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Session 3613

A Novel Use of HYSYS to Design an Industrial Refrigeration System K. Hing Pang California Polytechnic University, Pomona

Introduction

Industrial refrigeration systems such as those present in an ethylene plant or an ammonia plant are designed based on the demand of refrigerant in the process. Once the duties of the heat exchangers and the temperatures of the refrigerant are specified, the refrigeration system consisting of compressors, condensers and flash drums can be designed. Process simulators such as Provision or Aspen can be used to design the refrigeration system by using feed-forward or feed-back controllers to determine required refrigerant flow rates and the resulting compressor capacities. This technique often results in many control loops, each requiring many iterations to converge. This paper is the result of a senior project (1) undertaken by a chemical engineering senior at Cal Poly, Pomona. It describes how HYSYS has been used successfully to design a refrigeration system with 4 compressors, 13 heat exchangers and 4 flash drums without resorting to numerous control and recycle loops. The intent of this paper is not to provide a rigorous comparison between simulators, but to focus on a special technique used for solving industrial refrigeration design problems. It is expected that the same technique can be applied to other utility systems.

What is an industrial refrigeration system?

An ethylene manufacturing process usually consists of a hot section and a cold section. The primary equipment in the hot section is the pyrolysis furnace in which hydrocarbons are cracked at temperatures in excess of 1000o F to form ethylene and a slate of byproducts including methane, ethane, propylene, C4’s and C5 and heavier hydrocarbons. The cold section consists of a series of distillation columns, flash drums and exchangers which separate pure ethylene from the byproducts. The cold section operates under cryogenic conditions with temperatures ranging from -160o F to -25o F. Refrigerants, usually ethylene and/or propylene of several temperature levels, are used as coolants in exchangers and condensers.

Figure (1) shows the basic elements of a single level ethylene refrigeration system. Ethylene vapor is compressed in a compressor C1 from pressure P1 to P2 while temperature rises from T1 to T2. The compressed ethylene is cooled to temperature T3 and condensed to liquid in an exchanger E1. The liquid ethylene is expanded adiabatically in an expansion valve V1 to P3 while the temperature is further lowered to T4 and the the liquid is partially vaporized. The vapor is separated in a flash drum from the liquid which is used as a refrigerant in a process exchanger E2. The process stream is cooled and the liquid refrigerant absorbs the process duty Q and is vaporized. The vapor ethylene returns to the suction head of the compressor hence completing the cycle. The size of the compressor C1, exchanger E1, expansion valve V1 and the flash drum F1 depends on the temperature level of the refrigerant and the process duty Q.

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Hing Pang, K. (1998, June), A Novel Use Of Hysys To Design An Industrial Refrigeration System Paper presented at 1998 Annual Conference, Seattle, Washington. 10.18260/1-2--7317