Fischer Tropsch Synthesis in Supercritical CO_2

摘要

The results from studies on Fischer Tropsch synthesis wherein syngas was dissolved in CO_2 are presented. In addition CO_2 reduction by hydrogen under these conditions were also analyzed. It was found that the CO_2 behaves both a s a solven/diluent as well as a reactant. The syngas used was typical of that obtained from coal gasification, i.e. CO:H_2 of one. Under these conditions Co-based catalysts without any water gas shift catalyst does not perform well while Fe - based catalysts have been found to be useful. However, the Fe based catalysts have a propensity towards CO_2 selectivity via the primary FT reaction, Boudouard reaction and the water gas shift reaction. The use of CO_2 as a solvent was found to suppress the CO_2 and CH_4 selectivity while enhancing the hydrocarbon selectivity and CO conversion when FT synthesis was conducted using coal derived syngas on Fe-Zn-K catalysts. The effects were found to be significantly pronounced at pressures higher than or equal to 1200 psi. It should be noted that CO_2 is supercritical at pressures higher than 1050 psi and 31.4 °C. The effect of CO_2 partial pressure, reactor pressure, reaction temperature, catalyst loading and H_2:CO ratio in syngas on the liquid product distribution will be presented. Some of the notable findings include negative CO_2 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 CO_2:syngas in the reaction mixture is an important factor in the liquid product distribution. A higher value of the ratio is seen to favor hydrocarbon synthesis, while a lower value of the ratio favors oxygenate production particularly pentanols and butanols. In addition, the data on the once through fractionation of the products utilizing the solubilities in supercritical CO_2 and pressure tuning will be presented. It will be shown that the products can be easily fractionated into narrow carbon chain length distributions downstream of the reactor by simply reducing the pressures in each collection vessel. It was generally observed that the oxygenates and higher n-alkanes were collected in the higher pressure trap and lower hydrocarbons in the subsequent lower pressure traps. Residence time studies were conducted and the effect of conversion and reaction rates on the residence time and feed composition were evaluated. Long term catalyst life studies were conducted and it was found that the use of supercritical CO_2 enhanced the life of the catalyst as opposed to pure syngas feed. This research was made possible with support, in part, by the Illinois Department of Commerce and Economic Opportunity through the Office of Coal Development and the Illinois Clean Coal Institute and the Office of Vice Chancellor for Research at Southern Illinois University, Carbondale.