1. MASS TRANSFER OPERATION BASICS Presentation by VMM (CTS)

2. Mass transfer : Transfer of material from one homogeneous phase to another. The driving force for mass transfer is concentration difference or difference in activity coefficient. Mass transfer operation involves changes in composition of solution & mixtures. Transfer of substance through another on a molecular scale. Mass transfer coefficient : Rate of mass transfer per unit area per unit conc. difference. It depends on diffusivity,viscosity,density,velocity & linear dimension D. KcDKcD Dv = f (( DG /  ), (  /  D v ) ) N sh = f ( N Re, N sc ) Why Mass transfer operation: Any chemical process requires : 1)Purification of raw material. 2)Separation of product from byproduct.

3. Gas-Liq. Distillation Absorption Desorption Humidification. Dehumidification Gas-Solid Sublimation Drying Adsorption. Liq-Solid Crystallisation Liq-liq Extraction Position of operating line relative to the equilibrium line decides – 1.Direction of mass transfer. 2.How many stages required for given separation. Distillation Desorption Absorption Eq m line Operting line Eq m line Operting line Eq m line

4. Design principle (MTO) Equilibrium characteristics of system. Material balance. Diffusional rate. Fluid dynamics. Energy requirement. MTC : Regulate the rate at which equilibrium is approached. Control time required for separation,size & cost of equipment.

5. Basic equations & laws. Yi Mole fraction of component i in vapor phase. Xi Mole fraction of component i in liquid phase. Pvi Vapor pr of comp. i Pi Partial pr. Of component i Pt Total pr. k Henry’s constant  Relative volatility Yi Xi Ki = Pi = Pv * Xi Pi = k * Xi Pi = Yi * Pt  = Ki K j = Pvi Pvj Henry’s law Raoult’s law Distillation,Extractive distillation,Liquid-liquid extraction,Absorption are all techniques to separate binary & multicomponent mix. Of liquid & vapors.

6. Distillation : Vapor liquid equilibrium data Predict (Thermodynamic calculation) Experimentally calculate. These data need to relate temp, pressure & composition. Two types of system Ideal (Raoult’s law) Nonideal system Accurate experimental data necessary. For non-ideal system use of specific empirical relationship that predict with varying degree of accuracy the vapor pressure,concentration relationship at specific temp. & pr. Concept of fugacity & activity are fundamental to the interpretation of non-ideal system.

7. Distillation Types 1)Flash distillation. 2)Batch Distillation. 3)Steam Distillation. 4)Azeotropic & Extractive Distillation Batch Distillation : Used for 1)Feed composition may change from batch to batch. 2)Negligible hold up in the column & condenser relative to that in receiver & kettle. Flash Distillation : Vaporising a definite fraction of the liquid in such a way that the evolved vapor is in equilibrium with residual Liquid.

8. 3) Batches are relatively small where certain components are separated in pure form heating a heavier residue. Most Batch Distillation follow constant relative volatility vapor liquid equilibrium curve. y =  x x 1 + x (  - 1 ) Log F X f W X S =  Log F ( 1 – X f ) W ( 1 – X S ) F = D + W F X f = D X d + W X w Feed Bottom Distillate

9. Steam Distillation : Possible to distill an organic compound at much lower temp. At constant system pr.P T,steam lowers the partial & vapor pressure of organic compound & its corresponding boiling pt. Due to immiscibility of water,it can be separated from product by simple condensation & followed by decanting. Application : Purification of heat sensitive material as an alternative to vacuum distillation. Azeotropic & Extractive Distillation. Very close boiling mix can be separated economically by this technique. Solvent when added will increase the difference between volatilities of light & heavy component. The attraction of solvent to one of the component reduces the volatility of solvent & the component to which it is attracted.

