Separation theory, membranes

The attempt to show that surface tension phenomena were the cause of osmotic pressure was first made by Jager, and his theories were vigorously supported and developed by Traube, whose conclusions we shall state and examine briefly. He finds that the more a dissolved substance reduces the surface tension of water the greater is the velocity of osmosis of the solution. Hence he concludes that it is the difference in the surface tensions of solvent and solution which determines the direction and velocity of osmosis. The direction of flow Traube obtains by the following consideration let M (Fig. 7) be a membrane separating two liquids A and B. The molecules of each liquid are then drawn into its interior by the cohesion or intrinsic pressure. If the intrinsic... 

Membrane conformational changes are observed on exposure to anesthetics, further supporting the importance of physical interactions that lead to perturbation of membrane macromolecules. For example, exposure of membranes to clinically relevant concentrations of anesthetics causes membranes to expand beyond a critical volume (critical volume hypothesis) associated with normal cellular function. Additionally, membrane structure becomes disorganized, so that the insertion of anesthetic molecules into the lipid membrane causes an increase in the mobility of the fatty acid chains in the phospholipid bilayer (membrane fluidization theory) or prevent the interconversion of membrane lipids from a gel to a liquid form, a process that is assumed necessary for normal neuronal function (lateral phase separation hypothesis). 

In 4.4 the theory of 4.2 will be applied to study electro-diffusion of ions through a unipolar ion-exchange membrane, separating two electrolyte solutions. This will include the classical treatment of concentration polarization in a solution layer adjacent to an ion-exchange membrane under an electric current. The validity limits of this theory, set by the violations of local electro-neutrality and caused by the development of a macroscopic nonequilibrium space charge, will be indicated. (The effects of the nonequilibrium space charge are to be discussed at some length in Chapter 5.)... 

Advantages to Membrane Separation This subsection covers the commercially important membrane applications. All except electrodialysis are pressure driven. All except pervaporation involve no phase change. All tend to be inherently low-energy consumers in theory if not in practice. They operate by a different mechanism than do other separation methods, so they have a unique profile of strengths and weaknesses. In some cases they provide unusual sharpness of separation, but in most cases they perform a separation at lower cost, provide more valuable products, and do so with fewer undesirable side effects than older separations methods. The membrane interposes a new phase between feed and product. It controls the transfer of mass between feed and product. It is a kinetic, not an equilibrium process. In a separation, a membrane will be selective because it passes some components much more rapidly than others. Many membranes are very selective. Membrane separations are often simpler than the alternatives. 

This section will provide an overview of the principles of hydrogen separation and purification using membranes. More detailed discussions of the theory governing membrane separation processes can be found elsewhere.1... 

Maxwell-Stefan (dusty gas) approach by taking the membrane to be the additional component in the mixture. When the model is extended to account for thermodynamic nonidealities (what may be considered to be a dusty fluid model) almost all membrane separation processes can be modeled systematically. Put another way, the Maxwell-Stefan approach is the most promising candidate for developing a generalized theory of separation processes (Lee et al., 1977 Krishna, 1987). 

Neale J. Text. Inst., 1929, 373 1930, 225 1931, 349) attributes the attraction of cellulose towards sodium ions to the formation of non-diffus-ible anions of the type (CeHioOj)- or (CgH 9O5.H2O)-. This, it has been suggested, causes a distribution of sodium and hydroxyl ions on each side of the fibre boundary in accordance with the Donnan membrane equilibrium theory. The Donnan theory provides thermodynamic proof that if a solution of an electrolyte containing two diffusible ions is separated by a membrane from another solution containing a salt with a non-diffusible ion, then the equilibrium distribution of the former will not be equal on the two sides of the membrane . This state is illustrated diagrammatically in Fig. 3.12 where, on the side marked A of the membrane (which in this case is the... 

An attempt to unify the mass transport phenomena of liquid membrane separation underlying the basic LM configurations was presented in this chapter. The basic theory was developed in a simple physical-chemical-mathematical form and applied to the principal techniques in such a way to obtain comparable methods. Of course it is prehminary work once we start forging links between different methods there wiU be spiUover to further possibilities of integration. 

