Membrane separation process

This pdf contain every important things about MSP

1. Membrane Separation Process Mass Transfer 2 B.Tech . 3 rd Year Instructor U. K. Arun Kumar

2. Introduction Membrane : is a thin barrier, placed between two phases or mediums , It allows one or more species to selectively pass from one medium to the other while retaining the rest . It is done by a driving force. Membranes used for separation of mixtures are called semi-permeable .

3. Classification - Membrane Separation Processes Driving force for MSP Pressure difference Concentration difference Pressure Driven Processes Reverse Osmosis (RO) Nanofiltration (NF) Ultrafiltration (UF) Microfiltration (MF)

4. Concentration Gradient Driven Membrane process : Dialysis Membrane Extraction Electrodialysis Pervaporation

5. MSP – Advantages & Disadvantages Advantages Inherently simple Moderate operating temperature for high value heat sensitive substances No change of phase occurs (except in pervaporation ) Same basic principles apply to most MSP

6. MSP - Disadvantages Membrane fouling and resultant flux decline Polymeric membranes have limited chemical resistance to organic solvents A high degree of separation may not be possible for many mixtures .

7. Desired properties of a membrane A good membrane should have Good permeability High selectivity Chemical stability and compatibility Mechanical strength Resistance to fouling and adsorption Amenability to casting of a thin film Suitability for fabrication of a module

8. Membranes classification

9. Polymeric membrane Dense or non porous membrane and Porous membrane Dense or Non porous Membrane Isotropic or symmetric membrane Asymmetric membrane Composite membrane Porous Membrane Asymmetric membrane Microporous membrane

10. Dense Membranes A dense or non-porous membrane is a thin film that allows selective passage of one or more components of a mixture. No pores are present in the physical sense Permeating compound - dissolves at the membrane surface Diffuses through the intermolecular space or free volume within membrane Leaves at the opposite surface as permeate or product Used – RO, gas separation process, pervoporation

11. A dense membrane may have an asymmetric or a composite structure . Asymmetric Membrane An asymmetric dense membrane has a thin dense or non-porous permselective layer on a porous substructure . The dense layer allows selective permeation (i.e. perm-selective) The porous substructure offers necessary mechanical strength to the membrane Allows high permeate flux

12. Symmetric or Isotropic Microporous Membrane Symmetric microporous membranes are functionally similar to conventional filters . In symmetric membrane the materials are same But differ in their pore size and thickness Filters – separate relatively coarse particles in suspension

13. Microporous membranes : separate very fine particles or colloids or even dissolved solutes Membranes are thinner than filters Microporous symmetric membranes have interconnected pores High porosity Used for microfiltration Pore size range from 0.1 – 10 μ m . Particles >10 μ m are rejected completely. Those smaller than <0.1 μ m smallest pore size freely pass through the pores.

14. Asymmetric Membranes Membrane should be as thin as possible for high permeation flux Should have reasonable mechanical strength and defect free. If membranes are too thin and mechanical weak – Difficult to handle Develop pin-holes Practically impossible to have membrane <20 μ m

15. To overcome this problem Asymmetric membranes are made to contain A thin permselective layer (0.1 to 1.0 μ m) Supported on a highly porous substructure The thin layer may be non-porous (RO membrane) or with very fine pores (UF membrane) Entire material is an integral part of same material

16. Composite Membrane Composite membranes are functionally similar to asymmetric membranes It contains A porous or dense, thin permselective upper layer Cast on a thick mechanically strong support. The thin layer and the support are made up of different materials

17. Electrically Charged Membranes They have ionic groups attached to the membrane Forming fixed charged sites. In PEM fuel cell, the Nafion membrane is used Perfluoro ion exchange membrane has SO 3 - groups on a PTFE backbone This group give negatively charged fixed sites The cation can move freely within the membrane matrix

18. If the membrane contains a negative group attached - conducts the cations freely The fixed charges repel the negative charges and does not allow anions . Hence the name cation exchange membrane If the fixed charges are positive , The membrane is called anion-exchange-membrane .

19. Pressure driven membrane processes for liquid separation Micro-Filtration Ultra-Filtration Nano -Filtration Reverse Osmosis

20. Microfiltration Microfiltration refers to separation of fine particles and colloids from a liquid or particulates from a gas It uses porous membrane having pore sizes range 100 to 10 4 nm or (0.1 to 10 μ m) . Traditionally used for separation of microorganisms . Separation of yeast from fermentation broth

21. 1G – MF membranes were made from nitrocellulose. Separation occurs by sieving mechanism low pressure difference about 2 bar . Two types of flow and filtration arrangements are – Cross flow filtration Dead-end filtration .

22. Cross-flow filtration. Feed flows parallel to membrane surface Most of particles or solute are swept away with the flowing feed liquid A part of liquid is re-circulated if required. Dead-end filtration Feed flow is normal to membrane surface Retained particles or solute form a cake on membrane surface. Cake growth offers filtration resistance

23. Applications 1. Making small-scale lab separations for microbiological analysis – of soft drinks, wines, pharmaceutical products. Sterilization and clarification in food and beverage, clarification of cheese etc. Harvesting of cells from a fermentation broth Detection and analysis of particulate contaminants in air.

