Shaw’s Refinery Offgas (ROG) technology is used to purify fluid catalytic cracker (FCC) unit offgas. Normally, this contaminated gas is combined with the refinery fuel gas system. The ROG technology allows valuable ethylene and propylene to be recovered and sold primarily as polymer-grade product. However, FCC offgas often contains many light components and contaminants, making the contained ethylene unsalable as either a dilutant or polymer-grade product. An ROG unit can be designed to provide various levels of purification. At a minimum, the ROG unit provides a level of purity that allows the stream to be further processed in an ethylene plant recovery section.
The ROG unit is broken down into sections including feed contaminant removal, ethylene recovery and propylene recovery. Feed contaminants including acid gases, oxygen, NOx, arsine, mercury, ammonia, nitrites, COS, acetylenes and water must be removed. It is critical that the designer of the unit be experienced with feedstock pretreatment since many of the trace components in the offgas can have an impact on the ultimate product purity, catalyst performance and operational safety.
The ROG unit can include an ethylene recovery section that produces either dilutant or polymer-grade ethylene. Otherwise, the purified offgas stream is directed into the ethylene plant for further purification and recovery. Depending on the capacity constraints of the ethylene plant or the offgas quantity of methane, nitrogen and hydrogen, the ROG unit may include a cold box, followed by a demethanizer, a deethanizer and for polymer-grade ethylene, a C2 splitter as well. The ROG unit can be designed to remove contaminants including acid gases, COS, RSH, NO2, NH3, HCN, H2O, AsH3 and Hg. The difficult contaminants to remove are oxygen and NOx, which are typically removed by hydrogenation to H2O and NH3. Commercially available hydrogenation catalysts generally cause significant loss of the ethylene to ethane.
BASF and Shaw worked together to develop a reactor design based on the BASF copper-based catalyst PuriStar(R) R3-81, which is capable of essentially complete hydrogenation of the oxygen and NOx with minimal ethylene loss. The R3-81 catalyst is also more resistant to poisons and offers more operational stability than previous catalyst technologies. Elimination of oxygen and NOx is necessary for the safe operation of the subsequent processing steps of the ethylene plant. Oxygen and NOx can promote the formation of potentially explosive deposits or plug the cold box.
Economics: ROG streams from FCC units, deep catalytic cracking (DCC) units, catalytic pyrolysis process (CPP) units and coker units are normally used as fuel gas in refineries. 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.
Licensor: The Shaw Group