Pervaporation principles

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. 

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. 

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]. 

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...

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. 

Analytical pervaporation is a very mild process that can be operated at the required temperature and needs no high pressure or cross-flow velocity, and no additional chemicals. Because of its short life, the theoretical principles of analytical pervaporation have not yet been established except for liquid samples and gas-phase acceptors [160] there is, however, research in progress on solid and liquid samples processed with both liquid and gaseous acceptors [161]. 

The modules must be designed for a low pressure drop at the permeate side despite the increasing volume of the permeate due to the phase change since the principle of pervaporation is very sensitive to such pressure losses. 

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. 

Recently, new separation principles have been introduced and although these are very promising, they have not been extensively used for environmental analysis. Among them are FFF, pervaporation and biosorption. AU of them are easy to handle and not very expensive. In addition, FFF has very simple fundamental principles while pervaporation is very prone to automation and miniaturization. Biosorption is especially interesting for metal concentration because biosorbents can accumulate up to 25% of their dry weight in heavy metals. Some of the biosorbents are waste by-products of large scale industrial fermentations or certain abundant seaweeds. Analytes are easily released from the biosorbent and the biosorbent is regenerated for subsequent reuse. " ... 

Similarly to other traditional equipment used in separation processes, the main objectives when designing a vapor permeation or a pervaporation unit are the attainment of the highest possible mass-transfer surface to volume ratio, while maintaining adequate conditions to avoid detrimental mass-transport phenomena. These criteria, together with the need for simple operation and easy maintenance procedures, determine to a great extent the principles for module design. 

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. 

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. 

In this chapter, after some general considerations about the opportunities and alternatives offered by PVRs, the principles of pervaporation are briefly presented to identify the essential characteristics that can be exploited in a PVR. Following this, the fundamentals which are the basis of the existing studies and applications are surveyed with special attention to recent developments. Finally, indications are given regarding expected future trends. 

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. 

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. 

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... 

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