Membrane contactor

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). 

These gas transfer membranes or membrane contactors employ microporous polypropylene hollow fiber membranes arranged in a modular design. Oxygenated water flows on the shell side of the hollow fibers, and a strip gas (such as nitrogen) or a vacuum is applied to the inside (lumenside), with the hollow fibers acting as a support medium for intimate contact between the water and gas phases. 

Celgard. Liqui-Cel Extra-Flow Membrane Contactors. Technical literature. Celgard, Inc., USA, 2000. 

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]. 

The extractant was continuously regenerated by stripping with NaOH in a second membrane contactor. Thus, an impressive final butanoic acid concentration of more than 300 g L and a productivity of 7.37 g L" h were achieved. 

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... 

This book provides a general introduction to membrane science and technology. Chapters 2 to 4 cover membrane science, that is, topics that are basic to all membrane processes, such as transport mechanisms, membrane preparation, and boundary layer effects. The next six chapters cover the industrial membrane separation processes, which represent the heart of current membrane technology. Carrier facilitated transport is covered next, followed by a chapter reviewing the medical applications of membranes. The book closes with a chapter that describes various minor or yet-to-be-developed membrane processes, including membrane reactors, membrane contactors and piezodialysis. 

To-be-developed industrial membrane separation technologies Carrier facilitated transport Membrane contactors Piezodialysis, etc. Major problems remain to be solved before industrial systems will be installed on a large scale... 

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... 

Membrane contactors are typically shell-and-tube devices containing micro-porous capillary hollow fiber membranes. The membrane pores are sufficiently small that capillary forces prevent direct mixing of the phases on either side of the membrane. The membrane contactor shown in Figure 13.8 separates a liquid and a gas phase this is a liquid/gas contactor [18]. Membrane contactors... 

Contactors have a number of advantages compared to simple liquid/gas absorb-er/strippers or liquid/liquid extractors. Perhaps the most important advantage is high surface area per volume. The contact area of membrane contactors compared to traditional contactor columns is shown in Table 13.2. Membrane contactors provide 10-fold higher contactor areas than equivalent-sized towers. This makes membrane... 

Figure 13.9 Examples of membrane contactors and their applications...

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],... 

Table 13.3 Details of Liqui-Cel hollow fiber membrane contactor modules...

Figure 13.10 Oxygen removal from water with a 10-in.-diameter membrane contactor (135 m2 membrane area) [24]...

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. 

Figure 13.11 Flow schematic of the membrane contactor process developed by British Petroleum to separate ethylene/ethane mixtures by absorption into silver nitrate solution [28,29]...

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]...

Liquid/Liquid Membrane Contactors (Membrane Distillation)... 

The most important example of liquid/liquid membrane contactors is membrane distillation, shown schematically in Figure 13.13. In this process, a warm, salt-containing solution is maintained on one side of the membrane and a cool pure distillate on the other. The hydrophobic microporous membrane is not wetted by either solution and forms a vapor gap between the two solutions. Because the solutions are at different temperatures, their vapor pressures are different as a result, water vapor flows across the membrane. The water vapor flux is proportional to the vapor pressure difference between the warm feed and the cold permeate. Because of the exponential rise in vapor pressure with temperature, the flux increases dramatically as the temperature difference across the membrane is increased. Dissolved salts in the feed solution decrease the vapor pressure driving force, but this effect is small unless the salt concentration is very high. Some typical results illustrating the dependence of flux on the temperature and vapor pressure difference across a membrane are shown in Figure 13.14. 

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... 

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