FCC Reactor Vapor Line Coking
SUMMARY
Identifying the causes, penalties and possible solutions to coking in the FCC reactor vapor line, particularly with regard to inlet nozzle coking affecting unit capacity, conversion, reliability and pressure and heat balance.
TEXT
Coking in the FCC reactor vapor line (VPL) increases pressure drop between the air blower discharge and the wet gas compressor suction. As pressure drop increases, wet gas compressor suction pressure must be lowered or regenerator pressure increased to maintain pressure balance. Moreover, when it occurs at the main column inlet it has caused coke to accumulate in the bottom of the main column, increased main column bottoms (MCB) system fouling, higher rates of erosion of the MCB pumps, and premature flooding of the main column slurry PA section internals when column loading was high. With heavier feeds containing more aromatic species and higher reactor operating temperatures to produce light olefins as petrochemical feedstocks, coking problems in vapor lines have begun to reappear.
In one documented case, vapor line coking increased VPL pressure drop by 6 psi [Mauleon, J.L., Reactor and “Vapor Line Coking Problems in Fluid Catalytic Cracking Unit”, Grace Seminar on FCCU, June 28, 1989, Paris, France] when the main column inlet nozzle became partially blocked with coke. Because many FCC’s already operate against the air blower or wet gas compressor volume or driver limit, feed rate or conversion must be reduced to stay within compressor constraints. But when it forms in the main column inlet nozzle there are many adverse consequences that reduce unit profitability. Mauleon and others have previously covered VPL coking; however, the authors will review causes, penalties, and possible solutions to inlet nozzle coking.
Heavier feeds, cold spots along the VPL low VPL velocity, line configuration near the main column inlet, and main column bottoms liquid being sucked into the VPL all have caused coke to form. Higher boiling point reactor products can condense where there are cold spots or some reaction products can polymerize to form large molecules that are non-volatile...