Adsorption (mass transfer operations 2) ppt

Chemical engineering subject Mass transfer operations 2.

1. Adsorption Rushil Bhatt​

2. contents Introduction ADSORPTION CLASSIFICATION ​

3. Introduction Molecular forces at the surface of a liquid are in a state of imbalance or unsaturation. The same is true of the surface of a solid, where the molecules or ions on the surface may not have all their forces satisfied by union with other particles. As a result of this unsaturation, solid and liquid surfaces tend to satisfy their residual forces by attracting and retaining onto their surfaces gases or dissolved substances with which they come in contact. This phenomenon of the concentration of a substance on the surface of a solid (or liquid) is called adsorption. Thus, the substance attracted to a surface is said to be the adsorbed phase or adsorbate, while the substance to which it is attached is the adsorbent . 3

4. Adsorption should be carefully distinguished from absorption, the later process being characterized by a substance not only being retained on a surface , but also passing through the surface to become distributed throughout the phase.

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7. 8 The study of adsorption of various gases (or vapors) on solid surfaces has revealed that the forces operative in adsorption are not the same in all cases. Two types of adsorption are generally recognized: “physical” or van der Waals adsorption and “chemical” or activated adsorption

8. 9 Physical adsorption (physisorption) is the result of intermolecular forces of attraction between molecules of the solid and the substance adsorbed. When, for example, the intermolecular attractive forces between a solid and a gas (or vapor) are greater than those existing between molecules of the gas itself, the gas will condense upon the surface of the solid even though its pressure may be lower than the vapor pressure corresponding to the prevailing temperature. The adsorbed substance does not penetrate within the crystal lattice of the solid and does not dissolve in it but remains entirely upon the surface.

9. 10 Should the solid, however, be highly porous, containing many fine capillaries, the adsorbed substance will penetrate these interstices if it “wets” the solid. The partial pressure of the adsorbed substance at equilibrium equals that of the contacting gas phase, and by lowering the pressure of the gas phase, or by raising the temperature, the adsorbed gas is readily removed or desorbed in unchanged form. Physical adsorption is characterized by low heats of adsorption (approximately 40 Btu/ lbmol of adsorbate) and by the fact that the adsorption equilibrium is both reversible and established rapidly. This latter point allows one to simplify calculations in most real-world adsorption applications

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11. difference Presentation title 12 Physisorption Chemisorption

12. Although it is probable that all solids adsorb gases (or vapors) to some extent, adsorption, as a rule, is not very pronounced unless an adsorbent possesses a large surface area for a given mass. For this reason, such adsorbents as silica gel, charcoals, and molecular sieves are particularly effective as adsorbing agents. These substances have a very porous structure and, with their large exposed surface, can take up appreciable volumes of various gases. The extent of adsorption can be increased further by “activating” the adsorbents in various ways. For example, wood charcoal can be “activated” by heating between 350 and 1000 C in a vacuum or in air, steam, and certain other gases to a point where the adsorption of carbon tetrachloride, (e.g., at 248C) can be increased from 0.011 g/g of charcoal to 1.48. The activation apparently involves a distilling out of hydrocarbon impurities and thereby leads to exposure of a larger free surface for possible adsorption.

13. 14 The amount of gas adsorbed by a solid depends on a host of factors, including the surface area of the adsorbent, the nature of the adsorbent and gas being adsorbed, the temperature, and the pressure of the gas. Since the adsorbent surface area cannot always be readily determined, common practice is to employ the adsorbent mass as a measure of the surface available and to express the amount of adsorption per unit mass of adsorbing agent used. A concept, which becomes especially important in determining adsorbent capacity, is that of “available” surface, i.e., surface area accessible to the adsorbate molecule. It is apparent from pore size distribution data that the major contribution to surface area is located in pores of molecular dimensions. It seems logical to assume that a molecule, because of steric effects, will not readily penetrate into a pore smaller than a certain minimum diameter—hence, the concept that molecules are screened out.

14. This minimum diameter is the so-called critical diameter and is a characteristic of the adsorbate and related to molecular size. Thus, for any molecule, the effective surface area for adsorption can exist only in pores that the molecule can enter. Presentation title 15

15. The extent of adsorption of a given situation is reached once equilibrium is established between the adsorbent and its contacting solution. In practice, adsorption performance is also strongly influenced by the mass transfer of the species between the solution and the adsorbent surfaces and the adsorption reaction rate. Adsorption is, therefore, an equilibrium-diffusion-reaction process. Presentation title 16

16. ADSORPTION AS A SORPTION PROCESS The basic operating principle of adsorption: the preferential concentration of species onto surfaces of adsorbing solids also operates in two other processes; namely, chromatography and ion exchange. Similar to most adsorption operations, chromatography operates in fixed-bed mode, but is devised for separating liquid mixtures through an intermittent feed of the solution to be separated, followed by the passage of an elution solution. In ion exchange, the solid substance used contains charged groups that interact with the charged ions present in the liquid solution. If one views adsorption as an exchange process involving a fictitious species, the equivalence between adsorption and ion exchange becomes obvious. Presentation title 17

17. Adsorption Versus Absorption Gas absorption is an operation in which a gas mixture is brought into contact with a liquid for the purpose of dissolving one or more components of the mixture into the liquid. Absorption, therefore, is a bulk phenomenon, and the extent of separation is limited by the solubilities of the gases involved. Adsorption is a surface phenomenon, and the extent of adsorption is limited by the relevant adsorption isotherm relationship. Absorption may be carried out by passing the gas and liquid streams through a packed column concurrently or counter-currently. The operation consists of two moving phases (gas and liquid) and a stationary phase (column packing), which provides the interfacial area for liquid/gas contact. In fixed-bed adsorption, the fluid to be treated passes through a bed packed with adsorbent. The process involves two phases, a moving fluid and a stationary solid phase of adsorbents. Absorption, therefore, may be treated as a steady-state process, while adsorption in a fixed-bed operation is an inherently nonsteady state. As a result, the computational effort required for the design of fixed-bed adsorption is more extensive than that of absorption. Presentation title 18

18. Adsorption Versus Distillation Distillation also belongs to the equilibration-diffusion category of separation processes, and is used for the separation of homogeneous liquid mixtures. However, unlike adsorption or absorption, separation by distillation is effected by using energy instead of material as an agent of separation. In a hypothetical study comparing distillation versus adsorption, Ruthven (1984) showed that For separating an A-B mixture, the use of distillation becomes impractical if the relative volatility of A to B is less than 1.2. To separate light gas mixtures, adsorption was found to be preferential to cryogenic distillation, even when the relative volatility is high Presentation title 19

19. Adsorption Versus Deep-Bed Filtration Deep-bed filtration is a process designed for the removal of fine particles from diluted fluid suspensions. Its operation is carried out by passing the suspension to be treated through a column packed with granular or fibrous substances (filter media). Generally speaking, deep-bed filtration and fixed-bed adsorption share many common features, such as equipment configuration and modes of operation. Because of their similarities, the words ‘adsorption’ and ‘filtration’ are often used interchangeably. The removal of submicron colloidal particles from fluid to solid surfaces may be described as either filtration or deposition ( Hirtzel and Rajagopalan, 1985). Carbon columns used to remove dissolved organic solutes in water treatment are often referred to as ‘carbon filters’ by water engineers. Similarly, the term ‘charcoal filter’ is used to denote cartridges filled with granular activated carbon for personal protection Presentation title 20

20. A major difference between them resides in the fact that in deepbed filtration, removal of particles from the suspension to be treated results in particle deposition over the exterior surfaces of the filter media. In contrast, the adsorbed dissolved species in fixed-bed adsorption covers mainly the interior surfaces of adsorbents. The extent of separation achieved in adsorption is limited by the adsorption equilibrium relationship. On the other hand, particle retention in deep-bed filtration depends strongly upon the nature of particle-collector interaction forces, but there is no clear-cut limit on the extent of deposition (Tien and Ramarao, 2007). As adsorption processes may cease operation once the adsorbents become saturated, for deep-bed filtration, due to increasing particle retention, the pressure drops required for maintaining a specified throughput increase with time. The duration of operation is limited by the maximum allowable pressure drop. Presentation title 21

21. OPERATION MODES OF ADSORPTION PROCESSES Separation by adsorption is effected by contacting solutions to be treated with selected adsorbents. a) Adsorption in agitated vessels: Batch adsorption in agitated vessel represents perhaps the simplest way of bringing about fluid/adsorbent contact. A fixed amount of adsorbent of a known state is added to a volume of solution of a known solute concentration in a closed vessel. Agitation is provided by rotating stirrers in order to insure that adsorbent particles are fully suspended, and the adsorbate concentration is kept uniform throughout the solution. The data collected are the temporal evolution of the solute concentration of the solution. Not suitable for treating large volumes of solution (such as water supplies) Presentation title 22

22. B) Adsorption in continuous-flow tanks .: This type of operation is often used in waste water treatment. Adsorbents in powdered form, such as activated carbon, are added directly to a particular step of a treatment process (biological or physio-chemical) for the purpose of removing a particular species of contaminant. Presentation title 23

23. C) Fixed-bed adsorption consists of passing a solution through a column packed with adsorbents, and is commonly applied for eliminating trace contaminants from a liquid solution, or toxic and volatile vapors from gas streams. Fixed-bed adsorption, by nature, is a batch process. Starting with a fresh or newly regenerated state, adsorbents in fixed-bed operations become increasingly saturated with adsorbate to a point that reactivation (desorption) becomes necessary, and operation ceases. However, by employing a number of identical columns and arranging the adsorption/reactivation sequence properly, a continuous operation may be established. This, in fact, is the principle for the development of pressure swing and thermal swing processes for gas separation Presentation title 24

24. Moving-bed adsorption: It is carried out with both solid (adsorbent) and fluid phases in motion. The movements of both phases may be parallel, counter-current, or perpendicular (cross-flow); although the counter-current pattern is used in practice. Counter-current moving-bed adsorption is similar to gas absorption in packed beds. It is a steady-state process, and has the advantage of a more favorable utilization of the available driving force of mass transfer. However, maintaining a steady-state circulation of large quantities of solids provide serious problems in design, operation, and maintenance. Presentation title 25

25. In order to overcome this difficulty, a rather ingenious design based on the use of a rotation valve for directing and switching fluid flow and the addition of a desorbent , which enables the imitation of moving-bed behavior of fixed-bed adsorption, was developed. This simulated moving-bed (SMB) concept provides the basis for the development of the SORBEX processes for aromatic isomers’ separation Generally speaking, both moving-bed and simulated moving-bed are more costly in development, construction, and maintenance than fixed-bed; but offer more efficient separation. Presentation title 26

