F.T. reaction

Typical Mossbauer spectra for the fresh, reduced, carblded and used Fe/ZSM-5 system are shown in a composite Fig. 5. Similar spectra were obtained for the Fe-Co/ZSM-5 system. The product distribution for the F-T reaction, using the Fe and Fe-Co systems, are shown in Table 1. The gasoline range hydrocarbon yield increased from 75 to 94%, when the Fe-Co clusters were used in place of Fe only. In a typical CEMS (Conversion Electron Mossbauer Spectroscopy) of the Fe-Co system, no spectrum for 57pg vas observed even after one week from this. It was concluded that in the Fe-Co clusters Co was predominantly in the "mantle" and Fe species were In their "core," in the parlance of metallurgy/geophysics. This model Is sometimes referred to as the cherry model. 

Reactor Design The F-T reaction is highly exothermic and, for hydrogen-rich syngas, can be symbolically represented by C0-I-2H2 -CH2--I-H20 AH = -165kJ/molof-CH2- (24-34) For CO-rich syngas, the overall reaction is... 

The F-T reaction involves the following main steps at the catalyst surface ... 

Scope of the Review Paper. - From the above reasoning it is clear that over the past decades a large number of studies have been reported on supported cobalt F-T catalysts. All these studies indicate that the number of available surface cobalt metal atoms determines the catalyst activity and attempts to enhance the catalytic activity have been focusing on two interconnected issues (1) to reduce the cobalt-support oxide interaction and (2) to enhance the number of accessible cobalt atoms available for F-T reaction. It has been shown that the number of catalytically active cobalt atoms as well as their selectivity can be largely enhanced by the addition of small amounts of various elements, called promoters, to the catalyst material. The exact role of these promoters - as is the case for many other heterogeneous catalysts as well -remains often, however, unclear. 

Finally, it was observed by STEM-EELS that a Co/Mn/Ti02 catalyst prepared by IWI clearly decreased the Co particle size after activation. Co particles of 5-10 nm were covered by MnO after reduction, and this was manifested in the F-T reaction as a decrease in activity. Hence, it was concluded that even when Mn is not mixed with the C03O4 in the calcined catalyst, upon reduction it may undergo changes, which lead to a covering and blocking of the small Co particles by the MnO phase. 

Hydrocarbons from the Hydrogenation of CO the Fischer-Tropsch (F-T) Reaction... 

Still some controversy about the precise mechanism, over the years a consensus has emerged that the F-T reaction is a polymerization of surface CH2 species derived from the hydrogenation of adsorbed CO. This is for example shown by the product hydrocarbon distribution where there is a monotonic decrease in yield with increase in molecular size. Plotting og WjN) against W (W = mass fraction N = carbon number) gives a characteristic graph (dotted line, Figure 15) which is typical for polymerization kinetics. 

Detail studies were carried out on Fe-ZnO and Fe-ZnO/zeolite catalyst further more. Hydrocarbon synthesis from CO2 over various Fe-ZnO/zeolite catalyst is shown in Table 2. By the addition of HY zeolite, the formation of olefins and the selectivity of C2+ hydrocarbon increased and that of methane decreased. The hydrocarbon distribution of Fe-ZnO and Fe-ZnO/HY is shown in Figure 2. Hydrocarbon distribution followed Schulz-Rory rule over Fe-ZnO catalyst, indicating F-T reaction took place. However, it changed to the formation of higher hydrocarbons over Fe-ZnO/HY, indicating MTG reaction took place. 

The Fe-ZnO catalyst shows two kinds of catalytic sites, that is, iron species effective for F-T reaction formed from a -Fc203(Fe304) and Fe promoted ZnFe204 effective for methanol synthesis. In the absence of zeolite, the F-T reaction sites are very active to produce hydrocarbons with the Schulz-Anderson-Flory distribution. On the other hand, the sites for F-T reaction are deactivated and the sites for methanol synthesis ZnFe204 exhibit the catalytic activity in the case of the composite catalyst. Therefore, hydrocarbons were obtained by MTG reaction with a non-Schulz-Anderson-Florv distribution over Fe-ZnO/HY (Figure 4). 

The XRD analysis showed that Fe-ZnO has two active sites iron species formed from FejO for F-T reaction and ZnFc204 for methanol synthesis. When Fe-ZnO was used alone, F-T reaction over iron species was predominant. The addition of HY zeolite to Fe-ZnO deactivated the iron species for F-T reaction, and thus MTG reaction over ZnFe204 became predominant. 

