Ind. Eng. Chem. Res. 1989,28, 5-9 NH, t AIR
TEMP. NU.
B U R N E D HOLE D U E TO EXCESSIVE GAUZE TEMPERATURE
]
TEMP.
+ AIR
dNz
OXIDATION G A U Z E
N.0
1 NO
Figure 10. Ignition delay in the oxidation pack.
during start-up. In addition, Barelko et al. (1978) analyzed the speed for the propagation of ignition heat waves on Pt wires during the oxidation of ammonia. Accordingly, considerable ammonia bypass through the catalyst gauze can occur during the ignition period. I t is likely that bypass ammonia can react over Pd with the NO, produced by oxidation of ammonia at the gauze site heated by the hydrogen torch. The transient reaction of ammonia and nitrogen oxides is strongly catalyzed by palladium, resulting in highly exothermic reactions exceeding adiabatic equilibrium temperatures, leading to melting of the Pd recovery gauze. This has been observed at ammonia concentrations as low as 7.5% where the adiabatic temperature is far below that required to melt the gauze. This model is exhibited in Figure 10. Process and compositional modifications to improve the resistance of the getter gauze to deformation have been proposed by Lee (1986) and Farrauto et al. (1986). It should be noted, however, that the gauze surface temperature approaches the adiabatic equilibrium temperature, under heat-transfer-limiting conditions, which is substantially higher than the bulk gas temperature. Thus, significant temperature gradients between the catalyst surface and bulk gas stream exist, and any local variations in NH,/air can cause catalyst temperature flickering. Furthermore, in the case of overfueling beyond the stoichiometric ratio (14.38%),a temperature excursion at the catalyst surface is quite possible, causing catalyst
5
deformation by exceeding the adiabatic equilibrium temperature due to the limitations of the heat release rate from the catalyst surface to the bulk gas stream. The worst case for excessive a”onia/air ratio is often caused by liquid ammonia entrainment to a catalyst bed. But under those circumstances, both the oxidation and the recovery gauze are observed to melt. Hence, overfueling cannot explain the melting phenomena discussed in this paper. Registry No. NH3, 7664-41-7; Pt, 7440-06-4; Pd, 7440-05-3; NO, 10102-43-9.
Literature Cited Barelko, V.; Kurochka, I. I.; Merzhanov, A. G.; Shkadinsii, K. G. “Investigation of Travelling Waves on Catalytic Wires”. Chem. Eng. Sci. 1978, 33, 805-811. Chilton, T. H. “The Manufacture of Nitric Acid by the Oxidation of Ammonia”. Chem. Eng. Prog. Monogr. Ser. 1960,3, 56. Edwards, W. M.; Worley, F. L., Jr.; Luss, D. “Temperature Fluctuations of Catalytic Wires and Gauzes-11. Experimental Study of Butane Oxidation on Platinum Wires”. Chem. Eng. Sci. 1973, 28, 1479-1491. Ervin, M. A,; Luss, D. “TemperatureFluctuations of Catalytic Wires and Gauzes-I. Theoretical Investigation”. Chem. Eng. Sci. 1972, 27, 339-346. Farrauto, R. J.; Lee, H. C.; Hatfield, W. R. “Low Temperature Light-off Ammonia Oxidation”. Filed for US Patent Application No. 06887578, 1986. Hatfield, W. R.; Heck, R. M.; Hsiung, T. “Method for Recovering Platinum in a Nitric Acid Plant”. US Patent 4412859, 1983. Heck, R. M.; Bonacci, J.; Hsiung, T. “A New Research Pilot Plant Unit for Ammonia Oxidation Processes and Some Gauze Data Comparisons for Nitric Acid Process”. Znd. Eng. Chem. Process Des. Deu. 1982, 21(1), 73-79. Hegedus, L. L. “Temperature Excursions in Catalytic Monoliths”. AZChE J . 1975,21(5), 849-853. Hiam, L.; Wise, H.; Chaikin, S. “Catalytic Oxidation of Hydrocarbons on Platinum”. J. Catal. 1968,10, 272-276. Holtzman, H. “Platinum Recovery in Ammonia Oxidation Plant”. Platinum Met. Rev. 1969, 13, 2-8. Lee, H. C. “Platinum Recovery using Perforation Resistant Gauzes”. Filed for U.K. Patent Application No. 86 306 362.4, 1986. Pignet, T.; Schmidt, L. D. “Kinetics of NH3 Oxidation on Pt, Rh and Pd”. J. Catal. 1975, 40, 212-225. Tamman, G. Z. 2.Anorg. Allg. Chem. 1920, 111, 90.