26. APPLICATIONS OF ADSORPTION Recorded application of charcoal for purifying water two millennia ago was found in Sanskrit manuscripts: “It is good to keep water in copper vessels to expose it in sunlight and to filter it through charcoal” Modern use of adsorbents began with the discovery of the decoloring effect of charcoal on solutions ( Lowitz , 1786), followed by the invention of a charcoal cartridge for personal protection during the First World War. With the increasing availability of different types of adsorbents nowadays, and the more recent interests in biotechnology and green technology, there has been a great expansion in the applications of adsorption in many areas Presentation title 27

27. For liquid-phase adsorption • Decoloring, drying, or degumming of petroleum products • Removing of dissolved organic species from water supplies • Removing odor, taste, and color from water supplies • Advanced treatment of waste water (domestic and industrial) • Decoloring of crude sugar syrup and vegetable oils • Recovery and concentration of proteins, pharmaceuticals, and bio-compounds from dilute suspensions • Bulk separation of paraffin and isoparaffins For gas-phase adsorption • Recovering organic solvent vapors • Dehydration of gases • Removing toxic agents and odor for personal protection • Air separation • Separating normal paraffins from isoparaffin aromatics • CO2 capture for addressing climate change Presentation title 28

28. Adsorbents Presentation title 29

29. Presentation title 30 Activated carbon Activated alumina Activated silica zeolites

30. Most solid substances possess some adsorption capacity for gas and/or liquid, but only a few of them meet the necessary requirements to qualify as adsorbents for practical use. Some of the most widely used adsorbents are: a. Activated carbon. It may be produced from carbonization of materials such as coconut shells, coal, lignite, wood, and similar substances, followed by activation with air or steam. It is used widely for recovery of organic vapor, decoloring liquid solutions, and treatment and purification of water supplies and waste water. b. Activated alumina. Al2O3 is prepared by removing water from colloid alumina. It has a high capacity for water vapor, and is used mainly as a desiccant for gasses and liquids. Activated alumina can be reactivated for reuse. c. Activated silica . Activated silica is prepared from gel precipitated by acid treatment of a sodium silicate solution. Activated silica is used primarily for dehydration of air and removing toxic species from air for personal protection purposes (i.e., in gas masks). Presentation title 31

31. d. Molecular sieve zeolites . Most adsorbents, including those mentioned herein, have pores covering a range of sizes. Molecular sieve zeolites, however, are an exception. Molecular sieve zeolites are crystalline inorganic polymers of aluminosilicate, alkali, or alkali-earth elements such as Na, K, and Ca. The “cages” of the crystal cells can entrap adsorbed matter, and the diameter of the passageways, controlled by the crystal composition, regulate the size of the molecules that can enter. These “size-selection” characteristics differentiate molecular sieve zeolites from other adsorbents and make molecular sieve zeolites particularly useful in the separation of hydrocarbon mixtures. e. Polymeric adsorbents . These adsorbents are prepared from polymerizing styrene and divinyl benzene or acrylic esters for adsorbing nonpolar organics from aqueous solutions or polar solutes, respectively. Generally speaking, polymeric adsorbents are more expensive than other adsorbents, and their use is, therefore, more selective. Presentation title 32

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33. The structural adsorbents are a class of adsorbents fabricated in forms of simple ordered geometry, such as monolith, laminate, or foam. Such adsorbents provide simple, identifiable flow channels, and offer low resistance to fluid flow. Compared with granular adsorbents, they also offer low resistance to mass transfer. These adsorbents are, therefore, of particular interest for applications in pressure swing adsorption. Presentation title 34

34. Similar to commonly used zeolites, metal organic frameworks (MOFs) and covalent organic frameworks (COFs) are porous crystalline solids. The metal organic frameworks are compounds consisting of metal ions or clusters coordinated to organic ligands to form one-, two-, and three-dimensional structures. Covalent organic frameworks are a new type of organic material constructed with organic building units via covalent bonds. This well-defined crystalline porous structure, together with tailored functionalities, are known for their unique features. Both MOFs and COFs supposedly have great potential for gas storage, gas adsorption, and as catalysts as well as sensors Presentation title 35

35. Activated Carbon Charcoal, an inefficient form of activated carbon, is obtained by the carbonization of wood. Various raw materials have been used in the preparation of adsorbent chars, resulting in the development of active carbon, a much more adsorbent form of charcoal. Industrial manufacture of activated carbon is obtained from shells or coal that is subjected to heat treatment in the absence of air, in the case of coal, followed by steam activation at high temperature. Other substances of a carbonaceous nature also used in the manufacture of active carbons include wood, coconut shells, peat, and fruit pits. Zinc chloride, magnesium chloride, calcium chloride, and phosphoric acid have also been used in place of steam as activating agents. 36 Gas adsorbent carbons find primary application in gas purification, solvent recovery (hydrocarbon vapor emissions), and pollution control.

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37. Activated Alumina Activated alumina (hydrated aluminum oxide) is produced by special heat treatment of the precipitate of native alumina or bauxite. It is available in either granular or pellet form Activated alumina is primarily used for the drying of gases and is particularly useful for the drying of gases under pressure. 38

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41. 42 difference A ctivated A lumina Activated alumina is a white spherical substance Because of its unique skeleton structure, it has a strong affinity with active components. The product has a uniform micropore distribution, a suitable pore size, and a large pore volume. High, low bulk density, good mechanical properties, and good stability. for the dehydration and drying of industrial gases such as chemical, metallurgy, electronics, petroleum, etc., such as air, oxygen, nitrogen and other gases, smelting gas and petroleum cracking gas, M olecular S ieve The molecular sieve has a large specific surface area of ​​300～1000m2/g, and the inner crystal surface is highly polarized. It is a kind of high efficiency adsorbent and a kind of solid acid. The surface has a high acid concentration and acid strength, which can cause Catalytic reaction. When the metal ions in the composition are exchanged with other ions in the solution, the pore size can be adjusted to change its adsorption and catalytic properties, thereby preparing molecular sieve catalysts with different properties. Because of strong moisture absorption capacity and is used for gas purification treatment. Molecular sieves that have been stored for a long time and have absorbed moisture should be regenerated before use. Molecular sieve avoids oil and liquid water. 42

42. PROPERTIES AND PHYSICAL CHARACTERISTICS OF ADSORBENTS 43 Adsorption Equilibrium Adsorbent Characteristics Presentation title The equilibrium relationship between the solution and the adsorbed phase, and the equilibrium data of specific adsorbent-adsorbate systems give the limit of the performance of adsorption operations. Adsorbent size Adsorbent density Adsorbent porosity Pore size and size distribution Specific surface area

43. Adsorption Equilibrium Presentation title 44

44. Adsorbent Characteristics Presentation title 45 Adsorbent size, dp .: Based on sieve analysis results. Rate of adsorbate uptake is often dependent on it Determine the pressure drop required to sustain a given throughput in a fixed-bed operation. Adsorbent density, ρp . Extent of adsorption is often expressed on the basis of unit adsorbent mass, ρp is a factor of determining the height of fixed-beds in design. A related quantity, ρb , is given as ρp (1- ε), where ε is the fixed-bed porosity Adsorbent porosity, εp . Indication of the internal structure of the adsorbent, gives the value of the fraction of the void space of an adsorbent pellet (or granule). An associated quantity of εp is the specific pore volume Pore size and size distribution Specific surface area, Sg (surface area per unit mass of adsorbent): critical importance to the adsorption capacity of an adsorbent and, to a lesser degree, the uptake rate of the adsorbate

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46. Presentation title 47 Some surrogate quantities (indices) have also been used to represent the adsorption capacity of adsorbents. The iodine number is a rough measure of the adsorption of small molecule components, and correlates with the BET surface area. It is defined as the milligrams of iodine adsorbed by one gram of material when the iodine residual concentration of the filtrate is 0.02N (0.01 mol L-1) according to ASTM D4607 standard, which is based on a three-point isotherm. The molasses number was developed for cane sugar decoloration , and relates to the adsorption of large molecules from liquid solutions. It is ( DSTM 16) is a measure of the  decolorizing capacity  and is an indicator of the macro pore and transport pore structure of activated carbon. An amount of pulverised carbon is mixed and heated with a standard molasses solution for a period of time. The  Molasses number  is expressed as the relative amount of colour removed by an activated carbon compared to a standard carbon. So the higher the molasses number, the better the product.

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48. ADSORBENT SELECTION 49 In principle, an adsorbent may be selected based on a number of factors, including the adsorbent’s adsorption capacity and selectivity, adsorbate uptake kinetics, regenerability (for nowadays, discarding spent adsorbents is no longer an option), compatibility with operating conditions, and not the least, cost. While some of the information can be found in the literature and publications, other information must be obtained from one’s own investigations, including experiments. The circumstances one faces may not allow a thorough investigation. Generally speaking, because of the large number of variables present, and their incomplete information, there exists no general criteria for adsorbent selection. Instead selection is often made on a case-by-case basis. The decision is dictated largely by the type of application, and based on technical considerations, in combination with subjective judgment and past experience.

49. ADSORBENT SELECTION 50 The moisture (water vapor) isotherm data of several common commercial adsorbents are shown . Comment on their use in gas drying

50. 51 3 stages as the adsorbate concentration increases. Firstly, a single layer of molecules builds up over the surface of the solid. This monolayer may be chemisorbed and associated with a change in free energy which is characteristic of the forces which hold it. As the fluid concentration is further increased, layers form by physical adsorption and the number of layers which form may be limited by the size of the pores. Finally, for adsorption from the gas phase, capillary condensation may occur in which capillaries become filled with condensed adsorbate, and its partial pressure reaches a critical value relative to the size of the pore. In practice, all three may be occuring simultaneously in different parts of the adsorbent since conditions are not uniform throughout. Generally, concentrations are higher at the outer surface of an adsorbent pellet than in the centre , at least until equilibrium conditions have been established. ADSORPTION

51. 52 Adsorption equilibrium is a dynamic concept achieved when the rate at which molecules adsorb on to a surface is equal to the rate at which they desorb Most theories have been developed for gas–solid systems because the gaseous state is better understood than the liquid. Statistical theories are being developed which should apply equally well to gas–solid and liquid–solid equilibria, though these are not yet at a stage when they can be applied easily and confidently to the design of equipment. Adsorption Equilibrium

52. 53 The capacity of an adsorbent for a particular adsorbate involves the interaction of three properties — the concentration C of the adsorbate in the fluid phase, the concentration Cs of the adsorbate in the solid phase and the temperature T of the system. If one of these properties is kept constant, the other two may be graphed to represent the equilibrium. The commonest practice is to keep the temperature constant and to plot C against Cs to give an adsorption isotherm. When Cs is kept constant, the plot of C against T is known as an adsorption isostere. In gas–solid systems, it is often convenient to express C as a pressure of adsorbate. Keeping the pressure constant and plotting Cs against T gives adsorption isobars.

