To purify propylene-propane cuts from pyrolysis processes via selective catalytic hydrogenation of methylacetylene and propadiene (MAPD) impurities. Steam cracker C3 effluents typically contain over 90% propylene, with propane and MAPD making up the balance. Although distillation can be used to remove MAPD, it is often not practical or economical for achieving a propylene product meeting the parts-per-million levels required by chemical- and polymer-grade propylene specifications. Furthermore, distillation results in propylene losses. Selective hydrogenation is the route most commonly applied as it not only achieves the tight MAPD specifications, but it also produces more propylene.
The C3 cut is joined by recycled C3s and makeup hydrogen prior to entering the main reactor (1). There the MAPD is catalytically hydrogenated, forming propylene and traces of propane. A single reactor suffices for polymer-grade propylene (MAPD content < 10 ppm) when a C3 splitter is used. A finishing reactor (2) can be used to reduce MAPD content to five or even one ppm. A second reactor is advantageous when making chemical-grade propylene. With a typical specification of 95% propylene, 5% propane and < 5 ppm MAPD, a costly C3 splitter system is avoided. Licensor: Axens