Fischer-Tropsch process product selectivity control

Engineering aspects of the SMDS process are reviewed here. They include the manufacture of synthesis gas, the production of paraffinic Fischer-Tropsch waxes and the control of the chain length distribution by a selective hydrocracking step. The close interaction between the properties of the individual catalyst particles, the choice of the reactor technology and the overall process performance is discussed in detail. 

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

The test reactor was a 13 mm i.d. quartz tube fitted to a process unit equipped with gas supplies, flow and temperature controllers, a furnace, and a gas chromatograph with appropriate columns and detectors. The sample was loaded into the reactor, purged and/or reduced and heated to the test temperature (200-250°C). With the feed gas (C2H4 and O2, CO2 and H2, or CO and H2) flowing, conversion and product composition were measured. Conversions were maintained as low as possible so that differential rates could be determined. In the case of Fischer-Tropsch synthesis, measurements were made at higher conversions to check selectivities. 

This expression can indeed account for a positive, first order in hydrogen and a negative or close to zero order in CO as is experimentally observed. The expression is also valid for the Fischer-Tropsch synthesis of higher hydrocarbons. In this case the scheme of (3.8) has to be extended with chain-growth reactions, as discussed in Section 6.6.5. How to control the selectivity of this process is a key issue in CO hydrogenation catalysis. Methane and methanol are the only products that can be obtained with 100% selectivity. 

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