New separation methods used in industries

Separation methods

1. New Separation Processes

2. The word membrane originates from the Latin word membrana which means a skin. A  membrane  is a  selective barrier  that permits the separation of certain species in a fluid by combination of sieving and sorption diffusion mechanism. Separation is achieved by  selectively passing (permeating)  one or more components of a stream through the membrane while  retarding  the passage of one or more other components. Membranes can selectively separate components over a wide range of particle sizes and molecular weights, from macromolecular materials such as starch and protein to monovalent ions. Membranes have gained an important place in chemical technology and are used in a broad range of applications.

3. Membrane processes are characterized by the fact that a feed stream is divided into 2 streams: retentate and permeate . 

4. The  retentate  is that part of the feed that  does not pass through  the membrane, while the  permeate  is that part of the feed that  does pass through  the membrane. The optional "sweep" is a gas or liquid that is used to help remove the permeate. The component(s) of interest in membrane separation is known as the  solute . The solute can be  retained  on the membrane and removed in the retentate or passed through the membrane in the permeate.

5. Membrane Casting : The first step is to : Decide the polymer to use Select the appropriate solvent Non-solvent may also be needed Dry the polymer in vacuum oven at appropriate temperature to remove moisture (drying may be done overnight) Measure the quantity of polymer and solvent needed Dissolved the predetermined quantity of polymer in the solvent and stir for appropriate time: at specified temperature, stirring speed etc. A viscous solution is prepared.

6. Once the polymer is completely dissolved. Cast the solution onto a glass plate. This can be done manually or automatically by using casting machine. Place another plate on top of it. The gap is of the order of ~0.25 mm (Generally used doctor’s blade). Give a one directional motion to top plate (by doctor’s blade). A thin film is produced on the bottom plate. Take out the bottom plate and place it in water bath, which release the film.

7. The process that follows after casting depends on the type of membrane we intend to prepare : either dense (symmetric) flat sheet membrane or Asymmetric flat sheet membrane. T hree processes that can be performed after casting: Dry phase inversion, Wet phase inversion Dry/wet phase inversion Dry/wet phase inversion may give integrally skinned asymmetric membrane with dense top layer and porous support. W et phase inversion may give porous symmetric membranes. D ry phase inversion may give dense symmetric membranes.

8. There are 3 different mechanisms by which membrane can perform separations: By having holes or pores which are of such a size that certain species can pass through and others cannot. This mechanism is called size exclusion. By selective retardation by the pores when the pore diameters are close to molecular sizes. This mechanism is called pore flow. By dissolution into the membrane, migration by molecular diffusion across the membrane, and re-emergence from the other side. This is called solution diffusion.

9. Membrane selection depends on a variety of factors : Composition of the feed solution Operating parameters Application type Separation goals Chain interactions, chain rigidity, functional group polarity, and stereoisomerism also need to be factored into polymer choice and organic membrane manufacturing Organic and inorganic membranes have their own advantages and disadvantages, it is important to determine what type of membrane or polymer is most suitable for the application. 

10. A majority of industrial membranes consist of synthetic or natural polymers; membranes with both types of polymers are known as organic membranes. Examples of synthetic polymers include P olytetrafluoroethylene (Teflon PTFE), Polyamide- imide (PAI), and P olyvinylidenedifluoride (PVDF) Polystyrene and polytetrafluoroethylene (Teflon/ PTFE). Natural polymers include rubber, wool, and cellulose. 

11. Membranes can also be made from other non-polymeric materials. Such membranes include  inorganic  membranes (for example metal, ceramic, carbon and zeolites ) and  liquid  membranes. In addition, recent developments had led to the introduction of the so-called  Hybrid Membranes  (or Mixed Matrix Membranes), where both organic and inorganic components are used. Another variation in membranes application is the  Bipolar Membranes  (BPM). where membranes of different ionic charge are " sandwiched " together. 

12. Artificial / Synthetic polymers are synthesized by the polymerization of a monomer or co-polymerization of 2 monomers. The resulting polymer (Polymerization) is categorized as having: A long, linear chain such as polyethylene A, branched chain, such as polysulfone or polybutadiene A, three dimensional highly cross-linked structure, such as phenol-formaldehyde. moderately cross-linked structure, such as butyl rubber

13. Linear-chained polymers are more soluble in organic solvents. They become pliable or moldable with temperature increase and are known as thermoplastic polymers. On the other hand, cross-linked polymers are almost insoluble in organic solvents. They do not soften with temperature increase and are known as thermosetting polymers. 

