ION EXCHANGE

*DefinitionA reversible chemical reaction where an ion from a solution is exchanged for a similarly charged ion attached to an immobile solid particle

*Ion Exchange Chemical reactions between ions in solution and ions in an insoluble solid phaseExample: Water softening (to remove calcium ions)

Mechanisms*The techniques used in ion exchange so closely resemble those used in adsorption that for the majority of engineering purposes ion exchange can be considered as a special case of adsorption

*Water softening Hardness in water, caused by calcium and magnesium ions, which form insoluble compounds, is removed by ion exchange.

Ca2+ + Na2 R Ca R + 2 Na+ (12.4-1)(solution) (solid) (solid) (solution)The water is filtered through an artificial zeolite (ion exchange resin) , such as Permutite, and the sodium in the zeolite replaces the undesirable ions that are in the water.The positively charged ions in solution such as Ca2+ diffuse into the pores of the solid and exchange with the Na+ ions in the mineralWhen the zeolite is saturated with these metallic ions, it is washed with salt solution, which restores the sodium.

*Advantages

Increased value of a product because of improved propertiesRecovery of valuable componentsRecovery of water for re-useBetter quality control due to elimination of process variabilityReduced waste disposal costs due to elimination of objectionable components or reduced volumeReduction of corrosion or scaling in subsequent equipment

*Disadvantages Capital cost of equipmentResin costs which may be either capitalised or expensed depending on resin lifeCost of chemical regenerantsOperating labourDisposal costs for regenerant wastes

*Ion Exchange Resin Synthetic resins predominant can be tailored to specific applicationsProvides cation/anions to be exchangedGenerally solid gels in spherical or granular form consists:polymeric networkion functional group attachment counterions (exchangeable ions)solvent

* Ion Exchange ResinFixed and mobile ions in a cation exchanger (ref 1, pg 791)

*Ion Exchange Resin --SO3- groups permanently fixed to the polymeric network to give a negatively charged matrix and exchangeable, mobile hydrogen cations, H+This condition exist when solvent (e.g. water, methanol) is added.The H+ ion can be exchanged with other cations eg. Na+, Ca2+, K+ or Mg2+ to maintain neutrality of polymer.

Resin Each resin has a distinct number of mobile ion sites that set the maximum quantity of exchanges per unit of resin.

*Solid ion exchange particles An organic ion exchange resin is composed of high-molecular-weight polyelectrolytes that can exchange their mobile ions for ions of similar charge from the surrounding medium. e.g : Zeolites

Resin typesIon exchange resins are classified as cation exchangers, which have positively charged mobile ions available for exchange, and anion exchangers, whose exchangeable ions are negatively charged.

*Both anion and cation resins are produced from the same basic organic polymers.

Classification of resin *

Ion Exchangers (types)Natural: Proteins, Soils, Lignin, Coal, Metal oxides, Aluminosilicates (zeolites) (NaOAl2O3.4SiO2).

Synthetic zeolite gels and most common -polymeric resins (macroreticular, large pores).

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* Ion Exchangers Commercial ion exchangers

AmberliteDuoliteDowexIonacPurolite

Form of spherical beads from about 40m to 1.2mm diameter.

*Equilibrium Relations Ion exchange differs from adsorption in that one ion is exchanged for a solute ion.The exchange is governed by a reversible, stoichiometric chemical-reaction equation.

*Equilibrium RelationsNa+ + HR Na R + H+ (12.4-2)(solution) (solid) (solid) (solution)(12.4-4)

Total ionic concentration, = [NaR] + [HR] = constant

(12.4-6)

(Ref. 1)

*Ion-exchange process for strong acid or strong base exchangers by replacing ions in a solution with either: H+OH-

Regeneration, small amount of solution with high concentration of:H+ for cation exchangersOH- for anion exchangersOriginally in the resinRegeneration shift the equilibrium to the left, making the regeneration process more favorable

