Adsorption

Selected Chemical Engineering Operations

Adsorption (Introduction) • It is a separation process – certain component of a fluid phase are transferred to the surface of a solid adsorbents – Forming a distinct adsorbed phase

• Unlike absorption – in which solute molecules diffuse from the bulk of the gas phase to the bulk of the liquid phase

• Separation by adsorption depends on one component being more readily adsorbed than another • The selection of a suitable process may also depends on the ease with which the separated components can be recovered


Introduction • Usually the small particles of adsorbent are held in a fixed-bed – fluid is passed continuously through the bed – until the solid is nearly saturated

• The flow is then switched to a second bed – until the saturated adsorbents can be replaced or regenerated

• Ion exchange is another process – that is carried out in this semi-batch fashion in a fixed-bed – Water to be softened or deionized is passed over beds of ion exchange resin


Introduction • Chromatography is a process – similar to adsorption – in that gas or liquid mixtures are passed through a bed of porous particles – the feed is introduced in small pulses, rather than continuously

• The individual component move through the bed at different rates and are collected at the exit


Adsorbents and Adsorption Process • Most adsorbents are highly porous materials – adsorption takes place primarily on the walls of the pores or at specific sites inside the particle

• The internal surface area is orders of magnitude greater than the external area and is often 500 to 1,000 m2/g • Separation occurs because – differences in molecular weight, shape or polarity – cause some molecules to be held more strongly on the surface than others – or because the pores are too small to admit the larger molecules


Adsorbents and Adsorption Process • Applications (vapor and liquid phase) – Drying of gases – Separation of oxygen and nitrogen using molecular sieves – Separate normal paraffins from branched paraffins and aromatics – To remove organic components from drinking water aqueous wastes – Colored impurities from sugar solutions and vegetable oils and water from organic liquids

• Adsorbents with larger pores are preferred for use with liquids


Adsorption Equipment (Fixed-bed Adsorbers) • The adsorbent particles are placed in a bed deep supported on a screen or perforated plate • The feed gas passes down through one of the beds while the other is being regenerated – Downflow is preferred – Upflow at high rates might fluidize the particles, causing attrition and loss of fines

• When the concentration of solute in the exit gas reaches a certain value or at a scheduled time – valves are automatically switched to direct the feed to the other bed and initiate the regeneration sequence

• Regeneration can be carried out with hot inert gas – but steam is usually preferred if the solvent is not miscible with water


Fixed-bed Adsorbers


Fixed-bed Adsorbers • The bed may then be cooled and dried with inert gas – but it is not necessary to lower the entire bed to ambient temperature

• If some water vapor can be tolerated in the clean gas – evaporation of water during the adsorption cycle will help cool the bed and partially offset the heat of adsorption

• The size of the adsorbent bed is determined by the gas flow rate and the desired cycle time • By using the longer bed – the adsorption cycle could be extended to several days – but the higher pressure drop and the greater capital cost of the adsorber


Gas Drying Equipments • It is similar to that shown in fixed-bed adsorption – but hot gas is used for regeneration

• If the gas dryer operates at several atmospheres pressure during the adsorption cycle – nearly complete regeneration can be accomplished by passing part of the dry gas through the bed at atmospheric pressure without preheating

• Some of the heat of adsorption which was stored in the bed as sensible heat of the solid – becomes available for desorption and the bed cools during regeneration


Pressure Swing Adsorption • A simple PSA scheme for air separation uses two beds of molecular sieves – with one adsorbing at several atmospheres pressure – and the other being regenerated at 1 atm

• More complex schemes uses three or four beds – – – – –

with only one bed adsorbing the other being depressurized Purged or repressurized all under control of a sequence timer


Temperature Swing Adsorption • The spent adsorption bed is regenerated by heating it with embedded steam coils or with a hot purge gas stream to remove the adsorbate • Finally, the bed must be cooled so that it can be used for adsorption in the next cycle • The time for regeneration is generally few hours or more


Adsorption from Liquids • An important example – use of activated carbon to remove pollutants from aqueous wastes

