Adsorption

Adsorption

Alvin R. Caparanga

PIChE Handbook

Mapua Institute of Technology

J9 ADSORPTION J9.1 Introduction Sorption is a general term for any process in which a component (or components) of one phase, usually a gas or a liquid mixture, transfers to another phase, where the migrating component is accumulated. This is particular for situations in which the second phase is solid. Both adsorption and absorption are sorption processes. In adsorption, a component or components (atoms, molecules or ions) are extracted from the fluid phase (gas or liquid mixture) and are concentrated on the surface of the solid phase. A component, therefore, can be separated from a mixture if it selectively adsorbs onto a solid surface. For most applications, only one component is usually separated from the fluid phase such adsorption can occur in either a gas-solid or liquid-solid system. The component that concentrates (adsorbed) on the surface of the solid is called the adsorbate. The solid, which is the separating agent, is called the adsorbent. Most adsorbents are porous materials. Common examples are silica gel, alumina, zeolite, charcoal, activated carbon, synthetic resins and some types of adsorbing clay minerals. In so many cases of adsorption on these materials, the adsorbate is held strongly enough on the walls of the pores or at specific sites inside the particle to allow (nearly) complete removal of that component from the fluid. Because these materials are selective to some specific atoms, ions or molecules, there is usually very little adsorption of other components.

J9.2 Driving Forces in Adsorption A. Hydrophilic and hydrophobic character in an aqueous solution B. Specific affinity of the solute for the solid adsorbent via van der Waals forces attraction or chemical reaction

J9.3 Types of Adsorption .A. Physical adsorption Characterized by a relatively low energy of adsorption as a result of van der Waals forces of attraction, this adsorption type is predominant at low temperatures. The adsorbate is not fixed to a specific site; it is free to undergo translational movement within the interface.

B. Chemical adsorption A

Adsorption

Alvin R. Caparanga

PIChE Handbook

Mapua Institute of Technology

Also referred to as chemisorption or activated adsorption, chemical adsorption, having high energy of adsorption, is favored at higher temperature. Equilibrium is attained faster in chemisorption. C. Exchange adsorption With the charge of the ions being the determining factor, ions of one component concentrate on a surface as a result of electrostatic attraction The adsorbate reacts with the adsorbent. Chemisorption, having a high energy of adsorption, normally proceed rapidly at elevated temperatures.

J9.4 Mechanism of Adsorption The mechanism of adsorption of solute from solution by porous adsorbents can be summarized as follows: 1. Transport of adsorbate from the bulk phase through the surface film to the exterior of the adsorbent (rate-determining step for continuous-flow systems) 2. Sorption by the porous adsorbent 3. Adsorption of the solute on the interior surfaces bounding the pore and capillary spaces of the adsorbent (rate-determining step for agitated batch systems)

J9.5 Factors That Influence Adsorption A. Surface area The extent of adsorption is proportional to surface area (portion of the total area that is available for adsorption). B. Nature of adsorbate 1. Adsorption of solute is inversely proportional to its solubility in the solvent. Lundelius’ Rule: The greater the solubility, the stronger is the solute-solvent bond and the smaller is the extent of adsorption. Traube’s Rule: In general, the solubility of any organic compound in water decreases with increasing chain length, because the compound becomes more hydrocarbon-like as the number of carbon atoms increases. 2. Molecular sizes

B

Adsorption

Alvin R. Caparanga

PIChE Handbook

Mapua Institute of Technology

Adsorption generally proceeds at faster rate if adsorbate molecules are small and the process is controlled by intraparticle transport. This argument applies to a given class of compound. 3. Ionic charge Adsorption is at its minimum for the charged species and at maximum for the neutral species as long as the compounds are structurally simple. The effect of ionisation becomes less important as the compounds become more complex. 4. pH Hydrogen and hydroxide ions are adsorbed quite strongly; therefore, the adsorption of other ions is influenced by the pH of the mixture. 5. Temperature Adsorption, particularly physical adsorption, is normally exothermic; thus, the extent of adsorption generally decreases with increasing temperature. 6. Presence of other solutes Other solutes that may be present in the solution may compete with the desired solute for the available surface area. Therefore, relative adsorptive affinities and relative concentrations of the solutes are a factor. 7. Nature of the Adsorbent An adsorbent’s performance is generally better if its specific surface area available for adsorption is large and the general physico-chemical characteristics of the solid adsorbent complement the corresponding properties of the adsorbate.

