Pervaporation efficiency

The efficiency of a given pervaporation process must be evaluated in order to act on the overall system so as to either boost or reduce mass transfer to the acceptor chamber as required. Such evaluation can be done either in (a) relative terms by comparing signals provided under different conditions by the analyte (or its reaction product), previously collected in the upper chamber and driven to the detector, or in (b) absolute terms by comparing the signal obtained under the working conditions with that corresponding to 100% mass transfer. 

The relative procedure is typically used in optimization experiments in order to accommodate the sensitivity (usually by maximizing it), but also to ensure the best possible conditions for derivatization reactions (prior and/or subsequent to pervaporation) and dispersion along the continuous system, among others. No special alterations of the manifold other than those resulting from the optimization process are required in this case. 

The absolute procedure requires an auxiliary channel in the upper subsystem, through which the acceptor solution is also passed, furnished with an auxiliary injection valve (AIV in Fig. 4.19) of the same characteristics as the main injection valve (MIV), which is used to inject the sample for analyte separation and monitoring. Two sequential sample 

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Fig. 4.19. Simplified scheme of a hydrodynamic manifold for evaluation of pervaporation efficiency. Note that, when a derivatization reaction is required prior to or after analyte separation, one or more additional channels for the reagent solution(s) must be included in the donor and acceptor stream, respectively. AIV auxiliary injection valve, MIV main injection valve, PM pervaporation module, m membrane, a merging point, D detector, W waste, AUX auxiliary channel containing acceptor solution, S sample, AS acceptor solution, DS donor solution. (Reproduced with permission of Wiley Sons.)...

Fig. 4.21. Different ways of improving pervaporation efficiency. (A) By halting the flow in the acceptor chamber without disrupting the overall dynamic system. (B) By on-line retention of transferred volatile species and elution in the opposite direction. (C) By use of a packed flow-cell to integrate reaction and detection. E eluent, lEC ion-exchange column, PR preconcentration, EL elution. (For other abbreviations, see previous figures.) (Reproduced with permission of Wiley Sons.)...

The sensitivity of a method involving pervaporation essentially depends on the efficiency of the separation step. The efficiency of pervaporation can be adjusted in order to fit the signal provided by the analyte or its reaction product to the linear portion of the calibration curve for a given method, thus avoiding the need for a dilution or concentration step. A number of approaches to increasing and decreasing pervaporation efficiency exist. 

The concentration polarization is a phenomenon, which influences the pervaporation efficiency. The boundary-layer resistance to the solute transport was reported by Psaume et al. [9] for the recovery of trichloroethylene in aqueous solutions. They showed that under certain operating conditions, transport through the membrane was determined by the hydrodynamic conditions in the feed-side, and thus the resistance of the membrane becomes relatively negligible. Colman et al. [77] showed that resistance to the transfer of the boundary layer is a limiting factor of the dehydration of isopropanol by pervaporation. Various others studies [16,78,79] highlighted the importance of the hydrodynamics on resistance to the solute mass transport in pervaporation. 

MTBE is a well known enhancer of the number of octanes in gasoline and as excellent oxygentated fuel additives that decrease carbon monoxide emissions. Therefore, MTBE has been one of the fastest growing chemicals of the past decade. MTBE is produced by reacting methanol with isobutylene from mixed-C4 stream liquid phase over a strong acid ion-exchange resin as catalyst. An excess of methanol is used in order to improve the reaction conversion. This excess has to be separated from the final product. The pervaporation technique, more energy efficient and with lower cost process, has been proposed as alternative to distillation [74],... 

The same situation was evidenced in the case of xylene isomers separation. The evapomeation is more efficient than that of pervaporation [83],... 

The third application area for pervaporation is the separation of organic/organic mixtures. The competitive technology is generally distillation, a well-established and familiar technology. However, a number of azeotropic and close-boiling organic mixtures cannot be efficiently separated by distillation pervaporation can be used to separate these mixtures, often as a combination membrane-distillation process. Lipnizki et al. have recently reviewed the most important applications [53],... 

Solvent-resistant nanofiltration and pervaporation are undoubtedly the membrane processes needed for a totally new approach in the chemical process industry, the pharmaceutical industry and similar industrial activities. This is generally referred to as process intensification and should allow energy savings, safer production, improved cost efficiency, and allow new separations to be carried out. 

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. 

One approach to delivering increased performance in a membrane process is to complement one separation mechanism with another. Vapor-arbitrated pervaporation is an example of this strategy. In bioseparations, as will be covered in a later section, a similar integration of several process enhancements in High-Performance Tangential How Filtration is responsible for dramatic improvement in separation efficiency of protein mixtures once considered unachievable by means of conventional ultrafiltration. 

Separation of liquid mixtures with membranes is an intriguing new process. It is now an important problem to obtain absolute ethanol through fermentation of biomass. Instead of low-efficiency distillation, pervaporation is thought to be a promising method of separating ethanol from dilute aqueous solution. 

Pervaporation and vapor permeation are typical membrane processes with high application potential in chemical industry due to their high efficiency in the separation or the dehydration of organic solvents. Developed initially with organo-polymeric... 

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]. 

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]. 

The membranes used for analytical pervaporation are hydrophobic membranes of the types usually employed in ultrafiltration and gas-diffusion processes. In practice, PTFE is the most frequently used membrane material, followed by hydrophobic polyvinylidene-fluoride (PVDF). Ultrafiltration membranes are very thin, which, in combination with the large surface area of both the donor and acceptor chamber, leads to their easy bending. This results in changes in the ffux of the permeating component through an altered membrane area and hence in changes in the efficiency of the process. As a result, membranes must be replaced fairly often. Because of their thickness, gas-diffusion membranes are not so easily bent, so the same membrane can be used over long periods. The pore size of the... 

Preconcentration of pervaporated analytes is the most effective way of favouring mass transfer. The equilibrium between phases can be efficiently displaced by continuous removal of the transferred species from the upper chamber, followed by concentration (either in a minicolumn placed before the deteetor or in the flow-cell itself)-... 

Increasing the pervaporation temperature is quite often useful in order to improve the flux and separation efficiency of a membrane. This also holds for polyelectrolyte multilayer membranes [66]. For 2-propanol/water pervaporation across PVA/PVS membranes it was found, for example, that the total flux increases by a factor of ten upon changing the pervaporation temperature from 25 to 60 °C, whereas the water content of the permeate also increased slightly. The reason is that increasing the temperature mainly increases the flux of water across the membrane, which is favorable for the separation characteristics. For an optimum separation all pervaporation measurements were thus carried out at about 60 °C, the highest temperature technically available with our apparatus. 

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