Solid Liquid Adsorption for Wastewater Treatment:
Principle Design and Operation

By K. Vasanth Kumar, K. Subanandam, V. Ramamurthi and S. Sivanesan
February 2004

The Authors are Research Scholars at the Department of Chemical Engineering - A.C. College of Technology, Anna University, in Chennai - India.   → See also:

Introduction

Among the unit operations in water and wastewater treatment «adsorption» occupies an important position. Adsorption operations exploit the ability of certain solids preferentially to concentrate specific substances from solution onto their surfaces. In the field of gaseous separations, adsorption is used to dehumidify air and other gases, to remove objectionable odors and impurities from industrial gases such as CO2, to recover valuable solvent vapors from dilute mixtures. Liquid separations include the removal of objectionable taste and odor.

Sorption, which is a general term introduced by J.W. McBain (Seader and Henley, 1999), includes selective transfer to the surface, and for into the bulk of liquid. Thus adsorption of gas into a non-porous membrane is also sorption operations. In a general sorption process, the sorbed solutes are referred to as sorbate & the sorbing agent is the sorbent. Adsorption process may be classified as purification or bulk separation, depending on the concentration in the feed fluid to the component adsorbed. Although there is no sharp dividing concentration, Keller suggested 10wt% (Seader and Henley, 1999). Early applications of adsorption involved only purification, for e.g.: adsorption with charred wood to improve the taste of water has been known for at least five centuries. Adsorption of gases by a solid charcoal was first described by C.W. Chele in 1773 (Seader and Henley, 1999). Commercial applications of bulk separation by gas adsorption began in early 1920s. But did not escalate until the 1960s, following the invention by Milton of synthetic molecular sieve zeolites, which provide high adsorptive selectivity. Later the pressure swing cycle of Skarstrom, which made possible the efficient of operation of a fixed bed cyclic gas adsorption process (Seader and Henley, 1999). The commercial scale bulk separation of liquid mixtures also began in 1960s, following the invention by Broughton & Greshold of the simulated moving bed for adsorptive separation.

Adsorption Phenomena: Classification

Based on the nature of the bonding nature between the molecule and the surface, adsorption phenomena can be classified as physiorption and chemisorption. In physical adsorption, the only bonding is by weak Vander Waals - type forces. There is no significant redistribution of electron density in either the molecule or at the substrate surface. In chemisorption, a chemical bond, involving substantial rearrangement of electron density, is formed between the adsorbate and substrate. The nature of this bond may lie anywhere between the extremes of virtually complete ionic or complete covalent character. The important classification between chemisorption and physiorption is shown in Table 1.

Table 1. Typical Characteristics of Adsorption Processes
Characteristics Physical Adsorption Chemical Adsorption
Binding force Due to physical force of attraction, thus this process is also called as Vanderwaal´s adsorption Due to chemical forces or bonding, thus this process is also called as activated adsorption.
Saturation uptake Multilayer phenomena Single layer phenomena
Activation Energy No activation energy involved May be involved
Temperature Range
(over which adsorption occurs)
Adsorption is appreciable at lower temperature below boiling point of adsorbate Adsorption can take place even at higher temperature
Nature of sorbate Amount of adsorbate removed depends more on adsorbate than on adsorbent Depends on both adsorbent and adsorbate
Crystallographic specificity Virtually independent of surface atomic geometry Marked variation between crystal planes
Heat of adsorption 1 Kcal/mole 50 - 100 Kcal/mole

Adsorbents

Any solid has some tendency to adsorb fluid medium onto their surface, however only some solid materials have the selective adsorption capacity to adsorbate molecules. The adsorbate may be organic compound, color, odor, moisture etc. Keller (1995) lists the four most common adsorption processes in terms of their estimated annual sales, along with their characteristics, applications and disadvantages (Table 2).

Table 2. Characteristics of Different Adsorbents
Type Characteristics Use Disadvantages
Activated Carbon Hydrophobic, favors organics over water Removal of organic pollutants Difficult to regenerate
Zeolites Hydrophillic, polar, regular channels Air separation, dehydration Low total capacity
Silica gel High capacity, hydrophillic Drying gas streams Trace removal not effective
Activated alumina High capacity, hydrophiilic Drying gas streams Trace removal not effective

