Carbon number probability effects

Various mechanisms have been proposed for the FT reaction (j ) (2) and (4- ). A generalized mechanism is illustrated in Figure T. Irrespective of which mechanism is correct, or is dominant, there is general agreement that stepwise chain growth is involved. This inevitably results in a wide carbon number distribution of products, the particular distribution being determined by the probability of chain growth (<1). The calculated effect of different values on... 

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. 

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. 

E. Carbon Number Effects on Chain Growth Probability and Product Functionality... 

Fig. 9. Carbon number effects on olefin and paraffin chain termination probability (catalyst Ru/Ti02, 1.2 wt% Ru, 48% Ru dispersion 476 K, 560 kPa, H2/CO = 2.1). (a) < 2-s bed residence time, < 5% CO conversion (b) 12-s bed residence time, 60% CO conversion.

Ethylene and propylene intrinsic readsorption rates exceed those of larger olefins (4). This leads to the low apparent chain termination probabilities for C2 and C3 molecules shown in Fig. 8. These lower /8t values reflect a more effective reversal of chain termination to olefins (/3o) for these smaller but more reactive molecules (Figs. 9 and 10). The high rate of ethylene and propylene readsorption accounts for their appearance below the rest of the distribution in carbon number distribution plots on both Co and Ru catalysts (Fig. 5). 

Higher intrapellet residence times increase the contribution of chain initiation by a-olefins to chain growth pathways. This intrapellet delay, caused by the slow diffusion of large hydrocarbons, leads to non-Flory carbon number distributions and to increasingly paraffinic long hydrocarbon chains during FT synthesis. But intrapellet residence time also depends on the effective diameter and on the physical structure (porosity and tortuosity) of the support pellets. The severity of transport restrictions and the probability that a-olefins initiate a surface chain as they diffuse out of a pellet also de-... 

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 number distributions (Fig. 16) and chain termination probabilities (/3 and jSn) (Fig. 17) on Co catalysts at low values of bed residence time (<2 s, <10% CO conversion) are accurately described by the model. We reported previously a similar agreement on Ru catalysts (4). The model quantitatively describes the observed curvature of carbon number distribution plots (Fig. 16) and also the constant values of j3/y and the decreasing values of )3o observed as hydrocarbon size increases (Fig. 17). Such effects arise from the higher intrapellet fugacity and the higher residence time of larger a-olefins within transport-limited pellets. 

Fig. 17. Comparison of model and experimental results. Carbon number effects on olefin and paraffin chain termination probability (experimental/model same as in Fig. 16).

Many of the chain growth pathways and transport effects described above also occur on Fe-based FT synthesis catalysts. As on Co and Ru catalysts, FT synthesis on Fe often yields non-Flory carbon number distributions of products, where the chain growth probability and the paraffin content increase with hydrocarbon chain size (38-40). These effects were previously... 

The expertise with EOR was used for finding suitable microemulsion-forming systems for LNAPL. However, the high polarity of chlorinated hydrocarbons with very low or even negative equivalent alkane carbon numbers (EACN) required novel types of surfactants [56]. The enhanced solubility of surfactants in the oil phase makes most surfactants less effective for solubilisation. DNAPL extraction by mobilisation, however, is problematic owing to the high density of the pollutants, since they may be displaced into deeper soil compartments [57]. This probably happened in at least one field test [58]. 

Hall, Kokes, and Emmett (24), who used radioactive tracers to study the Fischer-Tropsch reaction, suggested that besides stepwise growth with single-carbon intermediates, multiple build-in could and probably did occur in the synthesis. They also showed that multiple build-in could not be distinguished from single-carbon stepwise growth below C12-C16 hydrocarbons, and thus the effect of multiple build-in could only be seen if detailed product distributions up to large carbon numbers were obtained. 

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