(140f) Optimization and Thermodynamic Analysis of Mixed Refrigerant System In Ethylene Plants

Optimization and Thermodynamic Analysis of Mixed Refrigerant System in Ethylene Plants

Karthik Krishnadevarajan, Jian Zhang and Qiang Xu

Dan F Smith Department of Chemical Engineering

Lamar University, Beaumont, TX 77710

Abstract

                Refrigeration is one of the most important and crucial system in chemical plants. A refrigeration system generally works by removing heat from low temperature streams through vapor compression cycles at the expense of mechanical work. Since a refrigeration system can cool down a process stream below the ambient temperature, it is indispensable to cryogenic cooling and used for separation process in many chemical industries, such as large scale productions of ethylene, oxygen, nitrogen and liquefied natural gas.

                Usually refrigeration systems employ single component as refrigerant as long as it is thermodynamically desirable and operationally feasible. But a multi component mixture can also be used as refrigerant, because a mixed refrigerant system provides a continuum of temperatures with smaller temperature differences even at the lower temperatures. This leads to a low energy loss in the system. Mixed refrigerant system has many inherent advantages than the traditional single component refrigerant system.

                In an ethylene plant, usually refrigeration is provided by cascading two or three single-component refrigeration systems. This process is not only expensive but also inefficient because it needs more facilities in comparison to single component refrigeration systems. The process streams in ethylene plant vary from 40 to -140 ^oC. For a single component refrigerant it is impossible to provide constant refrigeration at a particular temperature. Mixed Refrigerant is used in order to improve refrigeration process by using four components as mixed refrigerant.

                The present work deals with developing a mixed refrigerant system for an ethylene plant. The process is developed using a rigorous simulation model. Also the process is optimized for the optimal design and operating parameters of the system. The design and operating parameters include compressor work and total flow rate of the mixed refrigerants. This model has few challenging factors. First, the minimum temperature difference in the heat exchangers is taken as 2 ^oC. Second, the heat ­­­­­­duty of all the heat exchangers in the process is fixed based on the plant data. Finally, the process is non linear so optimization is more complex. To demonstrate this work, process streams thermodynamics are also analyzed using thermodynamic analysis. The base case results are compared with optimized results based on the rigorous simulation and optimization methods.