J9.6 Adsorption Equilibria and Isotherms At equilibrium, there is a defined distribution of solute between the fluid and solid surface. The distribution ratio may be a function of the concentration of the solute, the concentration and nature of competing solutes, etc. The dynamic phase equilibrium that is established for the distribution of solute between the fluid and the solid surface is somewhat similar to the equilibrium solubility of gas in a liquid. These equilibrium relations are called isotherms. Some of the common isotherms are summarized in Table J9-1. Table J9-1 Common adsorption isotherms Name of isotherm Equation

Comment Not common, but can be

C

Adsorption

Alvin R. Caparanga

PIChE Handbook

Mapua Institute of Technology

Linear

q = Kc

(J9-1)

Freundlich

q = Kc n

(J9-2)

applied to approximate data for dilute systems Empirical; Can be used to approximate data for many physical adsorption systems, particularly liquids

Langmuir

q=

(J9-3)

Has a theoretical basis

qoc K+c

When n = 1 in equation J9-2, the equation is reduced the Henry’s-law type equation (equation J9-1). Favorable isotherms permit higher loading (amount of adsorbate per unit mass of adsorbent) at lower solution concentrations. On the other hand, isotherms, which start out flat are unfavourable.; they only work well at high concentrations of solute. The Langmuir isotherm (equation J9-3) was derived considering the following assumptions: (a) The energies of adsorption on the surface are uniform. (b) Adsorption is reversible and maximum adsorption, which occurs at equilibrium, corresponds saturated monolayer of solute molecules on the adsorbent surface such that the energy of adsorption is constant. (c) There is no transmigration of adsorbate in the plane of the surface. Another important equilibrium relation is the Brunauer-Emmet-Teller (BET) isotherm, which has 5 forms (or types). This isotherm is based on the following assumptions: (a) The energies of adsorption on the surface are uniform. (b) A number of layers of adsorbate molecules form at the surface, where each layer is described by the Langmuir isotherm. (c) A given layer does not need to be completely formed before subsequent layers are formed such that the equilibrium condition will involve several types of surfaces. Given the q (solute loading on the adsorbent) versus c (concentration of solute in the fluid) data at equilibrium, the temperature-dependent constants K and n of equation J9-2 can be evaluated graphically from a ln q vs ln c plot, with slope equal to n and intercept equal to ln K. On the other hand, the constants qo and K can be evaluated from equation J9-3 by plotting 1/q against 1/c, with slope as K/qo and intercept as 1/qo. The solute loading on the solid adsorbent is usually expressed as mass, or moles, or volume of adsorbate per unit mass or per unit BET surface area of adsorbent. On the other hand concentration of solute in the fluid is usually in terms of partial pressure if the fluid is a gas; in the case liquid, the concentration may be in terms of moles, or mass, or volume per unit volume of the mixture.

D

Adsorption

Alvin R. Caparanga

PIChE Handbook

Mapua Institute of Technology

J9.7 Modes of Operation A. Batch or Single-Stage Adsorption Usually done with fresh (solute-free) adsorbent, batch adsorption is often used in small-scale operations. Similar to other separation processes, an equilibrium relation (appropriate isotherm) is required in addition to material balance (the equation of the operating line if to be solved graphically). Based on Figure J9-1, the material (solute) balance, which represents the operating line, for single-stage batch adsorption process for a binary mixture (liquid) is: _ S(c _ c*) = M (q * q ) (J9-4) o o The material balance equation, J9-4, also applies to (equilibrium) single-stage continuous adsorption.

