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New Pervaporation Membrane Holds Promise for Recycling and Waste Treatment
University of Waterloo
For over forty years, membrane technologies have been used for effective separation and filtration processes. Since the appearance of membrane separation in the early 1960s, the process has been broadly classified according to the driving force used; for example, filtration uses pressure, electrodialysis uses electrical potential, dialysis uses concentration gradients, and pervaporation uses partial pressure. Unlike other separation technologies, membrane processes don´t require a phase change so there are no heat requirements, making membrane separation simple and cost-effective in general. Even more attractive is its ability to concentrate, fractionate, and purify products all at the same time. This article focuses on a new pervaporation membrane developed by the University of Waterloo to address the limitations of current pervaporation membranes, including relatively poor water selectivity and low selectivity and flux rates.
What is Pervaporation?
Pervaporation is simply the combination of permeation and evaporation and is used extensively to separate miscible liquids. Pervaporation membranes are usually comprised of a very thin selective outer layer of material, typically supported on a more porous, structurally stronger material. The separation characteristics of the thin outer layer are determined by the composition of the material selected, including thickness and pore size, and its interaction with the liquid target to be separated. Typically the porous support layer of a pervaporation membrane has very little impact on the separation characteristics of the system. Unlike other membrane processes, pervaporation requires a brief phase change from liquid to vapor to separate the influent stream into two effluent streams -- the permeate and concentrate. Transport of the permeate through the membrane is driven by a difference in pressure maintained across the membrane. A partial vacuum on one side of the membrane is used to draw the permeate through the membrane barrier to create a permeate vapor which is immediately condensed back into liquid and drawn off, leaving an enriched concentrate solution.
an alternative to energy-intensive distillation processe
The majority of pervaporation systems have been used as an alternative to energy-intensive distillation processes because pervaporation techniques offer a number of advantages over other separation techniques, including:
- More cost-effective and thorough separation of azeotropic miscible liquids (liquids composed of components with close boiling points)
- No catalyst or entrainers required to affect separation of miscible mixtures
- Significant capital cost and operating cost savings over conventional systems
What makes pervaporation membrane technology so intriguing is its effectiveness in separating difficult materials, such as solvents and other azeotropic mixtures which are difficult to separate using other methods. Pervaporation is ideal for solvent recovery, recycling, ethanol-water separation or any application that may be either heat sensitive or a poor candidate for other membrane technology, such as microporous or asymmetric membranes. Pervaporation systems have found applications in the petrochemical, pharmaceutical, food and beverage, and wastewater remediation industries.
University of Waterloo´s Novel Composite Membrane
Broader use of pervaporation membranes in dehydration applications has been hindered by low selectivity attributed to the water swelling of commonly-used hydrophobic membranes such as a poly-vinyl-alcohol (PVA). UWaterloo has developed new membrane fabrication processes that rely on material casting techniques to produce novel 2-ply composite pervaporation membranes offering excellent mechanical strength, high flux, and outstanding water selectivity, making them a next-generation choice over commonly-used PVA membranes.
UWaterloo membranes open up new opportunities for dehydration applications
Several 2-ply composite membranes have been produced utilizing a variety of common membrane porous substrates (e.g., polyetherimide, polyvinylidene fluoride, polysulfone, etc.) upon which a very thin layer of chitosan material has been deposited. As chitosan is a hydrophilic material which also offers excellent chemical and mechanical stability. The UWaterloo membranes open up new opportunities for pervaporation dehydration applications (e.g., solvent recycling, alcohol dehydration). Several variations of this membrane production technique have been used to dramatically increase chitosan density (thus very thin layers), yielding membranes with high flux rates operating under low vacuum conditions. Yet another variation of the production technique has yielded a chitosan modified composite membrane that is suitable for separating difficult polar organic mixtures (e.g., methanol from MTBE).
Presently the UWaterloo process and resulting membranes are the subject of several patent applications and the technology is available for licensing and further development through the University of Waterloo.
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