System Optimization Of Turboexpander Process For Natural Gas Liquid(NGL) Recovery

Turboexpander process is the most widely-used method in Natural Gas Liquid(NGL) recovery. Turboexpander is a machine turning gas pressure energy to mechanical energy, while the natural gas obtains cryogenic cold quantity during depressurization, which can be used in NGL condensation and recovery.There are piles of process parameters in turboexpander process, e.g., operating pressure and feed precooling temperature. The parameters codetermine the recovery and energy consumption of the system. So either in design stage or in practice, choosing the appropriate values for the parameters can increase the light hydrocarbon recovery and process profit. However, due to highly integration in energy flows and topology, the restrictions and the interactions between the parameters are very complicated that the optimization of the NGL recovery system is especially difficult. Meanwhile, there are process restrictions such as ensuring CO2 freezing safe margin, keeping heat exchanger temperature approach, etc., turning into big obstacles in parameters optimization.Based on an NGL recovery engineering of 60×104 Nm3/h pipeline gas, this paper focuses on the optimization of NGL recovery system, including the downstream decarbonization process. Firstly, the system profit is chosen as the object function to build up optimization model. And based on the optimization idea of “decompose first, integrate later”, the total profit function is divided into the subfunction of cryogenic separation and the one of decarbonization process. Secondly, using process simulation technology, the inherent features of the turboexpander process are researched, which can uncover the influence of parameters on recovery process and the interaction between parameters. Then based on the analysis, taking all the factors and restrictions into consideration, a proper optimization strategy is put forward to solve the model. Finally, the two subprocesses are combined to get the system optimization results. After optimization, the NGL recovery system obtains largest process profit under 30 bar, which is RMB 83.8 thousand per hour.The in-depth analysis of turboexpander process, optimization idea and model of the NGL recovery system, and the solution algorithm suggested are helpful to understand the process features more clearly, contributing to the increase the light hydrocarbon recovery and process profit. Also, this research can provide theoretical guidance for design and production of turboexpander process, and is very significant to improve NGL recovery technology.