Environmental study of waste energy recovery by using exergy and economic analysis in a fluid catalytic cracking unit

An increase in fossil fuel consumption has significantly increased the concentration of greenhouse gases (GHGs). Waste energy recovery can reduce GHGs by reducing fossil fuel consumption. In the FCC unit in refineries, the catalyst is continuously regenerated by burning off the deposited coke with air and a large flux of waste gas with high temperature is generated which is vented into the atmosphere. The purpose of this study was to investigate the effect of waste heat/pressure recovery of the waste gas on the reduction of GHGs and air pollutant emissions. Based on this objective, exergy and economic analysis were carried out for two scenarios (S-1 and S-2). The S-1 scenario involved the installation of a Heat Recovery Steam Generator (HRSG), while S-2 applied the simultaneous usage of HRSG and a turbo-expander to evaluate electricity production using waste gas pressure. The exergy of waste gas was formulated and an in-house code was developed for solving the equations via a trial and error method. The results showed that exergy loss of the waste gas was higher than 660 MW and it was possible to recover about 64 MW and 75 MW in the S-1 and S-2, respectively. The amount of steam and the electrical energy produced were found to be about 88 ton/h and 8323 MWh/month, respectively. The results also showed that S-1 can reduce 72227 tCO2e of GHGs and 327 ton of air pollutant and S-2 can reduce 143464 tCO2e of GHGs and 649 ton of air pollutant annually. The economic indexes were evaluated and the results indicated that the internal rates of return (IRR) were found to be 33.18% and 36.76% for S-1 and S-2, respectively. This showed that the two scenarios were economically feasible, but from an environmental, economic and energy recovery standpoint, S-2 was the best scenario and the economic analysis on S-2 certified that there was no economic risk.