Pervaporation is derived from the two processes of permeation of water or an organic through a membrane and then evaporation to the vapor phase. Pervaporation is used in many industries, such as chemical and food processing.
The most important part of Pervaporation is the membrane. Membranes select one of the phases and passes it through. This means one the other phase would be richer over time. Although the other phase does pass through the membranes also but the transport rate for this phase would be lower. This transport rate would define the term of “selectivity” of the membrane which is a very important factor in membrane manufacture and design. Two values characterize a membrane:
- The permeate flux across the membrane
Parsian Green Chemical Industries Co. is the sole provider of Pervaporation systems in Iran reaching remarkable results in membrane manufacture and module design. We have been constructing dehydration units from 2012 with over 5 successful pervaporation units and one distillation-pervaporation unit for IPA recovery.
The driving force for the transport of one component through the membrane is by means of chemical potential difference of permeate (v) and the feed (l)/retenate. “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.” (From Wikipedia)
Raising the temperature in the liquid feed side would help the permeate component and thus increases membrane flux so in most Pervaporation processes the feed stream is heated and then is driven to the membrane module. Permeation of one component would cause temperature loss so the feed liquid should be re heated. Some pervaporation devices have internal heating devices like steam coils or heating elements and others have external heat exchangers that are in line with the circulation of the feed and keeps up the temperature at a desired level. Pervaporation devices also use vacuum pumps to reduce pressure on the permeate side of the membranes to remove the permeate side and to increase flux. To trap the permeate condensers are used which mainly reduce temperatures as low as -150 degrees with liquid nitrogen.
Vapor permeation is not that different from Pervaporation and it is governed by the same rules, yet. Using vapor instead of liquid would actually help the flux and selectivity by a reasonable degree.
Advantages of Pervaporation and Vapor permeation:
- Pervaporation could be used to separate azeotropic and close boiling mixtures such as water/ethanol, water/isopropyl alcohol that require high energy to separate with distillation.
- Pervaporation consumes up to 50% less energy in comparison with distillation and 20% less energy compared to PSA and TSA methods.
- Pervaporation devices are compacted into tubular modules and take a lot less space than conventional separation methods.
- Pervaporation can separate water from multi component mixtures even if they all have the same boiling point as water and form an azeotrope.
- Alcohols: methanol, ethanol, propanols, butanols and higher linear alcohols, as well as higher alcohols such as glycol, glycerin and glycol ether on consultation.
- Ketones :acetone, MEK and MIBK
- Ethers: diethyl ether, diisopropyl ether, tetrahydrofurane (THF) and dioxan.
- Esters: ethyl acetate, butyl acetate and isopropyl propionate.
- Hydrocarbons such as benzene, toluene and xylene
- Organic acids such as acetic acid, propionic acid and aqueous solvents higher acids.
- Aprotic solvents : DMF, DMSO , NMP(only with zeolite membranes)
- Dehydration of Organic solvents
Many solvents form azeotropes with water after some point in their production. Such as isopropyl alcohol in 89% or ethanol in 96% purity. Pervaporation can dehydrate solvents without adding other impurities, this makes pervaporation a good candidate for dehydrating of pharmaceutical grade ethanol and IPA. Also many industries use solvents as washing or moisture absorbing agents, this gives another chance to pervaporation to find a market within waste solvents.
- Fuel grade ethanol production
Ethanol has found its position in the fuel industry for decades now. In Brazil and US billions of gallons of ethanol are produced and burned as a fuel. Fuel grade ethanol should not have more than 0.8% of water. Pervaporation is one of the best methods for dehydrating ethanol
- Removal of organics from aqueous solutions
This method is mostly used in the food industry to produce nonalcoholic beer and beverages. It is also used in the ethanol production plants to constantly remove ethanol from mashes and waste waters.
- Separation of organics from each other
Separation of HAB from LAB, Separation of xylenes
- Methanol removal from methyl acetate and MTBE
Methanol is used in excess in the production of methyl acetate to increase efficiency. Removal of methanol from methyl acetate requires expensive methods of salting out which cause damages to equipment and consume too much energy. Methanol selective membranes can separate it from methyl acetate mixture by means of pervaporation under simpler facilities with higher lifetime.
- Constant water removal form esterification reactors
A byproduct of esterification reaction is water. Le Chatelier’s principle says that removal of one part of the product to change the concentration would shift the equilibrium forward thus increases efficiency of the reaction. By this logic removal of water from esterification reactions would increase efficiency.