Pervapouration – separation by selectively permeable membranes

Pervapouration is a membrane-based separation process used to separate mixtures of liquids by partial vapourization through a selectively permeable membrane.

How It Works: - Role of Vapour Permeability

In pervapouration, vapour permeability is the key performance factor. The membrane’s ability to selectively allow one component to permeate in vapour form is what drives separation.

Two Key Factors:

• Permeability (P): How easily a molecule passes through the membrane.
• Selectivity (α): The membrane’s ability to prefer one component over another.

Mechanism of Transport (Solution-Diffusion Model)

1. Sorption: The preferred component dissolves into the membrane at the feed side.
2. Diffusion: It diffuses through the membrane due to the concentration gradient.
3. Desorption: It evaporates on the low-pressure (permeate) side.

Solvent molecules that dissolve well and diffuse quickly have high vapour permeability in this context.

Membrane Materials Used
Depending on what you want to separate, the membrane material is tailored to favour certain types of solvents:

Membrane Material	Best For Separating	Permeation Preference
PVA (Polyvinyl alcohol)	Water from organics (dehydration)	Hydrophilic
PDMS (Polydimethylsiloxane)	Organics from water (e.g., toluene, ethanol)	Hydrophobic
PEBA (Polyether block amide)	Aroma recovery, alcohols	Moderate polarity
Zeolite or MOF mixed membranes	Enhanced selectivity	Customizable

Industrial Applications of Pervapouration (Using Vapour Permeability)

1. Dehydration of Alcohols

• Example: Ethanol–water mixtures
• Membrane: Hydrophilic (PVA)
• Why: Traditional distillation is inefficient near azeotropic points; pervapouration breaks the azeotrope.

2. Recovery of Volatile Organic Compounds (VOCs)

• Example: Toluene, acetone from wastewater
• Membrane: Hydrophobic (PDMS)
• Why: VOCs permeate faster due to higher solubility and diffusivity.

3. Flavour and Aroma Recovery

• Example: Orange juice or beer aroma recovery
• Membrane: Tailored for aroma compounds
• Why: Sensitive compounds can be recovered without high heat.

4. Fuel Processing

• Example: Removing water from bioethanol before blending with gasoline
• Membrane: Pervapouration membrane modules used inline with fuel production.

 Performance Metrics

• Permeation Flux (J): Amount of substance passing through per unit area and time (e.g., g/m²·h).
• Separation Factor or Selectivity (α): Ratio of concentrations in permeate vs. feed.
• Permeability Coefficient (P): Combines diffusivity and solubility (measured in Barrer or GPU).

Engineers balance high permeability (good throughput) and high selectivity (effective separation).

Challenges & Optimization

• Trade-off between permeability and selectivity: Higher permeability sometimes reduces selectivity.
• Membrane swelling: Solvent absorption can degrade mechanical integrity.
• Membrane fouling: Especially in industrial feed streams with impurities.
• Thermal and chemical stability: Important for long-term operation.

Comparison to Other Techniques

Method<	Energy Use	Selectivity	Scalability
Distillation	High	Moderate	High
Pervapouration	Lower	High	Moderate
Adsorption	Variable	High	Variable
Reverse Osmosis	Lower	Poor for organics	High

Pervapouration membranes exploit vapour permeability as a controlled and selective process for separating volatile solvents or removing water from mixtures. Unlike other fields where vapour permeability is a leakage concern, here it is an engineered advantage.