24. Theoretical principles In MF, the objective is to concentrate a suspension - by forcing the liquid through the pores Calculating solvent flux in membrane module is essential for design of MF device The flux may be expressed by Darcys law K’ – is a constant

25. If the flow of the fluid through the pore is laminar , Hagen- Poiseulle equation can be used

26. Sometime the membrane structure resembles like a very thin packed bed (with void spaces) The pressure drop relation for such a membrane is give by Kozeny -Karman equation

27. Ultra Filtration (UF) Separation or concentration of a large molecular weight solute or a colloidal suspension Approximate mol. wt. range 1000 to 80,000 Dalton (unit of mass equivalent to H atom) Membrane pores size - range 1 to 100 nm . Driving force for UF is - Δ P Separation occurs by sieving mechanism . Separates larger molecules from smaller ones which pass through the membrane

28. Application of Ultra Filtration

29. Configuration of a UF unit Since a single membrane module does not provide large area, A number of modules are used in parallels depending on feed rate. Two configurations are used Recycle configuration Tapered configuration

30. Recycle configuration A part of the retentate is reclycled back to the inlet of the module To achieve higher concentration of rententate Increases cross flow velocity Reduces membrane fouling

31. Tapered Configuration Modules are arranged in a parallel-series pattern Retentate volume decreases after liquid passes through a module Therefore, a lesser number of modules are provided in successive stages.

32. Reverse Osmosis When aqueous solution is kept separated from water by a semi-permeable membrane in two compartment cell, Water diffuses through the membrane into higher concentration compartment . This is due to the difference in chemical potential of water in two compartments . Chemical potential of pure water is larger than water containing a dissolved solute .

33. When the level of solution is maintained at a certain elevated position, the flow of solute to higher concentration side stops . This condition is called osmotic equilibrium . The extra pressure due to the elevated level of solution is – osmotic pressure ( π ) The chemical potential of water in a solution increases if it is maintained at an elevated pressure .

34. When an extra pressure higher than the osmotic pressure is applied , the chemical potential of water in solution becomes larger than pure water . So that , water flows from solution to the pure water side . This is reverse osmosis . Driving force : difference in chemical potential of water on both sides. RO is most important technique for desalination of brackish (1000 – 5000 ppm salt)

35. Or sea water (35,000 ppm or 3.5 % salt) Commercial exploitation was not possible until the 1960s The development of high flux, asymmetric cellulose acetate membrane by (Loeb and Sourirajan , 1963) made it commercially possible. Over 12,500 industrial scale desalination plants are operating worldwide. With an average production of 23 milliion m 3 per day of drinking water

36. Models for water/solute transport in RO – Solution-Diffusion Model This model assumes that sorption of both the solute and the solvent occurs at the upstream section of the membrane Followed by diffusion through the non-porous and homogeneous permselective layer And exits the membrane at the other side of membrane

37. Diffusion flux of solvent is given by The change in chemical potential of the solvent is given by Substituting equation 2, in equation 1. gives

38. Δ μ m - is the difference in chemical potential across the homogeneous membrane layer of thickness, l m Expressing the above equation in terms of measurable quantities

39. The overall equation, after substitution becomes,

40. For the salt (flux), - i.e., solute flux Contribution of pressure towards chemical potential difference is negligible, So the salt flux is viewed as diffusive and is expressed as

41. Maximum rejection of NaCl is about 70 % Permeation flux: 0.1 to 1 m3/m2.day Application: Water reclamation Food industries – concentration of dextrose syrup Chemical: metal recovery, recovery of textile dyes, cleaning paper mill efflueents

42. Concentration Driven Process Dialysis - is diffusional transport of one or more dissolved species – through a thin permselective barrier. The membrane is place between two aqueous solutions at different concentrations Separation is by concentration driving force No bulk flow of solvent or solution through membrane

43. Since smaller molecules diffuse faster than larger ones, Dialysis separate such molecules from solution. NaOH (17-20 %) in the viscose liquor can be recovered as 9-10 % solutions by dialysis A dialysis unit is called dialyser . The feed-side liquid leaving the unit is called dialysate ( rententate )

44. And that leaving the permeate side is diffusate (permeate) A membrane that swells substantially in contact with the solvent is selected, Because the process involves diffusion through dense membranes Swelling of a polymer increases the spaces between the polymer chains and facilitates diffusional motion of molecules through it.

45. Applications In medical purposes- haemodialysis Dialysis offers a passive envirionment for transport solutions are not in direct contact No pressure is applied as in RO Many breweries use dialysis to reduce alchol content of beer

46. Electrodialysis Electrodialysis (ED) is a process of removal of salts from an aqueous solution By transport through an electrically charged membrane. Cell is divided into a number of compartments by placing pieces of a cation exchange membrane and an anion exchange membrane alternatively

47. At the two ends are placed a cathode and an anode connected to a DC power supply Example NaCl , Na + ions pass through the cation-exchange membrane that has a fixed negative charges But anions are rejected Anion exchange membrane allows Cl - ions to pass through . Thus the solution passing through the compartment are depleted of the salt.

48. The adjacent compartments get enriched in the salt. Desalinated water and concentrated brine leave from adjacent compartments. Applied electrical potential is the driving force of the process. It is widely used for desalination of brackish water as an alternative to RO.

49. Pervaporation Pervaporation combines - permeation of one or more species and its subsequent vaporization. Pervaporation : permeation + vaporization A component in the feed, that has large affinity for the membrane gets sorbed (dissolved) on the feed-side membrane The solute then – diffused through the membrane Vaporizes at the product side of the membrane

50. Vaporization occurs at the product side under vacuum The vapour product – having a target compound at a much larger conc. than in the feed is condensed and recovered as liquid. Pervaporation cannot be an alternative for the conventional processes Like distillation and liquid extraction The diffusional flux through a membrane is generally low

51. Also right membrane that would provide high flux and satisfactory separation factor is not always available. Industrial application: Preparation of absolute alcohol Dehydration of solvents

52. Salt and Light “ You are the salt of the earth . But if the salt loses its saltiness , how can it be made salty again ? It is no longer good for anything, except to be thrown out and trampled underfoot. You are the light of the world .  A town built on a hill cannot be hidden. Neither do people light a lamp and put it under a bowl . Instead they put it on its stand, and it gives light to everyone in the house