The theory for BOHLM is developed for flat thin uncharged symmetric membranes without variation in porosity and pore sizes across the membrane thickness. To develop a three-phase system model [1,2], the transport model simpUfication analysis, developed by Hu [68] for the two-phase system, is used. Titanium(IV) was chosen, as an example for transport model verification, because of the extensive experimental data available on Uquid-Uquid extraction and membrane separation [1, 2, 64, 65] and for its extraction double-maximum acidity dependence phenomenon [63]. The last was observed for most extractant famUies basic (anion exchangers), neutral (complexants), and acidic (cation exchangers). So, it is possible to... 

In the following discussion, we consider an ideal situation where the flux of a molecular species is localized to a single pore the membrane is otherwise impermeable to the molecule. Although this model is only an approximation of real samples, the resulting theory remains quite useful in the quantitative analysis of porous membranes, provided that the pores are not too closely spaced. The membrane separates donor and receptor solutions the donor solution contains an electroactive molecule that is transported across the membrane and detected by the SECM tip on the receptor side of the membrane (Fig. 1). 

In this paper, it has been demonstrated experimentally that complex water proton relaxation can occur in a wide variety of biopolyraer systems. The detection of complex relaxation cannot therefore be used as a criterion for the existence of separate domains into which water or small solutes are compartmentalised by permeable or semi-permeable membranes. A theory has been developed which, to a first approximation, can explain the experimental observations and requires only that a heterogeneous distribution of the substrate be maintained over the timescale of... 

If a membrane separates two identical liquids or solutions and a potential difference is applied across the membrane, there results a flow of liquid through the pores of the membrane. This phenomenon is known as electro-endosmosis, or as electro-osmosis. A simplified version of the theory is as follows. 

A logical question that arises at this point is whether CPM theory can be applied to other separation systems. In an effort to illustrate this, we will consider membrane permeation because of its striking difference from distillation. By the nature of its operation, membrane separation is fundamentally different while in distillation, the separation is achieved by differences in boiling points, the driving force in membrane permeation is a difference between chemical potentials in the two phases. One of the most important consequences of this is that the constant molar overflow assumption, in general, cannot be employed in membrane permeating systems. 

Transport in dense discriminating layers is most commonly described using the well developed solution-diffusion theory [36]. The theory is based on the assumptions that 1) the driving force for transport is a gradient in chemical potential, 2) at a fluid-membrane interface the chemical potential in the two phases are equal (i.e., equilibrium exists), and 3) the pressure within the membrane is constant and equal to the highest value at either interface. Baker [37] summarizes the application of the theory to a variety of membrane separation processes ranging from dialysis to gas separation. 

In this chapter, we have presented the more used theories to explain the mass transfer mechanism in a polymer solid (porous or tight, charged or not) membranes. It is clear that the membrane structure and the nature of the solution forms a system with complex interactions between the solutes, solvent, driving force, and the polymeric membrane. Often many basic mass transfer mechanisms intervene in certain membrane separations, and the mass transfer modeling is not very easy, because we must take into account any interactions between the components of the system. 

Pangarkar VG, Ray SK (2014) Pervaporation theory, practice and apphcations in the chemical and allied industries. In Pabby AK, Rizvi SSH, Sastre AM, editors. Handbook of membrane separations chemical, pharmaceutical, food and biotechnological applications. 2nd ed. Marcel-Dekker (Taylor Francis) to be published March, 2015. 

More recent publications include Membrane Separations Technology Principles and Applications, published in 1995 and edited by Richard D. Noble and J. Douglas Way, who had coedited an earlier volume. Liquid Membranes Theory and Applications. The state of the technology is kept track of by the Business Communications Company, for example, in Membrane and Separation Technology Industry Review, " published in 1998. For continuing developments, consult Books in Print and WorldCat, a service in conjunction with OCLC (Online Computer Library Center). Additionally, there is, of course, the Internet. 

However, there are a number of important objectives where theory and practice are strongly interactive and which to some extent represent interesting models for other design objectives. Included in these are a) panels and load-bearing components for the automobile industry, (6) drug delivery systems, (c) membrane separation or barrier systems. 

Interestingly, Goldman [24] considers ENC in his theory of conditions around a membrane separating two electrolytes with different concentrations or composition, but also the further simplification of a constant potential field. This gave better results compared with his experiments. Mafe et al. have examined the conditions under which the Goldman condition is justified, and where it is not [29]. 

The electrical potential difference at both sides of a membrane separating two solutions of the same electrolyte but different concentrations (Ci, C2) is called membrane potential (AOm). The Teorell-Meyer-Sievers (or TMS) theory [37, 38] assumes the membrane potential can be considered as the sum of three terms associated with two different contributions ... 

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