Ioffe, I. I., Zauodsk. Lab. 16, 1252 (1950). Mars, P., Van Krevelen, D. W., Chem. Eng. Sci. 3, Spec. Suppl. 41 (1954). Morooka, Y . , Ozaki, A., Catalysis 5, 116-24 (1966). Novella, C., Bennloch, E., Anales Real. SOC.EspaA. Fii. Quim. 58B,783 (1962a). Novella, C., Bennloch, E., Anales Real. SOC.EspaA. Fii. Quim. 58B, 791 (196213). Novella, C.,Bennloch, E., Anales Real. SOC.EspaA. Fii. Quim. 59B, 669 (1963). Pichler, H., Obenaus, F., Brenmtof-Chem. 45, 97-103 (1964). Pichler, H., Obenaus, F., Brennstof-Chem. 46, 28-34 (1965).
Roberts, G . W., Satterfield, C . N., Ind. Eng. Chem. Fundamentals 4, 288 (1965). Roiter, W. A., "Actes du Deuxitme Congrts International de Catalyse," Part 1, p. 759, Ed. Technip, Paris, 1961. Simard, G . L., Steger, J. F., Arnott, R. J., Siegel, L. A,, Znd. Eng. Chem. 47,1424 (1955). Vrbaski, Th., J . Phys. Chem. 69, 3092 (1965). Vrbaski, Th., Mathews, W. K., J . Catalysis 5,125-34 (1966). Vrbaski, Th., Mathews, W. K., J . Phys. Chem. 69, 457 (1965). RECEIVED for review October 19, 1967 ACCEPTED April 29, 1968
SHIFT CONVERSION OF SYNTHESIS GAS CONTAINING SULFUR, DUST, AND CARBON DIOXIDE M. W. WILSON AND
K. D. PLANTS
U.S. Bureau of Mines, Morgantown, W. Va. 26505 CO in synthesis gas containing various sulfur compounds, dust, and COZ was converted over standard chromium-promoted iron catalyst a t temperatures to 1000" F., pressures to 300 p.s.i.g., and space velocities In 30-day continuous tests, CO conversions were lower than with purified gas (wet) to 5000 v./v./hour. over nonsulfided catalyst, but were comparable when the residence time was increased by raising the pressure. Dust in the gas did not appear to decrease catalyst activity, but dust concentrations of 400 grains per 100 s.c.f. increased the pressure drop and reduced the gas flow through the converter. Conversions of CO were generally independent of type of sulfur in the gas. Space velocities are given for specified conversions of sulfur-laden gas at 900' F. and 0, 100,200, and 300 p.s.i.g. AW
synthesis gas produced by the steam gasification of
R coal is a mixture of co, Hz, cos, HzS, various organic sulfur compounds, unreacted steam, and particles of carbon and ash. Only the C O and Hz enter into product synthesisthe sulfur, COz and solids are removed. Furthermore, the C O / H Z ratio must be decreased to synthesize certain products (such as high-B.t.u. gas), a process step that is accomplished by reaction of some of the C O with steam to form Hz and COz. This process step is called shift conversion. Several process steps precede conversion in modern industrial practice. T h e raw synthesis gas is cooled from about 2000' F. to 200' F. for removal of dust, COz, and sulfur, after which the purified gas is reheated to at least 350' F. for shift conversion. The gas is then cooled again to about 200' F. for removal of the COz produced during conversion. Gasprocessing costs might be reduced, however, if conversion could be carried out before the gas is purified. The unreacted gasification steam now lost in preconversion cooling and purification could then be utilized in C O conversion and the heating step eliminated. In the proposed system, the hot gas from the producer vessel could be washed by hot water scrubbing to make steam for the conversion, then sent directly to the converters. Both the COz and sulfur in the raw synthesis gas and the COz produced in the converters would be removed after conversion. Savings in steam, heat, and processing, if large enough to offset any cost increase incurred by decreased catalyst activity, would effect a net saving in over-all costs. Bohlbro (1963) described how the kinetic expression for C O conversion is modified by the presence of H2S in a feed 526
