Fischer-Tropsch synthesis selectivity control

Chain growth during the Fischer-Tropsch synthesis is controlled by surface polymerization kinetics that place severe restrictions on our ability to alter the resulting carbon number distribution. Intrinsic chain growth kinetics are not influenced strongly by the identity of the support or by the size of the metal crystallites in supported Co and Ru catalysts. Transport-limited reactant arival and product removal, however, depend on support and metal site density and affect the relative rates of primary and secondary reactions and the FT synthesis selectivity. 

Iglesia, E., Reyes, S. C., Madon, R. J., and Soled, S. L. 1993. Selectivity control and catalyst design in the Fischer-Tropsch synthesis Sites, pellets, and reactors. Adv. Catal. 39 221-302. 

Fischer-Tropsch synthesis could be "tailored by the use of iron, cobalt and ruthenium carbonyl complexes deposited on faujasite Y-type zeolite as starting materials for the preparation of catalysts. Short chain hydrocarbons, i.e. in the C-j-Cq range are obtained. It appears that the formation and the stabilization of small metallic aggregates into the zeolite supercage are the prerequisite to induce a chain length limitation in the hydrocondensation of carbon monoxide. However, the control of this selectivity through either a definite particle size of the metal or a shape selectivity of the zeolite is still a matter of speculation. Further work is needed to solve this dilemna. 

Fischer-Tropsch synthesis (continued) selectivity control, 39 222... 

Various catalytic reactions are known to be structure sensitive as proposed by Boudart and studied by many authors. Examples are the selective hydrogenation of polyunsaturated hydrocarbons, hydrogenolysis of paraffins, and ammonia or Fischer-Tropsch synthesis. Controlled surface reactions such as oxidation-reduction reactions ° or surface organometallic chemistry (SOMC) " are two suitable methods for the synthesis of mono- or bimetallic particles. However, for these techniques. 

Selectivity Control and Catalyst Design In the Fischer-Tropsch Synthesis Sites, Pellets, and Reactors... 

E. Transport Effects and the Control of Selectivity in Fc-based Fischer-Tropsch Synthesis... 

Many of the catalysts which are usefiil in Fischer-Tropsch synthesis are also capable of catalyzing the hydrogenation of CO2 to hydrocarbons. Our structure-function studies have shown that it is possible to control the selectivity of CO2 hydrogenation by specific iron-based catalysts to generate yields of C5+ hydrocarbons that are comparable to those produced with conventional CO based... 

Selectivity control continues to be a critical issue in Fischer-Tropsch chemistry, a catalytic process that dates back more than seventy years [1]. Operating conditions can be adjusted to control selectivities but overall effects are limited [2-4]. During Fischer-Tropsch synthesis with conventional bulk iron catalysts, various phases, including metal, metal carbides and metal oxides are present at steady-state catalytic conditions [5-7]. 

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. 

It is now superflous to point out the renewed interest for the Fischer-Tropsch (F-T) synthesis (j) i. . the conversion of CO+H2 mixtures into a broad range of products including alkanes, alkenes, alcohols. Recent reviews (292.9k ) emphasized the central problem in F-T synthesis1 selectivity or more precisely chain-length control. 

Hydrocarbon distributions in the Fischer-Tropsch (FT) synthesis on Ru, Co, and Fe catalysts often do not obey simple Flory kinetics. Flory plots are curved and the chain growth parameter a increases with increasing carbon number until it reaches an asymptotic value. a-Olefin/n-paraffin ratios on all three types of catalysts decrease asymptotically to zero as carbon number increases. These data are consistent with diffusion-enhanced readsorption of a-olefins within catalyst particles. Diffusion limitations within liquid-filled catalyst particles slow down the removal of a-olefins. This increases the residence time and the fugacity of a-olefins within catalyst pores, enhances their probability of readsorption and chain initiation, and leads to the formation of heavier and more paraffinic products. Structural catalyst properties, such as pellet size, porosity, and site density, and the kinetics of readsorption, chain termination and growth, determine the extent of a-olefin readsorption within catalyst particles and control FT selectivity. 

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. 

---