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56. 57 Single component adsorption At very low concentrations the molecules adsorbed are widely spaced over the adsorbent surface so that one molecule has no influence on another. For these limiting conditions it is reasonable to assume that the concentration in one phase is proportional to the concentration in the other

57. Concentration corresponding to any point on the equilibrium curve may be obtained either by adsorption of solute by a fresh adsorbent or by desorption from an adsorbent having higher adsorbate concentration. In some cases, however, different equilibrium curves, at least over a part of the isotherm, are obtained depending upon whether the gas or vapour is adsorbed or desorbed. This phenomenon known as hysteresis E quilibrium partial pressure corresponding to a certain amount of solute concentration in the adsorbent is high in case of adsorption than that in case of desorption. Hysteresis may be due to the shape of the openings to the capillaries and pores of the adsorbent or due to nonuniform wetting 58

58. 59 Different types of adsorption isotherms are exhibited by different adsorbate-adsorbent combinations Moles adsorbed have been plotted as ordinate against (p′/p), p′ being the partial pressure of the sorbate and p is its vapour pressure

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64. Charcoal, with pores just a few molecules in diameter, almost always gives a Type I isotherm. A non-porous solid is likely to give a Type II isotherm. If the cohesive forces between adsorbate molecules are greater than the adhesive forces between adsorbate and adsorbent, a Type V isotherm is likely to be obtained for a porous adsorbent and a Type III isotherm for a non-porous one. In the activated aluminaair -water vapour system at normal temperature, the isotherm is found to be of Type IV. Presentation title 65

65. 66 Freundlich Isotherm The amount of gas adsorbed per given quantity of adsorbent increases relatively rapidly with pressure and then much more slowly as the surface becomes covered with gas molecules. To represent the variation of the amount of adsorption per unit area or unit mass with pressure, Freundlich proposed a equation Although the requirements of the equation are met satisfactorily at lower pressures, the experimental points curve away from the straight line at higher pressures, indicating that this equation does not have general applicability in reproducing adsorption of gases by solids. Freundlich isotherm is particularly useful in cases where the actual identity of the solute is not known as in the removal of coloured substances from mineral and vegetable oils or from sugar solutions. NE

66. 67 The Langmuir isotherm At higher gas phase concentrations, the number of molecules absorbed soon increases to the point at which further adsorption is hindered by lack of space on the adsorbent surface. The rate of adsorption then becomes proportional to the empty surface available, as well as to the fluid concentration. At the same time as molecules are adsorbing, other molecules will be desorbing if they have sufficient activation energy. At a fixed temperature, the rate of desorption will be proportional to the surface area occupied by adsorbate. When the rates of adsorption and desorption are equal, a dynamic equilibrium exists NE

67. 68 For adsorption of gases on to plane surfaces of glass, mica and platinum. As well as being limited to monolayer adsorption, the Langmuir equation assumes that: (a) these are no interactions between adjacent molecules on the surface. (b) the energy of adsorption is the same all over the surface. (c) molecules adsorb at fixed sites and do not migrate over the surface NE

68. When adsorption first begins, every molecule colliding with the surface may condense on it. However, as adsorption proceeds, only those molecules that occupy a part of the surface not already covered by adsorbed molecules may be expected to be adsorbed. The result is that the initial rate of condensation of molecules on a surface is high and then falls off as the surface area available for adsorption is decreased. On the other hand, a molecule adsorbed on a surface may, by thermal agitation, become detached from the surface and escape into the gas. The rate at which desorption will occur will depend, in turn, on the amount of surface covered by molecules and will increase as the surface becomes more fully saturated. These two rates, condensation (adsorption) and evaporation (desorption), will eventually become equal and when this happens, an adsorption equilibrium will be established Presentation title 69 NE

69. 70 Which vapor would be most readily adsorbed using activated carbon? H2O at 140 F CH4 at 70 F C4H10 at 140F C4H10 at 70 F C4H10 at 70 F NE

70. 71 The carbon dioxide adsorption isotherms for Columbia (“Columbia” is a registered trademark of Union Carbide Corporation) activated carbon is presented for temperatures of 30, 50, and 95 C. 1) Determine the constants of the Freundlich equation at a temperature of 50C. 2) Determine the constants of the Langmuir equation. NE

71. 72 For the Freundlich equation (1), the plot of (log Y ) vs (log p) leads to the equation: Y = 30p^0.7 NE

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73. 74 Braunauer -Emmet-Teller Isotherm The BET theory allows different numbers of adsorbed layers to build up on different parts of the surface, although it assumes that the net amount of surface which is empty or which is associated with a monolayer, bilayer and so on is constant for any particular equilibrium condition. Monolayers are created by adsorption on to empty surface and by desorption from bilayers. Monolayers are lost both through desorption and through the adsorption of additional layers. The rate of adsorption is proportional to the frequency with which molecules strike the surface and the area of that surface. NE

74. 75 Spherical particles of 15 nm diameter and density 2290 kg/m3 are pressed together to form a pellet. The following equilibrium data were obtained for the sorption of nitrogen at 77 K. P/P0 M3 liq N2 * 10^6/kg solid 0.1 66.7 0.2 75.2 0.3 83.9 0.4 93.4 0.5 108.4 0.6 130.0 0.7 150.2 0.8 202.0 0.9 348.0 Obtain estimates of the surface area of the pellet from the adsorption isotherm and compare the estimates with the geometric surface. The density of liquid nitrogen at 77 K is 808 kg/m3. Area occupied by one adsorbed molecule of nitrogen = 0.162 nm2 Avogadro Number = 6.02 × 10^26 molecules/ kmol NE

75. 76 The Gibbs isotherm and H-J Model Assume that adsorbed layers behave like liquid films, and that the adsorbed molecules are free to move over the surface. It is then possible to apply the equations of classical thermodynamics. The properties which determine the free energy of the film are pressure and temperature, the number of molecules contained and the area available to the film. The H–J model may be criticised because the comparison between an adsorbed film on a solid surface and a liquid film on a liquid surface does not stand detailed examination BET equation may be applied to low surface coverage when it is thought that adsorbed molecules do not move. The H–J equation may be applied to the middle range of relative pressures (P /P0) corresponding to the completion of a monolayer and the formation of additional mobile layers. Neither theory, it seems, is equipped to deal with the filling of micropores that occurs with the onset of capillary condensation at higher relative pressures NE

76. 77 The potential theory In this theory the adsorbed layers are considered to be contained in an adsorption space above the adsorbent surface. The space is composed of equipotential contours, the separation of the contours corresponding to a certain adsorbed volume The theory was postulated by POLANYI, who regarded the potential of a point in adsorption space as a measure of the work carried out by surface forces in bringing one mole of adsorbate to that point from infinity, or a point at such a distance from the surface that those forces exert no attraction. NE

77. 78 The work carried out depends on the phases involved. Polanyi considered three possibilities: (a) that the temperature of the system was well below the critical temperature of the adsorbate and the adsorbed phase could be regarded as liquid, (b) that the temperature was just below the critical temperature and the adsorbed phase was a mixture of vapour and liquid, (c) that the temperature was above the critical temperature and the adsorbed phase was a gas. Only the first possibility, the simplest and most common, is considered here The potential theory postulates a unique relationship between the adsorption potential εp and the volume of adsorbed phase contained between that equipotential surface and the solid. It is convenient to express the adsorbed volume as the corresponding volume in the gas phase from adsorption data for one gas, data for other gases on the same adsorbent may be found NE

78. Characteristic curve for the adsorption of carbon dioxide on to charcoal 79 NE

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80. 81 STRUCTURE OF ADSORBENTS The equilibrium capacity of an adsorbent for different molecules is one factor affecting its selectivity. Another is the structure of the system of pores which permeates the adsorbent This determines the size of molecules that can be admitted and the rate at which different molecules diffuse towards the surface. Molecular sieves, with their precise pore sizes, are uniquely capable of separating on the basis of molecular size. In addition, it is sometimes possible to exploit the different rates of diffusion of molecules to bring about their separation. A particularly important example concerns the production of oxygen and nitrogen from air . Although it is sometimes possible to view pores with an electron microscope and to obtain a measure of their diameter, it is difficult by this means to measure the distribution of sizes and impossible to measure the associated surface area. NE

81. 82 Surface area The simplest measure of surface area is that obtained by the so-called point B method. Many isotherms of Type II or IV show a straight section at intermediate relative pressures. The more pronounced the section, the more complete is the adsorbed monolayer before multilayer adsorption begins. The constant of proportionality may be found by using the same adsorbate on a solid of known surface area. Since the equation was derived for mobile layers and makes no provision for capillary condensation, it is most likely to fit data in the intermediate range of relative pressures NE

82. 83 Pore sizes Where there is a wide distribution of pore sizes and, possibly, quite separately developed pore systems, a mean size is not a sufficient measure. There are two methods of finding such distributions. In one a porosimeter is used, and in the other the hysteresis branch of an adsorption isotherm is utilised . Both require an understanding of the mechanism of capillary condensation. The regions of Type IV and Type V isotherms which are concave to the gas-concentration axis at high concentrations correspond to the bulk condensation of adsorbate in the pores of the adsorbent. An equation relating volume condensed to partial pressure of adsorbate and pore size, may be found by assuming transfer of adsorbate to occur in three stages: ( i ) transfer from the gas to a point above the meniscus in the capillary. (ii) condensation on the meniscus. (iii) transfer from a plane surface source of liquid adsorbate to the gas-phase in order to maintain the first stage NE

83. isotherm for nitrogen on activated alumina . NE

84. 85 KINETIC EFFECTS It was assumed that equilibrium was achieved instantly between the concentrations of adsorbate in the fluid and in the adsorbed phases. Whilst it may be useful to make this assumption because it leads to relatively straightforward solutions and shows the interrelationship between system parameters, it is seldom true in practice. In large-scale plant particularly, performance may fall well short of that predicted by the equilibrium theory. NE

85. 86 There are several resistances which may hinder the movement of a molecule of adsorbate from the bulk fluid outside a pellet to an adsorption site on its internal surface Some of these are sequential and have to be traversed in series, whilst others derive from possible parallel paths. In broad terms, a molecule, under the influence of concentration gradients, diffuses from the turbulent bulk fluid through a laminar boundary layer around a solid pellet to its external surface. It then diffuses, by various possible mechanisms, through the pores or the lattice vacancies in the pellet until it is held by an adsorption site. During desorption the process is reversed NE

86. 87 In the process of being adsorbed, molecules lose degrees of freedom and a heat of adsorption is released. All the resistances to mass transfer also affect the transfer of heat out of the pellet, though to a different extent. If the heat cannot disperse fast enough, the temperature of the adsorbent increases and the effective capacity of the equipment is reduced. NE

87. 88 a) Boundary film Steady-state film-theory: It is assumed that the difference in concentration and temperature between the bulk fluid and the external surface of a pellet is confined to a narrow laminar boundary-layer in which the possibility of accumulation of adsorbate or of heat is neglected. b) Intra-pellet effects After passing through the boundary layer, the molecules of adsorbate diffuse into the complex structure of the adsorbent pellet, which is composed of an intricate network of fine capillaries or interstitial vacancies in a solid lattice. c) Adsorption NE

88. 89 c) Adsorption The final stage in getting a molecule from the bulk phase outside a pellet on to the interior surface is the adsorption step itself. Where adsorption is physical in nature, this step is unlikely to affect the overall rate. An equilibrium state is likely to exist between adsorbate molecules immediately above a surface and those on it. NE

89. 90 When chemisorption is involved, or when some additional surface chemical reaction occurs, the process is more complicated. Since the adsorption process results in the net transfer of molecules from the gas to the adsorbed phase, it is accompanied by a bulk flow of fluid which keeps the total pressure constant. The effect is small and usually neglected. As adsorption proceeds, diffusing molecules may be denied access to parts of the internal surface because the pore system becomes blocked at critical points with condensate. NE

90. 91 Simplifying by selecting what is termed the controlling resistance. This relates to the step whose resistance is overwhelmingly large in comparison with that for the other layers. The whole of the difference in adsorbate concentration, from its value in the bulk gas to its value at the adsorbent surface, occurs across the controlling resistance. It may be, however, that rate control is mixed, involving two or more resistances.

