Membrane technology like pervaporation (PV) and membrane filtration has been getting a lot of attention in the industry because of its energy efficiency and ability to break azeotropic systems.

The principle behind this technology is simple: the membrane behaves like a fixed filter that will allow water to pass through, while it catches suspended solids and other materials. Membranes are manufactured in a variety of configurations, such as hollow fiber, spiral, and tubular shapes. Each configuration offers a different degree of separation depending on the membrane process and the mixture to be separated.

PV in particular has been the only membrane process largely utilized for chemical purification over the last few decades with application in three primary areas: organophilic separation, removal of organic compounds from a dilute solution like water and dehydration of aqueous mixtures.
The process dates back to the late 1950s when researcher Robert Binning first took major research steps into PV commercialization. Binning reported on incorporating membrane pervaporation for dehydrating a ternary azeotrope (isopropanol-ethanol-water). This work marked the beginning of pervaporation research followed by others who covered topics like the separation of n-heptane and iso-octane and pyridine-water azeotrope separation, to name a few.

Membrane pervaporation as a clean technology for separating liquid mixtures was verified nearly 50 years ago, but commercial advancement did not occur due to the lack of market need.

Identifying Need

Water treatment is one of the many accomplishments of membrane technology to date. Worldwide consumption of water management specialty chemicals is forecast to expand at a constant average rate of 3.2% per year and is forecast to reach $12.4 billion in 2015, according to IHS Chemical.
Leading companies like Dow Chemical and BASF Chemical invest heavily in membrane technology, as described by IHS Chemical Week in a 2012 cover story on water treatment. Dow was featured for its sustainability target to cut desalination costs by 35% by 2015 as a result of utilizing the advancements in membrane filtration technologies, such as ultra-filtration (UF) and reverse osmosis (RO).

Other than water treatment, membrane technologies are being used to meet the global demands of cleaner energy with applications in the refinery, petrochemical and natural gas industries.

The problem of the late 1960s, namely the lack of a market, ceased to exist by the 1980s, when separation processes were regarded as vital elements of profitability. Fifty years ago, PV was just starting out with industrial systems introduced by small companies like California-based MTR. Today, Sulzer Chemtech Ltd. is among the leading vendors to provide such systems along with Canzler LLC, Petro Sep Membrane Technologies Inc., Zenon Environmental and others.

Current Reality

Recovering volatile organic compounds (VOCs) from aqueous solutions via PV has grown in importance over the past decade. Many efforts today focus on enhancing the potential of PV: new membrane materials are being created for specific VOC removal, more investment capital is being allocated to pilot-scale testing and the number of successful field trials is growing. While many membranes are being created on a lab scale, a relative few make it to commercialization. For a long time, the failure of many lab-scale developments to survive to industrial application was a matter of concern for researchers. Today, however, the situation has improved greatly.

Ceramic membrane module by Sulzer Chemtech. Source: ©Sulzer Chemtech Ltd. Ceramic membrane module by Sulzer Chemtech. Source: ©Sulzer Chemtech Ltd.
“The gap between lab-scale developments and industrial application is becoming shorter and shorter,” says researcher Patricia Luis, a Materials & Process Engineering expert from Université catholique de Louvain, Belgium. “Industries are very interested in PV due to the low energy consumption in comparison with distillation. This is clear since the research departments of large companies are working on it. At the universities, we try to collaborate and do more 'crazy' research since we are not so limited in time and applications.”

In comparison with conventional separation methods, PV does not add to the air emissions problem and is increasingly used by industry to conform to regulations on chemical emission limits.

Advanced ways of separating VOCs from water are being created through the use of new membrane materials and techniques such as filling, grafting and coating to achieve high selectivity and high flux. However, according to Luis, permeability is one of the most important elements in assessing the potential of a particular membrane material.

“The separation factor is a point, but the permeability is more important in order to minimize the membrane area,” she says.

Trade-offs are being made between permeability and using filling as a technique. Although filled polymeric membranes demonstrate improved physical properties (such as increased stiffness or reduced creep to achieve thermal stability improvement, high voltage resistance or radiation shielding), they also have permeabilities that are much lower than conventional unfilled membranes. Consequently, these properties create barriers for oxygen, water and other solutes.

Other than permeability and the separation factor, membrane material also plays a role in the separation of a particular mixture. As a rule of thumb, membranes used for VOC separation are mostly hydrophobic, or water shedding, materials. Over the years, polymeric membranes such as polydimethylsiloxane (PDMS), poly (trimethylsilyl) propyne (PTMSP), and polyvinyl chloride have been used to separate VOCs from water. PDMS in particular has been extensively used due to its hydrophobic nature and rubbery composition.

Ceramic membranes, although a less-popular choice, also can be used in this field. Even so, says Luis, polymeric membranes receive more attention because there is more research involving them. That focus does not mean that ceramics are not potential membranes, she says. Pervaporative removal of VOCs namely, ethyl acetate (EtOAc) and methyl tertiary-butyl ether (MTBE) from water was possible using ceramic membrane - hydrophobized titania (TiO2) with a separation factor of 84% and 56%, respectively.

Future Prospects

In the 1990s, researcher Peter Agre discovered water channel proteins (also known as aquaporins) that can make water delivery highly efficient. To build upon that discovery, other researchers have attempted to use biomimetic approaches while developing membranes to integrate biological elements or use concepts from biological systems.

Using engineered microporous support structures, this technology can be a more energy efficient alternative to conventional reverse osmosis and ultrafiltration membrane systems.

Due to the gift of selectivity and water permeability, biomimetic membranes may have the potential to achieve improved water permeability, reduced energy consumption and superior produced water quality. These characteristics, then, can be applied to ultra-pure water production, seawater desalination and water reuse.

Another possibility could be to apply biomimetic approaches to membrane fouling mitigation. Fouling has been a primary setback in membrane technology. Using concepts from biofilm, control and prevention in biological systems can be an important tool in efforts to prevent fouling.

However, this technology is in its infancy, with many obstacles in terms of lack of proficiency in understanding the relationship between functional molecules and matrix materials, the extent of attempts to synthesize biomimetic membranes, and the cost associated with producing large quantities of biomimetic materials.

With energy, water and environmental sustainability posing challenges around the world, membrane technology may experience wider use to address these challenges through additional research and process enhancements.