Lecture 10

Concentration driven processes:

• Dialysis

• Haemodialysis
• Pervaporaion
• Liquid membrane

Liquid membrane

• A liquid membrane (LM) is literally a membrane made of liquid.

• It consists of a liquid phase (e.g. a thin oil film) existing either in supported or
unsupported form that serves as a membrane barrier between two phases of aqueous
solutions or gas mixtures.

• One of the benefits of using a liquid membrane is that LMs are highly selective, and,
with the use of carriers for the transport mechanism, specific molecular recognition can
be achieved.

• LMs are relatively high in efficiency, and as such, are being looked into for industrial
applications.

• The major problem restricting widespread application is stability: liquid membranes

require stability in order to be effective, and if they are pushed out of the pores or
ruptured in some way due to pressure differentials or turbulence, then they just do not
work.

Types of LM

• Bulk liquid membrane (BLM),

• Emulsion Liquid Membrane (ELM)

• Immobilized Liquid Membrane (ILM), also called a Supported Liquid Membrane.

Bulk liquid membrane (BLM)

• Bulk liquid membrane (BLM) consists of a bulk aqueous feed and receiving phases
separated by a bulk organic which is water-immiscible liquid phase.

1
 • The phases may be separating by microporous supports which separate the feed and
receiving phases from the LM or module configuration may be without microporous
supports (layered BLM).

• Many of the LM subject reviewers considered only layered BLM and testified its
transport and selectivity inefficiency to be a potential for the practical application.

Immobilized Liquid Membrane (ILM)

• An ILM is much simpler to visualize. It is made of some kind of rigid polymer

membrane, with lots of microscopic pores in it which are filled with organic liquid.

• In the liquid are the carriers that perform the required separation.

• The ILM takes things from one side of the rigid membrane (the source phase) and carries
it to the other side (the receiving phase) through this liquid phase.

• Liquid impregnated (or immobilized) in the pores of a thin microporous solid support is
defined as a supported liquid membrane (SLM or ILM).

• The SLM may be fabricated in different geometries.

• Flat sheet SLM is useful for research, but the surface area to volume ratio is too low for
industrial applications.

• Spiral-wound and hollow-fiber SLMs have much higher surface areas of the LM modules
(103 and 104 m2/m3, respectively).

• The main problem of SLM technology is the stability: the chemical stability of the
carrier, the mechanical stability of porous support, etc

• In such a system, instability is caused by the removal of carrier or organic liquid in the
pores of that supporting membrane.

• There are two possible ways for this to occur:

• carrier or solvent evaporation

• a large pressure differential across the membrane that effectively pushes the fluid out

2
 •

Applications:
• Metal separation-concentration
• Biotechnological products recovery-separation

• Pharmaceutical products recovery-separation (4) Organic compounds separation, organic

pollutants recovery from wastewaters
• Gas separations

• Fermentation or enzymatic conversion-recovery-separation (bioreactors)

• Analytical applications

• Wastewater treatment including biodegradation-separation techniques

3
Pervaporation:
• Pervaporation (or pervaporative separation) is a processing method for the separation of
mixtures of liquids by partial vaporization through a non-porous or porous membrane.
• The term pervaporation is derived from the two steps of the process:
(a) permeation through the membrane by the permeate, then
– (b) its evaporation into the vapor phase.
• The membrane acts as a selective barrier between the two phases: the liquid-phase feed
and the vapor-phase permeate.
• It allows the desired component(s) of the liquid feed to transfer through it
by vaporization.
• Separation of components is based on a difference in transport rate of individual
components through the membrane.
• Typically, the upstream side of the membrane is at ambient pressure and the downstream
side is under vacuum to allow the evaporation of the selective component after
permeation through the membrane.
• Driving force for the separation is the difference in the partial pressures of the
components on the two sides and not the volatility difference of the components in the
feed
• The driving force for transport of different components is provided by a chemical
potential difference between the liquid feed/retentate and vapor permeate at each side of
the membrane. The retentate is the remainder of the feed leaving the membrane feed
chamber, which is not permeated through the membrane.

Applications:
• Solvent Dehydration: dehydrating the ethanol/water and isopropanol/water azeotropes
• Continuous ethanol removal from yeast fermenters.

• Continuous water removal from condensation reactions such as esterification to enhance

conversion and rate of the reaction.
• Membrane introduction mass spectrometry
• Removing organic solvents from industrial waste waters.
• Combination of distillation and pervaporation/vapour permeation

4
 • Concentration of hydrophobic flavor compounds in aqueous solutions (using
hydrophobic membranes)

Electrodialysis:

5
Dialysis:

The principle of dialysis

• Diffusion is the random, thermal movement of molecules in solution (Brownian motion)

that leads to the net movement of molecules from an area of higher concentration to a
lower concentration until equilibrium is reached.

• Dialysis is a procedure employed in a number of cases when a change in the

concentration or composition of solutes is necessary.

• In the biochemical practice, dialysis is often used to alter the concentration of salts and/or
small molecules in protein solutions—usually aimed at decreasing the concentration of
these solutes.

• However, the composition of the solution can also be changed in additional ways.

• Dialysis is based on diffusion during which the mobility of solute particles between two liquid
spaces is restricted, mostly according to their size.

• Size restriction is achieved by using a porous material, usually a semi-permeable

membrane called dialysis membrane.

•
• Dialyser: The dialysis unit is called as

• Dialysate: The feed side liquid leaving the unit is called

• Diffusate: The liquid leaving the permeate side is called

6
 • Dialysis: Dialysis is an operation to separate dissolved molecules based on molecular
weight.

• In practice, a biological sample is placed inside a tube of semi permeable membrane, and
placed inside a much bigger container.

• Dialysis membrane Concentrated solution Buffer.

• Only small molecules diffuse through the colloid ion membrane.

• At equilibrium, the concentration of small molecules is the same inside and outside the
membrane.

• Macromolecules remain in the bag.

•
Applications and limitation of Dialysis

• Removal of salts and low molecular weight compounds. Buffer exchange.

• Concentration of macromolecules.

• Purification of biotechnological products.

• Medical applications: kidney dialysis and Haemodialysis

Daily Application:

• kidney -blood's toxins and waste products-

• Kidney failure-release of nitrogenous containing waste products (urea and creatine) –

azotemia -causes metabolic acidosis leading to illness. Solutes -potassium and calcium -
Sodium Bicarbonate added to neutralize

7
Advantage of dialysis

• Dialysis is still in use today for it is very simple and is still the only way to deal
with large-volume samples.

• Characterization of a candidate drug in serum binding assays or detailed study of

antigen-antibody interactions

• Proves to be the most accurate method available.

• inexpensive and easy to perform

Disadvantage of dialysis

• Slow process several hours for completion, and thus, has been replaced by gel
filtration for most applications. Other forms of dialysis includes flow-dialysis and
pressure-dialysis

Theoretical principle:

Haemodialysis:

• DEFINITION

– A medical procedure to remove fluid and waste products or low molecular

compounds from the blood and to correct electrolyte imbalances.

8
 – This is accomplished using a machine and a dialyzer, also referred to as an
"artificial kidney

Application:

• Absorption of O2 and desorption of CO2 in lungs

• Removal of urea, creatinine, uric acid from blood

Construction:

Working principle:

9
10
Applications:

• Desalination of salt water

• Stabilisation of wine

• Whey demineralisation

• Pharmaceutical application

11