Principles of pervaporation

In Fig. 3.6 a principal scheme of a pervaporation process is shown. The liquid feed mixture is heated to the highest temperature compatible with its own stability, the stabihty of the membrane and all other parts (e.g. gaskets, module elements) in the system. All partial vapor pressures are at saturation and fi ffid by the temperature and composition of the liquid mixture, and by the nature of the components. On the permeate side all noncondensable gases are removed by means of a vacuum pump, and the permeated vapors are condensed at a sufficiently low temperature in order to maintain a sufficiently low vapor pressure 

This drop in temperature has several consequences. The partial vapor pressure of the critical (more-permeating) component is decreased not only by the reduction of its concentration, but also by the reduction in temperature. Furthermore, as the diffusional transport through the membrane is temperature dependent an additional reduction of the transmembrane flux results. Whereas the flux drops approximately linearly with the reduction in concentration and the concentration drop is unavoidable (it is the goal of the process), the flux is reduced exponentially with the decrease in temperature (Fig. 3.7). When the lost heat is not replaced the flux will soon drop to unacceptable levels. 

Flow Pass (Decreasing Concentration of Feed) Fig. 3.9 Isothermal and real flux in pervaporation. 

It is evident that the arrangement as shown in Fig. 3.8 reduces the energy consumption of the pervaporation process to a minimum value. Only the heat required for the evaporation of the permeate has to be supplied and is lost in the process, the sensible heat of the product can be recovered to any extent, limited only by the costs of the interchanger. 

Separation from mixtures is achieved because the membrane transports one component more readily than the others, even if the driving forces are equal. The effectiveness of pervaporation is measured by two parameters, namely flux, which determines the rate of permeation and selectivity, which measures the separation efficiency of the membrane (controlled by the intrinsic properties of the polymer used to construct it). The coupling of fluxes affecting the permeability of a mixture component can be divided into two parts, namely a thermodynamic part expressed as solubility, and a kinetic part expressed as diffusivity. In the thermodynamic part, the concentration change of one component in the membrane due to the presence of another is caused by mutual interactions between the permeates in the membrane in addition to interactions between the individual components and the membrane material. On the other hand, kinetic coupling arises from the dependence of the concentration on the diffusion coefficients of the permeates in the polymers [155]. 

For a binary mixture of components A and B, the flux can be expressed for the entire permeate (J, total flux) or each component and u, the flux of component A and B, respectively), having dimensions of mass/(area x time). The flux can be calculated provided the mass of the permeating component, the membrane area and the time of measurement are known. To this end, the following expression can be used  

The selectivity of pervaporation is given by the ratio of the mass fractions of components A and B for the permeate and feed. In the case of selective permeation of component A, the following equation applies  

The process is favoured by increased temperatures, which result in a vapour pressure difference that increases the permeability of the substances through the membrane by allowing the diffusion of permeated molecules and decreasing interactions between permeates. The morphology of the membrane (rubber, glassy polymer) also influences the separation efficiency. 

Pervaporation surpasses conventional industrial separation in several respects thus, it makes more efficient use of energy, allows the ready separation of azeotropic mixtures and dehydration of multicomponent mixtures, avoids contamination of the product with entrained compounds and the environmental pollution usually resulting from treatment of entrained substances, uses little space and is easy to implement and install on-site as the pervaporator is skid-mounted [155]. 

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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. 

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. 

Mulder, M.H. V. Thennodyaamics principles of Pervaporation in R.Y.M. Huang (ed.), Pervaporation Manbrane Separation Processes, Elsevier, Amsterdaxn, 1991,... 

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]. 

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. " ... 

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... 

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. 

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. 

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]. 

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. 

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. 

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. 

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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