Pervaporation

Question 1

What is pervaporation?

Answer 1

Processing method for the separation of mixtures of liquids by partial vaporization through a non-porous or porous membrane
Pervaporation can help overcome the azeotropic limit in separation.

Question 2

Distillation is the best method to separate when you do not need to worry about azeotropic mixture as it has
(3)

Answer 2

Distillation is the best method to separate when you do not need to worry about azeotropic mixture as it has

• high recoveries
• moderate energy usage
• lower capital costs when you have a higher feed concentration

There is a limit to distillation though therefore need to rely on other separation techniques.

Question 3

What is the key to energy efficiency?

Answer 3

The water evaporation is the key to energy efficiency, as more energy is required to evaporate the water compared to alcohol, so you want your feed to have lower concentration of water.

Question 4

What is the pervaporation equation?

Answer 4

Pervaporation = Permeation + evaporation

Question 5

What is the key to energy efficiency?

Answer 5

The water evaporation is the key to energy efficiency,

as more energy is required to evaporate the water compared to alcohol, so you want your feed to have lower concentration of water.

Question 6

What is the driving force across the membrane?

Answer 6

The driving force is the pressure gradient.

Question 7

How does permeation work?

Answer 7

1. Can use a dense non-porous or molecular porous membrane, which is selective for one species.
2. The feed is a liquid but the permeate side is a vapour.
3. Reduce the pressure on the permeate side, which vaporises the molecule and create a driving force to drive the permeable molecular across the membrane.
4. The liquid mixture is in direct contact with one side of the membrane.

The mass flux is brought about by maintaining the downstream partial pressure lower than the saturation pressure
This is by vacuum on the permeate side.

Question 8

How is the pressure on the feed side calculated?

Answer 8

The pressure on the feed side can be calculated by the saturation pressure multiplied the fugacity coefficient, so can model it as a real gas with gas behaviour

Question 9

What is fugacity?

Answer 9

Fugacity is the measure of the escaping tendency of a substance in a homogenous system.
This allows us to approximate the pressure based on the concentration of the liquid.

Question 10

How is the pressure on the permeate side calculated?

Answer 10

The partial pressure on the permeate side is the just the total particle pressure multiplied by the mole fraction.

Question 11

The membrane operates by a…

Answer 11

solution diffusion mechanism

Question 12

What are the 8 advantages of pervaporation?

Answer 12

1. Cost and performance advantages for the separation of azeotropic mixtures and mixtures with close boiling
2. Minimises thermal degradation of heat sensitive compounds
3. Reduces energy consumption
4. No entrainer is required
5. small units can operate economically
6. does not use sorberts
7. Continuous and offers immediate recovery of solvents
8. High Degrees of flexibility regarding the feed mixtures that may be accomodated, throughput and final product qualities

Question 13

What are the 8 advantages of pervaporation?

Answer 13

1. Cost and performance advantages for the separation of azeotropic mixtures and mixtures with close boiling
2. Minimises thermal degradation of heat sensitive compounds
3. Reduces energy consumption
4. No entrainer is required
5. small units can operate economically
6. does not use sorberts
7. Continuous and offers immediate recovery of solvents
8. High degree of flexibility regarding the feed mixtures

Question 14

What are the two things the performance is determined on?

Answer 14

the degree of separation

the permeate rate

Question 15

The choice of membrane depends on…

Answer 15

the feed solution and the permeate

There should be selective affinity between the permeate and the membrane.

Question 16

When should I…
1. Use a hydrophilic polymer…

2. Use organophilic polymers…

NOTE: Inorganic membranes like zeolites are more difficult to use on a large scale.

Answer 16

1. Use a hydrophilic polymer to remove water from organic solvents.
2. Use organophilic polymers to removed organics from solutions.

NOTE: Inorganic membranes like zeolites are more difficult to use on a large scale.

Question 17

Concentration gradient is not the most ideal concept

Answer 17

as the definition of concentration varies with the phase of the material. Need to ensure that the definition of concentration is consistent. There is also the discontinuity on the interface. Better to describe the driving force by the chemical potential gradient.

Discontinuity on interface

Question 18

What is chemical potential?

And the benefit

Answer 18

Chemical potential is the direct driving force for the change in the number of molecules.
The benefit of chemical potential is that the driving forces using the same definition even in different phases and there is the continuity on the interface without having to worry about the equilibrium

Question 19

Can increase the chemical potential gradient by…

Answer 19

Increasing the temperature or reducing the pressure on the permeate side by applying a vacuum pump.

This increases (viscosity_B in liq - viscosity of B in vapour).

Question 20

What does selectivity reflect?

Answer 20

Selectivity reflects the performance of the membrane.

The selectivity is another important parameter, you define it based on the more permeate component.

Question 21

What happens when the membrane is polymetric? and the temp increases

Answer 21

The membrane is polymetric so as the temperature increase the polymers become more mobile allowing both water and ethanol (more free volume) can push through the membrane, so therefore the flux increases.

Although flux increases, the selectivity decreases. So, at higher temperature both ethanol and water can easily pass through, while at colder temperature only water will pass through the membrane.

Question 22

What happens when the water content is really high (>80%)?

Answer 22

When the water content is really high (80%) then the membrane gets saturated so there is a reduction of flux. Therefore, is a limit to water content as the membrane will undergo membrane swelling which will permit the ethanol to pass through the membrane easily, so as mass fraction of water increases you have a decrease of selectivity.

Question 23

When you operate at lower temperature…

Answer 23

When you operate at lower temperature… will give lower flux but high selectivity due to low free volume in the membrane structure.

The pressure will not impact the mass fraction in the permeate.

Question 24

Why do pervaporation?

Answer 24

Potential for energy saving due to selectivity

Not limited by the thermodynamic VLE

Efficient at even small to medium scales