Permeate side, membrane extraction

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 ... 

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 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. 

Because caffeine extraction is an important industrial application of SC CO2 technology, different studies have been recently conducted on solvent recovery by membrane separation. As the molecular weight (MW) (194 g moP ) of caffeine is higher than CO2 (44 g moP ), classical suitable membrane for this application needs to reject caffeine while letting CO2 cross through the membrane. Thus, pure CO2 can be obtained on permeate side and recycled. 

Both theoretical and experimental studies have been performed on palladium-based membrane reactors for the water-gas shift reaction. Ma and Lund simulated the performance achievable in a high temperature water-gas shift membrane reactor using both ideal membranes and catalysts [18]. By comparing the results obtained with those related to the existing palladium membrane reactors, they concluded that better membrane materials are not needed, and that higher performances mainly depend on the development of a water-gas shift catalyst not inhibited by CO2. Marigliano et al. pointed out how the equilibrium shift conversion in membrane reactors is an increasing function of the sweep factor (defined as the ratio between the flow rate of the sweep at the permeate side and the flow rate of CO at the reaction side) [19]. The ratio is an index of the extractive capacity of the system. 

Raghuraman and Wiencek [11] developed a hybrid technique where an emulsion is fed into a hollow fiber contactor on the tube side. Since the solid membrane support is hydrophobic, the continuous phase of the water-in-oil emulsion easily wets the pores of the tube wall and permeates to the shell side. On the shell side of the hollow fiber, the aqueous feed phase is exposed and held at an elevated pressure that prevents the permeating liquid membrane phase from exiting the pores. Thus, extraction occurs on the shell side, and stripping on the tube side of the hollow fiber membrane module. This methodology is closely related to SLMs, but the key difference is the presence of the emulsion on the tube side, which allows for long-term stability because the membrane liquid is continuously replenished to make up for any loss by solubility. 

Pervaporation is a contraction of the terms permeation and evaporation because the feed is a liquid, and vapor exits the membrane on the permeate side. Pervaporation is a membrane process for liquid separation, and today, it is considered as a basic unit operation for the separation of organic-organic liquid mixtures because of its efficiency in separating azeotropic and close-boiling mixtures, isomers, and heat-sensitive compounds. It allows separations of some mixtures that are difficult to separate by distillation, extraction, and sorption. Pervaporation is one such type of membrane separation process with a wide range of uses such as solvent dehydration and separation of organic mixtures. When a membrane is in contact with a liquid mixture, one of the components can be preferentially removed from the mixture due to its higher affinity and quicker diffusivity in the membrane. 

The preheated feed is mixed with additional CH4 and steam and fed to the reforming/shift section, where CH4 is completely converted into CO, CO2 and H2 because of the selective H2 extraction through the Pd membranes which shifts methane SR. H2 extraction can be achieved by using dead-end Pd membranes and applying a vacuum on the permeate side. 

Finally, it is also interesting to report that in the literature it is possible to find studies dealing with esterification membrane reactor units coupled with other than distillation-based separation technologies. An example is the work of Park and Tsotsis (2004), who linked to the permeate side of the membrane reactor an adsorption step to increase byproduct extraction. Adsorption is a very powerful technology and leads to higher conversion in comparison with a conventional reactor (up to 10%). However, the cost of the adsorbent and of related equipment makes this design economically disadvantageous. 

The liquid-liquid membrane contactor is characterised by two liquid streams separated by a porous or nonporous membrane. In case of a porous membrane the feed phase may either wet or not wet the membrane. Firstly we will consider the case where the feed is an organic solvent from which a solute has to be removed while the permeate phase is an aqueous phase. If now a hydrofobic porous membrane is used the membrane will be wet and the pores will be filled. At the permeate side an aqueous stream is pumped now which does not wet the membrane and is not miscible with the organic solvent. An interface will formed at the permeate side (figure VI - 58a) and the actual liquid-liquid extraction will occur at this interface. If the feed is an aqueous stream and the membrane is hydrofobic then the feed will not wet the membrane. The (hydrofobic) organic solvent is now used at the permeate side and this will wet the membrane which implies that now the interface is forrried at the feed/membrane side (figure VI - 58b). Figure VT - 58 also... 

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). 

Hydrophobic membranes can be used to extract organic solvents or volatile organic compounds (VOCs) from water. The membranes are made of hydrophobic cross-Unked polymers. The membrane chemistry is designed to attract VOC molecules to the surface of the membrane. Pervaporation takes place in the membrane. VOCs diffuse through the membrane and evaporate on the permeate side with the help of a vacuum. These VOCs later condense in a condenser. Examples of these membrane applications include extraction of numerous types of VOCs from water, extraction of aromatics from water, ketones from water, esters from water, and many more. 

If the photoequilibrium concentrations of the cis and trans isomers of the photoswitchable ionophore in the membrane bulk and their complexation stability constants for primary cations are known, the photoinduced change in the concentration of the complex cation in the membrane bulk can be estimated. If the same amount of change is assumed to occur for the concentration of the complex cation at the very surface of the membrane, the photoinduced change in the phase boundary potential may be correlated quantitatively to the amount of the primary cation permeated to or released from the membrane side of the interface under otherwise identical conditions. In such a manner, this type of photoswitchable ionophore may serve as a molecular probe to quantitatively correlate between the photoinduced changes in the phase boundary potential and the number of the primary cations permselectively extracted into the membrane side of the interface. Highly lipophilic derivatives of azobis(benzo-15-crown-5), 1 and 2, as well as reference compound 3 were used for this purpose (see Fig. 9 for the structures) [43]. Compared to azobenzene-modified crown ethers reported earlier [39 2], more distinct structural difference between the cis... 

Pervaporation have been considered an interesting alternative process for the current industrial options for aroma recovery, distillation, partial condensation, solvent extraction, adsorption, or a combination thereof. It is considered a basic unit operation with significant potential for the solution of various environmental and energetic processes (moderate temperatures). This separation process is based on a selective transport through a dense membrane (polymeric or ceramic) associated with a recovery of the permeate from the vapour phase. A feed liquid mixture contacts one side of a membrane the permeate is removed as a vapour from the other side. Transport through... 

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