Cost-effective optimization design of light hydrocarbon recovery process based on exergy analysis

To solve the problems of high energy consumption and the difficulty of adapting to the changes in oil gas composition and state, which exist in the original light hydrocarbon recovery process that occurs in an oil gas field, the original "auxiliary cooling & direct heat exchanger" process is improved by taking advantage of the mainstream turbo-expander cryogenic process; this is based on the characteristics of the high pressure of the exploited inlet oil gas. With reduction of the total energy consumption as the goal, the newly designed process recovers the cold energy in the internal system to the greatest extent and efficiently achieves energy integration, which can eliminate the complex and energy-intensive propane refrigeration systems. Moreover, to obtain the optimal operating parameters of the new process for further evaluation of its advantages, an optimization model and solution strategy are established with the optimal economic benefit as the objective and the product quality and recovery as the constraints. To solve the optimization model to optimize the new process, an optimization algorithm based on Sequential Quadratic Programming in combination with the constraint boundary search is proposed, and the optimal operating conditions for the new process with optimal economic benefit are obtained. Compared with the original process, the new process is more reasonable in terms of energy utilization, and the exergy efficiency of the system is increased by 6.7%. The total operating cost is reduced by 18%, and the economic benefit is increased by approximately 12.5 million CNY/year owing to the increase in product recovery. In addition, the new process involves no risk of CO2 freezing; thus, it is safe and feasible. This study can provide theoretical reference for the design and actual production of light hydrocarbon recovery in oil gas fields.