Design and optimization of integrated membrane separation for natural gas decarbonization and light hydrocarbon recovery from LNG

Objective For natural gas with high CO₂ content, the use of a two-stage membrane system for separation has proven to be effective and economical for decarbonization. However, the membrane separators necessitate a considerable amount of cooling equipment and result in increased energy consumption. Additionally, the volume fraction of light hydrocarbons in natural gas increases after decarbonization. Recovering light hydrocarbons from the decarbonized natural gas can help reduce energy waste and minimize the need for cooling equipment.

Methods This paper proposes an approach that integrates membrane separation for natural gas decarbonization and light hydrocarbon recovery from LNG. HYSYS software was utilized to simulate both the single processes and the integrated processes, revealing the advantage of lower energy consumption in the integrated scenario compared to the single-process scenarios. Subsequent process optimization was conducted based on the simulation results, involving a comparative analysis of key parameters affecting process integration, such as comprehensive energy consumption, the volume fraction of CO₂ in the retentate gas, the volume fraction of methane, and the C₂₊ recovery rate. With the objective of minimizing comprehensive energy consumption, the Box-Behnken Design (BBD) response surface method was employed to establish a regression equation. A genetic algorithm was then used to solve this regression equation, ultimately yielding the optimized parameters.

Results Compared to the single-process scenarios, the integration of the processes resulted in lower energy consumption, specifically reducing comprehensive energy consumption by 2 364 kW. The volume fraction of CO₂ in the retentate gas decreased to 1.48％, while the volume fraction and output of methane reached 98.55％ and 5 422 kmol/h, respectively. Additionally, key parameters such as the primary membrane area, secondary membrane inlet temperature, secondary membrane area, and separator 2 inlet temperature were identified as influential factors affecting the comprehensive energy consumption, decarbonization efficiency, and light hydrocarbon recovery of the integrated processes. The final optimization results from solving the model are as follows: a primary membrane area of 11 200 m², a secondary membrane area of 11 200 m², and inlet temperatures of 40°C for the secondary membrane and −103°C for the separator 2.

Conclusion The design of integrating membrane separation for natural gas decarbonization and light hydrocarbon recovery fully leverages the LNG cooling capacity at LNG terminals. By reducing process energy consumption, this approach facilitates the simultaneous implementation of the natural gas purification process through decarbonization and the recovery of light hydrocarbons, maximizing the utilization of equipment and resources. The study outcomes offer insights into potential solutions for industrial applications focused on green and sustainable development.