ADSORPTION EQUILIBRIA AND REGENERATION VIVEK KUMAR
ADSORPTIONAdsorption is the process in which matter is extracted from one phase and concentrated at the surface ofa second phase. (Interface accumulation). This is a surface phenomenon as opposed to absorption wherematter changes solution phase, e.g. gas transfer. This is demonstrated in the following schematic. d
TYPES OF ADSORPTION•Exchange adsorption (ion exchange)– electrostatic due to charged sites on the surface.Adsorption goes up as ionic charge goes up and as hydrated radius goes down.• Physical adsorption: Van der Waals attraction between adsorbate and adsorbent. The attractionis not fixed to a specific site and the adsorbate is relatively free to move on the surface. This isrelatively weak, reversible, adsorption capable of multilayer adsorption.•Chemical adsorption: Some degree of chemical bonding between adsorbate and adsorbentcharacterized by strong attractiveness. Adsorbed molecules are not free to move on the surface.There is a high degree of specificity and typically a monolayer is formed. The process is seldomreversible.
SOME GENERAL ISOTHERMS IRREVERSIBLE FAVORABLE STRONGLY FAVORABLE UNFAVORABLEFLUID PHASE CONCENTRATION, MASS/VOL
ADSORPTION ON SOLID SURFACES Adsorption process Adsorbent and adsorbate Adsorbent (also called substrate) - The solid that provides surface for adsorption high surface area with proper pore structure and size distribution is essential good mechanical strength and thermal stability are necessary Adsorbate - The gas or liquid substances which are to be adsorbed on solid Surface coverage, The solid surface may be completely or partially covered by adsorbed molecules number of adsorption sites occupied define θ = θ = 0~1 number of adsorption sites available
LANGMUIR ISOTHERMAdsorption Isotherm: the mass of adsorbate per unit mass of adsorbent at equilibrium & at agiven temperatureRate of adsorption ra = k a p (1 − f ) (f: fraction of surface area covered)Rate of desorption rd = k d f ka p fAt equilibrium f = ka p + kd 1-fMono-layer coverage m = ka f ( m: mass of adsorbate adsorbed per unit mass of adsorbent)
For the Langmuir model linearization gives: Ce 1 C = + e qe K ⋅Q0 a Q0aA plot of Ce/qe versus Ce should give a straight line with intercept : 1 K ⋅Q 0 a 1 and slope: Q0 a Or: 1 1 1 1 = 0+ ⋅ q e Qa K ⋅ Qa Ce 0
FREUNDLICH ISOTHERM For the special case of heterogeneous surface energies (particularly good for mixed wastes) in which the energy term, “KF”, varies as a function of surface coverage we use the Freundlich model. 1 q e =K FCe n n and KF are system specific constants. 1Here a plot of 1/qe versus 1/Ce should give a straight line with intercept 1/Q and slope a o K ⋅Q0 a 1For the Freundlich isotherm use the log-log version : log q e = log K F + log C n A log-log plot should yield an intercept of log KF and a slope of 1/n.
The BET isotherm ● Theoretical development based on several assumptions: multimolecular adsorption 1st layer with fixed heat of adsorption H1 OT fig1.3 following layers with heat of adsorption constant (= latent heat of condensation) constant surface (i.e. no capillary condensation) givesBET method useful, but has limitations •microporous materials: mono - multilayer p 1 C −1 p adsorption cannot occur, (although BET surface = + ⋅ areas are reported routinely) va ( p0 − p) vm ⋅ C vm ⋅ C p0 •assumption about constant packing of N2 molecules not always correct? •theoretical development dubious (recent or molecular simulation studies, statistical mechanics) - value of C is indication o f the p p = I+ s⋅ shape of the isotherm, but not necessarily va ( p0 − p) p0 related to heat of adsorption
For the BET isotherm we can arrange the isotherm equation to get: Ce K B −1 C e 1 = ⋅ + (C S − C e ) ⋅ q e K B ⋅ Q 0 CS K B ⋅ Q 0 a a 1 Intercept = K B ⋅ Q0 a KB − 1 0 Slope = K B ⋅ Q a ⋅ Cs
Simplified method● 1-point method simplefied BET assuming value of C ≈ 100 (usually the case), gives p 1 C −1 p p = + ⋅ ≈ va ( p0 − p) vm ⋅ C vm ⋅ C p0 vm ⋅ p0 va ⋅ ( p0 − p) v m = p0 usually choose p/p0 ≈ 0,15 method underestimates the surface area by approx. 5%.