10. Solvent for Distillation should be Non-corrosive. Should not react with feed to form undesirable product. Non-toxic. Azeotropic solvent should have volatility near the major component desired in overhead product & in Extractive distillation its volatility should be lower than major component to be withdrawn at bottom. Extractive Distillation : An agent modifies the relative volatility between key component Without forming an azeotrope.The agent is nonvolatile & called Solvent. Extractive Distilation is simpler process than Azeotropic Distillation Solvent boils far above the system component. Because of the low volatility the solvent always leaves the Extractive column with the bottom product

11. In Extractive distillation attraction of solvent to one of the component Is attributed to Hydrogen Bonding. Polar characteristics of the solvent & members of the mix. Formation of weak unstable chemical complex. Chemical reaction between solvent & one of the component. y x Non-Azeotrope Min. boiling azeotrope Max. boiling Azeotrope. Azetropic Distilation

12. Extractive & Azeotropic Distillation. Water Crude MeOH Acetone MeOH water Oxide Impurities. Acetone water pentane

13. q = Moles of liquid flow in stripping section per mole of feed. a) Cold feed q > 1 b) Saturated liquid q = 1 c) Feed partially vapor 0 < q < 1 d) Feed at dew point q = 0 e) Feed superheated vapor q < 1 (a) (b) ( c ) ( d ) ( e )

14. q = 1 + C pL ( T b – T f ) q = - C pv ( T f – T d ) y = q ( 1 – q ) x + xfxf X D ( R D + 1 ) XFXF XBXB XDXD a b c d e Feed line R D = Reflux ratio TfTf TdTd TbTb = Feed temp. = Dew pt. = Bubble pt. C pL = Sp heat of liq. C pv = Sp heat of vapor

15. Feed line X D ( R D + 1 ) XBXB XFXF XDXD McCabe - Thiele

16. Fenske Equation N min = Log ( X D ( 1 – X B ) / ( X B ( 1 – X D ) Log  AB 1.0 Reflux ratio = L D = Reflux flow Distillate rate Eq of operating line of rectifying section. y n+1 = RDRD ( R D + 1 ) xnxn + XDXD

17. Minimum reflux ratio : No of plates infinite. Total reflux : Minimum no of plates C.S. area of column.  Flow rate of vapor. As the reflux ratio increases the vapor & liquid flow for given production rate increases Total cost Cost of heating & cooling Fixed charges Reflux ratio. RmRm Annual cost Total cost  Total plate area.  No of plates * C.S area of col Optium RR = (1.1-1.5) R min.

18. Distillation: Tower dia 0.3 m - 9.0 m No of plates few to 100. Plate spacing : 6 inch to several feet. Temp : upto 900 deg C. Material distilled : Viscosity,diffusivity,corrosive nature,tendency to foam,complexity of composition. Most plant operate at R.R. somewhat above the optimum.Total cost is not very sensitive to R.R. in this region & better operating flexibility is obtained. 70% to 80% of C.S area is used for bubbling / contacting. The vapor velocity should be high enough to create a frothy mix. of liquid & vapor that has a large surface area for Mass transfer. Misoperation such as weeping,foaming,entrainment,flooding Short circuiting,poor vapor distribution should be avoided.

19. Selection of contacting device is based on Tray type / packed type Vapor handling capacity. Liquid handling capacity. Mass transfer efficiency. Flexibility for wide range of operation. Pressure drop. Cost. Flooding pt. At flooding pt. Liquid continues to flow down the column,but builds up at greater rate from tray to tray. Flooding is associated with high liquid load over a wide range of vapor rates.Foaming tendency of the liquid influence the flooding.

20. Entrainment : When mist & liquid particles carry up in the vapor from the liquid from one tray to tray above,sufficient tray spacing should be available to prevent entrainment. Tray pressure drop : For normal operation pressure drop per tray Pressure Vacuum 2 – 4 inch water 2 – 4 mmHg Tray stability : A tray is stable when it can operate with acceptable efficiencies under condition that fluctuate, pulse or surge. Turndown ratio : Ratio of max. allowable vapor rate at or near flooding condition to the min. vapor rate when weeping or liquid leakage becomes significant.

