Porosity of membrane

Ultra- filtration Water sample la filtered under pressure through a membrane that will pass molecular constituents below a certain size and retain those above that size. Large molecules Porosity of membrane daterminaa the size of molecules concentrated. Usually used for compounds > 1000 molecular weight. Can concentrate large sample volumes at low temperatures. 

Porosities of membrane components vary widely and values are reported ranging from 20 to 60%. Commonly, values of 30-40% are used. Pore sizes range from macropores (>500 nm) via mesopores (20-500 nm) to micropores (<2 nm). A great problem is the lack of reliable measurement methods to measure the porosity and pore size distribution of supported membranes (see Chapter 4). 

Table I. Filtration properties and porosities of membranes prepared from different polymer-solvent systems by precipitation in water...

Table 1.2 Filtration Properties and Porosities of Membranes Prepared From Different Polymer-Solvent Systems by Precipitation in Water...

C, and Co represent the metal ion concentration in feed phase at time t and initial time V and 1 represent volumetric flow rate and volume of the feed solution, respectively e denotes the porosity of membrane material k denotes the mass transfer resistances, respectively... 

Khaisri et al. [23] used PTFE membrane (Markel Corporation) in a long-term CO2 desorption from aqueous monoethanolamine (MEA) solution for 200 h at 100°C. Membranes of different porosities of 23% and 40% were tested and the result showed that the desorption performance of membranes with 40% porosity reduced continuously due to the intrusion of MEA into the membrane pore. PTFE membrane is known as chemical and temperature resistant, due to the doubling of pore size and porosity of membrane that contributes to a high potential of wetting. Pressure drop along the module may aid this condition. Khaisri et al. [24] concluded that the membrane with low porosity and small pore size would be appropriate to be employed in a membrane contactor in terms of flux stability performance. 

Typically, 95% of dissolved salts are removed from the brine. All particulates are removed. However, due to their molecular porosity, RO membranes do not remove dissolved gases, such as CI2, CO2, and O2. 

Membrane Porosity Separation membranes run a gamut of porosity (see Fig. 22-48). Polymeric and metallic gas separation membranes, electrodialysis membranes, pervaporation membranes, and reverse osmosis membranes are nonporous, although there is hnger-ing controversy over the nonporosity of the latter. Porous membranes are used for microfiltration and ultrafiltratiou. Nanofiltration membranes are probably charged porous structures. 

The retention efficiency of membranes is dependent on particle size and concentration, pore size and length, porosity, and flow rate. Large particles that are smaller than the pore size have sufficient inertial mass to be captured by inertial impaction. In liquids the same mechanisms are at work. Increased velocity, however, diminishes the effects of inertial impaction and diffusion. With interception being the primary retention mechanism, conditions are more favorable for fractionating particles in liquid suspension. 

If the pressure drop across a tubular membrane is 2.8 bars, determine the permeat velocity across the membrane module. The thickness and the porosity of the deposit are 2 mm and 40 %, respectively. The average diameter of the partices is 5 microns. The initial membrane resistance is estimated to be 1.7 X 10 1/m. 

Bone is a porous tissue composite material containing a fluid phase, a calcified bone mineral, hydroxyapatite (HA), and organic components (mainly, collagen type). The variety of cellular and noncellular components consist of approximately 69% organic and 22% inorganic material and 9% water. The principal constiments of bone tissue are calcium (Ca ), phosphate (PO ), and hydroxyl (OH ) ions and calcium carbonate. There are smaller quantities of sodium, magnesium, and fluoride. The major compound, HA, has the formula Caio(P04)g(OH)2 in its unit cell. The porosity of bone includes membrane-lined capillary blood vessels, which function to transport nutrients and ions in bone, canaliculi, and the lacunae occupied in vivo by bone cells (osteoblasts), and the micropores present in the matrix. 

Button cells consist of cathode and anode cans (used as the terminals), powdered zinc anode, containing gelled electrolyte and the corrosion inhibitor, separator with electrolyte, thin (0.5 mm) carbon cathode with catalyst and PTFE, waterproof gas-permeable (teflon) layer and air distribution layer for the even air assess over the cathode surface. Parameters of battery depend on the air transfer rate, which is determined by quantity and diameters of air access holes or porosity of the gas-diffusion membrane. Air-zinc batteries at low rate (J=0,002-0,01C at the idle drain and J= 0,02-0,04C at the peak continuous current) have flat discharge curves (typical curve is shown by Figure 1). 

The lowest theoretical interparticular volume of perfectly packed uniformly sized spherical beads is calculated to be about 26% of the total available volume. In practice, even the best packed columns still contain about 30-40% void volume in addition to the internal porosity of the beads. The problem of interparticular volume does not exist in systems in which a membrane is used as the separation medium. Both theoretical calculations and experimental results clearly document that membrane systems can be operated in a dead-end filtration... 

This work was done in collaboration with Professor Hiroshi Yoneyama of Osaka University [124], The procedure used to prepare the LiMu204 tubules is shown schematically in Fig. 21. A commercially available alumina filtration membrane (Anopore, Whatman) was used as the template. Alumina is especially suited for this application because of its high porosity, monodispersity of pore size, and the fact that it can be heated to high temperature without degradation. This membrane contains 200-nm-diameter pores, is 60 p,m thick, and has a porosity of 0.6. A 1.5 cm X 1.5 cm piece of this membrane was mounted on a Pt plate (2 cm X 2 cm) by applying a strip of plastic adhesive tape (also 2 cm X 2 cm NICHIBAN VT-19) across the upper face of the membrane. The Pt plate will serve as the current collector for the LiMn204 battery electrode material. The strip of tape, which will be subsequently removed, had a 1.0 cm circular hole punched in it, which defined the area of the membrane used for the template synthesis of the LiMn204. 

The liquid-membrane electrode is another important type of ion-selective electrode. The internal filling solution contains a source of the ion under investigation, i.e. one for which the ion exchanger is specific, while also containing a halide ion to allow the reference electrode to function. The physico-chemical behaviour of the ISE is very similar to that of the fluoride electrode, except that ise and the selectivity are dictated by the porosity of a membrane rather than by movement through a solid-state crystal. 

There exist a maximum allowable thickness of the supported gel layers above which it is not possible to obtain crack-free membranes after calcination. For Y-alumina membranes this thickness depends on a number of (partly unknown) parameters and has a value between 5 and 10 /im. One of the important parameters is certainly the roughness and porosity of the support system, because unsupported membranes (cast on teflon) are obtained crack-free up to 100 )xm. The xerogel obtained after drying was calcined over a wide range of temperatures. At 390°C the transition of boehmite to y-AljOj takes place in accordance with the overall reaction... 

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