91. 92 ADSORPTION EQUIPMENT The scale and complexity of an adsorption unit varies from a laboratory chromatographic column a few millimeters in diameter, as used for analysis, to a fluidised bed several metres in diameter, used for the recovery of solvent vapours , from a simple container in which an adsorbent and a liquid to be clarified are mixed, to a highly-automated moving-bed of solids in plug-flow. All such units have one feature in common in that in all cases the adsorbent becomes saturated as the operation proceeds. For continuous operation, the spent adsorbent must be removed and replaced periodically and, since it is usually an expensive commodity, it must be regenerated, and restored as far as possible to its original condition.

92. 93 In most systems, regeneration is carried out by heating the spent adsorbent in a suitable atmosphere. For some applications, regeneration at a reduced pressure without increasing the temperature is becoming increasingly common. The precise way in which adsorption and regeneration are achieved depends on the phases involved and the type of fluid–solid contacting employed.

93. 94 3 types of contacting: Those in which the adsorbent and containing vessel are fixed whilst the inlet and outlet positions for process and regenerating streams are moved when the adsorbent becomes saturated. The fixed bed adsorber is an example of this arrangement. If continuous operation is required, the unit must consist of at least two beds, one of which is on-line whilst the other is being regenerated. Those in which the containing vessel is fixed, though the adsorbent moves with respect to it. Fresh adsorbent is fed in and spent adsorbent removed for regeneration at such a rate as to confine the adsorption within the vessel. This type of arrangement includes fluidised beds and moving beds with solids in plug flow. Those in which the adsorbent is fixed relative to the containing vessel which moves relative to fixed inlet and outlet positions for process and regenerating fluids. The rotary-bed adsorber is an example of such a unit.

94. 95

95. 96 Dual stationary-bed solvent recovery system with auxiliaries for collecting the vapor–air mixture from various point sources, then transporting through the particulate filter and into the on-stream carbon adsorber—in this case, bed 1. The effluent air, which is virtually free of vapors, is usually vented outdoors. The lower carbon adsorber (number 2) is regenerated during the service time of bed 1. A steam generator or other source of steam is required. The effluent steam–solvent mixture from the adsorber is directed through the condenser and the liquified mixture then passes into the decanter and/or distillation column for separation of the solvent from the steam condensate.

96. 97 2 designs of a stationary-bed solvent recovery system The type employs vertical cylindrical bends wherein the solvent laden air flows axially down through the bed. This particular design is advertised for use in the recovery of solvents used in degreasing and dry cleaning, although equally well-suited for recovery of solvents from other industries.(trichloroethylene, tetrachloroethylene, toluene, freon TF, and dichloromethane. ) Regeneration is accomplished with steam flowing upward through the bed and, since the above-mentioned solvents are essentially immiscible in water, decantation is used to separate the condensed solvent from the steam condensate.

97. 98

98. 99 The valves are of disk type, opened and closed by air-driven pistons. Water is used as coolant in the condenser. Steam, electric power to drive the blower, and cooling water are three operating-cost items. The cost of steam and cooling water increases with frequency of regeneration. This particular unit is a Vic Model 572 with two adsorber beds used alternately, i.e., while one unit is on-stream adsorbing, the other is regenerating. Dual adsorber systems can also be operated with both beds on-stream simultaneously, especially when solvent concentrations are low. In this situation, regeneration is less frequent than a full work shift and may be accomplished during off-work hours. Operation in parallel almost doubles the air-handling capacity of the adsorption unit and may be an advantage in terms of operating cost

99. 100 At the inlet end of the bed, the adsorbent has become saturated and is in equilibrium with the adsorbate in the inlet fluid. At the exit end, the adsorbate content of the adsorbent is still at its initial value. In between, there is a reasonably well-defined mass-transfer zone in which the adsorbate concentration drops from the inlet to the exit value. This zone progresses through the bed as the run proceeds.

100. 101 At t1 the zone is fully formed, t2 is some intermediate time, and t3 is the breakpoint time tb at which the zone begins to leave the column. For efficient operation the run must be stopped just before the breakpoint If the run extends for too long, the breakpoint is exceeded and the effluent concentration rises sharply

101. 102

102. 103

103. 104

104. 105

105. 106

106. 107 The rotary component of the system consists of four coaxial cylinders. The outer cylinder is impervious to gas flow except at the slots near the left end. They serve as air inlet ports where they are shown uncovered and as steam–vapor outlet ports as shown at the lower left end of the rotary. The carbon bed is retained between two cylinders made of screen or perforated metal and also segmented by partitions placed radially between the two cylinders.

107. 108 The inner cylinder is again impervious to gas flow except at the slots near the right end of the rotary. These slots serve as outlet ports for the discharge air and as inlet ports for the regenerating steam. On each rotation of the rotary, each segment of the bed undergoes adsorption and regeneration. The desorbed solvent can then be separated from the steam by decantation or distillation. Because of the continuous regeneration capability of the rotary bed, more efficient utilization of the carbon is possible than with stationary-bed systems.

108. 109 In most solvent recovery operations, the adsorption zone and the saturated bed behind it are idle but add to the bulk of the system and increase airflow resistance through the bed. In deep beds of 12 to 36 inches, as required in the stationary-bed systems, a large portion of the bed is idle at any one time. By continuous regeneration, the regeneration time for each segment of the bed is shortened, and thereby shorter bed lengths can be used. Advantages: a more compact system and a reduced pressure drop. Disadvantages are those associated with wear on moving parts and maintaining seals in contact with moving parts. The use of shorter beds also decreases the steam utilization efficiency.

109. 110 Fluidized-bed solvent recovery system The carbon is recirculated continuously through the adsorption–regeneration cycle. The spent carbon, saturated with solvent, is elevated to the surge bin and then passed down into the regeneration bed where it is contacted with an upward flow of steam. The regenerated carbon is then metered into the adsorber, where the carbon traverses nine beds while fluidized with the upward flow of the vapor-laden air. An air velocity of approximately 240 ft/min is required to cause fluidization of the bed. In both the regeneration and adsorption phases, the carbon is moved countercurrent to the gas or vapor. The countercurrent movement increases the efficiency of regeneration (i.e., the steam :solvent ratio is less than for a stationary bed under otherwise comparable conditions). In addition to the beneficial effects of the countercurrent movement, the bed length can be increased to further improve steam utilization

110. 111

111. 112 Countercurrent flow also increases the effective use of the carbon ; more solvent can be recovered with less carbon than with stationary- or rotary-bed systems. By adjustments or balance of the carbon and solvent input rates, the total carbon in the adsorber can be made part of the adsorption zone reaching saturation in the lowest bed just before it is discharged into the elevator. Very little of the carbon is then idle; hence, maximum utilization is made of the carbon. The fact that the carbon has reached saturation when delivered to the regeneration phase is another factor in the reduced steam requirement; in this respect, the fluidized bed system is most favorable. In rotary-bed operations, the carbon moved into the regeneration phase has reached the lowest state of saturation in the three systems.

112. 113 When large air volumes are treated and available space for the installation is at a premium, the smaller size and lower initial cost are definite advantages over the stationary-bed system. A serious disadvantage is that of high attrition losses of the carbon caused by the fluidization of the beds. Because of the attrition, filtration of the effluent airstream may be required. The influent airstream need not be highly filtered because plugging of the fluidized beds with particulate matter is minimal. The major manufacturer of these units has given consideration to discontinue operations.

113. 114 No adsorber system could operate alone without sufficient auxiliary equipment and the components required to collect, transport, and filter vapor-laden airstreams being delivered to the adsorber. The ducts and piping need to be sized properly for required air velocities to optimize the efficiency of the adsorber. If the air velocity is too high, the stationary-bed adsorber may become a fluidized-bed adsorber, or low flows may create severe channeling through the beds. The fan is the catalyst for forcing the gas stream in and out of the unit, so it is important that careful attention to design and sizing is given to this equipment.

114. 115 Because there are several adsorber configurations, the location of a filter can be varied: before the inlet airstream (in a stationary bed) to reduce possible contamination to the adsorbent, after the fluidized-bed adsorber to reduce particulate emissions. Monitoring of the filter efficiency can be accomplished by measuring the pressure drop across the filter using either a manometer or pressure gauge.

115. DESIGN AND PERFORMANCE EQUATIONS Presentation title 116 116

116. 117 The gas stream containing the solute at an initial concentration C0 is passed down through a (deep) bed of adsorbent material that is free of any solute. Most of the solute is readily adsorbed by the top portion of the bed. The small amount of solute that is left is easily adsorbed in the remaining section of the bed. The effluent from the bottom of the bed is essentially solute-free, denoted as C1. After a period of time, the top layer of the adsorbent bed becomes saturated with solute. The majority of adsorption (approximately 95%) now occurs in a narrow portion of the bed directly below this saturation section.

117. 118 The narrow zone of adsorption is referred to as the mass transfer zone (MTZ). As additional solute vapors pass through the bed, the saturated section of the bed becomes larger and the MTZ moves further down the length of the adsorbent. The actual length of the MTZ remains fairly constant as it travels through the adsorbent bed. Additional adsorption occurs as the vapors pass through the “unused” portion of the bed. The effluent concentration at C2 remains essentially zero, since there is still an unsaturated section of the bed

118. 119 Finally, when the lower portion of the MTZ reaches the bottom of the bed, the concentration of solute in the effluent suddenly begins to rise. This is referred to as the breakthrough point (or breakpoint)—where untreated vapors are being exhausted from the adsorber. If the air stream is not switched to a fresh bed, the concentration of the solute in the outlet will quickly rise until it approaches the initial concentration It should be noted that in most air pollution control applications even trace amounts of contaminants in the effluent stream are undesirable. To achieve continuous operation, adsorbers must be either replaced or cycled from adsorption to desorption before breakthrough occurs. In desorption or regeneration, the adsorbers solute vapors are removed from the used bed in preparation for the next cycle.

