Pervaporation and membrane distillation

Pervaporation and membrane distillation (MD) are distinguished from the above membrane separation processes since phase change, from liquid to vapor, takes place in the process. 

Schauer, J., Schwarz, H.H. and Eisold, C. 2003. Pervaporation and membrane distillation through membranes made of poly(2,6-dimethyl-l,4-phenylene oxide). Angew. Makromol. Chem. 206(1) 193-198. 

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. 

A product may be removed from its producing cell by five main possible techniques. Evaporation occurs via stripping, (vacuum) distillation or by membrane-supported techniques such as pervaporation and transmembrane distillation. Extraction into another phase includes the use of water-immiscible organic solvents, supercritical fluids, or an aqueous two-phase system. In addition, the... 

Velterop F (2011), The potential of the HybSi ceramic membrane in process intensification , Programme booklet of the International Scientific Conference on Pervaporation, Vapor Permeation and Membrane Distillation, Torun (Poland)... 

As a method of separation membrane processes are rather new. Thus membrane filtration was not considered a technically important separation process until 25 years ago. Today membrane processes are used in a wide range of applications and the number of such applications is still growing. From an economic point of view, the present time is intermediate between the development of first generation membrane processes such as microfiltration (MF), ultrafiltradon (UF), nanofiltration (NF). reverse osmosis (RO), electrodialysis (ED), membrane electrolysis (ME), diffusion dialysis (OD), and dialysis and second generation membrane processes such as gas separation (GS), vapour permeation (VP), pervaporation (PV), membrane distillation (MD), membrane contactors (MC) and carrier mediated processes. 

Gueneri G., Membrane alcohol separation process integrated pervaporation and fractional distillation, Trans. IChemE, 70(A), 501 508 (1992). 

Solvent fermentation produces a dilute solution of butanol, acetone, or isopropanol. The recovery of these products by distillation is an energy-intensive process. Unless the final product concentrations are elevated, the development of alternative methods for product recovery may improve the economics of this fermentation. Techniques for the alternative methods could include the use of membranes (reverse osmosis, perstraction, pervaporation, and membrane evaporation). 

Pervaporation, vapour permeation and membrane distillation Principles and applications... 

Membrane reactors for energy applications and basic chemical production Edited by Angelo Basile, Luisa Di Paolo, Faisal Hai and Vincenzo Piemonte 11 Pervaporation, vapour permeation and membrane distillation Principles and applications... 

The third application area for pervaporation is the separation of organic/organic mixtures. The competitive technology is generally distillation, a well-established and familiar technology. However, a number of azeotropic and close-boiling organic mixtures cannot be efficiently separated by distillation pervaporation can be used to separate these mixtures, often as a combination membrane-distillation process. Lipnizki et al. have recently reviewed the most important applications [53],... 

In membrane distillation, two liquids (usually two aqueous solutions) held at different temperatures are mechanically separated by a hydrophobic membrane. Vapors are transported via the membrane from the hot solution to the cold one. The most important (potential) applications of membrane distillation are in water desalination and water decontamination (77-79). Other possible fields of application include recovery of alcohols (e.g., ethanol, 2,3-butanediol) from fermentation broths (80), concentration of oil-water emulsions (81), and removal of water from azeotropic mixtures (82). Membrane (pervaporation) units can also be coupled with conventional distillation columns, for instance, in esterifications or in production of olefins, to split the azeotrope (83,84). 

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. 

Additional acetyl groups in the membrane decreased the flux but increased a Combination of reactive distillation and pervaporation allowed 100% conversion Experimental fluxes and selectivities of the permeating components were determined and evaluated on the basis of the feed mixtures and membranes. The effect of vacuum/pressure on total flux and selectivity is discussed Review Mass transfer analysis of pervaporation... 

The advantages of membrane distillation are associated with relatively lower energy costs as compared to competing processes such as distillation, reverse osmosis, and pervaporation (PV) much lower membrane fouling as compared with microhltration (MF), ultrahltration, and RO a quantitative rejection of nonvolatile solutes from the feed stream lower operating pressure and temperatures, without sacrihcing flux as compared with competing processes. 