l & E C PROCESS D E S I G N A N D D E V E L O P M E N T
gas at atmospheric pressure. H e did not evaluate the influence of dust, however, and the effect of organic sulfur compounds, while expected to be similar to that of HzS, was not actually determined. Additionally, in Bohlbro's tests the effects of elevated pressure and thermal aging were not determined. I n the work reported here, tests were conducted at elevated pressure with a laboratory-scale converter on gas containing dust and organic sulfur compounds (besides HzS) to determine the feasibility of converting raw synthesis gas over standard commercial catalysts. Conversions in terms of percentage of theoretical were determined at temperatures to about 1000' F., pressures to 300 p.s.i.g., and space velocities (wet) to approximately 5000 v./v./hour. Thermal aging of sulfided catalyst was also investigated. Apparatus and Procedure
Figure 1 is a flowsheet of the experimental system. The converter was 16 inches long, electrically heated, and constructed of l'/d-inch-diameter Vycor or schedule 80, type 347 stainless steel pipe. Catalysts were standard chromiumpromoted iron water-gas types marketed as G-3A, G-3B, and G-38. These catalysts, supplied by the Girdler Catalyst Division, Chemetron Corp., Louisville, Ky., had about the same iron and chromium content. Placek and Pedigo (1959) described the variation in method of preparation and activation. G-38 was found to perform best under sulfided conditions in the presence of sulfur-containing gas at elevated pressure, so it was used in the tests reported here. Simulated synthesis gas was utilized which contained CO, Hz, COz, dust, HzS, ethyl mercaptan ( C Z H ~ S H )thiophene , (CdHdS), carbon disulfide (CSZ), and carbonyl sulfide (COS). The carbon content of the 325-mesh dust utilized was about 50%.
Table 1.
CO Conversions for Purified (55% CO, 45% Hz) and Sulfur- and Dust-Laden Gases over Sulfided Catalyst
(Space velocity,a wet, 720 v./v./hr.) Sulfur as H S , Pressure, P.S.I.G. 0 0 0 0 0
Run 1
a
Conuerter Temp., ' F.
Duration oj Run, Hr.
Steam-CO Ratio
750 750 750 850 850 750 750 750 750 750 750 750
46 505 478 637 395 26 576 22 9 21 45 6 462
2.3 3.2 1.9 2.9 2.8 2.3 1.9 2.3 1.9 2.6 2.6
Gr./100 S.C.F. 0
2 3 4 5 6 100 7 100 8 200 9 200 10 300 11 300 1.8 12 300 Percentage Hourh volumejow of feed gas plus steam per unit oolume of catahst bed.
pass.
% actual conversi0n/7~theoretical conversion X 100.
0
540 520
132 87 402
0
0 0
0
0 0 0 0
480 405 0
700
CO Conuerted,b
Conversion,c
%
%
72.0 48.8 45.9 50.2 51 .O 78.1 74.7 77.5 73.8 77.5 78.6 63.3
77.8 52.0 61.8 58.6 59.7 88.5 88.0 92.9 91.3 92.3 93.5 85.1
250 310 of moles of CO feed converted to desiredproduct, or destroyed, per
response of pressure gages and was normally about =t5 p s i . I a t 300 p.s.i.g. Gas temperatures probably were within 5' F.
-
n Pressurized water tank
610
Dust, Gr./ 700 S.C.F. 0 0
Shift converter
A
Pressure letdown tank
Steam
generafor d
Condenser
Separator
To meter
Dust and condensate Figure 1 .