14. Polymer selection must be based on compatibility with membrane fabrication technology and intended application use. For example, the polymer may require a low affinity toward the permeate, while other times it may need to withstand harsh cleaning conditions due to membrane fouling. Chain interactions, chain rigidity, functional group polarity, and stereoisomerism also need to be factored into polymer choice and organic membrane manufacturing. 

15. Inorganic Membranes Inorganic membranes refer to membranes made of materials such as  ceramic, carbon, silica, zeolite , various oxides  (alumina, titania , zirconia ) and  metals  such as palladium, silver and their alloys. Inorganic membranes can be classified into 2 major categories based on its structure: Porous inorganic membranes and Dense (non-porous) inorganic membranes. Microporous inorganic membranes have 2 different structures: Symmetric and asymmetric; and include both amorphous and crystalline membranes. Microporous inorganic membranes can be obtained by coating of a porous support with a colloidal solution, called  sol . The sol can consist of either dense spherical particles (colloids of oxides such as Al 2 O 3 , SiO 2  or ZrO 2 ) or polymeric macromolecules.

16. Application of dense inorganic membranes is primarily for highly selective separation of gases such as hydrogen and oxygen. However, dense membranes have limited industrial application due to their low permeability compared to porous inorganic membranes. Therefore, today's commercial inorganic membrane market is dominated by porous membranes.

17. Advantages that inorganic membrane possesses are High thermal and chemical stability, Inertness to microbiological degradation, and Ease of cleaning after fouling compared to organic counterparts. However, inorganic membranes Tend to have higher capital costs due to specific thickness requirements needed to withstand pressure drop differences.

18. Metallic membranes are made from sintering metal powders such as tungsten , palladium or stainless steel and then depositing them onto a porous substrate. The main use of metallic membranes is for hydrogen separation with palladium (Pd) and its alloy being the primary choice of material, due to its high solubility and permeability for hydrogen. Palladium, however, is expensive. Alternative to palladium and less expensive are tantalum and vanadium, which are also quite permeable to hydrogen. Recent focus is on supported thin metallic membranes with thickness ranging from submicron to a few ten microns. The advantages include Reduced material costs, Improved mechanical strength and Possibly higher flux.

19. Another application is the use of these membranes to control the feed rate during partial oxidation reactions (e.g. addition of hydrogen). A major problem associated with metal membranes is the surface poisoning effects (e.g. by a carbon-containing source) which can be more significant for thin metal membranes.

20. Ceramic membranes consist of metal (Al or Ti) and non-metal (oxides, nitride, or carbide). They are generally used for highly acidic or basic environments due to inertness. T hey have the advantages of being chemically inert and stable at high temperatures. This stability makes ceramic microfiltration and ultrafiltration membranes particularly suitable for food, biotechnology and pharmaceutical applications in which membranes require repeated steam sterilization and chemical cleaning.

21. Ceramic membranes have also been proposed for gas separations. An example application of recent development is in the production and processing of syngas (synthetic gas - a mixture of hydrogen and carbon monoxide). The key part of the process involves the separation of oxygen from air in the form of ions to oxidize the methane. The downside of ceramic membranes is the high sensitivity to temperature gradient, which leads to membrane cracking. 

22. Zeolite membranes Zeolites are microporous crystalline alumina-silicate with a uniform pore size. Zeolites are used as catalysts or adsorbents in a form of micron or submicron-sized crystallites embedded in millimeter-sized granules. Zeolite membranes are used in highly-selective gas separation due to highly uniform pore size . This material also has a catalytic characteristic, which is beneficial for catalytic membrane reactor applications.

23. Few downsides of zeolite membranes include Relatively low gas flux and Thicker layer requirements to prevent cracks and pinholes.  Overcome: use thin layer supported on others. Other problem: Thermal effect of zeolites . The zeolite layer can exhibit negative thermal expansion, i.e. in the high temperature region the zeolite layer shrinks …. But the support continuously expands, resulting in thermal stress problems for the attachment of the zeolite layer to the support, as well as for the connection of the individual micro-crystals within the zeolite layer.

