Feed side, membrane extraction

A process referred to as vapor-arbitrated pervaporation addresses these issues by manipulating the transmembrane activity gradients of water and ethanol in a pervaporation system. Using a permeate side sweep stream that contains water vapor at a partial pressure corresponding to the activity of water on the feed side, permeation of water is halted while ethanol continues to diffuse through the membrane into the sweep stream and is removed. In this way, the native permselectivity of the membrane system can be altered in a controlled fashion to extract one or more volatile components from a solution. 

The transport of the substances from the feed solution to the strip side can be divided into the foUowing steps diffusion of substance S across the boundary aqueous layer in the feed (donor) phase, extraction (sorption) of substance on the donor/membrane phase interface, diffusion across the boundary layer on the feed (donor) side, convection transport in the liquid membrane zone, diffusion across the boundary layer on the strip (acceptor) phase of LM, re-extraction (desorption) on the membrane/strip phase... 

The magnitudes of individual titanium mass-transfer coefficients are similar at carrier or strip flow rates variations. Resistance to diffusion in the carrier solution layers and membrane pores is not a rate-controlling step, since the overall mass-transfer coefficients on the strip side of the BOHLM are two orders less than that on the feed side. Thus, ratecontrolling steps could act as resistance of the strip solution layer, or the interfacial backward extraction reaction rate. 

As a rule, the rates of ion-exchange reactions, or rates of complex formation and destruction on the interfaces or within the IBM, are fast compared to diffusion rates. Thus, corresponding concentration gradients of the counterions are related through the extraction equilibrium constants [46, 53, 56]. The averaged sums of the IBM and LMF potentials can be experimentally realized through distribution coefficients at membrane-based solvent forward and backward extraction Ep = Mfl/Mei on the feed side and Ep = Me/M i on the strip side (see Fig. 6.2B). 

Although the membranes used to extract hydrogen isotopes from plasmas meet the criterion for the definition of superpermeability, in that every atom of hydrogen incident upon the feed side surface of the membrane is transported through the dense membrane, it must be noted that superpermeability is achieved, in very large part, because of the relatively low flux of hydrogen incident upon the membranes. The plasma density in the quoted experiments was 5 x 10 cm-, and the total gas pressure was relatively low, 0.002-0.004 torr (0.3-0.6 Pa) [2]. 

It is important to stress again that MD is not purely a mass transfer operation in the way that, for example, direct osmosis is, because heat transfer is also a very important element of the process due to the water evaporation at the feed side and water condensation at the extract side. In this case, a simultaneous heat and mass transfer takes place through the membrane. Simultaneously here means that the heat transfer and mass transfer are intimately connected the heat transfer rate depends on mass flux and vice versa. 

Membrane extraction (ME) techniques are a set of solvent-free extractions, which have gained popularity for VOC analysis in water (for applications see Table 23.8). The sample is in contact with one side of the membrane surface called feed or donor side. Analytes permeate selectively (according to their membrane affinity) through the membrane to the other side, called permeate or acceptor side, where they are retained by an acceptor phase. This process is called pertraction (permeation-extraction). 

The transport process of As through the SLM using Cyanex 921 as extractant takes place in five steps in series (1) As(V) diffusion from feed bulk to membrane surface in the non-stirred boundary layer at the feed-membrane interface (2) complexation As(V) at membrane interface feed side, thus forming the As(V)-Cyanex 921 complex (3) diffusion of the As(V)-Cyanex 921 complex through the membrane thickness (4) stripping of As(V) from the As(V)-Cyanex 921 complex at membrane interface strip side (5) As(V) diffusion from membrane surface (strip side) to strip bulk, through the non-stirred boundary layer at the strip-membrane interface. 

M REC, as the TREC, does not depend on the reaction path. In addition, there is no dependence on the membrane-permeation properties (related to the time required for species permeation).1 In any case, the final value reached depends on the extractive capacity of the system, for example, the pressure and composition on the permeate side. The composition on the permeate side, similarly to the feed molar ratio, can be expressed by considering the ratio (named sweep factor) between the initial molar number of nonpermeating species (present on the permeate side) and the initial molar number of the reference reactant, for example, methane for methane steam reforming, or carbon monoxide for water gas shift). The sweep factor was defined for a closed M Ras ... 

Subsequently the pure extraction gas flows through a heat exchanger where it is indirectly cooled with water to rise its density. When possible, it is intended to get a liquid phase which is collected in a connected buffer vessel (KP). The installed membrane compressor is able to pump gas as well as highly compressible liquids, which are withdrawn from the buffer vessel To achieve a maximum flexibility with respect to extraction medium and operation parameters a cooler (WT6, for gas phase recompression) or a heater (WT4, for liquid phase recompression) can alternatively be used to bring the recompressed solvent back to extraction temperature Before it is entering the main column a side stream is removed for the feed presaturation. 

The extrapolation is to what is called pervaporation, where the feed mixture is a liquid, but the permeate vaporizes during permeation, induced by the relatively low pressure maintained on the permeate side of the membrane. Accordingly, the reject or retentate remains a liquid, but the permeate is a vapor. Thus, there are features of gas permeation as well as hquid permeation. The process is eminently apphcable to the separation of organics and to the separation of organics and water (e.g., ethanol and water). In the latter case, either water vapor may be the permeate, as in dehydration, or the organic vapor may be the permeate. The obvious, potential application is to the separation of azeotropic mixtures and close-boiling mixtures—as an alternative or adjunct to distillation or liquid-liquid extraction methods. 

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