Total Cost Optimization of a Single Mixed Refrigerant Process Based on Equipment Cost and Life Expectancy

This study mainly focuses on minimizing the total cost of the single mixed refrigerant (SMR) process based on the equipment cost equations considering equipment life expectancy. Moreover, the energy and cost analyses are performed by comparing optimization results with two different objectives. The objective functions to be minimized include the total compression energy and the total annual cost, which is the sum of the annual capital cost and annual operating cost. By the compression energy minimization, the operating cost is significantly reduced by 16.2% because the largest part of this cost is taken by electricity for the compression energy requirement. In addition, a 14.0% capital cost saving is realized by the energy minimization. The results of total cost minimization show a 16.0% operating cost reduction from the base case, which is a little lower compared to energy minimization, but the capital cost saving is dramatically higher at 28.3%. Through the cost analyses, it is found that the compressors greatly affect not only the operating cost but also the capital cost. Two major variables that determine the energy requirement and cost of compressor are the compression ratio and flow rate. The compression ratio affects the operating cost more than the flow rate but for the capital cost, it is the other way around. Therefore, finding the optimal design for the compressor is the key consideration in minimizing the cost. This study found the optimal design and operating conditions to obtain the minimum total cost. As a result of cost minimization, the total cost can be reduced by 19.2% with a 15.61 total compression ratio and 520.7 t/h refrigerant flow rate.