Section 4.19 Membranes Pervaporation

In the previous section the thermodynamic and kinetic relationships have been given to describe membrane formation by phase inversion processes. These relationships contain various parameters which have a large impact on the diffusion and demixing processes and hence on the ultimate membrane morphology. It has been shown that two different types of membranes may be obtained, the porous membrane (microfiltration and ultrafiltration) and the nonporous membrane (pervaporation and gas separation), depending on the type of formation mechanism, i.e. instantaneous demixing or delayed onset of demixing, involved. 

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). 

In this section the solution-diffusion model is used to describe transport in dialysis, reverse osmosis, gas permeation and pervaporation membranes. The resulting equations, linking the driving forces of pressure and concentration with flow, are then shown to be consistent with experimental observations. 

The analytical extraction systems related to points 1 and 2 are pervaporation-based techniques (such as those mentioned in Sections 4.3.1 and 4.3.2). Extraction based on the membrane separation of an aqueous phase and an organic phase (point 3 above) will be dealt with in Section 4.3.3. As the system concerning point 4 is very rarely used, it will not be considered here. 

One approach to delivering increased performance in a membrane process is to complement one separation mechanism with another. Vapor-arbitrated pervaporation is an example of this strategy. In bioseparations, as will be covered in a later section, a similar integration of several process enhancements in High-Performance Tangential How Filtration is responsible for dramatic improvement in separation efficiency of protein mixtures once considered unachievable by means of conventional ultrafiltration. 

These separations are usually carried out by pervaporation (see Section 10.5) or vapor permeation but they have also been performed using gas-phase feeds on organophihc [75] or hydrophilic membranes [76]. On organophdic membranes the permeation of the organic or less polar compound is favored, while the opposite trend is expected for hydrophilic membranes. 

The different mechanisms that operate in the separation of gases have been previously described in Section 10.4.1. In pervaporation, the transport mechanism can be described by an adsorption-diffusion mechanism [74,114] similar to one for polymeric membranes [115]. However, it is necessary to consider that the specific interactions between the permeating component and the zeolitic material are different in zeolites. Moreover, the diffusion through the ordered zeolite nanopores is different than in the dense organic matrix. 

The separation of -hexane/2,2-dimethylbutane, (DMB), another separation where size exclusion takes place, has been accomplished by Handers et al. [156], using pervaporation and vapor permeation. Both, the n-Cg and DMB fluxes are higher in the former case due to the higher driving force in pervaporation, leading to a lower selectivity compared to vapor permeation. The separation of xylene isomers on MH membranes has been described in Section 10.4.2.1 as a separation where size exclusion takes place. The results of the separation of these isomers using pervaporation with FER membranes [20] and MFl [21] were not successful, and very low fluxes of 10 and 10 mol/m s, accompanied by separation factors not greater than 16, respectively, were obtained. The best results for the pervaporation of xylenes were obtained by Yuan et al. [129] who prepared a template free sihcahte-1 membrane, the separation factor for a 50/50 wt% mixmre of p-xylene/o-xylene at 50°C was 60, and the flux of p-xylene was 13.7 x 10 kg/m h. 

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]. 

In spite of all these hurdles, there are already industrial-scale applications of zeolite membranes for solvent dehydration [106] by pervaporation plants using tubular zeolite A membranes with 0.0275 m of permeation area each (see Section 10.2.3). Li et al. [280] have prepared large area (0.0260 m ) ZSM-5 membranes on tubular a-alumina supports. This work is also interesting from the industrial point of view because the authors used inexpensive n-butylamine as template. Indeed, the cost required for industrial modules, on a general basis, is still far from clear. However, it must be noted that most of the costs can be ascribed to the module, and only 10%-20% to the membrane itself [3]. This underlines again the importance of preparation of zeolite membranes on cheaper, alternative supports that can also pack more area per unit volume. 

PV modules. The pervaporation membranes are placed on a stainless steel porous supports. Aqueous feed and strip solutions are intensively agitated. The MHS (without pervaporation section) and HLM are very similar systems. 

The second section refers to polyelectrolyte membranes prepared by alternating electrostatic layer-by-layer assembly of cationic and anionic polyelectrolytes on porous supports. Mass transport across ultrathin polyelectrolyte multilayer membranes is described. The permeation of gas molecules, liquid mixtures, and ions in aqueous solution has been investigated. The studies indicate that the membranes are excellently suited for separation of alcohol/water mixtures under pervaporation conditions and for ion separation, e.g. under nanofiltration conditions. 

