Ion Exchange 1

CPE591 PROCESS ENGINEERING I

ION EXCHANGE PROCESSES

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Resin

2 http://www.waters.com/waters/nav.htm?cid=10049076&locale=en_US

is a reversible chemical reaction wherein an ion (an atom or molecule that has

lost/gained an electron and thus acquired an electrical charge) from solution is

exchanged for a similarly charged ion attached to an immobile solid particle.

Resin

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

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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 types

Ion 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.

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

Classification of resin

Strong Acid Cation Resins.

Weak Acid

Cation Basins

Strong Base Anion Resins.

Weak Base

Anion Resins

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Ion Exchangers (types)

1. Natural: Proteins, Soils, Lignin, Coal, Metal oxides, Aluminosilicates (zeolites)

(NaOAl2O3.4SiO2).

2.Synthetic zeolite gels and most common -polymeric resins

(macroreticular, large pores).

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Ion Exchangers (types)

• Polymeric resins are made in 3-D networks by cross-linking hydrocarbon chains.

• The resulting resin is

insoluble, inert and relatively rigid.

• Ionic functional groups are attached to this framework

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Cross linking chains

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These resins are generally manufactured by

polymerizing neutral organic molecules such as styrene (to form polystrene) and then cross-linked with

divinyl benzene (DVB).

Functional groups are then added according to the intended use. For example the resin can be sulfonated by adding

sulfuric acid to get the structure shown above.

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Ion exchange in water & waste water

Ca, Mg (hardness removal) exchange with Na or H

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http://www.thewatertreatments.com/water-softener/softner-water-conditioner-water-treatment-filter

Removal of hardness from water

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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.

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Demineralization in waste water treatment

The effects of hardness in water are: 1. They react with soap to form a scum, which is unsightly, 2. The reaction of hard water with soap results in excessive use

of soaps and detergents.

3. Hard water may also cause taste problems in drinking water and may shorten the life of fabrics washed in hard water.

4. Hard water harms many industrial processes such as scales inside boilers and pipes.

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Ion exchange materials

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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

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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 resins

Acid 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 action General 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) 16

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

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

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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

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Equilibrium relations in ion exchange

Total concentration C in the liquid solution and total concentration Q is CONSTANT.

C = CA + CB Q = 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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Write down the equilibrium equation

Find the relative molar selectivity coefficient, K for Cation and Anion

exchangers

Write down and calculate the

equilibrium constant

Total concentration, C in liquid solution

Total concentration, Q in resin

Solve and calculate

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

[C]c [D]d -------- = Keq

[A]a [B]

b

C = CA + CB

Q = qAR + qBR

Table 12-4-1: if the resin used is 8% DVB crosslinking

Basic model in Ion exchange

• Rate of ion exchange depends on: 1. Mass transfer of ions from bulk

solution to the particle surface

2. Diffusion of the ions in the pores of solid to the surface

3. Exchange of ions at the surface

4. Diffusion of the ions back to the bulk solution

• Designs method similar to adsorption

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Concentration profiles in packed bed

Similar 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 zone

As 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.

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Capacity of column

Similar with adsorption process

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The total/ stoichiometry capacity

The usable capacity

The length of bed used up to breakpoint

The length of unused bed

The total bed length

References

1. http://www.tutorvista.com/content/chemistry/chemistry-iii/hydrogen/methods-hardness-water.php

2. http://www.science.uwaterloo.ca/~cchieh/cact/c123/massacti.html

3. http://www.remco.com/ix.htm

4. ceeserver2.cee.cornell.edu/jjb2/cee6560/9-Ion%20Exchange.ppt

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THANK YOU

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