Application: Purification and recovery of olefins from FCC, RFCC and DCC offgas.
Products: Hydrogen, methane, ethylene and LPG.
Description: Refinery offgas (ROG) streams from fluid catalytic cracker (FCC) units, deep catalytic cracking (DCC) units, catalytic pyrolysis process (CPP) units and coker units are normally used as fuel gas in refineries. Most FCC type units recover their propylene separately. But the ROG steam (typically C2 and lighter) may, in certain cases, also contain a portion of propylene. However, these streams contain significant amounts of olefins (ethylene and propylene), which can be economically recovered. In fact, many such streams can be recovered with project payout times of less than one year. As FCC type units use more zeolitic type catalysts, the respective proportion of both propylene and ethylene will increase.
Offgas-recovery units can be integrated with existing olefins units or, if the flows are large enough, stand-alone units may be feasible. ROG recovery units can be broken down into sections including feed contaminant removal, ethylene recovery and propylene recovery. Feed contaminants including acid gases, O2, NOx, arsine, mercury, ammonia, nitrites, COS, acetylenes and water must be removed. The ROG feedstock pretreatment is critical since many of the trace components in the ROG streams can impact the ultimate product purity, downstream catalyst performance and operational safety.
The ethylene recovery section can be a stand-alone unit where either dilute ethylene or polymer-grade ethylene (PGE) is produced; or a unit where partially recovered streams suitable for integration into an ethylene plant recovery section are produced.
Integration of treated ROG into an ethylene plant involves compression and treatment /removal of contaminants. If an ethylene product is required (dilute ethylene or PGE), an additional section is needed that cryogenically separates hydrogen, nitrogen and methane in a cold box, followed by a demethanizer, a deethanizer; and for PGE and C2 splitter.
Removal of contaminants including acid gases, COS, RSH, NO2, NH3, HCN, H2O, AsH3 and Hg is achieved by established processing methods, depending on the concentrations in the ROG feed. The difficult contaminants to remove in the ROG are the O2 and NOx, which are typically removed by hydrogenation to H2O and NH3 (DeOxo unit). Commercially available hydrogenation catalysts cause significant loss of ethylene to ethane. BASF together with Shaw have developed a copper-based De-Oxo catalyst (R3-81), which in sulfided form is capable of complete hydrogenation of the O2 and NOx.
DeOxo reactor. The DeOxo Reactor, in which the sulfided-copper catalyst (R3-81) is used, serves a dual function. By removing the O2, NOx and acetylene, it provides necessary purification of olefins but is also vital toward the safety of the process. Without it, the formation of explosive deposits in and around the downstream cold box can become an issue. An additional, economic benefit of the Shaw Stone & Webster solution, comes from the superior selectivity of the special catalyst— allowing deep removal of O2 and NOx, with negligible ethylene loss.