Microporous membrane separation types

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. 

In FLM, the LM organic solution flows in a thin channel between two hydrophobic microporous membranes separating the LM phase from an aqueous feed and strip solutions. The FLM differs from the HLM and MHS modules with hydro-phobic membranes by application of a spiral-type module. A schematic diagram of the spiral-type FLM module is shown in Figure 13.11. 

Although microporous membranes are a topic of research interest, all current commercial gas separations are based on the fourth type of mechanism shown in Figure 36, namely diffusion through dense polymer films. Gas transport through dense polymer membranes is governed by equation 8 where is the flux of component /,andare the partial pressure of the component i on either side of the membrane, /is the membrane thickness, and is a constant called the membrane permeability, which is a measure of the membrane s ability to permeate gas. The ability of a membrane to separate two gases, i and is the ratio of their permeabilities,a, called the membrane selectivity (eq. 9). 

These types of separators consist of a solid matrix and a liquid phase, which is retained in the microporous structure by capillary forces. To be effective for batteries, the liquid in the microporous separator, which generally contains an organic phase, must be insoluble in the electrolyte, chemically stable, and still provide adequate ionic conductivity. Several types of polymers, such as polypropylene, polysulfone, poly(tetrafluoroethylene), and cellulose acetate, have been used for porous substrates for supported-liquid membranes. The PVdF coated polyolefin-based microporous membranes used in gel—polymer lithium-ion battery fall into this category. Gel polymer... 

Development efforts are under way to displace the use of microporous membranes as battery separators and instead use gel electrolytes or polymer electrolytes. Polymer electrolytes, in particular, promise enhanced safety by eliminating organic volatile solvents. The next two sections are devoted to solid polymer and gel polymer type lithium-ion cells with focus on their separator/electrolyte requirements. 

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... 

The most important type of microporous membrane is formed by one of the phase separation techniques discussed in the next section about half of the isotropic microporous membrane used is made in this way. The remaining types are made by various proprietary techniques, the more important of which are described below. 

Figure 3.15 Polypropylene structures, (a) Type I open cell structure formed at low cooling rates, (b) Type II fine structure formed at high cooling rates [37]. Reprinted with permission from W.C. Hiatt, G.H. Vitzthum, K.B. Wagener, K. Gerlach and C. Josefiak, Microporous Membranes via Upper Critical Temperature Phase Separation, in Materials Science of Synthetic Membranes, D.R. Lloyd (ed.), ACS Symposium Series Number 269, Washington, DC. Copyright 1985, American Chemical Society and American Pharmaceutical Association...

Two membrane types that operate on different principles have been used in commercially available membrane separators microporous membranes and selectively permeable, nonporous polyimide or Nafion membranes. The micro-porous Teflon PTFE membrane can be used to remove water vapor or organic solvent vapor. Any gaseous component, including volatile analytes such as Hg, is partially or extensively removed. The sweep gas flow rate is typically similar to the sample carrier gas flow rate. 

Extraction of phenol from aqueous solution using hollow fiber membrane contactor was first investigated in Ref. [100]. However, the membrane used was not completely microporous. Instead, it was a dialysis-type membrane. A commercial plant to separate phenol from hydrocarbon fraction using microporous membrane contactors was reported in Ref. [101]. Soda lye was used to react with the phenol transferred from the feed phase to create and maintain the driving force for separation. This industrial-scale application enabled the processing of hydrocarbon fraction to a full-value raw material for phenol and acetone synthesis. 

To enhance the separation factor the average pore diameter should be decreased considerably. According to Eqs. (9.9a) and (9.15) the contribution to the total gas flux of the gas (Knudsen) diffusion decreases and at the same time that of surface flow (diffusion) increases with decreasing pore radius. In recent years modification of existing membranes for improving their separation efficiency has been actively pursued especially by attempts to decrease the pore size of membranes. This resulted in different types of microporous membranes. According to lUPAC convention these are porous systems with a pore diameter below 2 nm. In the literature the name microporous is frequently misused and this should be avoided. 

The ability to separate cells from a high molecular weight extracellular product is one of the touted advantages of using microporous membranes over ultrafiltration membranes. Microporous membranes compete with centrifuges and rotary vacuum filters for this type of recovery. High yield protein/cell separations with membranes, however, have not been well established in the literature. Rejection coefficients for extracellular proteins in bacterial cell broth have been reported to vary widely from 5 to 100% for microporous membranes of 0.1 to 0.6 micron... 