24. Types of motion of molecules through barrier 1) Permeation: a) Dissolution of permeating molecules in the membrane b) Diffusion of dissolved molecules c) Desorption of penetrant molecules to the downstream side. 2) Knudsen diffusion (d/λ < 0.2): Single gaseous molecules diffuse under rarefied conditions so that the mean free path is longer than the pore diameter. 3) Convection (d/λ > 20): Viscous flow through the pores of ultrafiltration and microfiltration.

25. Permeation mechanism Here we are talking about almost non porous membrane or pores are very small in case of reverse osmosis and nano filtration. The solute are getting dissolved in the dissolution steps, solute have getting dissolved in the membrane phase, and because of the concentration gradient, it will diffuse from upstream to permeate side (feed to the permeate side) through the membrane matrix then again, because of the concentration gradient in the permeate side the concentration is this nothing almost and it get dissolved in the permeate side these three steps including a in total they are called permeation mechanism.

26. Second one is, Knudsen diffusion (d/  <0.2) - d is the core diameter and  is the mean free path of the molecule. These are basically transport of gaseous species under rarefied condition. Third one is convection (d/ > 20) - if (d/ > 20) then pure convection will be taking place under the pressure gradient.

27. Permeation will be typical operation or typical mechanism transfer mechanism for reverse osmosis (RO) and nano filtration (NF) . Both diffusion and convection will be more or less predominant as we go towards the more pore size of the membrane /next relax pore size of the membrane. The mechanism is transfer from diffusion to the convection right for lower molecular, cut off ultra filtration membrane. For micro filtration membrane , convection is the only mechanism.

28. Transport mechanisms and performance parameters (Book Chapter) Sieving or size exclusion is the governing mechanism for MF or higher pore size UF membranes . Permeation , i.e., solute dissolution in membrane phase from feed, diffusion through the membrane and desorption in permeate is the governing mechanism of denser membranes, like, RO and NF. Performance parameters of any membrane based process are mainly permeability and retention . Permeability indicates how porous the membrane is and it is directly related to the throughput of the process.

29. Retention of membranes is an indicative of its selectivity. In fact, membranes with high permeability have lower selectivity. Therefore, there should be a judicious trade-off between permeability and retention of the membrane for a particular application. Hydrophilicity of membrane surface sometimes becomes important as it imparts the anti-fouling property to the surface, thereby lowering the membrane fouling and subsequently enhancing its life.

30. Generally, membranes have a pore size distribution and hence average pore size of membrane is denoted by the molecular weight of solute that is retained 90% by the membrane and this molecular weight is known as molecular weight cut off (MWCO). A membrane having a rating of 10000 MWCO means it retains solutes of molecular weight above 10000 Da and allows permeation of solutes having molecular weight less than that.

31. Modes of Membrane P rocesses Based on direction of flow field in the membrane flow channel, there are two modes of operation process : D ead E nd and C ross F low In dead end, the solution is pressurized over the membrane and permeation occurs.

32. On the other hand, in cross flow mode, pressurized feed is allowed to flow over the membrane surface tangentially and the permeate flows out through the membrane normal to the direction of retentate flow. Cross flow is advantageous as it imparts a shear on the membrane surface due to forced convection, thereby restricting the growth of the deposited layer of solutes, enhancing the throughput

33. Flow Pattern Advantages Disadvantages Dead end filtration Simple process set-up Laminar flow and most processes are batch type Low energy consumption Discontinuous concentrate discharge Low investment Risk of pore blocking and sensitivity to change in feed properties

34. Flow Pattern Advantages Disadvantages Cross flow filtration Turbulent flow More complex process layout Continuous concentrate discharge High energy consumption Control of cake-layer build up High investment cost

35. Membrane modules The practical equipment where the actual membrane based separation occurs is known as membrane modules. Housing of the membrane is known as membrane modules and these modules are generally expensive because they need to be leak-proof even at high operating pressure. The basic aim of development of these modules is to provide maximum filtration area in smaller volume, so that the design becomes compact, space saving and the permeate flux i.e., the productivity of the system is maximum.

