membrane technologies

Process Fundamentals

In the vapor permeation process, typically a pressurized and heated vapor phase mixture containing at least two components, for example, A & B, is fed to the membrane that has a higher permeation flux, for at least one of the components in the feed mixture. For example, the membrane in the illustration below has a higher flux for B.

The driving force for permeation of the components is the difference in partial pressure between the feed side and the permeate side of the membrane. Therefore, the permeate side of the membrane is maintained under vacuum.

The majority of component B and a small fraction of component A permeate the membrane in the vapor phase. Since, there is no phase change within the membrane module and the operating pressures are usually low, the feed, retentate, and permeate temperatures are essentially similar.

Feed: A-B Mixture (Vapor Phase)
Retentate: A-Rich (Vapor Phase)
Permeate: B-Rich (Vapor Phase)

In order to maximize the driving force for separation:

1. The feed should be pressurized and heated to the highest temperature so that the feed vapor does not condense. The maximum service temperature should not be exceeded under any circumstance.
2. Since the feed is in the vapor phase, the pressure drop in the membrane module is higher than the pressure drop in PV. Therefore, it is beneficial to use membrane modules that are in parallel inside the array.
3. The permeate should be cooled to the lowest possible temperature in order to maintain a deep vacuum.

Applications

• The dehydration of organic-water mixtures using hydrophilic zeolite membranes (for example, Zeolite A, T)
• The concentration of dilute organic mixtures using hydrophobic zeolite membranes (for example, Zeolite ZSM-5, Silicalite)
• The separation of organic-organic mixtures (for example, Zeolite X, Y)