Factors Affecting the Rate of Adsorption

Surface area of the adsorbent:
The rate of adsorption increases with increase in surface area of the adsorbent
Rate of adsorption ∝ 1/ Diameter of adsorbent for powdered activated carbon
Rate of adsorption ∝ (1/ Diameter of adsorbent) n for granular activated carbon
The parameter "n" is there because of porous structure of granular activated carbon (n= 1.85 – 2)
pH: depends on the nature of solvent solute system
Nature of solute (adsorbate):
Solubility of solute: Adsorption ∝ (1/Solubility of solute in solvent)
Chain length of molecules: Adsorption ∝ chain length of molecule
Molecular size of solute: Increase in molecular size of solute favors adsorption
Geometry of the molecule: Branched- lesser tendency to get removed, Coiled- more easily removed because easily entrapped in the interstices.
Degree of ionization: Adsorption ∝ (1/ Dissociation constant) this is the reason why salts are not much removed by activated carbon
Surface tension of the solvent:
Those substances which lower the surface tension of solvent in which they are dissolved become concentrated in surface layer (eg: organic substances), while those substances which raise the surface tension are less concentrated in the surface than in the bulk of solution. (eg.inorganic ions)
1   (1)
Where
S, is the amount of solute adsorbed per unit area of surface at equilibrium
C, is the concentration of solute in sulk of solution
T, absolute temperature
√, Variation of surface tension with concentration
- Sign indicates whether there is excess or deficit

Adsorption Equillibria

Generally adsorption process proceeds through varied mechanisms such as external mass transfer of solute onto sorbent followed by intraparticle diffusion. Unless extensive experimental data are available concerning the specific adsorption application, determining the rate-controlling step is impossible. Therefore, empirical design procedures based on adsorption equilibrium conditions are the most common method to predict adsorber size and performance. Adsorption equilibrium is a dynamic concept achieved when the rate at which molecules adsorb onto a surface is equal to the rate at which they desorb. The physical chemistry involved may be complex and no single theory of adsorption has been put forward to explain all the systems. Fortunately, engineer requires only the data at equilibrium conditions. Still now the oldest theories were used to predict the sorption process even though the assumption on which those models lie were found to be not entirely valid in later years. Also most of the adsorption theories have been developed for gas solid systems because the gaseous state is better understood than the liquid. Till now the statistical theories developed for gas – solid systems were applied for liquid solid systems with little confidence for designing of the equipment. The most commonly used equilibrium models to understand the adsorption systems was Freundlich and Langmuir isotherm equation which are explained as follows:

Freundlich isotherm: Herbert Max Finley Freundlich, a German physical chemist, presented an empirical adsorption isotherm for non ideal sorption on heterogeneous surfaces as well as multilayer sorption and is expressed by the equation:

Qe = KfCe1/nf   (2)

Equation (2) is used for the calculation of residual solute concentration at equilibrium (Ce) at a given volume of wastewater containing solute particles/quantity of adsorbent (Vo/Xo). Experimental values Ce and Qe values were compared to calculated ones. Adsorption in a batch reactor can be considered as a single staged equilibrium operation and it depends on two basic constraints, that of equilibrium and that of a mass balance. The mass balance for the sorbate particles is:

VoCo + XoQo = VoCe + XoQe   (3)

Co(Co – Ce) = Xo (Qe – Qo)   (4)

- Vo/Xo (Ce – Co) = (Qe – Qo)   (5)

equation (5) represents the straight line

Y = mX

(Qe – Qo) = -Vo/Xo (Ce – Co)   (6)

The line which passes through (Co, Qo) and (Ce, Qe) with slope (-Vo/Xo) is termed the operation line of this stage. So the single staged batch operation can be shown in Figure 1 (Ce versus Qe) and by drawing a operating line and the Freundlich equation respectively. Some of the important advantages of equation (6) are as follows:

Figure 1: Single - Stage Batch Adsorption

Single - Stage Batch Adsorption

By mass balance Co = Ce + Cx,e   (7)

Where Cx,e is the concentration of solute in the solid phase at equilibrium. Using equation (7), Cx,e can be calculated. If calculation is required Freundlich equation can be directly substituted in equation (6):

-Vo/Xo (Ce – Co) = (KfCe1/nf – Qo)   (8)

at time t = 0; Qo = 0; then equation (8) becomes:

-Vo/Xo (Ce – Co) = (KfCe1/nf – 0)   (9)

Therefore,

10   (10)

Vo/Xo can be calculated for desired purification (or) Ce/Cx,e at a given Vo/Xo for a given initial solute feed concentrations.

