Evolution and optimization of the dual mixed refrigerant process of natural gas liquefaction

This study unfolds the important developments in the evolution of dual mixed refrigerant (DMR) process of natural gas (NG) liquefaction followed by its optimization. The initial designs of DMR process involve direct intermixing of non-equilibrium streams that causes thermodynamic irreversibility and reduces efficiency. Major developments that improved DMR process efficiency were the use of coil-wind type heat exchangers followed by three stage throttling in NG pre-cooling and direct utilization of cold energy available to the boil-off gas. After enumerating major developments a generic design of DMR process is selected and optimized for specific compression energy (SCE) and overall heat transfer coefficient (UA) using Box methodology and controlled elitist genetic algorithm. Single objective optimization of SCE and UA with Box methodology results in savings of 36% and 15% respectively. There exists a partial trade-off between SCE and UA, thus the savings in SCE are offset by the increase of UA. Consequently a multi-objective optimization is performed that results in a simultaneous decrease of 24% and 3% in the SCE and UA values, compared to the base case.