Organophilic pervaporation

While vapour permeation and hydrophilic pervaporation have readily found well-established areas for industrial application, in the case of organophilic pervaporation a clear industrial breakthrough has not yet been achieved. The reasons for this situation derive from the intrinsic character of this process and from the way some problems have been approached so far ... 

Most research on aroma recovery by organophilic pervaporation has been conducted using aqueous aroma model solutions [25-28], although in recent years significant interest has been devoted to the recovery of aroma compounds from natural complex streams, such as fruit juices [29-31], food industry effluents [32] and other natural matrixes [33]. The increasing demand for natural aroma compounds for food use, and their market value, opens a world of possibilities for a technique that allows for a benign recovery of these compounds without addition of any chemicals or temperature increase. However, in most situations, dedicated requests by industrialists are formulated in cooperation with marketing departments, which translate into the need for a correct public perception. 

Pervaporation may certainly play an important role for replacement of evaporative techniques as well as aroma-recovery processes based on solvent extraction, in particular when the labelling natural is considered crucial. Some of the most relevant technical challenges discussed herein have to be addressed in order to render organophilic pervaporation a competitive process (Fig. 19.4). In particular, the way of capturing the target aromas from the permeate stream has to be reanalysed in terms of minimising energy consumption and labour-intensive operations. 

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...

Mass-transport limitations are common to all processes involving mass transfer at interfaces, and membranes are not an exception. This problem can be extremely important both for situations where the transport of solvent through the membrane is faster and preferential when compared with the transport of solute(s) - which happens with membrane filtration processes such as microfiltration and ultrafiltration - as well as with processes where the flux of solute(s) is preferential, as happens in organophilic pervaporation. In the first case, the concentration of solute builds up near the membrane interface, while in the second case a depletion of solute occurs. In both situations the performance of the system is affected negatively (1) solute accumulation leads, ultimately, to a loss of selectivity for solute rejection, promotes conditions for membrane fouling and local increase of osmotic pressure difference, which impacts on solvent flux (2) solute depletion at the membrane surface diminishes the driving force for solute transport, which impacts on solute flux and, ultimately, on the overall process selectivity towards the transport of that specific solute. 

Mass-transfer limitations due to poor hydrodynamic conditions in the feed-side/ membrane interface are common in organophilic pervaporation (as referred above). 

T. Schafer J. G. Crespo, Aroma Recovery by Organophilic Pervaporation. In Flavours and Fragrances Chemistry, Bioprocessing and Sustainability R. G. Berger, Ed. Springer-Verlag Berlin, 2007 pp 427-438. 

Schafer T, Bengston G, Pingel H, Boddeker KW, and Crespo JPSG. Recovary of aroma compounds from wine-must fermentation by organophilic pervaporation. Biotechnol. Bioeng. 1999 62(4) 412. 

Willemsen JHA, Dijkink BH, and Togtema A. Organophilic pervaporation for aroma isolation—industrial and commercial prospects. Memb. Tech. 2004 2 5-10. 

Willemsen, J.H.A., Dijkink, B.H., and Togtema, A., Organophilic pervaporation for aroma isolation-industrial and coimnercial prospects, Membr. TechnoL, February, 5-10, 2004. 

Ten, P.K. and Field, R.W., Organophilic pervaporation an engineering science analysis of component transport and the classification of behaviour with reference to the effect of permeate pressure, Chem. Eng. Set, 55 (8), 1425-1445, 2000. 

Polyakov, A.M., Starannikova, L.E. and Yampolskii, Y.P. 2004. Amorphous Telfons AF as organophilic pervaporation materials Separation of mixtures of chloromethanes. /. Memh. Sci. 238 21-32. 

Pervaporation, as a non-integrated process, is typically utilized for dehydration and for the recovery or removal of organics from aqueous solutions and sometimes also for the separation of organic mixtures (Neel, 1995). Also many hybrid processes have been developed where PV is coupled with other processes, such as different membrane processes (e.g., reverse osmosis, or organophilic pervaporation coupled with hydrophilic pervaporation), distillation, reactive distillation and, of course, reaction. With these aspects in mind, PV appears particularly suitable to keep the concentration of a by-product low, or to continuously recover a product while it is formed. Note that these are the main objectives typically pursued in membrane reactors. 

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. 

Schrader, J. (2005) Production of 2-phenylethanol and 2-phenylethylacetate from L-phenylalanine by coupling whole-cell biocatalysis with organophilic pervaporation. Biotechnol. Bioeng., 92, 624-634. 

Liu X, Li YS, Zhu G, Ban Y, Xu L, Yang WS. An organophilic pervaporation membrane derived from metal-organic framework nanoparticles for efficient recovery of bio-alcohols. Angew Chem Int Ed 2011 50 10636-9. 

M. Inal, Ethyl lactate production by hybrid processes determination of phase diagrams and evaluation of performance of organophilic pervaporation membranes. Master of Science Thesis,... 

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