Membrane operations pervaporation

Modules Every module design used in other membrane operations has been tried in pervaporation. One unique requirement is for low hydraulic resistance on the permeate side, since permeate pressure is very low (0.1-1 Pa). The rule for near-vacuum operation is the bigger the channel, the better the transport. Another unique need is for heat input. The heat of evaporation comes from the liquid, and intermediate heating is usually necessary. Of course economy is always a factor. Plate-and-frame construction was the first to be used in large installations, and it continues to be quite important. Some smaller plants use spiral-wound modules, and some membranes can be made as capillary bundles. The capillary device with the feed on the inside of the tube has many advantages in principle, such as good vapor-side mass transfer and economical construction, but it is still limited by the availability of membrane in capillary form. 

A flow scheme for an integrated distillation-pervaporation plant operating on a 5 % ethanol feed from a fermentation mash is shown in Figure 9.10. The distillation column produces an ethanol stream containing 80-90 % ethanol, which is fed to the pervaporation system. To maximize the vapor pressure difference and the pressure ratio across the membrane, the pervaporation module usually... 

Various membrane operations are available today for a wide spectrum of industrial applications. Microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), gas and vapor separation (GS, VS), pervaporation (PV), dialysis (D), electrodialysis (ED) and membrane contactors (MCs) are only some of the best-known membrane unit operations. 

Catalytic reactions can be combined in membrane-assisted integrated catalytic processes with practically all the membrane unit operations available today. Many examples of integration of membrane contactors, pervaporation, gas separation, nanofiltration, microfiltration, and ultrafiltration operations together with catalytic reactions, have been proposed in the literature. 

A membrane is usually seen as a selective barrier that is able to be permeated by some species present into a feed while rejecting the others. This concept is the basis of all traditional membrane operations, such as microfiltration, ultrafiltration, nanofil-tration, reverse osmosis, pervaporation, gas separation. On the contrary, membrane contactors do not allow the achievement of a separation of species thanks to the selectivity of the membrane, and they use microporous membranes only as a mean for keeping in contact two phases. The interface is established at the pore mouths and the transport of species from/to a phase occurs by simple diffusion through the membrane pores. In order to work with a constant interfacial area, it is important to carefully control the operating pressures of the two phases. Usually, the phase that does not penetrate into the pores must be kept at higher pressure than the other phase (Figure 20.1a and b). When the membrane is hydrophobic, polar phases can not go into the pores, whereas, if it is hydrophilic, the nonpolar/gas phase remains blocked at the pores entrance [1, 2]. 

Membrane operation is a specific, but not exotic, operation. In fact it is a hybrid of classical heat and mass transfer processes (Figure 4.1). Direct contact mass transfer operations tend to reach equilibrium due to a difference of chemical potential between two phases that are put into contact. In the same way, temperature equilibrium is aimed at during heat transfer operations, for which driving force is a temperature gradient. In contrast, for membrane operations, by using the specific properties of separation of the thin layer material that constitutes the membrane, under the particular driving force that is applied, it is possible to deviate from the equilibrium that prevails at fluid-to-fluid interphase with classical direct contact mass exchange systems and to reorientate the mass transfer properties. In particular, this is the case with classical operations such as microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), gas separation (GS), pervaporation (PV), dialysis (DI) or electrodialysis (ED), for which a few characteristics are recalled in Table 4.1. 

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. 

Membrane operations in wastewater treatment complexation reactions coupled with membranes, pervaporation and membrane bioreactors... 

Key words wastewater, membrane operations, supported liquid membranes, pervaporation, membrane bioreactors, complexation... 

The main problem in membrane processes, especially for UF and MF separations, is the decrease of permeate flux caused by concentration polarization and fouling, whereas other membrane processes such as gas separation and pervaporation are less affected. Different approaches have been studied to reduce fouling. Hybrid systems using different types of membrane operations (e.g., distillation, dialysis, NF, pervaporation, and osmosis) prevent microbial fouling, offering a strong potential for the use of new types of thin-film composite membranes. 

