Catalytic Reverse Shift Reaction A Kinetic Study L. W. Barkleyl, T. E. Corrigan, H. W. Wainwright, and A. E. Sands BUREAU OF MINES, U. S. DEPARTMENT OF T H E INTERIOR, AND ENGINEERING EXPERIMENT STATION, WEST VIRGINIA UNIVERSITY, MORGANTOWN, W. VA.
A study was made of the feasibility of adjusting the ratio of hydrogen to carbon monoxide in synthesis gas by the use of the reverse shift reaction COy(g) Hz(g) @ CO(g) HaO(g), using the standard shift reaction catalyst. Under certain conditions of gasification the ratio of hydrogen to carbon monoxide might be higher than the ratio required for hydrocarbon synthesis. A study of the reverse shift reaction as a means of adjusting this ratio was deemed logical. The kinetics of the reaction were studied, and the following rate equation was obtained:
+
k[pco2pat
-
= [11+ KAPCO,:
+
'91
+
COPZ
+ Hz(g)
Ft CO2
+ HzO(g)
(3)
3. The molecule of carbon monoxide is desorbed.
coz
* CO(g) + 1
(4)
4. The second step is the slowest and controls the rate of the reaction, The mte equation derived from the above mechanism agreed well TTith the data (Figure 5 ) .
KRPCOI
The constants of this equation were evaluated at 1000' F., and a probable mechanism for the reaction was postulated. The results of the investigation show that this method of adjusting the synthesis gas composition is feasible, but that its use would depend upon the economic advantage of employing a large excess of superheated steam in the coal-gasification step.
T
HE greater portion of the cost of producing synthetic liquid fuels from coal by the Fischer-Tropsch process is the cost of producing the synthesis gas (a mixture of carbon monoxide and hydrogen in the correct proportions). The United States Bureau of Mines a t Morgantown, IT. Va., in cooperation with \Vest Virginia University, is conducting extensive investigations toward reducing the cost of this step ( 4 ) . The ratio of hydrogen to carbon monoxide in the synthesis gas varies over a wide range, depending upon the operating conditions in the gasification step. The use of a high steam-coal ratio in the gasification step will produce gas of a high hydrogen-carbon monoxide ratio. The use of a large excess of superheated steam may have economic advantages under certain conditions, but the gas made in this manner may be richer in hydrogen than that desired for the Fischer-Tropsch synthesis. For this reason a study was made of the rate of the reaction CO?
2 . The adsorbed carbon dioxide reacts with a molecule of hydrogen, forming an adsorbed molecule of carbon monoxide plus a molecule of water in the gas phase.
+ Hn * H20 + CO
where r is the rate of the reaction
dx -__ d(W/F)'
The units of W j P
are pounds of catalyst per pound-mole of carbon dioxide fed per hour. The conversion, z, is expressed as pound-moles of carbon monoxide formed per pound-mole of carbon dioxide fed. W / F is a factor that is a function of residence time in the catalyst bed. p is partial pressure, and the subscripts A , B , R, and S refer to carbon dioxide, hydrogen, carbon monoxide, and wat,er, respectively. K is the equilibrium constant.
Experimental Methods Apparatus. h schematic diagram (Figure I) and a photograph (Figure 2 ) of the apparatus are shown. The niajor pircea of apparatus were:
A wet-test meter to measure feed rate A drying tower for the feed gas An isotherrual flow reactor A vertical condenser to remove mater from the product gases il second webtest meter t,o measure rate of product gases T, C.
Y
(1)
in the presence of an iron-copper catalyst at 1000° F. The kinetics ( 1 ) of the reaction were studied in an isothermal flow reactor using total conversion data. The pressure and temperature were kept constant, and the variables investigated were feed ratio, feed rate, and conversion. .4n assumed mechanism for the reaction, which fits t h r data reasonably well, was found to consist of the folloning steps:
II
TOWER
Figure 1.
TRAP
Diagram of Apparatus
1. -4 molecule of carbon dioxide is adsorbed on an active center on the catalyst. COa(g)
+ 1 * COZl
(2)
