Pore diameters, filter media

Two other deposition mechanisms, in addition to the six listed, may be in operation under particular circumstances. Some dust particles may be collected on filters by sieving when the pore diameter is less than the particle diameter. Except in small membrane filters, the sieving mechanism is probably limited to surface-type filters, in which a layer of collected dust is itself the principal filter medium. 

Nucleopore filters from the same lot should be used throughout the experiments. While pore diameter is uniform, there might be considerable variability in pore density. Therefore, filters should be checked before use by passing a certain amount of medium (20-50 ml), and measuring the time it takes to pass through this is proportional to the total pore area. Only filters giving similar flow values should be used in the experiments. 

Clarifying filters remove small amounts of solids or liquid droplets from either liquids or gases. The particles are trapped inside the filter medium or on its surfaces. Clarification differs from screening in that pores in the filter medium are larger—sometimes much larger—than the particles to be removed. The particles are caught by surface forces and immobilized on the surfaces or within the flow channels, where they reduce the effective diameter of the channels but usually do not block them completely. 

It is generally recommended that a suspension of filter aid be prepared in a pure liquid by agitation before filtration. Delivery to the filter medium can usually be done with a centrifugal pump. The filtrate can be recycled to the filter aid suspension feed until the finished precoat is formed. Depending on the ratio of particle size of the filter aid to the pore diameters of the medium, as well as the rate of suspension feed and its concentration, two processes arise on the filter. First, the medium retains only some particles and the remainder pass through with filtrate. Gradually, the medium s... 

Porosity and pore size are crucial properties of filter media, which determine efficiency as well as pressure drop and permeability. Small pore sizes at high porosity of a textile filter medium depend on the fiber size. A reduction of the pore size below the fiber diameter greatly reduces the porosity and diminishes the permeability, which means the filtration efficiency increases with a reduction in the fiber diameter. 

It has been stated that a filter medium is a porous (or at the very least semi-permeable) barrier placed across the flow of a suspension to hold back some or all of the suspended material. If this barrier were to be very thin compared with the diameter of the smallest particle to be filtered (and perforated with even sized holes), then all the filtration would take place on the upstream surface of the medium. Any particle smaller than the pore diameter would be swept through the pores, and any particle larger than that (assuming the particles to be rigid) would remain on the upstream surface. Some of the larger particles, however, would be of a size to settle into the individual pores and block them. The medium surface would gradually fill with pores blocked in this way, until the fluid flow reduced to below an acceptable level. At this point filtration would be stopped and the medium surface would be brushed or scraped clean (although many automatic filters have their surface continuously brushed or scraped). 

Most real media are, of course, not infinitely thin, but have a finite thickness in the direction of fluid flow, while most pores through such material vary in diameter along the fluid path. A second mechanism, termed depth straining, then applies when a particle moves through a pore until it meets a point where the pore is too small, and the particle is held entirely because of its size. The pore then is blocked, and remains so until the filter medium becomes too clogged in this way for it to have any further use. At this point it must be discarded, or, preferably, blown free of the trapped solids, by a reverse flow of fluid. 

The bubble point test is based on the fact that, for a porous filter medium, immersed in and thoroughly wetted by a specific liquid, the pressure required to force a gas bubble through a pore is inversely proportional to the diameter of the pore. In practice, this means that the pore size of a filter element can be established by wetting the element completely with the liquid and measuring the pressure at... 

Resistance to flow through a clean filter will be caused by the filter housing, cartridge hardware, and filter medium. For a fluid of given viscosity, the smaller the diameter of the pores or passages in the medium, the greater the resistance to flow will be. When a fluid meets resistance in the form of a filter, the result is a drop in pressure downstream of that filter, and the measurement of the pressure drop across the filter is called the differential pressure, or AP. Thus, for all practical purposes the terms "pressure drop," "differential pressure," and "AP" are synonymous. 

The sampling operation involves collection of an aerosol sample that is representative of the particle size distribution and concentration of the sampled atmosphere. The efficiency of particle transport and collection operations are dependent on the particle size, sampling velocity, the geometry of the sampling apparatus and the properties of the collection medium. In the present work, a 37 mm diameter membrane filter (0.3 ym pore size) is the primary collection medium under evaluation. The filter is housed in a standard filter cassette and effects of filter-holder inlet geometry are also being investigated. 

In a second series of experiments, Hanczyc et al. [52] found that the myristoleic acid vesicles could be induced to grow by addition of fatty acid to the medium, presumably by incorporating fatty acid molecules into the membrane, rather than by fusion of vesicles. If the resulting suspension of large vesicles was then filtered through a polycarbonate filter having pores 0.2 pm in diameter, the larger vesicles underwent a kind of shear-induced division to produce smaller vesicles. This process could be repeated several times (Fig. 5). 

Depending on the size of the particles and the pore size distribution of the porous medium, solid particles can be removed on the surface of the porous medium (filter cake) or inside the medium (deep filtration). If the particle size is larger than the pore throat diameter of the porous media, then particles will be separated on the face of the porous medium (i.e., form a skin) and will not deeply penetrate the porous medium (Figure 1). If the particle diameter is very small in comparison with pore... 

Fig. 6. Microgel analysis chromatograms of medium carboxyl content polyacrylamides. SI, a commercial polymer (Mw appx. 6x10 ) free of microgels S2 and S3, laboratory samples (Mw appx. 9x10 ) showing significant microgel content S3a, sample S3 after treatment with NaOH. The numbers refer to filtration test flow rates (through a 25-mm diameter 5/xm pore size Millipore filter).

---