Non-porous membrane

Buxbaum, R.E. and T.L. Marker, Hydrogen transport through non-porous membranes of palladium-coated niobium, tantalum and vanadium. /. Membr. Sci., 85, 29-38,1993. 

The main emphasis in this chapter is on the use of membranes for separations in liquid systems. As discussed by Koros and Chern(30) and Kesting and Fritzsche(31), gas mixtures may also be separated by membranes and both porous and non-porous membranes may be used. In the former case, Knudsen flow can result in separation, though the effect is relatively small. Much better separation is achieved with non-porous polymer membranes where the transport mechanism is based on sorption and diffusion. As for reverse osmosis and pervaporation, the transport equations for gas permeation through dense polymer membranes are based on Fick s Law, material transport being a function of the partial pressure difference across the membrane. 

It has recently been demonstrated that solutes can be extracted from ionic liquids by perevaporation. This technique is based on the preferential partitioning of the solute from a liquid feed into a dense, non-porous membrane. The ionic liquids do not permeate the membrane. This technique can be applied to the recovery of volatile solutes from temperature-sensitive reactions such as bioconversions carried out in ionic liquids (34). 

Pervaporation is a membrane separation process in which a dense, non-porous membrane separates a liquid feed solution from a vapour permeate (Fig. 19.2c). The transport across the membrane barrier is therefore based, generally, on a solution-difliision mechanism with an intense solute-membrane interaction. It... 

These tests were performed by measuring the rate of dapiprazole release from gel vehicles 6 and 8 to an aqueous sink, through a non-porous membrane. For this purpose, a thin nylon membrane, which had been preconditioned by extraction with ethanol (1 h at 60°C) and overnight hydration in distilled water at room temperature, was positioned between the receiving (5 ml) and the donating (4 ml) compartment of a glass GH flow-through diffusion cell [5], thermostated at 30°... 

Pervaporation (PV) is a membrane-based process used to separate aqueous, azeotropic solvent mixtures. This is done using a hydrophihc, non-porous membrane that is highly selective to water. Figure 3.9 shows a typical PV system that produces a dehydrated solvent stream (retentate) from a solvent/water feed. 

R.E. Buxbaum and T.L. Marker, Hydrogen Transport Through Non-porous Membranes of Palladium-coated Niobium, Tantalum, and Vanadium, 7. Membr. Sci. 85, 29 (1993). 

The third main class of separation methods, the use of micro-porous and non-porous membranes as semi-permeable barriers (see Figure 2c) is rapidly gaining popularity in industrial separation processes for application to difficult and highly selective separations. Membranes are usually fabricated from natural fibres, synthetic polymers, ceramics or metals, but they may also consist of liquid films. Solid membranes are fabricated into flat sheets, tubes, hollow fibres or spiral-wound sheets. For the micro-porous membranes, separation is effected by differing rates of diffusion through the pores, while for non-porous membranes, separation occurs because of differences in both the solubility in the membrane and the rate of diffusion through the membrane. Table 2 is a compilation of the more common industrial separation operations based on the use of a barrier. A more comprehensive table is given by Seader and Henley.1... 

Reverse osmosis Liquid Non-porous membrane with pressure gradient Desalination of sea water... 

Osmosis involves the transfer, by a concentration gradient, of a solvent through a membrane into a mixture of solute and solvent. The membrane is almost non-permeable to the solute. In reverse osmosis, transport of solvent in the opposite direction is effected by imposing a pressure, higher than the osmotic pressure, on the feed side. Using a non-porous membrane, reverse osmosis successfully desalts water. 

Although reverse osmosis can be used to separate organic and aqueous-organic liquid mixtures, very high pressures are required. Alternatively, pervaporation can be used in which the species being absorbed by, and transported through, the non-porous membrane are evaporated. This method, which uses much lower pressures than reverse osmosis, but where the heat of vapourisation must be supplied, is used to separate azeotropic mixtures. 

As an alternative to simple distillation, pervaporation could be used [124], This technique makes use of non-porous membranes with a selective layer consisting of hydrophilic or hydrophobic polymer. Those compounds, which are volatile and soluble in the membrane, are evaporated into the vacuum on the permeate side. By this means, selective separation, for example of volatile impurities from volatile auxiliary agents in the ionic liquid, should be possible. 

Water can permeate through non-porous membranes by a solution-diffusion process. The driving force for permeation is the net pressure difference, Ap -flux can be represented by the equation ... 

Mullins F. Evaluation of a novel non-porous membrane extraction probe to determine sulphonylureas in plasma with analysis by LC-MS/MS. J. Sep. Sci. 2001 24 593-598. 

In the case of non-porous membranes, the polymer should have sufficient solubility and permeability for drugs. 