1 Present address, E. I. d u P o n t de Nemours & Co., Ino., Loa Angelea, Calif.
1066
The reactor consisted of a split-type Heavi-Duty electric combustion furnace (750-watt capacity) mounted vertically, a section of 1-inch standard iron pipe placed in the furnace to facilitate temperature distribution, and a Vycor reactor tube 0.75 inch in inside diameter, placed inside the iron pipe. The catalyst WEW
INDUSTRIAL AND ENGINEEqING CHEMISTRY
Vol. 44, No. 5
Fuel Gasification supported on a perforated porcelain disk. Sample conncctions were provided in the feed line and in the product line. A promoted iron oxide furnished by the Girdlcr Corp. was used. The catalyst was a standard catalyst, which is generally used for the removal of organic sulfur from gas, as well as for the shift reaction (3). Its composition was: 67.6% 19.4% 13.0%
Iron oxide (FezOa) Copper oxide (CuO) Water (hydrate)
The catalyst was formed into cylindrical pellets 3 mm. in diameter and 1.75 mm. in length. It had a bulk density of 3.00 grams per cc. The bed height was varied from 0.5 to 2.5 inches,
The thermocouple was checked against the melting point of gold and was found to be accurate within 2"F. a t that point. The following procedure was used for each run.
Procedure.
The furnace was brought to reaction temperature while a stream of nitrogen was being passed through the catalyst. The carbon dioxide and hydrogen were fed in, and the rate and feed ratio were ad'usted to the desired value. Nitrogen was turned off, and about 30 minutes were allowed for steady-state conditions to be reached. Flow rates of feed and product &reams were measured. Samples of feed and product streams were taken and analyzed. Flow rates were checked again, and if they had changed the data were discarded and the run was repeated.
Experimental Results A summary of the data is shown in Tables I through 111, and plots of 1: versus W / F are shown in Figures 5 and 6. In Figure 5 conversion is expressed as pound-moles of carbon dioxide converted per pound-mole of carbon dioxide in feed with F as pound-moles of carbon dioxide in feed per hour. I n Figure 6 conversion is expressed as pound-moles of carbon dioxide converted per pound-mole of total feed, and F is expressed as poundmoles of total feed per hour.
Table I.
Values of x and W / F for Runs with 50% Carbon Dioxide in Feed
(Temperature 1000° F.)
c
Run NO.
101 102 103 104 105 106 107 108 109 110 111 112 113
Figure 2. Experimental Apparatus for Studying Reverse Shift Reaction
Lb.-Mole 30 Formed Lb.-Mole COZ Fed 0.337 0.322 0.352 0.160 0.058 0.i46 0.245 0.140 0.232 0.352 0.352 0.318 0.326
and a length of 7 inches preceding the bed was used for preheating. Feed rates were varied from 0.83to 7.9 cu. feet per hour. Method of Analysis. Samples of both feed and product were analyzed with a standard gas analyzer, manufactured by thc Fisher Scientific Co. Most of the samples were analyzed only for carbon dioxide, carbon monoxide, and hydrogen. The quantity of water vapor formed was obtained from a material balance. Complete analyses made on spot samples showed that the amounts of other gases present were negligible. A complete material balance was determined on each run, and those runs which did not check to within 5 % were discarded.
W/P,
Lb. Catalyst Lb.-Mole COP Fed/Hour 30.3 38.3 80.4 10.5 5 %
12.3 19.0' 9.8 14.6, 75.0 54.7 46.8 42 4 34.0 34.8 20.1 26.6 23.8 19.8 24.6
I
I
TIME IMINUTES)
Figure 4.
DISTANCE FROM BOTTOM OF
Figure 3.
BED I INCHES)
Variation of Temperature with Bed Depth
Temperature Control. The temperature of the reactor was controlled with a Tagliabue one- oint indicator-controller with a Chromel-Alumel thermocouple p%ced in a porcelain well extending into the catalyst bed. The temperature distribution within the catalyst bed and the fluctuation of temperature with time are shown in Figures 3 and 4. This measurement was made with a stream of nitrogen flowing through the bed. It can be noted from the periodic variation in temperature that the control characteristics of the furnace were not entirely satisfactory. May 1952
Temperature-Time Gradient in Catalyst Bed
The time factor, W / F (ratio of catalyst weight t o feed rate), was varied by changing both the amount of catalyst, W , and the feed rate, F. As the value of conversion was found t o be dependent upon ratio W / F only, and was not affected by the individual values of W and F making up the ratio, it was safe t o conclude that the effects of diffusion to the surface of the pellet were negligible. It was assumed that the rate of reaction was controlled by some chemical step. Development of a Typical Rate Equation. The reaction on a solid catalyst surface may be divided into the following steps
($1: 1. Diffusion of reactants to the catalyst, surface
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
1062
- F u e l
Gasification
Table 11. Values of x and W / F for Runs with 20% Carbon Dioxide in Feed (Temperature 1000° F.)
Run No.
