Fischer-Tropsch synthesis functionality

Concerning the Fischer-Tropsch synthesis, carbon nanomaterials have already been successfully employed as catalyst support media on a laboratory scale. The main attention in literature has been paid so far to subjects such as the comparison of functionalization techniques,9-11 the influence of promoters on the catalytic performance,1 12 and the investigations of metal particle size effects7,8 as well as of metal-support interactions.14,15 However, research was focused on one nanomaterial type only in each of these studies. Yu et al.16 compared the performance of two different kinds of nanofibers (herringbones and platelets) in the Fischer-Tropsch synthesis. A direct comparison between nanotubes and nanofibers as catalyst support media has not yet been an issue of discussion in Fischer-Tropsch investigations. In addition, a comparison with commercially used FT catalysts has up to now not been published. 

Cheng, J., Gong, X.-Q., Hu, P Lok, C. M Ellis, P and French, S. 2008. A quantitative determination of reaction mechanisms from density functional theory calculations Fischer-Tropsch synthesis on flat and stepped cobalt surfaces. J. Catal. 254 285-95. 

Tsubaki, N., Sun, S., and Fujimoto, K. 2001. Different functions of the novel metals added to cobalt catalysts for Fischer-Tropsch synthesis. J. Catal. 199 236 -6. 

Fig. 4 Effect of added water on the C5 + selectivity (filled symbols) and CH4 selectivity (open symbols) as a function of CO conversion at different conditions for Co/A1203 (A), CoRe/Al203 (B), Co/Si02 (C), CoRe/Si02 (D), Co/Ti02 (E), and CoRe/Ti02 (F). Before water addition ( , ), 20% water added ( , O), 33% water added (A, A) and after water addition ( , O).19 Reprinted from Journal of Catalysis, Vol. 231, S. Storsaeter, 0. Borg, E. A. Blekkan and A. Holmen, Study of the effect of water on Fischer-Tropsch synthesis over supported cobalt catalysts, pp. 405-419. Copyright (2005), with permission from Elsevier.

Fischer-Tropsch synthesis, 28 80, 97, 103, 30 166-168, 34 18, 37 147, 39 221-296 activation energy and kinetics, 39 276 added olefin reactions, 39 251-253 bed residence time effects on chain growth probability and product functionality, 39 246-250... 

The transient response of the catalyst to a step function in the concentration of reactant gases is simulated from the kinetics of the Fischer-Tropsch synthesis. 

Available reaction-transport models describe the second regime (reactant transport), which only requires material balances for CO and H2. Recently, we reported preliminary results on a transport-reaction model of hydrocarbon synthesis selectivity that describes intraparticle (diffusion) and interparticle (convection) transport processes (4, 5). The model clearly demonstrates how diffusive and convective restrictions dramatically affect the rate of primary and secondary reactions during Fischer-Tropsch synthesis. Here, we use an extended version of this model to illustrate its use in the design of catalyst pellets for the synthesis of various desired products and for the tailoring of product functionality and molecular weight distribution. 

CO reactants and the H2O product of the synthesis step inhibit many of these secondary reactions. As a result, their rates are often higher near the reactor inlet, near the exit of high conversion reactors, and within transport-limited pellets. On the other hand, larger olefins that are selectively retained within transport-limited pellets preferentially react in secondary steps, whether these merely reverse chain termination or lead to products not usually formed in the FT synthesis. In later sections, we discuss the effects of olefin hydrogenation, oligomerization, and acid-type cracking on the carbon number distribution and on the functionality of Fischer-Tropsch synthesis products. We also show the dramatic effects of CO depletion and of low water concentrations on the rate and selectivity of secondary reactions during FT 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... 

In order to produce ethanol by COj hydrogenation, the catalyst should have two functions C-C bond formation and C-0 bond partial preservation. In the case of the CO/Hj feed gas system, the former is industrially performed in Fischer-Tropsch synthesis, while the latter in methanol synthesis. K/Fe oxides catalyst, being effective in Fischer-Tropsch synthesis, was found to produce C-C bond in COj hydrogenation. It converted COj into CO, alcohols, and hydrocarbons. Cu-Zn oxides catalyst, practically used in methanol synthesis from CO/CO2/H2 mixture, was found unable to produce C-C bond it converted CO, to CO and methanol without any other detected compounds. 

Figure 2. Catalyst surface areas as a function of time on stream (TOS). Negative TOS denote catalyst pretreatment positive TOS denote Fischer-Tropsch synthesis.

Curve 1 in Fig. 8 sketches the variation of the molecular weight of the product of the Fischer-Tropsch synthesis over supported cobalt as a function of FE (138). Large particles favor a higher production of hydro-... 

Fig. 34. Rate of reaction of syngas (A = CO + H2) as a function of superficial gas velocity for Fischer-Tropsch synthesis in the slurry phase. Adapted from Krishna (1993)...

The decomposition of bulk formates has drawn special attention, in connection with the function of promoters in iron and cobalt catalysts for the Fischer-Tropsch synthesis. In the investigations made on this point the interest was, naturally, focused on the organic products formed during the decomposition of the formate [Hofmann and Schibsted (53), Marec and Hahn (126)]. 

The same metals function as catalysts for heterogeneous hydrogenation reactions and for Fischer-Tropsch synthesis. 

To investigate the role of readsorption and secondary conversion during Fischer-Tropsch synthesis, experiments were performed in which small amounts of ethylene were added to the synthesis gas before reaction. The fate of the olefin was then followed as a function of reaction time. In the case of ethylene (2.7 mol % in synthesis gas) under the present reaction conditions, 80-90% of the added olefin reacted. As shown in Figure 13, the predominant reaction was hydrogenation to ethane, but approximately 10% of the added ethylene was incorporated into growing chains. The incorporation of ethylene into chain products increased the relative amounts of C3 to C5 hydrocarbons as shown in Figure 14. To further demonstrate this effect, a series of experiments were performed in which the initial concentration of ethylene was varied while all other... 

The catalytic results are given in Table 6 (the conversion for the catalyst heat treated at 873 K was about six times lower than that for DPColO and DPColOSOO and is therfore omitted from the table). The main product formed is methane. The data in Table 6 shows that, at similar conversion, the olefin selectivity increases and the activity remains constant with increasing heat treatment temperature. This demonstrates the important influence of the surface-oxygen functionalities on the product selectivity in the Fischer-Tropsch synthesis. 

Branching probability (pbr) as a function of carbon number Nc (of the species to be branched) and time (texp) in Fischer-Tropsch synthesis with iron and cobalt as the catalysts... 

Figure 2 - The catalytic activity for Fischer-Tropsch synthesis as a function of temperature (adapted from Ref. lO)...

On the basis of the assumptions of model <22> and <23> the Fischer-Tropsch synthesis in a slurry phase BCR has been modeled [37, 38]. As this hydrocarbon synthesis from synthesis gas (CO + H2) is accompanied by considerable volume contraction, it is clear that gas flow variations have to be accounted for. The developed models are useful to evaluate experimental data from bench scale units and to simulate the behavior of larger scale Fischer-Tropsch slurry reactors. Though only simplified kinetic laws were applied, the predictions of the model are in reasonable agreement with data reported from 1.5 m diameter demonstration plant. Fig. 12 shows computed space-time-yields (STY) as a function of the inlet gas velocity. As the Fischer-Tropsch reaction on suspended catalyst takes place in the slow reaction regime, it is understood that STY passes through a maximum in dependence of uqo- The predicted maximum is in striking agreement with experimental observations [37]. 

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