Heat Exchangers in A C۳MR LNG Plant, Cooling Curves and Their Effects on Efficiency

In this paper, we are going to improve the energy efficiency of an industrial C۳MR base load LNG plant by changing its refrigerant components and its mole fractions in liquefaction and subcooling cycles based on the cooling curves generated in the main liquefaction and subcooling heat exchangers. Later on, selection features of the elements will be discussed. First, non-optimized C۳MR plant models and the obtained cooling curves reveals. The process is modeled by using the Hysys® software. The PRSV equation of state is used for thermodynamics properties calculations both for the natural gas and the refrigerants. Two methods for modeling and optimization are explained and the results are compared. The first optimization method is done by a try and error method, which uses temperature vs. enthalpy diagrams or thermodynamics second law (Exergy loss). This method is based on decreasing the temperature difference between hot and cold streams. In the second method, optimizer of Hysys® is used. In this method, a random mole fraction suggests by optimizer at each step and the plant’s consumption work calculates. By comparing the obtained consumption’works and choosing the least one, the relative mole fractions determines as the optimized refrigerants. The convergence of results of method ۱ and ۲ (The mole fractions of liquefaction and subcooling refrigerants) proves the accuracy of these methods and the models and primary predictions. The results show that by optimization of mixed refrigerants, it is possible to decrease the energy demand about ۱۰.۴ MW (۵.۳۶ %).