30
Lb.-Mole Formed Lb.-Mole Cog Fed
W/F
Lb. Cata’lyst Eb.-Mole COn Fed/Hour
1. A (carbon dioxide in this case) reacts with an active center, E , on the catalyst to form a free molecule of S (carbon monoxide in this case) and an adsorbed atom of D (oxygen), 2. The adsorbed D reacts with a molecule of B (hydrogen) to form adsorbed R (water), 3. R is then desorbed.
70.0 121.0 46.6 31.5 30.2 31.2 28.4 23.6 19.1 11.6 8.7 7.0 6.5 5.5 5.4 2.5
Table 111. Values of x and W / F for Runs with 80% Carbon Dioxide in Feed (Temperature 1000’ F.) Run No.
Lb.-Mole 20 Formed Lb.-Mole COz Fed
W/F Lb. Cata‘lyst Lb.-Mole COz Fed/Hour
314 315 316 317
0.0176 0.023 0.065 0.055 0.055 0.059 0.168 0.137 0.119 0.119 0.100 0.148 0,128 0.150 0.147 0.143 0.129
2.4 2.7 3.5 4.0 4.6 6.3 15.6 12.9 10.9 8.2 6.8 21.1 32.1 28.2 21.3 19.5 16.4
Figure 5.
One of the Proposed mechanisms Postulated in this work be used as an example of the method of deriving the rate equation corresponding to a given mechanism. Let us postulate that Reaction 6 takes place by the following steps:
1068
VS.
W/F
The above steps may be written symbolically thus:
1. A + E + D l + S
KI
cnps PAC1
2.
reaction. For the purpose of deriving a rate equation, it is assumed that one, and only one, of these steps is so slo~vas to control the rate of the over-all reaction. All other steps are assumed to be at equilibrium. To obtain an empirical rate law that represents the data it is necessary to:
Conversion
25 ZZ
Dl
+B e R 1
&
=cR CDPB
,B
NE
8 ,d
012
$a‘2
Equation 7 is the rate equation, but i t has no value until all variables are expressed in terms of the partial pressures of A , B, R, or S, which can be determined from experimental data. The terms C D and C R must be eliminated in favor of terms involving the partial pressures of the components in the gas phase. The equilibrium equations for steps 1 and 3 may be used for this purpose.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 5
Fuel Gasificationwhere a = l/k
From step 1
KI~ACI
=
ps
(8)
;z g$,
In Equation 14 all the unknown constants appear on the right-hand side, and all the measurable terms are on the lefthand side.
and from step 3
Rate Equations Used to Test the Data. Table 1V shows a list of some of the more plausible mechanisms that were considered in this treatment. The mechanisms are expressed in the symbolic form used previously. The corresponding rate equations for each mechanism are shown in Table V.
Thus Equation 7 becomes:
or
1"
r = k2cr
KzKs
Table IV. Postulated Mechanisms I. A + I F ? A I 111. A 1 d Dl S AI B F? RI S Dl B RZ RZeR+l RI7;),R+1 11. A 1 F? A1 IV. A 1eD1 8
but KIKsKa = K , the over-all equilibrium constant, (10) cz is the only variable left which cannot be measured experimentally. The term ci represents a concentration of unoccupied
RI@R+l Sl*S+l
active centers on the reactive surface. By letting L represent the total effective concentration of active centers, whether occupied or not, one may obtain the following expression:
L =
CI
f CD
+
+ + + + + BfLdBBE RI + 2 BZ + Dl
+ + + B+l@BBE Al + BI i=! RI -!-SI
RlF?R+l
Table V.
Rate Equations
CR
Again substituting for CD and CR from steps 1 and 3:
L=c1+-
or CI
L
=
[1+
K1
(E)+ E]
Substituting this value for c1 in Equation 10 results in:
or
where-
k = kzKiL KA = KI K R = 1/Ks K = the equilibrium constant of the over-all reaction. Equation 13 also may be put into another form that will facilitate the evaluation of its constants. Inverting Equation 13:
RpA(E)
I11
a
- PR
T
= a
+ (%)+ c P A (2)
IV
or
IV
IV
or
May 1952
IV
INDUSTRIAL AND ENGINEERING CHEMISTRY
1069
Fuel GasificationSelection of Best Rate Law A method has been proposed by Yang and Hougen (6) whereby initial rates are used to limit the number of possible mechanisms. This involves a plot of initial rates against the mole fraction of one of the reactants in the feed. The general shape of the resulting curve classifies the reaction into a limited group of mechanisms.
