Membrane filtration porous

Aliena, F. W. and Belfort, G. Chem. Eng. Sci. 39 (1984) 343. Lateral migration of spherical particles in porous flow channels application to membrane filtration. 

Enzymatic reactions are commonly observed or practiced in various kinds of food and biotechnology products. With the goals of reducing operating costs and improving product quality, a number of enzyme immobilization techniques have been developed in recent decades [Woodward, 1985]. The availability of robust membranes, particularly porous inorganic membranes, has improved the enzyme immobilization technology. One type of membrane bioieactors immobilizes enzyme in the membrane pores by dead-end filtration of the enzyme solution. 

Typical examples of one-phase techniques are filtration and dialysis. The membrane is porous, so there is a liquid (or gas) contact through the pores between the donor and acceptor phases, which are of similar chemical composition (i.e., both are either aqueous, organic, or gaseous). There is no phase boundary, and therefore, no partition between phases. Thus, physical and not chemical properties govern the process. This review will not consider one-phase systems further. Eor information on dialysis, especially its analytically interesting version microdialysis, see Refs. [23,24]. 

In membrane filtration, water-filled pores are frequently encountered and consequently the liquid-solid transition of water is often used for membrane pore size analysis. Other condensates can however also be used such as benzene, hexane, decane or potassium nitrate [68]. Due to the marked curvature of the solid-liquid interface within pores, a freezing (or melting) point depression of the water (or ice) occurs. Figure 4.9a illustrates schematically the freezing of a liquid (water) in a porous medium as a fimction of the pore size. Solidification within a capillary pore can occur either by a mechanism of nucleation or by a progressive penetration of the liquid-solid meniscus formed at the entrance of the pore (Figure 4.9b). 

The influence of metal oxide derived membrane material with regard to permeability and solute rejection was first reported by Vernon Ballou et al. [42,43] in the early 70s concerning mesoporous glass membranes. Filtration of sodium chloride and urea was studied with porous glass membranes in close-end capillary form, to determine the effect of pressure, temperature and concentration variations on lifetime rejection and flux characteristics. In this work experiments were considered as hyperfiltration (reverse osmosis) due to the high pressure applied to the membranes, 40 to 120 atm. In fact, results reproduced in Table 12.3 show that these membranes do not behave as h)qjerfiltra-tion membranes but as membranes with intermediate performances between ultra- and nanofiltration in which surface charge effect of metal oxide material plays an important role in solute rejection. 

J.J. Porter and R.S. Porter, Filtration studies of selected anionic dyes using asymmetric titanium dioxide membranes on porous stainless tubes, f. Membr. Sci., 101 (1995) 67. 

Problem 5-12. Flow Through a Porous Tube. Let us consider flow through a cylindrical porous tube, which occurs in many membrane filtration processes. The tube is very long with radius R. At the inlet of the tube, the pressure is/ /. Fluid permeates or leaks out through the wall of the tube with a velocity k(P — Ps) ///, where P is the local pressure in the fluid, Ps is the pressure on the other side of the membrane, A is a permeation coefficient, and // is the viscosity of the fluid. We wish to determine how much fluid is filtered as a function of the length of the tube. 

The breakthrough, which resulted in an anisotropic RO membrane in 1959, paved the way for the first anisotropic UF membrane in 1963. Though UF membranes are porous and RO membranes are not, the evolutionary development of both occurred in parallel. Before 1960, membranes showing the retention properties of RO and of UF were available, but both had impractical filtration rates (flux). 

Micro-filtration membranes are porous structures, which freely pass solvents (usually water) and molecularly dispersed solutes, including polymers. 

MICRO FILTRATION - A membrane filtration process, which forces water through a porous barrier. Pores are usually between 0.1 to 20 m, when used for water purification. For filtering purposes, pore sizes are. 045 m. 

Filtration can be used to distinguish between dissolved and dispersed components in a colloidal dispersion. Membrane filtration involves passing a suspension through a thin, porous membrane, which is usually polymer or ceramic in nature, but could also be woven fabric or metal fibres. Filtration is also a common method... 

Another very successful development for reverse osmosis is the energy-saving membrane series produced by Nitto Denko (Tab. 4.2) [37]. The membrane filtrating layer is also an aromatic polyamide. Due to its irregular surface, the actual membrane area available for the permeation is much larger than it would be in the case of a smooth surface on the same porous support. High fluxes are therefore obtained. 

The equipment for UF systems often looks very similar to RO systems although they operate at lower pressures. However, this similarity does not extend to the molecular level. Remeniber that RO membranes are nonporous and separate based on a solution-diffusion mechanism UF membranes are porous and separate based on size exclusion. Large molecules are excluded from pores in the thin membrane skin and thus, the large molecules are retained in the retentate. Small molecules fit into the pores and pass through to the permeate. Since there is usually a distribution of pore sizes, molecules within the range of pore sizes partially permeate and are partially retained. In a somewhat oversinplified picture, UF is cross-flow filtration at the molecular level. 

The filtration operation involves the separation, removal, and eolleetion of a discrete phase of matter existing in suspension. The undissolved solid partieles are separated from the liquid suspension by means of a porous medium (i.e., filter medium). Filtration leads to the formation of a eake containing a relatively low proportion of residual filtrate. Depending upon the meehanism for arrest and accumulation of particles, the filtration operation can generally be classified into three types cake filtration, deep-bed filtration, and membrane filtration (see Fig. 1). 

Microfiltration (MF) is a membrane filtration in which the filter medium is a porous membrane with pore sizes in the range of 0.02-10 pm. It can be utilized to separate materials such as clay, bacteria, and colloid particles. The membrane structures have been produced from the cellulose ester, cellulose nitrate materials, and a variety of polymers. A pressure of about 1-5 atm is applied to the inlet side of suspension flow during the operation. The separation is based on a sieve mechanism. The driving force for filtration is the difference between applied pressure and back pressure (including osmotic pressure, if any). Typical configurations of the cross-flow microfiltration process are illustrated in Fig. 2. The cross-flow membrane modules are tubular (multichannel), plate-and-frame, spiral-wound, and hollow-fiber as shown in Fig. 3. The design data for commercial membrane modules are listed in Table 1. 

Porous membranes separate according to the size of particles/molecules. Depending on their size, membrane filtration systems have pore diameters from 1 nm to 1 p,m (see Table 3.3.14 below). 

In membrane preconcentration (Fig.2e), gel or porous membranes are used to concentrate molecules bigger than the size of the pores. By adjusting the pore size, one can allow the passage of buffer ions and small molecules but exclude larger molecules of interest. With the formation of nanofllters or nanoporous membranes within the microfluidic systems, this strategy can be implemented easily. Membrane (flltration)-based preconcentration will not have any chemical bias (mainly dependent on the size of the molecule), but continuous membrane filtration could generate eventual clogging of the system, which is one of the main problems in this technique. 

Streaming potential as a function of the apphed pressure for different membranes is shown in Figure 9.5. Trans-membrane or filtration streaming potential (FSP) for composite HR95 membrane, its porous support (membrane PS-Uf) and the DEAE-dextran fouled PS-Uf porous membrane is shown in Figure 9.5, where different effects of interest can be observed, depending on the membrane structure and solution ... 

---