Besides OSMAHRD and TSMAHRD, several other possible retrofit designs are considered, and their feasibility is discussed in this section. The first one is to add a physical absorption system before the existing chemical absorption system, functioning as a pretreatment unit to reduce the acid gas concentration to a level similar to that in the base case. The main advantage is its low cost (as physical solvents are non-corrosive, requiring only carbon steel for construction). Besides, a higher CO2concentration in the feed can increase the efficiency of the physical solvent. However, the feed contains a non-negligible level of heavier hydrocarbons (C3+is around 7 mol%), which are co-absorbed and rejected in the waste acid gas stream. This heavier hydrocarbons loss overrides the benefit of low capital investment, making physical absorption before the existing chemical absorption system less attractive than membrane based retrofit designs.