Pervaporation organophilic membranes

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,... 

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). 

For the first time, siUca-filled poly(l-trimethylsilyl-l-propyne) (PTMSP) layers on top of UF membranes for the pervaporative separation of EtOH-water mixtures was reported by Claes et al. (2010). Reduction of the thickness of the separating PTMSP top layer and addition of hydrophobic silica particles resulted in a clear flux increase as compared with dense PTMSP membranes. The performances of the supported PTMSP-silica nanohybrid membranes were significantly better than the best conunercially available organophilic PV membranes. The developed composite PTMSP-silica nanohybrid membranes exhibited EtOH-water separation factors around 12 and fluxes up to 3.5 kg/m h, establishing a sevenfold to ninefold flux inCTcase as compared with dense PTMSP membranes. 

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. 

There are three kinds of pervaporation membranes (a) hydrophilic membranes, (b) hydrophobic membranes, and (c) organophilic membranes. 

Treatment of contaminated wastewater by pervaporation is superficially attractive, however, only a few commercial installations have been built. Phenol, which azeotropes with water at around 10wt%, is typically recovered by extraction in large plants, where measures to eliminate extractant loss can be economically applied. Pervaporation is an attractive and cost competitive alternative for small plants. Organophilic membranes in spiral wound modules are used in these applications. 

Hickey PJ, Juricic FP, Slater S. 1992. Effect of process parameters on the pervaporation of alcohols through organophilic membranes. Sep. Sci. Technol. 27 843-861. 

Separation of water or polar molecules from organic compounds These separations are usually carried out by pervaporation (see Section 11.5) or vapor permeation, but they have also been performed using gas-phase feeds on organo-philic [108] or hydrophilic membranes [109]. On organophilic membranes, the permeation of the organic or less polar compound is favored, while the opposite trend is expected for hydrophilic membranes. 

Generally speaking, two terms, hydrophobic and hydrophilic, are employed in pervaporation in zeolite membranes, to refer to the affinity of organophilic and water molecules, respectively, toward the zeolite. In this way, a hydrophilic zeolite adsorbs and preferentially permeates water. Giaya et al. [170] defined a hydrophobicity index (HI), which is the ratio between the amount of organic and the amount of water that a solid adsorbs ... 

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. 

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