Membrane Distillation Applications

The application of DCMD for breaking azeotropic mixtures was first proposed by Udriot et al. (1994), who looked at separating hydrochloric acid-water and propionic acid-water azeotrope mixtures. In fact, the azeotropic mixtures are impossible to separate by simple distillation. Retention selectivities of the solute between 0.6 and 0.8 were achieved instead of unity as implied by vapor-hquid equilibrium (VLE). In this case DCMD may be used to shift the selectivity above or below the one obtained by the VLE. 

It must be mentioned that up to now, MD is not applied in the nuclear industry. The process applications are still under evaluation due to its high-energy consumption and difficulties with long-time operation together with the membrane wettability. In fact, nuclear power industry should be a very convenient place for MD implementation because a lot of waste heat can be recovered in many places around the nuclear cycle. 

This temperature gradient along the membrane module length affects the local driving force and consequently the SGMD flux. 

Additionally, Khayet et al. (2002c) found the TPC in the feed side to be very close to unity, whereas the main temperature polarization in SGMD was located in the air phase, concluding that the mass flux in the SGMD process is mostly controlled by the heat transfer through the air boundary layer. 

Sweeping gas MD was applied successfully for desalination of aqueous solutions, first by Basini et al. (1987) and later by Khayet et al. (2003a, 2003c). Rejection of practically 100% was achieved. SGMD was used also for concentration of aqueous sucrose solutions by Komgold and Korin (1993) and separation of alcohol water mixtures by Calibo et al. (1987) and Lee and Hong (2001). 

---

Francis L, Maab H, AlSaadi A, Nunes S, Ghaffour N, Amy GL (2013) Fabrication of electrospun nanofibrous membranes for membrane distillation application. Desalin Water Treat 51 1337-1343... 

E. Drioli, A. Ali, S. Simone, F. Macedonio, S.A. AL-Jlil, F.S. A1 Shabonah, H.S. Al-Romaih, O. Al-Harbi, A. Figoli, and A. Criscuoli, Novel PVDF hollow fiber membranes for vacuum and direct contact membrane distillation applications. Separation and Purification Technology 115 (2013) 27-38. 

G. Zuo, R. Wang, Novel membrane surface modification to enhance anti-oil fouling property for membrane distillation application. Journal of Membrane Science, 447 (2013) 26-35. 

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). 

Membrane distillation is considered a promising separation method applicable primarily in environmental technologies. In membrane distillation a microporous and hydrophobic membrane separates aqueous solutions at different temperatures and compositions, as shown in Figure 9. The temperature difference existing across the membrane results in a vapor pressure difference. The molecules are transported through the pores of the membrane from the high-vapor-pressure side to the low-vapor-pressure side. At least one side of the membrane remains in contact with the liquid phase. Benefits offered by membrane distillation include (202) ... 

Currently, the most important application area for membrane distillation is water desalination technology. Figure 10 shows one of the water desalination processes developed by a Japanese organization, the Water Re-Use Promotion Center, in cooperation with Takenaka Corporation and Organo Corporation (204). The process uses solar energy and can therefore be installed at locations without an electricity supply. Other application areas for membrane distillation reported in the literature are summarized in Table 8. 

On the other hand, a pervaporation membrane can be coupled with a conventional distillation column, resulting in a hybrid membrane/distillation process (228,229). Some of the investigated applications of such hybrid pervaporation membrane/distillation systems are shown in Table 9. In hybrid pervaporation/ distillation systems, the membrane units can be installed on the overhead vapor of the distillation column, as shown in Figure 13a for the case of propylene/ propane splitting (234), or they can be installed on the feed to the distillation column,... 

Membrane reactors and contactors for extraction, gas absorption, or membrane distillation represent extensions of various types of the membranes in Table I and Table II. Nevertheless, these cases, along with controlled release of application, will be considered briefly to illustrate how the basic membrane types in Table I can be applied in unconventional, ever-expanding ways. 

Membrane processes termed as osmotic distillation or membrane distillation could be shown to be applications of membrane contactor technology also. Both of these processes are based on gas membranes. Osmotic distillation, sometimes called osmotic evaporation, involves transfer of water vapor across a gas-fiUed membrane, the process is driven by a difference in water vapor pressure maintained across the membrane [58-59] by separate aqueous hquids. Membrane distillation is a process where water vapor transfer is driven solely by a temperature difference across the gas-fiUed membrane [60-61]. Water evaporates from a hot aqueous phase and condenses on a cooler surface. This process may be useful in desalinating water or producing pure water if a good natural source of warm water is available, such as in a geothermal process. 

Wu B and Teo WK, Preparation and application of PVDF hollow fiber membranes in vacuum membrane distillation to remove TCE and TCA from aqueous solutions. Euromembrane 2004, Hamburg, Germany, September 28-October 1, 2004. 

Desalination of seawater is one of the important applications of membrane processes. There are various ways to produce fresh water such as distillation, electrodialysis, membrane distillation, freezing, membrane bioreactor, and reverse osmosis. Among them, distillation is the most used technique, but RO is becoming more popular in the desalination industry. A flow diagram of a single-stage RO system is shown in Fig. 4. 

Other potential uses for hollow fiber contactors include membrane distillation where microporous membranes form a barrier to liquid water but allow vapor to pass. Studies of these modules have been so far limited to laboratory scale investigations. Extensive reviews of the mass transfer correlations and applications for hollow fiber contactors are given in the literature. 

Hydrophilic MF membranes can be made by the dry-wet phase inversion technique. The latter technique is also applicable in making PVDF membranes. On the other hand, other hydrophobic MF membranes are made by the TIPS technique. In particular, semicrystalline PE, PP, and PTFE are stretched parallel to the direction of film extrusion so that the crystalline regions are aligned to the direction of stretch, while the noncrystalline region is ruptured, forming long and narrow pores. Hydrophobic membranes do not allow penetration of water into the pore until the transmembrane pressure drop reaches a threshold pressure called liquid entry pressure of water. These membranes can therefore be used for membrane distillation. Tracketching method is applied to make MF membranes from PC. 

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. 

The processes involving membrane contactors became one of the more developing modem unit operations with application of membranes. Such systems, like membrane emulsifiers, membrane extractors, membrane strippers and scmbbers, membrane distillation (MD), and crystallization systems, may overcome the limitations of conventional units and various more common membrane processes that have been already applied in nuclear industry [121,122], Both hydrophobic... 

Essalhi, M., Khayet, M., Surface segregation of fluorinated modifying macromolecule for hydrophobic/hydrophihc membrane preparation and application in air gap and direct contact membrane distillation, J. Membr. Sci. 417 18, 163, 2012. 

---