Shift conversion system
Approximately 200 cc. of I/d- by I/d-inch catalyst pellets formed the 7-inch high by ll/e-inch diameter bed. Before each test, the catalyst was reduced with HZor a mixture of HI, C O , and steam to temperature-pressure equilibrium. Before the tests with sulfur-laden gas, the catalyst was sulfided to temperature-pressure equilibrium for the operating condiis, it was exposed to sulfur-laden gas until the tions-that catalyst activity remained virtually constant. Sulfiding normally required about 16 hours. During this period C O conversion decreased, but a n equilibrium was eventually reached where no further decrease resulted. Dust-free and sulfur-free gases were used for reference tests. Gas velocities, steam-CO ratios, and reaction temperatures were controlled at predetermined values. Converter temperature was controlled with electrically heated coils to give a thermal balance across the unit. Temperature variations were small enough to consider the converter isothermal. During warmup, an equivalent flow of NZ was maintained through the converter at the experimental temperature, and steam was added when the preheater reached 250' F. Flow rates and temperatures initially set were reset after reduction and/or catalyst sulfiding and synthesis gas addition. After pressure letdown, gas from the converter was cooled in a water condenser, condensate was removed in a gas-liquid separator, and the gas was sampled, metered, and vented to the atmosphere. Inlet and exit gases were analyzed by standard methods for COZ,CO, Hz, HzS, organic sulfur, and dust. Concentrations of other constituents were negligible. T h e quantity of steam reacted was obtained from material balances of input and output water. Accuracy of Results
Measured variables subject to experimental error were pressure, temperature, gas flow rates, and gas composition. Accuracy of pressure measurement depended primarily on the
of true values-normal for thermocouples. Outlet gas flow rates, measured after pressure letdown, were within 1%. Gas composition percentages probably did not deviate from actual percentages by more than zt0.2. Analyses of gas samples obtained under presumably identical conditions agreed within 2 percentage points. C O conversions were based on the C O Z produced, since this method is least affected by experimental errors. Standard deviation of several tests a t virtually identical conditions was 3.7% for theoretical conversion and 6.1% for actual conversion. The steam-CO ratio varied somewhat, but was not considered to have had a significant effect. Variation in steamC O ratio affects the C O conversion but not the C O conversion efficiency. Steam-CO ratio was held as constant as possible and was not considered a variable. Variation in steam-CO ratio, within the range of these tests, is likely to affect only the space velocity, and this was held virtually constant. Conversion with Contaminated Gas
Tests with G-38 were conducted with synthesis gas at temperatures and pressures to 1200" F. and 300 p.s.i.g., respectively. Conversions were based on the apparent equilibrium constant, K,, which assumes the reaction of ideal gases. Sulfur in the gas significantly decreased conversions at atmospheric pressure, but had a lesser effect a t lower velocities associated with increased pressures at constant space velocity (Table I). Dust did not materially reduce conversion efficiency beyond that caused by the sulfur. The C O conversion, over sulfided catalyst, of gas containing dust (87 grains per 100 s.c.f.) and 1% sulfur as HzS during 637 hours of operation at atmospheric pressure was not materially decreased. At a higher dust loading, 402 grains per 100 s.c.f., the results were virtually the same, although this run was terminated after 395 hours by plugging of the converter outlet. Dust accumulated on the catalyst, as expected, increasing the AP across the bed. This increased AP did not materially reduce the CO conversion, but it would be a factor in converter design and would increase the amount of power required to circulate the gas, I n a typical run, 52% of the dust in the gas was trapped by the catalyst. Screening and water washing of used unsulfided catalyst (following a dust run and cooling in nitrogen) failed to restore its activity. No pyrophoricity was noted. Catalyst giving a 61.8% conversion at 750" F. and atmospheric pressure gave VOL. 7
