Pervaporation Flashcards
What is an issue with distillation of ethanol?
Distillation is very energy intensive at lower concentrations and can end up being more energy cost than the product produces.
Limited by azeotropes
Why is distillation hard to beat when VLE is favourable? (3)
It has high recoveries
Moderate energy usage (at >3 wt% EtOH)
Low capital costs
How does pervaporation work?
Uses a dense polymeric membrane which operates by a solution-diffusion mechanism.
Liquid mixture is in direct contact with one side of membrane, the permeated product is removed in the form of vapour by evaporation.
Mass flux is brought about by maintaining the the downstream partial pressure lower than saturation pressure by applying a vacuum on permeate side.
Evaporation at the vapour side has a major effect on observed selectivity.
Advantages of pervaporation? (8)
- With the low T and P involved in evaporation it often has cost and performance advantages for the separation of azeotropic mixtures
- Minimises thermal degradation of heat sensitive compounds
- Significantly reduces energy consumption
- No entertainer (separation helping additive) and therefore no contamination
- Due to modular design, small units can operate economically
- Uses no sorbents which must be regenerated
- Continous and offers immediate recovery of solvents for industrial applications
- High degree of flexibility regarding feed mixtures, throughput and final products
What is evacuation?
Increasing the chemical potential gradient
- a driving force for change in number of molecules
Why is upstream distillation required in pervaporation?
Want high fraction of EtOH in feed as this induces less swelling of the membrane, less permeation of ETOH (only want water), and induces a minimum yA
Issues with pervaporation?
Membrane becomes swollen on the liquid side and dries on the vapour side
Draw pervaporation process
x