Langmuir Isotherm: In general the rate equation for a heterogeneous reaction (adsorption) accounts for more than one process. Two main processes that occurs in heterogeneous systems are physical (heat and mass transfer) and reaction steps. For example reactions catalysed by solid catalyst, the process first is proceeded by the rupture of a bond of a molecules, followed by the formation of a new bond, but in this case the molecules are previously chemisorbed onto the catalyst surface. This physical transfer process (chemisorption) depends on the reaction occuring between the molecules and the co-ordinatively unsaturated atoms called "active sites" existing on the solid surface which can be considered elementary steps of the reaction mechanism. Such rates are called global or overall rates and their use allows us to the most of design equations. The global rate involved in such systems were predicted by formulating the rate constants corresponding to each step that occurs in the overall process. The sequence of steps involved in converting reactants to product is:

  1. Transport of reactants from the bulk fluid to the fluid-solid interface (external surface of catalyst particle).
  2. Intraparticle transport of reactants into the catalyst particle (if it is porous).
  3. Adsorption of reactants at interior sites of the catalyst particle.
  4. Chemical reaction of adsorbed reactants to adsorbed products (surface reaction – the intrinsic chemical step).
  5. Desorption of adsorbed products.
  6. Transport of products from the interior sites to the outer surface of the particle.
  7. Transport of products from the fluid interface into the bulk fluid stream.

At steady state all these rate steps will be the same. This equality can be used to develop a global rate equation in terms of the concentrations and temperatures of the bulk fluid. If there exiss any unsteady state behaviour then the slowest step contributes the global rate equation in order to apply in design equations. In the present communication the theory of hetergeous reactions are explained based on the theory behind adsorption reactions. Consider the reversible adsorption of single species A, which remains intact (undissociated) on adsorption, can be represented by

11   (11)

The rate of adsorption of A, is proportional to the rate at which molecules of A strike the surface, which in turn is proportional to the concentration in the bulk gas, and to the fraction of unoccupied sites.

rA = Ka CA (1- θA)   (12)

Where Ka is the adsorption rate constant. Equation (12) gets more meaningful if only one molecule of solute gets transferred onto the solid surface particle till time, t = 0 to t = te. Similarly the rate of desorption, rd is proportional to the fraction of surface of surface covered, θA:

rd = KdθA   (13)

where Kd is the desorption constant.

At equilibrium the rate of adsorption is equal to rate of desorption, with ra = rd.

Therefore, KaCA (1 - θ) = KdθA   (14)

Rearranging equation (14), the fraction of solid particles covered by solute, which is proportional to the amount of gas adsorbed is:

θA = (KaCA)/(Kd + KaCA) = (KACA)/(1 + KACA)   (15)

where KA = Ka/Kd, is the equilibrium constant and Ka values are given by:

Ka > 1; domination of adsorption process

Ka < 1; domination of desorption process

Ka = ∞; irreversible process

The equation expressing the amount of A adsorbed on the surface as a function of temperature is called Langmuir adsorption isotherm.

In case of multicomponent adsorption, say solute species A and B striking the solid surface, then for species A the rate of adsorption is:

raA = KaACA (1-θAB)   (16)

for the expression (16) it is assumed that a molecule of A from the bulk gas striking a site occupied by a B molecule is reflected, and does not displace the adsorbed molecule. The rate of desorption, as for a single species is:

rdA = KdAθA   (17)

at equilibrium, ra = rd, thus

KaACA (1-θAB) = KdAθA   (18)

or, if KA = (KaA/KdA), then for component A

θA = KACA (1 - θA - θB)   (19)

Similarly for component B,

θB = KBCB ( 1 - θA - θB) = θB   (20)

Where, KB in equation (20) is given by (KB = KaB/KdB)

Applying equation (18) in equation (19) and equation (20), we get

θA = (KACA)/(1 + KACA + KBCB)   (21)

θB = (KBCB)/(1 + KACA + KBCB)   (22)

The term KACA and KBCB in equation (21) and (22) indicates the inhibition of one component over another.

Based on equation (20) and (21), the mot general equation of Langmuir kinetic model for the adsorption of species "i" from a multiphase gas mixture is:

θi = (KiCi)1/ni/(1+∑(KjCj)1/nj); i, j = 1, 2, 3, ... n   (23)

where the term "n" in equation (23) indicates the no of fragments that the solute particles gets splitted, the significance of "n" are explained in the following example:

Consider the solute particles gets dissociated into two fragments as follows:

B2 + 2S → 2B.S   (24)

Thus, the rate of adsorption is given by

ra = Ka (1-θB)²   (25)

Similarly the rate of desorption with respect to B2 is given by:

rd = KdB)²   (26)

Thus the solute coverage rate can be calculated equating equation (25) and (26) at equilibrium conditions:

θB = (KBCB)1/2/1+(KBCB)1/2   (27)

where KB in equation (27) is the equilibrium rate constant and equal to Ka/Kd and the denominator of power term in equation (27) indicates the no of fragments that the solute components is dissociated.