ADSORPTION ON SOLID SURFACE Summary of adsorption isothermsName Isotherm equation Application Cs BP Chemisorption and Useful in analysis ofLangmuir θ= = 0 reaction mechanism C∞ 1 + B0 P physisorptionTemkin θ =c1ln(c2P) Chemisorption Chemisorption Chemisorption and Easy to fit adsorptionFreundlich θ = c1 p 1 / C2 physisorption data P / P0 1 c−1BET = + ( P / P0 ) V ( 1 − P / P0 ) cVm cVm Multilayer physisorption Useful in surface area determination
ADSORPTION ON SOLID SURFACE Five types of physisorption isotherms are found over all solids I Type I is found for porous materials with small pores e.g. charcoal. It is clearly Langmuir monolayer type, but the other 4 are not II Type II for non-porous materialsamount adsorbed Type III porous materials with cohesive force between adsorbate molecules III greater than the adhesive force between adsorbate molecules and adsorbent IV Type IV staged adsorption (first monolayer then build up of additional layers) Type V porous materials with cohesive force between adsorbate molecules V and adsorbent being greater than that between adsorbate molecules 1.0 relative pres. P/P0
DETERMINATION OF APPROPRIATE MODELTo determine which model to use to describe theadsorption for a particular adsorbent/adsorbateisotherms experiments are usually run. Data fromthese isotherm experiments are then analyzedusing the following methods that are based onlinearization of the models.
ADSORPTION-DESORPTION HYSTERESIS ● Hysteresis is classified by IUPAC (see fig.) ● Traditionally desorption branch used for calculation ● H1: narrow distribution of mesopores ● H2: complex pore structure, network effects, analysis of desorption loopHandbook misleadingfig 2 s 431 H2: typical for activated carbons ● H3 & 4: no plateau, hence no well- defined mesopore structure, analysis difficult H3: typical for clays
PORES AND POROUS SOLIDS Pore sizes micro pores dp <20-50 nm meso-pores 20nm <dp<200nm macro pores dp >200 nm Pores can be uniform (e.g. polymers) or non-uniform (most metal oxides) Pore size distribution Typical curves to characterise pore size: Cumulative curve Frequency curve wt dw dd Uniform size distribution (a) & ∆wt a non-uniform size distribution (b) a b b ∆d d d Cumulative curve Frequency curve
POROSITY AND PORE SIZE● The pore structure (porosity, pore diameter, pore shape) is important for the catalytic properties pore diffusion may influence rates pores may be too small for large molecules to diffuse into● Measurement techniques: Hg penetration interpretation of the adsorption - desorption isotherms electron microscopy techniques
Hg PENETRATION● Based on measuring the volume of a non-wetting liquid forced into the pores by pressure (typically mercury)● Surface tension will hinder the filling of the pores, at a given pressure an equilibrium between the force due to pressure and the surface tension is established: P ⋅ π ⋅ r 2 = −2π ⋅ r ⋅ γ ⋅ cos α where P = pressure of Hg, γ is surface tension and α is the angle of wetting● Common values used: γ = 480 dyn/cm and α= 140° give average pore radius r= 75000 2 [ Å] P[kp / cm ] valid in the range 50 - 50000Å
PORE SIZE DISTRIBUTION● If the Hg-volume is recorded as a function of pressure and this curve is differentiated we can find the pore size distribution function V(r)=dV/dr OT fig 2.3.
FACTORS WHICH AFFECT ADSORPTION EXTENT (ANDTHEREFORE AFFECT ISOTHERM) ARE:Adsorbate:SolubilityIn general, as solubility of solute increases the extent of adsorption decreases. This is knownas the “Lundelius’ Rule”. Solute-solid surface binding competes with solute-solvent attractionas discussed earlier. Factors which affect solubility include molecular size (high MW- lowsolubility), ionization (solubility is minimum when compounds are uncharged), polarity (aspolarity increases get higher solubility because water is a polar solvent).pHpH often affects the surface charge on the adsorbent as well as the charge on the solute.Generally, for organic material as pH goes down adsorption goes up.TemperatureAdsorption reactions are typically exothermic i.e., H rxn is generally negative. Here heat isgiven off by the reaction therefore as T increases extent of adsorption decreases.Presence of other solutesIn general, get competition for a limited number of sites therefore get reduced extent ofadsorption or a specific material.
DEFINITIONIf the adsorbent and adsorbate are contacted long enough an equilibrium will be established between theamount of adsorbate adsorbed and the amount of adsorbate in solution. The equilibrium relationship isdescribed by isotherms.Define the following:qe = mass of material adsorbed (at equilibrium) per mass of adsorbentCe = equilibrium concentration in solution when amount adsorbed equals q e.qe/Ce relationships depend on the type of adsorption that occurs, multi-layer, chemical, physical adsorption,etc.Adsorption heat:The increase in enthalpy when 1 mole of a substance is adsorbed upon another at constantpressure. •Adsorption is usually exothermic (in special cases dissociated adsorption can be endothermic) •The heat of chemisorption is in the same order of magnitude of reaction heat; the heat of physisorption is in the same order of magnitude of condensation heat.
