Ind. Eng. Chem. Process Des. Dev. 1985, 2 4 , 1043-1049
1043
Kinetics of the Fischer-Tropsch Synthesis in the Slurry Phase on a Potassium-Promoted Iron Catalyst Stanlslav LedakowIcz,+ Hubert Nettelhoff, RyoJI Kokuun, and Wolf-Dleter Deckwer' Fachbereich Chemie -Technische Chemie, University of Oldenburg, D-2900 Oldenburg, Federal Republic of Germany
Fischer-Tropsch synthesis (FTS) in the slurry phase was studied in a stirred autoclave at 1 MPa and 220-260 "C. A precipitated Fe catalyst promoted with K was employed. The catalyst had a high water gas shift activity. Hence, the water concentration in the reaction medium was negligible, and no inhibition of the rate by chemisorption of water could be detected. However, the rate data did not follow first-order kinetics in H, as expected under such circumstances. The experimental data can only be described by assuming an inhibition by chemisorption of CO,. Such an inhibiting action caused by CO, chemisorption on K-promoted Fe catalyst has been overlooked until now. I t can largely influence performance of FTS reactors.
In spite of the lot of literature on the Fischer-Tropsch synthesis (FTS), reliable kinetic data are relatively scarce. Recently, all the kinetic information available from the literature have been reviewed by Huff and Satterfield (1984). Anderson (1956) proposed the rate expression
which was successfully applied by other authors as well (Dry, 1976; Atwood and Bennett, 1979; Thomson et al., 1980; Kuo, 1983). Equation 1can be derived on the basis of the enol complex mechanism (Anderson, 1956; Dry, 1976; Huff and Satterfield, 1984). Other leading mechanisms of the FTS are the CO insertion mechanism (Pichler and Schulz, 1970) and the carbide theory (Rofer-DePoorter, 1981). As shown by Huff and Satterfield (1984), both of these mechanistic views yield by applying appropriate assumptions and simplifications the subsequent rate law k'CCOCH22 -rCO+Hp
=
CHpO
+ K'cCOCHp
(2)
Although the FTS in the slurry phase may provide serious advantages over other process modes (Kolbel and Ralek, 1977; Gray et al., 1980; Thompson et al., 1981; Kuo, 1983), the kinetics of the FTS in the slurry phase was only studied by Huff and Satterfield (1984), employing a reduced fused magnetite catalyst-a conventional catalyst for ammonia synthesis. These authors report that their comprehensivedata covering a wide range of experimental conditions can be described by eq 2. However, Huff and Satterfield (1984) point out that eq 2 reduces to eq 1 if K' is assumed to be inversely proportional to CHpas some experimental findings indicate. It should be noticed that eq 1 and 2 as well reduce to a first-order rate law in H2 if the conversion is small, say XCO+H, 5 0.6. This was also confirmed for FTS slurryphase operation by Sudheimer and Gaube (1983). Under such conditions, the partial pressure of water is small and hence Cco >> KCH 0 and K'CcoCH, >> CHZ0.A first-order rate law in H2 hasbeen used to evaluate conversion data of FTS slurry-phase reactors by Satterfield and Huff Present address: Chemical Engineering Department, Politechnikalodzka, Lodz, Poland. 0198-4305/85/1124-1043$01.50/0
(1980), Deckwer et al. (1981, 1982), and Bukur (1983). The catalyst applied by Huff and Satterfield (1984) was developed for ammonia synthesis and, therefore, is not a typical catalyst for the FTS. Usually, precipitated iron catalysts are employed for the FTS in the slurry phase as they have a high activity and sufficient long-term stability (Kolbel and Ralek, 1977; Kuo, 1983). If precipitated Fe catalysts are promoted with potassium, an increased activity with regard to the water gas shift (WGS) reaction can be observed (Kolbel and Ralek, 1977; Arakawa and Bell, 1983). Therefore, K-promoted Fe catalysts appear particularly promising to convert syngas from second generation gasifiers with a low H2/C0 ratio to hydrocarbons. The purpose of this study was to investigate the kinetics of the FTS in the slurry phase by using a precipitated Fe catalyst promoted with K. In particular, it was thought that due to the high WGS activity of such catalysts, the simple first-order rate law in H2 may be applicable even at higher syngas conversions as the water concentration in the reaction mixture can be kept low.