21. Plate efficiency : Plate efficiency is a function of rate of mass transfer between liquid & vapor. Any mis-operation of column such as excessive foaming or entrainment,poor vapor distribution,or short circuiting,weeping, dumping of liquid lowers the plate efficiency. Overall efficiency : Ratio of no. of ideal plates needed in the entire column to the no. of actual plates. Murphee sfficiency : The change in vapor composition from one plate to the next divided by change that would have occurred if the vapor leaving were in equilibrium with liquid leaving. Minimum reflux ratio. At this reflux ratio desired separation is just possible but infinite no. of plates is required.This min. reflux ratio is guide in choosing a reasonable R.R. for an operating column & in estimating no. of plates needed for given separation at certain value of R.R.

22. Bubble cap Sieve tray Capacity : Moderately high Higher than bubble cap at design at low throughput performance drops as efficiency falls. Efficiency : High As high as bubble cap in the region of design, falls to unacceptable value when capacity reduces < 60% Entrainment : Three times 1/3 rd of bubble cap tray. that of sieve tray Flexibility: Most flexible design Not suitable for column operating for high & low liquid rate. under variable load,falling < 60% Allows positive drain of of design.Tray weeps liquid at liquid from tray.Liquid low vapor rate. head is maintained by weir.

23. Bubble cap Sieve tray Tray spacing : 18 inch avg. Can be closer than bubble cap 24 to 36 inch for Vac condition due to improved entrainment 15 inch avg. 9,10,12 inch acceptable 20,30 inch for vacuum. Application : All service except Systems where high capacity extremely coking,polymer design rates to be maintained. formation or other high Handles suspended solid fouling condition. Particles.

24. Factors to Consider When Selecting High Performance Trays  Capacity & Hydraulic Limitations  Pressure Drop  Efficiency  Operating Range  Resistance to Fouling  Existing Column Configuration  Equipment Cost / Installation Cost

25. Various High Capacity / High Performance Trays available in the Market Norton Triton Trays Nutter MVG trays Koch-Glitsch Nye Trays Koch-Glitsch MaxFrac Trays Koch – Glitsch Superfrac Trays UOP E-MD trays SHELL High capacity trays

28. Saint-Gobain Norpro High Capacity Triton™ Trays increase the capacity of towers by increasing the area available for liquid / vapour contacting over conventional tray designs.

29. Conventional Tray High Capacity Triton™ Trays

30. Spray Height Profiles

31. General characteristics Trays : Usually made of sheet metals of special alloys. Thickness is governed by corrosion rate. Trays must be stiffened & supported & fastened to the shell to prevent movement owing to to surge of gas, With allowance for thermal expansion. Large trays must be fitted with Man-ways. Tray spacing : It should be such that insurance against flooding & excessive entrainment where tower height is important consideration.For all except smallest dia. tower,20 inch spacing is considered. For small dia tower spacing of 6 inch is considered. Tower diameter.: Tower dia should be sufficiently large to handle the gas & liquid rates within the region of satisfactory operation.

32. Flooding Weeping Coning Dumping Priming Excessive entrainment Satisfactory operation Gas rate Liquid rate VfVf =CfCf (  L -  G ) L L C f = Empirical constant VfVf = Superficial gas velocity. Tower dia can be decreased by use of increased tray Spacing,so that tower cost which depends on height & dia. passses through min. at some optimum tray spacing.

33. Downcomer :Liq. Is fed from one tray to next by downspot.Adequate Residence time must be allowed in the downspot to permit disengaging Gas from liquid so only clear liquid enters the tray below. Weirs : It maintains the depth of liquid required for gas contacting on the tray. In order to ensure uniform distribution of liquid flow on a Single pass tray a weir length of 60 to 80% of tower dia is used. Liquid flow : Small tower : Reverse flow. Most common : Single pass cross flow. Commertial col. Upto 15 m dia have been build. Two pass trays are common for dia of 3 to 6 m & more passes for larger dia. Reverse flow Single pass cross flow

34. Process Requirement of col. Internals. A)Primary Requirement. a) Efficiency b) Capacity. B) Secondary Requirement : a) Low pressure drop. b) Resistance to fouling. c) Resistance to corrosion. Type of internals : Trays Random packing. Structured packing.