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OCTOBER 1968
527
Table II. CO Conversions for Gas (55% CO, 45%
H2)
Containing Organic Sulfur Compounds, Sulfided Catalyst
[900" F., 720 v./v./hr. (wet), 0 to 300 p.s.i.g.1 Pressure, P.S. I.G. 0
Run
13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
0 0
0 0
150 150 150 150 150 300 300 300 300 300
Steam-CO Ratio
3.3 3.2 2.8 4.1 3.4 2.2 3.8 2.7 2.8 3.2 2.3 2.5 2.1 2.9 3.3
Sulfur ( S ) in Gas, Gr./IOO S.C.F. cs2 CzHsSH
cos 48
31
co
%
CaHaS
0
0
0
67 8
0
0 0
0 0
61.2 70.7
0
0 0
0
29 5
70.3 61.6 78.5 76.3
0
107
0 0 0
47 0 0
111
5
79
18
Conversion Ejiciency,
Converted,
% 93.0 70.6 89.1 78.4 82.9 93.0 94.3 91.3 89.9 97.1 97.5 91.5 93.7 92.9 94.4
100
90
L
W
2 W
n
-
80
2-
0 m
5
>
70 Feed gas, 44% H2, 55% CO, 1%H2S
z
8
-
Temperature, 750" F
60
50
80
a c
c
.Id
I
0
Space velocity, 720 vol/vol/hr (wet) Stearn.carbon monoxide ratio, 3 I 1 I I 100 200 300 PRESSUREl .psig
Figure 2. CO conversion, sulfur-laden and purified gases, as a function of pressure
Y 60
zE! m
40
>
z
,
Feed gas, 37% H2, 50% CO, 12% CO 1%H 2 Space velocity, 720 vol/vol/hr (wet) Steam-carbon monoxide ratio, 3 1 1 I I I I I 700 800 900 1,000
0
0
20
I
600
TEMPERATURE,
"F
Figure 3. CO conversion, sulfur-laden gas, at various temperatures and pressures
100
only 16.6% conversion following screening and washing. When the reaction temperature was raised to 1000" F., the CO conversion attainable with the cleaned and washed catalyst was 63.5%, still well below conversions achieved with nonsulfided catalyst on clean gas at 1000" F. No attempt was made to determine the reason for the decreased activity, although it is possible that washing the catalyst lowered the chromium content. Activity of sulfided catalyst at 900' F. appeared to be '1dependent of the type of organic sulfur compound in I' e synthesis gas-COS, CS2, CzHsSH, or C4H4S (Table 11). Results were similar at 750' F.
+ K
90
W
2 W
a 80
z
0 v)
5
70
>
z 0
" 60 0 Psig
50 Effects of Pressure, Temperature, and Velocity
Conversion of C O in sulfur-free gas over sulfur-free catalyst at 750' F. increased from 78y0 at 0 p.s.i.g., to 88% at 100 p.s.i.g., then went u p only an additional 4 percentage points when the pressure was raised to 200 and 300 p.s.i.g. (Figure 2). Conversion of gas containing about 1% sulfur (as H2S) over sulfided catalyst, while only 52y0 at 0 p.s.i.g., increased to 88% when the pressure was raised to 100 p.s.i.g., then increased only to about 91% at 300 p.s.i.g. This indicates that, at constant temperature, conversions comparable to those attainable with clean gas can be achieved with sulfurladen gas when operating a t pressures of 100 p.s.i.g. or more. Temperature effects were determined in tests with sulfided 528
I&EC PROCESS DESIGN A N D DEVELOPMEN1
I
O;1,O
I
2,dOO
' 3,dOO
\ 100 psig '4,0b0
' 5,dOO
6,jOO
SPACE VELOCITY, vol/vol/hr (wet)
Figure 4. Effect of space velocity on sion, sulfur-laden gas, sulfided catalyst
CO
conver-
catalyst treating sulfur-bearing gas at 0 to 300 p.s.i.g. and GOO0 to 1000° F. Conversions as a function of temperature at various pressures are given in Figure 3. With proper adjustment of temperature and pressure, desired conversions should be possible with sulfur-laden gas. Figure 4 presents conversions for HzS-laden gas over sulfided catalyst at 0 to 300 p.s.i.g., 900° F., and various space velocities.
