“…In many cases, it is displacing other, more classical gas separation processes, such as adsorption, absorption, or cryogenic treatment. Using membrane techniques, good results are obtained in the separation of air components [ 2 , 3 , 4 ], biogas components [ 5 , 6 , 7 ], the separation of helium from natural gas [ 8 , 9 ], hydrogen recovery [ 10 , 11 , 12 ], natural gas sweetening [ 13 , 14 , 15 ], or air dehydration [ 16 ] and natural gas dehydration [ 17 , 18 ]. In order to improve the efficiency of the membrane process and to optimize the energy consumption of such a process, single or multi-stage plants are used, along with recirculation of selected streams [ 19 , 20 , 21 ].…”