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
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Increasing FCC C3 and C4 reactor yields at the expense of gasoline requires catalyst formulation changes, higher reactor temperature, and/or converter technology modifications. 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 C3= recovery when all other variables remain constant. Higher reactor temperature also raises LPG yield, but increases reactor dry gas C2 minus yield, further increasing WGFR and making C3= 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/C4 yields. Product recovery flow schemes are unique with differing equipment designs, and unit specific constraints determine where capital needs to be employed. During early phases of the process 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 dictate 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 systems and equipment potentially impacted. For example, the wet gas compressor (WGC) system includes the compressor, 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 opportunities during pre-conceptual and conceptual project phases requires thorough process modelling and rigorous equipment 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 C3= recovery varies from 90-99% depending on operating parameters, equipment limits, reactor dry gas, and incremental LPG yields. The primary parameters affecting absorber C3= recovery and how to manipulate them will be discussed.
Regenerator and reactor
Regenerator, reactor, and catalyst technologies determine 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 technologies have lowered dry gas yield from thermal cracking, 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 execution to develop downstream scope with final yields tailored to capital investment appetite or desired investment return. Licensors also provide reactor overhead pressure and temperature, 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 represents the unit’s most critical system, which must operate 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 pressure control valve must have sufficient pressure drop available 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 regenerator 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 of the essential tools used to debottleneck when increasing LPG production. Lower pressure drop helps increase the WGC gas handling capacity because it raises suction pressure and reduces the actual gas volume.
Reactor temperature and catalyst formulation changes increase WGFR, which must stay within the stable operating range of the compressor while compressing the gas to maximum gas plant operating pressure. The air blower to WGC inlet is a single complex hydraulic system with many potential cost-effective modifications to allow higher LPG yield without replacing the WGC and/or driver. For instance, eliminating coke formation in the MC inlet nozzle (see Figure 2) can save 2-5 psi, which allows higher MC receiver pressure, thereby reducing WGFR.3 This is often ignored, yet it can help debottleneck the WGC.
Finding lower-cost opportunities
Replacing the WGC, its driver or paralleling operations are high-cost outcomes when increasing LPG yield. Although, this may be necessary when unit feed rate and LPG yield have increased over the years. Identifying opportunities to reduce system pressure drop starts with detailed pressure surveys, which isolate the various piping segments and equipment that generate pressure drop (see Figure 3). These measurements augment normal engineering tool calculations and are more accurate than any calculation tool result. The measured pressure profile helps to quickly identify debottlenecking options early in the work process. Even today, when technology allows such things as ‘digital twins’, old-school field measurements still have value.
Once the wet gas is compressed, the C3= plus must be condensed, absorbed, and stripped to allow further downstream processing. As more naphtha (gasoline) is converted to C3/C₄, condensing duty moves from the MC overhead to compressor inter-stage condensers, high pressure (HP) condensers, absorber inter-coolers, and debutanised gasoline recycle coolers (see Figure 4). Debutaniser gasoline recycle must be processed through the stripper and debutaniser, then cooled in fin-fan and/or cooling water (CW) exchangers.
Operating pressure and temperatures are important variables affecting condensing and recovery. For example, sometimes inter-stage condensers and after-condenser pressure drops are high, and bundle changes allow higher absorber operating pressure. In other cases, relatively short runs of piping and flow meters generate high pressure drop. Increasing absorber operating pressure by 20 psi improves recovery, especially with low operating pressure absorbers.
MC through absorber/stripper
This transcript reviews flow from the MC through the absorber/stripper bottoms. Other systems, including downstream distillation and treating systems, are all affected by higher LPG yield and must be considered during scope development. It is not unusual to limit LPG production to the ultimate capacity of the debutaniser, C3/C4 splitter, C3 splitter, or treating system vessels.
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