Membrane filter size distributions

Fig. 25. Reverse osmosis, ultrafiltration, microfiltration, and conventional filtration are related processes differing principally in the average pore diameter of the membrane filter. Reverse osmosis membranes are so dense that discrete pores do not exist transport occurs via statistically distributed free volume areas. The relative size of different solutes removed by each class of membrane is illustrated in this schematic.

It must also be emphasized that the major mass of a heterodispersed aerosol may be contained in a few relatively large particles, since the mass of a particle is proportional to the cube of its diameter. Therefore, the particle-size distribution and the concentration of the drug particles in the exposure atmosphere should be sampled using a cascade impactor or membrane filter sampling technique, monitored using an optical or laser particle-size analyzer, and analyzed using optical or electron microscopy techniques. 

Although most particles, larger than a given pore size, are normally retained, many smaller particles (sometimes 10 - 1000 times smaller than the pore size) may also be retained. If above and within a pore depth filtration occurs (see Chapter 7.6 and Fig. 7.12) colloidal particles smaller than the pore size become attached to the larger particles. Furthermore the pore size distribution of membrane filters is often non-narrow. 

Improved control devices now frequently installed on conventional coal-utility boilers drastically affect the quantity, chemical composition, and physical characteristics of fine-particles emitted to the atmosphere from these sources. We recently sampled fly-ash aerosols upstream and downstream from a modern lime-slurry, spray-tower system installed on a 430-Mw(e) coal utility boiler. Particulate samples were collected in situ on membrane filters and in University of Washington MKIII and MKV cascade impactors. The MKV impactor, operated at reduced pressure and with a cyclone preseparator, provided 13 discrete particle-size fractions with median diameters ranging from 0,07 to 20 pm with up to 6 of the fractions in the highly respirable submicron particle range. The concentrations of up to 35 elements and estimates of the size distributions of particles in each of the fly-ash fractions were determined by instrumental neutron activation analysis and by electron microscopy, respectively. Mechanisms of fine-particle formation and chemical enrichment in the flue-gas desulfurization system are discussed. 

The model has been used to predict the sampling efficiency of the VE for a wide range of MMAD and GSD values typical of what might be encountered in cotton textile processing. These parameter values are for the actual size distribution of particles in the sampled air, and not for those collected on the membrane filter. These results are summarized in Table II. A remarkable feature of this model is that it predicts that the VE will collect significant amounts of particles with aerodynamic diameters greater than 30 pm. 

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. 

Experimental Technique. The solid material (1-3 g) with known particle size and standard water (30-50 ml) containing the radionuclide of interest were shaken in glass bottles for 8-12 hours at constant temperature (25°C or 65 C). The phases were separated by centrifugation (50 min, 7000 rpm) and the distribution coefficient of the radionuclide was determined from measurements of the remaining activity of the water. Filtration of the samples through 0.2 pm membrane filter did not change the values. 

Figure 11 shows the wet-flow properties of three hypothetical membrane filter media. Each filter medium is made of the same material and has the same thickness and total void fraction. Media A and B have the same oversized pore size, but A has a broader pore size distribution. Medium C has a pore size smaller than A and B with a narrow pore size distribution. 

Conventional filters, such as a coffee filter, termed depth filters , consist of a network of fibers and retain solute molecules through a stochastic adsorption mechanism. In contrast, most membranes for the retention of biocatalysts feature holes or pores with a comparatively narrow pore size distribution and separate exclusively on the basis of size or shape of the solute such membranes are termed membrane filters . Only membrane filters are approved by the FDA for sterilization in connection with processes applied to pharmaceuticals. Table 5.3 lists advantages and disadvantages of depth and membrane filters. 

FIGURE 8.5 Size distribution of 300-nm and 5-pm beads in the original solution and in the solution after filtering through the ion-track membrane with 0.5-[im pores [228]. Reprinted with permission from the Institute of Physics Publishing. 

In order to prepare liposomes, the lipid preparation is dried at low temperature under an inert gas atmosphere (protect the lipid from oxidation). The lipid film is swollen with water or buffered aqueous solution and several freeze-thaw cycles are carried out to get optimal rehydration of the lipid. The rehydrated lipid preparation is filtered using membrane filters with defined pore size. After repeated filtration steps (extrusion) an unilamellar liposome preparation with a defined size distribution is obtained. Large unilamellar vesicles (LUV) are produced in this way. LUV s are about 100 nm in size the thickness of the lipid bilayer is about 4 nm. Even smaller liposomes can be derived from sonication (sonication probe or ultra-sonication bath). Separation of the prepared liposomes... 

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