119. 120 In regard to regenerable adsorption systems, three important terms are used to describe the equilibrium capacity (CAP) of the adsorbent bed. These capacity terms are measured in mass of vapor per mass of adsorbent. First, the breakthrough capacity (BC) is defined as the capacity of the bed at which unreacted vapors begin to be exhausted. The saturation capacity (SAT) is the maximum amount of vapor that can be adsorbed per unit mass of carbon (this is the capacity read from the adsorption isotherm) and thus is equal to CAP. The working capacity or working charge (WC) is the actual amount adsorbed in the adsorber. Thus, the working capacity is a certain fraction of the saturation capacity. Working capacities can range from 0.1 to 0.7 times the saturation capacity. (Note: A smaller capacity increases the amount of carbon required.) This fraction is set by the designer for individual systems by often balancing the cost of carbon and adsorber operation vs preventing breakthrough while allowing for an adequate cycle time. Another factor in determining the working capacity is that it is uneconomical to desorb all the vapors from the adsorber bed. The small amount of residual vapors left on the bed is referred to as the HEEL. This HEEL accounts for a large portion of the difference between the saturation capacity and the working capacity. In some cases, the working capacity can be estimated by assuming it to be equal to the saturation capacity minus the HEEL

120. 121 In regard to regenerable adsorption systems, three important terms are used to describe the equilibrium capacity (CAP) of the adsorbent bed. measured in mass of vapor per mass of adsorbent. First, the breakthrough capacity (BC) is defined as the capacity of the bed at which unreacted vapors begin to be exhausted. The saturation capacity (SAT) is the maximum amount of vapor that can be adsorbed per unit mass of carbon (this is the capacity read from the adsorption isotherm) and thus is equal to CAP. The working capacity or working charge (WC) is the actual amount adsorbed in the adsorber. Thus, the working capacity is a certain fraction of the saturation capacity. Working capacities can range from 0.1 to 0.7 times the saturation capacity. (Note: A smaller capacity increases the amount of carbon required.)

121. 122 Another factor in determining the working capacity is that it is uneconomical to desorb all the vapors from the adsorber bed. The small amount of residual vapors left on the bed is referred to as the HEEL. This HEEL accounts for a large portion of the difference between the saturation capacity and the working capacity. In some cases, the working capacity can be estimated by assuming it to be equal to the saturation capacity minus the HEEL

122. The determination of the state of the activated carbon (or any adsorbent) is done by monitoring the concentration of the adsorbate through the column.  At the top of the column the adsorbent is completely loaded, further down it is partially loaded, and towards the bottom it is unladen (contains no adsorbate). Presentation title 123 123

123. 124 Over time, as more water and adsorbate pass through the column, more of the adsorbent becomes loaded and the concentration profile changes At time  t 1  there is no adsorbate leaving the column (all gets adsorbed in the adsorbent).  However, at time  t 2  there is some adsorbent leaving the column with a concentration  C 2 , which increases to  C 3  at time  t 3 .

124. 125

125. 126 This outlet adsorbate concentration is extremely important in the operation of an adsorption process, as it often has a maximum allowable value imposed by process requirements or environmental regulations The evolution of this outlet concentration over time is called the  breakthrough curve  of the column.  It can be generated by collecting the data ( C 1 ,  C 2 , and  C 3 ) from the curves Alternatively, it is customary to rotate the concentration profiles by 90 degree counter-clockwise, plot them all in one graph and then use the outlet concentration to develop the  breakthrough curve

126. 127

127. From a breakthrough curve, the  time of breakthrough  can be calculated.  The  time of breakthrough  is the time t *  when the outlet concentration of the adsorbate reaches the acceptable maximum concentration C * Once that point is reached, the adsorption on this column must be stopped and the adsorbent regenerated, if possible, or replaced. Presentation title 128 128

128. 129

129. 130

130. 131 The region of falling concentration is called the mass transfer zone (MTZ). The gradient is called the adsorption wave front and is usually S-shaped. When its leading edge reaches the exit, breakthrough is said to have been attained. Practically, the breakthrough is not regarded as necessarily at zero concentration but at some low value such as 1% or 5% of the inlet concentration that is acceptable in the effluent. A hypothetical position, the average adsorbate content equals the saturation value, is called the stoichiometric front. The distance between this position and the exit of the bed is called the length of unused bed (LUB).

131. 132 The exhaustion time is attained when the effluent concentration becomes the same as that of the inlet, or some practical high percentage of it, such as 95 or 99%. The shape of the adsorption front, the width of the MTZ, and the profile of the effluent concentration depend on the nature of the adsorption isotherm and the rate of mass transfer. Practical bed depths may be expressed as multiples of MTZ, values of 5–10 multiples being economically feasible. Systems that have linear adsorption isotherms develop constant MTZs whereas MTZs of convex ones become narrower, and those of concave systems become wider as they progress through The narrower the MTZ, the greater the degree of utilization of the bed .

132. Factors that play important roles in dynamic adsorption, and the length and shape of the mass transfer zone are: ( i ) the type of the adsorbent (ii) the particle size of the adsorbent, which is dependent on maximum allowable pressure drop (iii) the depth of the adsorbent bed and the fluid velocity (iv) the temperature of the fluid stream and the adsorbent (v) the concentration of the contaminants to be removed and not to be removed (including moisture) (vi) the pressure of the system (vii) the removal efficiency required and (viii) possible decomposition or polymerization of contaminants on the adsorbent

133. The ideal adsorption time is Is by which the solute concentration in the fluid is reduced to 50% of the original value. The movement of the adsorption zone through the adsorbent bed and the effect of some process variables on the ideal adsorption time can be obtained in the following manner: The solute fed per unit time per unit area of bed cross section is given by

134. 135 Pressure swing adsorption (PSA) It is based on the principle that the adsorbing capacity of an adsorbent increases with increase in the solute pressure. In a pressure swing adsorption system, adsorption is carried out at an elevated pressure. The flow of the gas is stopped as the breakthrough point is reached and the adsorbent is regenerated by reducing the pressure. Sometimes, a hot inert gas is blown through the solid bed to carry away the desorbed solute molecules. Hot air or steams are frequently used for the purpose.

135. 136 A high dynamic adsorption capacity can be achieved in PSA by reducing the pressure to near vacuum and the PSA system is sometimes modified by putting a vacuum pump in the blow down line. This is called vacuum swing adsorption (VSA) or Vacuum pressure swing adsorption (VPSA). Sometimes in a single bed system, the adsorber may be rapidly pressurized and depressurized. It is called rapid pressure swing adsorption (RPSA).

136. 137 Some of the key industrial applications are as follows: ( i ) Gas drying (ii) Solvent vapour recovery (iii) Fractionation of air to produce oxygen and nitrogen enriched gases (iv) Production of hydrogen from steam-methane reformer (SMR) off-gas and petroleum refinery off-gases (v) Separation of methane and carbon dioxide from landfill gas (vi) Separation of carbon monoxide and hydrogen (vii) Normal isoparaffin separation (viii) Alcohol dehydration

137. 138 Reasons for PSA: An extra degree of thermodynamic freedom for describing the adsorption process introduces immense flexibility in the process design as compared with the other conventional separation processes such as distillation, extraction, absorption, etc. (ii) Numerous micro- meso porous families of adsorbents exhibiting different adsorptive properties for separation of gas mixtures are available. (iii) The adsorbate is recovered in a relatively concentrated form, and the energy cost is low since no heating is involved. (iv) The optimum combination between a material and a process in designing PSA separation scheme promotes innovation. (v) Different PSA process cycles.

138. 139 The RPSA processes are designed to increase the productivity (volume of product/volume of adsorbent/cycle) of separation devices by an order of magnitude or more by using total process cycle times of seconds instead of minutes as in the case of a conventional PSA cycles. It is achieved by simply running a conventional cycle faster using novel hardware or by changing both the adsorber and the process cycle designs. However, separation efficiency and performance may be compromised (lower product purity or recovery) due to time limitations in incorporating the complimentary cyclic steps and limitations caused by adsorption kinetics The example of the first type is the hydrogen purification, and that of the second type is production of oxygen enriched air from ambient air

139. 140 Skarstrom cycle In a single bed system, the output is not continuous as the bed is to be regenerated separately. However, in a twin bed adsorber the flow of output is kept continuous by operating the twin beds in tandem, one bed acts on adsorption mode while the other on regeneration mode. This is the basic Skarstrom cycle Each bed operates alternately in two half-cycles of equal duration: ( i ) adsorption followed by pressurization, and (ii) purging followed by depressurization or blow down

140. 141

141. 142 The feed gas is used for pressurization while a part of the effluent product gas is used for purge. Adsorption is taking place in bed 1 while a part of the gas leaving the bed 1 is routed to bed 2 to purge that bed in a direction counter-current to the direction of flow of the feed gas during the adsorption step. When moisture is to be removed from air, the dry air product is produced during the adsorption step in each of the beds. The adsorption and purge steps usually take less than 50% of the total cycle time.

142. 143 In many commercial applications of this technology, these two steps consume a much greater fraction of cycle time because pressurization and depressurization (blow down) can be completed rapidly. Therefore, cycle times for PSA and VSA are short, typically seconds to minutes. Thus, small beds have relatively large throughputs

143. 144 Since the introduction of the Skarstrom cycle, numerous improvements have been made to increase product purity, product recovery, adsorbent productivity and energy efficiency Among these modifications are the use of ( i ) three, four or more beds, (ii) a pressure equalization step in which both beds are equalized in pressure following purge of one bed and adsorption in the other, (iii) pretreatment or guard beds to remove strongly adsorbed components that might interfere with the separation of other components, (iv) purge with a strongly adsorbing gas, and (v) an extremely short cycle time to approach isothermal operation, if a longer cycle causes an undesirable increase in temperature during adsorption and an undesirable decrease in temperature during desorption.

144. 145 Separations by PSA and VSA are controlled by adsorption equilibrium or adsorption kinetics, where the latter refers to mass transfer external and/or internal to the adsorbent particle. Both types of control are important commercially. For the separation of air with zeolites, adsorption equilibrium is the controlling factor. Nitrogen is more strongly adsorbed than oxygen and argon. For air with 21% oxygen and 1% argon, oxygen of about 96% purity can be produced. When carbon molecular sieves are used, oxygen and nitrogen have almost the same adsorption isotherms, but effective diffusivity of oxygen is much larger than that of nitrogen. Consequently, nitrogen of very high purity (> 99%) can be produced.