Conclusions
100 + C
3
80
L
0)
a
H2
Feed gas, 46% Hpl 53% CO, 1%
-
201
7
'
Space velocity, 720 vol/vol/hr (wet) Steam-carbon monoxide ratio,. 3
8
I
I
I
I
1
I
9 10 11 12 11 10 9 8 TEMPERATURE, hundreds of degrees F
7
Figure 5. Thermal aging of sulfided catalyst, sulfurladen gas, atmospheric pressure
Thermal Aging
Thermal aging at atmospheric pressure of sulfided catalyst was evaluated with HeS-laden gas at 750', 900', 1000°, llOOo, and 1200'F. and back to 750" F. a t the same levels (Figure 5). Conversion was 52% at 750' F., increased to 9770 at 1000' and llOOo F., decreased to 79y0 when the temperature was raised to 1200' F., jumped to 93% when the temperature was reduced to 1100' F., then dropped to 27% at 750" F. Thus, CO conversions over catalyst heated to 1200' F. were lower when the temperature was returned to lower values that are normal for shift conversion.
Conversions of C O in dust- and sulfur-laden gas over chromium-promoted iron catalyst were lower than were attained with purified gas. Comparable conversions were achieved, however, by decreasing the gas velocity via increase in pressure. After temperature-pressure equilibrium with respect to sulfur was reached, C O conversions of sulfurand dust-laden gas remained relatively constant during 30day continuous tests. Dust did not decrease the conversions beyond those attributable to the sulfur. However, a dust loading of 400 grains per 100 s.c.f. restricted the gas flow through the test converter. Conversions were independent of sulfur compound in the gas. Screening and washing nonsulfided catalyst following exposure to dust-laden gas failed to restore the activity of the catalyst to levels attained with dust-free gas. Nomenclature
gr.
= grains = apparent equilibrium constant s.c.f. = gas volume, standard cubic feet
K,
v./v./hour = space velocity, volume of feed gas at standard conditions per volume of catalyst bed per hour literature Cited
Bohlbro, H., Acta Chem. Scand. 17, 7 (1963). Placek, C., Pedigo, C . L., Znd. Eng.Chem. 51,482 (1959). RECEIVED for review October 23, 1967 ACCEPTED May 28, 1968 Reference to brand names is for identification only.
KINETICS OF T H E R M A L C R A C K I N G OF HIGH M O L E C U L A R WEIGHT N O R M A L P A R A F F I N S S. G. W O I N S K Y C. F. Braun @ Go., Alhambra, Calif.
97802
The kinetics of thermal cracking of high molecular weight normal paraffins was studied using a digital sirnulation of an analog computer. Activation energies and frequency factors from the literature were used to determine rate constants for the intermediate free-radical chain reactions. Computer runs were then made to simulate cracking at 800°,900", 1000°,and 1100" K. and 0.2, 3, and 10 atm. The computer results give over-all reaction orders, activation energies, and frequency factors in good agreement with the available data, in the range of 750" to 900" K. and 0.2 to 70 atm. The program also predicts relative concentrations of free radicals in reasonable agreement with the data. Computer results can be correlated approximately over a wide range of conditions by an over-all reaction order of 1.3 and an activation energy of 53,000 cal. per mole. Examination of computer results leads to interesting observations about free-radical chain reactions, and points out the power of digital simulation in studies aimed at a fuller understanding of reaction mechanisms.
HERMAL cracking has long been a reaction of commercial T i m p o r t a n c e . Prior to the advent of catalytic cracking just before World War 11, it was the primary method of producing gasoline from petroleum (Shreve, 1945). More recently, it has become important in the production of acetylene, ethylene, and higher molecular weight alpha-olefins for use as petrochemical building blocks in the production of a
host of products (Chevron Chemical Co., 1965; Kamptner et al., 1966; Zdonik et al., 1967 a, b). The reaction is usually described in terms of first-order kinetics, even though it is definitely not a first-order reaction (Fabuss et al., 1964; Worre11 et d., 1967). Also, the most reliable correlation of firstorder kinetic constants a t atmospheric pressure is probably accurate only to within a factor of about 2 to 3 (Fabuss et al., VOL. 7
NO. 4
OCTOBER 1 9 6 8
529