Experimental Section A commercial l-dm3 stainless steel autoclave of 80-mm diameter (Type 1220, E. Haage Co., Muhlheim/Ruhr, FRG) was used. Stirring was done by a magnetically driven turbine agitator (48 mm in diameter). The autoclave was electrically heated and equipped with thermowell and various ports and valves for gas inlet and outlet, liquid charging, sampling, etc. The synthesis gas was composed of H2, CO, and N2, which were taken from cylinders. Their flow rates were controlled by thermal mass flow meters (Type 5810, Brooks Instruments Div.). For decomposing carbonyls possibly present in the CO, the syngas passed a bed of activated carbon particles heated up to 150 "C. The gas inlet into the catalyst slurry in the autoclave was directly beneath the impeller. The volatile products with unreacted syngas left the reactor at the top and passed a hot condenser (140 "C). After reduction to atmospheric pressure, liquid products were condensed in a cold separator (0 "C). The exit gas flow rate was measured by a wet test meter. The CO and C02 content in the exit gas was continuously monitored by IR analyzers. Thus, deviations from steady-state conditions could easily be detected. The composition of the inlet and outlet gases and the liquid samples in the hot and cold condensers were analyzed by GC techniques. Only traces of water were found in the cold 0 1985 American Chemical Society
1044
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
Table I. Experimental Conditions, P = 1 MPa
run
T, K
(STP)/min
yip
I
Yk,
YEo,
YL,o
YL,
YE0
U
Xc0+H2
--lo6rCO+Hp mol/(g of catalyst s)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
493 493 493 513 513 513 513 523 523 523 523 533 533 533 533
0.5 1.0 2.0 0.5 1.0 2.0 4.0 1.0 2.0 3.0 4.0 1.0 2.0 3.0 4.0
0.103 0.103 0.101 0.103 0.096 0.102 0.105 0,100 0.100 0.103 0.104 0.102 0.100 0.102 0.104
0.793 0.688 0.742 0.702 0.707 0.741 0.786 0.736 0.739 0.777 0.791 0.746 0.747 0.776 0.786
0.164 0.141 0.117 0.211 0.173 0.156 0.128 0.211 0.182 0.161 0.150 0.207 0.196 0.160 0.158
0.226 0.108 0.051 0.454 0.317 0.175 0.093 0.414 0.316 0.242 0.181 0.413 0.371 0.252 0.214
0.020 -0.007 0.006 4.002 -0.003 0.007 0.006 0.013 -0.001 0.006 0.006 0.002 0.009 0.011 0.005
0.298 0.345 0.357 0.222 0.290 0.327 0.362 0.255 0.302 0.321 0.342 0.240 0.275 0.312 0.333
0.274 0.404 0.457 0.106 0.220 0.330 0.402
0.596 0.482 0.612 0.527 0.524 0.559 0.581 0.555 0.527 0.550 0.557 0.571 0.549 0.564 0.548
0.601 0.392 0.206 0.882 0.687 0.520 0.301 0.814 0.699 0.586 0.498 0.816 0.766 0.609 0.547
1.845 2.404 2.685 2.522 4.248 6.389 7.377 5.012 8.607 10.774 12.203 5.013 9.430 12.660 13.394
G,dm3
condensate. The details will be given elsewhere (Nettelhoff, 1985). A precipitated Fe catalyst promoted with 1.3% K was employed in this study. The catalyst was prepared by Schering AG, Bergkamen, FRG. The particle size was less than 50 pm. One-hundred grams of unreduced catalyst was suspended in 350 g of sulfur-free molten wax (Vestowax SH 105, Veba Oil GmbH, Gelsenkirchen, FRG). The catalyst precursor was activated in the slurry phase at a constant pressure of 0.5 MPa (about 10 h under N2flow at 250 "C,followed by 12 h treatment with CO and reduction with Hz for 24 h at the same temperature). It was found in preliminary runs that the catalyst produced large amounts of higher hydrocarbons, i.e., wax. As withdrawal of wax without simultaneous withdrawal of suspended catalyst fines turned out to be very difficult, the produced wax was allowed to accumulate in the autoclave. Therefore, only a limited number of runs could be performed with one batch of catalyst. Table I gives the experimental conditions for the runs carried out. Also given are the H2-to-C0usage ratio, the syngas conversion, and the syngas consumption rate calculated therefrom. The mole fractions of N2, COz, Hz, and CO were calculated from the measured exit gas composition by correcting for the condensed hydrocarbons. The water mole fraction was not measured but calculated from n 2u-2 (3) YL,O = YAo2
