Propylene Production from Ethylene

The process is divided into three main areas: (1) Dimerization; (2) Metathesis; and (3) Purification.

Dimerization. Fresh ethylene is mixed with catalyst and reactor circulating liquid and then fed to the reactor, which is a heat exchanger in the loop designed to remove the exothermic heat of reaction from the circulating liquid using cooling water. The main reaction is dimerization of ethylene to form n-butenes (primarily 2-butenes). The reactor product is quenched with caustic solution to stop the reaction. The catalyst quench system is designed to remove greater than 99.9% of the catalyst from the net dimerized product. The crude butene product is fed to the butene column with C4 recycle from the metathesis unit downstream. This column overhead is condensed sent as feed to the metathesis unit and the bottoms are sent to the heavy ends column, which is designed to distill gasoline blend by-product from the C5 and heavier components feeding the column.

Metathesis. Fresh and recycled ethylene and butenes are mixed and vaporized and heated to the reaction temperature, between 280-320 °C. The metathesis reaction occurs in a fixed bed catalytic reactor, butene is commonly employed in excess to minimize side reactions, which eventually occur, producing mostly C5-C8 olefins. Coke, another by-product of the reaction, is deposited on the catalyst throughout the process. Each reactor can run for about 30 days before requiring regeneration, in which coke is burned in a controlled atmosphere.

Purification. Propylene purification is carried out in two columns. The stream leaving the reactor is cooled and sent to the deethylenizer column, the overhead stream is recycled back to the reactor and the bottom stream is fed to the depropylenizer column which produces polymer grade propylene in the overhead and a heavies product stream (C4+) that is also recycled.