(492d) Process Synthesis of Mixed Refrigerant Cascade System for Ethylene Plants

Refrigeration system generally works by indirectly transferring the heat from low-temperature sources to high-temperature sinks at the expense of electricity or compression work. Refrigeration system is one of the most important and critical operating system in chemical and petrochemical industries. It must satisfy different cooling tasks from different units working at different temperatures/pressures with a wide range; meanwhile it has to be integrated with other utilities (e.g., steam and cooling water) to accomplish an optimal plant-wide energy saving system. The traditional cascade refrigeration system (CRS) used in ethylene plants employs multiple refrigerants, each of which is circulated in a closed cycle and operated at multiple temperature and pressure levels. Unfortunately, the use of the multiple refrigerants require a very number of separate compressors, heat exchangers, pumps and associated pipes for each temperature and pressure level. Recently, mixed refrigerant cascade system (MRCS) has been introduced, where the pure working fluids are replaced by mixed working fluids. MRCS has many significant advantages than the tradition single-component refrigeration system. It needs fewer process units and fewer maintenances and thus less expenditures compared with CRS. The system operates within smaller temperature differences at the lower temperature which leads to a smaller increase in entropy and consequently a smaller work consumption.  In this paper, process synthesis model and operational conditions are developed in which the pure ethylene cycle of the conventional cascade refrigeration system is replaced by a mixed refrigerant. Meanwhile, a general MINLP (mixed-integer linear programming) model for the optimal synthesis of MRCS in ethylene plants has been solved. A case study about a mixed refrigerant cascade system used in a typical ethylene plant is employed to demonstrate the efficacy of the developed methodology.  Comprehensive thermodynamic analysis is also conducted to give deep understandings of the optimization results.