• Carbon adsorbents are also used to remove trace organics from municipal water supplies • Tall beds are needed to ensure adequate treatment – because the rate of adsorption from liquids is much slower than from gases

• There are number of commercial adsorbents • All are characterized by very large pore surface areas of 100 to 2000 m2/g


Different Type of Adsorbents • • • • •

Activated carbon Silica gel Activated alumina Molecular sieve zeolites Synthetic polymers and resins


Types of Isotherms • Linear Isotherm – the amount adsorbed is proportional to the concentration in the fluid W = kC

• Favorable Isotherm – That are convex upward are called favorable – because a relatively high solid loading can be obtained at low concentration in the fluid

• Unfavorable Isotherm – – – – –

That is concave upward is called unfavorable Because relatively low solid loadings are obtained It leads to quite long mass transfer zones in the bed they are worth studying to understand the regeneration process If the adsorption isotherm is favorable, mass transfer from the solid back to the fluid phase has characteristics similar to those for adsorption with an unfavorable isotherm


Types of Isotherms


Types of Isotherms • Langmuir Isotherm

kC W = Wmax 1 + kC

– This isotherm is of the favorable type

• The equation was derived assuming that – there are only a fixed number of active sites available for adsorption – only a monolayer is formed and the adsorption is reversible and reaches an equilibrium conditions – When k is large kC>>1, the isotherm is strongly favorable – When kC<<1, the isotherm is nearly linear

• The langmuir isotherm is derived assuming a uniform surface – not a valid assumption- but the relation works fairly well for gases that are weakly adsorbed


Types of Isotherms • Freundlich Equation – Strongly favorable isotherms

W = bC m – m<1 is often a better fit – particularly for adsorption from liquids


Types of Isotherms • Many applications involve adsorption of multicomponent mixtures • The Langmuir isotherm is easily modified for multiple adsorbates

  k1C1  W = Wmax   1 + k1C1 + k 2C2 + − − − − −  • However, this equation is not very satisfactory for strongly adsorbed materials


Principles of Adsorption • A fixed bed of granular particles – The fluid to be treated is usually passed down through the packed bed at a constant flow rate

• Mass transfer resistances are important – the process is unsteady state – The concentration of the solute in the fluid phase and of the solid adsorbent phase change with time and also with position in the fixed bed as adsorption proceeds

• The overall dynamics of the system – determines the efficiency of the process rather than just the equilibrium consideration

• At the inlet to the bed the solid is assumed to contain no solute at the start of the process


Principles of Adsorption • As the fluid first contacts the inlet of the bed – most of the mass transfer and adsorption takes place here

• As the fluid passes through the bed – the concentration in this fluid drops very rapidly with distance in the bed to zero way before the end of the bed is reached

• After a short time – the solid near the entrance to the tower is almost saturated – most of the mass transfer and adsorption now takes place at a point slightly farther from the inlet


Principles of Adsorption


Principles of Adsorption

t = t1

t = t2

t = t3

C/Co = 1

C/Co = 0


Principles of Adsorption • The solid at the entrance would be nearly saturated – and this concentration would remain almost constant down to the mass transfer zone – where it would drop rapidly to almost zero

• The difference in concentration is the driving force for mass transfer • The major part of the adsorption at any time takes place in a relatively narrow adsorption or mass transfer zone – which is S shaped, moves down the column

• The outlet concentration remains near zero until the mass transfer zone starts to reach the tower outlet


Principles of Adsorption • The breakpoint concentration – the maximum that can be discarded – often taken as 0.01 to 0.05 for Cb/C0

• For a narrow mass transfer zone – the breakthrough curve is very steep – most of the bed capacity is used at the break point

• When the mass transfer zone is almost as long as the bed – the breakthrough curve is greatly extended – and less than one half of the bed capacity is utilized

• This makes efficient use of the adsorbent – lowers energy costs for regeneration


Capacity of Columns • By material balance – the area between the curve and a line at C/C0 = 1.0 is proportional to the total solute adsorbed – if the entire bed comes to equilibrium with the feed

• For a symmetric curve – t* (ideal adsorption time) is also the time when C/C0 = 0.5

