PTQ Q2 2023 Issue

Mitigating FCC gas plant impacts when increasing reactor LPG yields Increasing FCC reactor LPG yields at the expense of gasoline increases profitability, but equipment downstream of the converter requires modifications

Darrell Campbell, Tony Barletta and Scott Golden Process Consulting Services

I ncreasing FCC C 3 and C 4 reactor yields at the expense of gasoline requires catalyst formulation changes, higher reactor temperature, and/or converter technology modi- fications. Equipment downstream of the converter typically requires modifications to handle higher wet gas flow rate (WGFR), to condense incremental LPG, and to recover it in the absorber/stripper system. Catalyst additive ZSM-5 cracks mid-boiling range gasoline to LPG, increases WGFR, and decreases liquid rate to the absorber, reducing C 3 = recovery when all other variables remain constant. Higher reactor temperature also raises LPG yield, but increases reactor dry gas C 2 minus yield, further increasing WGFR and making C 3= recovery more difficult. Major operating systems and critical equipment from the main column (MC) through the absorber/stripper system must be reviewed when increasing C3 /C 4 yields. Product recovery flow schemes are unique with differing equipment designs, and unit specific constraints determine where capi - tal needs to be employed. During early phases of the pro - cess work, it is important to narrow the reactor yield cases to those that fit the refiner’s investment appetite. Existing process systems and equipment limitations dic - tate potential yield shift opportunities and accompanying investment costs. Incremental LPG has to be compressed, condensed, recovered, fractionated, and treated. Identifying opportunities starts with a thorough field survey of the unit. Unlike the converter system where only a few pieces of equipment are affected, downstream there are major sys - tems and equipment potentially impacted. For example, the wet gas compressor (WGC) system includes the compres - sor, motor or turbine, inter-stage and after-condensers, and piping, which are all potential obstacles to increasing LPG production.1 Identifying and prioritising cost-effective opportuni - ties during pre-conceptual and conceptual project phases requires thorough process modelling and rigorous equip- ment evaluation, not cursory work based on assumptions, guesses, or rules of thumb. For example, shifting C3 = yields from current to very high may be desirable, but a more moderate increase may better fit the constraints or capital spending appetite. Reactor wt% C3 yields of 7.5-13.1 wt% will be used to show how variable changes affect the WGC. Typical gas plant C 3= recovery varies from 90-99% depend - ing on operating parameters, equipment limits, reactor dry

gas, and incremental LPG yields. The primary parameters affecting absorber C 3= recovery and how to manipulate them will be discussed. Regenerator and reactor Regenerator, reactor, and catalyst technologies deter- mine the reactor yields for a given feed quality. They are constantly being improved and innovated by catalyst and licensor technology providers. In recent years, these tech - nologies have lowered dry gas yield from thermal crack - ing, increased LPG, raised conversion, shifted LPG olefins percentages, and many other tailored solutions. FCC and/or catalyst technology providers generate reactor yields which must be processed through the downstream equipment. Reactor effluent composition includes feed products, steam, and inert gases entrained in the circulating catalyst. Typically, 2-4 yield cases are provided during project execu - tion to develop downstream scope with final yields tailored to capital investment appetite or desired investment return. Licensors also provide reactor overhead pressure and tem- perature, or alternately at the MC inlet nozzle. Air blower to WGC inlet The process flow diagram in Figure 1 shows the air blower to WGC inlet system. This interconnected equipment rep - resents the unit’s most critical system, which must oper - ate stably to avoid any number of potential problems.2 The blower supplies air to the regenerator, which burns coke off the catalyst. The pressure control valve on the main column overhead receiver sets the reactor pressure. The differential pressure between the reactor and regenerator must supply sufficient pressure drop for the limiting slide valve to ensure stable catalyst circulation. The MC overhead receiver pres - sure control valve must have sufficient pressure drop avail - able to ensure stable operation during normal pressure and temperature fluctuations. The regenerator maximum allowable working pressure (MAWP) or the air blower maximum discharge pressure limits regenerator operating pressure. Sometimes regenera - tor pressure can be increased to allow higher WGC suction pressure and more LPG yield without exceeding the WGC limit. The WGC inlet pressure and system pressure drop set the reactor operating pressure, which is limited by its MAWP. Reducing air blower to WGC pressure drop is one

PTQ Q2 2023