Membrane separation definition

AQUATECH Systems is a state-of-the-art bipolar membrane separation technology which exemplifies "pollution prevention" technology rather than "end of the pipe" regulation compliance. Consistent with SARA s definition of treatment, AQUATECH Systems is a technology "that in whole or in part will result in a permanent and significant decrease in the toxicity, morbidity or volume" of a hazardous waste material. 

In this chapter, we will introduce fundamental concepts of the membrane and membrane-separation processes, such as membrane definition, membrane classification, membrane formation, module configuration, transport mechanism, system design, and cost evaluation. Four widely used membrane separation processes in water and wastewater treatment, namely, microfiltration (MF), ultrafiltration (UF), nanofiltrafion (NF), and reverse osmosis (RO), will be discussed in detail. The issue of membrane foufing together with its solutions will be addressed. Several examples will be given to illustrate the processes. 

Although there is no commonly accepted definition of a membrane reactor (MR), the term is usually applied to operations where the unique abilities of membranes to organize, compartmentalize, and/or separate are exploited to perform a (bio)chemical conversion under conditions that are not feasible in the absence of a membrane. In every MR, the membrane separation and the (bio)catalytic conversion are thus combined in such a way that the synergies in the integrated setup entail enhanced processing and improved economics in terms of separation, selectivity, or yield, compared to a traditional configuration with reactor and separation separated in time and space. When the membrane itself carries the catalytic functions, it is mostly referred to as a reactive membrane. ... 

Inclusion of this technique to the BOHLM has to be explained. Solvent extraction or partition of the solute between two immiscible phases is an equilibrium-based separation process. So, the membrane-based or nondispersive solvent extraction process has to be equilibrium based also. Liquid membrane separation is a rate process and the separation occurs due to a chemical potential gradient, not by equilibrium between phases [114]. According to these definitions, many authors who refer to their works as membrane-based or nondispersive solvent extraction processes are not correct. 

Vapor permeation and pervaporation are membrane separation processes that employ dense, non-porous membranes for the selective separation of dilute solutes from a vapor or liquid bulk, respectively, into a solute-enriched vapor phase. The separation concept of vapor permeation and pervaporation is based on the molecular interaction between the feed components and the dense membrane, unlike some pressure-driven membrane processes such as microfiltration, whose general separation mechanism is primarily based on size-exclusion. Hence, the membrane serves as a selective transport barrier during the permeation of solutes from the feed (upstream) phase to the downstream phase and, in this way, possesses an additional selectivity (permselectivity) compared to evaporative techniques, such as distillation (see Chapter 3.1). This is an advantage when, for example, a feed stream consists of an azeotrope that, by definition, caimot be further separated by distillation. Introducing a permselective membrane barrier through which separation is controlled by solute-membrane interactions rather than those dominating the vapor-liquid equilibrium, such an evaporative separation problem can be overcome without the need for external aids such as entrainers. The most common example for such an application is the dehydration of ethanol. 

The scaled surface area and its variation with d> are of crucial importance in the definition and evaluation of the osmotic pressure , H, of a foam or emulsion. We introduced the concept in Ref 37, where it was referred to as the compressive pressure , P. It has turned out to be an extremely finitful concept (22,27,38). The term osmotic was chosen, with some hesitation, because of the operational similarity with the more familiar usage in solutions. In foams and emulsions, the role of the solute molecules is played by the drops or bubbles that of the solvent by the continuous phase, although it must be remembered that the nature of the interaetions is entirely different. Thus, the osmotic pressure is denned as the pressure that needs to be applied to a semipermeable, freely movable membrane, separating a fluid/fluid dispersion from its continuous phase, to prevent the latter from entering the former and to reduce thereby the augmented surface free energy (Fig. 4). The membrane is permeable to all the components of the continuous phase but not to the drops or bubbles. As we wish to postpone diseussion of compressibility effects in foams until latter, we assume that the total volume (and therefore the volume of the dispersed phase) is held constant. 