• The movement of the adsorption front through the bed and the effect of process variables on t* can be obtained by a simple material balance • For a unit area of bed cross section – the solute feed rate is the product of the superficial velocity and the concentration

FA = u0C0


Capacity of Columns • For an ideal breakthrough curve – all the solute fed in time t* is adsorbed – the concentration on the solid has increased from the initial value W0 to the equilibrium or saturation value, Wsat

u0C0t ∗ = Lρ b (Wsat − W0 ) t∗ =

Lρ b (Wsat − W0 ) u 0 C0

– For fresh or completely regenerated feed W0 = 0


Scale-up • The width of the mass transfer zone depends on – the mass-transfer rate – the flow rate – the shape of the equilibrium curve

• Usually adsorbers are scaled up from laboratory tests in a small diameter bed – the large unit is designed for the same particle size and superficial velocity

• The bed length need not be the same


Length of Unused Bed • For system with a favorable isotherm – the concentration profile in the mass transfer zone soon acquires a characteristic shape – and width that do not change as the zone moves down the bed

• Tests with different bed lengths give breakthrough curve of the same shape • With longer beds – the mass transfer zone is a smaller fraction of the bed length – greater fraction of the bed is utilized

• At the breakpoint – the solid between the inlet of the bed and the start of the mass transfer zone is completely saturated


Length of Unused Bed • The solid in the mass-transfer zone goes from nearly saturated to almost no adsorbate – the solid could be assumed to be about one half saturated

• The length of unused bed does not change with the total bed length • The total shaded area represents the total or stoichiometric capacity of the bed as follows  C tT = ∫ 1 − dt C0  0 ∞

• The usable capacity of the bed up to the break point time tb is the crosshatched area  C tu = ∫ 1 − dt C0  0 tb


Length of Unused Bed


Length of Unused Bed • The ratio of tu/tT is the fraction of total bed capacity or length utilized up to the breakpoint • If HT is total bed length and HB is the length of the bed tu used up to the break point H B = HT tT • The length of unused bed HUNB is then H UNB

 tu = 1 −  tT

  H T 

• The HUNB represents the mass transfer section or zone – Its depends on the fluid velocity – and is essentially independent of total length of the column


Length of Unused Bed • The full scale adsorber can be designed simply by – first calculating the length of bed needed to achieve the required usable capacity, HB, at the break point

• To obtain the total length HT , H T = H UNB + H B • This design procedure is widely used • The small tower unit must be well insulated to be similar to the large diameter tower, which operates adiabatically • The mass velocity in both units must be the same – the bed of sufficient length to contain a steady state mass transfer zone

• Axial dispersion or axial mixing may not be exactly same in both towers


Effect of Feed Concentration • The effect of moderate changes in feed concentration on the breakthrough curve can be predicted • The equilibrium capacity is determined from the adsorption isotherm • The breakpoint time is proportional to the capacity of the solid and to reciprocal of the feed concentration • Very large difference in concentration may lead to errors in scale-up – because of a change in the mass transfer coefficient or because of temperature effect

• In smaller beds, heat loss will limit the temperature rise, but a large unit will operate almost adiabatically – significant difference in performance could result


Rate of Mass Transfer • The differential mass balance equations – for an element of the adsorption column – and for an adsorbent particle within such an element – provide the starting point for development of a mathematical model to describe the dynamic behavior of the system

• The rate of accumulation in the fluid and in the solid is the difference between input and output flows • The change in superficial velocity is neglected


Rate of Mass Transfer ∂C ∂W + (1 − ε )dLρ P = u0C − u0 (C + dC ) ∂t ∂t ∂C ∂W ∂C ε + (1 − ε ) ρ P = −u0 ∂L ∂t ∂t

εdL

• Solute dissolved in the pore fluid is included with the particle fraction (1- ε) • For adsorption from a gas or a dilute solution – The accumulation in the fluid, is usually negligible compared to the accumulation on the solid

• The mechanism of transfer to the solid include – diffusion through the fluid film around the particle – diffusion through the pores to the internal adsorption sites


Rate of Mass Transfer • The actual process of physical adsorption is practically instantaneous – equilibrium is assumed to exit between the surface and the fluid at each point inside the particle

