Membrane contactors membranes used

The system could also be further improved if membrane contactors are used in hybrid systems integrated with vapor compression inverse cycle in this case the power saving can be as high as 50% [4]. 

Controlling temperature and humidity of process air or ambient air is another unique application of membrane contactors. Membranes are used to humidify or dehumidify air by bringing air in contact with water or a hygroscopic liquid. Mass transfer in such processes is very fast since mass transfer resistance in the liquid phase is negligible. Heat transfer and mass transfer are directly related to these processes, since latent heat of evaporation (or condensation) creates temperature profiles inside the contactor. Some of the references in Literature are shown in Refs. [78-79]. Application of such processes has been proposed for conditioning air in aircraft cabins [80], in buildings or vehicles [81], or in containers to store perishable goods [82]. 

Bothun GD, Knutson BL, Strobel HJ, and Nokes SE. Mass transfer in hollow fiber membrane contactor extraction using compressed solvents. J. Membr. Sci. 2003 227(1-2) 183-196. 

The traditional membrane separation processes (reverse osmosis, micro-, ultra- and nanofiltration, electrodialysis, perva-poration, etc.), already largely used in many different applications, are today combined with new membrane systems such as CMRs and membrane contactors. Membranes are applied not only in traditional separation processes such as seawater desalination but also in medicine, bioengineering, microelectronics, the life in the space, etc. 

A hollow fibre polypropylene membrane contactor is used in the gas-solvent absorption pilot plant the solvent is drawn directly from the solvent capture plant. The membrane contactor, with about 8 m surface area between the solvent and flue gas, is designed to handle 10 kg h i of... 

The Institute of Environmental and Energy Technology (TNO) in Netherlands have developed macroporous polypropylene membrane contactors and used it with alkali aminoacid salt as absorbant. No wetting or degradation of polypropylene membrane surface was observed and hence stable membrane performance was reported. The Kvaerner Process is used for Teflon membranes with aqueous amine solutions of MEA, DEA, MDEA and DIP A. The membrane system was used to recover acid gases from natural gas for offshore platform applications. No performance data are available for the flue gas treatment. 

The original expanded film membranes were sold ia roUs as flat sheets. These membranes had relatively poor tear strength along the original direction of orientation and were not widely used as microfiltration membranes. They did, however, find use as porous inert separating barriers ia batteries and some medical devices. More recentiy, the technology has been developed to produce these membranes as hoUow fibers, which are used as membrane contactors (12,13). 

Nonbiological methods for removal of trichloroethylene from water are also being studied. These include the use of a hollow fiber membrane contactor (Dr. A.K. Zander, Clarkson University), photocatalysis by solar or artificially irradiated semiconductor powders (Dr. G. Cooper, Photo-catalytics, Inc.), and micellar-enhanced ultrafiltration (Dr. B.L. Roberts, Surfactant Associates, Inc.). 

Generally, a distinction can be made between membrane bioreactors based on cells performing a desired conversion and processes based on enzymes. In ceU-based processes, bacteria, plant and mammalian cells are used for the production of (fine) chemicals, pharmaceuticals and food additives or for the treatment of waste streams. Enzyme-based membrane bioreactors are typically used for the degradation of natural polymeric materials Hke starch, cellulose or proteins or for the resolution of optically active components in the pharmaceutical, agrochemical, food and chemical industry [50, 51]. In general, only ultrafiltration (UF) or microfiltration (MF)-based processes have been reported and little is known on the application of reverse osmosis (RO) or nanofiltration (NF) in membrane bioreactors. Additionally, membrane contactor systems have been developed, based on micro-porous polyolefin or teflon membranes [52-55]. 

Thermolysine Synthesis of ZAlaPheOMe in a membrane contactor using Tris-HCl/ethyl acetate [54]... 

Liquid-liquid extraction of hydrophobic oil-laden surfactant solution was evaluated using counter-flow, porous hollow fiber membranes. Our liquid-liquid extraction experiments were conducted using Liqui-Cel Extra-Flow 2.5x8 Membrane Contactor purchased from Celgard LLC (Charlotte, NC). The dimensions of the column are 6.3 cm diameter and 20.3 cm. length with... 

