Capillary-pore membranes

FIGURE 22.13 Scanning electron micrographs of the surface (a) and the internal structure (h) of a Rotrac capillary pore membrane. (From Klobes et al.. Bull. Int. Dairy Fed., 311, 13, 1996. With permission.)... 

Figure 1.3 Schematic diagram illustrating the preparation of capillary pore membranes by a track-etching process.

Figure 1.5 SEM of asbestos filter accumulated on the surface of a capillary pore membrane.

Human red blood cells have a diameter of approximately 6 to 8 pm. The human body, however, contains capillaries approximately 3 pm in diameter. To pass through these vessels the blood cells have to deform correspondingly. Healthy cells will do this readily but malignant cells will not. By filtering blood through a 3 pm capillary pore membrane certain blood deficiencies can be monitored.16... 

All current MF membranes may be classified as either "tortuous-pore" or "capillary-pore" membranes (see Figure 2.1). The "capillary-pore" structure is distinguished by its straight-through cylindrical capillaries, whereas the "tortu-ous-pore" structure resembles a sponge with a network of interconnecting tortuous pores. 

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. 

A unique feature of the "capillary-pore" membranes is that the pore size... 

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. 

Many techniques have been proposed for making capillary-pore membranes including laser beams, electroforming, photochemical etching, and ionotropy to orient anisotropic gel particles to form ionotropic-gel membranes.3... 

Further, a new alumina capillary-pore membrane has just been introduced by Anotec Separations Ltd. The capillary-pore structure appears to result from controlled growth of alumina crystals. 

To date, none of these methods has produced submicron capillary-pore membranes at a reasonable cost in the large areas suitable for industrial applications except the "track-etch" membrane produced by Nudepore Corp. 

Figure 2.8 Doublet and triplet pores in capillary-pore membrane.

Figure 2.10 Doublet pore in 5.0 micron capillary-pore membrane.

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

Even the case of "capillary-pore" membranes where is only slightly larger than the thickness of the membrane, inertial losses, especially "front-and-back-face" losses, may be significant. 

To verify the theoretical equation for bubble point, a series of 0.2 H capillary pore membranes were made with extremely low pore densities. The lowest densities virtually eliminated all doublets and triplets and yielded bubble points equal to the theoretical value. Though the low density rendered the membrane useless for applications requiring a reasonable flow rate, it nevertheless demonstrated that a membrane can be made where a direct measurement of the pores (with S.E.M.) agreed with that calculated from the bubble-point. 

For example. Figure 2.1813 demonstrates the transmission of normal deformable red cells through various capillary-pore membranes. After hardening the red cells, they will not pass anything below a 9 y pore size. 

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. 

These results suggest that a "tortuous-pore" configuration is best for "cleanup" applications where the removal of all particles from the process stream is desired. On the other hand, the "capillary-pore" configuration is best for fractionation of particles. For example, capillary-pore membranes have been used in fractionating silver colloids to improve resoltuion on photographic films. 

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. 

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."... 

Figure 2.22 Aerosol retention on a capillary-pore membrane as a function of particle size and pore size.

Figure 2.24 Latex particles captured on a capillary-pore membrane by inertial impaction.

Figure 2.25 Silver (Ag) particles captured by a capillary-pore membrane by diffusional deposition.

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 2.26 Tap-water through-put for polycarbonate (PC) capillary-pore membranes and cellulose ester (CE), polytetrafluoroethylene (PTFE) totruous-pore membranes.

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. 

Figure 2.40 shows the cross-flow concentration of yeast with a "capillary-pore membrane. Since the yeast particles are considerably larger than the 0.2 ju pore size, internal pore fouling was nil. The sweeping action of the cross-flow stream tangential to the membrane surface maintained a stable flux at constant concentration. The ability to concentrate yeast from 0.1 to 10% with a stable flux would be impossible with TFF. 

Figure 2.37 Recovery of filtration rate on beer with periodic back-wash of capillary-pore membranes. 

Figure 2.40 Cross-flow concentration of yeast with a capillary-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. 

Figure 2.46 Capillary-pore membrane cassette (sandwiched between two support screens).

For reasons enumerated earlier, "capillary-pore" membranes, though pleat-able, will not yield satisfactory bubble-points and begin to pass Pseudomonas diminuta at challenge levels above 103-10s organisms even though the membrane area is 2 to 3 times greater. 

Figure 2.51 Increase in through-put for cross-flow filtration of 0.5% yeast with modified 0.2 micron capillary-pore membrane pleated cartridge.

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