35. Tray : Sieve Valve Bubble cap. High capacity trays. Features : Good efficiency. Good capacity Good resistance to fouling Reliable Relatively high pressure drop. Random packing. 1 st generation Rasching ring,saddles 2 nd generation Pall ring 3 rd generation Properietary packing Features : More capacity or more efficiency than trays. Low delta p than trays, Excellent revamp tool. Easy installation, Sensitive to vapor liquid maldistribution. Price higher than tray.

36. Structured packing Proprietary : Gempack (Glitch) Flexipack ( Koch) Mallapack (Sulzer) Intalox (Norton) Feature More capacity & more efficiency than trays. Very low pressure drop. Sensitive to vapor liquid maldistribution. Unreliable for high pressure distillation. High price.

37. Packed towerTray tower 1)Gas pr drop Smaller. 2)Liquid hold up Smaller. 3)Liquid/gas ratio High values can be Low values can be best handled best handled 4) Liquid cooling Cooling coils more readily built. 5) Side stream More readily removed. 6) Foaming system Operate with less bubbling of gas & hence more suitable. 7) Corrosion For difficult corrosion less costly. 8) Solid present. Not satisfactory. Not satisfactory. 9) Cleaning. Frequent cleaning difficult Easier. 10) Temp fluctuation Fragile packing tend to crush.

38. Control philosophy : Component segregation is achieved by control of heat load. Stream splitting is achieved by control of product flow. Column pr too high – Feed system may be unable to input feed to column.Allowable design pr may exceed. Column pressure too low : Product may not flow from system. Too high temperature (bottom) – Product degradation excess over equipment design. Rapid variation in flow or pr. – Control of d/s, u/s equipment become impossible. Pressure control : Pr can be controlled by manipulating vapor product or noncondensible vent stream. To get constant top vapor product composition,condenser outlet temp. needs to be controlled.

39. Vary the condenser cooling by manipulating the vapor product or non-condensable vent stream. Pr can be controlled by variable HTC in condenser. Condenser must have excess surface. Vacuum condenser (variable HTC). Very the amount of inerts leaving the condenser. Hot gas bypass. Reflux drum PT To flare Column

40. Temperature control : Measure of composition. Temp is controlled either at top or bottom depending on which product specification is important. For high purity operation temp. will be controlled at intermediate pt. The pt. Where dT/dC is max. is the best place to control temp. Column top temp. Controlled by manipulating reflux. For partial condenser it is typical to control condenser outlet temp. instead of column top temp. Column bottom temp. R/b outlet line temp. is used for controlling. For smooth control cascade arrangement,FRC on heating medium line Set pt. Of which is manipulated by TRC. Feed temp. This is critical as this decides the vapor & liq flow in feed zone. Often amount of heat available in the bottom is close to the optimum feed preheat.

41. Level control : For total condenser – accumulator level is controlled by varying distillate draw. For partial condenser,it can be controlled with condenser hot gas bypass. Bottom level is controlled by bottom draw. Flow control Feed flow often not controlled. Liquid product flow are on level than flow control. Top vapor product is usually on pr. control. Reflux is frequently on FRC, but also may be on column TRC or accumulator level. Differential temp. control : Used to control the column traffic. Good way is let the differential pressure control the heating medium to the r/b. Largest application is in packed tower where it is desirable to run at 80 to 100% of flood for best efficiency.

42. Differential temp. control : Diff temp control across several bottom section trays is key to maintain purity control. Column side draw flow was put on control by critical temp. difference. This controls the liquid reflux running down to critical zones by varying the liquid draw off at side draw. Optimisation: Hold reflux at minimum to deliver distillate purity. Boil up at minimum to deliver bottom purity. Higher throughput or improved product quality is balanced against higher operating cost such as labor energy or maintenance.

43. R/B selection : Forced circulation. Natural circulation. Vertical thermosyphon. Horizontal thermosyphon. Flooded bundle. Horizontal thermosyphon : Used for larger duties,dirty process & frequent cleaning is required. Process is usually on shell side. Vertical thermosyphon : Used for smaller duties,clean process & vaporisation < 30%. Viscosity of r/b feed should be < 0.5 cp. Forced circulation : Usually used where piping pr drop is high & Natural circulation is impractical. Kettle : Very stable & easy to control. No two phase flow,expensive.

