Report CopyRight/DMCA Form For : Adsorption Ion Exchange And Chromatography
Adsorption ion exchange andchromatography IntroductionSorption operations certain components of a fluid phase calledsolutes are selectively transferred to insoluble rigid particles.suspended in a vessel or packed in a column Sorption includes selective transfer to the surface and or into thebulk of a solid or liquid The substance that is transferred to the surafce is theadsorbate . The material on which the adsorbate deposits is theadsorbent Industrial ExamplePressure swing adsorption for thedehydration of air. SORBENTS 1 high selectivity to enable sharp separations 2 high capacity to minimize the amount of sorbent needed 3 favorable kinetic and transport properties for rapid sorption 4 chemical and thermal stability. 5 hardness and mechanical strength 6 a free flowing tendency for ease of filling or emptying vessels 7 high resistance to fouling for long life 8 no tendency to promote undesirable chemical reactions 9 the capability of being regenerated. 10 relatively low cost SORBENTSMost solids are able to adsorb species from gases and liquids However only afew have a sufficient selectivity and capacity to make them serious candidatesfor commercial adsorbents . Micro pore 20 A Meso pore 20 500 A Macro pore 500 A 50 nm The surface area to volume ratioThe specific surface area Sg is area per unit.mass of adsorbent is Specific surface area of an adsorbentSg is measured by adsorbing gaseous nitrogen Typically the BET apparatus operates at the normal boiling point of N2 195 8 C bymeasuring the equilibrium volume of pure N2 physically adsorbed on several grams of.the adsorbent at a number of different values of the total pressure in the vacuumrange of 5 to at least 250 mmHg Brunauer Emmett and assumed that the heat of adsorption during monolayerformation is constant and that the heat effect associated with subsequentlayers is equal to the heat of condensation The BET equation is. Specific surface area of an adsorbent Representative Properties of CommercialPorous Adsorbents Representative cumulative pore sizedistributions of adsorbents . Adsorption Mechanism 2 Chemical adsorption Results from a chemical interaction between theadsorbate and adsorbent Therefore formed bond ismuch stronger than that for physical adsorption. Heat liberated during chemisorption is in the range of20 400 kj g mole08 01 2020 Aerosol Particulate Research Lab 10 10 Adsorption ProcessClassified as .1 Physical adsorption2 Chemical adsorption Adsorption Process1 Physical adsorption The gas molecules adhere to the surface of the solid adsorbent as a.result of intermolecular attractive forces van der Waals forces between them The process is exothermic The process is reversible recovery of adsorbent material or adsorbedgas is possible by increasing the temperature or lowering the.adsorbate conc Physical adsorption usually directly proportional to the amount of solidsurface area Adsorbate can be adsorbed on a monolayer or a number of layers The adsorption rate is generally quite rapid. The amount of heat released during this process is equal to the heat ofcondensation Adsorption Process2 Chemical adsorption Occurs when there is sharing of electrons between adsorbent and adsorbate. The amount of heat released during this process is equal to the heat of reaction Heat liberated during chemisorption is much larger than the heat ofvaporization It is frequently irreversible On desorption the chemical nature of the originaladsorbate will have undergone a change . Only a monomolecular layer of adsorbate appears on the adsorbing medium Chemisorption from a gas generally takes place only at temperatures greaterthan 200 C may be slow and irreversible Commercial adsorbents rely on physical adsorption catalysis relies onchemisorption . Factors affecting adsorption Physical and chemical properties of the adsorbate Properties of the adsorbent Adsorption isotherm Solubility.pH often affects the surface charge on the adsorbent as well as the charge onthe solute TemperatureAdsorption reactions are typically exothermic as T increases extent ofadsorption decreases . Presence of other solutes Adsorption EquilibriumIf the adsorbent and adsorbate are contacted long enough an equilibrium will beestablished between the amount of adsorbate adsorbed and the amount ofadsorbate in solution The equilibrium relationship is described by isotherms . This equilibrium is usually expressed in terms of 1 concentration if the fluid is a liquid or partial pressure if the fluid is a gas ofthe adsorbate in the fluid 2 solute loading on the adsorbent expressed as mass moles or volume ofadsorbate per unit mass or per unit BET surface area of the adsorbent . This equilibrium isotherm places a limit on the extent to which a solute isadsorbed from a given fluid mixture on an adsorbent of given chemicalcomposition and geometry for a given set of conditions 1 Pure Gas Adsorption Brunauer s five types of adsorption. The simplest isotherm is Type I Corresponds to unimolecular adsorption Characterized by a maximum limit in theamount adsorbed This type applies often to gases at.temperatures above their critical temperature Desirable isotherms with strong adsorption Types II Multimolecular adsorption Observed for gases at temperatures below.their critical but approaching the saturationpressure vapor pressure The heat of adsorption for the first adsorbedlayer is greater than that for the succeeding Each layer is assumed to have a heat of.adsorption equal to the heat of condensation vaporization Desirable isotherms Strong adsorption Types III. Undesirable because the extent of adsorptionis low except at high pressures Corresponds to multimolecular adsorptionwhere the heat of adsorption of the first layeris less than that of succeeding layers . This type of isotherm is rarely observed an example being the adsorption of iodinevapor on silica gel In the limit as the heat ofadsorption of the first layer approaches zero adsorption is delayed until the saturation.pressure is approached Type IV and Type V the number of layers is restricted by pore sizeand capillary the maximum extent of adsorption occurs.before the