With the advent of process simulation packages, modeling of pervaporation and vapor permeation processes in a user added subroutine allows these unit processes to be included in overall separation schemes right from the conceptual stage. This enables many different combinations of pervaporation, for example, distillation to be studied and the optimum operating parameters for the preferred configuration to be selected very quickly. Such parameters include membrane feed temperature, which strongly influences the flux rate and, therefore. 

Benzene can be separated over cyclohexane with an revalue of 26 with a polvinyl alcohol-poly(allylamine) blend containing a cobalt(II) complex.213 An a-value of 60 has been obtained by pervaporation with a poly(acrylonitrile-co-methyl methacrylate) membrane.214 Membranes of porous polyethylene grafted with glycidyl methacrylate215 and poly(A,A-dimethylacrylamide-co-methyl methacrylate)216 have also been used in this separation with separation factors of 21-22. This is a separation that would be difficult to do by size and by distillation. The two boil only 2°C apart. The cyclohexane produced by the reduction of benzene is the starting material for nylon. The best solution to the problem is to run the reduction to 100% completion. 

Vapor permeation and pervaporation are membrane separation processes that employ dense, non-porous membranes for the selective separation of dilute solutes from a vapor or liquid bulk, respectively, into a solute-enriched vapor phase. The separation concept of vapor permeation and pervaporation is based on the molecular interaction between the feed components and the dense membrane, unlike some pressure-driven membrane processes such as microfiltration, whose general separation mechanism is primarily based on size-exclusion. Hence, the membrane serves as a selective transport barrier during the permeation of solutes from the feed (upstream) phase to the downstream phase and, in this way, possesses an additional selectivity (permselectivity) compared to evaporative techniques, such as distillation (see Chapter 3.1). This is an advantage when, for example, a feed stream consists of an azeotrope that, by definition, caimot be further separated by distillation. Introducing a permselective membrane barrier through which separation is controlled by solute-membrane interactions rather than those dominating the vapor-liquid equilibrium, such an evaporative separation problem can be overcome without the need for external aids such as entrainers. The most common example for such an application is the dehydration of ethanol. 

Membrane distillation is similar to pervaporation since phase change is involved in the process. When feed liquid (usually water) is in contact with a nonwetted porous hydrophobic membrane, water does not enter into the pores because the feed Hquid is maintained below a threshold pressure, the liquid penetration pressure of water. Only water vapor permeates through the pores from the feed to the permeate side. The driving force is the vapor pressure drop from the feed to the permeate side, since the permeate temperature is maintained below the feed temperature. Commercial hydrophobic membranes made of polypropylene, poly(vinylidene fluoride) and poly-... 

Khayet, M. and Matsuura, T. 2004. Pervaporation and vacnnm membrane distillation processes Modeling and experiments. 50(8) 1697-1712. 

Urtiaga, A.M., Gorri, E.D., Ruiz, G. and Ortiz, 1. 2001. Parallehsm and differences of pervaporation and vacuum membrane distillation in the removal of VOCs from aqueous streams. Sep. Purif. Technol. 22-23 327-337. 

AlO. Some azeotropic mixtures can be separated by sending the vapor mixture to a gas permeation system—designated as vapor permeation if the mixture is easily condensed fHuang. 1991 Neel. 1991)—and some (probably different) azeotropic mixtures can be separated by sending a liquid mixture to an RO system Why is pervaporation a much more popular method of separating azeotropic mixtures Note In all cases a h5 rid membrane-distillation system will probably be used. 

The most common configurations of PMRs with suspended photocatalyst are those utilizing pressure driven membrane processes, such as microfiltration (MF), ultraflltration (UF) and NF. In other types of PMRs, photocatalysis is combined with dialysis, pervaporation (PV) or direct contact membrane distillation (DCMD, MD) (Mozia, 2010). 

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