CHAIN-LIMITING OPERATION OF FISCHER-TROPSCH REACTOR

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The use of pulsing in Fischer-Tropsch (FT) synthesis to limit the hydrocarbon chain growth and maximize the yield of diesel-range (C{sub 10}-C{sub 20}) products was examined on high-chain-growth-probability ({alpha} {ge} 0.9) FT catalysts. Pulsing experiments were conducted using a stainless-steel fixed-bed micro-reactor, equipped with both on-line (for the permanent gases and light hydrocarbons, C{sub 1}-C{sub 15}) and off-line (for the heavier hydrocarbons, C{sub 10}-C{sub 65}) gas chromatography analysis. Additional experiments were performed using a highly active attrition-resistant iron-based FT synthesis catalyst in a 1-liter continuous stirred-tank rector (CSTR). On both a Co-ZrO{sub 2}/SiO{sub 2} and a Co/Al{sub 2}O{sub 3} FT synthesis catalyst 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 an instantaneous selectivity to CH{sub 4}. An optimum set of H{sub 2} pulse parameters (pulse frequency, pulse duration) is required for maximizing the yield of desirable diesel-range C{sub 10}-C{sub 20} products. Application of a suitable H{sub 2}more » pulse in the presence of added steam in the feed is a simple method to overcome the loss in activity and the shift in paraffin vs. olefin selectivity (increase in the olefin/paraffin ratio) caused by the excess steam. A decrease in syngas concentration has a strong suppressing effect on the olefin/paraffin ratio of the light hydrocarbon products. Higher syngas concentration can increase the chain growth probability {alpha} and thus allow for better evaluation of the effect of pulsing on FT synthesis. 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. FT reaction experiments using the Fe/K/Cu/SiO{sub 2} FT synthesis catalyst in a 1-liter CSTR indicate that both the catalyst activity and yield of all products (both favorable and unfavorable) are enhanced by increasing reaction pressure and H{sub 2}:CO feed ratio, as well as with decreasing reaction temperature. The selectivity to the desirable C{sub 5+} product fraction is favored by lower reaction temperatures and H{sub 2}:CO feed ratios. Based on the results of this study, the following recommendations should be considered: Pulsing experiments on FT synthesis catalysts (either cobalt-based or iron-based) should be performed under conditions that maximize the yield of the heavy hydrocarbon products (high chain-growth probability {alpha}), such as high synthesis gas partial pressure and low space velocity. More aggressive pulsing conditions (higher pulse frequency) should be examined, so as to establish the long-term impact of pulsing on product formation beyond experimental uncertainty. Also, more emphasis should be given to pulsing experiments in the CSTR which, due to its superior control of the catalyst temperature, would allow the evaluation of a more extensive range of pulsing parameters (pulse frequency and duration).« less