While the WGC ICFM can be managed with pressure and temperature changes, these changes affect gas density and typically raise the compressor power needs. Motor- driven compressors have a gearbox that sets speed based on the original design. Changing a compressor gearbox can be a low cost option to lower WGC speed, which reduces discharge pressure to stay within equipment MAWP and lower driver power. Other options include modifying the compressor rotors and/or reducing the number of wheels. An experienced process engineer can use the existing com- pressor polytropic head/flow and polytropic efficiency/flow curves to evaluate potential options during pre-conceptual and conceptual design, with the WGC OEM finalising the evaluation and generating details for cost estimating purposes. LPG condensing and recovery-operating variables Figure 4 shows the MC overhead through the absorber/ stripper bottoms process flow. Optimum reactor LPG yield depends on whether it can be compressed, condensed, and then recovered; otherwise, the incremental LPG simply moves from the refinery gasoline pool to fuel gas. Unloading the WGC requires higher receiver pressure and lower temperature because increased C 3 /C 4 yields come at the expense of gasoline. The majority of the gasoline is condensed in the MC receiver, whereas the C 3 /C 4’ s have to be condensed in the interstage and HP receiver condensers, with the remaining C 3 /C 4’s recovered by recycling debutanised gasoline to the absorber. It is impossible to materially increase C 3 /C 4 reac- tor yields and maintain high recovery without increasing debutanised gasoline recycle because there is not enough MC overhead liquid to recover the C 3 /C 4’ s produced. However, limiting debutaniser recycle is often necessary because both the absorber and stripper capacity can be
WGFR
28,000 26,000 24,000 30,000 32,000 34,000 40,000 36,000 38,000 44,000 42,000 46,000 22,000
130 ˚F 115 ˚F 100 ˚F
RX yield constant 13.1 wt% C3=
3.5 psig
6.5 psig
9.5 psig
Receiver pressure
Figure 11 Receiver temperature and pressure optimisation
Complete WGC system evaluation, including the driver Evaluating the WGC capacity and driver power require- ments is more complicated than the previous examples. These simply looked at WGC low stage ICFM manage- ment, which is only the starting point. Managing WGFR to the low stage does not address the driver power, devel- oped inter-stage discharge pressure, inter-stage condens- ing duty, high stage WGFR, and ultimately the discharge pressure. Low and high stage WGC discharge pressures cannot exceed the downstream equipment MAWP. Ideally, discharge pressure should be maximised against MAWP limits without exceeding driver capacity. Compressor motor horsepower can be calculated from efficiency curves assuming a nominal 3% gear loss and motor efficiency from the motor datasheet. Calculated motor horsepower should be close to the measured power consumption or steam consumption for the turbine.
Wet gas compressor discharge
98% recovery ~3% C =
Sponge absorber
Interstage liquid Interstage water
Liquid
Debutanised gasoline (lean oil) (No C =’S)
Condenser outlet
CW
Liquid
Primary absorber
Wet gas
(C = content varies 1.5–5.0+%)
Stripper
CW
CW
Main column receiver
LCO PA
Vapour
CW
High pressure receiver
Preheat
Debutanised gasoline
Debutaniser feed
Pressure
Figure 12 WGC discharge through stripper bottoms
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PTQ Q2 2023
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