44. Don’t use lower pr. Than desired,because separation efficiency & throughput decreases as pressure decreases. The requirement of bottom temp. to avoid overheating heat sensitive material may become predominant. Low pressure operation requires larger dia. Column.

45. Start Distillate & bottom composition known. Calculate bubble pt pr Pd at 49 deg C. Calculate dew pt pr Pd at 49 deg C Choose a refrigerant so as to operate partial condenser at 415 psig Estimate bottom pr. Pb Pd < 215 psia Pd < 365 psia Pd > 365 psia Pd > 215 psia Lower pr. Pd appropriately Calculate bubble pt. Temp. Tb of bottom at Pb Tb < bottom temp. or Critical temp. Method of estimation of col pr.

46. TI TRC REACTOR EFFLUENT 26 30 15 50 FRC MP STEAM CONDENSATE TRC COOLING WATER LIC TAME PRODUCT TO STORAGE COOLING WATER PRC COOLING WATER FR TO RAFFINATE WASH COLUMN FR SUMMER LIC ETHERMAX PROCESS PROCESS FLOW

47. Extraction : It is the separation of the constituent of a liquid solution by contact with another liquid. Feed : Solution which is to be extracted. Solvent : Liquid with which feed is contacted. Raffinate : Residual liquid from which solute has been removed. Distillation & Evaporation are direct separation method,the product of which is pure substance. Liquid Extraction produces new solution which must be separated by Distillation or Evaporation. Extraction : 1.Extraction is attractive alternative to Distillation under high vacuum & low temp. to avoid thermal decomposition. 2. Liquid Extraction incurs no chemical consumption or by-product formation & less costly. 3. Aromatic & paraffinic HC of nearly the same MW are impossible to separate by distillation due to vapor pr nearly same can be easily separated by Extraction.

48. Variables which influence the performance of Extraction. 1)Liquid System. Chemical identity & corresponding physical property. Concentration of solute. Direction of Extraction a) Aq. To organic. b) Continuous to dispersed phase Total flow rate of liquid. Ratio of liquid flow. What liquid is dispersed. 2) The Equipment. Design Packed / Mechanically agitated. Size shape of packing,arrangement of baffle. Nature & extent of mechanical agitation whether rotary or pulsating. MOC,which influence relative wetting by the liq. Height & end effect. Diameter of extractor & extent of axial mixing.

49. Solvent ExtractFeed Raffinate A R S E C : Solute in Feed A to be separated by extracting solvent S. Effect of temperature : Solubility of feed & solvent increases with increasing temp. & above certain temp. t 1, they dissolve completely. Liquid extraction operation which depends on formation of insoluble liquid phase must be carried out < t 1

50. Effect of Pressure : Except at very high pressure the influence of pressure on liquid equilibrium is so small that it can be generally ignored. Choice of Solvent : 1)Selectivity : For all Extraction operation selectivity must exceed unity,the more so better.If selectivity is unity no separation is possible. Selectivity = (wt. fr. of C in E)* ( wt fr. of A in E) (wt. fr. of C in R) * (wt. fr. of A in R) 2) Distribution coeff. Ratio of y* / x Higher the value,less solvent will be required for Extraction.

51. 3) Insolubility of solvent : Solvent which is more insoluble is preferable. 4) Recoverability : It is necessary to recover the solvent for reuse. 5) Density : Difference in density of saturated liquid phase is necessary, for stepwise & continuous contact equipment operation larger this difference better. 6) Interfacial Tension : Larger the interfacial tension more readily coalescence of emulsion will occur but more difficult the dispersion of one liquid in the other. Coalescence is usually of great importance hence Interfacial tension should be high. 7) Viscosity, Vapor pr., Freezing point. Should be low for ease in handling & storage. Solvent should be nontoxic,nonflammable & low cost.

