Continuous pervaporation process

Based on experimental results and a model describing the kinetics of the system, it has been found that the temperature has the strongest influence on the performance of the system as it affects both the kinetics of esterification and of pervaporation. The rate of reaction increases with temperature according to Arrhenius law, whereas an increased temperature accelerates the pervaporation process also. Consequently, the water content decreases much faster at a higher temperature. The second important parameter is the initial molar ratio of the reactants involved. It has to be noted, however, that a deviation in the initial molar ratio from the stoichiometric value requires a rather expensive separation step to recover the unreacted component afterwards. The third factor is the ratio of membrane area to reaction volume, at least in the case of a batch reactor. For continuous opera-... 

MHS with pervaporation of water from LM (MHS-PV) is presented in Figure 13.10. Contrary to the simple MHS with an agitated BLM, separated from the feed and strip solutions by flat hydrophobic or hydrophilic or ion-exchange membranes, the MHS-PV system exploits an FLM continuously flowing between the two flat cation-exchange and two pervaporation membranes. To couple the separation and pervaporation processes, the LM is simultaneously pumped through the MHS and... 

In the membrane reactor shown in Figure 13.16(c), the chemical reaction and the separation step use the same membrane. However, in some processes it is desirable to separate reaction and separation into two distinct operations. If the net result of the process is to change the products of the chemical reaction, the process is still classified under the broad heading of membrane reactor. Two examples in which chemical reaction and separation are physically separated are shown in Figure 13.17. Figure 13.17(a) shows the use of a pervaporation membrane to shift the equilibrium of the de-esterification reaction [39,40], A portion of the organic solution in the esterification reactor is continuously circulated past the... 

To this purpose, in a study on the photocatalytic degradation of 4-chlorophenol, Camera-Roda and Santarelli [89] proposed an integrated system in which photocatalysis is coupled with pervaporation as process intensification for water detoxification. Pervaporation represents a useful separation process in the case of the removal of VOCs and in this study it is used to remove continuously and at higher rate the organic intermediates that are formed in the first steps of the photocatalytic degradation of the weakly permeable 4-CP. 

A process referred to as vapor-arbitrated pervaporation addresses these issues by manipulating the transmembrane activity gradients of water and ethanol in a pervaporation system. Using a permeate side sweep stream that contains water vapor at a partial pressure corresponding to the activity of water on the feed side, permeation of water is halted while ethanol continues to diffuse through the membrane into the sweep stream and is removed. In this way, the native permselectivity of the membrane system can be altered in a controlled fashion to extract one or more volatile components from a solution. 

Operational Factors In industrial use, pervaporation is a continuous-flow single-stage process. Multistage cascade devices are unusual. Pervaporation is usually an adjunct separation, occasionally a principal one. It is used either to break an azeotrope or to concentrate a minor component. Large stand-alone uses may develop in areas... 

When the pervaporation unit is used for the continuous monitoring of a fermentation process with a view to determining analytes in samples containing suspended particles, or in slurries, the module is altered by increasing the diameter of both channels (inlet and outlet) of the donor chamber, which might otherwise be clogged. 

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 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. 

Another possibility is to locate the semi-permeable membrane well above the donor stream (Fig. 8.23, centre), thus establishing an air gap between the sample and the membrane [270]. This approach has been referred to as pervaporation, and was defined as "a combination of continuous evaporation and gas diffusion through a gas permeable membrane" [271], Both processes take place in a single step and in the same unit. The volatile chemical species present in the donor phase evaporates, crosses the air gap and the semi-permeable membrane and is collected in an acceptor stream. 

Friedl et al [3.70] studied ABE production in an integrated process using C. acetobutylicum immobilized onto a packed bed of bonechar coupled with continuous product removal by pervaporation. Using a concentrated feed solution containing lactose at 130 g/1, a lactose utilization value of 98.7 % was observed. The PVMBR system showed low acid loss, a high solvent yield of 0.39 g solvents/g lactose utilized, and a solvent productivity of 3.5 g/1. 

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