Natural gas is the fastest-growing fossil fuel and LNG is playing a growing role in the world’s gas supply. The liquefaction process is also by far the most energy-consuming part of the LNG chain. It is thus a priority today for the gas industry to decrease the cost and improve the efficiency of the liquefaction process of a plant. In this way, a novel techno-economic evolution of an existing NGL plant with adding an appropriate LNG production part is presented. Concerning the availability of propane, use of existing equipments and conditions of no structural changes in the existing installation, C3MR is used as the refrigeration system. For full recognition of the process, a high-accuracy surrogate model based on D-optimal approach is developed. MR composition (nitrogen, methane, ethane, and propane), inlet gas pressure of the LNG production part, demethanizer pressure, and high and low pressure of MR as the eight manipulated variables of the surrogate model predict the earned profit of the integrated plant. To increase profit, a hybrid GA-SQP optimization method is used. The results show that the earned profit of the optimized proposed plant with the LNG production capacity of 3.33 MTPA is 60.2% more than the existing NGL plant. In addition to increased earned profit, the thermodynamic efficiency is improved in the liquefaction section, too. Furthermore, the SPC value of 0.347 kWh kg^-1 LNG shows that the optimized plant has acceptable liquefaction efficiency. According to the optimization results, mixture variables are more effective than process variables on the earned profit. It is noticeable that increasing the ethane recovery not always increases profit in such integrated units.

Keywords