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The feasibility of high synthesis gas conversion over ruthenium promoted iron-based Fischer Tropsch catalyst
Author(s)
Fraser, Ian
Date Issued
2017
Type
Thesis
Publisher
Cape Peninsula University of Technology
Abstract
One of the very promising synthetic fuel production strategies is the Fischer-Tropsch
process, founded on the Fischer-Tropsch Synthesis, which owes its discovery to the
namesake researchers Franz Fischer and Hans Tropsch. The Fischer-Tropsch Synthesis
(FTS) converts via complex polymerisation reaction a mixture of CO and H2 over
transition metal catalysts to a complex mixture of hydrocarbons and oxygen containing
compounds with water as major by-product. The mixture of CO and H2 (termed syngas)
may be obtained by partial oxidation of carbon containing base feedstocks such as coal,
biomass or natural gas via gasification or reforming. The Fischer-Tropsch (FT) process
thus presents the opportunity to convert carbon containing feedstocks to liquid fuels,
chemicals or hydrocarbon waxes, which makes, for instance, the monetisation of
stranded gas or associated gas a possibility.
The FT-process is typically carried out in two modes of operation: low temperature
Fischer-Tropsch (LTFT) and high temperature Fischer-Tropsch (HTFT). LTFT is
normally operated at temperatures of 200 – 250 °C and pressures of 10 – 45 bar to target
production of high molecular weight hydrocarbons, while HTFT is operated at 300 –
350 °C and 25 bar to target gasoline production.
The catalytically active metals currently used commercially are iron and cobalt, since
product selectivity over nickel is almost exclusively to methane and ruthenium is highly
expensive in addition to requiring very high pressures to perform optimally. Fe is much
cheaper, but tends to deactivate more rapidly than Co due to oxidation in the presence of
high H2O partial pressures. One of the major drawbacks to using Fe as FT catalyst is
the requirement of lower per pass conversion which necessitates tail gas recycle to
extend catalyst life and attain acceptable overall conversions. A more active or similarly
active but more stable Fe-catalyst would thus be advantageous. For this reason
promotion of a self-prepared typical LTFT Fe-catalyst with Ru was investigated.
A precipitated K-promoted Fe-catalyst was prepared by combination of co-precipitation
and incipient wetness impregnation and a ruthenium containing catalyst prepared from
this by impregnation with Ru3(CO)12. The catalysts, which had a target composition of
100 Fe/30 Al2O3/5 K and 100 Fe/30 Al2O3/5 K/3 Ru, were characterised using XRD, SEMEDX,
ICP-OES, TPR and BET N2-physisorption, before testing at LTFT conditions of
250 °C and 20 bar in a continuously stirred slurry phase reactor.
process, founded on the Fischer-Tropsch Synthesis, which owes its discovery to the
namesake researchers Franz Fischer and Hans Tropsch. The Fischer-Tropsch Synthesis
(FTS) converts via complex polymerisation reaction a mixture of CO and H2 over
transition metal catalysts to a complex mixture of hydrocarbons and oxygen containing
compounds with water as major by-product. The mixture of CO and H2 (termed syngas)
may be obtained by partial oxidation of carbon containing base feedstocks such as coal,
biomass or natural gas via gasification or reforming. The Fischer-Tropsch (FT) process
thus presents the opportunity to convert carbon containing feedstocks to liquid fuels,
chemicals or hydrocarbon waxes, which makes, for instance, the monetisation of
stranded gas or associated gas a possibility.
The FT-process is typically carried out in two modes of operation: low temperature
Fischer-Tropsch (LTFT) and high temperature Fischer-Tropsch (HTFT). LTFT is
normally operated at temperatures of 200 – 250 °C and pressures of 10 – 45 bar to target
production of high molecular weight hydrocarbons, while HTFT is operated at 300 –
350 °C and 25 bar to target gasoline production.
The catalytically active metals currently used commercially are iron and cobalt, since
product selectivity over nickel is almost exclusively to methane and ruthenium is highly
expensive in addition to requiring very high pressures to perform optimally. Fe is much
cheaper, but tends to deactivate more rapidly than Co due to oxidation in the presence of
high H2O partial pressures. One of the major drawbacks to using Fe as FT catalyst is
the requirement of lower per pass conversion which necessitates tail gas recycle to
extend catalyst life and attain acceptable overall conversions. A more active or similarly
active but more stable Fe-catalyst would thus be advantageous. For this reason
promotion of a self-prepared typical LTFT Fe-catalyst with Ru was investigated.
A precipitated K-promoted Fe-catalyst was prepared by combination of co-precipitation
and incipient wetness impregnation and a ruthenium containing catalyst prepared from
this by impregnation with Ru3(CO)12. The catalysts, which had a target composition of
100 Fe/30 Al2O3/5 K and 100 Fe/30 Al2O3/5 K/3 Ru, were characterised using XRD, SEMEDX,
ICP-OES, TPR and BET N2-physisorption, before testing at LTFT conditions of
250 °C and 20 bar in a continuously stirred slurry phase reactor.
Additional information
Thesis (MTech (Chemical Engineering))--Cape Peninsula University of Technology, 2017.
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