1 + 2- - u
Equation 3 can be derived from stoichiometric considerations. n presents the average H-to-C atomic ratio in the products (see below). All the runs given in Table I were carried out at a total pressure of 1MPa. The stirrer speed was kept at 600 rpm where any mass-transfer limitation can be neglected (Ledakowicz et al., 1984). The solubility data, i.e., Henry constants of CO, CO,, and H2, were calculated from the data reported by Peter and Weinert (1955) and the measured densities of the liquid phase (Vestowax SH 105) (Nettelhoff, 1985). Stoichiometry The mean stoichiometry of the hydrocarbon synthesis reaction can be written as
CO
+ (1 + ,/2)HZ
-
CH,
+ HzO
The synthesis is followed by the WGS reaction CO + H 2 0 * H2 + C02
(4)
0.101 0.193 0.259 0.310 0.098 0.137 0.256 0.280
Table 11. Balance over Run 1-15 (Overall Syngas Consumption = 178.6 mol) hydrocarbons in mass, g mean C number H-to-C ratio gas phase 222.6 3 2.33 cold condens 91.7 10 2.10 hot condens 217.5 25 2.04 reactor 288.0 30 2.033 total
819.8
Let z be the fraction of water converted by the WGS reaction; then the overall stoichiometry is given by
CH,
The value of n was determined from an experimental balance and the product distribution. Table I1 gives the mass of the hydrocarbon fractions collected during all the runs. Their average C number and H-to-C product ratio was obtained from GC analysis. Therefrom the overall H-to-C atomic ratio in the product ( n )can be calculated and is found to be 2.123. Hence, it follows from eq 6 that (2 + 2.123/2) mol of syngas give 1 mol of CH2.123.As during all the runs given in Table I, 178.6 mol of syngas has been converted; the amount of hydrocarbons produced should be 823.9 g. The experimental value is 819.8 g which deviates by only -0.5%. From the water mole fraction in the reactor outlet gas given in Table I and calculated from eq 3, it can be concluded that the value of z in eq 6 must be close to 1. The occurrence of negative water mole fractions is a result of the data scatter. As can be discerned from eq 3, yh 0 is particularly sensitive with regard to U which could be determined with an accuracy of about f1%. For instance, if U is increased from 0.524 to 0.530 in run 5, yhz0changes to -2 X lo4. In spite of this high sensitivity, from -3 x one can conclude that the water content in the reaction mixture is small and negligible. In addition, the H2-to-CO exit ratio was analyzed, and the COPproduction rate was compared with the syngas consumption rate. Assuming irreversibility of the WGS reaction, the Hz-to-COexit ratio can be calculated from the inlet ratio I , the usage ratio U , and the syngas conversion according to
E=
(1 + 1)(1- XCO+H2) ~-
1+1 1- X
(5)
+ z C O ~+ (1 - Z ) H ~ O(6)
1
(7)
C O + H ~ F U
which was obviously also used by Satterfield and Huff
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
I 1.8
. 8
-
$
n
/
Fptd. Fe/K n = 2.12 1 i 220 260 "C P = 1 MPo
-
50
1045
1.6
1.4
0 "
--"
C
30
1.2
1.0
IO
/ 0.8 mole
-'co*+
cm
Figure 1. Parity plot of measured and predicted H2-to-C0 exit ratios.
