Natural gas liquefaction is an energy-intensive process in which energy reduction is a main concern. This research focused on minimizing the energy of the pure refrigeration cycle in natural gas liquefaction by improving the subcooling system. To minimize energy consumption, a pure refrigeration cycle with a subcooling system was simulated, and the result was thermodynamically analyzed. The thermodynamic analysis identified an opportunity to reduce the energy consumption, and a new design was proposed for the subcooling system. In addition, the proposed design was deterministically optimized to find the optimal compressing ratio, temperature, pressure, and flow rate. As the result, the optimal operating conditions were determined, and the energy consumption was reduced by 17.74%.

This study investigates the effects of multistage compression on single mixed refrigerant processes in terms of specific work. Comparison of specific work published in the literature is not straightforward due to the variety of compression configurations and the design bases. Therefore, four configurations (two-, three-, and four-stage and pump-added three-stage compressions) along with three natural gas compositions were considered. To compare with the simulation and optimization results in the literature, these 12 cases, having the same design basis, were optimized by adjusting the optimization variables such as the flow rate and composition of the refrigerant, the compression ratio of each compressor, the inlet pressure of the first compressor, and the outlet temperatures of the hot and cold refrigerant streams. There were two important findings: (1) adding a pump reduces specific work more than adding a compressor or decreasing the minimum temperature difference value in the compressors; (2) among the four configurations, the refrigerant composition does not significantly change, although it greatly affects the efficiency. The former results from the compressor constraint of the gaseous inlet and the latter from the minimum temperature constraint of the multistream heat exchanger. Furthermore, direct comparisons to other studies were also performed showing the importance of optimization and the effect of the design basis.

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Using an optimization technique, this study proposes a new decision support system to improve the economics of petrochemical industries. In achieving the goal, we first develop two optimization models to support the decisions on (1) the hedge trade by minimizing purchase costs of raw materials, and (2) the production planning by maximizing profits from product sales. We then integrated these two models into a single decision support system using a recursive two-stage programming framework. On the basis of the integrated framework, we systematically identify the optimal solution, that is, the maximum profit, by simultaneously determining timing, amount, and price of raw materials to purchase, and the strategies for facility operation and product sales. To illustrate the capability of the proposed system, it was then applied to the business model of the petrochemical company in Korea. As a result, compared to the base case, it is revealed that the profitability of the petrochemical company could be improved by up to 4.82% by optimized hedging trades and 4.30% by optimizing the production plan.

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