Basis isobutane alkylation

The highest quality component in the alkylation product when making aviation or motor alkylate is a mixture of trimethylpentanes. Although some of the other components have high octane, most are of inferior octane number. It is therefore desirable to make as much of the trimethylpentanes as possible. On a pure component basis, isobutylene and butylene-2 will alkylate isobutane quite readily to form trimethylpentanes. However, butylene-1 has a tendency to form dimethylhexanes. Most of these dimethyl-hexanes are of lower octane number, and their production is to be avoided... 

Although no specific analysis was made to Identify sec-butyl sulfate in the acid phase, there seems little doubt that this sulfate was the reaction product obtained in the n-butene runs. First, the n-butenes and acid reacted on essentially a 1 1 molar basis only a slight excess of acid was needed In all cases. Of Interest, formation of butyl sulfates has also been reported to occur In conventional alkylation reactors (13). Second, the reaction product was similar 1n many respects as compared to sec-propyl sulfate the product was unstable at higher temperatures and 1t reacted with isobutane in the presence of sulfuric aicd to form alkylate (9). Third, the reaction products from all three n-butenes alkylated in identical manners as will be discussed later (9). Fourth, McCauley (14) has shown that butyl fluorides can be formed and are quite stable at low temperatures such as used here he had contacted HF with n-butenes. 

To study the relative ease with which various hydrocarbons undergo thermal alkylation with ethylene, a mixture of propane, normal butane, isobutane, and neopentane (9.2 mol % each), 4.6 mol % C2H4, and 17 mol % HC1 were reacted at 399°C and at an initial pressure of 177 atm for 1 hr. The yields of the alkylation products (nC5Hi0 and iC5H10 from C3H6 nC6Hi4 and 3-methyl pentane from nC4Hi0 2-methyl pentane and 2,2-dimethyl butane from isobutane and 2,2-dimethyl pentane from neopentane) were measured and are shown in Table I on a relative basis. 

The mechanisms of alkylation reactions appear to be very complex. Analyses of typical alkylation products show that on basis of known reactions, secondary reactions of isomerization, cracking, and disproportionation, hydrogen transfer and polymerization must occur in the reaction. All these reactions are almost certain to occur by means of carbonium ion complexes including formation, addition, rearrangement, and proton and hydride ion transfer. The following reactions are at present beheved to occur as the main reactions in the alkylation of butene-1 with isobutane ... 

On the basis of this evidence, CPs and pseudo alkylate are produced in alkylation units mainly from olefins. Supporting evidence is that CP production increases with low ratios of isobutane to olefins and when increased amounts of branched olefins (isobutylene and isopentenes) are used. In both cases, mechanism 2 reactions increase in importance more olefins are reacted to produce C12—C20 oligomers. Such oligomers are likely precursors for the vmdesired side products. 

Fewer details have been published on the phenomena that occur during alkylation reactions with solid catalysts than with liquid catalysts. With solid catalysts, which are porous, most of the reactants diffuse into the catalyst pores, then various reactions occur, and finally most of the reaction products diffuse out of the pores. For the alkylation of isobutane, the catalyst quickly deactivates regardless of the catalyst chosen or the specific operating conditions employed. Such deactivation is apparently due to rather high molecular weight by-products that fail to diffuse from the pores. The diffusivity values for different molecules in the pores obviously depend on their molecular weight and shapes. For the alkylation of aromatics, solid catalysts have been found that undergo deactivation much slower. On the basis of commercial results, catalysts have been found that do not need to be reactivated or replaced for several years. 

Processes for the alkylation of benzene have evolved to very high extent in the last 30- 0 years on the basis of high quality scientific research, engineering, and administrative supervision. Although alkylation processes of isobutane have been practiced for more than 65 years, improvements with high probability can be realized in the near future. 

There is insufficient butylene in the raw material to alkylate all the isobutane produced by isomerization of n-butane. This surplus isobutane has therefore to be distributed between the alkylation and the dehydrogenation sections so as to eliminate surpluses of isobutane, butylenes and, also, isobutylene obtained through dehydrogenation of isobutane. Naturally, this problem has to be solved on the basis of the yields from the isomerization of butane and the dehydrogenation of isobutane. 

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