FCC-C3 SHU

Purpose
Acetylenes and diolefins are contaminants to all alkylation and dimerization processes. Eliminating methyl acetylene and propadiene (MAPD) from a propylene-rich C3 stream can produce a number of important benefits, including reductions in:

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

In a splitter, MAPD is removed from the propylene product by distillation, but the MAPD contaminates the corresponding propane product

Description
A C3-rich hydrocarbon enters the feed drum from the top of the LPG splitter. The feed is then pumped to 300 – 450 psig; mixed with hydrogen; heated; and introduced to the reactor. Hydrogen flow is regulated in proportion to the hydrocarbon feed rate.

After the purified product exits the reactor and is cooled, it can be routed to an alkylation unit, a dimerization unit, or a C3 Splitter. In most processes, it is not necessary to remove the small amounts of hydrogen and methane that remain in the product.

Catalyst Performance Metrics
Conversion of MAPD is the most important function of the catalyst. High conversion requires a combination of activity and selectivity.

ACTIVITY is gauged by the consumption of hydrogen at a given LHSV (liquid hourly space velocity — the inverse of residence time) and temperature. Ideal activity achieves high consumption at high LHSV at low temperature. Certain contaminants in the feed can adversely affect this outcome. A catalyst must be designed to overcome the inhibiting effects of these contaminants. Catalyst activity also determines how much unreacted hydrogen remains in the reactor effluent.

SELECTIVITY compares the relative amount of propylene saturated to the amount of MAPD converted. Because selectivity is a fundamental characteristic of the catalyst, selectivity becomes the focus of much design work. Without adjusting selectivity, all of the hydrogen could be consumed before all of the MAPD reacts. Selectivity also determines factors such as: the amount of hydrogen required to meet product specification; how much propylene is converted to propane while achieving the required MAPD specification; and the temperature rise in the reactor.