Economically recover olefins from FCC offgases

The concept of ethylene and propylene recovery from fluid catalytic cracking (FCC) offgases is not new; however, its application has been infrequent. For typical catalytic cracking of atmospheric and vacuum gas oils, ethylene yields range from 2.0 to 3.5 lb/bbl of FCC feed. The ethylene typically amounts to 8 to 18 vol% of FCC offgas and is normally routed to the fuel gas system. Variations in ethylene concentrations are affected by the FCC feed composition and cracking severity. This ethylene yield is anywhere from 0.7% to 1.1% of the FCC feed, as opposed to 26% to 36% for naphtha or gas oil cracking in conventional olefin plants. Due to high FCC unit feedrates (typically 25,000 to 85,000 bpsd for most North American refineries) even with a low ethylene yield, the olefins production can be significant. Here, two approaches to olefins recovery are addressed. In the first, ethylene is recovered as a dilute gas at a concentration of about 15 vol% and serves as raw material for ethylbenzene and, subsequently, styrene. In the second approach, ethylene is recovered as a pure polymer-grade liquid. Propylene recovery is identical for both approaches. The concept for producing polymer-grade liquid ethylene is described in detailmore » in terms of process technology, cost estimates and economic parameters.« less