Filter narrow size distribution

To obtain LUVs with a narrow size distribution, the solution is extruded several times (typically 20 times) through a polycarbonate filter. This results in LUVs with a diameter of 100 nm (42) (see Note 3). 

This method of ferrite formation is based on the formation of aqueous solutions of chlorides, nitrates or sulphates of Fe, and of divalent Ni, Co, Mg, Ba, Sr, etc., in the concentrations required for the ferrite composition, and their simultaneous precipitation in the form of hydroxides by NaOH. The precipitate is then filtered, washed and dried. The ferrite particles are obtained by calcination at 180-300 °C in air. Particles with a narrow size distribution in the range 50-500 nm may be obtained, with high purity. The final ferrite body is obtained by sintering at temperatures considerably lower than in the case of the ceramic method. 

Rapid filter. A coarser grain size, and sometimes a narrower size distribution than slow filters are used ofl en pressurised flow is employed. A variety of rapid filters exist, for example, downflow, upflow, mixed-media and continuous. These will be discussed in greater detail in the following sections. 

Dioctyl Phthalate—The traditional test aerosol used for determining HEPA filter efficiency was an aerosolized form of warm DOP, selected because, when generated in the specified manner, the aerosol had a very narrow size distribution. 

Filter aids should have a narrow fractional composition. Fine particles increase the hydraulic resistance of the filter aid, whereas coarse particles exhibit poor separation. Desired particle-size distributions are normally prepared by air classification, in which the finer size fractions are removed. 

Improved Filtration Rate Filterability is an important powder catalyst physical property. Sometimes, it can become more important than the catalyst activity depending on the chemical process. When a simple reaction requires less reaction time, a slow filtration operation can slow down the whole process. From a practical point of view, an ideal catalyst not only should have good activity, but also it should have good filtration. From catalyst development point of view, one should consider the relationship between catalyst particle size and its distribution with its catalytic activity and filterability. Smaller catalyst particle size will have better activity but will generally result in slower filtration rate. A narrower particle size distribution with proper particle size will provide a better filtration rate and maintain good activity. 

As described before, the pore size of porous material ranges widely from atomic size to millimeter order. Different pore sizes are required for different applications of porous materials. Most porous materials do not have uniform pores. Pore size distribution is also an important property. Narrow pore size distribution, i.e., uniform pore size, is required for instance for filters and bioreactor beds. Mercury porosimetry and gas adsorption methods are commonly used to measure pores size and pores distribution. 

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. 

In a relatively new process for production and fractionation of fine particles by the use of compressible media - the PGSS process (Particles from Gas-Saturated Solutions) - the compressible medium is solubilized in the substance which has to be micronized [58-61]. Then the gas-containing solution is rapidly expanded in an expansion unit (e.g., a nozzle) and the gas is evaporated. Owing to the Joule-Thomson effect and/or the evaporation and the volume-expansion of the gas, the solution cools down below the solidification temperature of the solute, and fine particles are formed. The solute is separated and fractionated from the gas stream by a cyclone and electro-filter. The PGSS process was tested in the pilot- and technical size on various classes of substances (polymers, resins, waxes, surface-active components, and pharmaceuticals). The powders produced show narrow particle-size distributions, and have improved properties compared to the conventional produced powders. 

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. 

Finally, track-etched MF membranes are made from polymers, such as polycarbonate and polyester, wherein electrons are bombarded onto the polymeric surface. This bombardment results in sensitized tracks, where chemical bonds in the polymeric backbone are broken. Subsequently, the irradiated film is placed in an etching bath (such as a basic solution), in which the damaged polymer in the tracks is preferentially etched from the film, thereby forming cylindrical pores. The residence time in the irradiator determines pore density, and residence time in the etching bath determines pore size. Membranes made by this process generally have cylindrical pores with very narrow pore-size distribution, albeit with low overall porosity. Furthermore, there always is the risk of a double hit, i.e., the etched pore becomes wider and could result in particulate penetration. Such filter membranes are often used in the electronic industry to filter high-purity water. 

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