52. Types : 1)Stage-wise contact a) Multistage crosscurrent Extraction. b) Continuous countercurrent Extraction with Reflux. c) Countercurrent multistage. 2)Differential (continuous contact extractor) a) Multistage crosscurrent Extraction Raffinate is successively contacted with fresh solvent. It can be continuous or in Batch. F E1E2E3 S1 S2 S3 R1 R2R3

53. b) Continuous countercurrent Extraction with Reflux. Solvent separator E1 E2E3E4 S R0 Extract product R1 R2 R3 R4 feed F Solvent c) Countercurrent Multistage F R1 R2 R3R4 S E

54. Single stage : Mixer – Settler Mixer : For contacting two liquid phase to bring Mass transfer. Settler : For mechanical separation. Mixer : Series of orifices or mixing nozzle through which liquid to be contacted are pumped co-currently. Agitated vessel : For continuous operation liquid enters at bottom & leave at top. In some cascade arrangement light & heavy liq. enters through side wall near the top & bottom of the vessel respectively & leave through The port in the wall opposite the impeller. For batch operation mixing vessel itself acts as settler. Impeller : Flat blade turbine type. Dia. Of impeller / Tank dia. = 0.25 to 0.33

55. Light liq. Heavy liq. Dispersion band. Settler : Decanter. a)Sufficient residence time b)Estimation of rate of flow to produce suitable dispersion band thickness. c)Calculation of time to settle individual drop through clear liq. above & below dispersion band. Multistage countercurrent : Recommended for systems of low interfacial tension. Light liquid out Heavy liquid in Light liquid in Heavy liquid out

56. Differential continuous contact Extractor : Countercurrent flow is produced by difference in density of liquid. 1)Spray towers 2)Packed towers. Mechanically agitated extractor : For systems of high interfacial tension where density difference are likely to be 1/10 th as large or less,good dispersion of system of high interfacial tension is impossible & mass transfer rates are poor.For such systems dispersion is brought about by mechanical agitation of liquid. Packed Towers : Packing reduces axial mixing.Void space is filled with continuous heavy liquid which flows down.Remainder of the void space is filled with droplets of light liquid. Packing should be sufficiently small & not greater than 1/8 th of tower dia. If the dispersed liquid wets the packing,it will pass through as rivulets & not as droplet & interfacial area produced will be small, for this reason,packing material should be wetted by continuous phase.

57. Economic Balance : Amount of solute extracted for a fixed solvent/feed ratio increases with increased no. of stages. For a fixed extent of extraction,the no. of stages required decreases as solvent or reflux ratio increases. Total cost,sum of investment & operating cost must pass through a min.,at a optimum solvent rate or Reflux ratio. As solvent rate increases,cost of solvent removal increases. Correlation for estimating flooding rates,axial mixing,Mass transfer rates are available.

58. ETHERMAX PROCESS PROCESS FLOW OLEFIN FEED FRESH WATER PIC LR LI TO STATIC MIXER LP Steam Condensate TRC FRC LIC WASTE WATER TO COKING UNIT Cooling Water LIC FRC ETHERMAX RAFFINATE RAFFINATE TO HYDROTREATING OLEFIN WASH COLUMN RECYCLE WASH COLUMN OLEFIN FEED SURGE DRUM DEGASSING DRUM

59. A D S O R P T I O N

60. Topics covered. What is Adsorption. Different terms & its technical significance. Physical & Chemical Adsorption. Adsorption Isotherm. Mass transfer characteristics, Equations. Industrial application.

61. ADSORPTION Adsorption : Tendency of a molecule from a fluid / gas phase to adhere to the surface of the solid. The molecule which adsorbs is called as Adsorbate & the surface on which it adsorbs is called Adsorbent. Separation occurs,due to difference in MW,shape or polarity which causes some molecules to be held strongly on the surface than others. For gas phase adsorption force field creates a regime of low PE near the solid surface,molecular density near the solid surface is generally > bulk gas density. Rate of Adsorption from liquid is much slower than from gas.