Figure 2. Comparison of C02 production rate and syngas consumption rate.
(1982). Considering the WGS equilibrium (Newsome, 1980)
interaction with Fe. Therefore, it is only reasonable that complete shift is observed already at a lower K concentration in the catalyst used in this study. In addition, the contact times and pressures employed in this study are both larger by about 1 order of magnitude compared to the conditions used by Arakawa and Bell (1983). Moreover, the H2/C0 inlet ratio was only 0.75. All these conditions promote approach of the WGS equilibrium. Examination of First-Order Rate Law If the water concentration in the reaction medium is close to 0, eq 1 and 2 reduce both to a simple first-order rate law
Ks
NC02NH,
= 0.0132 exp(4578/T)
(8)
NCdVHzO
and introducing the average H-to-C product ratio (n),eq 9 can be derived for the exit ratio
E=
-B
+ (B2- 4AC)lI2 2A
(9)
with A =2
+ -n2 - (1 + I)XCO+H,
(10)
=
-"CO+H,
(13)
klCcatCHz
At steady-state conditions, the H2mass balance in a mixed reactor neglecting mass-transfer resistances is given by RT (14) F~OGC&,,G - V&ck,,~ = klCcatV,l-C$ HeHz
Considering Figure 1gives a parity plot for the H2-to-C0 exit ratio. The theoretical E values calculated from eq 9 are in agreement with those observed experimentally. As the equilibrium constant of the WGS reaction is large (Ks = 71 at T = 533 K and Ks = 142 for T = 493 K), the equilibrium water concentration is small. Therefore, z in eq 6 is close to 1. The high WGS activity of the catalyst applied can also be recognized by comparing the rate of COzformation with the rate of syngas consumption. This is shown in Figure 2. It can be seen that by applying the appropriate stoichiometric factor (2 + (n/2)),these rates are the same. Once again, this result confirms that the water produced in the hydrocarbon synthesis reaction is immediately consumed by the WGS reaction. Obviously, the catalyst used has a significantly higher shift than FTS activity. The conclusion that the water generated in the FTS is consumed by the consecutive WGS reaction is in general agreement with the findings of Arakawa and Bell (1983). These authors studied an alumina-supported Fe catalyst promoted with various K concentrations. They observed that WGS equilibrium was reached only at higher temperatures and larger K concentrations (14 wt %). However as pointed out by Arakawa and Bell, at low K loadings, a significant amount of the promoted K is consumed by acid sites of the support, thus being not available for
Vk = and introducing the =
v8YR2/Yh*
(15)
H, conversion C&2,G(1
- xH,)(Yh2/&2)
(16)
eq 14 can be. written as
Therefore, a. plot of the left-hand side of eq 17 vs. ( y ~ , / y & , ) ( 1 / ~should G) give a straight line for data measured at the same temperature. As shown in Figure 3, this is only the case for the runs taken at 220 "C. For the other temperatures, linearity as predicted by eq 17 is evidently not fulfiied. Hence, the first-order rate law does not apply. New Kinetic Law As in this study, due to the high WGS activity of the catalyst, COP is one of the most abundantly available components, it was suspected that the syngas consumption rate may be reduced by competitive adsorption of COP. However, inspection of the literature showed that only Brotz and Rottig (1952) reported an empirical rate expression which takes into account a negative influence of oxidizing gases, i.e., CO, + H20on the reaction rate. On
1046
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
1
'
'
'
'
'
'
'
'
'
'
'
q ' I
26
20 16
12
08 04
0
0
Figure 3. Measured data plotted as to eq 17.
