(438a) Fischer Tropsch Synthesis in Compressed CO2 Atmosphere With Unreacted Gas Recycle

The key challenge in applying the Fischer-Tropsch process to produce transportation fuels is to make the capital and production costs economically feasible relative to the comparative cost of existing petroleum resources. To meet this challenge, it is imperative to enhance the CO conversion while maximizing carbon selectivity towards the desired liquid hydrocarbon ranges (i.e. reduction in CH₄ and CO₂ selectivities) at high throughputs. At the same time, it is equally essential to increase the catalyst robustness and longevity without sacrificing catalyst activity. This paper focuses on process development to achieve the above. The paper describes the influence of operating parameters on Fischer Tropsch synthesis (FTS) from coal derived syngas in supercritical carbon dioxide (ScCO₂).  In addition, the unreacted gas and solvent recycle was incorporated and the effect of unreacted feed recycle was evaluated.  It was expected that with the recycle, the feed rate can be increased.  The increase in conversion and liquid selectivity accompanied by the production of narrower carbon number distribution in the product suggest that higher flow rates can and should be used when incorporating exit gas recycle.  It was observed that this novel process was capable of enhancing the hydrocarbon selectivity, reducing the parasitic loss of carbon efficiency due to carbon dioxide formation during FT synthesis reactions and increasing the life of the catalyst.  The use of the supercritical CO₂ (ScCO₂) inhibited both methane and CO₂ selectivities while enhancing the rates of synthesis.  In addition, the use of supercritical CO₂ was found to prolong the life of the catalyst presumably by removing the heat of reaction from the catalysts surface and solubilizing the waxes that tend to deposit of the surface.  The effect of the process conditions on the life of the catalysts will also be presented. Fe based catalysts are known to have a high proclivity for producing CO₂ during FTS.  The data of the product spectrum and selectivity on Co and Fe-Co based catalysts as well as those obtained from commercial sources will also be presented.  The measurable decision criteria were the increase in CO conversion at H₂:CO ratio of 1:1 (as commonly found in coal gasification product stream) in supercritical phase as compared to gas phase reaction, decrease in CO₂ and CH₄ selectivity, overall liquid product distribution, and finally an increase in the life of the catalysts.  Some of the notable findings include negative CO₂ selectivities while obtaining nearly 98 % CO conversion. Increasing the reactor pressure was found to favor longer chain growth. In addition, it is noted that the ratio between CO₂:syngas in the reaction mixture is an important factor in the liquid product distribution.  The results from Fe-Zn-K, Fe-Co-Zn-K and Fe-Co-Ru-Zn catalysts on gamma alumina supports will be presented for recycled gas to feed gas flow rate ratio of 0, 0.5, 1, 2, 3, 4 and 5.