Nanofiltration in Organic Solvents

Department of Chemical Engineering, Imperial College, London SW7 2BY, United Kingdom 

Most polymeric OSN membranes have an asymmetric structure and are porous with a dense top layer. This asymmetry can be divided into two major types the integral type, where the whole membrane is composed of the same material, and the thin-film composite (TFC), where the membrane separating layer is made of a different material. 

Polymeric membranes generally fail to maintain their physical integrity in organic solvents because of their tendency to swell or dissolve. This is a major drawback since nonaqueous processes generally require polymers that are rigid and crystalline, thermally 

Advanced Membrane Technology and Applications. W. S. Winston Ho, and T. Matsuura Copyright 2008 John Wiley Sons, Inc. 

Some examples of classes of highly resistant polymers are presented in Table 16.1 (Critchley et al., 1983). 

---

A difficult problem that prevented the use of nanofiltration in organic solvents for a long time was the limited solvent stability of polymeric nanofiltration membranes, and the lack of ceramic nanofiltration membranes. For polymeric membranes, different problems occurred zero flux due to membrane collapse [54], infinite nonselective flux due to membrane swelling [54], membrane deterioration [55], poor separation quality [ 5 6], etc. I n an early study of four membranes thought to be solvent stable (N30F, NF-PES-10, MPF 44 and MPF 50), it was observed that three of these showed visible defects after ten days exposure to one or more organic solvents, and the characteristics of all four membranes changed notably after exposure to the solvents [15]. This implies that these membranes should be denoted as semi-solvent-stable instead of solvent stable. 

Solvent resistant nanofiltration membranes are a much more recent evolution. Historically, the membranes developed by Membrane Products Kyriat Weizmann (Israel) - now Koch - (MPF 44, MPF 50, MPF 60) were the first nanofiltration membranes intended for application in organic solvents, although other membranes (e.g., PES and PA membranes) also have a limited solvent stability. The Koch membranes are based on PDMS, similarly to pervaporation membranes, although the level of crosslinking is quite different. 

Due to recent advances in membrane development, nanofiltration membranes are nowadays increasingly used for applications in organic solvents [27, 58]. This narrows the gap between pervaporation and nanofiltration. It is even possible that the requirements for membrane structures completely overlap for the two processes whereas membrane stability becomes more important for nanofiltration membranes, the performance of pervaporation membranes could be improved by using an optimized (thinner) structure for the top layers. It might even be possible to use the same membranes in both applications. At this moment it is not possible to define which membrane structure is necessary for nanofiltration or for pervaporation, and which membrane is expected to have a good performance in nanofiltration, in pervaporation or in both. Whereas pervaporation membranes are dense, nanofiltration membranes... 

FIGURE 5.10 Scheme of MAX-DEWAX process. (Reprinted from J. Membr. ScL, 286, White, L.S., Development of large-scale applications in organic solvent nanofiltration and pervaporation for chemical and refining processes, 26-35, Copyright (2006), with permission from Elsevier.)... 

L.G. Peeva, E. Gibbins, S.S. Luthra, L.S. White, R.P. Stateva, A.G. Livingston, Effect of concentration polarization and osmotic pressure on flux in organic solvent nanofiltration, J. Memhr. 

Poly(dimethyl siloxane) (PDMS)/ hydrosilane Reduces die swelling of PDMS in organic solvent Nanofiltration [139]... 

When ionic liquids are used as replacements for organic solvents in processes with nonvolatile products, downstream processing may become complicated. This may apply to many biotransformations in which the better selectivity of the biocatalyst is used to transform more complex molecules. In such cases, product isolation can be achieved by, for example, extraction with supercritical CO2 [50]. Recently, membrane processes such as pervaporation and nanofiltration have been used. The use of pervaporation for less volatile compounds such as phenylethanol has been reported by Crespo and co-workers [51]. We have developed a separation process based on nanofiltration [52, 53] which is especially well suited for isolation of nonvolatile compounds such as carbohydrates or charged compounds. It may also be used for easy recovery and/or purification of ionic liquids. 

The separation of the auxiliary agent can be easily handled on a technical scale if it forms a pure phase. Otherwise more sophisticated separation methods are needed. In the case of ionic liquids a process termed organic solvent nanofiltration has been tested successfully [120,128]. 

Use of nanofiltration for non-aqueous separations is limited by membrane compatibility - a common material in composite nanofiltration membranes used for aqueous separations is polysulfone which possesses limited solvent resistance [134]. However, during the past two decades a number of materials have emerged with improved solvent resistance that have enabled a broad range of organic solvent nanofiltration (OSN) applications. These materials include polydimethylsiloxane, polyphenylene oxide, polyacrylic acid, polyimides, polyurethanes, and a limited number of ceramics. Commercial products are offered by Koch Membrane Systems, W.R. Grace, SolSep, and Hermsdorfer Institut fur Technische Keramik (HITK) [135]. 

---