All these equations from (11) to (26) are derived based on the assumption that the surface of solid are uniform and also only one molecule occupies one vacant site and also this assumption holds for catalytic reactions. i.e., the enthalpy change for the adsorption of molecule is same at all active sites. But true homogeneous surface particles are nowhere used in chemical systems (whether with or without reaction), as even a polished surface shows a rough surface in a microscopic scale. This shows the limitation of Langmuir model. This model also suffers from the background that reaction mechanism occurs only due to solid liquid reactions and doesn´t consider the liquid interactions at solid liquid interface. However previous investigations in literature and textbooks confirm the Langmuir type model is the best model to analyze the gas solid reactions.

Isotherm shape: The equilibrium curve relating Qe and Ce is also mainly used to determine the nature of the sorption process. The different curve shapes explaining the sorption process is shown in Figure 2.

Figure 2: Isotherm Shape
Isotherm Shape

Adsorption Thermodynamics

For designing adsorption columns or batch adsorption systems, the designer should be able to answer the following two questions (Levenspiel, 1995):

  1. What changes can we expect to occur
  2. How fast will they takes place

The fast of reaction or the reaction rate can be calculated from the knowledge of kinetic studies. But the changes in reaction that can be expected during the sorption process require the brief idea of thermodynamic parameters. The three main thermodynamic parameters includes enthalpy of adsorption (ΔH), free energy change (ΔG) due to transfer of unit mole of solute from solution to the solid-liquid interface and entropy (ΔS) of adsorption.

Thermodynamic function which is most commonly used due to its practical significance is enthalpy of adsorption or heat change in adsorption. The apparent heat change or net enthalpy, ΔH of adsorption is related to the Langmuir Constant, KL as follows:

KL = KL x e-ΔH/RT   (28)

(Or)

Ln KL = Ln KL* - ΔH/RT   (29)

Where K* is a constant. The function ΔH is very useful whenever a differential change occurs in the system. Enthalpy is an extensive property that is its value is additive. The ΔH value in equations (28) and (29) can be calculated from the plot of Ln.KL versus 1/T. The negative value of ΔH will indicate the process is exothermic and the sorption behavior may be physical in nature and can be easily reversed by supplying the heat equal to calculated ΔH value to the adsorption system. The positive value of ΔH indicates the process is endothermic also the process may be due to chemical bonding or Chemisorption. Also positive ΔH indicates process is irreversible. Negative values of ΔH indicates the free diffusion of molecules through bulk solution and boundary layer is less than compensated by bulky groups and the stereo-tactic arrangements of adsorbates on the surface and in the pores of adsorbents. Another important thermodynamic parameter is the Free energy change ΔG can be calculated using the relation as follows:

ΔG = RT.Ln KX   (30)

Where, KX = CAe/Ce   (31)

From the calculated ΔH and ΔG, the change in entropy ΔS can be calculated as follows (Myers, 2002):

ΔS = ΔG-ΔH/T   (32)

In crease in negative value of ΔS will indicate the spontaneity in the adsorption process with reduction in molecular size and increased randomness at solid-liquid interface. Also value of ΔS less than 1 indicates that the process is highly reversible.

The values of thermodynamic function ΔS and ΔH can also be evaluated using VantHoff´s equation, which is given by:

Log KX = (ΔS/2.303 R) – (ΔH/2.303 RT)   (33)

Thus the value of ΔS and ΔH can be calculated from the intercept and slope of plot between Log KX versus 1/T.

Adsorption Systems

The contact between solid adsorbent and the liquid can be made by atleast five systems: Batch contact, Fixed bed (up flow or down flow), pulsed bed, steady state moving bed and fluidized bed.

Batch Systems:

Batch systems are used if the volume of wastewater to be treated is less. The various parameters involved in batch sorption systems are shown in figure 2. Where G Kg of solvent and the pollutant concentration is reduced from Yo to Y1 Kg of solute/ Kg of solvent. The amount of adsorbent added is L Kg of adsorbate free solid and the solute concentration increase from Xo to X1 Kg of solute/ Kg of adsorbent. If fresh adsorbent is used, Xo = 0. Taking mass balance one gets:

G(Yo – Y1) = L(X1 – Xo) = LX1   (34)

The Freundlich isotherm given by equation (35) may now be applied to equation (34):

Qe = KfCe1/nf   (35)

Where, Kf, nf are temperature dependent Freundlich constants. Xe represents the equilibrium concentration of solute (Kg) on adsorbent (Kg) and Ye is the equilibrium solute concentration.

