Optimal process design for recovering effluent gas at subambient temperature

Effluent gas is common in the petrochemical industry. The recovery of effluent gas is of great importance to resource recycle and environmental protection. However, low concentration, low boiling point feature of petrochemical effluent decreases the recovery economy. This article presents a systematic approach to design process networks for recovering effluent gas at subambient temperature. An effluent recovery superstructure that contains HENS (Heat Exchanger Network Synthesis) operator, pressure operator and separation operator is proposed. In addition, multicomponent nonisothermal phase change of streams are allowed inside the HENS operator. Since the process streams exhibit nonlinear temperature-enthalpy relations (T-H curve), we correlate the enthalpy (H) with temperature (T), pressure (P) and compositions (m) of the process streams by a cubic polynomial function to balance the nonlinearity and accuracy of the state equations. The objective of the model is to maximize total annualized revenue by regulating connecting structures as well as operating parameters. Two industry cases are presented to show the accuracy and effectiveness of the proposed approach.