*Relative-Molar-Selectivity Coefficients (K)The equilibrium constant or selectivity coefficient for exchange of any two ions A and B (Table 12.4-1) and the formula below:Table 12-4-1 :Relative molar selectivity coefficient for polystyrene with 8 % DVB (ref 1)

Strong-Base Anion Exchange (Relative to Cl- as 1.0)Strong-Acid Cation Exchanger (Relative to Li+ as 1.0)Cl-IodideNitrateAcetateSulfateHydroxide1.08.73.80.20.150.05 0.07Li+H+Na+NH4+K+Mg2+Cu2+Ca2+1.01.271.982.552.903.293.855.16

*A + BR A R + B (solution) (solid) (solid) (solution)

KA,B = KA/KB

KA,B = cBqAR/cAqBR (c = ion concentration in equivalents/L volume of solution)(q = ion concentration in equivalents/L of bulk bed volume)

C = cA + cB (total ion concentration in liquid solution)Q = qA + qB (total ion concentration in resin)

*ExampleA waste acidic stream contains copper in solution which is being removed by a strong acid-cation resin. The cation Cu2+ (A) is displacing the cation H+ (B) in the resin. A polystyrene resin similar to the Table given is being used. The total resin capacity Q is approximately 1.9 equivalents/l of wet bed volume. For a total concentration C of 0.10 N (0.10 equivalents/L) in the solution, calculate at equilibrium the total equivalents of Cu2+ in the resin when the concentration is Cu2+ in solution is 0.02M (0.04 N).

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Cu2+ + 2HR Cu R2 + 2H+(solution) (solid) (solid) (solution)

* Design ConsiderationsFixed-Bed Ion-Exchange Columns

Rate depends on

Mass transfer ions from bulk to particle surface

Diffusion in the pores of solid to surface

Exchange of ions at the surface

Diffusion of the exchange ion back to the bulk solution.

*Modes of Operation (Similar to adsorption)

a) Continuous countercurrent: packed bed

*Modes of Operation

a) Continuous countercurrent: fluidized-bed

*Ion exchange in water & waste water

Ca, Mg (hardness removal) exchange with Na or H*http://www.thewatertreatments.com/water-softener/softner-water-conditioner-water-treatment-filter

Removal of hardness from water*The water obtained after passing through the ion-exchangers is called deionised water or demineralised water. In the ion exchange process, hard water is passed through two tanks 'A' and 'B'.Tank- A contains acid resin and tank- B is filled with basic resin. All the cations present in hard water (except H+) are removed by the acid resin present in Tank- A, and the basic resin present in Tank- B removes all the anions (except OH-) present in hard water. Water obtained after passage through both the tanks is free from all the cations and anions that make it hard.

*Demineralization in waste water treatmentThe effects of hardness in water are:They react with soap to form a scum, which is unsightly,The reaction of hard water with soap results in excessive use of soaps and detergents.Hard water may also cause taste problems in drinking water and may shorten the life of fabrics washed in hard water.Hard water harms many industrial processes such as scales inside boilers and pipes.

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Ion exchange materials*

Basic resins contain the basic group [(-NH3+)OH-], i.e., substituted ammonium hydroxide group.

Acid and basic ion exchange resins are represented as RCOO-H+ and RNH3+ + OH- respectively.

Giant organic molecules having acidic or basic groups are known as Ion-exchange resins.

Ion exchange materials*Basic resins exchange their OH- ions with the other anions such as HCO3-, Cl-, SO42-, present in hard water.

Basic resins, therefore, are also known as acid exchange resinsAcid resins exchange their H+ ions with other cations such as Ca2+, Mg2+, etc., present in hard water. Acid resins are, therefore known as base-exchange resins.

Equilibrium relations in ion exchange Developed using the law mass of actionGeneral chemical reaction equation in which reactants A and B react to give product C and D.

a A + b B --> c C + d D

where a, b, c, d are the coefficients for a balanced chemical equation.

The mass action law states that if the system is at equilibrium at a given temperature, then the following ratio is a constant. [C]c [D]d -------- = Keq [A]a [B]b

This is the ideal law of chemical equilibrium or law of mass action. The units for K depend upon the units used for concentrations.

