Byproducts isobutane alkylation

The main products formed by the catalytic alkylation of isobutane with ethylene (HC1—AICI3, 25-35°C) are 2,3-dimethylbutane and 2-methylpentane with smaller amounts of ethane and trimethylpentanes.13 Alkylation of isobutane with propylene (HC1—AICI3, — 30°C) yields 2,3- and 2,4-dimethylpentane as the main products and propane and trimethylpentanes as byproducts.14 This is in sharp contrast with product distributions of thermal alkylation that gives mainly 2,2-dimethylbutane (alkylation with ethylene)15 and 2,2-dimethylpentane (alkylation with propylene).16... 

Propane as a degradation product of polyethylene (a byproduct in the reaction) was ruled out because ethylene alone under the same conditions does not give any propane. Under similar conditions but under hydrogen pressure, polyethylene reacts quantitatively to form C3 to C6 alkanes, 85% of which are isobutane and isopentane. These results further substantiate the direct alkane alkylation reaction and the intermediacy of the pentacoordinate carbonium ion. Siskin also found that when ethylene was allowed to react with ethane in a flow system, n-butane was obtained as the sole product, indicating that the ethyl cation is alkylating the primary C—H bond through a five-coordinate carbonium ion [Eq. (5.66)]. 

Although aviation gasoline can be made by the alkylation of isobutane with propylene, butylenes, and amylenes, the butylenes are by far the predominant feed. In a few cases a mixture of di- and triisobutylene polymers, which is a byproduct of butadiene manufacture, is used as part of the olefinic charge. Either sulfuric acid or hydrofluoric acid can be used as a catalyst when aviation gasoline is produced by the alkylation reaction. 

The input-output structure of the flowsheet is presented in Figure 9.1. Butene (feed rate FA,0) and isobutane (feed rate FB-0) are the raw materials. The butene feed is impure with quite large amounts of propane (FI 0). The main product is the alkylate C8Hi8, at the rate FP. The selectivity of the process is not 100%, therefore heavy products are formed at the rate FR. The inert fed into the process must also leave the plant, the flow including light byproducts that are formed in secondary reactions. Often, significant quantities of n-butane are mixed with the isobutane fresh feed. For this case, development of the flowsheet and the design of the main units is left as an exercise for the reader. 

In addition to agitation, the interfacial areas are significantly affected by the following acid/hydrocarbon ratio, acid composition (and especially the amounts of dissolved conjunct polymers), and temperature. Conjunct polymers are surfactants that collect in appreciable concentrations at the interface. Here, they act as a reservoir of H s in the transfer steps from isobutane or other isoparaffins to the i-Cs to i-Cie cations. Conjunct polymers also have a major effect on the viscosity and other physical properties of the acid phase. In the alkylation reactor, the preferred sulfuric acid and HF phases contain appreciable amounts of conjunct polymers optimum amounts result in higher RON values, higher yields, less byproducts, etc. 

Aluminas may be used for the dehydrofluorination of alkylfluorides, which are byproducts of the HF-catalyzed isobutane—butylene alkylation process. Fluoroalkanes are converted to olefins on alumina at temperatures of 170—220 °C. FIF is adsorbed on the alumina and aluminum fluoride is formed as a consequence, regeneration is needed every 6 months (596). 

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