Adsorbent M kg adsorbent qo kg adsorbate/kg adsorbent

Feed solution S kg solution co kg solute/kg solution

Lean solution S kg solution c* kg solute/kg solution

Spent adsorbent M kg adsorbent q* kg adsorbate/kg adsorbent

Figure J9-1 Batch or single-stage adsorption Solving simultaneously equation J9-4 and the applicable isotherm equation, one can determine the equilibrium values of q and c. Since in most cases, either the Langmuir or Freundlich isotherm applies, either being non-linear, it is suggested that q and c be solved graphically, with the intersection of the curves of the isotherm and material balance (equation J9-4) as the coordinates of equilibrium values q* and c*. From these values the extent of separation can be determined.

q

Operating line Equilibrium curve (isotherm)

q*

E

Adsorption

Alvin R. Caparanga

PIChE Handbook

Mapua Institute of Technology

c*

c

Figure J9-2 Graphical solution to batch (single-stage) adsorption Over small concentration ranges in dilute solution, however, the Freundlich isotherm (J9-2) usually applies. If process starts with an adsorbent initially free of adsorbate (q0 = 0), then c 0 _ c* M = S K (c * ) n

(J9-5)

At different values of n, the solutions of J9-5 are given in Figure J9-3. operating lines (slope: -M/S) n<1

c0

equilibrium curves / isotherms n =1

c*

n>1

q Figure J9-3 Single-stage adsorption at different values of n in Freundlich isotherm B. Two-Stage Cross-Current Adsorption In practice and for economic reasons, split-feed treatment is normally used; it uses smaller batches of adsorbent rather than one large batch. However, two stages are generally optimal; otherwise, additional filtration auxiliaries would eliminate any economic gain. M1 qo

M2 qo

Feed solution S kg solution co kg solute/kg solution

c1 Stage 1

F

c2 Stage 2

Adsorption

Alvin R. Caparanga

PIChE Handbook

Mapua Institute of Technology

M2 q1

M2 q2

Figure 9-4 Two-stage cross-current adsorption On the assumption that the amount of solution is practically constant (S ≈ S1 ≈ S2) like in usual cases, the material balances are as follows. For stage 1: For stage 2:

c 1 ) = M 1 (q 1

_

q0 )

S(c1 c 2 ) = M 2 (q 2

_

q0 )

S(c 0

_ _

(J9-6) (J9-7)

If fresh adsorbent is used in each stage (q0 = 0) and equilibrium is of the Freundlich type, then equations J9-6 and J9-7, respectively, become

M 1 c o _ c1 = S Kc1n

(J9-8)

M 2 c1 _ c 2 = S Kc n2

(J9-9)

The total amount of adsorbent used is

M 1 + M 2 c o _ c1 c1 _ c 2 = + S Kc1n Kcn2

(J9-10)

At specified extent of separation or c2, the minimum total adsorbent can be obtained by setting d M1 + M 2 the derivative to zero. Considering that K, n, c0 and c2 are constants, then dc 1 S equation J9-10 becomes

c1n c n2

_

n

c0 =1_ n c1

(J9-11)

from which c1 at minimum total adsorbent can be evaluated. This c1 is then used to evaluate the minimum total adsorbent using equation J9-10. C. Multistage Countercurrent Adsorption G

Adsorption

Alvin R. Caparanga

PIChE Handbook

Mapua Institute of Technology

S c1

S c0 Stage 1

S c2

S cN-1 Stage N

Stage 2

q1

q2

S cN

MN+1 q3

qN

qN+1

Figure J9-5 Multistage countercurrent adsorption 1. Determination of Number of Stages Using Kremser Equation The Kremser-type equation as used in other stage processes (gas absorption, extraction and distillation) may also be applied to evaluate the number of equilibrium stages in countercurrent adsorption provided that the following conditions are satisfied: (a) equilibrium relation is of the Freundlich-type with n = 1 (or linear isotherm, equation J9-1); (b) flow rate of solution is practically constant (S = S0 = S1 = S2 = … = SN); and, (c) adsorbent flow rate is constant (M = M1 = M2 = … = MN+1). Balance around any stage n: cn-1

cn Stage n

qn

qn+1

M (qn – qn+1) = S (cn-1 – cn)