Received for review May 6, 1988 Accepted September 6, 1988
Kinetics of Absorption of Carbon Monoxide in Aqueous Solutions of Sodium Hydroxide and Aqueous Calcium Hydroxide Slurries Anand V. Patwardhan and Man Mohan Sharma* Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India
The kinetics of absorption of carbon monoxide in aqueous sodium hydroxide solutions and aqueous calcium hydroxide slurries was studied in a stirred autoclave with a plane gas-liquid interface, in the temperature range 100-160 “C. The partial pressure of carbon monoxide was varied from 10 to 100 atm. In the case of aqueous sodium hydroxide solutions, the reaction was found to occur in the diffusion film and was first order in carbon monoxide; the reaction was also found to be first order in hydroxyl ion concentration. The values of the second-order rate constant were found to be in the range 3-141 m3/(kmol.s). In the case of calcium hydroxide slurries, the values of the second-order rate constant were found to be in the range 2-60 m3/(kmol-s). The absorption of carbon monoxide in aqueous solutions of sodium hydroxide and aqueous slurries of calcium hydroxide is relevant for the manufacture of sodium or calcium formate/formic acid.
Previous Studies Khalifa (1965) has suggested a mechanism for the for0888-5885/89/2628-0005$01.50/0
mation of formate ion from carbon monoxide and hydroxyl ion. The mechanism involves attack on the carbon atom of the carbon monoxide molecule by a lone pair of electrons of the hydroxyl group. The complex which is formed is of a resonating type and rearranges to give formate ion. Carbon monoxide in gases available from a variety of sources, which do not contain carbon dioxide, has been 0 1989 American Chemical Society
-
6 Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989 Htj:
+
:C=O
- ...._
HEC=G:
-__ ......- - .... CG:=HO+ --- .... Cc:=o:
HOCO:
+
HOi)'fi:
HC
-
- CHCf=:G (formate ton)
claimed to be utilized for the production of sodium formate by absorption either in pure aqueous sodium hydroxide solutions or in aqueous sodium hydroxide solutions containing some additives like methanol (Taguchi et al., 1973; Dorfman et al., 1974; Unni, 1974; Awane et al., 1975; Procek and Stolka, 1978; Mel'nikov et al., 1981; Venkateswarlu, 1985). Sodium formate has been claimed to be prepared in quantitative yield by reacting carbon monoxide with aqueous sodium hydroxide solutions containing 10-40% (w/w) methanol at 90-110 "C a t 10-30 atm of pressure (Ostertay et al., 1978). Sirotkin (1953) has studied the effect of partial pressure of carbon monoxide on the rate of reaction of carbon monoxide with sodium hydroxide. From the foregoing, it is clear that very limited data are available on the absorption of carbon monoxide in aqueous solutions of sodium hydroxide. Further, there are no data available on the absorption of carbon monoxide in aqueous slurries of calcium hydroxide. This work was, therefore, undertaken to study the kinetics of absorption of carbon monoxide in aqueous solutions of sodium hydroxide and aqueous slurries of calcium hydroxide. Materials and Methods of Analysis. A pure carbon monoxide gas cylinder was obtained from BOC Ltd., U.K. The purity of the gas was stated to be 99.9%. Sodium hydroxide pellets were obtained from S.D. fine-chem Pvt. Ltd., Boisar, India, and Polypharm Ltd., Bombay, India, and were of AR grade. Calcium hydroxide powder was obtained from S.D. fie-chem Pvt. Ltd., Boisar, India, and its purity was found to be 95% (w/w). The particle size of calcium hydroxide in aqueous slurries was measured by an optical microscope and found to be -40 pm (4 x m). All the reagents used in various analyses were of AR grade. The concentration of formate ions in the product was determined by a redox iodometric titration (Encyclopedia of Industrial Chemical Analysis, 1971).
