Pervaporation membrane principle

The resulting spectra from El usually contain a number of fragments, providing extensive structural information about the analyte. A disadvantage of the observed fragmentation is eventually occurring isobaric overlay from different compounds in the analysis of sample mixtures, which often requires a separation step prior to the MS analysis. For this purpose the coupling of a GC with the ion source of the mass spectrometer via capillary inlet is a well established technique. Volatiles can be selectively introduced into El mass spectrometers via pervaporation membranes. The principle and application of this technique, called membrane introduction (MI) MS was recently reviewed [45]. The accuracy of intensity ratio measurements by El MS is about 0.1 -0.5% [4,8]. 

Mulder, M. Basic Principles of Membrane Technology, Kluwer Academic, Dordrecht, 1997. Mulder, M.H.V. in Pervaporation Membrane Separation Processes, R.Y.M. Huang, ed., Elsevier, Amsterdam, 1991. 

Dip-coating is a very simple and useful technique for preparing composite membranes with a very thin but dense toplayer. Membranes obtained by this method are used in reverse osmosis, gas separation and pervaporation. The principle of this technique is shown schematically in figure EH -11. 

Modules Every module design used in other membrane operations has been tried in pervaporation. One unique requirement is for low hydraulic resistance on the permeate side, since permeate pressure is very low (0.1-1 Pa). The rule for near-vacuum operation is the bigger the channel, the better the transport. Another unique need is for heat input. The heat of evaporation comes from the liquid, and intermediate heating is usually necessary. Of course economy is always a factor. Plate-and-frame construction was the first to be used in large installations, and it continues to be quite important. Some smaller plants use spiral-wound modules, and some membranes can be made as capillary bundles. The capillary device with the feed on the inside of the tube has many advantages in principle, such as good vapor-side mass transfer and economical construction, but it is still limited by the availability of membrane in capillary form. 

Fig. 23.4 Organophilic pervaporation (PV) for in situ recovery of volatile flavour compounds from bioreactors. The principle of PV can be viewed as a vacuum distillation across a polymeric barrier (membrane) dividing the liquid feed phase from the gaseous permeate phase. A highly aroma enriched permeate is recovered by freezing the target compounds out of the gas stream. As a typical silicone membrane, an asymmetric poly(octylsiloxane) (POMS) membrane is exemplarily depicted. Here, the selective barrier is a thin POMS layer on a polypropylene (PP)/poly(ether imide) (PEI) support material. Several investigations of PV for the recovery of different microbially produced flavours, e.g. 2-phenylethanol [119], benzaldehyde [264], 6-pentyl-a-pyrone [239], acetone/buta-nol/ethanol [265] and citronellol/geraniol/short-chain esters [266], have been published...

The use of silicone membranes as an interface in MIMS for direct extraction and analysis by MS has fostered their implementation for extraction purposes that can be combined off-line or on-line with other analytical instrumentation, such as GC. The technique of membrane extraction with sorbent interface (MESI) (Figure 4.2) employs the pervaporation principle in a nonporous polymeric membrane unit, where the membrane is used as a selective barrier for the extraction of VOCs and SVOCs in gaseous or liquid samples. 

A good example of separation on the basis of affinity is the separation of alcohol/ water mixtures using a hydrophobic, silicalite membrane. Pervaporation of an ethanol/ water mixture through such a membrane resulted the removal of the alcohol from the mixture [16]. The separation selectivities achieved are between 10 and 60, depending on temperature and the alcohol content in the feed. In this way azeotropes can be broken. The reason for this is that the principle of separation, namely, differences in adsorptive behavior, is different from separation based on vapor pressure differences, used in distillation. Another example of such a separation is the pervaporation of an acetic acid/water mixture through a silicalite membrane, resulting in the removal of acetic acid [17]. 

FIGURE 6.27 General working principle of a pervaporation or vapor permeation module equipped with tubular ceramic membrane elements. 

Most importantly non-porous membranes such as ion exchange membranes, membranes for reverse osmosis, pervaporation, etc. should not be used in systems in which insoluble compounds precipitate on and in the membranes because this will destroy them and their functionality will be lost. Secondly all separation membranes, including ion exchange membranes, can achieve excellent performance by use of an appropriate apparatus and under optimum operation. For example, because solute and solvent transport speeds in the membrane phase are different from those in the solution, membrane-solution interfaces play an important role in separation, which depends on the structure of the apparatus and its operation. In this chapter, many examples of applications of ion exchange membranes are explained together with the principles on which they rely to achieve separation. 

The principles of using pervaporation for removing water from solvent are covered in Chapter 7 and involve the use of a hydrophilic membrane. The removal of solvents from water acts in an identical... 

See also Extraction Solvent Extraction Principles Solid-Phase Extraction Solid-Phase Microextraction. Flow Injection Analysis Principles Instrumentation. Ion Exchange Principles. Ion-Selective Electrodes Liquid Membrane Gas Sensing Probes Enzyme Electrodes. Membrane Techniques Dialysis and Reverse Osmosis Ultrafiltration Pervaporation. Solvents. 

Ndel J (1995), Pervaporation , in Noble R D and Stern S A eds., Membrane separations technology, Principles and Applications, Elsevier Science, Amsterdam,The Netherlands. Ch.5. 

Ionic membranes are characterised by the presence of charged groups. Charge is, in addition to solubility, diffusivity, pore size and pore size distribution, another principle to achieve a separation. Charged membranes or ion-exchange membranes are not only employed in electrically driven processes such as electrodialysis and membrane electrolysis. There are a number of other processes that make use of the electrical aspects at the interface membrane-solution without the employment of an external electrical potential difference. Examples of these include reverse osmosis and nanofiltration (retention of ions), microfiltration and ultrafdtration (reduction of fouling phenomena), diffusion dialysis and Donnan dialysis (combination of Dorman exclusion and diffusion) and even in gas separation and pervaporation charged membranes can be applied... 

The thermodynamics of irreversible processes are very useful for understanding and quantifying coupling phenomena. However, structure-related membrane models are more useful than the irreversible thermodynamic approach for developing specific membranes. A number of such transport models have been developed, partly based on the principles of the thermodynamics of irreversible processes, both for porous and nonporous membranes. Again, two types of structure will be considered here porous membranes, as found in microfUtration/uItrafiltration, and nonporous membranes of the type used in pervaporation/gas separation. 

Pervaporation is a complex process in which both mass and heat transfer occurs. The membrane acts as a barrier layer between a liquid and a vapour phase implying that a phase transition occurs in going from the feed to the permeate. This means that the heat of vaporisation of the permeating components must be supplied. Because of the existence of a liquid and a vapour pervaporation is often considered as a kind of extractive distillation process with the membrane acting as a third component. The separation principle in distillation is based on the vapour-liquid equilibrium whereas separation in pervaporation is based on differences in solubility and diffusivity. The vapour-liquid equilibrium influences... 

Pervaporation, vapour permeation and membrane distillation Principles and applications... 

Membrane reactors for energy applications and basic chemical production Edited by Angelo Basile, Luisa Di Paolo, Faisal Hai and Vincenzo Piemonte 11 Pervaporation, vapour permeation and membrane distillation Principles and applications... 

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