145. 146 Regeneration of the Adsorbent Often it becomes necessary to regenerate the adsorbent and reuse the same. This is particularly necessary if the adsorbent is costly or the adsorbate is valuable and has to be recovered to the extent possible. There are several methods for regeneration of adsorbents of which the pressure swing and temperature swing methods are commonly used.

146. 147 Temperature Swing Regeneration An adsorbate loaded solid can be regenerated by passing a relatively hot inert gas like hot air or steam through the bed. The temperature of regeneration is decided on the basis of adsorption-desorption equilibrium or isotherm and the characteristics of the adsorbent.

147. 148 Feed stream containing a small quantity of an adsorbate at a partial pressure p′1 is passed through the adsorption bed at a temperature T1. The initial equilibrium loading on the adsorbent is X1, generally expressed in weight of adsorbate per unit weight of adsorbent. After equilibrium between the adsorbate and the adsorbent is reached, regeneration requires raising the temperature of the bed to T2. Desorption occurs as more feed is passed through the bed and a new equilibrium loading of X2 is established.

148. 149 The net removal capacity of the adsorption bed is given by the difference between X1 and X2. When the bed is again cooled to T1, purification of the gas stream can be started. Typical adsorbate removal in a temperature swing cycle is generally in excess of 1 kg/100 kg of adsorbent. Since the regeneration time associated with the temperature swing cycle is relatively long, the cycle is used almost exclusively to remove small concentrations of adsorbate from feed streams.

149. 150 This allows the on-stream time to be a significant fraction of the total cycle time of the separation process. In addition, temperature swing cycles may require a substantial quantity of energy per unit of adsorbate. However, if the adsorbate concentration in the feed is small, the energy cost per unit of feed processed can be reasonable.

150. 151 In the inert-purge cycle, the adsorbate during regeneration is removed by passing a nonadsorbing gas containing no adsorbate through the bed. This lowers the partial pressure of the adsorbate and initiates the desorption process. If sufficient pure purge gas is directed through the bed, the adsorbate will be completely removed and adsorption can be resumed. Cycle times, in contrast to temperature swing processes are often only a few minutes rather than hours. However, because adsorption capacity is reduced as adsorbent temperature rise, inert-purge processes are usually limited to 1 to 2 kg of adsorbate/100 kg of adsorbent.

151. 152 The displacement purge cycle differs from the inert-purge cycle since the cycle uses a gas or liquid in the regeneration step that adsorbs as strongly as the adsorbate. Thus, desorption is facilitated both by adsorbate partial pressure or concentration reduction in the fluid, and by competitive adsorption of the displacement medium. Typical cycle times are usually a few minutes. The use of two different types of purge fluid negatively affects the contamination level of the less adsorbed product. However, since the heat of adsorption of the displacement purge fluid is approximately equal to that of the adsorbate as the two exchange places on the adsorbent, the net heat generated or consumed is essentially negligible and thus maintains nearly isothermal conditions throughout the cycle.

152. 153 The latter condition makes it possible to increase the adsorbate loading of the displacement purge cycle over that for the inert-purge cycle One of the important considerations for a TSA system is the temperature of the adsorbent bed during regeneration, others being quantity and flow of the regenerating gas. The regeneration temperature can be estimated by a simple graphical procedure from the isosteres The isosteres are a set of curves which may be drawn from the isotherms by plotting the partial pressures or dew points or concentrations against the corresponding temperature or reciprocal of temperature for a equilibrium loading as parameter. The particular form of the parameters, viz. p′, log p′, T, or 1/T is chosen so as to make the isosteres almost linear for facilitating any interpolation.

153. 154

154. 155 Pressure Swing Regeneration In this method, the adsorbent bed is regenerated by reducing the pressure of the adsorbent bed to atmospheric or even lower pressure when the bed is stripped of the adsorbed solute. If required, the adsorbent bed may be further cleaned by passing hot air or steam through the bed. In the process, the partial pressure of an adsorbate can be reduced by lowering the total pressure of the gas. The lower the total pressure of the regeneration step, the greater is the adsorbate loading during the adsorption step

155. 156 The time required to load, depressurize, regenerate and repressurize a bed is generally only a few minutes. Thus, even though practical adsorbate loadings are almost always less than 1 kg of adsorbate per 100 kg of adsorbent, to minimize thermal gradient problems, the very short times makes the pressure swing cycle a viable option for bulk gas separations

156. 157 In the pressure swing process one can have 3-12 adsorber vessels, one or more can be used in adsorption step while others are used for various stages of regeneration. Polybed pressure swing adsorption process for commercial production of hydrogen with a purity of 99.99% by steam reforming of methane.

157. 158

158. 159 It is quite common in a pressure swing cycle to use a fraction of the less adsorbed gas product as a low pressure purge gas. Often the purge flow is in the opposite direction from that of the feed flow. The simplest version of this process is designated as PSA. Skarstrom has developed the rules for the minimum fraction of less adsorbed gas product required for displacing the adsorbate. Although the concept of PSA is very simple, the process in commercial application is fairly complex because it involves a multicolumn design where the adsorbers operate under a cyclic steady-state using a series of sequential nonisothermal , nonisobaric and nonsteady state steps. These include adsorption, desorption and a multitude of complementary steps which are designed to control the purity of product along with its recovery as well as to optimize the overall separation performance.

159. 160 Unique PSA cycles are also used to produce simultaneously two pure products from a multicomponent feed, e.g. oxygen and nitrogen from air, carbon dioxide and hydrogen from SMR off-gas, carbon dioxide and methane from landfill gas, and to produce a product containing a component which is not present in the feed, e.g. ammonia synthesis gas from SMR off-gas

160. 161 The RPSA process for hydrogen purification developed by Questar Industries, Canada utilizes the conventional Hydrogen-PSA process consisting of the following steps: Step 1: adsorption of feed gas over a packed bed of adsorbent at super-ambient pressure in order to produce a hydrogen-enriched gas Step 2: co-current depressurization of adsorber to produce additional amount of hydrogen-enriched gas for pressurizing an accompanied vessel Step 3: further co-current depressurization to produce another stream of hydrogen-enriched gas for counter-currently purging an accompanied vessel Step 4: counter-current depressurization to desorb a part of the feed-gas impurities Step 5: counter-current purge with hydrogen from Step 3 to further desorb the impurities Step 6: counter-current pressurization with hydrogen from Step 2, and Step 7: counter-current pressurization to feed pressure level with hydrogen from Step 1.

161. 162 The cycle runs rapidly using a total cycle time of ~1 min compared to cycle times of 10-30 minutes in a conventional process. Six parallel adsorbers packed with layers of activated carbons and zeolites are used in conjunction with two rotating flow control valves for introduction and removal of gases to and from the adsorbers. The process produces 99.95% pure hydrogen with a recovery of 67–73% from the feed gas at 8.5 atm pressure.

162. 163 The peculiarity of a novel adsorber design shows the use of two or more (multiples of two) layers of a zeolite adsorbent inside a single vessel separated by a perforated plate (similar to the arrangement of packings or trays in gas/ vapour -liquid contacting equipment). The RPSA process consists of the following steps: ( i ) simultaneous pressurization and adsorption of nitrogen from air by one layer of the adsorbent in order to produce the oxygen-enriched gas (ii) simultaneous depressurization and purging with oxygen product of the other adsorbent layer of the pair for its regeneration. Using 5A zeolite as the adsorbent and total cycle time of 10 s (compared to 60-240 s. for a conventional PSA-oxygen process) the process can produce 27.5% oxygen with recovery of 64.1% at feed air pressure of 2.22 atm

163. 164 High Temperature PSA Process A chemisorbent , Na2O supported on alumina, is used in a PSA process for direct removal and recovery of bulk carbon dioxide from a high temperature feed gas (wet) without pre-drying or cooling of the feed. The adsorbent reversibly chemisorbs carbon dioxide in presence of steam in the temperature range 200-300°C A gas stream at ~200°C saturated with water, is treated for simultaneous production of CO2 depleted stream and CO2 enriched stream without removing water. The PSA cycle consists of the following steps: Step 1: adsorption at ~125 psia Step 2: co-current CO2 rinse at ~125 psia with recycle of effluent gas as feed Step 3: counter-current evacuation to ~2.5 psia with simultaneous steam purge at feed gas temperature and Step 4: counter-current pressurization with a part of CO2 depleted product gas from Step 1.

164. 165 The effluent gas from Step 3 is partly used as the purge gas in Step 2 after recompression, and the balance is withdrawn as the recovered CO2 product gas. The inert gas and CO2 recoveries from the feed gas are ~100 and 78%. This application may be attractive for greenhouse gas emission control, i.e. CO2 sequestration from a hot gas or CO2 removal from combustion gases for power generation without cooling the gas and removing the water. This technology is a promising one for future development and possesses high potential for tackling many practical problems in the fields of natural gas supply and conservation, reduction of greenhouse gas emissions, production and conservation of hydrogen, etc.

165. 166 Choice between the adsorption cycles The total quantity of adsorbent required for a particular application is equal to the total load of the adsorbate or contaminant in the cycle divided by the dynamic adsorption capacity. The higher is the dynamic adsorption capacity, the less is the size of the bed and hence, less is the capital cost of the equipment as well as the cost of the adsorbent replacement. Thus, prima facie PSA appears to be a better choice than TSA. But if the throughput is large, the capital cost of the equipment and the cost of the compression would be very high. Also, in PSA a purge is required to drive off the residual gas from the void spaces of the bed for improving the product quality, and often this purge has to be the product gas itself or a high purity inert gas. Since this cannot be reused due to its level of impurity, it is often vented out which affects the overall recovery or the economics of the operation

166. In TSA too, a regenerating gas is required and that has to be heated to a sufficiently high temperature for regenerating the adsorbent bed. In large industries TSA is usually in operation due to having spare resources for use in regeneration. Very large throughputs cannot be handled by PSA. If the feed rate is low or moderate but the feed contains contaminants in large quantity, PSA system is suitable.

167. PSA and VSA cycles have been modelled successfully for both equilibrium and kinetic controlled processes. Of particular importance in PSA and TSA is the determination of the cyclic steady state. In TSA, following the desorption step the regenerated bed is usually clean. Thus, a cyclic steady state is closely approached in one cycle. In PSA and VSA, this does not often happen where complete regeneration is seldom achieved or necessary. It is only required to attain a cyclic steady-state whereby the product obtained during the adsorption step has the desired purity and at cyclic steady-state, the difference between the loading profiles after adsorption and desorption is equal to the solute entering in the feed.