Experimental Apparatus and Procedure All experiments were conducted in a stainless steel 316 autoclave of 0.101-m i.d., with a plane gas-liquid interface. The autoclave was provided with a 0.05-m-diameter, four-bladed turbine agitator. The volume of liquid used was 4 X lo4 m3 in the case of aqueous sodium hydroxide solutions and 5 X m3 in the case of aqueous calcium hydroxide slurries. The stirring speed was varied from 2.083 to 3.833 rev/s. The effective interfacial area of the stirred autoclave was 7.854 X m2. It was ascertained from cold models that calcium hydroxide particles did not settle and the gas-liquid interface was plane at the specified stirring speeds. The liquid (or slurry) was fed in the reactor and was heated to the desired reaction temperature in the presence of air. Carbon monoxide was then introduced into the reactor through the gas inlet valve, and air was flushed out; the partial pressure of carbon monoxide was adjusted to the desired value by taking into account the vapor pressure of the liquid in the reactor. The temperature was controlled within f l "C throughout the reaction period with the help of a temperature controller. A t the end of an experiment, stirring was stopped, ice cold water was circulated through the cooling coil, and after the temperature of the liquid (or slurry) reached around 25 "C, carbon monoxide was released from the reactor. The reactor was then opened, the contents were sparged with nitrogen to
remove any dissolved carbon monoxide, and then samples were taken. In the case of aqueous sodium hydroxide solutions, the samples taken were of 2.5 X lo4 mm3 size, and the specific rates of absorption were based on the concentration of sodium formate formed. The fall in concentration of sodium hydroxide was also determined, whenever this change was substantial for analytical purposes. In the case of aqueous calcium hydroxide slurries, the particles of calcium hydroxide were filtered, and samples of 2.5 X lo4 mm3 size of the clear liquid were taken. The specific rates of absorption were based on the concentration of calcium formate formed. The contribution to the overall rate during the heating period was negligible, as no carbon monoxide was present. Further, the period of flushing was too small (of the order of 60 s) in relation to a typical batch time of 1 h. The rate of reaction during the cooling period was also expected to be negligible, as the temperature could be brought down by 40-50 "C within 60 s. Thus, in view of the long batch times, the procedure adopted for measuring rates of reaction is sound.
