Optimal Synthesis of Cascade Refrigeration in Liquefied Natural Gas Cycles by Pinch-Exergy

Abstract

Iran's vast common natural gas resources and the necessity to extract and export it as Liquefied Natural Gas (LNG) to distances more than 3000Km opens a lucrative field for researchers to optimize LNG cycles. In this article heat integration in cryogenic cycles by determining interacycle partition temperature and optimizing refrigeration features like Subcooler, Presaturator, aftercooler, reboiler, superheater and economizer is investigated to reduce compressor shaftwork. Better conceptual understanding of design improvement is illustrated on Composite¬ Curve (CC) and Exergetic Grand Composite Curves (EGCC) of Pinch-Exergy analysis diagrams. A program (LNG-Pro) is developed by integrating Visual Basic Application, Refprop(Reference Fluid Thermodynamic and Transport Properties) and Excel MINLP Solver to automate the methodology on an LNG plant with the capacity equal to a south pars gas field phase. Shaftwork is step by step reduced from 1479.36Kj/Kg to 1255.5Kj/Kg and 1158.8Kj/Kg which is 2.46% less than required energy for liquefaction in LNG industry.