Pervaporation, phase transfer processe

Membrane operation is a specific, but not exotic, operation. In fact it is a hybrid of classical heat and mass transfer processes (Figure 4.1). Direct contact mass transfer operations tend to reach equilibrium due to a difference of chemical potential between two phases that are put into contact. In the same way, temperature equilibrium is aimed at during heat transfer operations, for which driving force is a temperature gradient. In contrast, for membrane operations, by using the specific properties of separation of the thin layer material that constitutes the membrane, under the particular driving force that is applied, it is possible to deviate from the equilibrium that prevails at fluid-to-fluid interphase with classical direct contact mass exchange systems and to reorientate the mass transfer properties. In particular, this is the case with classical operations such as microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), gas separation (GS), pervaporation (PV), dialysis (DI) or electrodialysis (ED), for which a few characteristics are recalled in Table 4.1. 

Advantages to Membrane Separation This subsertion covers the commercially important membrane applications. AU except electrodialysis are pressure driven. All except pervaporation involve no phase change. All tend to be inherently low-energy consumers in the-oiy if not in practice. They operate by a different mechanism than do other separation methods, so they have a unique profile of strengths and weaknesses. In some cases they provide unusual sharpness of separation, but in most cases they perform a separation at lower cost, provide more valuable products, and do so with fewer undesirable side effects than older separations methods. The membrane interposes a new phase between feed and product. It controls the transfer of mass between feed and product. It is a kinetic, not an equihbrium process. In a separation, a membrane will be selective because it passes some components much more rapidly than others. Many membranes are veiy selective. Membrane separations are often simpler than the alternatives. 

The evaporation velocity at ambient pressure and, say, 60 °C, which corresponds to a mole fraction in the sweep gas of about 30 % water vapor, is about uuq = IO-5 m s-i. This results in Kj , = 0.904 which is rather close to 1, so that the effect of the liquid phase mass transfer resistance on the selectivity of an open distillation process with a free gas-liquid interface in most cases can be ignored. If, however, kuq becomes very small (as in the pervaporation process described in the next example), Kuq might become very small and thus reduce the selectivity of the open distillation process practically down to zero. 

This example illustrates the distillation of a binary mixture in an open-batch distillery with flowing sweep gas and pervaporation by having a porous plate floating on top of the liquid hold up, as shown in Fig. 4.20. The porous plate was made from inert sintered metal with various pore sizes between 100 and 1 mfi, and had a thickness of 1 mm. The porosity was 40 % and the tortuosity factor was about 2. This results in an effective liquid phase mass transfer coefficient of about hiq = 2 X 10-7 m s-i, which results in Kiiq = 1.9 X 10 22. Therefore, one would expect the distillation process to be nonselective - that is, Si = xi - xi = 0. 

Although clean-up and preconcentration help to improve the selectivity of dissolution and offset the dilution effeot, they lengthen the analytical process. This drawback should always be borne in mind in view of the growing tendency to shorten the analytical process so as to analyse as many samples as possible in the shortest time. It is always preferable to use selective steps such as leaching, pervaporation or headspace to remove the analytes from a solid sample. However, very frequently, they fail to provide quantitative results owing to inadequate efficienoy and (or) preoision. In this situation, USASD is an effective alternative to ensure complete transfer of analytes to a liquid phase and hence the quality in the results. 

The qualifiers continuous and discrete as applied to pervaporation refer to different aspects of the process. In fact, analytical pervaporation is a continuous technique because, while the sample is in the separation module, mass transfer between the phases is continuous until equilibrium is reached. Continuous also refers to the way the sample is inserted into the dynamic manifold for transfer to the pervaporator. When the samples to be treated are liquids or slurries, the overall manifold to be used is one such as that of Fig. 4.18 (dashed lines included). The sample can be continuously aspirated and mixed with the reagent(s) if required (continuous sample insertion). Discrete sample insertion is done by injecting a liquid sample, either via an injection valve in the manifold (and followed by transfer to the pervaporator) or by using a syringe furnished with a hypodermic needle [directly into the lower (donor) chamber of the separation module when no dynamic manifold is connected to the lower chamber]. In any case, the sample reaches the lower chamber and the volatile analyte (or its reaction product) evaporates, diffuses across the membrane and is accepted in the upper chamber by a dynamic or static fluid that drives it continuously or intermittently, respectively, to the detector — except when separation and detection are integrated. 

The word pervaporation is a contraction of two words, permeation and evaporation. This process is different from the other membrane processes in that there is a phase change as the solute permeates across the membrane. Thus, both heat and mass transfer are important... 

In vapor permeation, feed-side concentration polarization is much less prone to occur than in pervaporation, owing to the high mass-transfer rate of the solute in the vapor feed phase. In fact, this feature is one of the main factors that distinguish the two processes. 

Beaumelle, D. and Marin, M., Effect of transfer in the vapor phase on the extraction by pervaporation through organophilic membranes experimental analysis on model solutions and theoretical extrapolation, Chem. Eng. Process., 33 (6), 449-458, 1994. 

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... 

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