Physicochemical Data Solubility of Carbon Monoxide in Aqueous Solutions. The solubility data for carbon monoxide in water from 0 to 100 "C have been reported by Lange (1985). These data were used for extrapolation to higher temperatures by fitting the following type of equation (Reid et al., 1977): In H , = CI/T + C2 (1) where C, = 1099 and Cz = -10.65. The solubility of carbon monoxide in electrolyte solutions was corrected for the electrolyte concentration (Danckwerts, 1970). In the range of pressure employed in this work, Henry's law holds. Solubility of Calcium Hydroxide in Water. The solubility data for calcium hydroxide in water from 0 to 100 "C have been reported by Lange (1985). These data were used for extrapolation to higher temperatures by fitting the following type of equation (Reid et al., 1977): In [B,] = C 3 / T + C, (2) where C3 = 955.1 and C, = 7.057. Diffusivity of Carbon Monoxide in Aqueous Solutions. The diffusivity of dissolved carbon monoxide in water, a t various temperatures, is available from the literature (Wise and Houghton, 1968). The value of diffusivity in aqueous solutions of sodium hydroxide was estimated by multiplying the value' given by Wise and Houghton by the ratio viscosity of water at 30 "C/viscosity of NaOH at 30 "C. This involves an assumption that the ratio of viscosities a t 30 "C is the same as that at higher temperatures. This ratio at 30 "C is -0.1 for 10 M aqueous sodium hydroxide solution. The values of diffusivity in aqueous slurries of calcium hydroxide were assumed to be the same as that in water a t the same temperature, as the concentration of hydroxyl ions was relakmol/m3). tively low (C7.8 X Physical Mass-Transfer Coefficient. Values of the physical mass-transfer coefficient, kL, are needed to check the conditions for different controlling regimes and also they may be required for correlating the specific rates of absorption with the pertinent variables (Doraiswamy and Sharma, 1984). The kL values at different stirring specs.. were obtained from the earlier work in this laborator:; (Yadav and Sharma, 1981), for essentially the same configuration of the contactor. The system used was carbon dioxide-water at 30 "C and the kL values were in the range
Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989 7 Table I. Carbon Monoxide Absorption in Aqueous Solutions of Sodium Hydroxide in a Stirred Autoclave with Plane Gas-Liquid Interface 107~*,
duration of expt, h
temp,
[Boli, M
k2,m3/(kmol-s)
160 160 160
2.5 2.5 2.5
p A , atm 100 100 100
stirring speed, rev/s
0.5 0.5 0.5 9 9 9 9
100 120 140 160
10 10 10 10
100 100 100 100
3.833 3.833 3.833 3.833
9 1
160 160
7.5 5.0
100 100
3.833 3.833
60.2 174
141 149
1 0.5 0.5 0.5
160 160 160 160
2.5 2.5 2.5 2.5
20 40 60 80
3.833 3.833 3.833 3.833
81.6 158 233 304
145 136 140 144
O C
(3.4-5.95) X m/s in the range of stirring speeds 2.5-3.833 rev/s. These kL values were corrected by multiplying by the factor (DA in solution at higher temperature/DA in water at 30 0C)1/2.
Results and Discussion Absorption of Carbon Monoxide in Aqueous Solutions of Sodium Hydroxide. The specific rate of absorption of carbon monoxide at 100 atm of partial pressure and 160 "C in 2.5 M aqueous sodium hydroxide solution was found to be independent of the hydrodynamic factors in the range of stirring speed from 2.083 to 3.833 rev/s; the maximum variation was 3.5% (Table I). The estimated values of (RA/kL[A*])were in the range 30.9-17.6. Table I also gives the values of the specific rate of absorption of carbon monoxide at 100 atm of partial pressure in 10 M aqueous sodium hydroxide solution at 100, 120, 140, and 160 "C; values of the specific rate of absorption of carbon monoxide at 100 atm of partial pressure at 160 "C in 7.5 and 5 M aqueous sodium hydroxide solutions; and values of the specific rate of absorption of carbon monoxide in 2.5 M aqueous sodium hydroxide solution at 160 "C at partial pressures ranging from 20 to 80 atm. From Table I, it can be seen that the specific rate of absorption at 160 "C decreased as the concentration of aqueous sodium hydroxide solution increased from 2.5 to 10 M, at a constant stirring speed of 3.833 rev/s. This is due to the reduction in the solubility of carbon monoxide with an increase in the concentration of sodium hydroxide and reduction in diffusivity of the dissolved gas with an increase in the viscosity of the solution. However, with the corrections applied for the reduction in the solubility, the specific rate was found to be proportional to - (t~O~iean)"~*
The specific rate of absorption at 160 "C in 2.5 M aqueoussodium hydroxide solutions was found to be directly proportional to the partial pressure of carbon monoxide (Figure 1). Thus, the dissolved gas was found to undergo a pseudo-first-order reaction. The specific rate of absorption, RA (kmol/(m2-s)),is then given as (Doraiswamy and Sharma, 1984) RA = [A*](D,k)'J2 (3) The condition to be satisfied is
to
kmol/ (m2.s)
But since the specific rate was found to be proportional eq 3 can be written as (5)
2.083 3.000 3.833
121 132 138
348 354 361 2.94 5.94 12.1 21.0
~
10
9
20
0t m .
