Fischer-Tropsch synthesis rate constants

In case of Fischer-Tropsch synthesis, we have to consider that the first-order reaction rate constant is related to the concentration in the gas phase (e.g., ce2), and that the diffusive flux in the liquid-filled pores is related to the concentration in the liquid (ce21). Thus, instead of Equation 12.10, we have to use... 

Dautzenberg et al. (3) have determined the kinetics of the Fischer-Tropsch synthesis with ruthenium catalysts. The authors showed, that because the synthesis can be described by a consecutive mechanism, the non steady state behaviour of the catalyst can give information about the kinetics of the process. On ruthenium they found that not only the overall rate of hydrocarbon production per active site is small, but also that the rate constant of propagation is low. Hence, Dautzenberg et al. find that the low activity of Fischer-Tropsch catalysts is due to the low intrinsic activity of their sites. On the other hand, Rautavuoma (4) states that the low activity of cobalt catalysts is due to a small amount of active sites, the amount being much smaller than the number of adsorption sites measured. 

Studies of the Fischer-Tropsch synthesis on nitrided catalysts at the Bureau of Mines have been described (4,5,23). These experiments were made in laboratory-scale, fixed-bed testing units (24). In reference 5, the catalyst activity was expressed as cubic centimeters of synthesis gas converted per gram of iron per hour at 240°C. and at a constant conversion of 65%. Actually, the experiments were not conducted at 240°C., but the activity was corrected to this temperature by the use of an empirical rate equation (25). Conditions of catalyst pretreatment for one precipitated and two fused catalysts are given in Table IV. 

According to the Sachtler-Biloen mechanism, the Fischer-Tropsch reaction is initiated through CO adsorption followed by CO dissociation. Experimental evidence for the involvement of an oxygen-free intermediate exists it was observed that predeposited C is incorporated into the product during Fischer-Tropsch synthesis when CO was included in the feed gas (3). It is important to distinguish whether during the Fischer-Tropsch s)mthesis CO dissociation is strictly monomolecular or instead involves a reaction with Hads to produce an intermediate "HCO" formyl species that in a subsequent reaction decomposes to "CH" and Oads-Another question is how the rates of CO dissociation, chain growth, and termination depend on the catalyst surface structure. Thus, it is essential to know the relative values of the rate constants for these three elementary reactions. 

Three reaction systems of industrial importance, i.e. Fischer-Tropsch synthesis (FTS) and the methema-tion of CO in batch slurries of molten wax and the continuous hydrogenation of butynediol, were used which obey first, half and second order rate laws, respectively. Kinetic expressions, rate constants and the reaction conditions are given in Tedjle 1. 

To simulate the dynamic behavior of the Fischer-Tropsch synthesis a reactor description and a set of detailed kinetic equations and constants are needed. In literature much is known about Fischer-Tropsch reactors (e.g. [1]), but the detailed kinetics is lacking. For calculation of conversions or selectivities towards certain (light) products or fi actions rather simple reaction kinetics is enough, but the description of the reaction rates of both reactants and products requires more detailed information about the reaction mechanism and the constants in the rate equations. 

This case occurs rarely. It usually happens at the cases of considerably small catalyst particles, less external mass transfer coefficient and relative large reaction rate constant. This will happen when the porousless netlike materials are used for catalyst as platinum-net for ammonia oxidation, or thin layer distribution of active component at external surface of catalysts. Besides, it is commonly considered that Fischer-Tropsch synthesis reaction to produce hydrocarbons is also controlled by external diffusion on iron catalysts. 

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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