Permeability, of filter

Frazier A test to measure the air permeability of filter septums. Expressed in cfm or air at a pressure drop of 1/2 in. WC. 

The problem of correlating the permeability of filter media with the bade dimendons of the mat ials con odng the septum has received atterdon p avies, 1952]. Badcaify, the problem is to relate B, as defined in Equation 4.4 to variables such as fibre diameter, weave constmetion, etc. Whilst the permeability of random system is not the princ al interest here, it is inq)ortant to record some work rqported in this area, in view of the growing application of nonwovens in the area of pressure filtratum. 

The permeability of filter cakes, as described by a, can depend on pressure applied, the filter cake can be compressible, resulting in an increasing resistance with the pressure applied. 

The resistance (or permeability) of a filter medium direetly affects the capital and operating costs. Most manufacturers also employ the permeability of filter media as a measure of particle retention, which is related to its pore size and porosity. A high permeabil-... 

Addition of Inert Filter Aids. FUtet aids ate rigid, porous, and highly permeable powders added to feed suspensions to extend the appheabUity of surface filtration. Very dilute or very fine and slimy suspensions ate too difficult to filter by cake filtration due to fast pressure build-up and medium blinding addition of filter aids can alleviate such problems. Filter aids can be used in either or both of two modes of operation, ie, to form a precoat which then acts as a filter medium on a coarse support material called a septum, or to be mixed with the feed suspension as body feed to increase the permeabihty of the resulting cake. 

Use of filter aids is a technique frequently applied for filtrations in which problems of slow filtration rate, rapid medium blinding, or un-satisfactoiy filtrate clarity arise. Filter aids are granular or fibrous solids capable of forming a highly permeable filter cake in which veiy fine solids or slimy, deformable floes may be trapped. Application of filter aids may allow the use of a much more permeable filter medium than the clarification would require to produce filtrate of the same quahty by depth filtration. 

Diatomaceous Silica Filter aids of diatomaceous silica have a dry bulk density of 128 to 320 kg/m (8 to 20 Ib/fU), contain paiiicies mostly smaller than 50 [Lm, and produce a cake with porosity in the range of 0.9 (volume of voids/total filter-cake volume). The high porosity (compared with a porosity of 0.38 for randomly packed uniform spheres and 0.2 to 0.3 for a typical filter cake) is indicative of its filter-aid ability Different methods of processing the crude diatomite result in a series of filter aids having a wide range of permeability. 

For incompressible cake, the pressure distribution and the rate depend on the resistance of the filter medium and the permeability of the cake. Figure L8-150 shows several possible pressure profiles in the cake with increasing filtration rates through the cake. It is assumed that r /i i = 0.8 and /p//i = 0.6. The pressure at / = ri, corresponds to pressure drop across the filter medium Ap, with the ambient pressure taken to be zero. The filtration rate as well as the pressure distribution depend on the medium resistance and that of the cake. High medium resistance or blinding of the medium results in greater penalty on filtration rate. 

Actually the design of the dc EMI filter inductor is an easy matter. The core manufacturer provides a graph entitled Normal Magnetization Curves for the MPP cores, as seen in Figure 3-22. A permeability of 60 or below is recommended. 

Different filter media, regardless of the specific application, are distinguished by a number of properties. The principal properties of interest are the permeability of the medium relative to a pure liquid, its retention capacity relative to solid particles of known size and the pore size distribution. These properties are examined in a laboratory environment and are critical for comparing different filter media. 

For a given suspension rheology and flow rate there is a critical permeability of the filter, below which no cake will be formed. The model also suggests that the equilibrium cake thickness can be precisely controlled by an appropriate choice of suspension flow rate and filter permeability. 

Filter-cakes are hard to remove and thus can cause considerable formation damage. Cakes with very low permeability can be broken up by reverse flow. No high-pressure spike occurs during the removal of the filter-cake. Typically a high-pressure spike indicates damage to the formation and wellbore surface because damage typically reduces the overall permeability of the formation. Often formation damage results from the incomplete back-production of viscous, fluid loss control pills, but there may be other reasons. 

