Purpose
Diolefins and mercaptans are contaminants to all alkylation and dimerization processes as well as to most etherification processes. Eliminating acetylenes and diolefins from an olefins-rich stream can produce a number of potential benefits, such as a reduction in:

• acid consumption in alkylation;
• catalyst fouling in dimerization;
• product gum concentration; and
• energy required for redistillation of the product.

Description
To eliminate these contaminants, Selective Hydrogenation Units (SHU) can be located at various points in the FCC recovery train.

In all cases, the extent of the reaction (e.g., thioetherification, selective hydrogenation, hydroisomerization, or olefin saturation) is determined by the choice of catalyst, and by process operating conditions.