Product functionality, Fischer-Tropsch synthesis

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... 

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. 

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-... 

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)]. 

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. 

Fig. 4. Product compositions as a function of carbon number for the Shell middle distillate synthesis process (a) the Fischer-Tropsch product following...

The Fischer-Tropsch (FT) synthesis leads to a broad range of products, i.e. hydrocarbons, alcohols, acids, esters, etc. The increasingly stringent regulations on the sulfur and aromatics content in fuels are the reasons for renewed interest in this reaction [1]. More efficient catalysts are required to improve FT activity and selectivity to the desired products. Cobalt catalysts have been found to be most suitable for the production of higher hydrocarbons [2]. Optimization of the metal function (Co, Fe, Ru, Mo) in supported FT catalysts has been studied in a large number of papers [3-6]. 

Early SBCR models were reviewed by Ramachandran and Chaudhari (5) and by Deckwer (9). They require hold-up correlations as an input and do not compute flow patterns. The most complete and useful of these models applied to the Fischer-Tropsch (F-T) conversion of synthesis gas in a SBCR is that of Prakash and Bendale (79). They sized commercial SBCR for DOE. They gave syngas conversion and production as a function of temperature, pressure and space velocity. Input parameters with considerable uncertainty that influenced production rates were the gas hold-up, the mass transfer coefficient and the dispersion coefficient. Krishna s group (77) extended such a model to compute product distribution using a product selectivity model. Air Products working with Dudukovic measured dispersion coefficients needed as an input into such model. The problem with this approach is that the dispersion coefficients are not constant. They are a function of the local hydrodynamics. 

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