More recently the use of membrane contactors to solve the stability problem of liquid membranes has been proposed [19-21], The concept is illustrated in Figure 11.4. Two membrane contactors are used, one to separate the organic carrier phase from the feed and the other to separate the organic carrier phase from the permeate. In the first contactor metal ions in the feed solution diffuse across the microporous membrane and react with the carrier, liberating hydrogen counter ions. The organic carrier solution is then pumped from the first to the second membrane contactor, where the reaction is reversed. The metal ions are... 

The main disadvantages of contactors are related to the nature of the membrane interface. The membrane acts as an additional barrier to transport between the two phases that can slow the rate of separation. Over time, the membranes can foul, reducing the permeation rate further, or develop leaks, allowing direct mixing of the two phases. Finally, the polymeric membranes are necessarily thin (to maximize their permeation rate) and consequently cannot withstand large pressure differences across the membrane or exposure to harsh solvents and chemicals. In many industrial settings, this lack of robustness prohibits the use of membrane contactors. 

Despite these caveats, the use of membrane contactors is growing rapidly. Positive reviews are given by Reed et al. [20], Qi and Cussler [21], Gabelman and Hwang [22] and Prasad and Sirkar [23],... 

In Europe, the TNO [27] and Kvaerner [19] are both developing contactors to remove water and carbon dioxide from natural gas. Glycol or amines are used as the absorbent fluid. The goal is to reduce the size and weight of the unit to allow use on offshore platforms, so oftentimes only the absorber, the largest piece of equipment in a traditional absorber/stripper, is replaced with a membrane contactor. Kvaerner has taken this technology to the demonstration phase and commercial units are expected to be introduced soon. 

Another type of gas exchange process, developed to the pilot plant stage, is separation of gaseous olefin/paraffin mixtures by absorption of the olefin into silver nitrate solution. This process is related to the separation of olefin/paraffin mixtures by facilitated transport membranes described in Chapter 11. A membrane contactor provides a gas-liquid interface for gas absorption to take place a flow schematic of the process is shown in Figure 13.11 [28,29], The olefin/paraffin gas mixture is circulated on the outside of a hollow fiber membrane contactor, while a 1-5 M silver nitrate solution is circulated countercurrently down the fiber bores. Hydrophilic hollow fiber membranes, which are wetted by the aqueous silver nitrate solution, are used. 

To reduce the relatively large volume of silver nitrate solution held in the flash tank portion of the plant shown in Figure 13.11, Bessarabov et al. [30] have proposed using two membrane contactors in series, as shown in Figure 13.12. One contactor functions as an absorber, the other as a stripper. The first contactor removes ethylene from the pressurized feed gas into cold silver nitrate solution. The solution is then warmed and pumped to the second contactor where ethylene is desorbed from the silver nitrate solution into a low-pressure product ethylene gas stream. The regenerated silver nitrate solution is cooled and returned to the first contactor. 

Figure 13.12 Flow schematic of process using two membrane contactors for the separation of ethylene/ethane mixtures proposed by Bessarabov et al. [30]...

Of the processes described in this chapter, membrane contactors and membrane reactors have the greatest potential to develop into large-scale commercial processes. Both technologies are already used on a small scale in niche applications, and both are being developed for much larger and more important processes. Membrane contactors are currently most widely used to deaerate liquids, but the... 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

Additionally, membranes have the unique advantage of allowing the simultaneous contact with two different media, at each membrane side, creating compartments with different properties. Therefore, membranes offer the potential to promote the spatial organization of catalytic compartments and selective barriers. This feature is used with advantage in new concepts of membrane multiphasic (bio)reactors and membrane contactors. 

Other types of membrane separation processes that can be useful when they are coupled with a photocatalytic system are the membrane contactors. 

Dialysis - photocatalysis Azrague et al. [91] described a particular type of membrane contactor photoreactor in which a dialysis membrane (used as a contactor) was... 

In particular, our research is addressed to the simultaneous one-step production and separation of phenol by selective oxidation of benzene in a membrane photoreactor using Ti02 as catalyst and a membrane contactor for the separation process. 

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]. 

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