Ivo Vankelecom of the Katholieke Universiteit of Leuven, Belgium, looks at the potential for solvent-resistant filtration - from the lab to the plant.

Membrane technology, being a waste- and energy-efficient process, has experienced a significant growth over recent years, spurred by increasing environmental concerns and energy prices. This technology has set the new standard for treating aqueous streams, which still forms the major membrane market. Meanwhile, new technical achievements and a growing acceptance of membrane technology in industry have recently increased interest in using membranes to separate organic streams. As new membranes are able to separate organic mixtures at the molecular level things get particularly exciting. Such membranes inherently have extremely small pores. Permeation often has to take place through the available polymer free volume only.

Nanostructured polymer and ceramic membranes (~100 µm) can separate organic mixtures at the molecular level

External pressures above 5 bar are thus needed to realize reasonable membrane fluxes. Membrane processes at such pressures are referred to as reversed osmosis and nanofiltration. In the context of organic feeds, solvent resistant nanofiltration (SRNF) is the name most commonly used.The main advantages of SRNF over other unit operations are the ease of combining it with existing unit separations into a hybrid process, and the possibility to recycle solvents, reduce waste streams and decrease energy consumption. The main challenges that remain are: (1) the required robustness of the membranes to survive long-term performance with organic solvents, even under reactive conditions, and (2) the right combination of sufficiently high fluxes with acceptable rejections for a given solvent/solute pair. Efforts to address these have been increased recently and several new types of better performing membranes - both ceramic- and polymer-based - are being developed, some of which are already commercially available.

SRNF has the potential to be applied in a wide variety of industrial processes, in lab scale organic synthesis and in the more fundamental unravelling of chemical reactions. The last two applications have yet to be exploited, often because pressurized filtration cells are simply not available in organic synthesis labs, or because the separation power of the membranes is not known to the organic chemist running the experiments. Much better documented are the industrial processes, where SRNF forms an alternative or complement to existing unit operations to de-bottleneck a process, increase its efficiency or capacity.

Even though many process streams in the food industry are water-based, SRNF has great potential in, for instance, the vegetable oil industry where acetone and hexane are often used in refining the oil, or in synthesising food additives. In the chemical industry, SRNF can eliminate often destructive post-reaction work-up needed to remove catalysts from reaction mixtures. Many opportunities and challenges are for instance in the energy and separation-intensive petrochemical industry. Since 1998 SRNF has complemented the existing distillation plant in the 11 500 m3 per day recycling of solvents used in lube oil dewaxing at an ExxonMobil refinery in Texas. The hybrid installation requires only 25% of the heat consumption, 20% of the size and 10% of the conventional refrigeration capacity. Even under these harsh conditions, the applied polymeric membranes proved to be stable for many years. Other refinery applications potentially benefiting from SRNF implementation are gasoline desulfurization and crude oil deacidification. Pharmaceutical production can benefit from SRNF in downstream processing, or in between two different reaction steps via athermal solvent exchange and intermediate purification or recycle steps.

As is expected from a young technology, the effective number of large-scale industrial SRNF-applications is still limited. However, several potential applications have recently made the move from lab tests to pilot plant, so new industrial implementations can surely be anticipated in the near future.

Read the full Critical Review 'Solvent resistant nanofiltration: separating organics on a molecular level' in Chemical Society Reviews.