168. Starting with a clean bed, the attainment of a cycle steady-state for a fixed cycle time may require tens or hundreds of cycles. For example, ethane and carbon dioxide are separated from nitrogen using 5A zeolite at ambient temperature with adsorption and desorption for 3 min each at 4 atm and 1 atm, respectively, in beds of 0.25 m in length The computation of the loading and gas concentration profiles at the end of each adsorption step for ethane starting with a clean bed, shows that at the end of the first cycle the bed is still clean beyond about 0.11 m and by the end of the 10th cycle, a cyclic steady-state has almost been attained with a clean bed existing only near the very end of the bed. Experimental data for ethane loading at the end of 10th cycle agree reasonably well with the computed profile

169. The cycle time in PSA is very short as the adsorbent bed becomes saturated quickly due to large concentration of the contaminant (adsorbate). So the adsorbent must be rugged enough to withstand millions of pressurization and depressurization cycles without disintegration. The carbon molecular sieve in nitrogen generation plant suffers more than 100 million such cycles in its life of 10 years.

170. On the other hand, TSA handles a very large quantity of feed and its cycle time is long because of the sufficient time required to heat and cool the bed. The bed is to be cooled after heating, and brought to adsorption temperature otherwise the adsorption capacity would be less. Naturally, an adsorbent suffers several thousand cycles of thermal expansion and cooling in its life time, and has to be mechanically and chemically rugged enough to withstand such repeated thermal shocks For adsorbent like silica gel, the regeneration temperature is very low, usually less than 150°C. But for molecular sieves and activated carbons, the regeneration temperatures are quite high depending on the application. Normally the molecular sieves require regeneration temperatures more than 220°C because of the higher heat of adsorption

171. Gas phase Liquid phase Liquid phase Process Condition Temperature swing Inert purge Displacement purge PSA TSA Feed: gas or vapour y y y y n Feed: liquid, vaporise at < 200°C n y y y y Feed: liquid, vaporise at > 200°C n n n n y Adsorbate concentration in feed, < 3% y y n n y Adsorbate concentration in feed, 3-10% y y y y n Adsorbate concentration in feed, > 10% n y y y n High product purity Required y y y Y ( hAp , lDp ) y Thermal generation Required y y n n n Difficult purge/ adsorbate separation Y (no recovery of adsorbate) n n NA y

172. Adsorption Equipment Stage-wise Contact Adsorption from gases Fluidized beds are widely used for adsorption from gases like recovery of vapours from vapour -gas mixtures, desorption, fractionation of light hydrocarbon vapours and several other applications. Adsorption is usually carried out at gas velocities just above the quiescent fluidizing velocity when the bed expands and looks like boiling liquid. The solid particles move about freely and are thoroughly circulated. Well defined bubbles of gas rise through the bed. Only fine particles below 20 mesh are suitable for use in fluidized beds and the superficial gas velocity has to be kept below 0.60 m/s.

173. They can, however, retain large range of particle sizes. Teeter beds are more frequently used for adsorption from gases since they provide better gas-solid contact by accommodating coarser particles, up to 10 mesh at superficial gas velocities in the range of 1.5 to 3 m/s. In these beds, however, there is little gas bubbling and the size range of particles retained is relatively small. Multistage operations are generally not practised for gas-liquid contact in commercial scale

174. Adsorption from liquids Agitated vessels are commonly used for adsorption from liquids. The liquid depths vary between 0.75 to 1.5 tank diameters. The tanks are provided with wall baffles to avoid vortex formation and lift the solid particles through vertical liquid motion. Flat blade or pitched blade turbines, fitted to an axial shaft are generally used for efficient mixing. Multiple turbines on the same shaft are sometimes used to get more uniform slurry throughout the tank. The clearance of the lowest impeller from the tank bottom should be more than the settled height of the bed, so that the impeller is not embedded if it stops.

175. A typical batch equipment for liquid-solid contact consists of an agitated vessel as above but having a filter press and a collection tank. The liquid to be treated and the adsorbent particles are thoroughly mixed in the tank, at the desired temperature for the required length of time. The slurry is then filtered in a filter press to separate the clear liquid from the solid adsorbent containing the adsorbed solute. The operation can be readily converted to multistage one by using additional tanks and filter presses as required. The operation can also be made continuous by using continuous rotary filters or centrifuges instead of a filter press. Alternatively, the adsorbent may be allowed to settle and continuously withdrawn from bottom

176. Multistage contact : Removal of a given amount of solute can be done with greater economy of adsorbent by treating the solution separately with small batches of adsorbent and filtering between stages rather than employing the entire adsorbent in a single stage. This can be practiced both in batch and continuous operations. As in other mass transfer operations, greater economy of adsorbent can be achieved by counter-current operation where the solution to be treated and the adsorbent flow in opposite directions, either from stage to stage or in a single equipment.

177. Continuous Contact Steady-state moving bed adsorbers Adsorption from gases : Steady state operation requires continuous movement of both fluid and adsorbent through the equipment at constant rates with no change in conditions at any point with passage of time. In order to have better performance, it is necessary to have counter-current operation so that several stages can be accommodated within a single device, since parallel flow of solids and fluids can at best produce a single stage. In view of the difficulties in moving solids through equipment continuously and uniformly, it has not been possible to successfully develop continuous flow counter-current adsorber in commercial scale particularly for gas-solid contact.

178. Adsorption from liquids : The difficulties in moving solids continuously and uniformly through equipment, as in case of adsorption from gases, stand in the way of developing successful continuous counter-current equipment for adsorption from liquids also. In case of liquids, however, much less elaborate arrangements are required to overcome the difficulties. For instance, a screw feed or simple gravity flow from a bin may introduce the adsorbent at the top of the tower and the same may be removed from the bottom by a revolving compartmented valve.

179. In the pulsed or moving bed, the liquid enters the bottom and leaves through the top of the column. The flow of liquid is stopped periodically, spent adsorbent is withdrawn (pulsed) from the bottom, and virgin or regenerated adsorbent is added into the top of the adsorber.

180. The Higgins contactor, originally developed for ion-exchange has been more or less successfully used for adsorption from liquids. The equipment though intermittent and cyclic fashion made by the action of piston, approaches counter-current operation in its performance. Adsorption takes place in the upper part of the equipment where the solution flows down through a stationary bed of the adsorbent.

181. At the end of adsorption, the solid particles are transferred to the lower part for regeneration and the upper part is filled with regenerated adsorbent when the cycle is repeated. Adsorption, regeneration and material shifting from adsorption to regeneration zone are carried out in an intermittent and cyclic way by regulating the valves. Out of three valves in the equipment, one valve remains open for each operation.

182. Unsteady-state fixed bed adsorbers The inconvenience and relatively high cost of moving solid particles as required in steady-state operations often make it more economical to use fixed bed adsorbers where the gas or liquid is made to pass through a stationary bed of solid adsorbent. Fixed bed adsorbers are frequently used in recovery of solvent vapours from air or other gases, purification of air in gas masks, dehydration of air or other gases, etc.

183. Adsorber vertically placed with six trays. A thin bed of adsorbent particles is spread on a screen or perforated plate. The depth of the bed usually varies from 0.3 to 1.3 m. Filtered feed gas is admitted into the chamber and flows through the bed. If the effluent gas is substantially free from solvent vapour , it is discharged into the atmosphere. If, on the other hand, the gas contains appreciable amount of vapour , the same is diverted to a subsequent adsorber in series. When the adsorbent particles in any absorber become nearly saturated they are subjected to regeneration

184. The enclosures for fixed-bed adsorbers may be horizontal or vertical, conical or cylindrical shells Conical fixed-bed adsorbers are used when a low pressure drop through the cone-shaped bed is less than one-half of that through a conventional flat-bed adsorber, while the air volume is more than double of that through the flat bed.

185. When multiple fixed beds are needed, the usual configuration is a vertical cylindrical shell The type of enclosure used is normally dependent on the gas volume and the permissible pressure drop. The gas flow can be either down or up. Downflow allows for the use of higher gas velocities, while in upflow the gas velocity must be maintained below the value which causes flooding of the bed. When large volume of gas is to be handled, cylindrical horizontal vessels are suitable.

186. For the continuous operation of fixed bed adsorbers, it is desirable to have two or three units. With two adsorber units, one unit adsorbs while the other regenerates or desorbs. Under most situations, two units are sufficient if the regeneration and cooling of the second bed can be completed prior to the breakthrough of the first unit. The move to three units makes it possible for one bed to be adsorbing, one cooling and the third regenerating. The vapour -free air from the first bed is used to cool the unit which was just regenerated. Occasionally a fourth bed is used, when two units adsorbing in parallel, discharge the exhaust to a third unit on the cooling cycle and a fourth unit is being regenerated.

187. Regeneration is done by hot inert gas or low pressure steam, the latter being preferred if the solvent is not miscible with water or if presence of small amount of water is not objectionable. The size of the adsorbent bed is determined on the basis of gas flow rate and desired cycle time. The cross sectional area of the bed is fixed to give a superficial gas velocity of 0.15 to 0.45 m/s.

188. For very large flow rates, rectangular beds in horizontal cylinders are generally used. The operation can be made more or less continuous by placing the adsorbent particles in several small chambers contained in an inner drum rotating inside a stationary compartmented outer shell. As the inner drum rotate, the adsorbent is successively subjected to adsorption, regeneration, drying and cooling. Moist gases can be dried by passing them through beds of silica gel, alumina, bauxite or molecular sieves. If the gases are under pressure, towers may be 10 m or more in height, but in such cases, the adsorbent should be supported on trays at intervals of 1 to 2 m in order to minimize compression of the bed

189. Adsorption from liquids Fixed bed adsorbers are often more economical in case of adsorption from liquids also. Liquids such as gasoline, kerosene, transformer oil, etc., can be dehydrated by activated alumina in fixed bed adsorbers High temperature steam along with vacuum can be used for regenerating the adsorbent. For the bulk separation of liquid mixtures, continuous counter-current systems are preferred over batch systems because they maximize the mass transfer driving force. This advantage is particularly important for difficult separations where selectivity is not high and/or where mass transfer rates are low. Ideally, a continuous counter-current system involves a bed of adsorbent moving downward in plug flow and the liquid mixture flowing upward in plug flow through the void space of the bed. Unfortunately , such a system has not been successfully developed because of problems of adsorbent attrition, liquid channeling and nonuniform flow of adsorbent particles. Successful commercial systems are based on a simulated counter-current system using a fixed bed.

190. Chromatographic separation It is based on the partition coefficient between a mobile phase and a stationary phase. Chromatography techniques have long been used for chemical analysis and separation. The reasons for finding increasing use of chromatographic separation can be as follows: ( i ) Highest number of theoretical plates it offers per unit length at perhaps the lowest cost per plate (ii) Maximum flexibility in designing separation factor for a given separation through selectivity engineering in the form of infinitely possible surface modifications (iii) Give multicomponent separation in a single operation

191. Chromatography is the technique for the separation, purification, and testing of compounds The term  “chromatography”   is derived from Greek, chroma meaning,  “ colour ,”  and graphein meaning “ to write.” In this process, we apply the mixture to be separated on a stationary phase (solid or liquid) and a pure solvent such as water or any gas is allowed to move slowly over the stationary phase, carrying the components separately as per their solubility in the pure solvent.