L (Ye/Kf)1/nf = G (Yo - Y)   (36)

37   (37)

Substituting equation (35) in equation (34), gives equation (37):

Equation (37) is mainly used to calculate adsorbent to solution ratio for a given change in solution concentration, (Yo – Ye).

Fixed Bed Adsorption:

Batch type sorption is usually limited to the treatment of small volumes of effluent, whereas fixed bed systems have an advantage over this limitation. In fixed bed the adsorbate is continuously in contact with a given quantity of fresh adsorbent thus providing the required concentration gradient between adsorbent and adsorbate for adsorption. Though a lot of methods are available for designing adsorption column, a practical generalized method of designing adsorption column is bed depth service time (BDST) model. The design and theory of fixed bed adsorption systems centers on establishing the shape of the breakthrough curve and its velocity through the bed. The breakthrough curve and the influence of volumetric throughput rate on the shape of the breakthrough curve is shown in Figure 3.

Figure 3: Adsorption Zone Progression in a Fixed Bed Adsorber

Adsorption Zone Progression in a Fixed Bed Adsorber

From figure 3 it was observed that the concentration of the adsorbate exist an S shaped curve in the adsorption zone with ends asymptotically approaching zero and the influent concentration Co. The breakthrough concentration, Cb, is selected based on effluent criterion with a safety factor. A fixed bed adsorption can be operated in series (Figure 4) or in parallel (Figure 5).

Figure 4: Packed Bed Adsorption in Series Figure 5: Packed Bed Adsorber in Parallel
Packed Bed Adsorption in Series Packed Bed Adsorber in Parallel

Pulsed Bed:

Figure 6: Pulsed - Fed Adsorption Unit

Pulsed - Fed Adsorption Unit

Solid liquid adsorption can be carried out in pulsed bed (Figure 6) in which some carbon is removed from the bottom of the column at a constant time intervals and are replaced by fresh adsorbent. This type has an advantage of better utilization of adsorbent, because the adsorbents were kept for regeneration as soon as the adsorbent gets saturated. A typical application was the recovery of ethylene from gas composed mainly of hydrogen and methane, but with some propane and butane using hypersorber. The hypersorber unit is shown in Figure 7. The mixture to be separated is fed to the center of the column down which activated carbon moves slowly. Immediately above the feed, the rising gas is stripped of ethylene and discharged as a product. The adsorbent with its adsorbate continuos down the column into an enriching section where it meets an upward stream of recycled top product. The least strongly adsorbed ethylene is desorbed and recovered in a side stream. The heavy components continue downward on the carbon until these are also stripped of all the adsorbate is lifted to the top of column where it is cooled before the cycle starts again.

Figure 7: Hypersorber (Coulsan and Richardson, Chemical Engineering, Volume II)

Hypersorber Unit

Steady State Moving Bed Adsorption:

The moving bed adsorption column is a steady state countercurrent operation since the adsorbent solid is moving downward through he column while the liquid is flowing upward. It is a common method of operation and is most widely used in most of wastewater treatment plants. A schematic diagram of continuos countercurrent adsorption system is shown in Figure 8.

Figure 8: Continuous Contact Moving Bed Adsorption
Continuos Countercurrent Adsorption System

A solute balance around the entire tower is given by equation (38):

G(Yo – Y) = L (Xo – X)   (38)

The mass balance around the upper part of the column is:

G(Y – Y1) = L(X – X1)   (39)

From the mass balance equations, the operating line can be constructed on the equilibrium curve as shown in Figure 9. The operating line is the straight line of slope L/G, joining the terminal conditions (Xo, Yo) and (X1, Y1). The concentration of solute, Y and Y1, at any height within the column will lie on this line.

Figure 9: Operating Diagram for Continuos Contact Adsorption
Operating Diagram for Continuos Contact Adsorption

By assuming that the column operates under approximately isothermal conditions and that film and internal diffusional resistance can be represented by an overall mass transfer term (K), then the incremental mass transfer rate is:

L.dX = G.dY = Ka(Y-Ye).dZ   (40)

Where dZ is an incremental height in the adsorber, "a" is the external surface of the adsorbent particle and Ye is the equilibrium solute concentration in liquid corresponding to composition X. The vertical distance between the temperature and operating line of figure 9 gives the driving force ΔY = Y – Ye. the number of mass transfer units, N, may be defined by equation (41):

41   (41)

