Constrained non-linear optimisation of a process for liquefaction of natural gas including a geometrical and thermo-hydraulic model of a compact heat exchanger

•Optimisation of a LNG-process including a full cryogenic heat exchanger model.•The objective function is to minimise compression work.•Size and weight of the heat exchanger are additional constraints in the optimisation.•Results compared minimal internal approach temperature (MITA) – formulation.•Discusses how low MITA value implicitly assumes unrealistic large heat exchangers.

A great deal of effort has been put into improving natural gas liquefaction processes, and a number of new process configurations have been described. Recent literature has identified a need for more realistic heat exchanger models to obtain optimum design and operating conditions that do not compromise safety, or that are unrealistic. Here we describe a concept for finding the design and operating conditions of a single mixed-refrigerant process which gives minimum power consumption under given space or weight constraints. We use a sophisticated heat exchanger modelling framework that takes into account system geometry and resolves the details of the heat exchanger through conservation equations coupled with accurate models of thermo-physical properties. First, we find the feasible region which does not compromise safety with Ledinegg instabilities. We then identify the optimal operating conditions for a specific design within this region, before identifying the process design that requires least power consumption. We illustrate how this differs from a purely thermodynamic optimisation, and discuss our key results.