9 - New generation vapour permeation membranes

Abstract

In vapour permeation the feed is a vapour, in contrast to pervaporation, where the feed is a liquid. The process employs a membrane to provide a semipermeable barrier between the feed side under high pressure and the permeate side under low pressure. Separation is achieved by the different degrees to which components are dissolved in and diffuse through the membrane; the system works according to a solution-diffusion mechanism. The materials used in the membrane depend on the types of compounds being separated, and polar polymers are preferred for water transport. Therefore hydrophilic membranes, whether organic or inorganic, have been thoroughly explored; in particular those made from poly(vinyl alcohol), which is the basis for several commercial membranes. Incorporation of inorganic nanoparticles such as silica allows operation at high fluxes and temperatures, as required for steam separation for example. Furthermore, adding particular nanoparticles can improve the antiviral and antibacterial properties of polymeric membranes. Other mixed matrix membranes include an annealed sodium alginate/poly(vinyl alcohol) membrane that is complexed with calcium ions. The annealing process markedly affected the membrane morphology and led to shrinkage of the free volume between the polymer chains. Purely inorganic zeolite membranes have been prepared by depositing a zeolite on a porous α-alumina support seeded with zeolite NaA crystals. Dense intergrown zeolite crystals were formed on the outer surface, to give a system that was highly permeable to water. Novel metal–organic frameworks in nanoparticle form have been incorporated in silicone rubber to produce membranes suitable for use in biorefining. The species employed was a zeolitic imidazolate framework of exceptional thermal and chemical stability. The membranes produced are capable of avoiding severe plasticisation under aggressive feed conditions. They can have very selective adsorption characteristics that are capable of being tailored at the pore level to create unique interactions with guest molecules to meet a particular need. So far only a limited number of types of hydrophobic frameworks have been examined, with most effort going toward gas separation applications. Some may give rise to fouling, so this is an area that needs to be evaluated, and at worst, an appropriate pretreatment devised; additives with hydrophilic pore surfaces seem to be an essentially unexplored territory.