Filter permeability

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. 

Furthermore, the transplants are often packed in aluminum boxes, which are sealed by sterile filters permeable to water vapor. Even if the box can be designed with a negligible resistance to water vapor flow, the heat transfer is substantially reduced. 

Fig. 8. (a) Velocity fields at different sections through the reconstructed filter wall (gray denotes the solid material, violet denotes lowest and red denotes highest velocity) and visualization of flow paths in the reconstructed filter wall and (b) comparison of experimental and simulated filter permeabilities (see Plate 8 in Color Plate Section at the end of this book). 

Konstandopoulos, A. G., and Kladopoulou, E. The optimum cell density for wall-flow monolithic filters Effects of filter permeability, soot cake structure and ash loading. SAE Technical Paper No. 2004-01-1133 (2004). 

T. Urbanski, Malendowicz and Dybowicz [68] examined the behaviour of nitroglycerine (and of other nitric esters) exposed to ultra-violet rays and established that nitroglycerine irradiated once for a short period with a quartz lamp started to undergo slow decomposition which stopped only after an interval of 2-3 days. A sample of 3 g nitroglycerine was irradiated for 1 hr with rays from quartz lamp passing through a filter permeable to rays of 3200-4100 A whilst maintained at a temperature of 15°C. In order to test the decomposition of the specimen, from time to time 0.25-0.5 g sample of the substance were removed, shaken with water and the pH values determined. The following results have been obtained immediately after irradiation pH = 6.86 after 6, 24, 48 and 72 hr—6.12, 4.66,4.48 and 5.22 respectively. 

In Figure 3 a typical result of the transient evolution of size specific collection efficiency for the cordierite sample No. 2 is presented. The decrease of the filter permeability due to the coating is beneficial for the collection efficiency of the coated filters and makes them approach the cake filtration regime more quickly. 

The microbial rating is generally used for membrane filters that are used in the sterilisation industry and is expressed as the ability of the filters to sterilise liquids. The filter permeability is the expression of resistance to flow provided by the filter media. This direct method establishes the permeability and can provide flow and pressure drop data with respect to fluid temperature and viscosity, filter size, and time. The effect of pulsating flow is the loosening of fine particles by agitating the filter. 

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. 

An additional benefit of prethickening is reduction in cake resistance. If the feed concentration is low, there is a general tendency of particles to pack together more tightly, thus leading to higher specific resistances. If, however, many particles approach the filter medium at the same time, they may bridge over the pores this reduces penetration into the cloth or the cake underneath and more permeable cakes are thus formed. 

Other designs squee2e the cake between two permeable belts or between a screw conveyor of diminishing diameter, or pitch, and its permeable enclosure. The available filters which use mechanical compression can be classified into four principal categories, ie, membrane plate presses, tube presses, belt presses, and screw presses. 

Because gravity is too weak to be used for removal of cakes in a gravity side filter (2), continuously operated gravity side filters are not practicable but an intermittent flow system is feasible in this arrangement the cake is first formed in a conventional way and the feed is then stopped to allow gravity removal of the cake. A system of pressure filtration of particles from 2.5 to 5 p.m in size, in neutralized acid mine drainage water, has been described (21). The filtration was in vertical permeable hoses, and a pressure shock associated with relaxing the hose pressure was used to aid the cake removal. 

The three disadvantages described can be avoided by using soHd elements, instead of permeable ones, which create the shear to prevent or reduce cake formation. Only the stationary surface inside the filter is then available for filtration and this means a reduction in capacity. This is not a problem because the soHd disks can be slimmer and the collection of filtrate does not have to be through a hoUow shaft. 

Air Permeability. Air permeabiUty is an important parameter for certain fabric end uses, eg, parachute fabrics, boat sails, warm clothing, rainwear, and industrial air filters. Air permeabiUty of a fabric is related to its cover, or opacity. Both of these properties are related to the amount of space between yams (or fibers in the case of nonwovens). The most common method for specifying air permeabiUty of a fabric involves measuring the air flow per unit area at a constant pressure differential between the two surfaces of the fabric. This method, suitable for measuring permeabiUty of woven, knitted, and nonwoven fabrics, is described in ASTM D737. Units for air permeabiUty measured by this method are generally abbreviated as CFM, or cubic feet per square foot per minute. 

The experimental procedure and method of treatment of compression-permeability data have been explained by Grace [Chem. Eng. Prog., 49, 303, 427 (1953)], who showed that the values of a measured in such a cell and in a pressure filter were the same, and by Tiller [Filtr Sep., 12, 386 (1975)]. 

Filter-medium selection embraces many types of construction fabrics of woven fibers, felts, and nonwoven fibers, porous or sintered solids, polymer membranes, or particulate solids in the form of a permeable bed. Media of all types are available in a wide choice of materials. 

Filter Papers These papers come in a wide range of permeability, thickness, and strength. As a class of material, they have low strength, however, and require a perforated backup plate for support. 

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. 

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