36. Commonly used membrane modules are Plate and Frame, Hollow Fiber Module, Spiral Wound Module and Tubular Module. Suitable module is used for separation or clarification purpose based on the operating conditions and process parameters

37. Characteristics Plate and Frame Spiral wound Tubular Hollow fiber pH range 4-7 4-7 4-7 4-7 Mechanical resistance Good Good Poor Poor Packing density (m 2 /m 3 ) 200 to 400 (Moderate) 300 to 1000 (Moderate) 100 to 300 (low) 1000 to 10000 (High) Power consumption based on membrane area Good Good Poor Good Energy consumption Medium Medium High Low Membrane replacement cost Low Moderate High Low Hold up volume Medium Low High Low Cleaning in place Fair Fair Excellent Good Other comments Dead spots Mesh spacer creates dead spots to flow Can handle high solid content, high resistance to pH. Cannot withstand high pressure Major applications RO, PV, UF, MF RO, UF, MF RO, UF RO, MF, UF, NF

38. Driving Forces in Membrane Separation Processes Separation in membrane is the result of differences in the transport rates of chemical species through it. Transport rate is determined by the driving force acting on individual components, their mobility, concentration of solute in membrane phase, etc. Mobility : Depending on solute size and structure of membrane. Concentration : Chemical compatibility of solute & interface material.

39. Categorization of various membrane based processes Among the different membrane separation techniques, pressure-driven processes are simplest in terms of their ability to separate particulates in liquid and gas feed streams according to size. Through utilizing pressure as a driving force for separation, with a membrane acting as a semipermeable barrier, pressure-driven processes are also associated with higher flux compared to their thermal and concentration-based separation counterparts.

40. Types of pressure-driven membrane separation techniques are categorized according to membrane pore size, which, in turn, dictates the degree of separation achieved. These categories are Microfiltration (MF), Ultrafiltration (UF), Nanofiltration (NF), and Reverse osmosis (RO).  

41. Reverse Osmosis Reverse osmosis (RO) membranes contain the smallest pores of the pressure-driven membrane processes and are capable of retaining all dissolved particles within a feed stream, including monovalent ions. This degree of separation results in a permeate consisting of a pure solvent, which, in many cases, is water. Separation using RO is accomplished not only through size exclusion but utilizes a diffusive mechanism as well.

42. Pore size is very small (2-10A ), therefore, it will be used for separation of very low molecular size material (we are going to separate small solute particles which will be having a molecular weight typically less than 100, that means various types of salts, e.g.: Sodium chloride have molecular weight 58.5). Since the pore size is very small, the osmotic pressure will become predominant. As osmotic pressure has two characteristics: It is directly proportional to the concentration (that’s why it is known as the colligative property) and inversely proportional to the molecular weight.

43. Therefore, if we encounter solute which is having very low molecular weight, then osmotic pressure become very high (and for higher molecular solute, osmotic pressure becomes low, it is not very important). Therefore, in this case, we have to apply pressure in the feed side to overcome the osmotic pressure. Then only the first step of permeate coming in the other in the downstream side. So, pressure requirement in reverse osmosis becomes highest . Pressure requirement is in the order of 25-40 atmosphere. The most common applications for RO are in the preparation of drinking water and beverage concentration.

44. Nano Filtration Both size and charge play a role in nanofiltration (NF) separation processes (in contrast to MF and UF, in which solutes are separated according to size). P ore size are slightly higher than RO. With a average pore size between 5 - 20A , NF membranes are capable of retaining low molecular weight, uncharged solutes, such as sugars and other organic molecules. S ince the pore size is higher, therefore, we can separate the particles of higher molecular weight (in the range of 200-1000).

45. As the pore size is higher and the molecular weight of the particle to be separated is higher, therefore, pressure requirements will be slightly lower in this case. I t causes a partial retention of salts NF membranes also retain charged species, such as polyvalent ions and large monovalent ions, whereas smaller monovalent species pass through. Applications for NF membranes range from theremoval of natural organic matter in wastewater treatment, hardness reduction in water purification, and whey demineralization in dairy processing. 

46. T he applications of nano filtration are dyes separation: dye molecules have the molecular weight in the range of 200-900. These are various dye solution / dyes, which is having molecular weight in these ranges. T he small low molecular weight organics like, polyphenols having a typical molecular weight between 400-600 and they can be separated by the nano filtration completely so, we can select appropriate cut off or characterized nano filtration membrane and can separate the polyphenols , dye. Therefore, nano filtration has tremendous application in the in treatment of the textile effluent. So, it can separate out the dyes.

47. Ultra Filtration Within the family of pressure-driven membrane processes, ultrafiltration (UF) lies between microfiltration and nanofiltration in terms of pore size, which can range from 20 - 100A . Molecular weight of solutes that is separated will be in the range of 1000-10 5 . Since, we are talking about the higher pore sized and separation of higher molecular solute, the pressure requirements will be less and it will be 6-8 atmosphere. Transport mechanism is, a mixture of convection (main mechanism) as diffusion.

