Synthesis Of Refrigeration System With Continuous Optimization Of Temperature Levels

As the main energy consumer in the chemical industry,the refrigeration system is featured with complicated integrated networks and intensive energy distribution.For this reason,the synthesis and optimization of refrigeration system is critical to the energy saving of chemical processes,and it is also a challenging systems engineering task.Based on the mathematical programming method,this research developed a novel methodology for handling the synthesis of multi-stage compression refrigeration systems with continuous temperature levels(number of levels,temperature levels,heat transfer duties,etc).The procedure comprises four steps:ⅰ)A superstructure is firstly established which could be used to describe all possible and meaning configurations of a compression refrigeration system;ⅱ)With the goal of minimizing the total compression shaft works,the optimal synthesis of compression refrigeration system can be formulated as a mixed-integer nonlinear program(MINLP)model;ⅲ)Solving the model and obtaining the optimal configuration and operating conditions;ⅳ)Via strictly steady simulation in Aspen Plus,the obtained configurations and parameters could be verified.Instead of pre-specifying discrete temperature levels as candidates in previous works,the temperature levels in this paper were set as continuous variable to search optimal solution,which contributed to a better match between the compression refrigeration system and process system.During the research,the exergy-analysis was introduced as an evaluation tool to discover the space for energy conservation and validation of the feasible solution;“the extended transshipment model” was introduced to describe the heat exchanged between compression refrigeration system and process system,and further accomplish the simultaneous optimization of refrigeration system and heat exchanger networks when the continuous refrigeration system temperature is considered.The superstructure-based MINLP method was illustrated through a case of ethylene compression refrigeration system in an industry ethylene plant,and demonstrated that,the continuous setting of temperature levels often led to an energy-saving matching with a result of 8.08% compression shaft works reduction,and the feasibility and consistency were verified and supported by Aspen Plus,which proved that the method is effective.