As a furthor It is realized that, the arbitrary space velocity in an empty reaccheck on this method runs were made on a catalyst which had tor is not strictly comparable to that in a reactor filled with 200 cc. been pretreated by heating in an atmosphere of dry air ut reof catnbpt. However, the actual time of contact in the reacaction temperatures for a long period of. time. Conversions t,ion Eone of the empt,y reactor is somewhat greater than in one were experienced which checked with those of runs in which filled with catalyst. As a further demonstration of catalytic space velocities were corrected as described above. Figure 7 activity, values of earlier runs ( 1 4 ) have been included in this shows this agreement. Runs 114 to 117, the resu1t.s of which are tsble, in which the catalytic effects of silica gel and Alfrax are given in Table 111, were carried out over the pretreatSedcatalyst. compared. Alfras showv a negligible conversion and thus might be considered as an inert filler.
Table VI. Conversion with Silica Gel Versus No Catalyst
QUALITY OF CARBON DISULFIDE PRODUCED
20
IS 0 f.
4
IO
." 5
0
100
203
300
400
500
TIME, hours
Figure 6. Pretreatment of Silica Gel Catalyst at 525" C. LIFE OF CATALYST
Over a long period of use the d i c u gel catalyst used ill this investigation showed a considerable volume decrease. Assuming that unit activity of the catalyst was not impaired, it was still necessary to correct space velocities to the bwis of 200 cc. of charged catalyst. In order to find the existing volume of catalyst in the reactor at a given time aftcr charging, a separate unit was ,set up in which dry air was passed over fresh catalyst a t reaction temperatures. This catalyst was removed periodically and its volume and weight were measured. Table VI1 and Figure 6 show the results of this investigation. Upon discharging used catalyst from the reactor, it8 shrinkage was compared to that of the catalyst over which air had been passed for the same duration Under catalyst charges I and I1 of Table VI1 are the values of volume and weight losses of silica gel which had been heated in air at the temperature and duration of time indicated. Charges 111 and IV show the volume losses of catalysb discharged from the reactor. Satisfactory checks were obtained, as indicated by the cromes in Figure 6. On this basis the volume of catalyst present for any run was estimated and space velocities for all runs in this paper have been corrected accordingly. On making this correction for catalyst volume, little if any loss of 80tivity of the catalyst per unit volume waa noted over long periods of use. Figure 7. Comparison of Untreated and Pretreated Catalyst Check runs under a
Throughout the research and development program of this process, analyses were macle, on the carbon disulfide produced, to check for impurities and for possible products of side re:ictions. Routine tests on laboratory products were carried out by the chemical methods of Bell and Agruss (3) to determine if obher sulfides were present in the carbon disulfide. Products from many runs were composited and t.csted by the physical method8 as outlined by the specifications for analytical reagents of the AMERICANCHEMICAL SOCIETYand specifications for visco~e grade carbon disulfide (18), as well as by the chemical methods of Bell and Agruss. Results of these tests on a laboratory sample and on one from pilot plant production are given in Table VJII. The laboratory sample is a composite of many runs carried out under widely varying operating conditions. The dry ice condensates of individual runs werc allowed to stabilize themselves by exposure to the air and the resulting stabilized carbon disulfide was composited, The free sulfur contentis high because
Table VII. Pretreatment of Silica Gel Catalyst Catalyst Charge
I
I1
111
11-
Time, Hours 6.5 22.6 42.5 64.6 84.2 119.8 192.6 211.4 232.4 278.0 337.8 373.6 395.1
0
Residuer H?S
Volume 6.60 8.00 8.80 10.60 11.80 12.40 13.33 14.13 14.40 15.33 16.13 16.67 17.07 12 18
Weight 3.56 3.90 3.90 3.96 4.09 4.56 4.38 4.29 4.50 4.67 4.67 4.84 4.87
... ...
Analysie of Carbon Disulfide
Specific gravity. 15.6' C./16.6' C. Boiling ran e, ' C. Residual ofor
Color
c.
526 527 524 526 527 527 526 523 525 525 626 524 524 525 500-650
123.2 406.9
Table VIII.
% Loss
Temp.,
Laboratory' 1.270 45 2-45.7b Some Water-white 0.02 0.18 0.12
....
Pilot Piant 1,272 46-46.3 None Water-white 0.002 None
n....on
0.00
0.00 0.00 Done on composite sample. Inchldes products from wide variation of operating conditions. Unoorrected for barometric pre88ure. C Mostly free sulfur. Calculated a8 methyl compodnd.
November 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
of the condensation of sulfur dust in the liquid carbon disulfide. The hydrogen sulfide content is due to incomplete stabilization. In most runs no mercaptans or sulfides were found. Although. under certain conditions of operation, small amounts of these impurities are produced, in no case did the mercaptan or sulfide content of the carbon disulfide exceed 0.1 and 0.6%, respectively. Another indication of the absence of side reactions waa found in the appearance and analyses of recycle sulfur. This material was bright yellow in color and generally had a carbon content lower than that of the fresh sulfur charged to the system.