i
Figure 7,
Plot of Equation 1-2 for a Feed of 20% Carbon Dioxide
The method of Yang and Hougen (6) was used as a preliminary evaluation of these data. Some of the mechanisms selected by the initial rate method were found t o yield equations that would require negative adsorption constant8 to fit the experimental data. In addition, it was decided to check all of the mor? probable mechanisms by evaluating the rate constants for each equation and eliminating those equations that required large negative constants. The method of least squares was used to evaluate the constants. The curves of conversion versus TY/F were used t o obtain values of 5 and corresponding rates (slopes) at various values of W / F . The partial pressures were calculated from the stoichiometry of the reaction. The equilibrium constant, K , was evaluated from a plot of log K versw the reciprocal of thc absolute temperature
(6). JVith thew nieasured values of mte, equilibrium constant, arid partial pressures, the constants of the equations of Table V T$ ere evaluated by the method of least squares. The values of these constants are tabulated in Table VI. The constants a, b, c, etc., are either ratios or products of rate and equilibrium constants. Therefore, they must be positive if the equation is to represent a physicallv possible mechanism. Those mechanisms for which any of the constants in the rate equation are large negative numbers are immediately eliminated as being physically implausible. Of all the mechanisms listed in Table IT, only two had a11 positive constants. These were No. I, step 3 controlling, and So. 111. step 2 controlling. However, mrchanisms I, step 2 controlling, and 111, step 3 controlling, each had one very small negative constant. The constants for these two mechanisms were obtained from the preliminary evaluations by the method of initial rates with the supplementary use of trial and error on the integrated form of the rate equation. This gave all positivr constants for both.' As a final check to select one of the four mechanisms that E U I vived the previous test, the rate equations were integrated graphically and the calculated values of conversion were plotted against W / F for each set of experimental conditions. The equations of two merhanisms (1-2 and 111-3) fitted the data well. However, that of mechanism 1-2 fitted the data slightly better, Mechanism 1-2 was chosen as being more plausible. This choice was also verified by preliminary intlications of further work on the project. Figures 7 through 9 show hou the calculated values from the, selected rate equations compare with the experimental valurs. I
Table VI. Rate Constants a
44.1 16.4 24.3 2.29 36.2 78.9 24.6
I11 I11 I11
IV
IV IV IV
b
-435.1 67.06 -0.009= -3500.0 0.10 -0.81 -1.32
1
1
ClRCLLI I R E LXPER
Polls 10
20
-
30
40
80
60
I1,", I
70
BG
LE. CATALYST- HR
( LB.-MOLE GOn IN FEED ) Plot of Equation 1-2 for a Feed of 50y0 Carbon Dioxide
Summary d - e
d
C
113.5 0.90 0.67 112.0 946.0 -204.0 7.6
I
e
Figure 8.
ConMecha- trolling nism Step
I
..... .
.
I
.
.
-16.3 -383.0 -135.0 0.12
... ... ... ... ,.. ... I
.
.
The results of this work may be summarized as follows: I t is possible to make adjustments in the hydrogen-carbon monoxide ratio of synthesis gas using the reverse shift reaction over a standard shift catalyst at 1000 O F. The following mechanism of the reaction is indicated: ( a ) A molecule of carbon dioxide is adsorbed on a single active center. ( b ) The adsorbed carbon dioxide reacts with hydrogen to form a molecule of adsorbed carbon monoxide and a molecule of water in the gas phase. ( c ) The molecule of carbon monovide is desorbed. ( d ) Step b is controlling. The following rate equation applies to the reaction:
a The constants for mechanisms 1-2 and 111-3 were later re-evaluated as described on the following pages. The revised values are:
I I11
2 3
1.68 12.8
7.5 2.43
70 2.94
b Exact values of these constants were not calculated. The following identities were established: a c = -38.18 b c = 10.95 (d e) 2c = 6.18 Careful inspection of these values will prove that the constants cannot all be positive.
+ + --
1070
The values of the constants at 1000" I;. are: ic
= 0.595
K = 0.267 K - 446 K," 41:65
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 5
-Fuel The units of these constants are such that the equation will bold when conversion is expressed as the fraction of carbon dioxide converted, partial pressures are in atmospheres, feed rate is in pound-moles of carbon dioxide per hour, the weight of catalyst is in pounds, and rate is defined as d r / d ( W / F ) . The effect of diffusion through the gas film at the surface of the catalyst ia negligible. This was shown by the fact that at a constant W / F value the conversion does not vary with mass velocity. The reaction will not proceed a t all at this temperature except in the presence of the catalyst. Runs were made using an inert filler in place of the catalyst and no conversion was obtained.
0 I6
,
012
0 04
g
0
o
0
Figure 9 .
4
8
12 L
16 20 CATILYST-HR
24
28
32
LI ( L B 8MOLC GO, IN FEED Plot of Equation 1-2 for a Feed of 80% Carbon Dioxide
Gasification-
nection with the preparation of the manuscript for this paper. Acknowledgment is also made of the assistance rendered by L. J. Kane of the Bureau of Mines staff in connection with the laboratory work.