Zeolites are traditionally used in catalysis/purification and separation applications in the petrochemical industry but are rapidly finding new uses. This section discusses membranes for low-dielectric-constant, corrosion-resistant, hydrophilic and antimicrobial, and pervaporation applications. 

Fig. 10 Cross section of silica pervaporation membrane. (View this art in color at www.dekker.com.)...

Synthetic membranes with calibrated pores are used for various operations in the wine industry ultrafiltration, front-end microfiltration, tangential microfiltration and reverse osmosis. Electrodialysis and pervaporation, special separation techniques described elsewhere in this book (Section 12.5.1), also make use of membranes. 

When k c = kg = 1, the off-diagonal elements of [k ] are equal to zero and the RD process discussed in Section 5.3.2 (case b) is recovered. Then, the location of the attainable product composition only depends on the vapor-liquid equilibria. In Fig. 5.24, singular point curves for the attainable bottom products of the depicted counter-current pervaporative reactor are given as a function of ky c (Fig. 5.24a) and kg (Fig. 5.24b). For each set of membrane permeabilities there is one curve of possible singular points. The ftill circles on the singular point curves indicate stable nodes, that is th mark attainable bottom products. For the system considered, stable nodes are only obtained below the chemical equilibrium line. 

Cation-exchanged zeolites show a strong tendency to adsorb water compared to organic molecules, as described in Chapter 7. As a consequence, water can be preferentially removed from solvents. By the use of porous tubes coated in zeolite membranes, water can be removed from the solvent on one side of the membrane and evaporated from the other side. The Japanese Mitsui company, for example, have commercialised the first large-scale pervaporation plant that produces over 500 Ih of solvents with less than 0.2 wt% water from simple alcohols containing 10 wt% water. The plant makes use of over 100 individual sections of NaA zeolite membrane operating at 120 °C. 

A range of membrane processes are used to separate fine particles and colloids, macromolecules such as proteins, low-molecular-weight organics, and dissolved salts. These processes include the pressure-driven liquid-phase processes, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), and the thermal processes, pervaporation (PV) and membrane distillation (MD), all of which operate with solvent (usually water) transmission. Processes that are solute transport are electrodialysis (ED) and dialysis (D), as well as applications of PV where the trace species is transmitted. In all of these applications, the conditions in the liquid boundary layer have a strong influence on membrane performance. For example, for the pressure-driven processes, the separation of solutes takes place at the membrane surface where the solvent passes through the membrane and the retained solutes cause the local concentration to increase. Membrane performance is usually compromised by concentration polarization and fouling. This section discusses the process limitations caused by the concentration polarization and the strategies available to limit their impact. 

Figure 11.19b. The most studied structure is the MFI, followed by LTA, and a similar number of publications could be found about FAU and MOR. The section others corresponds to FER, BETA, MEL, ZSM-11, and related materials like ETS-10 or ETS-4. The distribution of the zeotypes studied in the period 2(X)5-2011 does not change that much, although the proportion of mixed matrix membranes or composites decreases to approximately 10%. The number of publications referred to MFI, FAU, LTA, or MOR is multiplied by three compared to the period 1995-2(X)5 and CHA structure has also been introduced. After the excellent review published by Falconer and Noble in 2004 [158], the work on pervaporation and zeolites has been reviewed in specific or general reviews of zeolite membranes and pervaporation [1,3,159].

The criteria to choose between pervaporation or vapor permeation have been discussed in Section 3.2.6. In Fig. 3.10 the principal features of a standalone vapor-permeation plant are shown. The liquid feed stream from a storage tank is completely evaporated the composition of the vapor entering the membrane modules equals that of the feed entering the evaporator. If the feed is an azeotrope the composition of liquid feed, vapor, and evaporator content are identical. 

Section 4.15 describes membranes and introduces a range of membrane separation options. Molecular geometry is exploited in separations of gases via gas permeation. Section 4.16. Dialysis and electrodialysis are considered in Sections 4.17 and 4.18 respectively. Other methods to separate species in liquids are given in Section 4.19, pervaporation Section 4.20, reverse osmosis Section 4.21, for nanofiltration Section 4.22, for ultrafiltration Section 4.23, for microfil-tration and Section 4.24 for chromatographic separations. Separations of larger sized species are considered heterogeneous systems and are considered in Chapter 5. 

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