Pervaporation is used to separate the liquid mixture. A phase transition occurs at the phase boundary on the permeate side, allowing desorption by vaporization. According to the solution-diffusion model, selectivity is primarily achieved because not all components in the mixture of substances can be dissolved equally well in the membrane material. Pervaporation involves a second selectivity step as a result of the required vaporization of the permeating components. For this, the partial pressure on the permeate side of the components must be lower than the saturated steam pressure. If only some of the components dissolved by the membrane boil at the operating point, the remainder of the components are not desorbed. 

The performance of membrane operations is diminished by polarisation phenomena, although the extent to which these phenomena can occur differ considerably. Thus, in microfiltration and ultrafiltration the actual flux through the membrane can be only a fraction of the pure water flux, whereas in pervaporation the effect is less severe. 

Poly-4-methyl-l-pentene MDEA Liquid/liquid extraction, membrane distillation, pervaporation. Operational abscsbent a... 

Pervaporation operates under constraints similar to low pressure gas-separation. Pressure drops on the permeate side of the membrane must be small, and many prevaporation membrane materials are mbbery. For this reason, spiral-wound modules and plate-and-frame systems ate both in use. 

Pervaporation. Vapor arbitrated pervaporation is used to remove ethanol from whiskey by selective passage of the alcohol through a membrane. Whiskey flows on one side of a membrane. A water-vapor stream flows on the other side and sweeps away the ethanol that permeates the membrane. Thus alcohol reduction and selective retention of flavor and aroma components can be achieved usiag membranes with a particular porosity. The ethanol can be recovered by condensing or scmbbiag the vapor stream. Pervaporation systems operate at or slightly above atmospheric pressure (Fig. 

In this process the addition of water vapor to the sweep stream can be controlled so that the water activity of the gas phase equals that of the beverage. When this occurs, there is no transport of water across the membrane. The water content of both the beverage feed and the sweep stream is kept constant. These conditions must be maintained for optimum alcohol reduction. The pervaporation system controls the feed, membrane, airstream moisture level, and ethanol recovery functions. An operational system has been developed (13). 

Advantages to Membrane Separation This subsertion covers the commercially important membrane applications. AU except electrodialysis are pressure driven. All except pervaporation involve no phase change. All tend to be inherently low-energy consumers in the-oiy if not in practice. They operate by a different mechanism than do other separation methods, so they have a unique profile of strengths and weaknesses. In some cases they provide unusual sharpness of separation, but in most cases they perform a separation at lower cost, provide more valuable products, and do so with fewer undesirable side effects than older separations methods. The membrane interposes a new phase between feed and product. It controls the transfer of mass between feed and product. It is a kinetic, not an equihbrium process. In a separation, a membrane will be selective because it passes some components much more rapidly than others. Many membranes are veiy selective. Membrane separations are often simpler than the alternatives. 

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... 

The preferred choice of a water-selective membrane up to now has been hydrophilic membranes because of their high water affinity. However, recently Kuhn et al. reported an all-silica DDR membrane for dehydration of ethanol and methanol with high fluxes (up to 20kg m h ) and high selectivities (H20/ethanol 1500 and H20/methanol 70 at 373 K) in pervaporation operation. The separation is based on molecular sieving with water fluxes comparable to well-performing hydrophilic membranes [51]. 

Figure 20-48 shows Wijmans s plot [Wijmans et al.,/. Membr. Sci., 109, 135 (1996)] along with regions where different membrane processes operate (Baker, Membrane Technology and Applications, 2d ed., Wiley, 2004, p. 177). For RO and UF applications, Sj , < 1, and c > Cl,. This may cause precipitation, fouling, or product denatura-tion. For gas separation and pervaporation, Sj , >1 and c < ci. MF is not shown since other transport mechanisms besides Brownian diffusion are at work. 

---