271079 Fischer-Tropsch Synthesis: Effect of Process Conditions On Performance of 0.48% Re-25%Co/Al2O3 Catalyst in a Stirred Tank Slurry Reactor
Fischer-Tropsch synthesis: Effect of process conditions on performance of 0.48% Re-25%Co/Al2O3 catalyst in a stirred tank slurry reactor
Wenping Maa, Gary Jacobsa, Branislav Todicb, Dragomir Bukurb, and Burtron H. Davisa*
a Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, Kentucky, 40511, USA.
b Department of Chemical Engineering, Texas A&M University at Qatar. PO Box 23874, Doha, Qatar.
It is well known that process conditions have significant effect on performance of Fischer-Tropsch (FT) synthesis catalysts, including activity, selectivity, and stability. However, few studies have focused on investigating the changes in Co catalyst behavior with process conditions. Therefore, it is of importance to systematically investigate the dependence of Co FT behavior on operating conditions. Due to variation of concentration with position and possibility of hot spot formation during FT synthesis in a fixed-bed reactor, the slurry phase reactor is a better choice to study these effects.
In the present study, the effects of process variables (temperature, pressure, space velocity, and H2/CO ratio) on FT synthesis over 0.48% Re-25%Co/Al2O3 catalyst were studied in a 1-L continuously stirred tank reactor (CSTR). Three tests were conducted at 205, 220 and 230 oC, respectively. At each temperature, space velocity (2.0- 22.0 NL/g-cat/h), pressure (1.5-2.5 MPa), and H2/CO ratio (1.4 and 2.1) were varied in order to achieve different CO conversion levels in the range of 10-75%. In each run 220 oC was used during the startup period (120-173 h) in order to attain the same steady state CO conversion (i.e., within experimental error) before process conditions were changed. The Re-Co catalyst was highly active, displaying an initial CO TOF value of 0.094 s-1 under the conditions of 220 oC, 1.5 MPa, H2/CO = 2.1 and 8.0 NL/g-cat/h. A comparison of catalyst activity and selectivity (i.e., CH4, C5+, CO2, olefin content or olefin/paraffin ratio, 1-olefin/2-olefin content at different carbon numbers, oxygenate selectivity) under different process conditions was made at the same conversion level, except in the cases illustrating the effect of conversion on selectivity. Low temperature, low H2/CO ratio, high pressure, or high CO conversion (i.e. lower SV) inhibited the formation of CH4 and simultaneously improved the production of heavier hydrocarbons and increased olefin selectivity. High pressure and high H2/CO ratio also inhibited secondary reactions of 1-olefins, which resulted in lower extent of isomerization at higher CO partial pressure or higher hydrogenation rate of internal olefins at higher partial pressure of hydrogen. Oxygenate selectivity on the Re-Co catalyst was small (<1%). Low CO conversion, high H2/CO ratio and low pressure appear to favor oxygenate formation. However, it is also suggested that catalyst activity in term of CO rate of per gram of catalyst and selectivity exhibit greater sensitivity to the parameters of temperature, H2/CO ratio, or CO conversion, while pressure only moderately changed CO conversion and slightly adjusted selectivity. The extent of secondary reactions of olefins (percentage of internal olefins with carbon number i in all hydrocarbons with carbon number i) and CO2 selectivity were found to be low for this catalyst (<5% and <1.3%, respectively). Furthermore, the Co-Re catalyst deactivation rates during 150-250 h of testing under changing process conditions were moderate (0.9-1.4%/day) and selectivities at the baseline process conditions were relatively stable.
Keywords: Fischer-Tropsch synthesis; Re-Co/Al2O3 catalyst; process conditions; hydrocarbons; oxygenates;1-olefin; 2-olefin;CSTR
* Corresponding author: Burtron H. Davis, firstname.lastname@example.org, Tel: 859-257-0251.