3.29 12.2 47.0 135
-
30
LO
50
I
d
2.5M Aq~leouS MOH a 5% ( w / w ) Aqueous
0
'0
I
I
I
I
I
20
LO
60
80
100
PA,
otm.
-c
Figure 1. Effect of partial pressure of carbon monoxide on specific rate of absorption.
The absorption of carbon monoxide in aqueous sodium hydroxide solutions, therefore, appears to conform to the fast pseudo-fiist-order reaction regime. The average values of the second-order rate constant, k2,were estimated as 3.29, 12.2,47.0, and 141 m3/(kmol-s)at 100, 120, 140, and 160 "C, respectively (Figure 2). The standard deviation of the k2 values at 160 "C was estimated as 6.2. To assess the effect of ionic strength of solution on the second-order rate constant of the reaction, experiments were conducted at a total ionic strength of 7.5 kion/m3 and sodium hydroxide concentration of 2.5 and 5 M, at 160 "C with a carbon monoxide partial pressure of 100 atm. The ionic strength of the solution was adjusted by the addition of appropriate quantities of sodium iodide. The values of the second-order rate constant, k2,were estimated as 135 and 145 m3/(kmol.s) for 2.5 and 5 M sodium hydroxide solutions, respectively. These values compare very well with those obtained without the addition of sodium iodide, under otherwise the same conditions. This shows that there is no effect of ionic strength on the second-orderrate constant of the reaction, in the range of concentration studied in this work. The value of activation energy was
8 Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989 Table 11. Carbon Monoxide Absorption in Aqueous Slurries of Calcium Hydroxide in a Stirred Autoclave with Plane Gas-Liquid Interface 106RA, stirring kmol/ duration of expt, h temp, "C PA, atm W , (w/wj % speed, rev/s (m%j RA/(k~[A*I) k2,m3/(kmo14 2.11 1.03 1.05 2.083 1.5 100 30 5 2.35 1.02 1.47 3.000 5 100 30 1.5 2.40 1.01 1.79 3.833 5 30 100 1.5 1.09 6.50 1.11 2.083 5 30 120 1.5 7.07 1.05 1.52 3.000 5 30 120 1.5 7.89 1.04 1.84 5 3.833 30 120 1.5 21.9 1.26 1.29 2.083 5 30 1 140 1.15 23.0 1.66 5 3.000 30 140 1.5 23.5 1.96 1.10 3.833 5 30 140 1 1.28 69.7 2.26 5 3.833 30 1 160 1.24 59.1 2.19 3.833 10 30 1 160 64.6 2.23 1.25 3.833 15 30 1 160 1.92 1.32 56.1 3.000 5 30 1 160 1.29 50.5 1.87 3.000 30 10 1 160 58.3 1.93 1.33 3.000 15 30 1 160 1.61 59.1 1.65 2.083 30 5 1 160 1.56 53.0 1.59 2.083 5 30 1 160 60.0 1.65 1.61 2.083 10 30 1 160 53.1 1.56 1.59 2.083 15 30 160 1 72.3 1.28 2.28 3.833 10 30 1 160 62.7 1.23 0.72 5 3.833 10 160 2 61.8 1.24 1.47 3.833 5 20 1 160 58.7 1.23 2.92 3.833 5 40 160 0.5
found to be independent of solid loading in the range 5-1570 (w/w), 51.40-163.4 kg of solid/m3 of slurry, at 160 "C. In view of the relatively very low concentrations of hydroxyl ions, the condition given by expression 4 is not satisfied and hence the reaction conforms to the intermediate regime between slow and fast reaction (Doraiswamy and Sharma, 1984). The following equation should hold:
Here, the specific rate of absorption depends on the hydrodynamics of the system. The specific rate of absorption of carbon monoxide in aqueous calcium hydroxide slurries can be correlated by eq 6. The specific rate of absorption at 160 "C, a t a stirring speed of 3.833 rev/s, was found to be directly proportional to the partial pressure of carbon monoxide in the range 10-40 atm (Figure 1). The average values of the secondorder rate constant, k2,were estimated as 2.28,7.15, 22.8, and 59.6 m3/(kmol.s) at 100, 120, 140, and 160 "C, respectively (Figure 2). The corresponding standard deviations were estimated as 0.13,0.57,0.67,and 5.9. The value of the activation energy was found to be 7.36 X lo4 kJ/ kmol, which compares reasonably well with that found for sodium hydroxide solution. The k , values for calcium hydroxide are lower than the corresponding values for sodium hydroxide. This difference may be attributed to the large difference in the ionic strength in two cases. Figure 2. Effect of temperature on the second-order rate constants for the reactions CO + NaOH .-c HCOONa and CO + 1/2Ca(OH)2 1/2(HC00)2Ca.
