Alan R. English
Your C3 in fuel gas looks to be on the high side of typical, but this doesn’t really tell us how much you are losing. Many refiners also monitor the percentage of total C3 production lost to fuel gas.
Pressure and absorbent liquid flow rate are the main control variables. Pressure should be maximized for best recovery but this must be weighed against any capacity reduction which might result from reduced wet gas compressor capacity. Absorbent liquids should enter at the lowest temperatures achievable. Any attempt to increase C3 recovery will also increase C2 recovery, forcing a corresponding increase in stripper bottom temperature to control C2 in the LPG product. Therefore, the optimum C3 recovery is a balance between the value of recovered C3, wet gas compressor capacity and stripper energy cost. The best way to identify this optimum is with a rigorous simulation of the entire system (wet gas compressor to debutanizer) using any of the available programs (HYSYS, PetroSIM, PROII, etc.). Close monitoring of the LPG C2 level is also critical. You should operate as close to the product specification as possible as anything lower indicates excessive stripper bottom temperature which would waste energy and reduce C3 recovery.
A low cost capital improvement would be to recycle debutanizer bottoms to the top of the absorber with the High Pressure Separator liquid routed to a lower tray (perhaps 3 or 5, numbered from the top). This improves recovery of the C3 in the HPS liquid.