Dimerization, Oligomerization, and Condensation

Owing to the uncertainty in the MTBE market, many new projects are being placed on hold. Some of the existing plants are being modified to produce an alternate product such as alkylate (typically the mixture of branched octanes). Alkylate can be an ideal gasoline blendstock as it has high octane and paraffinic nature. A re-

cent patent from UOP describes a process based on indirect alkylation, which uses RD in the production of isobutylene dimer, disobutylene, which on hydrogenation gives the alkylate [78]. This process uses the same feedstock, that is, the C4 stream along with methanol. The advantage of conducting dimerization in the presence of methanol or MTBE is to protect the catalyst from coking and avoid formation of the undesired oligomers. The motive behind using RD is to increase the catalyst life, achieve better selectivity, avoid formation of C12 oligomers and reduce C4 recycle. Oligomerization of linear butenes and removal of butadiene through dimerization in RD unit, as an alternative to selective hydrogenation, has also been described [79, 80].

The condensation reactions of aldehydes, for example acetaldehyde to paraldehyde or formaldehyde to trioxane, also exhibit similar features and can become important candidate reactions for RD [76, 77]. An RD column with total reflux and removal of condensed product through the bottom stream is a recommended configuration for such systems. Aldol condensation of acetone to diacetone alcohol has been widely studied as a reaction that may give better conversion and higher selectivity to diacetone alcohol in the RD column, by suppressing the further formation of mesityl oxide. The reaction takes place in the presence of anion-exchange resins [38]. Similarly condensation of butaraldehyde to 2-ethyl hexanal has also been investigated in RD, with a view to increasing selectivity [39]. In all these reactions the condensation product is normally heavier than the rest of the components and can be conveniently separated out during the course of the reaction.

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