(J9-12)

Substituting equation J9-1 into the preceding equation and rearranging yields: cn+1 = cn – Acn-1 + Acn where A =

(J9-13)

S , a quantity analogous to absorption factor; hence, it will also be referred to MK

as ‘absorption’ factor for adsorption.. For stage 1, using n = 1, equation J9-13 becomes c2 = c1 – Ac0 + Ac1 = c1 (1 + A) – Ac0

(J9-14)

For stage 2, using n = 2, and eliminating c 2 from the resulting equation, equation J9-13 becomes c3 = c1 (1 + A + A2) – c0 (A + A2) H

(J9-15)

Adsorption

Alvin R. Caparanga

PIChE Handbook

Mapua Institute of Technology

These equations can be generalized for the nth stage to yield cn+1 = c1 (1 + A + A2 + … + An) – c0 (A + A2 + … + An)

(J9-16a)

cn+1 = c1 + (c1 – c0) ( A + A2 + … + An)

(J9-16b)

or The terms in the parenthesis represent a sum of a geometric series: A ( 1_ A n ) 2 3 n A + A + A + ... + A = 1_ A

(J9-17)

Therefore, for stage N, equation J9-16 becomes _

A(1_ A N 1 ) ( ) c N = c1 + c1 c 0 1_ A

(J9-18)

_

This is a form of the Kremser equation for multistage countercurrent adsorption, which is applicable as long as the previously mentioned conditions are satisfied. Normally, q1 is known; therefore, the corresponding value of c1 can be estimated from equation J9-1. Given the number of stages, the exit concentration of the solution from the last stage can be estimated. Also, if desired extent of separation or c N is known, the number of stages required to effect the separation can be calculated. 2. Determination of Number of Stages by Graphical Method In case when the isotherm that describes the equilibrium is not linear, a graphical method similar to McCabe-Thiele is applicable. Equilibrium isotherm can be that of Freundlich (with the parameter n ≠ 1) or Langmuir type. Considering figure J9-5 to represent a multistage countercurrent system and the assumptions b and c in the preceding section, the following figure (J9-10) illustrates the procedure for determining the number of stages by graphical method:

c0

1

c1 2

I

Adsorption

Alvin R. Caparanga

PIChE Handbook

Mapua Institute of Technology

N

cN qN+1

q2

q1

Figure J9-6 Graphical solution for multistage countercurrent adsorption D. Fixed-Bed Adsorption Columns The Freundlich and Langmuir isotherms are applicable to batch adsorption systems where sufficient time is provided to allow equilibrium between solute in the solution and the adsorbate to occur. Many applications use a fixed-bed approach where the fluid stream flows through a stationary mass of adsorbent. During the flow through the fixed-bed of adsorbent, the solute components of the solution come into contact with active surface sites and are retained on the surface of adsorbing media. 1. Design Parameters In the design of these fixed bed adsorbers two significant parameters – minimum contact time and the life of the bed – are required. Minimum contact time The minimum contact time is determined through a series of kinetic tests in which a known mass of adsorbent is exposed to a solution of known solute concentration for specific amounts of time. At the end of each time increment, the concentration of the solute in the solution is measured; these values are plotted against time. The time at which the curve begins to flatten is the minimum contact time required to attain pseudo-equilibrium. Separation or removal of solute from the solution may still continue past this minimum time; however, longer contact times in general are not economically efficient because the size of the fixed-bed adsorber will need to be much greater for minimal additional removal. Service life of the bed The service life of the bed can be determined using the bed-depth-service-time (BDST) model. The model is based on the Bohart and Adams quasi-chemical rate law, using the following assumptions: (a) equilibrium is not instantaneous; and, (b) the rate of sorption is proportional to the fraction of sorption capacity still remaining on the adsorbent. The linearized BDST equation is

tb =

N0 C0 _ Cb 1 D_ ln 1000 ενC 0 kC 0 Cb

J

(J9-19)