In the SLM process, like in all membrane processes, the membrane plays a key role in the transport and separation efficiency. The permeation rate and separation efficiency depends strongly on the type of liquids and supports used for SLM construction. However, the transport properties depend on the type of liquids used as a membrane phase the hquid membrane stability and mechanical stability depend, to a large extent, on the microstructure like pore shape, size, and tortuosity of the membrane used as a support. Therefore, many types of polymeric and inorganic microporous membrane supports are studied for the liquid membrane phase immobilization. 

A new type of configuration, the flowing liquid membrane (FLM) was studied by Teramoto et al. [20]. In this case, the membrane liquid phase is in motion as the feed and strip phase. In this type of system a plate-and-frame and spiral-wound configuration with flat membrane was used. The scheme of the FLM configuration is drawn in Fig. 7.3A. The hquid phase flows (FLM) between two hydrophobic microporous membranes. The two membranes separate the hquid membrane phase from feed and strip phases. In Fig. 7.3B, it is reported the classical plate-and-frame module employed for the separation of ethylene from ethane [20]. The liquid membrane convection increased the membrane transport coefficient in gas separation. However, the membrane surface packing density (membrane surface area/ equipment volume) is much lower in spiral-wound system than in hollow fiber. 

The same authors proposed a flowing hquid membrane [20], for the separation of ethylene over ethane, in which a liquid membrane solution flowed in a thin channel between two microporous membranes. Also in this case a silver nitrate was used as carrier of ethylene. The selectivity for ethylene over ethane was about 460 at a higher concentration of 4 X 10 inol/in This type of membrane was stable for more than 11 days. 

Figure 4.10 Membrane and electrochemically regenerated suppressors. There are two types of membranes, one type permeable to cations (H and in this example Na ), the other permeable to anions (OH and here CP), (a) The microporous cationic membrane is adapted to the elution of anions. Only cations can cross the membrane (corresponding to a polyanionic wall which keeps away the anions in the solution), (b) An anionic membrane suppressor placed, contrary to the preceding model, at the outlet of a cationic column. Ions are regenerated by the electrolysis of water. Note in both cases the counter flow circulation between the eluted phase and the solution of the post-column suppressor, (c) An example of a separation of inorganic cations (concentrations of the order of ppm) using a suppressor of this type.

The effect of reactant loss on membrane reactor performance was explained nicely in a study by Harold et al [5.25], who compared conversion during the cyclohexane dehydrogenation reaction in a PBMR equipped with different types of membranes. The results are shown in Fig. 5.4, which shows the cyclohexane conversion in the reactor as a function of the ratio of permeation to reaction rates (proportional to the ratio of a characteristic time for reaction in the packed bed to a characteristic time for transport through the membrane). Curves 1 and 2 correspond to mesoporous membranes with a Knudsen (H2/cyclohexane) separation factor. Curves 3 and 4 are for microporous membranes with a separation factor of 100, and curves 5 and 6 correspond to dense metal membranes with an infinite separation factor. The odd numbered curves correspond to using an inert sweep gas flow rate equal to the cyclohexane flow, whereas for the even numbered curves the sweep to cyclohexane flow ratio is 10. 

Alkaline Mn02-Zn primary cells in general use macroporous separators made from woven, bonded or felted materials. The choice of the separator material is not so sensitive as for rechargeable alkaline Mn02-Zn cells, where microporous membrane-type materials have to be used additionally to prevent zinc dendrites from causing cell shorts. Pore diameters range from 2.5 to 10 nm (membranes) to several hundred nanometer for porous-sheet separators [8,32]. 

Membrane separators offer the possibility of compact systems that can achieve fuel conversions in excess of equilibrium values by continuously removing the product hydrogen. Many different types of membrane material are available and a choice between them has to be made on the basis of their compatibility with the operational environment, their performance and their cost. Separators may be classified as (i) non-porous membranes, e.g., membranes based on metals, alloys, metal oxides or metal—ceramic composites, and (ii) ordered microporous membranes, e.g., dense silica, zeolites and polymers. For the separation of hot gases, the most promising are ceramic membranes. 

Plexiglas sandwich-type gas-diffusion separator with PTFE microporous membrane (cf. Fig. 5.1). 

PTFE sandwich-type membrane phase separator with PTFE microporous membrane. 

Chemical grafting polymerization represents another efficient method to modify the structure and property of the microporous membrane, which can generate a strong coat on the surface through covalent bonds so as to change the surface polarity permanently. For instance, a porous separator was immersed into a mixed solution of photoinitiator and monomer, followed by polymerization under UV irradiation (Senyarich and Viaud, 2000). In this case, the electrolyte retention capacity and wettability depends on the degree of the polymerization and the type of the used monomer. 

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