-
found to be 8.57 X lo4 kJ/kmol. Absorption of Carbon Monoxide in Aqueous Slurries of Calcium Hydroxide. Table I1 shows the values of the specific rate of absorption of carbon monoxide in aqueous slurries of calcium hydroxide at various temperatures ranging from 100 to 160 "C. The specific rate was found to depend on hydrodynamic factors at all the temperatures and solid loadings. Further, the specific rate was
Conclusions (1) The absorption of carbon monoxide in aqueous sodium hydroxide solutions is accompanied by fast reaction in the diffusion film. (2) The reaction is first order in sodium hydroxide concentration. (3) The absorption of carbon monoxide in aqueous slurries of calcium hydroxide falls in the transition regime between slow and fast reaction where only part of the reaction occurs in the diffusion film.
Ind. Eng. Chem. Res. 1989, 28, 9-12
Literature Cited
Acknowledgment A.V.P. is thankful to the University Grants Commission, New Delhi, for the award of Research Fellowship.
Nomenclature [A*] = concentration of solute gas A at the gas-liquid interface, kmol/m3 [B0li= concentration of liquid-phase reactant B at the beginning of the experiment, kmol/m3 [B0lf= concentration of liquid phase reactant B at the end of the experiment, kmol/m3 [Bolmean = (([BOI~’/~ + [BoI~‘/~)/~)~ [B,] = saturation concentration of solid reactant in the liquid phase, kmol/m3 C1, C2 = constants in eq 1 CB,C4 = constants in eq 2 DA = diffusivity of dissolved gas in the liquid phase, m2/s DB = diffusivity of liquid-phase reactant, m2/s H , = Henry’s constant for carbon monoxide-water system, kmol/ (m3-atm) k = pseudo-first-order reaction rate constant, s-l k2 = rate constant for second-order reaction, m3/(kmol.s) kL = physical mass-transfer coefficient on liquid side, m/s pA= partial pressure of carbon monoxide, atm RA = specific rate of absorption of gas A, kmol/(m2.s) T = absolute temperature, K W = solid loading, 70 (w/w) 2 = number of moles of the liquid-phase reactant reacting with 1 mol of dissolved gas Greek Symbol p
9
Awane, Y.; Otsuka, S.; Nagata, M.; Tanaka, F. Ger. Offen. 2 436 979, 1975; Chem. Abstr. l975,82,155380r. Danckwerts, P. V. Gas-Liquid Reactions; McGraw-Hill: New York, 1970; Chapter 1. Doraiswamy, L. K.; Sharma, M. M. Heterogeneous Reactions; Wiley: New York, 1984; Vol. 11. Dorfman, E. Ya.; Kunin, A. M.; Gluzman, S. S.; Faingol’d, N. I. Khim. Prom. (Moscow) 1974, 6, 424; Chem. Abstr. 1974, 81, 49220h. Encyclopedia of Industrial Chemical Analysis; Snell, F. D., Ettre, L. S., Eds.; Interscience: New York, 1971; Vol. XIII, p 135. Khalifa, M. J.Pharm. Sci. U.A. R. 1965,6,133; Chem. Abstr. 1968, 68, 1 2 0 6 9 ~ . Lange, T. Lunge’s Handbook of Chemistry, 13th ed.; Dean, J. A., Ed.; McGraw-Hill: New York, 1985; Chapter 10. Mel’nikov, K. A,; Rogoznyi, V. V.; Karmazina, T. P.; Sergienko, I. D.; Pushkin, A. G. U.S.S.R. 810663,1981; Chem. Abstr. 