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Membranes Relative volatility

If the system forms azeotropes, then the azeotropic mixtures can be separated by exploiting the change in azeotropic composition with pressure, or the introduction of an entrainer or membrane to change the relative volatility in a favorable way. If an entrainer is used, then efficient recycle of the entrainer material is necessary for an acceptable design. In some cases, the formation of two liquid phases can be exploited in heterogeneous azeotropic distillation. [Pg.650]

Concurrently with the work on carbon dioxide and hydrogen sulfide at General Electric, Steigelmann and Hughes [27] and others at Standard Oil were developing facilitated transport membranes for olefin separations. The principal target was the separation of ethylene/ethane and propylene/propane mixtures. Both separations are performed on a massive scale by distillation, but the relative volatilities of the olefins and paraffins are so small that large columns with up to 200 trays are required. In the facilitated transport process, concentrated aqueous silver salt solutions, held in microporous cellulose acetate flat sheets or hollow fibers, were used as the carrier. [Pg.455]

Several authors have already developed methodologies for the simulation of hybrid distillation-pervaporation processes. Short-cut methods were developed by Moganti et al. [95] and Stephan et al. [96]. Due to simplifications such as the use of constant relative volatility, one-phase sidestreams, perfect mixing on feed and permeate sides of the membrane, and simple membrane transport models, the results obtained should only be considered qualitative in nature. Verhoef et al. [97] used a quantitative approach for simulation, based on simplified calculations in Aspen Plus/Excel VBA. Hommerich and Rautenbach [98] describe the design and optimization of combined pervaporation-distillation processes, incorporating a user-written routine for pervaporation into the Aspen Plus simulation software. This is an improvement over most approaches with respect to accuracy, although the membrane model itself is still quite... [Pg.57]

PV-assisted catalysis in comparison with reactive distillation has many advantages the separation efficiency is not limited by relative volatility as in distillation in pervaporation only a fraction of the feed is forced to permeate through the membrane and undergoes the liquid- to vapor-phase change and, as a consequence, energy consumption is generally lower compared to distillation. [Pg.279]

Many industrially important liquid systems are difficult or impossible to separate by simple continuous distillation because the phase behavior contains an azeotrope, a tangent pinch, or an overall low relative volatility. One solution is to combine distillation with one or more complementary separation technologies to form a hybrid. An example of such a combination is the dehydration of ethanol using a distillation-membrane hybrid, as shown in Figure 6.30. [Pg.415]

The ideal dense membrane has a high permeance for the penetrant molecules and a high selectivity between the components to be separated. The selectivity is expressed in terms of the separation factor, aAB, defined similarly to the relative volatility in distillation ... [Pg.510]

Different permeabilities enable passage of volatile organic compounds through a silicone membrane (relative to water or inorganic gases)... [Pg.242]

Although this is not an equilibrium process it looks very similar to a flash distillation with a large relative volatility f Hoffman. 20031. Membrane separators are useful because they are a practical way of generating favorable RT curves. [Pg.741]

In pervaporation, as shown in Figure 34.5, when feed mixtures are added on one side of the membrane and the other side is evacuated, a certain component in the feed mixture can be preferentially permeated through the membranes (Binning et al. 1961 Choo 1962). In the pervaporation, the difference in the solubility of permeants into the membrane, the diffusivity of permeants in the membrane, and the relative volatility of permeants from the membrane can influence the characteristics of permeation and separation. This pervaporation technique is advantageous for the separations of azeotropic mixtures, close-boiling point mixtures, and structural isomers. [Pg.486]

In order to automate the analysis, these methods frequently combine immobilized enzymes with flow or sequential injection techniques. These methods may include a separation step such as solid-phase extraction, gas diffusion, or pervaporation. The latter is a nonchromatographic separation technique, which selectively separates a liquid mixture by partial vaporization through a nonporous polymeric membrane. Separation is not based on relative volatilities as in distillation, but rather on the relative rates of permeation through the membrane. [Pg.1541]

Pervaporation is a nonchromatographic membrane-based separation technique extensively used in industry. The separation is not based on relative volatilities, as in the case of distillation or evaporation, but rather on the relative rates of permeation through the polymeric membrane. It is based on the diffusion of volatile substances from a donor stream, through a porous hydrophobic membrane, to an acceptor stream caused by a temperature or pressure gradient. [Pg.2994]

As previously indicated, the permeability values within mixtures are generally less than those for the pure components. Or, the degree of separation in mixtures is less sharp than expected from the permeability of the pure components. This can perhaps be traced to projecting the idea of relative volatility to membrane relative permeability or selectivity. [Pg.84]

