Simulation of a Dual Mixed Refrigerant Process using i Nonsmooth Framework

Abstract

Natural gas liquefaction is an energy intensive process with very small driving forces at cryogenic temperatures. Small temperature differences arise from the excessive exergy destruction that occurs from irreversible heat transfer at low temperatures. As a result, even a small change in driving forces in the low temperature region can propagate into large exergy losses that must be compensated by additional compression power. Along with the significant investments and operating costs associated with these processes, this demands a robust and accurate simulation tool. Nonsmooth simulation models for single mixed refrigerant processes already exist in the literature. However, these processes are relatively simple, and are normally only considered for small-scale production or floating operations. Other processes, such as dual mixed refrigerant processes, are therefore normally considered for large-scale production of LNG. It is necessary to investigate whether the nonsmooth flowsheeting strategy is capable of also handling these more complex liquefaction processes. This article describes a simulation model for a dual mixed refrigerant process. The model is solved for two cases using the Peng-Robinson equation of state, each solving for a different set of unknown variables. Both cases converged within a few iterations, showing nearly identical results to simulations run in Aspen Plus.