Pervaporation systems continuous

Besides wastewater treatment, pervaporation systems have also been tested on a development scale for continually removing volatile organic products (e.g., ethanol, volatile acids) from fermentation broths. 

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. 

A choice of batch or continuous pervaporation systems, or continuous vapor permeation depending on the duty. 

For a rapid conversion of lab-scale results into an economically viable reaction-pervaporation system, an optimum value can be determined for each parameter. Based on experimental results as well as 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 an Arrhenius law, whereas the pervaporation is accelerated by an increased temperature also. Consequently, the water content fluctuates much faster at a higher temperature. The second important parameter is the initial molar ratio. 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 operation, the flow rate should be considered as the determining factor for the contact time of the mixture with the membrane and subsequently the permeation... 

O Brien, D.J. and Craig, J.C. Jr.l996. Ethanol production in a continuous fermentation membrane pervaporation system. 44 699—704. 

Gubicza, L., Belafi-Bako, K., Feher, E., Frater, T. (2008). Waste-free process for continuous flow enz3fmatic esterification using a double pervaporation system. Green Chemistry, 10, 1284-1287. 

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. 

Does the system form an azeotrope If so, is an entrainer used Can one replace the entrainer by a more benign agent Can the azeotrope be broken by other methods such as pervaporation (see paper by Wynn, 2001) or membrane separation and then continue with distillation to get the final purity ... 

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... 

The analytical pervaporator can be used in combination with a flow-injection manifold, either in the upper chamber when the pervaporated species must be derivatized for adaptation to the detector and/or in the lower chamber for the pervaporation of analytes from liquid samples or slurries. Alterations of either the auxiliary dynamic manifold or the pervaporator itself are required when the pervaporation step is assisted by focused microwaves, the separation step assists in the continuous monitoring of an evolving system, untreated solid samples are used or pervaporation is integrated with detection. 

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. 

Mol sieves are routinely used for drying a number of reaction feeds to very low water levels. This is particularly important where expensive catalysts are susceptible to wet feed. Pervaporation has the advantage of continuous, steady operation, minimizing operational upsets, which can result from sieve regeneration. Simpler systems are especially attractive where toxic materials are involved. 

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. 

Groot WJ, Luyben KCAM. 1987. Continuous production of butanol from a glucose/xylose mixture with an immobilized cell system coupled to pervaporation. Biotechnol. Lett. 9 867-870. 

Calculated values of selectivity and yield in a continuous system for the production of benzaldehyde with and without pervaporation. 

Ding W W, Wu XT,Tang X Y,Yuab L, Xia Z Y (2011), Continuous ethanol fermentation in a closed-circulating system nsing an immobilized cell coupled with PDMS membrane pervaporation, / Chem. Technol. BiotechnoL,S6,82-. ... 

Gao, Yue, and Li (1996) studied the same reaction using a zeolite A-PVA composite membrane (at temperatures ranging from 20 to 50 °C). In this work, together with the pervaporation-aided catalytic esterification of acetic acid with ethanol, the reaction between salicylic acid with methanol was also treated. Among other results, it showed that the continuous removal of water from the system displaced the equilibrium limit (79%), making possible a 95% conversion, when using PVA, PVA -I- KA, and PVA -I- CaA membranes for 20.0, 11.3, and 10.0 h, respectively. 

Problems may arise from the poor water solubUity of the starting substrate, or from the inhibition of cell growth due to either the substrate or the product over a certain threshold concentration. Methods for the continuous addition of substrate at non-harmful concentrations and removal of the product(s) have been developed. Among these, the most effective ones are the pervaporation method [30], or the addition to the culture media of a nonsoluble solid or nonmiscible hydrophobic liquid phase to create a two-phase system in which the desired volatile is sequestered by the solid or the hydrophobic hquid phase. 

Alternative and more sophisticated approaches based on biphasic in situ product removal have also been proposed and demonstrated recently, which include the use of ionic liquids as the nonaqueous phase [88], the inclusion of an organophilic pervaporation step [89], and the coupling of product removal with a continuous culture system [90]. The latter approach, which comprises two different units for culture and adsorption, separated by a ceramic membrane to prevent the cells from polluting and clogging the resin, allowed achieving the highest space-time yield (0.9 g 1 h) ever reported for this bioprocess. 

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