ADSORPTION EQUILIBRIA If the adsorbent and adsorbate are contacted long enough an equilibrium will be established between the amount of adsorbate adsorbed and the amount of adsorbate in solution. The equilibrium relationship is described by isotherms. Heats of AdsorptionGas adsorption to a solid is exothermic.The magnitude and variation as a function of coverage may reveal informationconcerning the bonding to the surface. Q Calorimetric methods determine heat, Q evolved. qi = n V qi = integral heat of adsorption δQ qD = qD = differential heat of adsorption δ n V ,T
HEAT OF ADSORPTION .. CONTINUED● Since ∆G=∆H-T∆S, it is clear that for ∆G to be negative, ∆H of adsorption process must be negative. That is, the adsorption is an exothermic process.● the amount of gas adsorbed will decrease as the temperature is increased.● The molar enthalpy, ∆adsHm, of adsorption in reversible system will adhere to the Clausius-Clapeyron equation ∂ ln p ∆ ads H m ÷ =−08/23/12 ∂T n RT 2● The subscript n represents an isosteric adsorption. ∆adsHm is called the molar isosteric enthalpy of adsorption.
ENTHALPY OF ADSORPTION Heats of adsorption change as a function of surface coverage M g + S surface ⇔ M − S surface ∆G 0 AD = − RT ln K = ∆H 0 0 AD − T∆ S 0 AD ∆H ∆S0 0 ln K = − 0 + AD AD RT R δ 0 ∆H AD 0 δT ln K = RT 2differentiate θ Van’t Hoff equation
DESORPTION AND REGENERATION OF ADSORBENTSAdsorbent particles have finite capacity for fluid phase molecules. An extendedcontact with the feed fluid will ultimately lead to the creation of athermodynamic equilibrium between the solid adsorbent and the fluid phases. Atthis equilibrium condition the rates of adsorption and desorption are equal andthe net loading on the solid cannot increase further, It is now becomes necessaryeither to regenerate the adsorbent or to dispose of it.In certain applications it may be more economical to discard the adsorbent afteruse. Disposal would be favoured when the adsorbent is of low cost, is verydifficult to regenerate, and the non-adsorbed component is the desired product ofvery high value. In the majority of applications, the disposal of adsorbents aswaste is not an economic option and therefore regeneration is carried out eitherin situ or external to the adsorption vessel to an extent that the adsorbents can bereused.
PRACTICAL REGENERATION METHODSPractical methods of desorption and regeneration include one, or more usually acombination, of the following: Increase in temperature Reduction in partial pressure Reduction in concentration Purging with an inert fluid Displacement with a more strongly adsorbing species Change of chemical conditions, e.g. pHThe final choice of regeneration method(s) depends on technical and economicconsiderations.
FIXED-BED ADSORPTION PROCESS AND THEIR REGENERATION METHODSA. Pressure Swing Adsorption (PSA)Regeneration in a PSA process is achieved by reducing the partialpressure of the adsorbate. There are 2 ways in which this can be achieved:(1) a reduction in the system total pressure, and (2) introduction of aninert gas while maintaining the total system pressure. In the majority ofpressure swing separations a combination of the 2 methods is employed.Use of a purge fluid alone is unusual.
PICTORIAL EXPLAINATIONThe Figure below shows the effect of partial pressure on equilibrium loading fora Type I isotherm at a temperature of T1. Reducing the partial pressure from p1to p2 causes the equilibrium loading to be reduced from q1 to q2. Changes in pressure can be effected very much more quickly than changes in temperature, thus cycle time of pressure swing adsorption (PSA) processes are typically in the order of minutes or even seconds. PSA processes are often operated at low adsorbent loadings because selectivity between gaseous components is often greatest in the Henrys Law region. It is desirable to operate PSA processes close to ambient temperature to take advantage of the fact that for a given partial pressure the loading is increased as the temperature is decreased. Typical PSA processes consist of 2-Bed system, although other systems (e.g. 1- Bed system or complex, multiple-beds system) had also been developed.
USES OF PSA PROCESSESPSA processes is a popular process for performing bulk separations of gases.Separations by PSA and VSA are controlled by adsorption equilibrium oradsorption kinetics. Both types of control are important commercially. Forthe separation of air with zeolites, adsorption equilibrium is the controllingfactor. Nitrogen is more strongly adsorbed than oxygen. For air with about21% oxygen and 79% nitrogen, a product of nearly 96% oxygen purity canbe obtained. When carbon molecular sieves are used, oxygen and nitrogenhave almost the same adsorption isotherms, but the effective diffusivity ofoxygen is much larger than nitrogen. Hence more oxygen is adsorbed thannitrogen, and a product of very high purity nitrogen ( 99%) can be obtained.