2
6
4
8
'
cco, cco
Figure 4. Measured data plotted as to eq 24.
Table 111. Evaluated Rate Constants and Ratios of Adsorption Constants
T,K
k, cm3/(g of catalyst s)
Kco,JKCO
493 513 523 533
0.161 0.428 0.836 0.965
0.117 0.103 0.116 0.124
-l--
the other hand, Dry et al. (1969) studied the chemisorption of CO and C 0 2 on promoted Fe catalyst. They found comparable heats of chemisorption for CO and COz. It is therefore assumed that competitive chemisorption of both CO and C 0 2 occurs on the active sites of the catalyst Kco co + s = s-co
(18)
Kco*
(19)
The enol complex theory postulates that the hydrogenation of chemisorbed CO is the rate-limiting step of the FTS
Hence, the rate law for syngas consumption is given by =
(21)
kCH$CO
Assuming Langmuir behavior for CO and C 0 2 ,it follows from eq 18 and 19
Neglecting the 1 in the denominator of eq 22 and substituting for Bco in eq 21 leads to KCOCH2CC0 -rCO+H,
=
KCOCCO
-rCO+H2
'
+ KCOzCCOz
(23)
- -1 + 1 KCOz KCO
cCO~ cCO
(24)
Therefore, a plot of CH2/(-rCO+H ) vs. C c o , / C C o should be a straight line. Figure 4 shows tkat the data measured in this study can well be approximated by straight lines. The rate constants and the ratios of the adsorption constants of COPand CO can be calculated from the intercepts and slopes. These data are given in Table 111. Figure 5 presents an Arrhenius diagram of the rate constants. An
'
I
'
1.9
"
220
240*C
250T '
'
'
'
*
'
1.95
'
'
'
2.0
'01
I T ,K-'
Figure 5. Arrhenius diagram for rate constants evaluated under consideration of C 0 2 inhibition.
activation energy of 103 kJ/mol is obtained therefrom. This value is in the expected range for the FTS. No trend can be discerned for the ratios of the adsorption constants determined at different temperatures. Therefore, it is assumed that Kco,/Kco is independent of temperature. A mean value of 0.115 is obtained from the data given in Table 111. The result that Kco,/Kco does not significantly depend on temperature is in accordance with the findings of Dry et al. (1969) who observed almost the same heats of chemisorption for CO and COz on Fe catalysts promoted with potassium. For the catalyst used in this study, the rate law is given by = 1.4 x 1010e-12407/~
CH2
(25) Go2 1 + 0.115cco For checking the applicability of eq 25, syngas conversions were calculated from this rate law. They are compared with the experimental ones in Figure 6. Agreement is fair. Influence of C 0 2 Inhibition on Reactor Performance The above results demonstrate that when using FTS catalysts having simultaneously a high activity for the WGS reaction, an inhibition caused by C 0 2chemisorption can be observed. This is important in evaluating rate data. The ratio of actual rate involving COz inhibition to a first-order rate law in H2 is given by -rCO+Hz
Rearrangements of eq 23 lead to the linearized relation
--CH2
'
I. 85
cot + s rs-co2
-rCO+H2
2W'C -2
No. 4, 1985 1047
Ind. Eng. Chem. Process Des. Dev., Vol. 24,
-rco+H,(actual) -rco+H,(first order)
-
1 KCO, CCO,
I Fe I K
(26) Xco.n2 calc.
1+--
Kco cco
%%/
/ 0
This ratio may become low, particularly, at high conversions of CO. For estimating the effect of C 0 2 chemisorption, let us consider a well-mixed reactor without mass tranasfer limitations. Then the overall mass balance can be written as
I
0'5
d 1
0 0
0.5
Kco cco Assuming that all water produced by the FTS is converted by the WGS reaction the following relation holds
%Eo, = f/2(%Y80 - %YEO) = f/Z%Y8OXCO
1
Xco.n2
1+--
Figure 6. Parity plot of measured and calculated conversions. I .o
(28)
With consideration of Henry's law, introduction of eq 28 into eq 27 gives
exp
0a 0.05 0.10
-
0.50 0.8
1.0
A .