48. It has a wide variety of application, e.g., separation of high molecular weight protein. This size range allows for the concentration of high molecular weight proteins, macromolecules, and other small, suspended solids. In contrast to MF, UF membranes are categorized with respect to their molecular weight cutoff , i.e., their ability to retain a molecule of a given size, rather than by the size of their pores. Nevertheless, the pore size range of UF membranes makes them well-suited for use in a wide variety of ultrafiltration applications across multiple industries.

49. In the automotive industry, UF is used in the recovery of undeposited paint for reuse in the electrocoating process. In the food and beverage industries, it is used in applications ranging from the concentration of whey protein to the clarification of fruit juices.  Protein separation purification or fractionation blood, red blood cells, polymeric solution separation on purification of polymeric solution, it can be done under ultra filtration process.

50. Membrane can be utilized for separation / purification / fractionation. All the purpose can be solved. One can separated out particular solute, one can purified a particular solute by separating, and one can fractionated. Suppose, we are having 2 solutes, (Let say molecular weight 60,000 and another solute having molecular weight 7000) then we can select a particular membrane. Let say, 40,000 or 30,000 cut off that will retain in the higher molecular solute, in the upstream side and it will allow lower molecular solute in the downstream side, so, it can be utilized for the case of fractionation.

51. Micro Filtration Microfiltration (MF) lies on the upper end of the spectrum of pressure-driven membrane techniques, with membranes containing the largest pore size of the aforementioned processes. Pore size is very high (in the order of more than 1000 A ), 0.1  m, 0.2  m, 1  m, 2  m like that, and molecular weight of solutes to be separated is greater than 1 lakh . Therefore, pressure requirement is lower (2 to 4 atm ). Ex: filtration of clay solution, latex, paint etc. As the pore size of the membrane becomes higher, the pressure requirement is going to be lower and lower. Therefore, we do not require very high pressure for effect.

52. As we are talking about the low pore size membrane, then the osmotic pressure becomes pretty important and becomes very high. It is often used as a precursor step to downstream filtration applications in order to achieve the desired degree of separation within a given feed stream. Due to the larger pore size of MF membranes, many of these processes are capable of being run at lower pressures than those with membranes containing smaller pores. Common MF applications involve the separation of large macromolecules in clarification steps, such as in the removal of bacteria from cellular broths and in fat removal processes in the dairy industry. 

53. Osmotic Pressure (  ) Suppose we are having a chamber separate by a semi permeable barrier (between two solutes, it will selective to a particular species, i.e., it will allows water, but it will not allows salt) Let say some volume of water in both chambers. One is solution side (add some salt here) and another is solvent (pure water) side. Then the solvent (water) activity is less in the solution chamber and more in the solvent side, therefore, water will be transported from the solvent to the solution side, because driving force of chemical potential gradient.

54. So, after some time, the equilibrium will be taking place, (it may be after 24 hours, may be occur 36 hour) finally, the level of water under solvent side will go down and level of water in the solution side will go up that will calls a hydrostatic development of a hydrostatic head ( gh ) and this is nothing but the osmotic pressure . As osmotic pressure is colligative property ( Colligative property means, any property means, any property that will depend on the amount of solute present in the system). It mean, if we increase the concentration of the salt in the solution side, the concentration difference will be higher, so water activity difference of activity higher, so more water will be permeating from the solvent side to the solution side.

55. In that case, the hydrostatic pressure ( gh ) between the final equilibrium position in the solution side and solvent side will be more, so osmotic pressure develop will be more. Therefore, osmotic pressure is directly proportional to the concentration of the solute and inversely proportional to the molecular weight of the solute. Therefore, for solute having lower molecular weight, the osmotic pressure will be very high. Therefore, for dilute solution  = RTC / M ; this is known as Vant Hoff relation. This is for the monovalent salt . The relation for the divalent salt :  = (  + -  - ) RTC / M, i.e., for CaCl 2 these basically valence,  + is 2 and Cl - is 1.

