FISCHER-TROPSCH SYNTHESIS IN SUPERCRITICAL PHASE CARBON DIOXIDE: DEACTIVATION STUDIES

Author

David Carron, Southern Illinois University CarbondaleFollow

Date of Award

8-1-2011

Degree Name

Master of Science

Department

Mechanical Engineering

First Advisor

Mondal, Kanchan

Abstract

ABSTRACT This thesis presents the results of investigations regarding the effect of supercritical CO2 on the long term activity, life and deactivation rates of an Fe-Zn-K catalyst during Fischer Tropsch Synthesis from syngas (H2:CO =1:1) typically produced from coal gasification. Previous studies at SIUC on FTS in Supercritical CO2 (SC-CO2) have shown that CH4 selectivity was inhibited and with the presence of excess CO2, the WGS reaction was reversed. This increased the carbon economy as result of the reduction in parasitic loss of CO to CO2. In addition, it was observed that the conversion of CO, under these pressures and CO2 dilution, was significantly enhanced. Studies in a continuous flow system showed the use of SC-CO2 affected the distribution of hydrocarbons, mainly producing heavier hydrocarbons (diesel fuel). In this thesis, results from four long term experiments (21-28 day) varying the CO2:syngas ratio are reported. The experiments were conducted at 350 oC, 1200 psi with a feed rate of 200sccm in a fixed bed supercritical reactor with a volume of 150 cc. The results show that the conversion of syngas increased from 47% to 95% at the optimum ratio 5:1 (CO2:Syngas). The steady state reaction rate constant also increased 4.756 times the baseline run from 0.021215 min-1 to 0.100907 min-1, for pure syngas and a CO2:syngas ratio of 5:1 respectively. The deactivation rate did not improve with the use of supercritical CO2; however, the life span of the catalyst more than doubled that of the base line run with an increase in SC- CO2. Product tailoring can also be performed by simply changing the SC-CO2:Syngas ratio. Ratios less than 5:1 will yield a product distribution of predominately alcohols, ratios greater than 5:1 produce heavier hydrocarbons. Both of these product distributions can be beneficial, but for this research a ratio of 5:1 yielded the desired product distribution of light to heavy hydrocarbons generically known as gasoline and diesel fuel. Liquid selectivity was observed to increase with CO2 content in the feed upto a CO2:syngas ratio of 5:1, thereafter it declined slightly. CO2 is produced in the experiment of pure syngas with no Sc-CO2, however the introduction of Sc-CO2 resulted in the consumption of CO2 for the production of hydrocarbons. The methane selectivity was found to monotonically decrease with the increase in CO2 content in the feed. With oil prices increasing, the use of SC- CO2 as a reaction media for FTS is showing more promise in providing liquid fuels more effectively. The evidence of consumption of CO2 means that CO2 does not need to be removed from the syngas feed stream after the gasification and water gas shift unit processes. The increase in the observed life of the catalyst under supercritical conditions will ultimately reduce the operating cost as less material will be needed to produce the same amount of product allowing for FTS to become economically competitive.