Membrane separation processes component transport

Under the influence of pressure, the membrane permits specific components to pass through (or permeate). The membrane also inhibits transport of some components. This selective transport forms the basis of the membrane separation process. Rejection is a bulk separation capability of the membrane. The observed solute rejection coefficient II for a given species i is given by... 

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. 

Process Description Pervaporation is a separation process in which a liquid mixture contacts a nonporous permselective membrane. One component is transported through the membrane preferentially. It evaporates on the downstream side of the membrane leaving as a vapor. The name is a contraction of permeation and evaporation. Permeation is induced by lowering partial pressure of the permeating component, usually by vacuum or occasionally with a sweep gas. The permeate is then condensed or recovered. Thus, three steps are necessary Sorption of the permeating components into the membrane, diffusive transport across the nonporous membrane, then desorption into the permeate space, with a heat effect. Pervaporation membranes are chosen for high selectivity, and the permeate is often highly purified. 

Membranes are semipermeable barriers that permit the separation of two compartments of different composition or even condition, with the transport of components from one compartment to another being controlled by the membrane barrier. Ideally, this barrier is designed to let pass selectively only certain target compounds, while retaining all others—hence the denotation semipermeable . Membrane separations are particularly suitable for food applications because (1) they do not require any extraction aids such as solvents, which avoids secondary contamination and, hence, the necessity for subsequent purification (2) transfer of components from one matrix to another is possible without direct contact and the risk of cross-contamination (3) membrane processes can, in general, be operated under smooth conditions and therefore maintaining in principle the properties and quality of delicate foodstuff. 

Pervaporation is a separation process in which a multicomponent liquid is passed across a membrane that preferentially permeates one or more of the components. A partial vacuum is maintained on the permeate side of the membrane, so that the permeating components are removed as a vapor mixture. Transport through the membrane is induced by maintaining the vapor pressure of the gas on the permeate side of the membrane at a lower vapor pressure than the feed liquid. The gradients in chemical potential, pressure, and activity across the membrane are illustrated in Figure 2.12. 

While both of these devices use hollow fiber membranes similar to the primary components of kidney dialyzer units, the difference between the two techniques lies in how the analyte undergoes mass transport into the device. Microdialysis sampling is a diffusion-based separation process that requires the analyte to freely diffuse from the tissue space into the membrane inner lumen in order to be collected by the perfusion fluid that passes through the inner lumen of the fiber. Ultrafiltration pulls sample fluid into the fiber lumen by applying a vacuum to the membrane (Figure 6.1). 

The term electromembrane process is used to describe an entire family of processes that can be quite different in their basic concept and their application. However, they are all based on the same principle, which is the coupling of mass transport with an electrical current through an ion permselective membrane. Electromembrane processes can conveniently be divided into three types (1) Electromembrane separation processes that are used to remove ionic components such as salts or acids and bases from electrolyte solutions due to an externally applied electrical potential gradient. (2) Electromembrane synthesis processes that are used to produce certain compounds such as NaOH, and Cl2 from NaCL due to an externally applied electrical potential and an electrochemical electrode reaction. (3) Eletectromembrane energy conversion processes that are to convert chemical into electrical energy, as in the H2/02 fuel cell. 

Viewed in this way, chemical potential profiles (along with flow) govern separation different phases, membranes, and applied fields are simply convenient media for imposing the desired profiles. The media are selected on pragmatic grounds chemical compatibility with the components and the system, selectivity between components, noninterference with detectability, ease of solvent removal (another separation process), facilitation of rapid transport, and so on. 

Desorption on the downstream side of the membrane is generally considered to be rapid and nonselective. The gas phase diffusivities in the final step of transport are very high and hence this step offers the least resistance in the overall transport process. As a separation process, pervaporation relies on the difference in membrane permeabilities, which are dependent on the thermodynamics activities of the components to be separated. 

Way, Noble and Bateman (49) review the historical development of immobilized liquid membranes and propose a number of structural and chemical guidelines for the selection of support materials. Structural factors to be considered include membrane geometry (to maximize surface area per unit volume), membrane thickness (<100 pm), porosity (>50 volume Z), mean pore size (<0.1)jm), pore size distribution (narrow) and tortuosity. The amount of liquid membrane phase available for transport In a membrane module Is proportional to membrane porosity, thickness and geometry. The length of the diffusion path, and therefore membrane productivity, is directly related to membrane thickness and tortuosity. The maximum operating pressure Is directly related to the minimum pore size and the ability of the liquid phase to wet the polymeric support material. Chemically the support must be Inert to all of the liquids which It encounters. Of course, final support selection also depends on the physical state of the mixture to be separated (liquid or gas), the chemical nature of the components to be separated (inert, ionic, polar, dispersive, etc.) as well as the operating conditions of the separation process (temperature and pressure). The discussions in this chapter by Way, Noble and Bateman should be applicable the development of immobilized or supported gas membranes (50). 

Dialysis is a diffusion-based separation process that uses a semipermeable membrane to separate species by vittue of their different mobilities in the membrane. A feed solution, containing the solutes to he separated, flows ou one side of the membrane while a solvent stream, die dialysate, flows on die other side (Fig. 21. -1). Solute transport across the membrane occurs by diffusion driven by the difference in solme chemical potential between the two membrane-solution interfaces. In practical dialysis devices, no obligatory transmembrane hydraulic pressure may add an additional component of convective transport. Convective transport also may occur if one stream, usually the feed, is highly concentrated, thus giving rise to a transmembrane osmotic gradient down which solvent will flow. In such circumstances, the description of solute transport becomes more complex since it must incorporate some function of die trans-membrane fluid velocity. 

Rate processes, on the other hand, are limited by the rate of mass transfer of individual components from one phase into another under the influence of physical shmuli. Concentrahon gradients are the most common stimuli, but temperature, pressure, or external force fields can also cause mass transfer. One mass-transfer-based process is gas absorption, a process by which a vapor is removed from its mixture with an inert gas by means of a liquid in which it is soluble. Desorption, or stripping, on the other hand, is the removal of a volatile gas from a Hquid by means of a gas in which it is soluble. Adsorption consists of the removal of a species from a fluid stream by means of a solid adsorbent with which it has a higher affinity. Ion exchange is similar to adsorption, except that the species removed from solution is replaced with a species from the solid resin matrix so that electroneutrality is maintained. Lastly, membrane separations are based upon differences in permeability (transport through the membrane) due to size and chemical selectivity for the membrane material between components of a feed stream. 

In order to reduce such interferences, successful efforts have been made to isolate the cell membranes, or even their transport-active constituents. One way to achieve this is by preparation of isolated membranes which have a natural tendency to form closed and homogenous vesicles [31,32]. Another approach is by reconstituted systems , i.e. to isolate membrane components involved in specific transport processes, and to incorporate them in artificial lipid membranes, usually liposomes [33,34]. Vesicles have been successfully prepared from various cells and tissues and tested for transport activities. Whenever membranous material has been isolated from other cellular components it tends to form vesicles spontaneously, sometimes with an uniform orientation, right-side-out or inside-out vesicles, respectively. For mixtures of vesicles of the two orientations, methods were developed to separate the two polarities. Furthermore, one can separate vesicles from different cell types or even from different regions of the cell, e.g. brush-border membranes form basal lateral ones [35,36]. 

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