PVA membrane

The selectivity of pervaporation membranes varies considerably and has a critical effect on the overall separation obtained. The range of results that can be obtained for the same solutions and different membranes is illustrated in Figure 41 for the separation of acetone from water using two types of membrane (89). The figure shows the concentration of acetone in the permeate as a function of the concentration in the feed. The two membranes shown have dramatically different properties. The siUcone mbber membrane removes acetone selectively, whereas the cross-linked poly(vinyl alcohol) (PVA) membrane removes water selectively. This difference occurs because siUcone mbber is hydrophobic and mbbery, thus permeates the acetone preferentially. PVA, on the other hand, is hydrophilic and glassy, thus permeates the small hydrophilic water molecules preferentially. 

Fig. 41. The pervaporation separation of acetone—water mixtures achieved with a water-selective poly(vinyl alcohol) (PVA) membrane and with an acetone-selective siUcone mbber membrane. The PVA membrane is best suited to removing small amounts of water from a concentrated acetone solution, whereas the siUcone mbber membrane is best suited to removing small amounts of acetone from a dilute acetone stream (89).

Using pervaporation through PVA membranes, J. W. Rhim et al. [17] have studied the separation of water-phenol mixtures. 

Another approach to enhance separation performance of membrane for dehydration of isopropanol is the modification of PVA membranes in gaseous plasma [30], The modification of membrane properties in nitrogen plasma environment lead to increase in selectivity by about 1477 at 25 °C such increase in the selectivity is justified by an increase of cross-linking on membrane surface provoked by plasma treatment. 

The same authors reported the possibilities of using a membrane made by PVA modified by LiCl, whose surface has been modified by exposure to low-pressure nitrogen plasma [31], The best results have been obtained for 0.05 wt% of LiCl in PVA membrane at 25 °C (selectivity 14 and flux 250 g m"2lT ). 

Table 2. The characteristics of the iso-propanol (IPA) dehydration process, by using pervaporation technique, through (PAA.HCl)-PVA membrane...

In a previous section, the effect of plasma on PVA surface for pervaporation processes was also mentioned. In fact, plasma treatment is a surface-modification method to control the hydrophilicity-hydrophobicity balance of polymer materials in order to optimize their properties in various domains, such as adhesion, biocompatibility and membrane-separation techniques. Non-porous PVA membranes were prepared by the cast-evaporating method and covered with an allyl alcohol or acrylic acid plasma-polymerized layer the effect of plasma treatment on the increase of PVA membrane surface hydrophobicity was checked [37].The allyl alcohol plasma layer was weakly crosslinked, in contrast to the acrylic acid layer. The best results for the dehydration of ethanol were obtained using allyl alcohol treatment. The selectivity of treated membrane (H20 wt% in the pervaporate in the range 83-92 and a water selectivity, aH2o, of 250 at 25 °C) is higher than that of the non-treated one (aH2o = 19) as well as that of the acrylic acid treated membrane (aH2o = 22). 

Solid PVA-Co2+ composite asymetric membranes have been prepared starting from PVA and two different salts Co(N03)2 and Co(CH3COO)2, respectively, in order to separate cyclohexene/cyclohexan mixtures. A facilitated transport mechanism has been evidenced, due to the capacity of Co2+ ions to coordinate the olefin molecules [82], The authors reported stronger complexation of Co2+ ions with cyclohexene in the case of PVA/ Co(CH3COO)2 mixtures then in the case of PVA/ Co(N03)2 mixtures. It was found that for a concentration ratio of ([Co2+]/[OH]) by 0.75 mol/mol, the permeation flux of PVA membrane containing Co2+ increases 2-3 times and the separation factor increses 50 times compared with pure PVA membrane. 

It was observed that the n-PrOH concentration in the permeate through the CD/PVA membrane by pervaporation was approximately same as that in the feed solution, namely, the PrOH isomers could hardly be separated through these membranes by pervaporation. The n-PrOH concentration in the permeate obtained by evapomeation was higher than that in the... 

The n-PrOH concentration in the permeate and the normalized permeation rate increased with the increasing CD content in the CD/PVA membrane. The addition of CD in the PVA membrane determined the increasing of the swelling degree and preferential sorption of n-PrOH and p-xylene, due to the fact that the affinity of CD for these isomers was stronger than that for i-PrOH and o-xylene respectively [84],... 

The influence of the CD content in the membrane and the n-PrOH respectively p-xylene content in the feed mixture on the separation factors and sorption and diffusion selectivities of the CD/PVA membranes for the n-PrOH/I-PrOH and p-xylene and o-xylene mixtures by evapomeation are presented in tables 12 and 13. 

