Alkene Feed Composition

The choice of appropriate reaction conditions is crucial for optimized performance in alkylation. The most important parameters are the reaction temperature, the feed alkane/alkene ratio, the alkene space velocity, the alkene feed composition, and the reactor design. Changing these parameters will induce similar effects for any alkylation catalyst, but the sensitivity to changes varies from catalyst to catalyst. Table II is a summary of the most important parameters employed in industrial operations for different acids. The values given for zeolites represent best estimates of data available from laboratory and pilot-scale experiments. 

Patents assigned to Mobil (217) describe the use of boron trifluoride supported on several porous carriers. BF3 supported on silica was found to exhibit a slightly higher performance with added water in the alkylation of a mixed alkene feed at 273 K. It was also shown that self-alkylation activity was considerably lower than that with HF as catalyst. Another patent (218) describes the use of a pillared layered silicate, MCM-25, promoted with BF3 to give a high-quality alkylate at temperatures of about 273 K. BF3 was also supported on zeolite BEA, with adsorbed water still present (219). This composite catalyst exhibited low butene isomerization activity, which was evident from the inferior results obtained with 1-butene. At low reaction temperatures, the product quality was superior to that of HF alkylate. 

The catalyst is reported to be a true solid acid without halogen ion addition. In the patent describing the process (239), a Pt/USY zeolite with an alumina binder is employed. It was claimed that the catalyst is rather insensitive to feed impurities and feedstock composition, so that feed pretreatment can be less stringent than in conventional liquid acid-catalyzed processes. The process is operated at temperatures of 323-363 K, so that the cooling requirements are less than those of lower temperature processes. The molar isobutane/alkene feed ratio is kept between 8 and 10. Alkene space velocities are not reported. Akzo claims that the alkylate quality is identical to or higher than that attained with the liquid acid-catalyzed processes. 

Table 4 also shows that although the products obtmned wnth catalyst B contain lower total sulfur than with catalyst A, the mercaptan sulfiir content was significantly higher. The mercaptans were about 13 percent of the total sulfur at 220°C and increased with temperature to 80 percent at350°C. It can be inferred that catalyst B has higher HDS activity, but it also favors the formation of mercaptans by H2S-alkene recombination reactions. However, a maximum of 300°C was also observed for optimum catalyst performance. The data indicate that the optimum operating temperature is dependent on the feed composition and not the catalyst used (both catalysts were of Co-Mo type). 

Butene as the feed alkene would thus—after hydride transfer—give 2,2,3-TMP as the primary product. However, with nearly all the examined acids, this isomer has been observed only in very small amounts. Usually the main components of the TMP-fraction are 2,3,3-, 2,3,4-, and 2,2,4-TMP, with the selectivity depending on the catalyst and reaction conditions. Consequently, a fast isomerization of the primary TMP-cation has to occur. Isomerization through hydride- and methyl-shifts is a facile reaction. Although the equilibrium composition is not reached, long residence times favor these rearrangements (47). The isomerization pathways for the TMP isomers are shown schematically in Fig. 3. 

Using 1-butene as the feed alkene in most cases does not lead to dimethylhexanes as expected, but also to a mixture of TMPs. These are formed in a rapid isomerization of the linear butenes, almost to equilibrium compositions, in which the 2-butenes are strongly favored. On the other hand, some of the DMH-isomers produced in 2-butene alkylation also stem from a rapid isomerization of the feed. 

Table III provides a comparison of alkylate compositions for both the liquid acid-catalyzed reactions with various feed alkenes. The data show that H2SO4 produces a better alkylate with 1-butene, whereas HF gives better results with propene or isobutylene. The products from 2-butene and also from pentenes (not shown in Table III) are nearly the same with either acid.

Compositions of alkylates obtained with various feed alkenes and various acid catalysts (50,228)... 

Table 4 gives a typical mass composition of the products formed during the steamcracking of ethane, propane and naphtha. This shows that the alkenes decrease and that the aromatics and the petrols increase when the mean molar mass of the feed increases. 

Compositional modulation has been practiced for the FT synthesis in catalytic reactors [126]. It was found that the cyclic feeding of synthesis gas (CO/IT2) had an influence on the selectivity of the FT products. In the early studies, only low conversions could be utilized due to the exothermic nature of the reaction. It was concluded that for an iron catalyst, the methane selectivity increased with periodic operation as did the molar ratio of alkene/alkane. Higher conversion studies were conducted in a CSTR, and it was found that periodic operation had an influence on the selectivity of the products from the FT synthesis using an iron catalyst [127]. First, there was a decrease in the alpha value for synthesis with increasing period. In addition, the alkane/alkene ratio increased with an increase in the period. There was a change in the CO2 production but this could be attributed to the change in CO conversion and not the... 

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