192. Chromatography separates mixtures into components by passing a fluid mixture through a bed of adsorbent material called stationary phase. The two most common adsorbents used in chromatography are porous alumina and porous silica. Sometimes carbon, magnesium oxide and various carbonates are also used. Alumina is a polar adsorbent and is preferred for the separation of components that are weakly or moderately polar. In addition, alumina is a basic adsorbent preferentially retaining acidic compounds. Silica gel is less polar than alumina and is an acidic adsorbent preferentially retaining basic components such as amines. Carbon is a nonpolar stationary phase with the highest attraction for larger nonpolar molecules. Adsorbent type sorbents are better suited for the separation of a mixture on the basis of chemical type (e.g. olefins, esters, acids, aldehydes, alcohols, etc.) than for separation of individual members of a homologous series.

193. For the separation of individual members of a homologous series, partition chromatography is preferred wherein an inert solid support like silica gel, is coated with a liquid phase. For application to gas chromatography the liquid must be nonvolatile. For liquid chromatography, the stationary liquid phase must be insoluble in the mobile phase. The stationary liquid phase may however be bonded to the solid support if required. An example of a bonded phase is the result of reacting silica with a chlorosilane. If the resulting stationary phase is more polar than the mobile phase, the technique is referred to as normal-phase chromatography. Otherwise, the name reverse-phase chromatography is used.

194. 195 In liquid chromatography, the order of elution from the column of the solutes in the mobile phase can also be influenced by the solvent carrier of the mobile phase by matching the solvent polarity with the solutes and using more polar adsorbents for less polar solutes and less polar adsorbents for more polar solutes. Operation modes for large-scale, commercial application of chromatography are mainly of two types: the first and the most common is a transient mode that is a scaled-up version of an analytical chromatograph, referred to as large scale, batch (elution) chromatography. If properly designed, the batch chromatograph operates somewhat like a distillation column, producing a nearly pure cut for each component in the feed and slop cuts for recycle;

195. the second major type of large scale chromatograph is the counter-current flow or simulated counter-current flow mode as discussed in adsorption. This mode is more efficient but complicated and can only separate a mixture into two products. A third mode not yet commercialized, is the continuous cross-current chromatograph. In this technique, a packed annular bed rotates slowly about its axis, past the feed inlet point. Eluant enters the top of the bed uniformly over the entire cross sectional area. Both feed and eluant are fed continuously and are carried downward and around by the rotation of the bed. Because of the different selectivities of the feed components for the sorbent, each component traces a different helical path since each spends different amount of time in contact with the sorbent. Thus, each component is eluted from the bottom of the packed annulus at a different location.

196. The adsorbent materials which may be solid, gel, or liquid phase immobilized in or on a solid phase, are kept in a column. When a mobile phase or fluid phase is injected into the packed column, the pulse generated is followed by a solvent or eluent. The pulse enters as narrow concentrated peak but exits dispersed and diluted by additional solvent. Different solutes in the mixture interact differently with the adsorbent which is considered as stationary phase, some solutes interact weakly while some others act strongly. Solutes that interact weakly with the solid matrix pass out of the column rapidly while others exit slowly due to their strong interaction with the matrix.

197. The differential rates of migration separate the solutes into different peaks that exit at a characteristic retention time. In fact, chromatographic separation is achieved through unequal partitioning of solutes between two phases, i.e. the stationary and mobile phases. The elution chromatography in which the substances emerge from a column into narrow peaks, is being used to achieve maximum purification of a product due to wide separation of peaks. It is used to purify the product even it is diluted. It is different from fixed-bed adsorption (sometimes called frontal chromatography) in which the solute is entrapped in the adsorbent and then eluted as a concentrate.

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199. Adsorption chromatography ( ADC): In this chromatography, the stationary bed contains solid particles and the solute molecules get adsorbed on solid particles such as alumina, silica gel, by weak van der Waal forces and steric interactions. Gel permeation chromatography: It is also known as Size exclusion chromatography or Gel filtration chromatography. It separates molecules according to their sizes and shapes. Solute molecules penetrate into small pores of packing particles, get trapped and are removed from the mobile phase. Unequal rate of migration of charged molecules in a buffer through a gel phase under electric field is the basis of electrophoresis separations. Affinity chromatography (AFC): It is based on the selective non-covalent interactions between solute molecules and ligands. It is very specific, e.g. enzyme-substrate interaction which may depend on covalent, ionic forces or hydrogen bond formation, but not very robust. Affinity binding may be molecular size and shape specific.

200. Ion-exchange chromatography (IEC ): Adsorption of ions or electrically charged compounds including amino acids, peptides and proteins takes place on the ion-exchange resin particles by electrostatic forces. It is used to separate analytes on the basis of their respective charges. Liquid-liquid partition chromatography (LLC): It is a separation technique while the mobile phase is a liquid. This process is based on the different partition coefficients of solute molecules between an adsorbed liquid phase and passing solution. The adsorbed liquids used are often nonpolar. Hydrophobic chromatography (HC): The hydrophobic interaction between solute molecules (e.g. proteins) and functional groups (e.g. alkyl residues) on support particles is the criterion of the process. This method is a type of reverse phase chromatography, which requires the stationary phase to be less polar than the mobile phase.

201. High performance liquid chromatography (HPLC ): In this technique a liquid (mobile phase) at high pressure is passed through a packed column containing solid particles, porous monolithic layer or porous membrane. It is operated on the principle of chromatography, the only difference being high pressure applied to the packed column. Due to flow of liquid at high pressure (high liquid flow rate) and dense column packing, HPLC provides fast and high resolution of solute molecules. Considerable developments in the fifties and sixties led to the emergence of high performance chromatography through use of adsorbent beads smaller than 10 mm. This has improved the fluid dynamics performance of chromatographic separations by more than three orders of magnitude. A 250 mm long column provides thousand theoretical plates required for separation of molecules having a separation factor close to unity. Recent emergence of ultra performance liquid chromatography (UPLC) using smaller particles is a further step towards improving resolution and speed of separation.

202. The choice of the stationary phase and consequently the type of chromatography depend on the nature of the solutes and process goals. Ion-exchange chromatography is widely used particularly for recovery of proteins. This method offers good resolution of peaks, high capacity, and good speed. HC and LLC share many of the attributes but are best applied when the aqueous phase is at high ionic strength. Also organic solvents may be needed, which may denature proteins or create environmental problems. Adsorption chromatography is relatively inexpensive but the resolutions are often not very sharp. Gel-filtration chromatography is good for buffer exchange and desalting and offers descent resolution. Because gels are compressible, capacity and throughput are often low; new rigid packing for size exclusion chromatography allows faster flow rates. Affinity method offers the possibility of very high selectivity and good capacity and speed. However, such methods can be extremely expensive especially if based on antibodies. For real bioprocesses like recovery of proteins, multiple types of chromatography are used since greater purity is possible if more than one basis of separation is used. Ion-exchange and Affinity chromatography are the most widely used methods to recover proteins from bioprocesses.

203. Another type of affinity chromatography is IMAC (immobilized-metal-affinity chromatography) which exploits the different affinities that solutes have for metal ions chelated to a support surface. Usually the analysis of actual columns is more complicated owing to dispersion, wall effects, and lack of local equilibrium. Wall effects can be reduced by keeping the column diameter to bead diameter ratio larger than 10. But compression can occur under some operating conditions and can further complicate the analysis of the system by causing flow irregularities. Understanding these hydrodynamic and kinetic effects is also critical for scaling-up of chromatographic processes. These effects alter the shape of the peaks exiting the column. In addition, the step controlling the rate of adsorption will alter peak size. For a porous support internal diffusion, diffusion through the external film or external dispersion in laminar flow can control the rate of mass transfer. Recent development in the design of packings for chromatography columns helps in scale-up. Rigid mechanically strong particles with macropores allow convective flow both around and through particles. The macropores are initially connected with the micropores thus resulting in good mass transfer. These particles allow fast flow of solvent and solutes, and reduce problems of bed compression allowing column length to be increased.

204. Some other chromatographic separation techniques like Pyrolysis gas chromatography, Reverse phase chromatography, Counter-current chromatography and Chiral chromatography are being practiced. Of many separation techniques practiced, perhaps the most powerful is the chromatographic separation. Not surprisingly therefore, chromatography is often combined with mass spectroscopy (LC-GC-MS) for high performance separation and identification. a typical chromatography column in which a solution is flowing in downward direction. During the movement of the solution, the solute is adsorbed on adsorbent solids and a band is usually formed. The solvent that carries the solute downwards moves along the axis of the column. It is assumed that a rapid equilibrium is established between the solute and the adsorbent; the radial gradients are considered negligible.

205. Chromatography is a separation method where the analyte is combined within a liquid or gaseous mobile phase., which is pumped through a stationary phase. Usually one phase is hydrophilic and the other is lipophilic. The components of the analyte interact differently with these two phases. Depending on their polarity they spend more or less time interacting with the stationary phase and are thus retarded to a greater or lesser extent. This leads to the separation of the different components present in the sample. Each sample component elutes from the stationary phase at a specific time called as retention time. As the components pass through the detector their signal is recorded and plotted in the form of a chromatogram.

206. Adsorption Chromatography D ifferent compounds are adsorbed on the adsorbent to different degrees based on the absorptivity of the component. Here also, a mobile phase is made to move over a stationary phase, thus carrying the components with higher absorptivity to a lower distance than that with lower absorptivity. Presentation title 207

207. Thin Layer Chromatography M ixture of substances is separated into its components with the help of a glass plate coated with a very thin layer of adsorbent, such as silica gel and alumina The plate used for this process is known as chrome plate. The solution of the mixture to be separated is applied as a small spot at a distance of 2 cm above one end of the plate. The plate is then placed in a closed jar containing a fluid termed as an eluant, which then rises up the plate carrying different components of the mixture to different heights. Presentation title 208

208. Column Chromatography U sed to separate the components of a mixture using a column of suitable adsorbent packed in a glass tube The mixture is placed on the top of the column, and an appropriate eluant is made to flow down the column slowly. Depending upon the degree of adsorption of the components on the wall adsorbent column, the separation of the components takes place. The component with the highest absorptivity is retained at the top, while the other flow down to different heights accordingly. Partition chromatography In this process, a continuous differential partitioning of components of a mixture into a stationary phase and mobile phase takes place. In this process, chromatography paper is used as a stationary phase which is suspended in a mixture of solvents that act as a mobile phase. Here, we put a spot at the base of the chromatographic paper with the mixture to be separated and as the solvent rises up this paper, the components are carried to different degrees depending upon their retention on the paper. The components are thus separated at different heights.

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210. THANK YOU Rushil Bhatt