The no of transfer units is obtained by graphical integration and the height of bed, Z, is obtained by combining equation (40) and (41) to give:

42   (42)

Where the height of mass transfer unit, Ht is defined by:

Ht = G/Ka   (43)

Fluidized Bed:

For any adsorption process, it is advantageous to keep the adsorbent articles as small as possible to achieve higher rate of adsorption. Using smaller adsorbent particles in fixed bed creates problems like excessive head loss, air binding and fouling with particulate matter whereas these advantages can be overcome in fluidized systems. Also a fluidized system has an advantage of higher mass transfer rate between adsorbate and adsorbents. The designing of fluidized bed mainly involves the determination of mass transfer coefficient, K. Figure 10 shows the fluidized systems for adsorption.

Figure 10: Fluidized Bed Adsorber in Operation
Fluidized Bed Adsorber in Operation

By assuming steady state conditions, the rate of mass transfer to the adsorbent across a film is given by:

Rate = Driving force/ Resistance   (44)

Driving force is the product of the concentration gradient between adsorbent and solution the total area across which transfer is taking place. The resistance is given as the reciprocal of mass transfer coefficient, K. Therefore:

Rate, r = KAM.ΔY   (45)

Where: A is the effective transfer area per unit mass of adsorbent; M is the mass of adsorbent; ΔY is the concentration difference across the film; r is the rate of transfer.

The value of K in equation (45) is calculated using the relation:

46   (46)

Where: AT = total surface area of adsorbent; E in equation (46) is calculated using equation (47):

47   (47)

Where: Q is the mass flow rate of solution; P is the partition coefficient; W is the total weight of adsorbent.

The overall height of a transfer unit, Ht, is then obtained from equation (48):

Ht = G/Ka   (48)

The Ht obtained is based on a single resistance model and neglects two additional processes, namely: (i) transfer between the fluidized bed and the wall (other particles); (ii) transfer between lean and dense phases in fluidized beds.

Fixed Bed Adsorber Design:

Various models are available for the design of fixed bed adsorption column, out of them MTZ, HETU and BDST model are the most commonly used which are discussed as following:

Mass transfer zone model (MTZ): In the MTZ model concentration of solute in the adsorbent will change from the initial value Co to the equilibrium value Ce. The area below the wave front reflects the unused adsorbent capacity. The resultant design equation is:

Zu = Zo –Ze = Zo(1 - θbs)   (49)

Bed depth service time: In continuous flow experiments, it is essential to predict the exhaustion rate of adsorbent bed on how long the bed will last before regeneration is necessary. In this method the service time of a fixed bed adsorbent, treating a solution of single adsorbate, can be expressed as a function of operational variables as:

θb = No/Cou[Z – (u/KNo) Ln(Co/Ct – 1)]   (50)

Equation (51) can be represented as:

θb = A.Z+B   (51)

Thus a plot between service time against bed depth can be used to test the model.

Height of an equivalent transfer unit: This column design method is based on two concepts: Height of an equivalent transfer unit and maximum permissible space at which full adsorption takes place. The minimum bed height for maximum product quality is suggested as:

Zmin = (55 – 60) HETU   (52)

Low Cost Adsorbents

Activated carbon is the most commonly used adsorbent for the treatment of wastewater. But due to its high cost and 10 – 15% loss during regeneration, alternative low cost adsorbents have attracted the attention of several investigators to provide an alternate for the high cost activated carbon. Table 3 summarizes the potential of several low cost adsorbents to remove the various pollutants such as color, heavy metals, COD from the effluents. Though several studies were available explaining the utilization of several low cost adsorbents, most of these work stand at the laboratory level and only a very few cases have been directly implemented in practical applications at industrial level.