saturation pressure is reached Type IV is the capillary condensation versionof Type II Type V is the capillary condensation versionof Type III. Hysteresis can also occur throughout anyisotherm when strongly adsorbed impuritiesare present Adsorption isotherms for NH3 on charcoalFor ammonia boiling point is 33 3 C and the.critical temperature is 132 4 Cisotherms are of Type I When the amount adsorbed is low 25cm3 g the isotherms are almost linear andthe following form of Henry s law called the.linear isotherm where q is amount adsorbed unit mass of adsorbentk is an empirical temperature dependent constantp is the partial pressure of the component in the gas Adsorption isobars for NH3 on charcoal.As the temperature increases theamount adsorbed decreases becauseof Le Chatelier s principle for anexothermic process Adsorption isotherms for pure propane vapor.at 298 303 Kadsorption isotherms for a given puregas at a fixed temperature varyconsiderably with the adsorbent Adsorption isotherms for.water in air at 20 to 50 C Generalized adsorption correlationfor Pittsburgh Chemical Co BPLcarbon 1040 m2 g Comparison of Equilibrium Adsorption of.Pure Gases20 40 mesh Columbia L Activated CarbonParticles S 1 152 m2Ig at 38 C and 1 atmfor a given adsorbent theloading depends strongly on. Freundlich Isotherm correlate isotherms of Type I k and n are temperature dependent 1 n 5constants n 1 Henry s law equation. K decreases with increasing temperature n increases with increasing temperature andapproaches a value of 1 at high temperatures assumptions a heterogeneous surface. nonuniform distribution of the heat ofadsorption over the surface Langmuir Isotherm correlate isotherms of Type I Assumtion .chemisorption the Langmuir adsorptionisotherm is restricted to a monomolecular the surface of the pores of the adsorbent ishomogeneous the forces of interaction between adsorbed.molecules are negligible Although originally devised by Langmuir forchemisorption this eq has been widelyapplied to physical adsorption data K should change rapidly with temperature .but q should not because it is related throughv by to the specific surface area of theadsorbent Sg Other Adsorption IsothermsValenzuela and Myers Isotherm.m b and t constants a givenadsorbate adsorbent system andtemperatureHonig and Reyerson Isotherm Adsorption isotherms for multicomponent.Adsorption isotherms are generally presented for a single component but manyapplications involve multicomponent mixtures The Langmuir isotherm is easily modified for multiple adsorbates by adding terms tothe denominator This equation is not very satisfactory for strongly adsorbed materials . 2 Liquid Adsorption When porous adsorbent particles are immersed in a pure gas the amount ofadsorbed gas is determined by the decrease in total pressure With a liquid the pressure does not change and no simple experimentalprocedure has been devised for determining the extent of adsorption of a.pure liquid If the liquid is a homogeneous binary mixture it is customary to designateone component the solute 1 and the other the solvent 2 adsorption of the solvent is tacitly assumed not to occur adsorption isotherms. no adsorption of solvent a negligible change in the total moles of liquid mixture adsorption isothermsWhen the solvent is not aComposite curve without.negative Concentration Composite isotherms or isothems of concentration When data for the binary mixture are only available in the dilute region the amountof adsorption of the solvent may be constant and all changes in the total amountadsorbed are due to just the solute In that case the adsorption mass of adsorbent isotherms are of the form of Figure.a which resembles the form obtained with pure gases It is then common to fit with adsorbent the data with concentration forms of theFreundlich equation SORPTION SYSTEMS Common Commercial Methods for.Adsorption Separations 1 Stirred tankslurry operation the rate of adsorption is rapid With good agitation and small particles the external resistance to mass transfer.from the bulk liquid to the external surface of the adsorbent particles is small The main application of this mode of operation is the removal of very smallamounts of dissolved and relatively large molecules such as coloring agents from the spent adsorbent is removed from theslurry by sedimentation or filtration and. can operated continuously 2 Cyclic fixed bed batch operation Bed pressure drop decreases with increasing particle size The solute transport rate increases with decreasing particle size For purification or bulk separation of gases the adsorbent is almost always.regenerated in place 3 Continuous countercurrent operationIt is difficult to circulate the solid adsorbent as a moving bed to achieve asteady state operationOnly a few units were installed. UOP Sorbex processfeed entrydesorbent entryextract adsorbed removalraffinate non adsorbed .EC extract column RC raffinate column 45 Regeneration methods1 Thermal temperature swing adsorption TSA method2 Inert purge swing method.3 Pressure swing adsorption PSA cycle4 Vacuum swing adsorption5 Displacement purge displacement desorption cycle 1 thermal temperature swing adsorption TSA method. the adsorbent is regenerated by desorption at a temperature higher than thatused during the adsorption step of the cycle The temperature of the bed is increased 1 heating coils located in the bedfollowed by pulling a moderate vacuum. 2 heat transfer from an inert non adsorbing hot purge gas such as steam heating and cooling of the bed requireshours and the quantity of adsorbent inthe bed is reasonable thus TSA is practical.only for purification involving small ratesof adsorption Fluidized bed TSAPurasiv process with a fluidized bed foradsorption and moving bed for desorption .Application remove small amounts of solvents from air removal of moisture C02 and pollutantsfrom gas streams 2 inert purge swing method. Desorption is at the same temperature and pressureAdsorption, ion exchange, and chromatography Introduction Sorption operations: certain components of a fluid phase, called solutes, are selectively transferred to insoluble, rigid particles suspended in a vessel or packed in a column.