Membrane, general description

Process Description Gas-separation membranes separate gases from other gases. Some gas filters, which remove hquids or sohds from gases, are microfiltration membranes. Gas membranes generally work because individual gases differ in their solubility and diffusivity through nonporous polymers. A few membranes operate by sieving, Knudsen flow, or chemical complexation. 

A. General description Denileukin dif-titox is a recombinant, DNA-derived, interleukin-2 receptor specific ligand, cytotoxic fusion protein consisting of diphtheria toxin fragments A and B fused to interleukin-2. It is produced by expression of a recombinant fusion protein in Escherichia coli that contains nucleotide sequences for human interleukin-2, and sequences for the enzymatically active fragment A of diphtheria toxin and the membrane-translocating portion of diph-... 

Alternatively, /cuptake, may represent the rate constant associated with transfers from immediately outside the cell to the interior spaces where the relevant enzymes occur. In this case, we need to focus on how long it takes for microorganisms to be equilibrated with medium in which they occur (step 1 in Fig. 17.1). This requires an evaluation of the time for chemicals to be transported from the outside of the microorganisms across their cell envelope to the relevant enzymes. The cell envelope of bacteria differs from species to species (Sikkema et al., 1995) and can even be changed by a single microbial strain in ways that affect cross-membrane transport in response to environmental conditions (e.g., Pinkart et al., 1996 Ramos et al., 1997). However, a general description is useful in the context of molecular... 

The basic hydrodynamic equations are the Navier-Stokes equations [51]. These equations are listed in their general form in Appendix C. The combination of these equations, for example, with Darcy s law, the fluid flow in crossflow filtration in tubular or capillary membranes can be described [52]. In most cases of enzyme or microbial membrane reactors where enzymes are immobilized within the membrane matrix or in a thin layer at the matrix/shell interface or the live cells are inoculated into the shell, a cake layer is not formed on the membrane surface. The concentration-polarization layer can exist but this layer does not alter the value of the convective velocity. Several studies have modeled the convective-flow profiles in a hollow-fiber and/or flat-sheet membranes [11, 35, 44, 53-56]. Bruining [44] gives a general description of flows and pressures for enzyme membrane reactor. Three main modes... 

The water distribution within a polymer electrolyte fuel cell (PEFC) has been modeled at various levels of sophistication by several groups. Verbrugge and coworkers [83-85] have carried out extensive modeling of transport properties in immersed perfluorosulfonate ionomers based on dilute-solution theory. Fales et al. [109] reported an isothermal water map based on hydraulic permeability and electro-osmotic drag data. Though the model was relatively simple, some broad conclusions concerning membrane humidification conditions were reached. Fuller and Newman [104] applied concentrated-solution theory and employed limited earlier literature data on transport properties to produce a general description of water transport in fuel cell membranes. The last contribution emphasizes water distribution within the membrane. Boundary values were set rather arbitrarily. 

The solution-diffusion model is currently being used by most of the membrane community. The most general description of the mass transport across a membrane is based on irreversible thermodynantics (10) ... 

Isobaric applications in the continuum regime, making use of molecular bulk diffusion and/or some viscous flow are found in catalytic membrane reactors. The membrane is used here as an intermediating wall or as a system of microreactors [29,46]. For this reason some attention will be paid to the general description of mass transport, which will also be used in Sections 9.4 and 9.5. 

GENERAL DESCRIPTION Cap smooth, white, thick fleshed, convex to broadly convex to plane with age. The cap margin is incurved at first but soon decurves. The gills are pinkish at first, soon darkening to chocolate brown with spore maturity. The stem is thick, relatively short and adorned with a double membranous annulus. (The lower ring is often a thin annular zone). Its spores are dark chocolate brown In mass. 

GENERAL DESCRIPTION A robust, thick fleshed Agaricus species, with thin gills that are pinkish when young, and darkening to sepia and then chocolate brown in age. The cap is characteristically brownish, whitish or cream colored. The cap surface is smooth to appressed squamulose and dry. This species has a short, thick stern which is adorned with a persistent membranous annulus from a well developed partial veil. Its spores are chocolate brown in mass. 

One of the most general description of ion flow across a membrane is given by the Nernst-Planck equations. This is a partial differential equation where the independent variables represent space (x) and time (t). The main dependent variable is the concentration of the ion (c(x, t)). The potential (u(x, t)) is usually a fixed function but can be made a dependent variable in which case an additional equation is required. The Nernst-Planck equations can be written as ... 

Hence, M. EikerUng, A. A. Kornyshev, and E. Spohr start out in Volume 215, Chapter 2 with a general description of proton-conduction in polymer membranes, elucidating the influence of water and charge-bearing species in the polymer environment. Y. Yang, A. Siu, T. J. Peckham, and S. Holdcroft give an... 

Detailed studies on thermo-osmosis using highly selective cellulose acetate membrane in the presence and absence of osmotic pressure difference have also been carried out [25]. Using general description of thermo-osmosis based on irreversible thermodynamics, it was shown that coupling between the flow of heat and the flow of water is quite loose possibly on account of thermal leak between the compartments. Whatever the detailed stmctural interpretation, it was argued that in annealed, less-permeable membranes, the water-matrix interaction is increased relative to the water-water interaction and with only this type of interaction strong thermo-osmosis is expected. 

One of the important membrane processes is pervaporation, that is, the partial vaporization through a membrane. The mass transfer of a component through a membrane to the permeate side depends mainly on its affinity to the membrane material. General description of mass transfer in pervaporation consists of the following five steps [4] ... 

To address the theoretical limitation of the Nikolsky-Eisemnan equation, a more general description of the equihbrium responses of hquid membrane ISEs in mixed ion solutions was proposed (41). The model is based on phase boundary potentials under an equilibrium exchange of an analyte and an interfering co-ion at the membrane/sample solution interface. With ionophore-based membranes, the ion-exchange process is followed by complexation of the ions with an ionophore, where free ionophore was assumed to be always present in excess to simplify the model. The charge of the ions was not fixed so that their effect on the potentiometric responses can be addressed by the model. Under equilibrium conditions, the model demonstrated that the Nikolsky-Eisemnan equation is valid only for ions with the same charge (zj = Zj). The selectivity coefficient, however, can still be used in the new model to quantify the potentiometric responses in the mixed ion solution. For example, the potentiometric responses to a monovalent cation in the presence of a divalent cation are given as... 

The most general description of the mass transport across a membrane is based on irreversible thermodynamics. 

Bruining, W.J., 1989. A general description of flows and pressures in hollow fiber membrane modules. Chem. Eng. Sci. 44, 1441-1447. 

This chapter presents a general description of the three kinds of measurements (SP, MP, IS) used for electrical characterization of membrane in working conditions , that is, with the membranes in contact with dectrolyte solutions and the information achieved from them is briefly indicated. The main attention focuses on the characterization of those membranes commonly used in traditional separation processes (from diverse materials and with different structures), and IS measurements are considered in more detail. Membrane and matrix material electrical parameters are obtained, but the measurements also provide thermodynamics and geometrical/structural information. 

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