Table 3. Treatment of Different Wastewater by Adsorption
S.No. Authors Adsorbate Adsorbent Variables Studied Remarks
1 Sekar and Murthy. (1998) Distillery Spent wash Powdered activated carbon Initial conc., Adsorbent dosage. Removal of 18% is noted and when spent was pretreated with polyelectrolyte as a flocculating agent, the color removal was increased up to 99%.
2 Yeh and Thomas (1995) Synthetic dye wastewater of Disperse-red-60 Powdered activated carbon Effect of contact time,concentration of PAC. 95-98% of COD removal for 25-200 ppm & 88-98% for various particle sizes. Freundlich, Langmuir, Dziubek & Kowals adsorption isotherms fit well. Film-pore double resistance diffusion model describes the mass transfer. External film mass transfer coefficient increases with decrease in particle size.
3 Yeh and Adrian Thomas (1995) Disperse-red-60 Powdered activated carbon, Activated alumina, Molecular Sieves & Diatomite Contact time, Adsorbent Dosage, Particle size Decrease in particle size gave only a minor improvement for activated alumina, finer molecular sieve materials reduce the dosage requirement by half. Performance of adsorbents are as PAC> Activated alumina> molecular sieve.
4 Mahesh et al. (1998) Catechol Granular Activated carbon [Industrial grade; Laboratory grade] Initial Concentration, pH, Adsorbent dosage, Particle Size. Equilibrium studies showed that IGGAC has the maximum adsorption capacity. Diffusion studies showed that initial part of the adsorption is attributed to external mass transfer effects followed by intraparticle diffusion.
5 Kapadia et al (2000) Petrochemical Effluent (Raw) Fly Ash [Coal] Initial Concentration, adsorbent dosage, contact time Adsorption dose of 3-3.5% shows significant color removal. Contact period of 30-40 minutes gives optimum removal.
6 Kapadia et al (2001) Petrochemical Effluent (Raw) Powdered activated carbon, Granular Activated carbon, Carbon soot, Fly ash Bed Height, Particle size. Batch studies show that flyash also reduces suspended solids, ammonical nitrogen, COD, and nitrophenol apart from color. Efficiencies of adsorbents in decreasing order PAC>Carbon soot>GAC> Flyash.
7 Mall and Upadhyay. (1998) Malachite green, Methylene blue. Fly ash I& II Initial Concentration, Adsorbent dosage, Contact time, Bed Height. Maximum color removal was attained with flyash containing high carbon content. Column studies show the applicability of BDST theory.
8 Sharma et al (1999) Malachite green Bagasse pith, Orange peel, Sawdust, Eichornia shoot & root Effect of initial dye conc., Initial adsorbent dosage & Ph PH Studies shows that there exists some chemical interaction between adsorbent and adsorbate. Efficiency in decreasing order Eichornia root>Eichornia shoot>Orange peel>Saw dust>Bagasse.
9 Nassar and Geundi (1991) Astrazone Blue, Maxillon Red, Telon Blue. Activated carbon, Natural clay, Bagasse pith, Maize cob. Initial Concentration, Adsorbent dosage. Activated carbon has maximum adsorption capacity. Cost analysis showed that Natural clay, bagasse pith &maize cob were economically attractive than activated carbon.
10 Balasubramanim et al (1998) Cr (IV). Lignite Adsorbent dosage. Kinetic showed the applicability or Lagergren model. The datas tend to follow first order rate kinetics.
11 Annadurai.G (1998) Direct Scarlet. B Chitin pH, Adsorbent dosage, Particle Size, Temp. Diffusion studies showed adsorption mechanism involves an initial rapid uptake of dye due to surface adsorption followed by intraparticle diffusion.
12 Sankar et al (1999) Direct Red 31, Acid Black 1 & Acid Blue 16 Rice bran based activated carbon Contact time, Initial concentration, Temp., Particle size Maximum removal is seen at acidic pH and 20-30 degree C. The enthalpy value show that adsorption is a physical phenomena
13 Nassar and Magdy (1999) Basic Red 22 Natural clay Initial Concentration, Adsorbent dosage, Air flow rate. Mode of contact: Fixed bed Mass transfer study showed thar external mass transfer is the only rate-controlling step.
14 McKay and McConvey (1981) Astrazone Blue Carbonized wood Initial conc., Agitating speed. Mass transfer study showed thar external mass transfer is the only rate-controlling step.
15 McKay and Allen (1980) Telon Blue (Acid) Peatv Initial Concentration, PH, Adsorbent dosage, Contact time, Agitation speed. Temperature has the most pronounced effect on mass transfer.
16 McKay and Ho (1998) Astrazone Blue BG Peat Initial conc., Adsorbent dosage. Pseudo second order mechanism has been explained.
17 Annadurai and Krishnan (1997) Verofix Red Chitosan Initial Concentration, pH, Adsorbent dosage, Temp. The datas tend to follow First order kinetics
18 Inbaraj and Sulochana (2002) Malachite green Jack fruit peel (Carbonized by chemical method) Agitation time, Initial conc., Adsorbent dosage, Temp, pH. Equilibrium datas tend to follow Freundlich, Langmuir & Redlich Peterson isotherms. Desorption studies show that adsorption is a chemical phenomena.
19 Das and Patnaik (2001) Raw Paper mill effluent Blast furnace dust & Slag Initial COD conc., pH, Temp.,Adsorbent dosage. COD removal efficiency of BFD is significantly better than Basic slag.
20 McKay (1982) Deorlene Yellow, Telon Blue, Victoria Blue Activated carbon Initial conc., pH, Particle size., Adsorbent dosage, Salt. Batch study shows that addition of salt in the form of NaSo4 increases the rate of adsorption.
21 McKay (1982) Deorlene Yellow, Telon Blue. Activated carbon Bed Height, Particle size. Column studies show the applicability of BDST theory.
22 McKay (1983) Deorlene Yellow, Telon Blue, Victoria Blue,Disperse Blue Activated carbon Adsorbent dosage, Particle size, Initial conc. Diffusion study show that intraparticle diffusion is not a rate controlling step.
23 McKay (1983) Deorlene Yellow, Telon Blue, Victoria Blue,Disperse Blue Activated carbon Adsorbent dosage, Particle size,Initial conc. Mass studies show that resistance to external mass transfer is mainly controlled by initial conc. And adsorbent dosage.
24 McKay (1984) Telon Blue, Deorlene Yellow Activated carbon Initial conc. Homogenous solid phase diffusion coefficient had been estimated using Two resistance mass transfer model.
25 Allen et al (1989) Maxilon Red, Astrazone yellow Lignite Initial conc. Equilibrium dats tend to follow Freundlich, Langmuir & Redlich Peterson isotherms. The equilibrium isotherm exhibited deviates from the theory.
26 Kapadia et al (2000) Copper(Synthetic) Fly ash Initial conc.,pH,Adsorbent dosage. Maximum efficiency was at the pH of 6.0. The fly ash treatement raises the pH of effluent.
27 De (2001) Cadmium(Synthetic) Coal Fly ash Bed Height, Initial conc., pH. 100% removal is seen at lower initial conc. of 3 mg/l.
28 Rao et al (2001) Copper & Lead (Synthetic) Bagasse, Fly ash Particle size, Adsorbent dosage, Initial conc., pH. Adsorption capacity decrease in the order Fly ash > Bagasse> PAC for the removal of copper ions and PAC > Bagasse>Fly ash for removal of lead ions under optimum conditions.
29 Rai et al (2000) Magenta Rice husk Fly ash (Boiler feed) Initial conc. When compared to other adsorbents Chitin has the maximum adsorbent potential, Nickel shows more preference for adsorption sites of chitin than Zinc.
30 Viswanathan et al Zinc & Nickel ions Chitin, Saw dust, Clay, Fly ash pH, Particle size, Initial conc., Calcium ion. When compared to other adsorbents Chitin has the maximum adsorbent potential, Nickel shows more preference for adsorption sites of chitin than Zinc.