• The transfer process is approximately using an overall volumetric coefficient and overall driving force ∂W ρ p (1 − ε ) = K c a (C − C ∗ ) ∂t • The mass transfer area a is taken as the external surface of the particles, which is 6(1- ε)/DP for spheres • The concentration C* is the value that would be in equilibrium with the average concentration W in the solid


Internal and External Mass Transfer • The overall coefficient Kc depends on – the external coefficient Kc,ext – and on an effective internal coefficient Kc,int

• Diffusion within the particle is actually an unsteady process – the value of Kc,int decreases with time – as solute molecule must penetrate farther and farther into the particle to reach adsorption site

• An average effective coefficient can be used to give an approximate fit to uptake data for spheres K c ,int ≈

10 De DP

1 1 D ≈ + P K c K c ,ext 10 De


Internal and External Mass Transfer • The effective diffusion coefficient De depends on – – – –

the particle porosity the pore diameter the tortuosity and the nature of diffusing species

• Diffusion of adsorbed molecules along the pore walls – surface diffusion – often contributes much more to the total flux than diffusion in the gas phase

• In the adsorption of solute from aqueous solutions, surface diffusion also occurs – but its effect is hard to predict


Internal and External Mass Transfer • In some cases surface diffusion is slow, and the internal diffusion resistance dominates – but in others the internal and external resistances are nearly equal

• There are many solutions to for the above equations – for different isotherm shapes and controlling steps

• All solutions involve a dimensionless time τ and a parameter N representing overall number of transfer unit u0C0 (t − Lε u0 ) τ= ρ P (1 − ε ) L(Wsat − W0 ) K c aL N= u0


Internal and External Mass Transfer • The term Lε/u0 – time to displace fluid from external voids in the bed – which is normally negligible – ρP(1-ε) is the bed density ρb

• Then τ is the ratio of the time to the ideal time t* τ = t t ∗ • If there were no mass transfer resistance – the adsorber could be operated with complete removal of solute up to τ = 1, – and then the concentration would jump from 0 to C/C0 = 1

• With a finite rate of mass transfer, – breakthrough occurs at τ<1.0 – steepness of the breakthrough curve depends on the parameter N and the shape of the equilibrium curve


Adsorber Design • Design involves – choosing the adsorbent and the particle size – selecting an appropriate velocity to get the bed area – determining the bed length or calculating the breakthrough time

• Using a shorter bed length – smaller inventory of sorbent and lower pressure drop – frequent regeneration and higher regeneration costs

• For gas purification – larger particle size is used – but smaller sizes can be used when better mass transfer is needed and pressure drop is not a problem

• Reduction in particle size gives a much steeper breakthrough curve


Adsorber Design • For adsorption from liquids – smaller particle sizes are chosen – the fluid velocity is much lower than with gases

• It is difficult to mach conditions for a large adiabatic bed in a laboratory adsorber – modeling is complicated because the desorption isotherm is nonlinear and unfavorable

• For some processes where adsorption is used to recover a valuable product – regeneration by thermal swing desorption is not feasible – because high temperature needed for desorption would degrade the product


Ion Exchange • Solid particles containing exchangeable cations or anions are contacted with electrolyte solution • Major applications are seen in softening water – by exchanging calcium ions for sodium ions – demineralization of water by removing both cations and anions

• The technique used in ion exchange so closely resemble those used in adsorption – Ion exchange can be considered as a special case of adsorption

• In ion exchange certain ions are removed by the ionexchange solid • Since electroneutrality must be maintained – the solid releases replacement ions to the solution


Ion Exchange • The first ion-exchange materials were natural-occurring porous sands called zeolites and or cation exchangers

Ca 2+ + Na2 R ⇔ CaR + 2 Na + (solution)

(solid)

(solid) (solution)

– This is the basis for “softening of water”

• To regenerate, a solution of NaCl is added – which drives the reversible to reaction above the left

• Almost all of these inorganic ion-exchange solids exchange only cations • Mostly, ion exchange solids are synthetic resins or polymers


