Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 609-615
CATALYST SECTION
Steady-State Study of the Fischer-Tropsch Reaction Joseph L. Felmer, Peter L. Sllveston,' and Robert R. Hudglns' Department of Chemical Engineering, Universw of Waterloo, Waterloo, Ontario, Canada, N2L 3G 1
In this study, the influence of carbon monoxide, hydrogen, water, and temperature on the steady-state product distribution from the Fischer-Tropsch reaction were investigated using a precipitated, copper-potassium promoted, iron catalyst. The product distribution was measured at five points along the length of a fixed-bed reactor. The initial reaction rates of the light hydrocarbons and alcohols were linearly proportional to the hydrogen concentration but inhibited by both CO and water. Increasing the reaction temperature (220-270 "C) shlfted the hydrocarbon product distribution toward the lighter hydrocarbons and selectively increased CO converted to carbon dioxide. The hydrocarbon product distribution, however, was found to be independent of the P, lP, feed ratio in the range studied (from 1 to 6). The development of the product distribution along the reactor length indicated the presence of secondary reactions.
Introduction With the mounting energy crisis, the Fischer-Tropsch (F-T) synthesis has drawn renewed attention. Although the hydrogenation of carbon monoxide over various catalysts was studied extensively before and after World War 11,there are relatively few published kinetic data available unmasked by transport effects. The objective of this study was to obtain such data by monitoring the steady-state, Fischer-Tropsch product distribution as it developed axially in a fEed-bed reactor. The experiments were carefdy designed to measure reaction rates unaffected by transport steps. Iron catalysts have been widely used for the FischerTropsch (F-T) synthesis. The copper-containing, potassium promoted catalyst employed was one of the group studied by the Exxon Corporation for the U.S.Department of Energy (Madon et al., 1977). The copper promoter makes it possible to reduce the catalyst at reaction temperatures (about 250 "C). In recent years, kinetic data for well-defined crystal planes have been obtained using both differential and gradientless reactors. Data of this type (Krebs et al., 1979; Dwyer and Somorjai, 1978; Dry et al., 1972),however, are not easily translated into industrial contexts. This is especially true of complex reactions such as the F-T synthesis where side reactions are known to occur (Anderson, 1956). Industrial reactors are invariably integral reactors; yet Atwood and Bennett (1979) are the sole authors to report the progression of the steady-state product distribution within an integral reactor prior to this study. In their study, the data were obtained by varying the velocity of the feed gas over a fixed catalyst bed. In contrast, in the present study the catalyst bed length was varied for a given feed-gas velocity, a technique which is closer to industrial practice. Varying the volumetric feed rate should have no effect on the primary or chain-building reaction, provided the reaction rate is not disguised by transport effects. Nevertheless, Dry et al. (1972) have shown that changes 0196-432 118111220-0609$01.25/0
in volumetric feed rate do affect side reactions and, therefore, the overall product distribution. In this work, the totalpressure of the reactor was varied over a range from 10 to 20 atm. This is typical of the pressure range encountered in industrial practice. Mechanistic proposals for the F-T synthesis have recently been reviewed (Mills and Steffgen, 1973; Ponec, 1978; Vannice, 1976). Since the present paper is directed at kinetics rather than mechanism, we omit further comment on mechanism. Experimental Methods A schematic diagram of the experimental arrangement is shown in Figure 1. The desired pressure in the system was obtained from pressurized cylinders of reactant gases. Since water inhibits the reaction rate, moisture levels in the feed gases were kept in the parts-per-billion range by means of in-line molecular sieve dryers. Teledyne-Hastings mass flowmeters were used to measure flows independently of the total pressure. By means of a packed-bed mixer and a pre-heater feed, gases were thoroughly mixed and brought to the desired temperature. This was done in order to achieve an isothermal bed and reproducible measurements. The reactor was divided into a series of five segments so that the product distribution could be observed as it progressed along the reachr length. The first segment of the reactor was designed to provide differential reaction rates. One-sixteenth inch 0.d. chromel-alumel thermocouples were placed at the entrance to the first reactor segment as well as at the outlet of each remaining reactor segment. the thermocouples were packed in approximately 8 cm of quartz beads to prevent direct contact of the thermocouples with the walls and to minimize the effect of heat conduction along the sheath to the thermocouple tip. Such a packing arrangement should ensure a measured temperature closer to that of the gas than to the surrounding heating medium. An oil bath was utilized as the heating medium. Measurements from the various points along the reador showed that the entire catalmt-bed 0 1981 American Chemical Society
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Figure 1. Experimental apparatus.
temperature was within f l "C of the oil bath temperature. High-temperature valves whose stems extended above the oil bath cover could be opened and closed manually to direct the flow of gas through each segment of the total fixed bed reactor. Figure 1 shows, for example, that closing valve 1 and opening valve 2 allowed the gas to pass only through the first section of the reactor. By opening and closing appropriate valves, the product distribution could be obtained at various points along the reactor. Liquid products from the first segment were not analyzed because the differential conversion rates employed during the experimental run did not produce enough liquid sample for accurate analysis. A gas analysis was determined for all five segments. The high molecular weight compounds produced in the synthesis were kept from condensing by heating the line leaving the reactor by means of a nichrome wire. The heavy waxes were knocked out in a separator operated at reaction pressure and approximately 130 "C. The lighter oils, alcohols, and water were collected in a second separator operated at reaction pressure and room temperature. The pressure was reduced to atmospheric using a Matheson pressure reducing valve, while a finemetering valve, located further downstream, was used to control the total outlet flow. Since the reducing valve was easily blocked by liquid and solid substances (especially water), the incoming gas was first filtered through Drierite. Some preliminary experiments showed that carbon monoxide, carbon dioxide, and light hydrocarbon gases were taken up by the Drierite only to a small extent and saturation was rapid. The influence of water was studied by passing the CO line through a saturator which was added to the system downstream from the fine-metering valve (see Figure 1). Preliminary trials in which nitrogen was used in place of the reactant stream indicated that saturation was achieved. According to Dwyer and Somorjai (1979), preoxidizing an iron surface not only increases the F-T reaction rate but also shifts the product distribution slightly toward the higher molecular weight hydrocarbons. Bohlbro (1964) recommended that the bulk-stream oxygen concentration
Table I. Purity of Feed Gases
CO (CP grade) H, (prepurified) minimum purity Ar CH, 0 2
N, H* CO, He moisture H2S
cos
99.97%
99.95%
< 10 ppm < 5 PPm < 1 5 ppm < 200 ppm < 3 0 ppm
< 1 PPm
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