(22a) 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 Ma^a, Gary Jacobs^a,
Branislav Todic^b, Dragomir Bukur^b, and Burtron H. Davis^a*

^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.

 Abstract

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,
davis@caer.uky.edu, Tel:
859-257-0251.