While unpromoted Fe catalysts show mainly the formation of light alkanes in hydrocarbons, the addition of potassium to iron results in increased selectivities towards light olefins and to liquid hydrocarbons. This is in agreement with the fact that potassium has long been known as an effective promoter of iron-based catalysts for the production of olefins and long chain hydrocarbons in F-T reaction [15]. 

There are mainly two types of F-T reactors. The vertical fixed tube type has the catalyst in tubes that are cooled externally by pressurized boiling water. The other process uses a slurry reactor in which preheated synthesis gas is fed to the bottom of the reactor and distributed into the slurry consisting of liquid and catalyst particles. As the gas bubbles upwards through the slurry, it is diffused and converted into liquid hydrocarbons by the F-T reaction. The heat generated is removed through the reactor s cooling coils where steam is generated for use in the process. 

Reactions which have rale-determining FT have transition slates with charge development and should be accelerated in the more polar solvent THF. Since the various coordination equilibria existing in the reaction mixtures are seriously shifted, reactivity changes have to be carefully analysed. Although the FT reaction with azobenzene is accelerated in Till, the F.T reaction between t-butylmagnesium chloride and di-r-hulylperoxide is extremely slow in THF 1441... 

The Fischer-Tropsch (F-T) reaction, which is conducted as a solid-catalyzed gas-phase reaction, and which is commercially operated in several countries, is inevitably accompanied by local overheating of the catalyst surface as well as by the production of heavy wax (alkanes higher than C2o)- Local overheating of the catalyst may lead to catalyst deactivation and also to an increase in methane selectivity. Heavy wax may plug micropores of the catalyst and the catalyst bed itself, also resulting in catalyst deactivation. 

Fujimoto and Fan developed an F-T synthesis in the supercritical phase and compared its reaction performance to that in the liquid phase and the gas phase. The supercritical phase F-T reaction, as described here, shows unique characteristics such as rapid diffusion of reactant gas, effective removal of reaction heat, and in situ extraction of high-molecular-weight hydrocarbons (wax). 

Figure 4.8-3 Arrhenius activation energy of the F-T reaction in various phases. Co-La/ Si02 catalyst, standard reaction conditions, W/F(CO+H2) = 3 g-cat h/mol.

Bukur et al. conducted the F-T reaction on an iron-based catalyst with supercritical phase propane [25-27], and similar conclusions were obtained, indicating the above analysis to be independent of the catalyst itself and the SCF. 

Figure 4.8-5 Catalyst pellet size influence on the alkene content in the supercritical phase F-T reaction. Conditions RU/AI2O3 catalyst, Rp = pellet radius,...

Figure 4.8-6 Addition of heavy alkenes to SCF F-T reactions gives an anti-ASF distribution. Conditions Co-La/Si02 catalyst, 220 °C, p(n-pentane) =...

It is difficult to selectively synthesize waxy hydrocarbons through an F-T reaction. The main reason is that the F-T products follow the Anderson-Schultz-Flory (ASF) distribution [23,24]. Development of a new type of F-T reaction, free from ASF constraints on product selectivity, is of great importance for wax production, as modifrcation of the F-T catalyst alone is not enough to increase wax selectivity significantly. 

Addition of a small amount of heavy 1-alkene into supercritical phase F-T reaction can significantly promote the chain growth and greatly enhance the selectivity of waxy products. As a matter of interest, this phenomenon does not occur in the gas phase reaction [28-30]. 

Shown in Figure 4.8-6 is the F-T product distribution profile after a reaction with 4 mol% (based on CO) addition of 1-tetradecene or 1-hexadecene in SCF -pentane (Tc = 196.6 °C Pc = 33.7 bar). It is clear from the figure that the product distributions in the aUcene-added systems are very flat, in marked contrast to the supercritical phase F-T reaction without addition of alkene. The selectivities for hydrocarbons lower than Ch were higher in the F-T reaction without addition of alkene. The reverse was true, however, for heavy products with carbon numbers higher than 14 the selectivity to waxy products was remarkably enhanced in the alkene-added reactions. Another remarkable phenomenon was the suppression of methane formation in the alkene-added F-T reactions. Similar behavior was observed with addition of 1-heptene or 1,7-octadiene to the supercritical phase reaction [29,30]. 

The characteristics of the alkene-added reaction systems are summarized in Figure 4 8-7. Methane selectivity in any alkene-added reaction was lower than half that in the same reaction without addition of alkene. CO conversion was also higher in the 1-alkene added reactions, except for 1,7-octadiene. However, the CO2 selectivity in all F-T reactions with heavy alkene addition was lower than that in the absence of added alkene. 

Figure 4.8-7 Reaction performance of the supercritical phase F-T reaction with addition of various long-chain alkenes (CorLa/SiOa = 25 3/100). Reaction conditions as for Figure 4.8-6.

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