Nomenclature a, b, c, etc. = empirical constants ca, c b , etc. = effective concentration
of active centers occupied by components A , B elc. CI = effective concentra&on of unoccupied active centers on catalyst surface F = pound-moles of carbon dioxide feed per hour h: = reaction rate constant kl,kp = specific rate constants for individual steps of a reaction K = equilibrium constant K A ,KB, etc. = adsorption equilibrium constants K,, K , = equilibrium constants for individual steps in a reaction I = symbol for an active center on the catalyst surface L = effective concentration of all active centers (occupied or unoccupied) on catalyst surface P = partial pressure; subscripts A , B , R, and S refer t o carbon dioxide, hydrogen, carbon monoxide, and water, respectively r = reaction rate, pound-moles converted per hour per pound of catalyst W = pounds of catalyst W / F = pounds of catalyst per pound-mole of carbon dioxide feed per hour x = pound-moles carbon dioxide converted per pound-mole of carbon dioxide in feed
Literature Cited
It is realized that the work of this investigation could well represent the beginning of a long-time project involving a study of the kinetics of this reaction over wide ranges of operating conditions. Further work is being done to investigate the effect of temperature on the constants of the rate equation. It is hoped that in the future this study may be advanced by determining the effect of pressure and by investigating other catalysts.
(1) Hougen, 0.A., and Watson, K. M., “Chemical Process Principles,’’ Vol. 3. New York, John Wiley & Sons, 1947. ( 2 ) Hougen, 0.A., and Watson, K. M , IND.ENG.CHEM.,35, 529 (1943). (3) Sands, A. E., Wainwright, H. W., and Egleson, G. C., U. S. Bur. Mines, Re@. Invest. 4699 (1950). (4) Strimbeck, G. R.,Holden, J. H., Rockenbach. L. P., Cordiner, J. B., and Schmidt, L. D., Zbid., 4733 (1950). (5) Wagnian, D. D., Kilpatrick J. E., Taylor, W. J., Pitzer, K. S., and Rossini, F. D., Natl. Bur. Standards, Research Paper RP1643 (1945). (6) Yang, K. H., and Hougen, 0. A., Chem. Eng. Progress, 46, 146 (1950).
Acknowledgment The authors wish to express their deep appreciation t o E. D. Arnold, West Virginia University Engineering Experiment Station, for the extremely valuable assistance rendered in con-
RECEIVED for review July 31, 1951.
ACCEPTEDFebruary 8,1952.
Gasification of Solid Fuels at Elevated Pressures Wilhelm Gumz, BATTELLE
MEMORIAL INSTITUTE, COLUMBUS, OHIO
Gasification of solid fuels under pressures of 20 to 30 atmospheres offers advantages in high capacity and high fuel rates, low oxygen consumption, and decrease in cost of labor and product gas. It may have applications in pressurized boiler furnaces, gas turbines, and in the production of synthesis gas and high-B.t.u. fuel gas. The effect of pressure on gas composition, pressure drop in the fuel bed, and terminal velocity of suspended particles is calculated and compared with experimental data.
G
ASIFICATION under pressures of 20 to 30 atmospheres has become a proved technique and offers distinct advantages with respect to high capaoity and high fuel rates, low oxygen consumption, and decrease of labor and product-gas costs. Prospective applications are in pressurized boiler furnaces, gas turbines, and produetion of synthesis gas and high-B*t.u. fuel gas. May 1952
*
The effect of pressure and other operating variables can be clearlydemonstrated by calculation (5,6,9,11,12,14,16),by solving a set of simultaneous equations representing thematerial balances of carbon, hydrogen, oxygen, nitrogen, and sulfur, theequilibria of the Boudouard or producer-gas reaction, of the heterogeneous water-gas reaction, and of the formation of methane and sulfur compounds, hydrogen sulfide, carbonyl sulfide, carbon disulfide, and sulfur vapor, and finally the heat balance. The required data on the C-H2--02 reactions are well known ( 6 , 1 6 ) , and data on equilibria of sulfur compounds can be derived from the work of Ferguson (S), Lewis and Randall ( l S ) , Lepsoe ( I I ) ,and Cross ( 2 ) . Table I gives an excerpt of equilibrium constants of sulfur compounds. The solution of a set of simultaneous equations involving the 13 unknowns-carbon monoxide, carbon dioxide, hydrogen, water,
INDUSTRIAL AND ENGINEERING CHEMISTRY
1071