If M is used for all concentrations, K has units Mc+d-(a+b)*

Equilibrium relations in ion exchange Convenient tables for relative-molar selectivity coefficient, K for various types of ion-exchange resins

Table 12.4.1 (textbook) for polystyrene resin with 8% DVB cross-linking (B8, P4) for strong acid and base.

The equilibrium constant/selectivity coefficient for ions A and B :KA,B = KA/KB

*

Table 12.4.1 (textbook) for polystyrene resin with 8% DVB cross-linking (B8, P4) for strong acid and base.*

Equilibrium relations in ion exchange For dilute solutions, the equilibrium constant is :Activity coefficient = CONSTANT

KA,B = cB qAR/cA qBR = cH+ qKR/ cK+ qHR

Resin phase : concentration qKR and qHR the unit are in equivalents/L of bulk bed volume of water-swelled resin

Liquid phase : concentration of cH+ and cK+ the unit are in equivalents/L of volume of solution

*

Equilibrium relations in ion exchange Total concentration C in the liquid solution and total concentration Q is CONSTANT.

C = CA + CBQ = qAR + qBR

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Example 12.4.1

Removal of Cu+ Ion From Acid Solution by an Ion-Exchange Resin

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*a A + b B --> c C + d D [C]c [D]d -------- = Keq [A]a [B]b C = CA + CBQ = qAR + qBRTable 12-4-1: if the resin used is 8% DVB crosslinking

Basic model in Ion exchangeRate of ion exchange depends on:Mass transfer of ions from bulk solution to the particle surfaceDiffusion of the ions in the pores of solid to the surfaceExchange of ions at the surfaceDiffusion of the ions back to the bulk solution

Designs method similar to adsorption

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Concentration profiles in packed bedSimilar as adsorption

Typical S-shaped curves occur and pass through the bed

Major part of the ion exchange at any time takes place in narrow mass transfer zone (MTZ)

The MTZ moves down the column

The Breakthrough curve (BC) is similar to BC of adsorption

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Mass transfer zoneAs MTZ travels down the column, the height of MTZ becomes constant

The ion to be removed from the feed stream has a greater affinity for the solid resin than the ion originally present in the solid.

The constant Height of MTZ allows for scale-up purposes. *

Capacity of columnSimilar with adsorption process*

References http://www.tutorvista.com/content/chemistry/chemistry-iii/hydrogen/methods-hardness-water.phphttp://www.science.uwaterloo.ca/~cchieh/cact/c123/massacti.htmlhttp://www.remco.com/ix.htmceeserver2.cee.cornell.edu/jjb2/cee6560/9-Ion%20Exchange.ppt

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*Industrial Applications

Water softening

Recovery of antibiotics/vitamin from fermentation broth

- Decolourisation of sugar

*Chromatorgraphy Chromatorgraphy - greatest advantage of the chromatography method over other analytical procedures is the ability of separating specific analytes (the entities you are analyzing for), and gives that ability to discover and analyze unknown elements and chemical compounds.

All chromatography techniques consists of two phases; mobile and immiscible stationary phases.

*Chromatorgraphy

The components that are strongly retained move slowly, while the components that are weakly retained travel more rapidly. A detector should be placed at the end of the chromatograph to quantify the analyte.

Based on the mobile phase, chromatorgraphy can be classified into major two categories; liquid chromatography and gas chromatorgraphy.

*Classification of Chromatorgraphy

*Ion Exchange ChromatorgraphyIon exchange chromatography the stationary phase consists of an insoluble porous resinous material containing fixed charge-carrying groups.

*ReferencesGeankoplis C. J., Transport Processes and Unit Operations, 4th Edition, Prentice Hall, 2003. 2. Seader J. D. and Henley E. J., Separation Process Principles, John Wiley, 19983. http://pasture.ecn.purdue.edu/~eql/H2Oanalyseanion

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