Olefins readsorption

Different explanations have been proposed to explain this effect, including the influence of water on the adsorbed carbon species on the surface and the reduction of secondary hydrogenation of primary olefins by water, thereby facilitating olefin readsorption and chain initiation.8... 

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. 

Recently, we reported detailed descriptions of hydrocarbon chain growth on supported Ru catalysts (7,8) we showed that product distributions do not follow simple polymerization kinetics and proposed a diffusion-enhanced olefin readsorption model in order to account for such deviations (7,8). In this paper, we describe this model and show that it also applies to Co and Fe catalysts. Finally, we use this model to discuss a few examples from the literature where catalyst physical structure and reaction conditions markedly influence hydrocarbon product distributions. 

Readsorption of a-olefins is also influenced by bed residence time (i.e., space-velocity). As we discussed previously, space-velocity controls the readsorption rate of small a-olefins (7,8). As a-olefin size increases, the influence of bed residence time decreases and the influence of pore residence time increases until the latter totally controls the secondary reaction chemistry. In some cases, where only bed residence times affect a-olefin readsorption and pore residence times are much smaller than bed residence times, straight line Flory plots with a single value of a can be obtained such a values, however, are larger than in the absence of secondary readsorption (7,22). 

The trends in carbon number distribution and in a-olefin/paraffin ratio on Ru, Fe, and Co, three very different catalytic surfaces, are remarkably similar. All catalysts show a curved Flory plot and an a-olefin/paraffin ratio that decreases with increasing carbon number until only paraffins are observed at high carbon numbers. In each case, diffusion-enhanced olefin readsorption accounts for such trends. Its contribution depends on the catalytic surface, its physical structure, and reaction conditions. 

The effective diffusivity Dn decreases rapidly as carbon number increases. The readsorption rate constant kr n depends on the intrinsic chemistry of the catalytic site and on experimental conditions but not on chain size. The rest of the equation contains only structural catalyst properties pellet size (L), porosity (e), active site density (0), and pore radius (Rp). High values of the Damkohler number lead to transport-enhanced a-olefin readsorption and chain initiation. The structural parameters in the Damkohler number account for two phenomena that control the extent of an intrapellet secondary reaction the intrapellet residence time of a-olefins and the number of readsorption sites (0) that they encounter as they diffuse through a catalyst particle. For example, high site densities can compensate for low catalyst surface areas, small pellets, and large pores by increasing the probability of readsorption even at short residence times. This is the case, for example, for unsupported Ru, Co, and Fe powders. 

Relative rate constants for a-olefin readsorption decrease as follows kr c0>kr Ru> r Fe (7)- Although kr on Fe catalysts is smaller than on Ru or Co, the other parameters in Eq. (2), such as the low diffusivity of large hydrocarbon and the high site density on unsupported Fe catalysts, ultimately increase the probability of a-olefin readsorption therefore, pore diffusion effects also play a crucial role in Fe-catalyzed FT synthesis (Figures 3 and 4). Fe catalysts, however, give lower C20+ selectivity because of lower intrinsic values of kr- even though asymptotic chain termination probabilites are lower on Fe. 

Results for the Synthol entrained-bed process (16) are plotted in Figure 8. The available C to C15 data follow the conventional Flory plot with a equal to 0.7. The Synthol process uses a fused Fe catalyst of low surface area and porosity and operates at high temperatures ( 590K). The products in the reactor are mainly gaseous, wax formation is minimal, and the pellet pore structure remains free of liquid products therefore, diffusion-enhanced a-olefin readsorption is much less likely than in the ARGE process. Whereas the product selectivity in the ARGE process is altered by diffusion-enhanced a-olefin readsorption, that in the Synthol process is not. 

Many studies address the effect of promoters such as K and Mn on Fe-based catalysts. Dry et al (23) suggest that the alkali promoter weakens the C-O bond and enhances its rate of dissociation it also strengthens the metal-C bond, the surface residence time of adsorbed chains, and the probability of chain growth. In the presence of Mn, termination to olefins predominates (24-26). Our results suggest that we must also consider the effect of promoters and of catalyst treatment on a-olefin readsorption. Perhaps the presence of alkali also enhances a-olefin readsorption reactions leading to heavier products whereas Mn does not. 

As summarized below and described in detail elsewhere (4,5), the olefin readsorption model requires mass conservation equations for growing chains at catalyst sites [Eq. (8)] and for reactive olefins [Eqs. (9) and (10)] and un-reactive paraffin products [Eqs. (11) and (12)] within isothermal spherical pellets. 

As in the olefin readsorption model, the Thiele modulus for reactant diff-sion (4>o) can also be expressed as the product of two components ... 

The second term is identical to the parameter x described in Eq. (15), where it accounts for the effect of catalyst structural properties on olefin readsorption rates. Not surprisingly, structural catalyst properties that restrict the removal of a-olefins from catalyst pellets also limit the arrival of reactants at catalytic sites. The i/>co term reflects the diffusivity and reactivity properties of reactant molecules. 

Carbon number distributions are similar on all Co catalysts. As on Ru catalysts, termination probabilities decrease with increasing chain size, leading to non-Flory product distributions. The modest effects of support and dispersion on product molecular weight and C5+ selectivity (Table III) reflect differences in readsorption site density and in support pore structure (4,5,14,40,41), which control the contributions of olefin readsorption to chain growth. Carbon number distributions obey Flory kinetics for C30+ hydrocarbons the chain growth probability reaches a constant value (a ) as olefins disappear from the product stream. This constant value reflects the intrinsic probability of chain termination to paraffins by hydrogen addition it is independent of support and metal dispersion in the crystallite size range studied. 

At lower reactant pressures, olefins are more effectively intercepted by secondary hydrogenation reactions that prevent them from initiating surface chains. Primary chain growth pathways and olefin readsorption and chain initiation rates are not influenced strongly by reactant pressure thus, total... 

As expected from the key role of diffusion-enhanced olefin readsorption, C5+ selectivity increases with increasing site density (Fig. 13a curve A, 1.0... 

The initial increase in C5+ selectivity as x increases arises from diffusion-enhanced readsorption of a-olefins. At higher values of CO transport restrictions lead to a decrease in C5+ selectivity. Because CO diffuses much faster than C3+ a-olefins through liquid hydrocarbons, the onset of reactant transport limitations occurs at larger and more reactive pellets (higher Ro, 0m) than for a-olefin readsorption reactions. CO transport limitations lead to low local CO concentrations and to high H2/CO ratios at catalytic sites. These conditions favor an increase in the chain termination probability (jSr, /Sh) and in the rate of secondary hydrogenation of a-olefins (j8s) and lead to lighter and more paraffinic products. 

The diffusion-enhanced olefin readsorption model described in Section III,C was used to predict the effect of carbon number on chain growth probability and paraffin selectivity. The model requires only one adjustable parameter the exponent c in a hydrocarbon diffusivity equation that depends on molecular size ( ), but that is identical for paraffins and olefins of equal size ... 

Experimental results are described accurately without the need for multiple chain growth sites, secondary hydrogenation functions, or a-olefin readsorption kinetics that depend on chain size. The model also predicts... 

Bed residence time effects can also be described by the convective terms included in the olefin readsorption model [Eqs. (13) and (14)]. Convection-limited removal of a-olefins, characterized by the Peclet number, accounts... 

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