These results show the CD/PVA membranes are good candidates for isomers separation from organic liquid mixtures by evapomeation. 

Phenol red immobilized PVA membrane for an optical pH sensor is developed based on the same approach, since the molecular structure of phenol red is similar to that of phenolphthalein. Phenol red was first reacted with the formaldehyde to produce hydroxymethyl groups, and then it was attached to PVA membrane via the hydroxymethyl groups. The changes of spectra characteristics after immobilization, the ionic strength effects, response time, reproducibility and long-term stability of the sensor membrane are discussed by Z. Liu et al. [170],... 

In this article, results of studies of the water and salt transport properties of PVA membranes and a method for preparing thin skinned, high flux PVA membranes are reported. 

Radiation Crossllnked PVA Meinbranes. The water and salt permeability coefficients of the radiation crossllnked PVA membranes, obtained by using equations 1 and 2 were found to decrease with increasing the applied pressure. A linear correlation was found between the reciprocal of the water permeability coefficient and the pressure, as shown in Figures 1 and 2, at various temperatures. This linear correlation can be expressed by the following equation ... 

The temperature dependence of the water permeability of radiation crossllnked PVA membranes Is shown In Figure 5. From this dependence an activation energy of 6.0i0.2 kcal/mole can be derived, In good agreement with data reported earlier by Peter and Mlttelstadt (7) and by PaulW. ... 

Diffusive Salt Permeability Through PVA Membranes. In order to verify the Interdependence between the water and salt transport through PVA, the diffusive permeability of salt through an un-pressurlzed PVA membrane was measured. The diffusion system employed In this experiment Is described schematically In Figure 8. The diffusion cell consists of an upper cell and a lower cell, separated by the tested membrane, a 150 micron thick, untreated PVA membrane. Initially, the lower cell contalnes distilled water, which Is circulated through a conductivity cell, and the... 

Figure 4. Pressure dependence of water fluxes through PVA membranes as determined by Ehra-Lima and Paul (6) and in this work ( ) Ebra-Lima and Paul s data at 24°C ( = SSg/n) (O) data for radiation cross-linked, 115,000 mol wt, 100% hydrolyzed PVA (X = 4.0 /im).

Figure 5. Temperature dependence of the water permeability of radiation cross-linked PVA membranes under various pressure differentials ( ) AP = 200 psi ...

Diffusive salt permeability through u treated PVA membranes measured at atmospheric pressure ( car Is x 10 )... 

Heat Treated PVA Membranes. The concept of independent water and salt transport through PVA Is further supported by the permeability properties of heat treated PVA membranes. It was found that by subjecting the PVA membranes to heat treatment, a sharp decrease of the water and salt permeabilities Is caused. The... 

Study of the temperature dependence of the salt and water permeabilities through heat treated PVA membranes indicates that the heat treatment effect is expressed mainly in changes in permeability activation energies. The activation parameters for water and salt permeability of PVA membranes following various treatments are presented in Table IV. It is evident that the salt permeability activation energy is increasing about 2-3 times as... 

Temperature dependence of water and salt permeabilities of heat treated PVA membranes -E /RT P=P e ... 

The salt and water permeabilities of these membranes were compared with those of homogeneous PVA membranes heat treated under similar conditions. The salt rejections of several homogeneous and asymmetric membranes are presented in Figure 10. 

Figure 10. Salt rejection of heat-treated asymmetric and homogeneous PVA membranes as a function of feed concentration at aP = 1000 psi and t = 30°C ( <>) ASS ( J AS23 (O) AS4 (9) AS3. The solid lines represent the data obtained for the heat-treated homogeneous PVA membranes.

The results of a reverse osmosis study of radiation crossllnk-ed and heat treated polyvinyl alcohol(PVA) membranes are reported. In the framework of this study the permeability of water and salt through these membranes was investigated. In parallel, the diffusive transport of salt through PVA was also studied. The results suggest that the transport of salt and water through PVA is uncoupled, The salt transport data can be rationalized in terms of a modified solution-diffusion model. 

Figure 11. SEMs of asymmetric PVA membranes at X3000 magnification (a) the dense side (skin) of the membrane (b) the porous side of the membrane.

Polymers may exhibit non-Fickian diffusion below the Tgt but a transition to Fickian diffusion is noted as the temperature is raised above the Tr The permeability of a gas, such as oxygen, is much greater through a silicone membrane than through a polyvinyl alcohol (PVA) membrane. Permeability of specific gases may be controlled by the use of composite membranes. 

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