Table IX. Aatwl O m u l r Volumea in S p t e m C& US= Cst +Wn8
+
% con-
venion
0 10 20
ao
40
so
60 70
DISCUSSION OF SPACE VELOCITY CONTACT
AND TIME OF
In the data presented in this paper, space velocity has been defined aa the total volume of reactant gases at 0" C. and 760 mm. of mercury which passes over unit volume of catalyst per hour. Total volumes have been calculated on the baeis of sulfur vapor existing as the Sa modification and under ideal gas law conditions. Because sulfur vapor a t reaction temperatures is composed of a mixture of SS,Sa, and Sa molecules, the relative concentrations of each component modification being dependent upon the conditions of operation, the space velocities given w e
1
-
P
2207
80
90
5
(Bseia, unit volume of C H I ) Mnloc. 1 710 1:m 2.017 2.164 2.290 2.424 2.11116 2.688 2.814 2,929
52SOC. 1.777 1. 960 2.091 2,224
a .a&
2.485 2.614 2.736 2.850 2.949
575'C. 2.027 2.216 2.336 2.462 2.563 2,663 2.768 2.863 2.939 2.985
55OOC.
1.875 2.071 2.201 2.327 2.452 2.671 2.688 2.797 2.897 2.971
W0C.
65OOC.
2.244 2.895 2.489 2.1188 2.681 2.769 2 . w 2.917 2.971 2.994
2.653 2.663 2,732 2.794 2.852 2,903 2.945 2.976 2.094
3,990
partial molal volumes of SS, Ss,and Sa and hence the totd molal volume of the real system may be approximated at diierent conversion levels. Table IX summarizes the results of these calculations at 1 atmosphere total pressure for different temperatures and conversion levels. Compressibility factors have been disregarded for carbon disulfide, hydrogen sulfide, and methane at these high temperatures and low partial preseures. Utilizing the inlet and outlet volumes from Table IX, times of contact have been arrived at by assuming a logarithmic mean average of the two. In thia treatment total volume of the catalyst bed haa been used, and no correction has been made for the void space between catalyst granules, as such a correction would be a constant factor. The data have been treated kinetically on the above basis. It is found that the over-all reaction is approxiniatcly second order. Close adherence to second order is obtained a t temperatures up to 600' C. I n the higher temperature ranges a deviation from the second order occurs. Figure 8 shows a plot of second-order rate constants against the reciprocal of absolute temperature in degrees Kelvin. From this plot an activation energy of 38,300 oalories per gram mole is obtained and from which the equation Log k = 22.853
38,300 - 2.303RT -
may be used to express the reaction. Figure 8. Variation of Rate borntanto with Temperature
LITERATURE CITED (1)
not strictly comparable to each other. The true spaae velocity, upon which contact time is directly based, is governed by some mean between that calculated for the volume of reactants entering the catalyst bed and that calculated for the volume of products leaving the catalyst bed, due correction being made for the temperature and preseure for the reaction zone. Calculation on the basis of f& will give the maximum value, whereas calculation on the exact volume of reactants will give the minimum value because the sulfur modification approaches & as the partial pressure of sulfur vapor decreases or as conversion increases. Thus in order to obtain actual contact time it is necessary to consider the sulfur vapor modification under reaction conditions, and tho temperature and pressure of reaction, as well as the degree of conversion attained. In Tables I and I1 the partial pressures of the different species of sulfur vapor and of the reactant mixtures of methane and sulfur vapor have been calculated for different temperatures and pressures. Using these data it is possible to express the weight fractions of SS,Sa, and SI molecules in terms of theoretical partial pressures of S* existing at different levels of coversion in the theoretical system CH4 2 8 ~= CSt 2 H d . Assuming various conversion levels in this system and assuming a total molal volume of the system, which in this caRe would not change with conversion, the
+
+
Bacon, R. F., and Boa, E. S., IND.ENQ.CHEM.,37,
469-74
(1QW
F.,and Fanelli, R., J . Am. Chem. Soc., 65,639 (1943). Bell, R. T., and Agrusa, M. S., IND.ENQ.CREM.,ANN,. ED., 13,
(2) Baaon, R. (3)
297-9 (1941).
Bodenatein, P. H.,U. 9. Patent 1,981,161 (Nov. 20, 1984). (5) desimo, M., W.., 2,187,393 (Jan. 16, 1940). (6) Fanelli, R.,b. ENQ.CHEM.% 3 8 , 3 9 4 3 (1946). (7) I. G. Farbenlndustrie, A.-G., Brit. Patent 293,172 (May 26, (4)
1927).
(8) Kelley, K.K., U. 8.Bur. Mines, Bull. 406 (1937). (9) (10)
IComlOs, J., K6mloa, A., and Engelb, E. F., Brit. Patent 265,994
(Feb.15,1928). Pier, M., ltnd Winkler, K., U. S. Patent
1,735,409
(Nov.
12,
1929). (11) (12) (13) (14)
Preuner, Q., and Schupp, W.,Phy8. Chenz., 68, 129 (1909). Stull, D.R.. IND. CH~OU., 41, 1968-73 (1949). Thscker, C. M., U. 8. Patent 2,330,934 (Oat. 5, 1943). Thscker,'C. M., and Miller, E., IND. ENQ.CAEM.,36, 1 8 2 4
(15)
Waterman. H.I., and vanvlodrop, C., J . SOC.Chum Itul., 58,
&(I.
(1944).
109-10 (1939).
Weet, J. R., paper presented at Symposium on Propertiea, Structure, and Thermodynamic8 of Inorganic Compoundn, before Division of Physical and Inorgenio Chemistry, AM. CEEM.Soo., Syracuse. N. Y.,June 1948. (17) Wheeler, T. S., and Franois, W.,U. S. Patent 1,907,274 (May 2,
(16)
1933). (18)
White, A. C., Rayon & Melldand Textile Monthlg, 16, 512 (1935).
RECISWED Maroh 87, 1980.
