Membrane Dynamics

Dynamic membranes are concentration—polarization layers formed in situ from the ultrafiltration of coUoidal material analogous to a precoat in conventional filter operations. Hydrous zirconia has been thoroughly investigated other materials include bentonite, poly(acryhc acid), and films deposited from the materials to be separated (18). 

Tubes for dynamic membranes ate usually smaller (ca 6-mm ID). Typically, the tubes ate porous carbon or stainless steel with inorganic membranes (sihca, zirconium oxide, etc) formed in place. 

Another type of membrane is the dynamic membrane, formed by dynamically coating a selective membrane layer on a finely porous support. Advantages for these membranes are high water flux, generation and regeneration in situ abiUty to withstand elevated temperatures and corrosive feeds, and relatively low capital and operating costs. Several membrane materials are available, but most of the work has been done with composites of hydrous zirconium oxide and poly(acryhc acid) on porous stainless steel or ceramic tubes. 

Autofiltration The retention of any material at the surface of the membrane gives rise to the possibility of a secondaiy or a dynamic membrane being formed. This is a significant problem for fractionation by ultrafiltration because microsolutes are partially retained by almost all retained macrosolutes. The degree of retention is quite case-specific. As a rule of thumb, higher pressure and more polarization resiilts in more autofiltration. Autofiltration is particularly problematic in attempts to fractionate macromolecules. 

You may be surprised, but fouling is not always detrimental. The term dynamic membrane describes deposits that benefit the separation process by reducing the membrane s effective MWCO Molecular Weight cut-off) so that a solute of interest is better retained. Concentration polarization refers to the reversible build-up of solutes near the membrane surface. Concentration polarization can lead to irreversible fouling by altering interactions between the solvent, solutes and membrane. 

Generally, the effectiveness of the separation is determined not by the membrane itself, but rather by the formation of a secondary or dynamic membrane caused by interactions of the solutes and particles with the membrane. The buildup of a gel layer on the surface of an ultrafiltration membrane owing to rejection of macromolecules can provide the primary separation characteristics of the membrane. Similarly, with colloidal suspensions, pore blocking and bridging of... 

E Pefferkorn, A Schmitt, R Varoqui. Helix-coil transition of poly(a,L-glutamic acid) at an interface Correlation with static and dynamic membrane properties. Biopolymers 21 1451-1463, 1982. 

Carotenoid molecules incorporated into the lipid membranes considerably interfere with both the structural and the dynamic membrane properties. Both effects are directly related to the chemical structure of carotenoid molecules. Importantly, it is the rigid, rod-like backbone of the carotenoids,... 

We would like to point out that an order parameter indicates the static property of the lipid bilayer, whereas the rotational motion, the oxygen transport parameter (Section 4.1), and the chain bending (Section 4.4) characterize membrane dynamics (membrane fluidity) that report on rotational diffusion of alkyl chains, translational diffusion of oxygen molecules, and frequency of alkyl chain bending, respectively. The EPR spin-labeling approach also makes it possible to monitor another bulk property of lipid bilayer membranes, namely local membrane hydrophobicity. 

Hybrid supramolecular dynamic membranes as selective information transfer devices. Desalination, 199, 521-522. 

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. 

Living cells visualization of membranes, lipids, proteins, DNA, RNA, surface antigens, surface glycoconjugates membrane dynamics membrane permeability membrane potential intracellular pH cytoplasmic calcium, sodium, chloride, proton concentration redox state enzyme activities cell-cell and cell-virus interactions membrane fusion endocytosis viability, cell cycle cytotoxic activity... 

Porous metals have long been commercially available for particulate filtration. They have been used in some cases as microfiltration membranes that can withstand harsh environments, or as porous supports for dynamic membranes. Stainless steel is by far the most widely used porous metal membrane. Other materials include silver, nickel. Monel, Hastelloy and Inconel. Their recommended maximum operating temperatures range from 200 to 650°C. Elepending on the pore diameter which varies from 0.2 to 5 microns, the water permeability of these symmetric membranes can exceed 3000 L/h-m -bar and is similar to that obtained with asymmetric ceramic microfiltration membranes. Due to the relatively high costs of these membranes, their use for microfiltration has not been widespread. 

Figure 26 presents their results for the rejection of 0.01 M MgCJl2 400 psi on a dynamic membrane as a function of where U is the linear velocity down the tube, v is the permeation rate, and N is the Reynolds number. It will be noted that the greatest effect of the turbulence promoter was observed at the lowest velocities, where the rejection increased from 25 to 72 percent. At the highest tangential velocities, the improvement was much less, from 90 to 93 percent. In addition, Thomas and Watson observed an increase in permeation rate varying from 10 to 50 percent. Thus, with turbulence promoters, the same rejection and flux, as in an unpromoted system, may be obtained at a considerable reduction. 