The dual-phase (DP) membrane used in analytical separation usually consists of a polymer, or in some cases a ceramic solid-phase support impregnated with a fluid (i.e., gaseous or liquid phase). If the fluid is air the DP membrane is known as a gas-diffusion membrane. DP membranes incorporating a liquid phase can be considered in a broader sense as liquid membranes. The liquid phase in a liquid DP membrane can be identical to the feed and/or receiver solution (e.g., dialysis membranes, membrane-assisted LLE (MALLE)) or it can form a third immiscible liquid phase in the membrane separation system (e.g., supported and polymer liquid membranes). Membranes incorporating a liquid phase immiscible with the feed and receiver solutions are usually referred to as liquid membranes. This narrower definition of liquid membranes, currently accepted in the literature, will be used in subsequent discussions. The... 

By definition the membrane liquid phase of a liquid membrane must be immiscible with the solutions in contact with it. This allows the chemical composition of all three liquid phases (including the membrane liquid phase) to be altered independently of one another. For this reason liquid membranes offer a higher degree of control over the membrane separation process compared to other types of separation membranes. 

In electron micrographs of thin-sections of organisms the cell wall appears as a triple-layered structure, 60-100 angstroms thick (outer membrane) separated by 1 or more layers of variable electron density from the plasma membrane, but showing in addition 1 or more layers of definite order of structure extracellular to the outer membrane. Electron micrographs of negatively stained, freeze-etched or shadowed organisms show a smooth surface structure. 

Quite obviously, then, for true equilibrium, it must be required that P. = that is, the pressure must be uniform throughout the system, on both sides of the membrane. Furthermore, the composition is uniform throughout the system. Since, by definition, in membrane separations, P P, a condition of true equilibrium cannot be reached. 

Often, the active substance is released from its administration form in a dissolved state. If this is not the case, the active substance must first dissolve in aqueous environment after it has been released. Only in the dissolved state, can an active substance pass biological membranes separating the site of administration from the systemic circulation (the blood circulation) via which transport to the site of action occurs. The fraction of the administered active substance that dissolves in the aqueous fluid adjacent to the biological membranes and thereby becomes available for passing them is called the pharmaceutical availability. The fraction of the total amount of the administered active substance that ultimately reaches the systemic circulation in an unchanged form is called bioavailability. By definition, an intravenously injected medicine will have a bioavailability of 1.0 (or 100 %). When a medicine is administered via a different route, its bioavailability will be reduced, due to, for example, incomplete dissolution or losses during the transport of dissolved active substance to the systemic circulation. 

The great potential and numerous advantages of carbon membranes will definitely lead to their wide application in the gas separation industry over the eoming years. 

The definitive and up-to-date monographs on membrane separation are by the following authors ... 

A fundamental difference exists between the assumptions of the homogeneous and porous membrane models. For the homogeneous models, it is assumed that the membrane is nonporous, that is, transport takes place between the interstitial spaces of the polymer chains or polymer nodules, usually by diffusion. For the porous models, it is assumed that transport takes place through pores that mn the length of the membrane barrier layer. As a result, transport can occur by both diffusion and convection through the pores. Whereas both conceptual models have had some success in predicting RO separations, the question of whether an RO membrane is truly homogeneous, ie, has no pores, or is porous, is still a point of debate. No available technique can definitively answer this question. Two models, one nonporous and diffusion-based, the other pore-based, are discussed herein. 

Misunderstandings arise when membrane users assume that MWCO means what it seems to say. The definition implies that a 50 kD membrane will separate a 25 kD material from a 75 kD material. The rule of thumb is that the molecular mass must differ by a factor of ten for a good separation. Special techniques are used to permit the separation of proteins with much smaller mass ratio. 

Definitions Following the practice presented under Gas-Separation Membranes, distillation notation is used. Literature articles often use mass fraction instead of mole fraction, but the substitution of one to the other is easily made. 

If a solution and the pure solvent are separated by a semipermeable membrane, the solvent tends to pass through the membrane into the solution, and the osmotic pressure is the pressure that must be applied to the latter to keep the solvent from entering into it. The term osmotic pressure of the solution is, therefore, strictly speaking, incorrect, as osmotic pressure is, according to the definition, produced only when the solution is separated from the solvent by a semipermeable membrane. If this is remembered, it disposes of the objection sometimes raised that osmotic pressure works the wrong way, in that it causes motion from places of lower to places of higher osmotic pressure. It is osmosis which causes osmotic pressure, and not osmotic pressure which produces osmosis. 

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