Xco.n2 0.6
I
0.6
:
U i 0.53
0.4
with
a: - 2 1 3
0.2
"." 0
IO
5
20
15
Substituting
Da
Figure 7. Effect of C 0 2 inhibition on syngas conversion in completely mixed reactor (calculated from eq 38). X:oi
and 08 xco.., 06
and considering
-
U
:0.53
a s- z / 3
owIO 0 53 I
-
1 50
-
-
2w-
leads in eq 29 to
0
2
I
3
I
5
Do
(34)
with
(35) By converting X H , and Xco to XCO+H, using the relations 1+1 (36) xCO = X C O + H 2 1 + ~
(37) one obtains from eq 34
+ "XCO+H,)(1 + v)(1 + n X I(1+ v)- X C O + H z U ( l + I) 1 + u - XCO+H,(1 + 1)(1- 1) = Da (38) 1+ u - XCO+Hz(l + n
XCO+H,(1
Figure 7 shows a plot of XCO+H, vs. Da for various values of A. I, U , and a have been set constant. A marked effect
Figure 8. Effect of H2-to-C0 inlet ratio on conversion for a given value of A.
of the inhibition of COz characterized by h can be discerned. Already at low values of h, the influence is significant if the desired conversion is high. For the catalyst used in this study, h is about 0.1. For this value, the Damkohler number must increase from 5.75 to 14.5 in order to maintain a syngas conversion of 0.9. The inhibition by COz can be reduced to some extent by increasing the H,-to-CO inlet ratio as shown in Figure 8. Thus, the H, concentration is increased while C02 concentration is decreased. However, the positive effect on reactor performance by increasing I can only be observed for low values of XCO+H, and Da, respectively. The reason for this is that Xco runs to 1;hence the limiting syngas conversion follows from eq 36 lim
=
1+u
(39) 1+1 Therefore, a compromise has to be found between the inhibiting action of CO, and the limiting conversion. As xco-1
XCO+H,
~
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Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
a rule of thumb, one can conclude that for a certain usage ratio, a catalyst is adopted to best results; i.e., high conversions can be obtained if the inlet ratio is slightly larger than U. Proposal of Generalized Kinetic Model Huff and Satterfield (1984) clearly demonstrated that chemisorption of water drastically reduces to the rate of the FTS in the slurry phase. In accompanying investigations to this study, the FTS on an unpromoted precipitated Fe catalyst and a fused Fe catalyst promoted among others with KzO was studied in the slurry phase (Nettelhoff et al., 1984). It was found that the unpromoted catalyst revealed relatively little WGS activity. Hence, significant inhibition by HzOproduced in the FTS could be observed; on the other hand, the promoted fused Fe catalyst had a high WGS activity, and similar to the catalyst used in this study, COz inhibition had to be considered to describe the rate data. Therefore, any mechanism of the FTS should not only consider chemisorption of CO and HzO but also that of COz. If one assumes that in accordance with the enol complex theory eq 20 presents the rate-limiting step of the FTS, it follows for the syngas consumption rate with consideration of chemisorption of CO, HzO, and COz
kCH, -rCO+H2
=
(40) CH20
KC02 c C O ~
Kco cco
Kco cco
KH20
1+--+-----
As reported by Huff and Satterfield (1984), the heat of chemisorption of water is high compared to that of CO and C 0 2 . Hence, KHZo/Kcowill be strongly temperature-dependent, while temperature dependency of Kco2/Kcois almost negligible. Furthermore, the adsorption constant of water is large compared to that of COz, K H ~>> O &02. In the slurry phase, the inhibiting action of product water is yet augmented by the fact that water has an higher solubility than CO. Thus, the inhibition of the reaction rate by water is very pronounced and easily conceals an inhibition due to C 0 2 . This may explain why only Brotz and Rottig (1952) reported on a negative effect of "oxidizing" gases (COz + HzO). An inhibition by COz becomes significant, however, if the catalyst applied has a high activity for the WGS reaction. In