Pervaporation
Pervaporation (or pervaporative separation) is a processing method for the
Theory
The term pervaporation is a portmanteau of the two steps of the process: (a)
The membrane acts as a selective barrier between the two phases: the liquid-phase feed and the vapor-phase permeate. It allows the desired components of the liquid feed to transfer through it by vaporization. Separation of components is based on a difference in transport rate of individual components through the membrane.
Typically, the upstream side of the membrane is at ambient pressure and the downstream side is under vacuum to allow the evaporation of the selective component after permeation through the membrane. Driving force for the separation is the difference in the
The driving force for transport of different components is provided by a chemical potential difference between the liquid feed/retentate and vapor permeate at each side of the membrane. The retentate is the remainder of the feed leaving the membrane feed chamber, which is not permeated through the membrane. The chemical potential can be expressed in terms of fugacity, given by Raoult's law for a liquid and by Dalton's law for (an ideal) gas. During operation, due to removal of the vapor-phase permeate, the actual fugacity of the vapor is lower than anticipated on basis of the collected (condensed) permeate.
Separation of components (e.g. water and ethanol) is based on a difference in transport rate of individual components through the membrane. This transport mechanism can be described using the solution-diffusion model, based on the rate/degree of dissolution of a component into the membrane and its velocity of transport (expressed in terms of diffusivity) through the membrane, which will be different for each component and membrane type leading to separation.
Applications
Pervaporation is effective for dilute solutions containing trace or minor amounts of the component to be removed. Based on this,
Pervaporation is an efficient energy conserving alternative to processes such as distillation and evaporation. It allows the exchange of two phases without direct contact.[2]
Examples include solvent dehydration: dehydrating the ethanol/water and isopropanol/water azeotropes, continuous ethanol removal from yeast
Recently, a number of organophilic pervaporation membranes have been introduced to the market. Organophilic pervaporation membranes can be used for the separation of organic-organic mixtures, e.g.: reduction of the aromatics content in refinery streams, breaking of azeotropes, purification of extraction media, purification of product stream after extraction, and purification of organic solvents.
Materials
Hydrophobic membranes are often polydimethylsiloxane based where the actual separation mechanism is based on the solution-diffusion model described above.
Hydrophilic membranes are more widely available. The commercially most successful pervaporation membrane system to date is based on polyvinyl alcohol. More recently also membranes based on polyimide have become available. To overcome the intrinsic disadvantages of polymeric membrane systems ceramic membranes have been developed over the last decade. These ceramic membranes consist of nanoporous layers on top of a macroporous support. The pores must be large enough to let water molecules pass through and retain any other solvents that have a larger molecular size such as ethanol. As a result, a molecular sieve with a pore size of about 4 Å is obtained. The most widely available member of this class of membranes is that based on zeolite A.
Alternatively to these crystalline materials, the
See also
References
Further reading
- Fontalvo Alzate, Javier (2006). Design and performance of two-phase flow pervaporation and hybrid distillation process. Technische Universiteit Eindhoven, The Netherlands: JWL boekproducties. ISBN 978-90-386-3007-6.
- Matuschewski, Heike (2008). MSE — modified membranes in organophilic pervaporation for aromatics/aliphatics separation. www.desline.com: Desalination.
- Eslami, Shahabedin; Aroujalian, Abdolreza; Bonakdarpour, Babak; Raeesi, Ahamdreza (2008). "Coupling of Pervaporation system with Fermentation Process" (PDF). International Congress on Membrane and Membrane Technology (ICOM2008) Honolulu, Hawaii, USA.
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