1981,95, 42382t. Ostertay, W.; Wunsch, G.; Kiener, V.; Hetzel, E.; Schreiner, S.; Leutner, B.; Schlimper, H. v.; Voelkl, E. Ger. Offen. 2716032,1978; Chem. Abstr. 1979,W 170937m. Procek, E.; Stolka, A. Pol. 99851, 1978; Chem. Abstr. 1979, 91, 140352~. Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. The Properties of Gases and Liquids, 3rd ed.; McGraw-Hill: New York, 1977. Sirotkin, G. D. Zh. Priklad. Khim. 1953,26,340; Chem. Abstr. 1954, 48, 6085c. Taguchi, I.; Oya, K.; Tanaka, F. Japan. Kokai 73 75 509,1973; Chem. Abstr. 1974, 80, 47464~. Unni, E. B. Indian Chem. Manuf., Annu. Number 1974, 13, 67; Chem. Abstr. 1977,86, 16265b. Venkateswarlu, Y. Indian IN 156050,1985; Chem. Abstr. 1986,105, 135907h. Wise, D. L.; Houghton, G. Chem. Eng. Sci. 1968,23, 1211. Yadav, G. D.; Sharma, M. M. Chem. Eng. Sci. 1981,36, 599.
= viscosity of the reacting solution, P
Received for review March 14, 1988 Revised manuscript receiued July 29, 1988 Accepted August 29, 1988
Registry No. CO, 630-08-0; NaOH, 1310-73-2; Ca(OH)*, 1305-62-0.
Effect of Carbon Dioxide on the Kinetics of the Fischer-Tropsch Synthesis on Iron Catalysts Ian C. Yates and Charles N. Satterfield* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
A recently proposed kinetic expression applicable to feed ratios H 2 / C 0 of 0.8 or less correlates the synthesis rate by an equation containing an inhibition term for C 0 2 and assdmes that water vapor concentrations are too low to offer significant inhibition. Experimental studies here with synthesis gas of H2/C0 ratios of 0.67-0.72 to which C 0 2 was added show, in contrast, that C 0 2 is relatively inert. The data are well correlated by an equation developed by Huff that contains an inhibition term for H 2 0 but not for COS. We suggest that the inhibition attributed to C 0 2was instead actually caused by H20 formed by the reverse water gas shift reaction. Except for one early publication discussed below, all the kinetic expressions that have been published until recently for the rate of synthesis assume that there is no inhibiting effect of COz on the rate. The rate expression of Anderson (1956, p 227), frequently referenced, is -RHz+CO
-
Huff and Satterfield (1984), using a well-mixed continuous flow slurry reactor, more recently developed an improved rate expression:
aPH~PCO
pco + bPH,O
(1)
Anderson noted that for a fixed temperature the two constants showed a definite, but undisclosed, trend with feed gas composition. 0888-5885/89/2628-0009$01.50/0
In most previous kinetic studies, the partial pressure of hydrogen did not vary significantly with conversion because the consumption of hydrogen was nearly offset by the contraction that accompaniesreaction. Consequently, the dependence of the rate on hydrogen pressure was not 0 1989 American Chemical Society
