Microporous membranes examples

Cellulose acetate Loeb-Sourirajan reverse osmosis membranes were introduced commercially in the 1960s. Since then, many other polymers have been made into asymmetric membranes in attempts to improve membrane properties. In the reverse osmosis area, these attempts have had limited success, the only significant example being Du Font s polyamide membrane. For gas separation and ultrafUtration, a number of membranes with useful properties have been made. However, the early work on asymmetric membranes has spawned numerous other techniques in which a microporous membrane is used as a support to carry another thin, dense separating layer. 

Surface media Captures particles on the upstream surface with efficiencies in excess of depth media, sometimes close to 100% with minimal or no off-loading. Commonly rated according to the smallest particle the media can repeatedly capture. Examples of surface media include ceramic media, microporous membranes, synthetic woven screening media and in certain cases, wire cloth. The media characteristically has a narrow pore size distribution. 

Dasgupta and Jacobs [29] patented a concept of using a gel layer in combination with a microporous membrane. The gel layer acts as an adhesive bridge between separator and electrodes, just as in the flat pack Zn/MnC cell [30], The microporous membrane (for example, Celgard membrane) provides excellent mechanical... 

Terms such as symmetric and asymmetric, as well as microporous, meso-porous and macroporous materials will be introduced. Symmetric membranes are systems with a homogeneous structure throughout the membrane. Examples can be found in capillary glass membranes or anodized alumina membranes. Asymmetric membranes have a gradual change in structure throughout the membrane. In most cases these are composite membranes... 

An example of the track-etch membrane was given in Section 2.2 (Quinn et al. 1972). Booman and Delmastro (1974) have also described the layer deposition method to produce a microporous membrane by a track-etch method. 

To overcome the poor mechanical properties of polymer and gel polymer type electrolytes, microporous membranes impregnated with gel polymer electrolytes, such as PVdF. PVdF—HFP. and other gelling agents, have been developed as an electrolyte material for lithium batteries.Gel coated and/ or gel-filled separators have some characteristics that may be harder to achieve in the separator-free gel electrolytes. For example, they can offer much better protection against internal shorts when compared to gel electrolytes and can therefore help in reducing the overall thickness of the electrolyte layer. In addition the ability of some separators to shutdown... 

Particles smaller than the largest pores, but larger than the smallest pores are partially rejected, according to the pore size distribution of the membrane. Particles much smaller than the smallest pores will pass through the membrane. Thus, separation of solutes by microporous membranes is mainly a function of molecular size and pore size distribution. In general, only molecules that differ considerably in size can be separated effectively by microporous membranes, for example, in ultrafiltration and microfiltration. 

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. 

In these systems, the interface between two phases is located at the high-throughput membrane porous matrix level. Physicochemical, structural and geometrical properties of porous meso- and microporous membranes are exploited to facilitate mass transfer between two contacting immiscible phases, e.g., gas-liquid, vapor-liquid, liquid-liquid, liquid-supercritical fluid, etc., without dispersing one phase in the other (except for membrane emulsification, where two phases are contacted and then dispersed drop by drop one into another under precise controlled conditions). Separation depends primarily on phase equilibrium. Membrane-based absorbers and strippers, extractors and back extractors, supported gas membrane-based processes and osmotic distillation are examples of such processes that have already been in some cases commercialized. Membrane distillation, membrane... 

Soft and hard contact lenses with good oxygen permeability, optical clarity, flexibility, and biocompatibility dental composites denture linings surgical adhesives catheters and hydrophobic microporous membranes are examples of applications of fluorinated polyurethanes in the medical and dental fields.53... 

Scopolamine was the first drug to be marketed as a transdermal delivery system (Transderm-Scop) to alleviate the discomfort of motion sickness. After oral administration, scopolamine has a short duration of action because of a high first-pass effect. In addition, several side-effects are associated with the peak plasma levels obtained. Transderm-Scop is a reservoir system that incorporates two types of release mechanims a rapid, short-term release of drag from the adhesive layer, superimposed on an essentially zero-order input profile metered by the microporous membrane separating the reservoir from the skin surface. The scopolamine patch is able to maintain plasma levels in the therapeutic window for extended periods of time, delivering 0.5 mg over 3 days with few of the side-effects associated with (for example) oral administration. 

The principle of these electrodes is a little different, and is normally based on the measurement of the pH of a solution of electrolyte placed between a membrane and a glass electrode, the membrane being porous to the species it is desired to determine (Table 13.3). The dissolved gas conditions the pH of the solution behind the membrane. Membranes can be microporous (for example PTFE) or homogeneous (for example silicone rubber). 

Membrane Techniques The interest in membrane techniques for sample preparation arose in the 1980s. Extraction selectivity makes membrane techniques an alternative to the typical sample enrichment methods of the 1990s. Different membrane systems were designed and introduced into analytical practice some more prominent examples are polymeric membrane extraction (PME), microporous membrane liquid-liquid extraction (MMLLE), and supported liquid membrane extraction (SEME) [106, 107]. Membrane-assisted solvent extraction (MASE) coupled with GC-MS is another example of a system that allows analysis of organic pollutants in environmental samples [108-111] ... 

From outward appearance membrane contactors look similar to other membrane devices. However, functionally the membranes used in contactors are very different. They are mostly nonselective and microporous. Membrane contactors can be made out of flat sheet membranes and there are some commercial apphcations. Most common commercial membrane contactors are, however, made from small-diameter microporous hollow fiber (or capillary) membranes with fine pores (illustrated in Figure 2.1) that span the hoUow fiber wall from the fiber inside surface to the fiber outside surface. The contactor shown as an example in Figure 2.1 resembles a tube-in-sheU configuration with inlet/outlet ports for the shell side and tube side. The membrane is typically made up of hydrophobic materials such as Polypropylene, Polyethylene, PTFE, PFA, and PVDF. 

This brief overview of mass transfer and separation mechanisms involved in ceramic membrane processes will be useful not only for a better understanding of actual operating conditions of ceramic membranes, but also for anticipating future applications. For example, a same microporous membrane can serve theoretically as liquid or gas separation membrane. However, transport mechanisms and operating conditions being totally different, a good membrane permeability and selectivity in the former case cannot be systematically transposed to the second case. 

The support shown in Fig. 6.3 serves as a support for a microporous membrane, for example, a silica membrane. Figure 6.5 is a SEM photograph of such a membrane layer. The silica layer is orUy 200 nm thick and is supported by the system shown in Fig. 6.3. The substrate for the silica membrane is the very smooth mesoporous y-alumina layer 4. 

A Illustrative Examples of Permeation and Separation with Microporous Membranes... 

Molecular models can considerably impact the chemical process industry. Obviously, numerous problems fall beyond the realm of conventional molecular simulation (see the example above on zeolitic membranes). Examples include dynamics of protein folding, diffusion through microporous membranes and human cells, formation of quantum dots in heteroepitaxial growth of semiconductors, and pattern formation on catalyst surfaces. 

When the microporous membrane is in the form of a hollow fiber (see Fig. 2.7), the interfacial areas on the two sides of the hollow fiber are different. The overall mass-transfer coefficient may be defined based on the surface area calculated using either the inside diameter (ID) or the outside diameter (OD) of the hollow fiber. For calculating an overall mass-transfer coefficient, the interfacial area should be based on the diameter where the aqueous-organic phase interface is located. Consider, for example, the aqueous feed and strip phases in hydrophobic fiber lumen (tube side) and organic LM phase on the shell side. The rate of solute extraction per unit fiber length with the aqueous-organic interface located on the fiber ID ... 

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