Multilayered filtration

Multimedia filtration (also called prefiltration, sand filtration or multilayered filtration) is mainly aimed at removing sediments and suspended matter. Suspended contaminants are trapped in small crevices and, as a result, water turbidity is improved. A number of media are distinctly layered with the coarsest on top so the suspended matter is collected throughout the depth of the filter according to size. 

The U.S. military specification, M1L-P-27201B, requires 95% para content, 99.995% minimum hydrogen by difference, 50 vppm maximum total imputities, 9 vppm maximum combined nitrogen, water, and volatile hydrocarbons, 1 vppm maximum combined oxygen and argon, 39 vppm maximum helium, 1 vppm maximum carbon monoxide and dioxide, and a 10/40 micrometers nominal /absolute particulate filtration level. Liquid hydrogen is stored in double-walled vessels with evacuated pedite or multilayer insulation and transported in similarly insulated 50,000-L trailers or 900,000-L barges. 

The SBP membrane filtration system concentrates contaminants and reduces the volume of contaminated groundwater, surface water, storm water, landfill leachates, and industrial process water. This hyperfiltration system consists of stainless steel tubes coated with a multilayered membrane, which is formed in-place using proprietary chemicals. The membrane filtration system can be used with an SBP bioremediation system or another technology as part of a treatment train. 

Figures 4A and 4B are scanning electron micrographs of a an ultrafilitration polysulfone membrane with a 30,000-Dalton mol wt cutoff at low and high magnification, respectively. The image was taken during the protein-filtration portion of cycle 10 ( 3100 s), for an SMY experiment with 5.0 g/L of cellulase in the primary feed, 5.36 of g/L yeast in the secondary feed, and Py=Ph = 15 psi. The majority of the membrane is covered by an SMY, but in a few places the SMY is absent or has been eroded. Note that the SMY is mostly monolayered, compared with the multilayered SMY seen during microfilitration. This difference is apparently due to the lower flux in ultrafiltration.

Pores, and espjecially mesopores and micropores, play an essential role in physical and chemical properties of industrially important materials like adsorbents, membranes, catalysts etc. The description of transport phenomena in porous materials has received attention due to its importance in many applications such as drying, moisture transport in building materials, filtration etc. Although widely different, these applications present many similarities since they all depend on the same type of transport phenomena occurring in a porous media environment. In particular, transport in mesoporous media and the associated phenomena of multilayer adsorption and capillary condensation have been investigated as a separation mechanism for gas mixtures. 

To determine the effect of a pressure field, resulting from cavitation bubbles compressing, on the penetration of a melt into a capillary channel of a filter, one should estimate the character of the pressure impulse propagation through the capillary. The mechanism of the flow is somewhat different under conditions of fine filtration of a melt through a porous medium (a filter from multilayer fiberglass with a cell of 0.6 x 0.6 or 0.4 x 0.4 mm in size) with rather short (about several mm) but complexly curved channels, but the main conditions remain. 

Assuming that the melt flow is laminar, its flow rate through a multilayer filter does not depend on time. But in the case when a molten metal contains dispersed particles with a size less than the section of the channel, the flow rate becomes dependent on time mainly due to the adhesion of the particles to the channel walls. With this, those particles which have the size larger than that of the capillary channel section are retained at the entrance of the filter in a form of a cake which increases the apparent length of the channel and decreases the active surface of the filter. The input of intensive ultrasonic oscillations in the mode of developed cavitation results in the appearance of active acoustic streams near the filter surface and in washing-out the cake. In the ideal case, the value of the flow rate through the filter can be sustained constant for a sufficiently long period of fine filtration due to the action of acoustic cavitation and streams. 

S. Sarrade, C. Bardot, M. Carles, R. Soria, S. Cominotti and R. Gillot, Elaboration of new multilayer membrane for nanofiltration. Proceedings of the 6th World Filtration Congress, 18-21 May 1993, Nagoya, Japan. 

Most inorganic membrane supports exhibit a tubular shape. This is a well-adapted geometry for cross-flow filtration in which the feed stream is circulated across the surface of the membrane and the permeated flux passes through the membrane in a perpendicular direction. Stainless steel, carbon, and ceramic are the most frequently used materials in the preparation of supports. As shown in Fig. 2, tubes or multichannel substrates can act as membrane supports. A well-designed support must be mechanically strong, and its resistance to fluid flow must be very low. Aiming at enhancing flux performances, multilayered substrates have been prepared that exhibit an asymmetric structure... 

Membrane separation applications with higher selectivity, capacity, and flow rates are driven by the need for more economical and robust manufacturing processes providing higher-purity products. In the area of virus and sterile filtration, membrane providers are developing high-capacity and high-flux membranes by the use of multilayer structures. 

C. Kaya, A. R. Boccaccini and P. A. Tmsty, Processing and Characterisation of 2-D Woven Metal Fibre-Reinforced Multilayer Silica Matrix Composites Using Electrophoretic Deposition and Pressure Filtration, J. Europ. Ceram. Soc. 19, 2859-2666 (1999). 

Single-collector efficiency for monolayer filtration vreis estimated with the expression developed by Rajagopalan and Tien [128], obtained by the combination of the trajectory analysis of a spherical particle in the vicinity of a spherical collector with the contribution of the Brownian diffusion. For fine-fine capture step, filtration becomes driven by the fine-fine interaction forces yielding a multilayer deposit for which the filter coefficient no longer remains constant in time. The change of the filter coefficient as a function of the specific deposit was estimated using the correlation developed by Tien et al. [129]. Extra information about trickle-bed deep-bed filtration model is given in Iliuta and Larachi [130] and Iliuta et al. [119]. 

Some 3D fabrics have a long history. Velvets are made as two woven fabric layers linked by spacer threads before cutting to expose a pile. Multilayer weaves with two or three layers are used for some technical fabrics, such as those used in filtration or papermaking. Hand-knitted socks have a 3D shape, turning at the heel. 

One of the functions of geotextiles is to separate one layer, such as the stone base of a roadway, from another layer, such as the underlying soil, while allowing water to pass through. This is another filtration operation for which multilayer fabrics are used. 

Commercial deep-bed filters consist of a cylindrical or rectangular packed bed through which the suspension to be filtered is passed. The common types of deep-bed filter include the slow sand filter, the rapid filter, and direct filtration with a flocculated mixture. Typical deep-bed filters are 0.5-0.3 m in height and 1 m in diameter. Smaller packing material (filter media) provide a greater surface area and result in a more effective capture of suspended solids, but the bed pressure drop and clogging tendency also increase. Often, the design of the deep-bed filter is to employ mixed size media packed in multilayers, as shown in Fig. 7 (Cheremisinoff, 1998). 

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