Pervaporation membrane water/organic selective membranes

In the case of pervaporation of dissolved volatile organic compounds (VOCs) from water, the magnitude of the concentration polarization effect is a function of the enrichment factor. The selectivity of pervaporation membranes to different VOCs varies widely, so the intrinsic enrichment and the magnitude of concentration polarization effects depend strongly on the solute. Table 4.2 shows experimentally measured enrichment values for a series of dilute VOC solutions treated with silicone rubber membranes in spiral-wound modules [15], When these values are superimposed on the Wijmans plot as shown in Figure 4.12, the concentration polarization modulus varies from 1.0, that is, no concentration polarization, for isopropanol, to 0.1 for trichloroethane, which has an enrichment of 5700. 

A third possibility, illustrated in Figure 9.7(c), is to sweep the permeate side of the membrane with a counter-current flow of carrier gas. In the example shown, the carrier gas is cooled to condense and recover the permeate vapor, and the gas is recirculated. This mode of operation has little to offer compared to temperature-gradient-driven pervaporation, because both require cooling water for the condenser. However, if the permeate has no value and can be discarded without condensation (for example, in the pervaporative dehydration of an organic solvent with an extremely water-selective membrane), this is the preferred mode of operation. In this case, the permeate would contain only water plus a trace of organic solvent and could be discharged or incinerated at low cost. No permeate refrigeration is required [36],... 

K.W. Boddeker and G. Bengtson, Selective Pervaporation of Organics from Water, in Pervaporation Membrane Separation Processes, R.Y.M. Huang (ed.), Elsevier, Amsterdam, pp. 437-460 (1991). 

The inorganic silica membranes, also commercial, have solved the problem of thermal and chemical stability however, these membranes are only used for dehydration purposes, leaving the problem of separation of organic mixtures unsolved. As we have seen previously, due to the versatility and special feamres of zeolites, new applications in pervaporation that are not possible with other membranes could be developed with zeolite membranes. GaUego-Lizon et al. [110] compared different types of commercial available membranes zeolite NaA from SMART Chemical Company Ltd., sUica (PERVAP SMS) and polymeric (PERVAP 2202 and PERVAP 2510) both from Sulzer Chemtech GmbH, for the pervaporation of water/f-butanol mixtures. The highest water flux was obtained with the silica membrane (3.5 kg/m h) while the zeolite membrane exhibited the highest selectivity (16,000). 

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. 

The selectivity of a pervaporation membrane is defined in different ways. Most commonly found in Hterature is the so-called a-value. This is calculated as the ratio of the more-permeable component (e.g. water) to the less-permeable component (e.g. organic) in the permeate divided by the respective ratio in the feed. 

Pervaporation membranes were developed for the dehydration of ethanol and other organic solvents. Therefore, the dense selective layer is made of polyvinyl alcohol that is one of the most hydrophilic materials. Water is preferentially sorbed to polyvinyl alcohol and also preferentially transported. To suppress the excessive swelling of polymer in water, polyvinyl alcohol is partially cross-linked by dialdehydes such as glutaraldehyde [23]. 

The phenomenon of solvent transport through solid barriers has three aspects which discussed under the heading of permeability. These are the permeation of solvent through materials (films, containers, etc.) the use of pervaporation membranes to separate organic solvents from water or water from solvents the manufacture of permeate selective membranes. 

As usual with membrane separations, the membrane is critical for success. Currently, two different classes of membranes are used commercially for pervaporation. To remove traces of organics from water a hydrophobic membrane, most commonly silicone rubber is used. To remove traces of water from organic solvents a hydrophilic membrane such as cellulose acetate, ion exchange men )rane, polyacrylic acid, polysulfone, pol5 inyl alcohol, composite membrane, and ceramic zeolite is used. Both types of membranes are nonporous and operate by a solution-diffusion mechanism Selecting a membrane that will preferentially permeate the more dilute conponent will usually reduce the membrane area required. Membrane life is typically about four years tBaker. 20041. 

Bacterial cellulose can be used as a selective pervaporation membrane in water-organic mixtures. An easy experiment can be set up if a bacterial cellulose membrane is available. The membrane should be tightly fixed between two transparent containers, like a partition. Both containers should have an aperture for the introduction of the... 

Hydrophilic pervaporation membranes can be very selective, mainly because the materials for this type of membrane show both sorption selectivity and diffusion selectivity much larger than unity. So they are widely utilized in the dehydration of organic solvents, whenever water is the minor component. 

Table Al.ll Organic Pervaporation Behavior of Organic/Water Systems for Selected Membranes...

Pervaporation is a process in which organic solvent water mixture or organic solvent mixture can be separated by partial vaporization through a nonporous permeate selective membrane. In this process liquid feed mixture circulates in contact with the active nonporous side of the membrane while a vacuum is applied on the other side of the membrane. A phase change of membrane-selective permeate takes place in the membrane. The membrane-selective permeate diffuses through the membrane and desorbs on the posterior side of the membrane. Later, it evaporates with the help of a vacuum from the posterior side of the active nonporous membrane. The transport of the permeate through a nonporous permeate-selective membrane is quite complex. This could be explained in three steps, which are as follows ... 

The mechanical properties of these membranes were improved by including a crosslinker, methylene bisacrylamide, in the aqueous phase, and by using a styrene/butyl acrylate (BA) mixture as the continuous phase [185]. The styrene/BA mixture had to be prepolymerised to low conversion to allow HIPE formation. The permeation rate of the membrane was improved by including a porogen (hexane) in the organic phase, generating a permanent porous structure [186]. The pervaporation rate was indeed increased, however a drop in selectivity for water from water/ethanol mixtures was also observed. 

H.H. Nijhuis, M.V.H. Mulder and C.A. Smolders, Selection of Elastomeric Membranes for the Removal of Volatile Organic Components from Water, in Proceedings of Third International Conference on Pervaporation Processes in the Chemical Industry, R. Bakish (ed.), Bakish Materials Corp., Englewood, NJ, pp. 239-251 (1988). 

In certain cases it is desirable to selectively remove a volatile solute from a solution that contains other, less volatile, solutes as well as the solvent. Some examples are the reduction of ethanol content from alcoholic beverages or from dilute alcoholic extracts of aromatic flavors and fragrances from plant sources such as fruits or flowers. Conventional pervaporation would facilitate removal of water from such mixtures while retaining ethanol and the higher molecular weight organics that comprise the characteristic aroma and flavor profile of the products of interest. On the other hand, membrane distillation or osmotic distillation cannot retain the volatile components at all. 

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