Pervaporation hydrophilic membranes

Klamklang, S., Soontarapa, K. and Damronglerd, S. 2002. Preparation and characterization of hydrophilic pervaporation membranes from natural rubber latex based polymer. Sci. Asm 28 135-143. 

Removal of water in PVRs PVRs with hydrophilic pervaporation membranes... 

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. 

Although hydrophobic pervaporation membranes can be used to recover alcohol from fermentation broth, hydrophilic pervaporation membranes allow for the dehydration of water/alcohol mixmres (Figure 11.7). Hydrophilic pervaporation membranes can be applied to separate water from highly concentrated alcohol (>85%) (Abels et al., 2013). Two types of membranes have been explored in recent studies (1) zeoUte-based membranes and (2) polymeric composite membranes with polyvinylalcohol or polyimide as the active layer. 

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. 

Pervaporation membranes are of two general types. Hydrophilic membranes are used to remove water from organic solutions, often from azeotropes. Hydrophobic membranes are used to remove organic compounds from water. The important operating charac teris-tics of hydrophobic and hydrophihc membranes differ. Hydrophobic membranes are usually used where the solvent concentration is about... 

Novel hydrophilic polymer membranes based on crosslinked poly(allylamine hydrochloride) (PAA.HCl)-PVA have been developed in order to dehydrate different organic compounds by pervaporation [33], The characteristics of the acetone dehydration process,... 

A list of typical commercial pervaporation membranes [23] is given in Table 3.1. Commercial hydrophilic membranes are very often made of polyvinyl alcohol (PVA), with differences in the degree of crosslinking. Commercial hydrophobic membranes often have a top layer in polydimethyl siloxane (PDMS). However, a wide variety of membrane materials for pervaporation can be found in the literature, including polymethylglutamate, polyacrylonitrile, polytetrafluoroethylene, polyvinylpyrrolidone, styrene-butadiene rubber, polyacrylic acid, and many others [24]. A comprehensive overview of membrane materials for pervaporation is given by Semenova et al. [25],... 

Some efforts have already been made to develop ceramic pervaporation membranes, especially silica and zeolite membranes, which are both hydrophilic membranes. Silica pervaporation membranes have been developed by ECN, The Netherlands. The membranes were tested in a pilot installation of 1 m2 membrane surface at Akzo Nobel and other companies in the Netherlands [34, 35]. 

In general, most of the high-separation factors reported for zeolite membranes are associated with pervaporation processes (see Section 10.5) or with vapor-separation applications where the permeated component is preferentially adsorbed. This has given rise to a variety of works in which the membranes have been used for equilibrium displacement by selective product permeation. The largest group probably corresponds to esterification processes, where hydrophilic zeolite membranes are employed to remove the product, water, replacing the extensively studied polymer membranes [187-192]. 

MHS with pervaporation of water from LM (MHS-PV) is presented in Figure 13.10. Contrary to the simple MHS with an agitated BLM, separated from the feed and strip solutions by flat hydrophobic or hydrophilic or ion-exchange membranes, the MHS-PV system exploits an FLM continuously flowing between the two flat cation-exchange and two pervaporation membranes. To couple the separation and pervaporation processes, the LM is simultaneously pumped through the MHS and... 

Another appUcation of membrane processes for MTBE removal studied in literature is the pervaporation, which represents an alternative to the stripping process discussed in chapter 3. Hydrophilic pervaporation is commonly used for the absolutation of alcohol which forms an azeotropic mixture at high ethanol content [128]. By the vaporization of the mixture through a thin membrane layer, the azeotrope is cleaved and water is evaporated leaving the pure ethanol in the retentate. 

THF is an important indnstrial solvent and forms an azeotropic mixture at 5.3 wt.% with water (see Table 11.2). In order to separate water/THF, Li et al. [206] tested the pervaporation performance of different hydrophilic zeolite membranes, zeolite A, zeolite Y, MOR, and ZSM-5. The preliminary test showed that the separation factor increased as the Si/Al ratio of the zeolite decreased, except for the case of zeolite A. This observation is probably due to the lower quality of this membrane with respect to the others. In fact, the permeation of TIPB showed the highest flux, 3.1 g/(m h), indicating the presence of nonselective defects. Therefore, the best results were obtained with zeolite Y, rendering a separation factor of 300 with a water flux of 2.24 kg/(m h) at 60°C. The water flux increased with water concentration in the feed, up to a value of 15 wt.%, indicating that the zeolite was saturated, as was the same for the case of water/etha-nol mixtures in zeolite A, previously described. At the same time, the separation factor decreases as water concentration decreased. The stability of the membrane was also studied, showing a stable performance after 35 h of operation. 

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]. 

Pervaporation, as a non-integrated process, is typically utilized for dehydration and for the recovery or removal of organics from aqueous solutions and sometimes also for the separation of organic mixtures (Neel, 1995). Also many hybrid processes have been developed where PV is coupled with other processes, such as different membrane processes (e.g., reverse osmosis, or organophilic pervaporation coupled with hydrophilic pervaporation), distillation, reactive distillation and, of course, reaction. With these aspects in mind, PV appears particularly suitable to keep the concentration of a by-product low, or to continuously recover a product while it is formed. Note that these are the main objectives typically pursued in membrane reactors. 

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