KlapaMI, Park SM, Sinskey AJ, Stephanopoulos G (1999) Biotechnol Bioeng 62 375 Lirxenburger H, Wittmann C, Heinzle E (1998) Metabolic flux studies in Saccharomyces cerevisiae using dynamic membrane mass spectrometry. 2" Conference on Metabolic Engineering, Elman, Germany. 

In order to interpret the physicochemical steps of retinal transduction as well as membrane excitability, we analyze macroscopic properties of membranes within biological components. Such membranes separate two aqueous ionic phases the chemical compositions of which are kept constant separately. The total flux through the membrane is directly deduced from the counterbalance quantities in order to maintain the involved thermodynamical affinities constant. From such measurement, we calculate the dynamical membrane permeability. This permeability depends not only on membrane structure but also on internal chemical reactions. 

We can say, following these experiments, that the structural permeability loses its physical meaning. In fact, such nonlinearities between fluxes and forces and permeability variations establish the very existence of a chemical effect, leading us to the concept of the dynamical membrane permeability. 

Ultrafiltration and microfiltration can be backwashed occasionally to remove accumulated solids from membranes. UF and MF membranes may be used to remove micrometer-sized and upper suspended particles, namely bacteria, algae, and so on, they can also be used to remove Guardia and Cryptosporidium, as well as most viruses found in surface water. In fact, the solid layer ( cake ) adhering to the membranes in the latter two techniques acts like a dynamic membrane [8, 9], removing smaller particles even at colloidal and virus levels. 

Rumyantsev, M., Shauly, A., Yiantsios, S.G., Hasson, D., Karabelas, A.J. and Semiat, R. (2000) Parameters affecting the properties of dynamic membranes formed by Zr hydroxide colloids. Desalination, 131, 189-200. 

The membrane emulsification can be considered as a case of microdevice emulsification process [17, 18] in which the porous membrane is used as microdevices. Membrane emulsification carried out in quiescent conditions is also referred to as static membrane emulsification, while membrane emulsification carried our in moving conditions (either the membrane, i.e., rotating module, or the phase, i.e., crossflow) is also referred to as dynamic membrane emulsification (Figure 21.2(b)). 

In certain cases, the separation medium of a membrane is a liquid that is immiscible with the feed stream. The very high permeability of liquids relative to solid materials offers a productivity advantage. Usually the selectivity of liquids derives from differential partitioning of permeants. Liquids may also be used as a solvent for specific com-plexing agents that do not form membranes themselves. Finally, transient deposits of colloids can be used as selective barriers in the so-called dynamic membranes, which offer very high productivities when moderate degrees of separation are adequate. 

Textile sizing agents such as polyvinyl alcohol may also be reclaimed from hot process water. Here, both polymeric membranes and inorganic, dynamic membranes are appropriate choices. Systems based on polymeric membranes operate at lower fluxes and require less recirculation pumping, and are somewhat more economical. Plants with treatment capacities as high as 60 m3 per hour are in operation. 

Dialysis operates by the diffusion of selected solutes across a nonporous membrane from high to low concentration. An early industrial application of dialysis was caustic soda recovery from rayon manufacturing. It had been a viable process because inexpensive but alkali-resistant cellulose membranes were available that were capable of removing polymeric impurities from the caustic. Gradually however, dialysis is being replaced by dynamic membrane technology for caustic soda recovery because of the latter s much higher productivity. 

Biomaterials, Synthesis, Fabrication, and Applications Bioreactors Distillation electrochemical Engineering Fluid Dynamics Membrane Structure Membranes, Synthetic (Chemistry) Molecular Hydrodynamics Nano-structured Materials, Chemistry of Pharmaceuticals, Controlled Release of Solvent Extraction Wastewater Treatment and Water Reclamation... 

Kempken R, Rechtsteiner H, Schafer J, Katz U, Dick O, Weidemeier R, Sellick I (1997), Dynamic membrane filtration in mammalian cell culture harvest, In Carrondo MJT (Ed.), Animal Cell Technology, Kluwer, Dordrecht, pp. 379-382. 

The aims of this book are to highlight and summarize for medicinal and pharmaceutical chemists some important properties of phospholipid bilayers to explain, using examples, analytical tools for determining thermotropic and dynamic membrane properties and the possible effects of drugs on such membrane properties and, finally, to discuss examples of the importance of drag-membrane interactions for drug pharmacokinetics (absorption, distribution, accumulation) as well as drag efficacy, selectivity, and toxicity. 

As well as being the causative organisms of a number of major human and animal diseases (e.g. cysticercosis, hydatidosis), cestodes serve as elegant experimental models for the study of fundamental biological phenomena. These include not only problems of specific parasitological interest, such as host-specificity, but also more basic problems such as enzyme dynamics, membrane transport and cell and tissue differentiation (especially asexual/sexual differentiation), common to many other biological fields. 

Collins, J. W., Boggs, L. A., Webb, A. A., and Wiley, A. A. (1973). Spent sulfite liquor reducing sugar purification by ultrafiltration with dynamic membranes. Tappi 56(6), 121 -124. 

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