56. Note Any solution have its own osmotic pressure, for example, if we have a glass of saline solution, or sugar solution, it will be having its own osmotic pressure, but we cannot realize the osmotic pressure, because osmotic pressure can be realized if one only if, semi permeable barrier is present in the solution. Otherwise, we cannot realize the osmotic pressure. That’s why, whenever we are talking about membrane base separation process, since, a semi permeable barrier is present in the solution itself. The osmotic pressure becomes very important.

57. When we drink a glass of saline water, it is having osmotic pressure, but before drinking it we cannot feel it, but when it goes inside, the vessels etc, in the body are basically semi permeable barrier; so, it creates an increasing blood pressure therefore, the doctor also, advise do not take saline water or decrease the intake of salt, because it will increase osmotic pressure in the blood vessels and it may ruptured.

58. Observed and Real Retention Observed retention indicates the selectivity of the membrane, i.e., how much solute it can retain. It indicates extent of separation. This is defined as where C p is concentration of solute in permeate and C is solute concentration in the feed. So, this gives directly the extent of separation. We know the concentration of feed solution, as we can experimentally measure it. Why it is known as observed retention : because the permeates concentration or the permeate stream is compared with the feed stream that’s why it is called observed retention.

59. Therefore, there exists another counter part of observed retention, i.e., real retention . Difference between the observed retention and real retention where C m is the solute concentration on membrane surface in feed side. Membrane surface concentration will be always higher than the feed concentration, because it is pressure driven process. Under pressure solutes will be convective towards the membrane surface and they will be return by the membrane. So, concentration gradient starting from C up to C m near the membrane surface and C m is always greater than C . Therefore, R r > R . As C < C m.

60. Dialysis Dialysis is a separation technique that relies on selective diffusion of molecules across a semi-permeable membrane to separate molecules based on size. In the feed side, a specific set of solutes are permeated through the membrane (which contains pores of a manufactured size-range) to the other side. The upstream feed is known as the feed side and the downstream is known as the dialysate . Typically, dialysate stream is pure distilled water. Thus, the concentration gradient between the two streams is the maximum.

61. The transport is effected by the concentration gradient between two streams. The duration of separation entirely depends on the rate of the solutes through the membrane. Sample molecules that are larger than the pores are retained on the sample side of the membrane, but small molecules pass through the membrane, reducing the concentration of those molecules in the sample. Alternatively, desired components in the external buffer solution can be slowly brought into the sample. Dialysis is used for a wide variety of applications: desalting, buffer exchange, removal of labeling reagents, drug binding studies, cell growth and feeding, virus purification, and blood treatment. An example is removal of urea, creatinin from blood stream.

62. Electro Dialysis (ED) Electro Dialysis (ED) is a membrane process, during which ions are transported through semi permeable membrane, under the influence of an electric potential. T h e membranes are cation - or anion-selective, which basically means that either positive ions or negative ions will flow through. Cation -selective membranes are polyelectrolytes with negatively charged matter , which rejects negatively charged ions and allows positively charged ions to flow through.

63. By placing multiple membranes in a row, which alternately allow positively or negatively charged ions to flow through, the ions can be removed from wastewater. In some columns concentration of ions will take place and in other columns ions will be removed. The concentrated saltwater flow is circulated until it has reached a value that enables precipitation. At this point the flow is discharged. T his technique can be applied to remove ions from water. Particles that do not carry an electrical charge are not removed.

64. Cation -selective membranes consist of sulphonated polystyrene, while anion-selective membranes consist of polystyrene with quaternary ammonia. Sometimes pre-treatment is necessary before the electro dialysis can take place. Suspended solids with a diameter that exceeds 10 µm need to be removed, or else they will plug the membrane pores. There are also substances that are able to neutralize a membrane, such as large organic anions, colloids, iron oxides and manganese oxide. These disturb the selective effect of the membrane. Pre-treatment methods, which aid the prevention of these effects are active carbon filtration (for organic matter), flocculation (for colloids) and filtration techniques.

65. Applications Desalination of salt water Stabilisation of wine Whey demineralisation Pharmaceutical application Pickling bath recycling

66. Concept of concentration polarization and membrane fouling Concentration Polarization : Accumulation of solute particles over the membrane surface is defined as concentration polarization. When pressure is applied to the feed side of a membrane during the filtration process, the solute is partially or totally retained by the membrane and will accumulate on the surface, while the solvent passes through the membrane more freely. Due to the membrane’s solute retention, the concentration of the solute in the permeate (C p ) is lower than the concentration in the bulk (C b ).

