TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred System: The system (S1) further comprises a compressor to compress a permeate stream from the primary membrane unit to the secondary membrane unit.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The pre-membrane comprises one polybenzoxazole polymer membrane or at least one crosslinked polybenzoxazole polymer membrane. The secondary membrane unit comprises at least one glassy polymer. The secondary membrane unit comprises at least one polybenzoxazole polymer membrane or at least one crosslinked polybenzoxazole polymer membrane. The polybenzoxazole polymer membrane is prepared from a hydroxyl-containing polyimide with pendent hydroxyl groups ortho- to a heterocyclic imide nitrogen. The hydroxyl-containing polyimide is a compound having units of formula (I). The crosslinked polybenzoxazole polymer membrane is prepared from a polyimide comprising crosslinkable functional groups and pendent hydroxyl groups ortho to a heterocyclic imide nitrogen. The polyimide comprising crosslinkable functional groups and pendent hydroxyl groups ortho to a heterocyclic imide nitrogen is a compound having units of formula (II). The primary membrane and the secondary membrane are made from a glassy polymer selected from: polysulfones; sulfonated polysulfones; polyetherimides; cellulosic polymers; polyamides; polyimides; polyamide/imides; polyketones, polyether ketones; poly(arylene oxides) such as poly(phenylene oxide) and poly(xylene oxide); poly(esteramide-diisocyanate); polyurethanes; polyesters (including polyarylates), comprising poly(ethylene terephthalate), poly(alkyl methacrylates), poly(acrylates), and poly(phenylene terephthalate); polysulfides; poly(ethylene), poly(propylene), poly(butene-1), poly(4-methyl pentene-1), polyvinyls, poly(vinyl chloride), poly(vinyl fluoride), poly(vinylidene chloride), poly(vinylidene fluoride), poly(vinyl alcohol), poly(vinyl esters), poly(vinyl pyridines), poly(vinyl pyrrolidones), poly(vinyl ethers), poly(vinyl ketones), poly(vinyl aldehydes), poly(vinyl amides), poly(vinyl amines), poly(vinyl urethanes), poly(vinyl ureas), poly(vinyl phosphates), and poly(vinyl sulfates); polyallyls; poly(benzobenzimidazole); polyhydrazides; polyoxadiazoles; polytriazoles; poly(benzimidazole); polycarbodiimides; polyphosphazines; microporous polymers; interpolymers, and grafts and blends containing any of these polymers. The pre-membrane unit, primary membrane unit and the secondary membrane unit are in the form of sheets, disks, tubes or hollow fibers.X1=moiety selected from 8 moieties as given in the specification, e.g. moiety of formula (i), (ii), or naphthalene-tetrayl, biphenylene-tetrayl or phenylene-1,2,4,5-tetrayl;X2 and Y1=2,4-dihydroxy-phenylene-1,5-diyl, 4,4'-dihydroxy-1,1'-biphenylene-3,3'-diyl, or a moiety of formula (iii);Q1=-O-, -C(O)-, or -S(O)2-;Q2=-C(CH3)2-, -C(CF3)2-, or -S-;R=-C(CH3)2-, -C(CF3)2-, -S(O)2-, -O-, or -S-;X3=moiety of formula (i);X4=moiety selected from 7 moieties as given in the specification, e.g. moiety of formula (ii), naphthalene-tetrayl, biphenylene-tetrayl or phenylene-1,2,4,5-tetrayl. EXAMPLE - No suitable example is given.