ION EXCHANGE. Presentation Outline Ion Exchange Reactions Unit Operations of Ion Exchange Sodium,...

ION EXCHANGE

Presentation Outline

Ion Exchange Reactions Unit Operations of Ion Exchange Sodium, Hydrogen Cycle and Regeneration Production of Pure Water Active or Exchange Zone Design of Ion Exchangers Quantity of Regenerant Wastewater Production

Ion Exchange Reactions

Ion Exchange is the displacement of ion by another

Ion Exchange is a reversible chemical reactions wherein an ion from solution is exchanged for a similarly charged ion attached to an immobile solid particles

The displaced ion moves into solution and the displacing ion becomes a part of the insoluble materials (ResinResin)

Ion Exchange

Two types of ion exchange materials are used The cation exchange material

The anion exchange material

Ion Exchange

Ion Exchange

The insoluble part of the exchange materials is called the host

The cation exchange materials may be represented by rr : the number of active sites in the insoluble

material r n/mr n/m: the number of charged exchangeable

particles attached to the host materials -n-n: is the charge of the host +m+m: is the charge of the exchangeable cation

mmrn

mr

n CR /


cation exchange reaction as well as the anion exchange reaction are as follows

Ion exchange reaction are governed by Equilibrium. For this reason, effluents from ion exchange processes never yield pure water

mqrn

qsr

nqsmrn

mr

n Cm

rnCRC

q

rnCR //

ptro

tsr

otspro

pr

o Ap

roARA

t

roAR //


Displacement Series for Ion ExchangeDisplacement Series for Ion Exchange

• Displacement series for ion exchange materials is shown in left side table

• when an ion species high in table is in solution, it can displace ion species in the insoluble material below it in the table.

• to remove any cation in solution, the displaceable cation must be the proton, and to remove any anion, the displaceable anion must be the hudroxyl ion.

ION Exchange Reactions

Examples of exchange materials Zeolites (Natural Material) Synthetic resins

Synthetic resins are insoluble polymers

These polymers are either acidic or basic group, and they are called functional group


These groups are capable of performing reversible exchange reactions with ions in solution

The total number of these groups determine the exchange capacityexchange capacity of the exchange material

The type of functional group determines ion ion selectivityselectivity

The exchanger may be regenerated by the reverse reactions (upon exhaustion)

Unit Operation of Ion Exchange

Unit Operation of Ion Exchange

In both units, the influent is introduced at the top of the vessel

the bed of ion exchanger materials would be inside the vessels

As the to be treated passes through, exchange of ions takes place

This exchange of ions is the chemical reaction of the unit process of ion exchange

Sodium, Hydrogen cycle, And Regeneration Sodium and Hydrogen are the logical choices for the

exchangeable ions.

The cation exchange resin using sodium to remove the Ca+2 may be represented by the following reactions

NarnCaRCarn

NaR rnrn

rnrn

2/22

2

Sodium, Hydrogen Cycle, And Regeneration As soon as the resin is exhausted, it may be regenerated

The resin is regenerated by using a concentration of NaCl of a bout 5 to 10%, thus, driving the reaction to the left

Operations where regeneration is done using NaCl, the cycle is called the Sodium CycleSodium Cycle

Operations where regeneration is done using acids (H2SO4), the cycle is called the Hydrogen CycleHydrogen Cycle

Sodium, Hydrogen Cycle, And Regeneration

The following table shows approximate exchange capacities and regeneration requirements for ion exchangers

Sodium, Hydrogen Cycle, And RegenerationExchanger, cycleExchanger, cycle Exchange Exchange

CapacityCapacity (geq/m3)

RegenerantRegenerant Regenerant Regenerant RequirementRequirement

)geq/m3(

Cation exchangers:

Natural zeolite, Na 175-350 NaCl 3-6

Synthetic zeolite, Na 350-700 NaCl 2-3

Resin, Na 350-1760 NaCl 1.8-3.6

Resin, H 350-1760 H2SO4 2-4

Anion exchanger:

Resin, OH 700-1050 NaOH 5-8


In order to determine the exchange capacities and regeneration requirements we have to do the following:

Perform an actual experiment

Obtain data form the manufacturer


Table below shows some additional properties of exchangers


The strongly acidic (cation) exchangers readily remove cations from solutions

The weakly acidic exchangers have limited ability to remove certain cations

The strongly basic (anion) exchangers can readily remove all the anions

The weakly basic one remove mainly the anions of strong acid such as SO4

-2 and Cl

Production of “PURE WATER’’