Cools and Janssen [545] studied the effect of background salt on the permeability of warfarin through octanol-impregnated membranes (Millipore ultrafiltration filters, VSWP, 0.025-pm pores). At a pH where warfarin was in its ionized form, it was found that increasing background salt increased permeability (Fig. 7.7). This... 

Ghosh [548] used cellulose nitrate microporous filters (500 pm thick) as scaffold material to deposit octanol into the pores and then under controlled pressure conditions, displace some of the oil in the pores with water, creating a membrane with parallel oil and water pathways. This was thought to serve as a possible model for some of the properties of the outermost layer of skin, the stratum comeum. The relative proportions of the two types of channel could be controlled, and the properties of 5-10% water pore content were studied. Ibuprofen (lipophilic) and antipyr-ine (hydrophilic) were model drugs used. When the filter was filled entirely with water, the measured permeability of antipyrine was 69 (in 10 6 cm/s) when 90% of the pores were filled with octanol, the permeability decreased to 33 95% octanol content further decreased permeability to 23, and fully octanol-filled filters indicated 0.9 as the permeability. 

An infrequently used method (in pharmaceutical research) for determining the UWL permeability involves measuring transport of molecules across a high-porosity microfilter that is not coated by a lipid. The molecules are able to diffuse freely in the water channels of the microfilter. The filter barrier prevents convective mixing between the donor and acceptor sides, and an UWL forms on each sides of the microfilter. Camenisch et al. [546] measured the effective permeabilities of a series of drug molecules in 96-well microtiter plate-filterplate (Millipore GVHP mixed cellulose ester, 0.22 pm pore) sandwich where the filters were not coated by a lipid. The permeabilities were nearly the same for all the molecules, as shown in Fig. 7.8a. Our analysis of their data, Fig. 7.8b, indicates / aq = 460 pm (sandwich stirred at 150 rpm). We have been able to confirm similar results in our laboratory with different microfilters, using the lipid-free method. 

Avdeef et al. [556] measured the PAMPA permeabilities of a series of drug molecules and natural products using both dodecane- and (dodecane + 2%DOPC)-coated filters. It was proposed that a new H-bonding scale could be explored, based not on partition coefficients but on permeabilities. 

It is assumed that the convective flow of water across the ABL, cell mono-layer, and filter owing to pressure gradients is negligible and that the cell mono-layer is uniformly confluent. When these conditions are not met, Katz and Schaeffer (1991) and Schaeffer et al. (1992) point out that mass transfer resistances of the ABL and filter [as described in Eq. (21)] cannot be used simply without exaggerating the permeability of the cell monolayer, particularly the paracellular route. An additional diffusion cell design was described by Imanidis et al. (1996). 

In Section III, emphasis was placed on flux kinetics across the cultured monolayer-filter support system where the passage of hydrophilic molecular species differing in molecular size and charge by the paracellular route was transmonolayer-controlled. In this situation, the mass transport barriers of the ABLs on the donor and receiver sides of the Transwell inserts were inconsequential, as evidenced by the lack of stirring effects on the flux kinetics. In this present section, the objective is to give quantitative insights into the permeability of the ABL as a function of hydrodynamic conditions imposed by stirring. The objective is accomplished with selected corticosteroid permeants which have been useful in rat intestinal absorption studies to demonstrate the interplay of membrane and ABL diffusional kinetics (Ho et al., 1977 Komiya et al., 1980). 

Table 13 Influence of Stirring Flow Rate on the Permeability of [14C] Testosterone Across Caco-2 Cell Monolayer/Filter Support in 02/C02 Gas Lift Side-by-Side Diffusion at 37°C...

The identification and characterization of cell culture systems (e.g., Caco-2-cells) that mimic in vivo biological barriers (e.g., intestinal mucosa) have afforded pharmaceutical scientists the opportunity to rapidly and efficiently assess the permeability of drugs through these barriers in vitro. The results generated from these types of in vitro studies are generally expressed as effective permeability coefficients (Pe). If Pe is properly corrected to account for the barrier effects of the filter (PF) and the aqueous boundary layer (PAbl) as previously described in Section II.C, the results provide the permeability coefficient for the cell monolayer... 

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