B. TEMPERATURE SWING ADSORPTION (TSA)Regeneration of adsorbent in a TSA process is ahieved by an increase in temperature.The Figure below showed schematically the effect of temperature on the adsorptionequilibrium (Type I isotherm) of a single adsorbate. For any given partial pressure of the adsorbate in the gas phase (or concentration in the liquid phase), an increase in temperature leads to a decrease in the quantity adsorbed. If the partial pressure remains constant at p1, increasing the temperature from T1 to T2 will decrease the equilibrium loading from q1 to q2. A relatively modest increase in temperature can effect a relatively large decrease in loading. It is therefore generally possible to desorb any components provided that the temperature is high enough. However, it is important to ensure that the regeneration temperature does not cause degradation of the adsorbents.
TSA.. CONTD........A change in temperature alone is not used in commercial processesbecause there is no mechanism for removing the adsorbate from theadsorption unit once desorption from the adsorbents has occurred.Passage of a hot purge gas or steam, through the bed to sweep out thedesorbed components is almost always used in conjunction with theincrease in temperature.A very important characteristic of TSA processes is that they are usedvirtually exclusively for treating feeds with low concentrations ofadsorbates.
C. DISPLACEMENT PURGE ADSORPTION (DPA)Adsorbates can be removed from the adsorbent surface by replacing them with amore preferentially adsorbed species. This displacement fluid, which can be agas, a vapour or a liquid, should adsorb about as strongly as the componentswhich are to be desorbed. If the displacement fluid is adsorbed too strongly thenthere may be subsequent difficulties in removing it from the adsorbent.The mechanism for desorption of the original adsorbate involves 2 aspects: (1) partial pressure (or concentration) of original adsorbate in the gas phasesurrounding the adsorbent is reduced (2) there is competitive adsorption for the displacement fluid. Thedisplacement fluid is present on the adsorbent and thus will contaminate theproduct.One advantage of the displacement fluid method of regeneration is that the netheat generated or consumed in the adsorbent will be close to zero because theheat of adsorption of the displacement fluid is likely to be close to that of theoriginal adsorbate. Thus the temperature of the adsorbent should remain more orless constant throughout the cycle.
PICTORIAL EXPLAINATIONWith neither pressure nor temperature changes from adsorption to desorption,regeneration by displacement purge depend solely on the ability of the displacementfluid to cleanse the bed in readiness for the next adsorption step. A typicalDisplacement Purge Adsorption (DPA) process is shown in the Figure below. A is the more strongly adsorbed component in the binary feed mixture of (A and B) while D is the displacement purge gas. The feed mixture of (A and B) is passed through Bed 1 acting as the adsorber, which is preloaded with D from the previous cycle (when Bed 1 was the regenerator). A is adsorbed and the product of a mixture of (B and D) emerges from the top of the column. (B and D) are easily separated by distillation so that B is collected in a relatively pure state. The displacement gas D then enters Bed 2 acting as regenerator and from which emerges a mixture of (A and D). (A and D) can be separated without difficulty in another distillation column.
DPA.. CONTD........In effect the original mixture of (A and B), which would have been difficult to separate by PSA or TSA, is separatedby the "intervention" of another strongly adsorbed component D. The ease of separation of A from D, and B from D,in the additional distillation stages, is crucial in determining the economies of displacement purge cycle operation.Examples of commercial processes include the separation of linear paraffins from mixtures containing branchedchain and cyclic isomers in the range of C10 - C18 hydrocarbons.Other Adsorption CyclesVirtually all adsorption processes use changes in temperature, pressure, concentration of a competitvelyadsorbing component to effect adsorption and desorption. But presumably any other variables which couldeffect changes in the shape of an adsorption isotherm could also be used.One such variable is the pH. The bonding between some adsorbents and adsorbates such as amino acids inwater can be changed remarkably as the pH is changed from above the isoelectric point of the amino acid tobelow its isoelectric point. The isoelectric point is the pH at which the amino acid molecule has zero charge.The economic problem of using pH swing as a means to drive a cyclic process is the cost of the acid and baserequired to change the pH, as well as the cost of disposal of the salt by-product.Another means for changing the shape of the adsorption isotherm is the use of electric charge. Electrosorptioninvolves adsorption when the adsorbent is subjected to one voltage and the desorption when the voltage ischanged. Typically the voltage can be small, such as 1V or less. This process can only be accomplished incases in which both the adsorbent and the feed stream are highly conductive. An example is EDA(ethylenediamine), which demonstrates different loading at different voltages.