Adsorption

Alvin R. Caparanga

PIChE Handbook

where tb C0 Cb ν ε k N0 D

Mapua Institute of Technology

= time until breakthrough (min) = initial concentration of solute (mg/L) = breakthrough concentration of solute (mg/L) = fluid velocity or loading rate (m/min) = porosity of the bed = quasi-chemical rate constant from Bohart and Adams theory (L/mg-sec) = capacity of the adsorbent for each solute in a multi-component solution (mg/m3 of adsorbent bed) = depth of the adsorbent bed

2. Breakthrough Concentration Curve The amount of material adsorbed within the fixed bed depends time of contact and position. For example: as fluid enters the bed, it comes in contact with the first few layers of adsorbent to a certain position. Instantaneously, solute adsorbs, filling up some of the available surfaces or sites. After some time, the adsorbent near the entrance is saturated and the fluid penetrates further into the bed before all solute is removed. The active region, therefore, shifts through the bed progressively (see Figure J9-7). Initially, the fluid emerging from the exit of the bed will have no solute remaining and this will continue at least until the bulk of the bed becomes saturated. At the point when the bed is almost saturated with the solute, little amount of solute starts to emerge with the fluid exiting. When the concentration of the fluid leaving the bed spikes as unadsorbed solute, a break point occurs. At this point the bed is already ineffective. See Figure J9-8. The corresponding concentration at the break point is called the breakpoint or breakthrough concentration (cb). This concentration corresponds to the maximum amount of acceptable solute lost and is normally set at values ranging from 0.01 to 0.05co.

Figure J9-7 Concentration profiles at various positions and times in a fixed bed

K

Adsorption

Alvin R. Caparanga

PIChE Handbook

Mapua Institute of Technology

Mass transfer zone

Figure J9-8a concentration curve in

Breakthrough the solution at the outlet of bed

Mass transfer zone

C/Co

1

0.5

break point, cb

0

time, t

L

Adsorption

Alvin R. Caparanga

PIChE Handbook

Mapua Institute of Technology

Figure J9-8b Breakthrough concentration curve in the solution at the outlet of bed As the

G.4 Fixed-Bed Adsorption Columns G.4.1 Breakthrough Concentration Curve G.4.2 Capacity and Scale-Up Design

J9.8 Regeneration and Adsorption Cycles Adsorption cycles are classified according to the method of regeneration. Most systems use a combination of the following: A. Temperature swing, which is used to regenerate bed by raising the vapor pressure of contaminant by preheating sweep gas B. Pressure swing, which vaporizes contaminant by pressure reduction C. Stripping, which sets up a contaminant lean (low partial pressure) condition that results to its diffusion from solid to sweep gas D. Removal of contaminant to a sweep liquid J9.9 Applications Table J9-2 lists some of the applications of adsorption as a separation process. Table J9-2 Some applications of adsorption Liquid-Solid Systems • Removal of organic components from drinking water or aqueous waste • Removal of colored impurities from sugar solution and vegetable oils • Removal of water from organic liquids • Recovery of reaction products that

• • • • • M

Gas-Solid Systems Drying of gases by adsorbing the water on silica gel or alumina Separation of oxygen and nitrogen on molecular sieves Removal of CO2 from natural gas Removal of H2S, CS2 and other odorous compounds from air circulating in ventilation systems Separation of normal paraffins

Adsorption

Alvin R. Caparanga

PIChE Handbook

• •

Mapua Institute of Technology

are not easily separated by distillation or crystallization Removal of sulfur compounds from organic solutions Removal of heavy metals from wastewater using biosorbents

from branched aromatics

N

paraffins

or