Costing and Optimization

The minimum costs for the system can be obtained by optimizing the size of the adsorption and regeneration units using the basic design equations. Bed depth and flow rate determines residence time. Figure 11 shows this effect and indicates a minimum total operating cost for a specific residence time.

Figure 11: Optimisation to Determine Residence Time

Optimisation to Determine Residence Time

Adsorbent Regeneration

Regeneration is an important process consideration and because it is usually an exothermic process, higher temperature do not favor adsorption and this is advantageous for regeneration purposes. That is as long as the surrounding fluid contains little or no adsorbate, raising the temperature of the adsorbent will drive off the adsorbate. This method of regeneration is called "Temperature swing adsorption" (TSA). There are two other common methods for regeneration, "Pressure swing adsorption" (PSA) and "Displacement purge adsorption" (DPA). In PSA, the adsorption is usually accomplished at elevated pressures and the bed is regenerated by lowering the pressure in bed. This has an advantage over TSA in that pressure can be lowered more rapidly, allowing for faster regeneration. In DPA the adsorbent is first saturated with an adsorbate that is easily replaced by the desired adsorbate. The decision to regenerate an adsorbent bed is based on the "breakthrough" point. The breakthrough point is the maximum allowable concentration of adsorbate in the gas exiting the adsorber and is determined from the breakthrough curves.

Conclusion

Various equilibrium models used to better explain the sorption process have been briefly analyzed. The significance of various models have been discussed. Methods available to calculate thermodynamic functions were also presented. Different methods used for designing of adsorption columns were also presented. The most commonly used equilibrium model Freundlich and Langmuir isotherm were explained in detail. Design concepts behind different sorption contact system were discussed. Though both equilibrium and thermodynamic approach is used to explain the sorption systems, it is the duty of designer to select whether the kinetic or thermodynamic models to design adsorption systems. For example adsorption systems with negligible heat effects, thermodynamic studies are not required and only enough kinetic data can provide the design parameters.

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