Principles - Membrane Applications

Question 1

What’s the flux and selectivity of an ideal membrane like?

Answer 1

High flux and high selectivity

Question 2

What determines the performance/transport rate of a component through a membrane?

Answer 2

–The structure of the membrane
–The size of the component
–The chemical nature of the component
–The electrical charge of the membrane material
–The chemical composition of the membrane material
–The driving force applied (i.e. pressure)

The performance of the membrane is determined by the hydrophobicity, charge and roughness of the membrane, as well as the size and number of its pores

Question 3

How do membranes split liquids?

Answer 3

Into retentate (concentrated) and permeate (low conc’)

Question 4

What’s flux?

Answer 4

The amount of liquid permeate passing through the membrane, per unit area, per unit time.

• Usually expressed in litres m-2 hr-1 ( LMH in the literature)
• Alternatively, expressed as m s-1 (a superficial velocity) through the membrane).

Question 5

How’s membrane rejection calculated?

Answer 5

R = 1 - Cp/Cf

Question 6

How is VCR (volume concentration ratio) calculated?

Answer 6

VCR = Initial feed volume / retentate volume

Question 7

How does transmembrane pressure affect flux?

Answer 7

Flux starts off in pressure dependant region.

Increasing the pressure results in a decline in the rate of increase of flux.

Pressure independent region is quickly reached - increases in pressure do not increase the flux, and almost certainly lead to fouling. This is the ‘limiting flux’ value.

Increases in the limiting flux can be brought about by increasing the cross flow velocity, or decreasing the viscosity (by reducing the feed concentration, or by increasing the feed temperature.

Question 8

When is batch filtration used?

Answer 8

Batch filtration is commonly used in lab and pilot scales

• It is the fastest method of concentrating a given amount of material, and will require the smallest membrane area
• Used when the permeate is the required product and the retentate is usually waste (as it is damaged by long hold up times and shear)

Question 9

When is ‘batch filtration with partial retentate recycle’ used?

Answer 9

Used when a continuous feed stream needs to be processed.

• When feed is added continuously at the same rate as the flux, it is called ‘batch with recirculating loop, topped off’
• Also done when permeate is the required product (eg for fruit juice production).
• The additional cross flow velocity of the retentate helps reduce concentration polarisation and fouling

Question 10

What occurs in single pass continuous filtration?

Answer 10

This involves no recycling of retentate.

• A single pass results in a very low residence time, and hence a very low recovery unless a very large membrane area is used
• Thus this type of operation is only used when concentration polarisation is not a factor, and high flow rates are not needed
• Typical applications are water treatment

There are no cycles

Question 11

What occurs in a Feed and Bleed filtration system?

Answer 11

This is the most common type of configuration used for continuous full scale operation

• The recycling of retentate gives the desired separation (concentration factor) by increasing the residence time in the system
• By varying the fraction or the retentate stream that is recycled per pass, the cross flow velocity can be controlled (reducing fouling and concentration polarisation)

Question 12

What are the main applications of membrane technology?

Answer 12

Electrocoating
Dairy industry
- filtration of milk
- ultrafiltration of cheese
Beverage production
- filtration of juice
Water treatment and purification

Question 13

Wha are the (3) main areas where membranes are used in fruit juice processing?

Answer 13

Clarification - the production of sparkling clear beverages using UF or MF

Concentration - using RO to produce concentrates above 42o Brix

Deacidification - using NF to reduce the acidity of citrus juices.