Ion Exchange • These anionic groups can exchange cations

Na + + HR ⇔ NaR + H + (solution) (solid) (solid) (solution) • Similar synthetic resin containing amine group can be used to exchange anions and OH- in solution Cl − + RNH 3OH ⇔ RNH 3Cl + OH − (solution) (solid)

(solid)

(solution)


Ion Exchange Equilibria • The capacity of the ion exchanger is the number of exchangeable groups per unit mass of dry resins • For cation resins – the capacity is often expressed as meq per gram of dry hydrogen form resin or meq/gH+

• For anion resin – the basis is a gram of dry chlorine form resin for practical purposes – the capacity may be given in equivalents per liter of bed

•

Ion exchange is reversible reaction – in which the counterion in the resin is replaced by a different ion from the external solution

A+ + NaR ⇔ AR + Na +

Cl − + RNH 3OH ⇔ RNH 3Cl + OH −


Ion Exchange Equilibria • The equilibrium constant for the reaction

K eq =

C Na + C AR C A+ C NaR

γ Na γ AR × γ A γ NaR +

+

• For dilute solutions – the activity coefficients do not change much with concentration times activity coefficients

K eq =

C Na + C AR C A+ C NaR


Ion Exchange Equilibria • Diffusivity for ions in the resin are lower than in the external solution – because of the limited internal void fraction, the tortuosity, and to some extent the hindered diffusion in small pores

• For monovalent ions – the effective diffusivity is about an order of magnitude lower than normal

• For divalent ions – it may be lower that two order of magnitude

• Fortunately the internal resistance to diffusion is often no large than the external resistance and may even be negligible


Ion Exchange Equilibria • Regeneration of exhausted bed is carried out – by passing a concentrated solution of acid, base or salt through the be – By using a concentrated solution, the minimum regeneration time is decreased


Chromatography • Class of processes for separation of multi component mixtures of gases or liquids • It uses a bed of solids or immobilized liquid as a stationary phase – intermittent feed of the materials to be separated

• Components of the mixture are moved through the bed or eluted by continuous flow of carrier gas or liquid – mobile phase

• Feed component partition between the mobile and stationary phases – and move at different rates through the beds because of different distribution coefficient


• If the bed or column is long enough – all the components emerge sequentially as separate pulses – an analyzer at the exit shows the concentration of each component in the mobile phase

• The term chromatography arose from the bands of color seen – when a glass column was used to separate liquid mixtures of plant cell pigments

• It is now applied to other similar separation • The column analyzer and associated equipment for controlling the flow rate and temperature are called a chromatograph


• Chromatographs are classified by the nature of the mobile and stationary phases – GSC (gas solid chromatograph) – GLC (gas liquid chromatograph) – LLC (Liquid-liquid chromatograph)

• In GSC – if one component is strongly adsorbed with a non-linear isotherm – the resulting peak is asymmetric with a long tail – which makes the separation from other peaks difficult

• In GLC – the feed can be a gas but it is often a liquid injected as a small sample into a vaporizer – swept into the column by the carrier gas


• The separation of liquid mixtures by chromatography (LC) – can be carried out in a column containing a solid stationary phase, LSC, – or with an immiscible liquid as the stationary phase, LLC

• Diffusion resistances are important in liquid chromatography – performance is improved by using very small particles in the stationary phase

• The term high performance liquid chromatography (HPLC) – is applied to separations carried out at high pressure with very fine particles and high flow rates


Gel Permeation Chromatography • A different separation principle is used • A mixture of high molecular weight polymers or biomaterials dissolved in a liquid is fractionated • The largest molecules are excluded from some of the pores in the stationary phase • The molecules are separated by size – with the largest ones coming out first and smallest eluted last


Preparative Chromatography • When a chromatography process is scaled up for commercial production • Increasing the column diameter usually increases the peak broadening – caused by uneven flow distribution and wall effects

• The height of theoretical plate, HETP may increase several fold – unless great care is taken in packing the column and getting uniform distribution of the feed

• Using a narrow range of particle sizes and vibrating the column while it is being packed – help keep the HETP close to the laboratory values

• Baffles may be introduced at regular intervals to redistribute the flow


Thank You

Adsorption

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