Carbon number distributions selectivity

The carbon number distribution of the synthesis run with co-fed 1-hexene shows an increased C selectivity of the C7 fraction (Figure 11.5). However, the increase of the following fractions strongly declines with increasing carbon number, so that the distribution approaches, within a few carbon numbers, the one obtained without co-feeding 1-hexene. This result suggests that readsorbed and... 

For this purpose selectivities of hydrocracking of the n-alkanes are expressed in terms of probabilities of overall cracking reactions. The latter are calculated from the carbon number distributions of the cracked products (n. = number of moles of cracked products with carbon number i, from Figures 7, 8 or 9). Examples for the mode of calculation are represented in the following scheme which is valid for pure primary cracking. 

Selectivity in the Fischer-Tropsch (FT) synthesis is directly related to carbon number distributions. These distributions were first discussed by Friedel and Anderson (i), who used an approach suggested by Herrington (2) to relate the rates of formation of hydrocarbon chains to the rates of chain termination. Later, it was shown (5) that the approach in Ref. (i) was similar to that used in... 

Table V illustrates the carbon-number distribution in selected series. Similar trends were observed in most others. With one exception, the homolog present in the largest concentration within a series has three to four carbon atoms more than the first member of the series. This could indicate short sidechains or, possibly, the appearance of isomeric naphthenoaromatic series...

Table V. Carbon-Number Distribution in Selected Homologous Series in EDS Asphaltenes...

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. 

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. 

The apparent dispersion and support effects on FT selectivity (Table III) will be described in a later section. In the following sections, we first describe the reaction pathways and the structural catalyst properties that control the carbon number distribution and the paraffin selectivity during FT synthesis. 

A value of c equal to 0.3, previously used to describe FT selectivity data on Ru catalysts (4), was also chosen here to describe the behavior of cobalt catalysts. This equation for hydrocarbon diffusion in melts reflects the strong influence of molecular size in reptation and entanglement models of transport in such systems (IJ6). Our model also requires the input of intrinsic values for jSn (given by the asymptotic j8r), jSo, j8r, and j8s, measured independently. After such parameters are specified, the model yields a non-Flory carbon number distribution of increasingly paraffinic hydrocarbons that agrees well with our experimental observations (Fig. 16). 

The high asymptotic value of C5+ selectivity at large values of occurs on pellets that restrict the removal of reactive a-olefins and allow many readsorption events in the time required for intrapellet olefin removal by diffusion. Yet, transport restrictions within these pellets must not significantly hinder the rate of arrival of CO and H2 reactants to the active sites. Carbon number distributions also obey Flory kinetics for high values of because even the smaller olefins significantly react within a catalyst pellet. 

Diffiisional restrictions increase the effectiveness of olefin interception sites placed within catalyst pellets. Very high olefin hydrogenation turnover rates or site densities within pellets prevent olefin readsorption and lead to Flory distributions of lighter and more paraffinic hydrocarbons. Identical results can be obtained by introducing a double-bond isomerization function into FT catalyst pellets because internal olefins, like paraffins, are much less reactive than a-olefins in chain initiation reactions. However, light paraffins and internal olefins are not particularly useful end-products in many applications of FT synthesis. Yet, similar concepts can be used to intercept reactive olefins and convert them into more useful products (e.g., alcohols) and to shift the carbon number distribution into a more useful range. In the next section, olefin readsorption model simulations are used to explore these options in the control of FT synthesis selectivity. 

Fig. 27. Effect of diffusion-enhanced a-olefin cracking catalytic function on carbon number distribution (simulations experimental/model parameters as in Fig. 15, 10% CO conversion). (A) FT synthesis without cracking function (B) with intrapellet cracking function, jS = 1.2 (C) with extra pellet cracking function, jS = 1.2. (a) Carbon selectivity vs. carbon number (b) Flory plots.

Fig. 30. The effect of secondary reactions on FT synthesis selectivity. Composite Fe-Mn/ Y-zeolite and Fe-Mn/SiOi catalysts (501 K, 2500 kPa, H2/CO = 2.1, 45-47%.5% CO conversion). (a) Carbon number distribution (b) a-olefin to n-paraffin ratio.

Chain growth during the Fischer-Tropsch synthesis is controlled by surface polymerization kinetics that place severe restrictions on our ability to alter the resulting carbon number distribution. Intrinsic chain growth kinetics are not influenced strongly by the identity of the support or by the size of the metal crystallites in supported Co and Ru catalysts. Transport-limited reactant arival and product removal, however, depend on support and metal site density and affect the relative rates of primary and secondary reactions and the FT synthesis selectivity. 

Diffusion-limited removal of products from catalyst pellets leads to enhanced readsorption and chain initiation by reactive a-olefins. These secondary reactions reverse chain termination steps that form these olefins and lead to heavier products, higher chain growth probabilities, and more paraffinic products. Diffusion-enhanced readsorption of a-olefins accounts for the non-Flory carbon number distributions frequently observed during FT synthesis on Co and Ru catalysts. Diffusion-limited reactant (H2/CO) arrival leads instead to lower selectivity to higher hydrocarbons. Consequently, intermediate levels of transport restrictions lead to highest selectiv-ities to C5+ products. A structural parameter containing the pellet diameter, the average pore size, and the density of metal sites within pellets, determines the severity of transport restrictions and the FT synthesis selectivity on supported Ru and Co catalysts. 

MOGD olefin product distribution is determined by thermodynamic, kinetic, and shape-selective limitations. The equilibrium calculation was greatly simplified by assuming the isomers for a given carbon number to be at equilibrium (ref. 19). At low pressure and high temperature, olefin equilibrium is reached, while at higher pressure kinetic limits prevent equilibrations at commercially feasible space velocities. Isomerization reactions are fast at all carbon numbers, and isomer equilibrium is achieved for low carbon numbers. Shape selectivity determines isomer equilibrium for higher carbon... 

Guczi et al. (305) also find antipathetic structure sensitivity for the CO/H2 reaction over Ru/Al203 and Ru/Si02. Alloying with iron has little effect on TOF but increases selectivity toward olefins. In contrast to some previous reports (128, 306), it has been found (307) that the product carbon number distribution is sensitive to particle size. 

This leaves only the increase in C1-C5 aliphatic selectivity unexplained. Since acid sites regulate the carbon number distribution, the increase in light material would indicate unimpaired acid site activity. The increase in C1-C5 selectivity can also be explained in... 

Figure 11 illustrates the expected molar carbon-number distribution from primary methanolysis of n-Ci4 as calculated for an a-value of 0.5. The figure exhibits trends that are very similar in shape to what was obtained experimentally at low to medium conversion (Figures 1 - 5), namely very high molar select vities of methane and high tail ends of the product carbon-number distribution. 

Allen and coworkers ° introduced a structural lumping approach based on group contribution concepts and pure-compound data. An oil fraction is assembled with a finite number of selected compound classes to capture key structural features of the oil. They calculated the number of CH, CH2, CH3 as well as terminal and nonterminal olefmic and aromatic carbons. They then followed the evolutions of carbon number distributions and carbon types in each compound class. 

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