PE is a crystalline polymer with many grades, with a wide variety of crystallinity and molecular weight. A porous PE membrane is more drug-permeable than non-porous membranes of low-crystalline PE, both of which are available for the reservoir system. EVA, as a copolymer of ethylene and vinyl acetate with 9-40 wt.% vinyl acetate, is favorably used for the reservoir membrane. However, it should be noted that EVA, especially the copolymer with high vinyl acetate content, is resistant to hydrophilic liquid substances such as water and glycerin. But this copolymer swells and deforms itself in lipophilic liquid substances. Hence, paraffin, squalene, and IPM could not be used as reservoir liquids in combination with EVA. 

Membranes play an important role in natural science and for many technical applications. Depending on their purpose, their shape can be very different. For instance, membranes include porous or non-porous films, either supported or non-supported, with two interfaces surrounded by a gas or by a liquid. Important properties of non-porous membranes are their permeability for certain compounds and their stability. In biological cells their main task is to stabilize the cell and to separate the cell plasma from the environment. Furthermore, different cells and cell compartments have to communicate with each other which requires selective permeability of the membranes. For industrial applications membranes are often used for separation of gases, liquids, or ions. Foams and emulsions for instance are macroscopic composite systems consisting of many membranes. They contain the continuous liquid phase surrounded by the dispersed gas phase (foams) or by another liquid (emulsions). Beside these application possibilities membranes give the opportunity to investigate many questions related to basic research, e.g. finite size effects. 

For dense (non-porous) membranes used for oxygen separation the flux becomes insensitive for a decrease of the layer thickness for a critical thickness which is of the order of 0.1-0.3 mm depending on the permeant-membrane... 

Air separation by membranes to produce N2 or O2 is interesting especially for small to medium capacities and medium (up to 99%) purity. The highest purities of oxygen (>99%) are to be obtained with dense (non-porous) membranes. The economics of gas separation processes is discussed in detail by Spilman [7] together with a number of potentially interesting applications. 

Dense (non-porous) membranes and surface reaction limitation... 

In dense, non-porous membranes, surface limitations to oxygen permeation are a common phenomenon as can be understood from the very low adsorption levels and large activation energies on the dense membrane materials (see Chapter 10). For hydrogen permeation in dense metal membranes estimates have been made by Govind [105]. 

Finally, dense (i.e. non-porous) membranes permeate O2 or H2 only and so are important only in applications where these gases play a role such as in air... 

Autoclave moulding is typically used in the aerospace industry for the production of high-value composites from prepregs. The laminate, which is covered on both sides by a fine polyester cloth peel-ply (for enhancing the surface effect), is built up on the mould surface. The top surface of the laminate is covered by a porous release film and bleeder cloth. The whole assembly is then covered with a non-porous membrane, which is sealed to the mould, and then placed inside an autoclave as shown in Figure 6.19. 

Non-porous membranes can be used for extraction of polar and non-polar compounds from liquid samples using only minimal amount of organic solvent. A non-porous membrane is a liquid or a solid (e.g. polymeric) phase sandwiched between two other phases, usually aqueous but can also be gaseous (8). One of these two phases contains the components to be extracted, i.e. the donor phase. On the other side of the membrane is the acceptor phase, i.e. where the extracted components are collected. Usually, the membrane unit is made of two blocks of inert material with a machined groove in each. The membrane is placed in-between these blocks and clamped together, so that a channel (typically 10-1000... 

As mentioned earlier, the mechanism of gas separation by non-porous membranes basically is different from the one in microporous membranes. Gas molecules actually dissolve and diffuse in the dense membrane matrix. Differences in permeability, therefore, will result not only from diffusivity (mobility) differences of the various gas species but also from differences in physico-chemical interactions of these species within the polymer, determining the amount of gas that can be accommodated per unit volume of the matrix. 

The ion exchange membrane, a non-porous, membranous polymer having ionic groups, is a typical functional polymer. The characteristics of the membrane are (1) ion conductivity, (2) hydrophilicity and (3) the existence of fixed carrier (ion exchange groups). According to these characteristics, various applications have arisen. Table 1.1 shows example applications. 

Most importantly non-porous membranes such as ion exchange membranes, membranes for reverse osmosis, pervaporation, etc. should not be used in systems in which insoluble compounds precipitate on and in the membranes because this will destroy them and their functionality will be lost. Secondly all separation membranes, including ion exchange membranes, can achieve excellent performance by use of an appropriate apparatus and under optimum operation. For example, because solute and solvent transport speeds in the membrane phase are different from those in the solution, membrane-solution interfaces play an important role in separation, which depends on the structure of the apparatus and its operation. In this chapter, many examples of applications of ion exchange membranes are explained together with the principles on which they rely to achieve separation. 

Membrane separators offer the possibility of compact systems that can achieve fuel conversions in excess of equilibrium values by continuously removing the product hydrogen. Many different types of membrane material are available and a choice between them has to be made on the basis of their compatibility with the operational environment, their performance and their cost. Separators may be classified as (i) non-porous membranes, e.g., membranes based on metals, alloys, metal oxides or metal—ceramic composites, and (ii) ordered microporous membranes, e.g., dense silica, zeolites and polymers. For the separation of hot gases, the most promising are ceramic membranes. 

---