Basic Factors in Alkylation

There are some basic factors in alkylation which should be considered in addition to the catalyst, feed stock, and normal operating variables. Although thermal alkylation can be accomplished without the presence of a catalyst, essentially all commercial processes today employ a catalyst. The alkylation reactions considered here normally take place in the liquid state. Sufficient pressure is therefore required to insure that the hydrocarbons in the reactor remain in the liquid state. This is only partially true for the autorefrigeration process, where cooling of the reaction zone takes place as a result of vaporizing some of the light hydrocarbons in the reactor. The ethylbenzene reaction is also an exception, since it normally takes place in the partial vapor state, especially when the feed contains considerable ethane. The alkylation reaction is exothermic, and this heat of reaction must be removed from the reaction zone. This heat can be removed by cooling coils, by vaporization of hydrocarbons, or in the effluent stream. 

Numerous side reactions take place in the reaction zone, some of which are beneficial, but many of which are harmful (4). Some catalysts act to promote olefin isomerization to some extent as well as alkylation. In aviation and motor alkylate production, where more highly branched products are desirable, this isomerization reaction is of considerable benefit. This is especially true when the isomerization of butylene-1 to butylene-2 precedes the alkylation reaction. Harmful side reactions include hydrogen transfer and polymerization. The production of propane from propylene and of normal butane from butylene occurs by hydrogen transfer. Polymerization is very harmful because it not only produces the tar which spends the catalyst, but also reduces the yield of valuable products. 

Alkylation catalysts may have the ability of breaking down certain olefins and alkylating isobutane with the resulting olefins. A useful example of this ability is the charging of diisobutylene to an alkylation unit, wherein the diisobutylene is broken down and reacts with two molecules of isobutane to form two molecules of trimethylpentane. 

The first catalyst to be used widely in commercial alkylation for the production of aviation alkylate was sulfuric acid. The catalyst in the reaction zone is normally 88 to 92 wt. % H2SO4 with the makeup acid usually having a strength of 98 % or higher. 

With the advent of the alkyl aryl sulfonate type of detergent, aluminum chloride, as well as hydrofluoric acid, was used to a great extent for the production of dodecylbenzene. 

---