If membrane permeability or selectivity is introduced in lieu of relative volatility, then the effect would seemingly enhance the separation, as follows ... [Pg.85]

The last example given involves a case where the fference in relative volatilities of the components to be separated is reladvely small. If membranes are available with higher selectivities than the vapour-liquid equilibrium, pervaporation can be combined with distillation as shown in figure VI - 28. This approach is very attractive in case of debottlenecking of an existing distillation plant Most pervaporation applicadons can be found in the chemical process industry but they are also other areas such as the food and... [Pg.337]

Many experiments have demonstrated the ability of Ag+-exchanged Nafion membranes to separate unsaturated hydrocarbons (alkenes and arenes) from saturated organics. This ability is potentially useful in the context of hybrid distillation-membrane processes and can be further demonstrated by performing separations of bi-component mixtures containing similar boiling points, i.e with relative volatilities close to 1. Table I is a compilation of several representative close boiling mixtures which would be difficult and expensive to separate by distillation alone. Each mixture consists of one alkane and one alkene. Therefore, the separation of the mixtures should be achievable based on their difference in reactivity with Ag+. [Pg.296]

Selectivity a, on the other hand, is a property of the membrane and deals with the relation between compositions, usually expressed as mole fractions i/i, on either side of the barrier. Its definition, at least in its form, is identical to that of relative volatility, which was seen in coimection with vapor-liquid equilibria (see Equation 6.20a). For a two-component system, it is given by the relation... [Pg.342]

The relative permeability of the compounds present in the feed mixture is a key material characteristic and it is called the ideal separation factor, or membrane selectivity (a ). It is recalled that, similarly to a convention used in distillation for the relative volatility, it is usually expressed so as to be larger than 1 ... [Pg.62]

This shows that pervaporation depends on two sources of selectivity. The first source, given in the square brackets, is the relative permeability across the membrane. As expected, this relative permeability is the product of diffusion and partition coefficients. Second, the selectivity of the pervaporation is influenced by the relative volatility given in the braces. This volatility, a thermodynamic factor, is independent of any dynamic concerns. This combination of dynamic and equilibrium factors explains why the less volatile species may be concentrated in the permeate stream of a pervaporation process. [Pg.538]

This equation shows that the separation achieved in pervaporation is proportional to the product of the separation achieved by evaporation of the Hquid and the separation achieved by permeation of the components through a membrane. To achieve good separations both terms should be large. It follows that, in general, pervaporation is most suited to the removal of volatile components from relatively involatile components, because will then be large. However, if the membrane is sufficientiy selective and P g is large, nonvolatile components can be made to permeate the membrane preferentially (88). [Pg.86]

ILs, on the other hand, are uniquely suited for use as solvents for gas separations. Since they are non-volatile, they cannot evaporate to cause contamination of the gas stream. This is important when selective solvents are used in conventional absorbers, or when they are used in supported liquid membranes. For conventional absorbers, the ability to separate one gas from another depends entirely on the relative solubilities (ratio of Henry s law constants) of the gases. In addition, ILs are particularly promising for supported liquid membranes, because they have the potential to be incredibly stable. Supported liquid membranes that incorporate conventional liquids eventually deteriorate because the liquid slowly evaporates. Moreover, this finite evaporation rate limits how thin one can make the membrane. This... [Pg.90]

The heterogeneity of the VN primary neurones is reflected in their modes of chemosensory preferences. The relative binding efficiencies for distinct odourant types onto the membrane sites is indeed functionally partitioned. When urinary fractions from male mice were applied to VN cells of females, stimulation by a lipophilic and volatile odourant fraction activated only the Gi protein-expressing cells. In contrast, Go activation was elicited by one of the lipocalin superfamily the MUP fraction containing an a-2-globulin (Krieger, 1999). This observation... [Pg.142]


See other pages where Membranes Relative volatility is mentioned: [Pg.177]    [Pg.383]    [Pg.575]    [Pg.360]    [Pg.128]    [Pg.321]    [Pg.118]    [Pg.391]    [Pg.839]    [Pg.167]    [Pg.289]    [Pg.488]    [Pg.70]    [Pg.183]    [Pg.435]    [Pg.431]    [Pg.341]    [Pg.311]    [Pg.149]    [Pg.91]    [Pg.518]    [Pg.438]    [Pg.435]    [Pg.293]    [Pg.463]    [Pg.91]    [Pg.428]    [Pg.57]   
See also in sourсe #XX -- [ Pg.27 , Pg.225 ]




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Relative volatility

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