Vapour permeation

Gas separation Hollow-fibre for high-volume applications with low-flux, low-selectivity membranes in which concentration polarisation is easily controlled (nitrogen from air) Spiral-wound when fluxes are higher, feed gases more contaminated, and concentration polarisation a problem (natural gas separations, vapour permeation). 

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... 

While vapour permeation and hydrophilic pervaporation have readily found well-established areas for industrial application, in the case of organophilic pervaporation a clear industrial breakthrough has not yet been achieved. The reasons for this situation derive from the intrinsic character of this process and from the way some problems have been approached so far ... 

Water vapour permeation, which is inversely proportional to thickness and increases with air velocity... 

Moisture transfer through fibres, which increases with the moisture absorption. Of these mechanisms the water vapour permeation seems to be the most important... 

Moisture Vapour Transmission The rate at which water vapour permeates through a plastic... 

Zhu, H.Y., W.H. Gao and E.F. Vansant 1994, The pillared clay-carbon composite membrane the preparation, vapour permeation and separation of gas mixtures, presented at 3rd Int Conf. Inorg. Membr., Worcester, MA, USA. 

In particular they are not good for fuses, because their vapour permeates the fuses as time passes and damages them. The pieces must be assembled after the vapour is thoroughly dried out. 

Jafar JJ, Budd PM, and Hughes R. Enhancement of esterification reaction yield using zeolite-A vapour permeation membrane. J Membr 50 2002 199 117-123. 

Chmielewski, A.G. et al., and H/D Separation factors for liquid/vapour permeation of water through an hydrophobic... 

Baert, L. Remon, J.P. Water vapour permeation of aqueous based ethylacrylate methylmethacrylate copolymer films. Int. J. Pharm. 1993, 99, 181-187. 

S. Sommer, 2003, Pervaporation and vapour permeation with Mieroporous Inorganie Membranes, PhD Thesis, RWTH Aaehen, Germany. 

A. Jonquieres, R. Clement, P. Loehon, J. Neel, M. Dreseh, B. Chretien, 2001, Industrial state-of-the-art of pervaporation and vapour permeation in the western eountries. Journal of Membrane Science, vol. 206, 87 -117. 

According to a recent conference given by Prof. Kita [162], the classical synthesis method currently used by Mitsui allows to produce about 250 zeolite membranes per day. Both the LTA and T types (Na K) membranes are now commercial and more than 80 pervaporation and vapor permeation plants are operating in Japan for the dehydration of organic liquids [163]. A typical pervaporation system, similar to the one described in [8], is shown in Fig. 11. One of the most recent applications concerns the production of fuel ethanol from cellulosic biomass by a vapour permeation/ pervaporation combined process. The required heat is only 1 200 kcal per liter of product, i.e. half of that of the classical process. Mitsui has recently installed a bio-ethanol pilot plant based on tubular LTA membranes in Brazil (3 000 liters/day) and a plant with 30 000 liters/day has been erected in India. The operating temperature is 130 °C, the feed is 93 % ethanol, the permeate is water and the membrane selectivity is 10 000. 

For solid products the risk of migration is low and therefore interaction studies are not required over and above the normal stability test results. For semi-solid products it is necessary to look particularly at the migration of additives, vapour permeation and the effects of the product on the physical parameters of the pack. For liquid products the migration potential for the specific formulation is required, and the determination of active ingredient content under simulated use, along with extractives data is required for parenteral and ophthalmic products. Moisture permeation is important, particularly for solid products packaged in blister packs. 

Generally all plastic films are water-repellent (water-resistant). Various degrees of barrier are offered against water vapour permeation, gases (organic and inorganic), oils, solvents, aromatics, preservatives, etc. All films, if free of pinholes, provide a barrier to moulds, bacteria, etc., i.e. are a hygiene barrier. 

A similar diffusion mechanism applies for the other membrane processes mencioiied above - of a vapour out of a liquid mixture for pervaporation, or of one gas out of a mixture for gas or vapour permeation. These diffusion processes all require the continuity of material represented by a polymeric membrane, in sheet or tubular form, and cannot be undertaken in ceramic materials. 

Vapour permeation is used for the separation of a mixture of saturated vapours, and is often combined W ith distillation, to separate solvents from exhaust air streams, especially in degassing operations such as resin production. 

For a film of a single material, the water vapour permeation rate increases more rapidly with temperature than the permeability, because the vapour pressure of water increases rapidly with temperature (2.3 kPa at 20 °C, 7.4 kPa at 40 °C, 19.9 kPa at 60 °C, 47.2 kPa at 80 °C). From the definition... 

PV combines the evaporation of volatile components with their permeation through a membrane. It involves the use of a Hquid feed to produce a vapour permeate and a hquid reject. The feed is usually suppHed at above 100°C and at a pressure sHghdy above 1 bar. The permeate pressure is maintained at vacuum well below atmospheric. Vapourisation occurs as the permeating species pass through the membranes. Mosdy asymmetric composite hydrophihc membranes such as composite poly vinyl alcohol (PVA)/PS (or polyacrylonitrile) are used [43,44]. Membranes made from a thin layer of PDMS cast on PAN have been found to be usefiil in separating polar compounds from non-polar ones in deal-coholisation of Hquors. Flat sheet and spiral wound modules are commonly used. 

Solak, E.K. and Sanh, O. 2009. Permeation and separation characteristics of dimethylfor-mamide/water mixtures by vapour permeation and vapour permeation with temperature difference methods through a sodium alginate-g-n-vinyl-2-pyrrohdone membrane. Desalin. Water Treat. 2 148-155. 

In this work preliminary vapour permeation measurements were carried out with two different species, the rather bulky dichloromethane (DCM) molecules and the much smaller methanol molecules. Two typical permeation curves are displayed in Figure 4.8. The transport parameters, determined on the basis of the tangent method and Equations (4.9)-(4.11), are listed in Table 4.3. It contains the parameters dehned above as well as solubility C in the membrane in equilibrium with the feed pressure of penetrants. 

A similar example of a promising application of solar heat for intensified process systems is pervaporation. In pervaporation, a selective membrane is used as barrier between two phases, the liquid feed and the vapour permeate. The process depends on the sorption equilibrium and the mobility of the components through the membrane and is rather independent of the vapour liquid equilibrium. The desired component, which is in liquid form in the feed, permeates through the membrane and evaporates while passing the membrane, because the partial pressure of the permeating component is kept lower than the equilibrium vapour pressure [21]. Permeabilities depend on the solubility and diffusion rates through the membrane. 

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