62. Adsorbent : Selectivity Capacity Mass transfer rate Long term stability. Equilibrium capacity : How much of adsorbate will be adsorbed under given system. Adsorption rate : How fast the adsorbate be adsorbed under these condition. Life : How many times the operation repeated.

63. Adsorptive properties : Pore size / Pore size distribution Nature of solid surface. Size of bed : Gas / Liquid flow rate. Desired cycle time. Superficial velocity is usually 0.15 – 0.45 m/s. Height of bed : Pressure drop. Depending on the adsorbent the distribution of the pore dia. within the adsorbent particle may be narrow ( 20 to 50 0 A) or it may range widely ( 20 to several thousand 0 A)

64. Downward flow allows use of higher velocity. If upward flow is selected it should be less than that at Which lifting of Adsorbent occurs. Fluidisation or violent agitation leads Adsorbent attrition & loss of Adsorbent,higher pressure drop. To avoid distribution & channeling use proper L/D ratio. Use proper device to distribute flow uniformly. Avoid local high velocity & eliminate particle movement & channeling. Adequate bed support should be provided.

65. Short cycle time : 1) Loss of operating flexibility. 2) Life of Adsorbent shortened. 3) High operating cost. 4) Less efficient use of Total Adsorbent.

66. Adsorbent must have high specific area & highly porous structure with micropores. Adsorbent surface adsorbs different components with different affinities. Depending on the nature of surface forces the adsorption is of two types : 1) Physical Adsorption. 2) Chemisorption. Adsorption process types : Continuous / Batch Efficiency of Adsorption process is higher in continuous mode of operation than in Batch mode of operation.

67. Physical Adsorption Chemical Adsorption Forces Weak Van der walls Strong forces,Electron transfer,Bond formed bet’n adsorbate & surface. Heat of 2 or 3 times latent Adsorption heat of evaporation. heat of evaporation Nature of Monolayer/multilayer Adsorbed No dissociation of phase adsorbed species. Specificity Non specific Highly specific. Monolayer. Dissociation of Adsorbed species.

68. Physical Adsorption Chemical Adsorption Reversibility rapid, non-activated activated,may be slow reversible & irreversible. Temp. range Significant at relatively Possible over a wide low temp. Range of temp.

69. Selectivity in Physical Adsorption Majority is based on Equilibrium based selectivity. Kinetic selectivity is restricted to molecular sieve adsorbent. Normally down-flow is preferred as,up-flow at high rates might fluidise the particles,causing attrition & loss of fines. Equilibrium Kinetics. Adsorption is Exothermic process. Heat of adsorption depends on loading.

70. Crystalline adsorbent : e.g Zeolite,alumna phosphate No distribution of pore size. Bed of adsorbent is supported on screen or perforated plate. High area : lacks physical strength. & hence limits the application. Regeneration --- Hot inert gas is used. Amorphous adsorbent : Silica, Alumna, activated carbon Specific area : 200 – 1000 m 2 /g max. 1500 m 2 /g Adsorbent Crystalline. Amorphous

71. For gas concentration is expressed in mole% or partial pr. For Liquid phase concentration is expressed in ppm. All system show a decrease in the amount adsorbed with an increase in temperature.Working capacity of adsorbent depends on fluid conc. & temperature. Adsorption Isotherms : Linear Concave upward Convex upward. Adsorption Isotherm : Equilibrium relationship between Concentration in the fluid phase & the concentration in the adsorbent particles at given temp. Extent of adsorption is greater,the smaller the solubility in Solvent.

72. W g adsorbed g solid C ppm Equilibrium capacity is determined from the adsorption isotherm. Conc. of adsorbate on solid Mass adsorbed per unit mass of original adsorbent.

73. Irreversible adsorption : Very favourable Amount adsorbed is independent of conc. down to very low value. Linear isotherm : Amount adsorbed is directly proportional to concentration in fluid phase.. Concave upward isotherm : Unfavourable. Relatively low solid loadings are obtained. It leads to long Mass transfer Zone in the bed. Convex upward : Favourable Relatively high solid loading obtained at low conc in fluid.

