Another retrofit design is to add an amine absorptionregeneration system in the upstream instead of physical absorption system. However, utility cost (steam cost of 4,299,410 $/year for both regenerators compared to 1,791,994 $/year in OSMAHRD)is high due to large amine circulation rate, resulting in a SCPUF of 0.578 $/kmol compared to 0.421 $/kmol in OSMAHRD. There-fore, this solution is not economical than membrane based retrofit designs. On the other hand, although parallel system is a popular way for retrofitting existing plants, it is not suitable in the present case study because the sharp increase in CO2partial pressure will cause severe corrosion of the original column built from carbon steel.