this case, the water produced in the synthesis reaction will be almost completely converted under COz formation. In accordance with the results reported by Kolbel and Ralek (1977), Dry et al. (1969), and Arakawa and Bell (1983),the high WGS activity of the catalyst used in this study has to be attributed to its promotion with K. Also Huff and Satterfield (1984) used a fused Fe catalyst promoted with K which obviously had a high WGS activity (Satterfield and Huff, 1982). However, when evaluating their kinetic data, these authors do not report about any effect of COP It appears thinkable that neglect of the COz chemisorption might explain why Huff and Satterfield (1984) obtained a better description of their experimental data on the basis of eq 2 instead of an extension of eq 1. Summary Syngas consumption rates of the FTS were measured in a stirred autoclave employing a precipitated Fe catalyst promoted with K. The catalyst revealed a high WGS activity. Thus, almost all the water produced by the FTS has been converted under COz formation. Therefore, inhibition of the rate by competitive chemisorption of water could be neglected. In spite of this, the rate did not follow first-order behavior in Hz as expected. Instead, a significant inhibition by COz chemisorption was postulated, on the basis of which a fair description of the experimental
data could be achieved. The occurrence of COz inhibition can largely influence FTS performance in the slurry phase. Acknowledgment We acknowledge financial support from Deutsche Forschungsgemeinschaft. S.L. thanks Alexander von Humboldt-Foundation for granting a research fellowship. Thanks are also due to Schering AG, Bergkamen, FRG, for supplying the catalyst. Nomenclature C = concentration in slurry phase, mol/cm3 Da = Damkohler number, defined by eq 35 E = H,-to-CO exit ratio FTS = Fischer-Tropsch synthesis He = Henry constant, Pa cm3/mol I = H,-to-CO inlet ratio k = rate constant, cm3/(g of catalyst s) k , = first-order rate constant, cm3/(g of catalyst s) K = adsorption (chemisorption) constant Ks = equilibrium constant of WGS reaction mcat= mass of catalyst, g n = stoichiometric coefficient, refer to eq 4 N = mole flow rate P = total pressure, Pa -r'CO+H, = first-order rate for syngas consumption, mol/ (cm3 S) -rCO+H2
= syngas consumption rate, mol/(g of catalyst s) R = universal gas constant, Pa cm3/(mol K) T = temperature, K U = H,-to-CO usage ratio V = volumetric flow rate, cm3/s V,, = volume of slurry, phase, cm3 WGS = water gas shift X = conversion y = mole fraction in gas phase z = fraction of water converted by WGS reaction
Greek Symbols a = volume contraction factor X = inhibition parameter, defined by eq 30 0 = fractional surface coverage
Subscripts CO = carbon monoxide CO, = carbon dioxide G = gas phase
H2 = hydrogen HzO = water Nz= nitrogen Superscripts 0 = reactor inlet 1 = reactor outlet Registry No. CO, 630-08-0; COP,124-38-9; Fe, 7439-89-6; K, 7440-09-7.
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Received for review April 16, 1984 Revised manuscript received December 17, 1984 Accepted January 21, 1985
Equilibrium Vaporization of a Coal Liquid from a Kentucky No. 9 Coal Ho-Mu Lln, Hwayong Kim, Tianmln Guo,+ and Kwang-Chu Chao' School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907
Vapor-liquid equilibrium (VLE) has been experimentally determined for a coal liquid obtained from a Kentucky No. 9 coal at the Wilsonville plant of Catalytic, Inc. The measurements, including VLE of the coal liquid in mixtures with hydrogen, were performed at temperatures up to 7 10 K and pressures to 25 MPa. Inspections are reported for the normal bolllng point, molecular weight, and specific gravity of the vaporized overhead and bottom fractions as well as for the true boiling point (TBP) fractions of the total coal liquid. Experimental data are correlated with several equations of state.
