[Objective] Helium is a strategic resource, which is a by-product of natural gas processing. The tail gas from the nitrogen rejection unit (NRU) of the liquified natural gas (LNG) plant is a main helium-containing gas with a helium mole content of about 1.0% - 5.0%. The existing process uses cryogenic distillation to enrich it into crude helium (> 50. 0 %) and obtains high-purity helium (> 99.99%) through catalytic oxidation and pressure swing adsorption (PSA). However, the existing helium enrichment process based on cryogenic distillation requires severe operating conditions with high pressure and low temperature, leading to complex operations, high energy consumption and equipment investment. Besides, new impurities (e.g., O2 and H2G) can be introduced in the helium separation process under catalytic oxidation dehydrogenation, which increases the load of the adsorption device significantly and leads to the loss of potential hydrogen resources. [Methods] Therefore, this paper proposed a novel helium separation process from the NRU tail gas based on a membrane coupling electrochemical hydrogen pump (EHP). Whole helium separation process can be divided into two parts with enrichment and purification. A two-stage membrane separation was used to achieve efficient enrichment of low-content helium. Due to the advantages of membrane with no phase change, small footprint and simple operation, it can realize energy saving in the helium enrichment process. The crude helium was refined through PSA to remove N2 and CH4. Then, the EHP was used to separate hydrogen and helium. Moreover, high-purity hydrogen and helium can be obtained without introducing impurities. Processes modeling and data analysis were conducted on Aspen HYSYS. Due to the strong interactions between process parameters in the two-stage membrane process, the response surface method (RSM) was used to optimize four key process parameters with membrane areas and feedstock pressures. Since the helium purification unit based on EHP is located at the end of the process and has no interaction with the previous units. The single-factor sensitivity analysis was used for parameter optimization of EHP. [Results] The optimization results of the membrane separation process show that the helium mole purity and recovery rate of the crude helium can reach 64. 94% and 95. 67% under the optimal operating conditions (M-101 : 4759. 5 m2, M-102: 435. 3 m2; M-101 and M-102 feedstock pressures arc 6010. 3 and 4352. 5 kPa* respectively). Furthermore* high-purity helium and hydrogen can be achieved simultaneously through EHP under the optimal parameters. The applied potential of two-stage EHP is 1 V* and the MEA areas of two-stage EHP arc 39 and 17 m2, respectively. Economic evaluation results show that the production cost of the helium in the coupling process is 125. 47 CNY/m3, The financial evaluation of the new helium separation process was conducted based on the economic evaluation data. The dynamic payback period is 2. 09 years, and the internal rate of return is 79%. [Conclusions] In summary, the proposed membrane coupling EHP helium separation process has significant economic and social benefits. It provides a feasible route for the independent industrial production of high-purity helium in China. © 2023 Press of Tsinghua University. All rights reserved.
