*Membrane SeparationByNitish GuptaChemical Engineering 3rd year Meerut Institute of Engineering and Technology

*1. OverviewMembrane separation process is a feed consisting of a mixture of two or more components are partially separated by means of semi permeable barrier (the membrane) through which one or more species move faster than another or other species.

*Part of the feed that does not pass through membrane, i.e. retained is called Retentate.Part of the feed that passes through the membrane is called permeate.

*2. Why Membrane SeparationSaves large amount of energy.Long life membrane.Defect free.More compactMore easily operated, controlled and maintained.

*3.Desirable attributesGood permeability.High selectivity.Chemical and Mechanical compatibility with the processing environment.Ability to withstand large pressure differences across the membrane thickness.

4. Membrane materialsAll the membrane materials are made from synthetic and natural polymer.These polymers can be formed by addition, condensation or by the copolymerization processes.Thermoplastic polymer membranes can be used for the temperature upto glass transition temperature.Thermosetting polymers never melts.*

Membranes made of glassy polymers can operate below or above the glass transition temperature.Aromatic polyamides (also called aramids) are high melting crystalline polymers that have better long term thermal stability and higher resistance to solvents than do aliphatic polyamide, such as nylon.Polycarbonate differs from most other polymers because they can be extruded into various shape,films and sheets.

*

*4. Permeance factorTo be effective for separating a mixture of chemical component a polymer membrane should have high permeance and a high permeance ratio.The permeance of the given species diffusing through membrane of given thickness is analogous to mass transfer coefficient. i.e. flow rate of that species per unit cross sectional area of membrane per unit driving force ( pressure, concentration etc.)

*The membrane which consist of thin dense skin about .1 to 1 micrometer in thick is called perm selective layer. Ni = Pm / lm (driving force)where,Pm is Permeability lm is Membrane thickness.

*5. Transport in membranesIt is important to understand the nature of transport phenomenon in membranes because both the permeance and driving force depends on the mechanism of transport.Membranes can be macro porous, micro porous and dense ( non-porous).

*5 (a) Porous MembranesIf the pore diameter is larger as compared to the molecular diameter and a pressure differences exists across the membrane, bulk or convective flow through the pores occurs. Such a flow is generally undesirable because it is not perm selective and therefore no separation between component and feed occurs. Contd.

*If fugacity, activity, concentration differences exists across the membrane but the pressure is same on the both side, perm selective diffusion of the component through the pores will takes place.

Contd.

*

5 (b) Non porous MembranesIn this gas or liquid components absorbs into the membrane at the upstream face diffusion through the solid membrane and desorbs at the face.Diffusivities of solid is less than gas as well as liquid.water vapor in air is .25water in ethanol liquid is 1.2*10-5 water in cellulose acetate is 1.0*10-8*

6. Some Industrial Application of Membrane SeparationDialysisGas permeationReverse osmosisPervaporation

*

DIALYSISDialysis is membrane processes that use pressure difference as the driving force.It is of great importance in hemodialysis, in which urea, creatine, uric acid, phosphate and chlorides are removed from blood without removing essential higher molecular weight compounds and blood cells. This dialysis is called as artificial kidney.*

*

REVERSE OSMOSISIt refers to the passage of solvent such as water, through a dense membrane i.e. permeable to the solvent, but not the solute.It was first experimented by Nollet with water, an alcohol and an animal bladder.Reverse osmosis is applied to the desalination and purification of sea water, brackish water and waste water.*

*

PERVAPORATION pervaporation is the combination of two words perm selective and evaporation.Here vaporization may occur near the downstream face of the membrane, such that the membrane can considered to operate with two zones, a liquid phase zone and a vapor phase zone as shown in the figure.*

*Contd.

It is used for the removal of volatile organic compounds ( VOCs).Dehydration of ethanol.Removal of organics from water.It is best applied when the heat solution is dilute in the permeant because sensible heat of feed mixture provides the enthalpy of vaporization to permeate. *

Thank you*