System optimization of turbo-expander process for natural gas liquid recovery

Turbo-expander process is a complex technology to recover the light hydrocarbon in natural gas where energy is highly integrated and some important parameters such as demethanizer pressure and precooling temperature interact with each other, which also codetermine both the recovery and energy consumption of the system. Based on an NGL recovery engineering of 60 × 104 Nm3/h pipeline gas, this paper focuses on the optimization for NGL recovery system. The overall profits of whole process are chosen as objective function and optimization model is developed. Considering all the influential factors and restrictions such as CO2 freezing, a proper optimization strategy is put forward which is based on SQP (Sequential Quadratic Programming) along with constraint boundary research. The optimization model is solved by Aspen Plus combined with Excel VBA to determine the optimal value of parameters which satisfy both the constraints of the process and the maximum economic benefits of the system. After optimization, it turns out that the overall profits have been improved by 13.5%. The optimization model and the corresponding solution algorithm can provide theoretical guidance for process design and production practice of the turbo-expander process.