Filter media membrane filters

Surface Filters. In surface filters (Fig. 2), the goal is to achieve separation on the upstream side of a relatively thin filter medium. The particles to be separated must be larger than the pores in the medium, ie, in strainers, membrane filters, etc, or the particles must approach the pores in large numbers and bridge over the pores, as in cake filters. 

Insoluble Matter. Insoluble matter in sugar is deterrnined as the dry weight of material left on a filter or membrane after passage of a sugar solution. This may include bits of sand, filtration medium, plant material, and polymeric material. 

Another standard test, which is much simpler and more convenient, is the membrane filter technique. A suitable volume of sample is filtered through a sterile, 0.45-p.m membrane filter. The filter is placed in a petri dish containing a specific growth medium (M-Endo nutrient broth, M-Endo medium) and incubated for 24 h at 35°C. If after this time the colonies show the characteristic green sheen, this is taken as positive evidence for the presence of the coliform group (see Water, sewage). 

A membrane filter technique can also be used to determine the presence of fecal coliforms, and this procedure is said to be 93% accurate (20). A sample is passed through a membrane filter, and this filter is placed in a petri dish containing an enriched lactose medium. The dishes are incubated at... 

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. 

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. 

Process Concept The application of a direct elec tric field of appropriate polarity when filtering should cause a net charged-particle migration away from the filter medium. This electrophoretic migration will prevent filter-cake formation and the subsequent reduction of filter performance. An additional benefit derived from the imposed electric field is an electroosmotic flux. The presence of this flux in the membrane and in any particulate accumulation may further enhance the filtration rate. 

One potential difficulty with CF-EF is the electrodeposition of the particles at the electrode away from the filtration medium. This phenomenon, if allowed to persist, will result in performance decay of CF-EF with respect to maintenance of the electric field. Several approaches such as momentaiy reverses in polarity, protection of the electrode with a porous membrane or filter medium, and/or utilization of a high fluid shear rate can minimize electrodeposition. 

Filtration is the concentration of solids (or clarification of liquor) from slurry by fluid flow through a permeable medium. This normally takes the form of a membrane, filter leaf or packed bed, which restricts the particles, more than the fluid (Figure 4.4). 

The cross-flow filtration method is applied mainly to hyper- and ultrafiltration as well as to some microfiltration.8 In cross-flow filtration the slurry solution or suspension fed to the filter flows parallel to the filter medium or membrane. The filtration product (permeate or filtrate) leaves the filtration module at right angles to the filter medium (the membrane). The traditional perpendicular flow filtration (where the flow of the suspension is directed at right angles to the filter medium and the permeate leaves the filter medium in the same direction) entails filter cake buildup, whereas cross-flow filtration is intended to prevent such filter... 

The design of a cross-flow filter system employs an inertial filter principle that allows the permeate or filtrate to flow radially through the porous media at a relatively low face velocity compared to that of the mainstream slurry flow in the axial direction, as shown schematically in Figure 15.1.9 Particles entrained in the high-velocity axial flow field are prevented from entering the porous media by the ballistic effect of particle inertia. It has been suggested that submicron particles penetrate the filter medium and form a dynamic membrane or submicron layer, as shown in... 

Therefore, when operating in the filter cake mode, the axial velocity should be maintained at a level such that an adequate shear force exists along the filter media to prevent excessive caking of the catalyst that could cause a blockage in the down-comer circuit. For the separation of ultrafine catalyst particles from FT catalyst/wax slurry, the filter medium can easily become plugged using the dynamic membrane mode filtration. Also, small iron carbide particles (less than 3 nm) near the filter wall are easily taken into the pores of the medium due to their low mass and high surface area. Therefore, pure inertial filtration near the filter media surface is practically ineffective. 

Each filter has demonstrated the capacity to filter the full brine flow of 195 m3 h 1. The pressure drop through the filter medium is measured and monitored continuously. Typically, it is nearly constant over a 2-h filtration at 195 m3 h 1. Back-pulse cleaning restores the initial pressure drop from cycle to cycle, with only a slow increase over time. After 12 months running time, the initial pressure drop at the beginning of the filter cycle had increased by 0.6 bar. The filter membranes were chemically cleaned with 5 % hydrochloric acid. After a cleaning time of 2 h the filter was started again and the pressure drop was less than 0.1 bar greater than that of new filter socks. 

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. 

Three commercially available filtering media were evaluated for the collection of airborne particulates of the organoarsenical compounds. These were cellulose ester, Fluoropore, and Mitex membrane filters. The evaluation was based on the following requirements l) the collection medium must quantitatively collect the particulates 2) the collection medium must be capable of. retaining the compounds without loss or chemical change before analysis 3) the collected species must be extracted quantitatively from the collection medium for analysis and b) the method must be efficient at temperatures up to 50°C and a relative humidity of... 

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