67. The concentration of the solute gradually increases on the surface of the membrane, due to solute accumulation from convective flow. At some point, the convective solute flow to the surface of the membrane will be balanced by the solute flux through the membrane and the diffusive flow from the membrane surface to the bulk. A concentration polarization profile, in which the concentration at the membrane surface (C m ) is typically higher than the C b , will be established in the boundary layer.

68. Effect of Concentration Polarization : Increase in osmotic pressure of the solution. Formation of gel over the membrane surface. Increases the viscosity of the solution. Solute enters into the pores and pores are blocked partially or completely. First phenomena decrease in driving force. Second and third increases the resistance against flux. Fourth decreases the membrane permeability. All these effects lead to decrease in permeate flux. Concentration polarization cannot be avoided, it can only be minimized.

69. Fouling of membrane : Fouling of membrane is of two types reversible and irreversible. Reversible Fouling : It can be washed away by adopting an appropriate cleaning protocol, like membrane washing. After cleaning, membrane permeability is restored. Concentration polarization is reversible fouling. Irreversible Fouling : In this case, membrane pores are blocked permanently and they cannot be removed, even after proper washing. Permeability is lost permanently.

70. Other Types Fouling I : Biofouling - Biofouling is a term for an undesirable accumulation of microorganisms on the membrane surface. May be caused by algae growth stimulated by light, by microorganisms embedded in the membrane ( Biofilms ) or module or even by sulphate reduction by anaerobic bacteria present in raw waters and eventually causes possible degradation of membrane material Particulate fouling is the build-up of particulates such as suspended solids, colloids and microorganisms on the membrane

71. Fouling II : Organic fouling : occurs by the chemical or physical adsorption of organic compounds on to the membrane, which may be followed by the formation of a cake or gel layer Scaling : It is the term for agglomeration of particles (salts) on the membrane, which ends up in a total blockage of the filtration process. This negative effect can occur during nanofiltration or reverse osmosis.

72. Factors Affecting Membrane Fouling Physicochemical properties of the membrane, e.g. hydrophobicity , electrostatic charge, reactive groups Physicochemical properties of the solute, like molecular weight, electrostatic charge, hydrophobicity The physicochemical parameters of the feed solution, e.g., pH, solute concentration Membrane morphology, i.e. pore size, pore shape, etc. Operating parameters, e.g. TMP, permeate flux, system Hydrodynamics, etc. Concentration polarization Membrane operation history

73. Fouling control Prevention of fouling by pre-treatment of feed water Optimize nutrient limitation techniques( Biofilms ) Periodic cleaning (e.g. Backwashing, anti-fouling-agents) Optimization of filtration operating conditions Improve cleaning efficiency

74. Industrial Membrane-Separation Processes (Applications) 1. Reverse osmosis Desalinization of brackish water Treatment of wastewater to remove a wide variety of impurities Treatment of surface and groundwater Concentration of foodstuffs Removal of alcohol from beer 2. Dialysis Separation of nickel sulfate from sulfuric acid Hemodialysis (removal of waste metabolites and excess body water, and restoration of electrolyte balance in blood)

75. 3. Electrodialysis Production of table salt from seawater Concentration of brines from reverse osmosis Treatment of wastewaters from electroplating Demineralization of cheese whey Production of ultra-pure water for the semiconductor industry 4. Microfiltration Sterilization of liquids, gases, and parenteral drugs Clarification and biological stabilization of beverages Bacterial cell harvest and purification of antibiotics Recovery of mammalian cells from cell culture broth

76. 5. Ultrafiltration Preconcentration of milk before making cheese Clarification of fruit juice Purification of recombinant proteins and DNA, antigens, and antibiotics from clarified cell broths Color removal from Kraft black liquor in papermaking 6. Pervaporation Dehydration of ethanol–water azeotrope Removal of water from organic solvents Removal of organics from water

77. 7. Gas permeation Separation of CO 2 or H 2 from methane Separation of uranium isotopes Adjustment of the H 2 /CO ratio in synthesis gas Separation of air into nitrogen- and oxygen-enriched streams Recovery of helium Recovery of methane from biogas 8. Liquid membranes Recovery of zinc from wastewater in the viscose fiber industry Recovery of nickel from electroplating solutions

78. Characterization of Membranes Membrane processes can cover a wide range of separation problems with a specific membrane being required for every problem. Membranes may differ significantly in their structure and consequently in their functionality. To know what membrane to use in a particular separation process, different membranes must be characterized in terms of structure and mass transport properties. Because very different membranes are used, different techniques are required for characterization.