Knowledge of vapor-liquid equilibrium (VLE) of coal liquid by itself and in mixture with hydrogen is needed in the technology of fuels and oils. Experimental measurements of VLE of mixtures of hydrogen and model compounds have been reported by Simnick, Sebastian, Lin, Kim, and co-workers. The literature sources are summed up in Sebastian et al. (1981a-c) and Radosz et al. (1982). These data have been used to develop and test correlation methods (El-Twaty and Prausnitz, 1980; Sebastian et al., 1981a-c; Wilson et al., 1981; Radosz et al., 1982; Watanasiri et al., 1982; Gray et al., 1983). Experimental VLE data on coal liquids are scarce and limited to reports by Henry (1980), Lin et al. (1981), Sung (1981), and Wilson et al. (1981). We report here experimental results of VLE for a coal liquid and its mixtures with compressed hydrogen at elevated temperatures. The coal liquid studied in this work was supplied by Catalytic, Inc., from processing of a Kentucky No. 9 Fies mine coal at the Advanced Coal Liquefaction R&D Facility in Wilsonville, AL. The process is a combination of three units: (1) a solvent-refined coal (SRC) unit in which a slurry of coal and recycled solvent is reacted with hydrogen at elevated temperature and pressure; (2) a critical solvent deashing unit; and (3) an H-oil ebulated bed hydrotreater to raise the hydrogen content of the oil. The coal liquid was labeled sample No. 74-534 from run No. 234. A detailed description of the process, operating conditions, and the physical properties of this oil was reported by Lewis (1981). The VLE measurements were made in a flow apparatus with sapphire windows for visual observation of the liquid level. All condensate samples from cell effluents were collected and inspected for the boiling point, molecular weight, and density. The coal liquid was fractionated by Graduate Division, East China Institute of Petroleum Technology, Beijing, China. 0 196-430518511124-1049$01.50/0
distillation under vacuum and the resulting fractions were inspected. The experimental VLE data are correlated with the Cubic Chain-of-Rotators (CCOR) equation of state (Kim et al., 1983), the Grayson-Streed correlation (1963), the Soave equation (1972), and the modified Soave equation of Radosz et al. (1982). Experimental Section A flow apparatus was used in this work to measure VLE while minimizing thermal decomposition of the coal liquid at high temperature. The heart of the apparatus is a new equilibrium cell equipped with transparent sapphire windows to permit visual observation of the liquid level in the cell. The old cell described by Simnick and co-workers (1977) which was used extensively in this laboratory proved useless for coal liquids. Detection of the liquid level in the old cell was by means of an electric capacitor which did not function for coal liquids because of their high electric conductivity at the higher temperatures of interest. The new cell and apparatus have been described by Lin and co-workers (1985). The residence time of the coal liquid in the equilibrium cell depends on the volume of liquid maintained in the cell and on the flow rate. At normal operating conditions, there is a holdup of about 5 mL of liquid in the cell. The normal flow rate of the liquid feed is 500-1000 mL/h, giving a plug flow residence time of approximately 18-36 s. Condensates from the cell effluents were collected for inspection of the boiling point (Tb), molecular weight (MW), and density ( p ) at 298.2 K by procedures that are commonly used for heavy petroleum fractions. Normal boiling point was determined by simulated distillation on a Varian 3700 gas chromatograph with a flame ionization detector. The calibration was prepared from 50 model coal liquid compounds, and the Tb of an oil fraction was calculated as the area average from the gas chromatograph. A Mechrolab 301A vapor pressure osmometer was used to 0 1985 American Chemical Society