74. Langmure Isotherm: W = W max ( Kc / (Kc + 1) ) W = Adsorbate loading K = Adsorption constant. C = Conc in the fluid. Freundlich Equation : W = b c m b & m are constants. For liq. M < 1.0

75. t1t1 t2t2 t3t3 t4t4 L Bed length C / C 0 time tbtb

76. Break point time œ Capacity of solid Break point time œ 1/ Feed conc. Mechanism of transfer to solid includes diffusion through the fluid film around the particle & diffusion through the pores to internal adsorption sites.Actual process of physical adsorption is instantaneous & equilibrium is assumed to exists between surface & the fluid at each point inside the particle. For mass transfer to take place the conc.in the fluid phase in equilibrium with the solid phase should be less than the actual fluid conc.

77. Where the conc. Profile is steep the difference in conc. is large & mass transfer is rapid. Width of mass transfer zone depends on the mass transfer rate, flow rate & the shape of the equilibrium curve. Narrow Mass Transfer Zone : Efficient use of adsorbent Energy cost of regeneration is low. Performance of Adsorbent bed is predicted from equilibrium data & mass transfer calculation.

78. Usually adsorption are scaled up from lab tests in small dia. bed & large unit is designed for the same particle size & superficial velocity. ρpρp (1- έ) dW dt = K c a (C – C*) (1- έ) ρpρp = ρb ρb = Bed density. KcKc = Overall mass transfer coefficient. K c int & K c ext K c depends on : provides insight into the mechanism by which adsorption occurs.Difficult to determine & tedious to use. KcKc Higher the conc.of solute,higher the equilibrium adsobate conc.on the adsorbent.

79. Diffusion within the particle is unsteady state process. K c int = 10 D e DpDp DeDe = Effective diffusivity DpDp = Dia. Of particle. Effective diffusion coefficient depends on particle porosity, pore dia. & nature of diffusing species. For adsorption of solute from aq. Solution internal diffusion resistance often determine the transfer process & surface migration is much less important.

80. 1) External film resistance. 2) Intraparticle diffusional : Macropore diffusional resistance Micropore diffusional resistance Depending on the particulate system any one of these resistance may be dominant or the overall rate of mass transfer is determined by the combined effect of more than one resistance. True driving force for diffusion transport is gradient of chemical potential rather than conc. difference.

81. Usually mass transfer is governed by pore diffusion inside the particle. The size range of particle should be narrow. Largest particle control the rate & gives lowest adsorption performance.Smallest particle controls pressure drop. Particle size of Adsorbent affects 1) Mass transfer rate. 2) Pressure drop. 3) Maximum lifting velocity.

82. Adsorption is always Exothermic. From 1 st & 2 nd law of Thermodynamics. F = H – ST All adsorption process proceeds spontaneously hence F is always - ve. S is – ve since system prior to adsorption exists in less orderly state. HenceH will always be negative.

83. De-sorption require much higher temp. when adsorption is strongly favourable or irreversible than when isotherms are Linear. Process specific concerns: Adsorbent age. Loosing capacity because of fouling. Loss of surface area or crystallinity. Oxidation. Mass transfer resistance increase over time.

84. UOP adsorbent used in PP plant for removal of sulfur species Design conditions : Feed rate : 100 MT/hr., Temperature = 40 deg C, pressure = 22 bar a & composition 100% propylene. AZ300 is better for heavier Sulfur species. The adsorbent charged in C 2018 : SG 731 : 26 T AZ 300 : 12 T UOP has suggested us to charge SG731 : AZ300 in the ratio 2.26 : 1.0.

85. The worst case referred to UOP was as follows. At inlet H2S 2.0 ppm max. Mercaptan 1 ppm ( max) DMS < / = 0.1 ppm COS < / = 0.1 ppm Others < / = 0.1 ppm ( Others : DMDS/Thiopene/CS2/higher mercaptans ) At outlet : Total Sulfur : 0.1 ppm As per UOP, for worst case of S impurities,the bed will be due for regeneration after 12 days.This indicates that the adsorbent bed has capacity of removing 83.52 kg of total sulfur species.

86. T H A N K Y O U