79. Membrane characterization is a very important part of membrane research and development because the design of membrane processes and systems depends on reliable data relating to membrane properties. Characterization of  Porous membranes 2. Characterization of  Dense, Homogeneous membranes 3. Characterization of  Charged membranes

80. Characterization of Porous Membranes Porous micro- or ultrafiltration membranes are generally characterized in terms of their trans-membrane flux, pore size, pore size distribution, and molecular mass cut-off. Electron Microscopy : The structure of porous membranes can be determined by electron microscopy. Scanning Electron Microscopy (SEM) gives an especially clear picture of membrane structure and requires minimum sample preparation; however, resolution is limited to about 50-nm. Higher resolution can be obtained with Transmission Electron Microscopy (TEM), but sample preparation is significantly more complex and the structure is not nearly as clear as that obtained by scanning electron microscopy.

81. Bubble-Point Test The " bubble-point " test is a simple method for determining the maximum pore size of a membrane. One side of the membrane is filled with liquid such as water, although i-propanol is often used as the standard liquid. If the other side of the membrane is exposed to air at a certain pressure, air bubbles will penetrate through the pores of the membrane when the radius of the air bubble is equal to the radius of the pore. The pressure needed to penetrate a pore is inversely proportional to pore size. Thus, penetration occurs first through the largest pores.

82. When the pressure is increased further, pores with smaller diameters are also penetrated. Because the surface tension between water and air is rather high, high pressure is required for the determination of small pores. In practice, pore sizes between 0.1 and 10 mm are determined by hydrostatic pressures of 1500 - 15 kPa . The main application of the bubble-point test is to determine pinholes and leaks in micro- and ultrafiltration membranes and modules.

83.   Filtration Tests In filtration tests, trans-membrane flux and membrane solute retention are determined. To avoid the influence of concentration polarization or any other boundary layer phenomenon, the trans-membrane flux is generally measured as a function of applied hydrostatic pressure with ultra-pure water. Membrane flux often decreases with time during the filtration test due to compaction of the membrane structure under pressure. A compaction factor has been defined, which is determined from the slope of the curve obtained when the trans-membrane flux at constant pressure is plotted versus time on a semi-logarithmic scale.

84. Unfortunately, the fluxes measured with ultra-pure water, at least in ultra- and microfiltration, often bear little relation to those obtained with solutions containing macromolecules or suspended particles.

85. C ommon characterization of membrane and instrument used to measure those characteristics Contact angle measurement by Goniometer Zeta potential measurement by Zeta sizer Surface roughness by Atomic Force Microscopy (AFM) Structure of porous membrane by Scanning electron microscopy (SEM) or Transmission electron microscopy (TEM) Chemical Force Microscopy (CFM)

86. Identification and quantification of components (functional group) present by FTIR (Fourier Transform Infrared Spectroscopy) Pore size and surface area of membrane by BET Analyzer ( Brunauer -Emmett-Teller) Porosity measurement by Porometer

87. Effect of Contact angle on the nature of membrane ( Hydrophilicity or Hydrophobicity )

88. 17-Nov-16 Registration Seminar 100

89. Driving forces for transport : Gradient of electro chemical potential of species is the driving force of transport of species. This gradient may be caused by pressure difference, concentration, temperature or electrical potential between two phases separated by the membrane. (a) Passive Transport : In such cases, the upstream chemical potential of a component is more than that in the downstream

90. In this case also the chemical potential of a species in upstream is more than that in the downstream. (b) Facilitated transport However, as shown in Figure, components to be transported are coupled with a carrier in the membrane phase. So, it is a special form of passive transport and very selective and at the same time, the transport is facilitated by the carrier component.

91. (c) Active Transport : As shown in Figure, components are transported against driving force. Driving force for transport is provided by the activation energy of chemical reaction in the membrane phase (Living Cell). In this case, the upstream chemical potential is higher than that of downstream potential.

92. Test Link:        http://test.thegatecoach.com   

93. Permeation will be typical operation or typical mechanism transfer mechanism for reverse osmosis (RO) and nano filtration (NF) .

94. https://www.slideshare.net/pallem1986/membrane-separation-processe http://www.thermopedia.com/content/948/