MAPD

The C3 byproduct of ethylene production via steam cracking contains a high content
of propylene. Eliminating methyl acetylene and propadiene (MAPD) can produce a number of important benefits:

• Improve the distillation yield of propylene
• Reduce fouling in the pyrolysis furnaces from reduced MAPD in the propane
• Reduce energy required to reprocess undesirable C3 byproduct.
• Improve the yield of petrochemicals over fuel

A 2-bed selective hydrogenation unit can reduce the MAPD concentration to less than 10 ppm

Description
A C3 rich hydrocarbon from the top of the depropanizer enters the feed drum. The feed is pumped to 300 – 450 psig, mixed with hydrogen and then introduced to the primary reactor. The hydrogen flow is regulated in proportion to the hydrocarbon feed rate. A portion of the reactor effluent is recycled to the feed to limit the temperature rise and vaporization across the catalyst bed. Additional hydrogen is added to the guard reactor feed as required to meet the final MAPD specification.

The guard reactor effluent may be distilled to remove green oil and unreacted hydrogen, allowing the purified product to meet chemical grade propylene specifications, in most cases. Alternately, the reactor effluent is routed to a C3 splitter to produce polymer grade propylene.

THE FOLLOWING REACTIONS TAKE PLACE ACROSS THE CATALYST BED:
Selective hydrogenation — The desired reactions are the conversion methyl acetylene and propadiene to propylene.

Saturation – An undesirable reaction is the conversion of propylene to propane.

Dimerization – The least desirable reaction is the formation of heavier compounds. One example is the conversion of methyl acetylene to hexadiene.

Catalyst Performance Metrics
Conversion of MAPD is the desired 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, which is the inverse of residence time) and temperature. The ideal is high consumption at high LSHV at low temperature. Activity is adversely affected by sulfur, nitrogen, and oxygenates in the feed. The catalyst must be designed to overcome the inhibiting effects of these contaminants. Catalyst activity also determines how much unreacted hydrogen will be in the reactor effluent.

SELECTIVITY is the net amount of propylene produced compared to the amount of MAPD removed. Selectivity is a fundamental characteristic of the catalyst and therefore the focus of much design work. Without selectivity, all of the hydrogen fed could be consumed before all of the MAPD reacts. Catalyst selectivity also dictates the amount of hydrogen required to meet the product specification, how much propylene is converted to propane while achieving the required MAPD specification, and the temperature rise in the reactor. Additionally, selectivity also determines how much MAPD is converted to green oil.