CHAIN-LIMITING OPERATION OF FISCHER-TROPSCH REACTOR

Abstract

The use of pulsing to limit the chain growth of the hydrocarbon products of the Fischer-Tropsch (FT) synthesis in order to maximize the yield of diesel-range (C{sub 10}-C{sub 20}) products was examined on three high-chain-growth-probability ({alpha} {ge} 0.9) FT catalysts. On a Co-ZrO{sub 2}/SiO{sub 2} FT synthesis catalyst the application of H{sub 2} pulsing causes significant increase in CO conversion, and only an instantaneous increase in undesirable selectivity to CH{sub 4}. Increasing the frequency of H{sub 2} pulsing enhances the selectivity to C{sub 10}-C{sub 20} compounds but the chain-growth probability {alpha} remains essentially unaffected. Increasing the duration of H{sub 2} pulsing results in enhancing the maximum obtained CO conversion and the instantaneous selectivity to CH{sub 4}. An optimum set of H{sub 2} pulse parameters (pulse frequency and duration) is required for maximizing the yield of desirable diesel-range C{sub 10}-C{sub 20} products. On a high-{alpha} Fe/K/Cu/SiO{sub 2} FT synthesis catalyst H{sub 2} pulsing enhances the yield of C{sub 10}-C{sub 20} but at the same time decreases the catalyst activity (CO conversion) and increases the selectivity to CH{sub 4}. On the other hand, pulsing with CO also increases the yield of C{sub 10}-C{sub 20} but has no impact on the selectivity to CH{sub 4} or CO{sub 2} and decreases catalytic activity only moderately. In contrast to these catalysts, H{sub 2} pulsing on a high-{alpha} Ru/alumina FT synthesis catalyst has only minimal effect on activity and product distribution, showing enhanced activity towards methanation and water-gas-shift at the expense of FT synthesis. However, these observations are based on experiments performed at a significantly lower reaction pressure (ca. 26 atm) and higher reaction temperature (210-250 C) than those commonly used for supported-Ru FT catalysts (typically 100-1000 atm, 160-170 C).