TORTUOUS-PORE MEMBRANES

Fouling Fouling affec ts MF as it affects all membrane processes. One difference is that the fouling effect caused by deposition of a foulant in the pores or on the surface of the membrane can be confounded by a rearrangement or compression of the sohds cake which may form on the membrane surface. Also, the high, open space found in tortuous-pore membranes makes them slower to foiil and harder to clean. 

Membrane thickness is a factor in microbial retention. Tortuous-pore membranes rated at 0.22 pm typically have surface openings as large as 1 pm (Fig. 20-67). Narrower restrictions are found beneath the surface. In challenge tests, P. diminuta organisms are found well beneath the surface of a 0.2-pm membrane, but not in the permeate. 

The microstnictuie of a porous membrane can vary according to the schematic in Figure 1.2. The shape of the pores is strongly dictated by the method of preparation which will be reviewed in Chapter 3. Those membranes that show essentially straight pores across the membrane thickness are referred to as straight pore or nearly straight pore membranes. The majority of porous membranes, however, have interconnected pores with tortuous paths and are called tortuous pore membranes. 

The "tortuous-pore" membranes are the most common and include typical cellulosic membranes and virtually all other polymers. The "capillary-pore" membranes are currently manufactured commercially only by Nuclepore Corp. and Poretics Corp. They are available as polycarbonate or polyester membranes. 

Figure 2.1 Capillary-pore and tortuous-pore membranes.

A look at the open area of the two membranes in Figure 2.1 indicates that the "tortuous-pore" membranes are more porous-having a porosity over 75%. The "capillary-pore" membranes generally have porosities less than 5%. However, the fact that the latter are /is the thickness of the "tortuous pore" membranes means that the flow rates are often comparable. 

Phase-Inversion Process. Most tortuous-pore membranes are made by a casting process known as "phase inversion." Figure 2.2 is a simplified schematic of a casting machine which makes cellulose ester membranes. Typically, a casting solution made up of the polymer and a multicomponent solvent system is metered onto a stainless steel belt or web. The belt passes through a series of environmental chambers usually containing water vapor at elevated temperatures. The more volatile solvents evaporate and the water vapor precipitates the polymer around the less volatile solvent which becomes the "pore-former." Subsequently, (not shown in Figure 2.2), after the membrane is formed, the residual solvents are washed out of the pores, surfactants are added, and the membrane is dried. 

It is easy to meesure the pore size of capillary-pore membranes with a scanning electron microscope, but tortuous pore membranes are more difficult. 

Membranes of equal porosity and thickness will show the same "diffusional-flow." This is why the so-called "forward-flow" test advocated by some manufacturers is so misleading. They claim they can correlate bacteria retention with the diffusional-flow measured. Yet, most pore sizes of tortuous-pore membranes have approximately the same porosity and thickness. Equation (4) makes it clear... 

Davis et alls investigated the retention of 0.05 and 0.005 y Au colloids by "capillary-pore" and "tortuous-pore" membranes. Table 2.4 shows clearly that the "tortuous-pore membranes" retain particles much smaller than the rated pore size. Indeed, a 5 y pore size will retain 60% of the 0.05 y colloidal particles and 18% of the 0.0005 y colloid. On the other hand, "capillary-pore" membranes retain less than 1% of either. "Tortuous-pore" membranes have 25 to 50 times more internal surface area for adsorption than "capillary-pore" membranes, and the tortuous path also results in a greater likelihood of small particles contacting the pore-wall. 

For dilute process streams, product may be lost via adsorption on the membrane. The recovery" of this product may be improved by pretreating the membrane such that most of the adsorption sites are occupied. For example, in the data of Hahn et al16 (Table 2.5), polio virus adsorption on cellulosic "tortuous-pore" membranes was significantly higher than that on polycarbonate "capillary-pore membranes, (i.e.. The virus recovery is low due to adsorption.) The recov ery was improved from 5 to 76% by pretreating the membrane with a beef extract solution. 

Negatively charged membranes are also available. The fractionation capability of "tortuous-pore membrane" is enhanced considerably by using filters with a zeta potential of the same sign as the particles (usually negative). It should be noted that untreated nylon membranes have a negative zeta potential at pH values above 6.5 (see Figure 2.21 ).18... 

The filtration of particles in a gas stream can be quite different from the filtration of the same particles in a liquid stream. The three mechanisms of aerosol particle retention may be illustrated from the data of Spurny et al20 in Figures 2.22 and 2.23. The U-shaped curves are characteristic of the efficiency of aerosol particle collection as a function of particle size. However, "capillary-pore" membranes have a deeper minimum in the curves than do "tortuous-pore membranes."... 

For "tortuous-pore membranes" the minimum in the curves of Figures 2.22 and 2.23 is much less pronounced. This is because the tortuous path results in more and smaller particles captured by inertial impaction. Further, the longer path length through the pore results in more and larger particles captured by diffusional deposition. 

Thus, if the objective is to capture as many particles as possible on the membrane, "tortuous-pore" membranes are preferred. 

On the other hand, for some specialized analytical applications where fractionation of the aerosol particles is the objective, "capillary-pore" membranes are preferred. For example, it has been found that an 8 ju "capillary-pore" membrane will collet air-pollution particles that are normally deposited in the upper respiratory tract (nasopharynx.)21 Air sampling stations have used this membrane in the first stage particles passing are collected on a tortuous-pore membrane in a second stage to simulate what is deposited in the lungs. 

Figure 2.32 Cross-section photomicrograph-anisotropic tortuous-pore membrane.

Incidentally, this also explains how "capillary-pore" membrane cartridges can equal the throughput of "tortuous-pore" cartridges. Two to three times the area of the "tortuous-pore" membranes can be pleated into a similar cartridge because the capillary-pore membranes are so much thinner. 

As might be suspected, "capillary-pore" membranes appear to be more am-menable to backwashing than "tortuous-pore" membranes. However, some process streams deposit particulates on the membrane that cannot be backwashed from either type. Figure 2.37 shows a relatively successful backwash experiment on "capillary-pore" membranes used to filter beer. 

For suspensions of particles with sizes nearer to the pore size, some internal pore fouling will occur but at a greatly reduced rate. Figure 2.4122 shows cross-flow filtration of a single cell protein suspension on a "tortuous-pore" membrane. The flux declines rapidly at first, as boundary layer conditions are established, and then levels off with a diminishing rate of flux decay. 

Figure 2.41 Cross-flow filtration of single cell protein suspension (0.83 wt %) with 0.22 micron tortuous-pore membrane.

Only "tortuous-pore" membrane discs may be used in such a stack. The reason is that the polycarbonate or polyester "capillary-pore" membranes currently available are very thin (typically 10 ju thick) and easily pick up an electrostatic charge. It is almost impossible to load 293 mm discs of these membranes onto the plates. They cling to hands and wrinkles cannot be totally eliminated. It is possible to buy (in Japan) a heat sealed "sandwich" with polyester screens on both sides of the "capillary-pore" membrane (see Figure 2.46) which facilitates handling. The "sandwich" is sealed around the periphery to prevent lateral leakage. 

Various "tortuous-pore" membranes are also available in tubular and hol-... 

These tortuous pore membranes resemble porous sponges where permeant moves through a convoluted path as it passes between bulk phases. Membranes are considered dense when they have less than 50% void volume. However, even dense membranes can be relatively porous and might require additional treatment to produce a more selective membrane. The membrane characteristics are often altered by treating the membrane surface to produce a region with the requisite properties. The bulk of the membrane then functions simply as a porous support. 

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