Objective Optimization of Multi-Stage

Multi-Objective Optimization of Multi-Stage Gas-Phase Refrigeration Systems 

Nipen M. Shah, Gade Pandu Rangaiah and Andrew F. A. Hoadley Department of Chemical Engineering, Monash University, Clayton, VIC 3800 Australia 

Keywords Gas-phase Refrigeration, Dual Independent Expander Process, Liquefied Natural Gas, NSGA-II. 

Refrigeration systems are extensively used in the chemical industries in low temperature processes such as liquefaction of natural gas, ethylene purification and cryogenic air separation. In these kinds of low temperature systems, heat is rejected from the process by refrigeration to heat sinks being other process streams or refrigeration systems at the expense of mechanical work. The refrigeration systems employed are complex, and energy and capital intensive, and therefore, play a critical role in the overall plant economics. 

Liquefied Natural Gas (LNG) is a clean burning fossil fuel. It offers an energy density comparable to petrol and diesel fuels, and produces less pollution. LNG is about 0.2% of the volume of natural gas at standard temperature and pressure, thus making it more economical to transport over long distances. The refrigeration and liquefaction are the key sections of the LNG plant, which typically account for 30-40% of the capital cost of the overall plant (Shukri, 2004). There are several licensed processes available for LNG, but most fall into either a cascade refrigeration or mixed refrigerant scheme. 

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