Theoretically, It would seem possible to produce pure water by combining the cation exchanger and the anion exchanger

The following equation for the hydrogen cycle is

rnHCRCq

rnHR qrn

qsr

nqsrnr

n/


Letting the molar concentration ofbe

The corresponding concentration in geq / L is

qsC

gmol/L][ qsC

][][ q

sqs

qs

qs Cq

q

C

CC


Therefore, the total concentration in gram equivalents per liter of removable cations in solutions is the sum of all the cations. Thus,

As, [CatT]eq of cations is removed form solution, a corresponding number of equivalent concentrations of anions pair with the H+ ions displaced from the cation bed

mi

i

qsieq

i

iCqCatT

1

][][


The total anions and the hydrogen ions displaced is expressed as follows

Practically, we may say that “ pure water” is

produced and expressed as follows

eqeqeq TCatHTTAnion ][][][

The units of ti are equivalents per mole

mi

i

tsieq

i

iAtTAnion

1

][][


Example: A wastewater contains the following ions:

mg/L12024 CrO

mg/L 15 Zmg/L, 30 2n

2 Cumg/L 202 Ni

Calculate the total equivalents of cations and anions, assuming the volume of the wastewater is 450 m3.


Solution:Solution:

Ions (mg/L) Equiv. Mass Cations (meq/L) Anions (meq/L)

58a __ 2.069b

31.75 0.945 __

32.7 0.469 __

29.35 0.681 __

12024 CrO

302 Cu

152 Zn

202 Ni

a Equiv. mass b120/58 = 2.069 .582/)]16(452[24 CrO

395.2 069.2


Solution (Cont’d) Solution (Cont’d)

Total equivalents of cations = 2.395(450) = 1077.75 1077.75 Ans

Total equivalents of cations = 2.069(450) = 931.05931.05 Ans

Active or Exchange Zone

Active zone is a segment of exchanger bed engaged in exchanging ions

ultps

nnnnobx

M

XA

CCC

2

][][)(][)(2 1

1


Where:

x

b

][ oC

1n

= total volume of water or wastewater treated at complete exhaustion of bed

= = length of active zone

= volume treated at breakthrough

= influent concentration to

= total volume treated at time

1nt


(Cont’d)

n = total volume treated at time nt

= concentration of solute at effluent of at time 1nt][ 1nC

= concentration of solute at effluent of at time nt][ nC

= surficial area of exchanger bed sA


Active zone at various times during adsorption and the breakthrough curve


Example 2: A breakthrough experiment is conducted for a wastewater producing the results below. Determine the length of the active zone. The diameter of the column used is 2.5 cm. and the packed density of the bed is 750 kg/m3. is equal to 2.2 meq/L. And

The experiments results are tabulated on the next slide

][ oCmeq/g5.6)/( ultMX


C, meq/L

0.06 1

0.08 1.2

0.09 1.3

0.1 1.4

0.2 1.48

0.46 1.58

1.3 1.7

1.8 1.85

2.1 2

L,


Solution:

ultps

nnnnobx

M

XA

CCC

2

][][)(][)(2 1

1


0.06 1.0 0.20 0.07 0.014

0.08 1.20 0.10 0.085 0.0085

0.09 1.30 0.10 0.095 0.0095

0.10 1.40 0.08 0.15 0.012

0.20 1.48 0.10 0.33 0.033

0.46 1.58 0.12 0.88 0.1056

1.30 1.70 0.15 1.55 0.2325

1.80 1.85 0.15 1.95 0.2925

2.10 2.00

meq/L,C L, )( 1 nn

2

][][ 1 nn CC )( 1 nn

2

][][ 1 nn CC


22

00049.04

025.0mAs

Therefore,

= 1.2 mm= 1.2 mm

Design of Ion Exchangers

Designs of ion exchangers should include the following:

Quantity of exchange materials

Quantity of regenerant

Quantity of Exchange Materials

The amount of exchange bed materials required can be determined by the calculating the amount of displacing ions in solution to be removed

The equivalents of ion displaced from the bed is equal to the equivalents of displacing ion in solution

The mass of bed materials CatTBedMass CatTBedMass in kilograms is


24

1000)()(])[(

24

1000)()()]([

int1

int

ult

qs

mi

i i

ult

eq

M

X

tQCq

M

X

tQCatTsCatTBedMas

i

i

Q is the m3/d of flow and tint is the interval of regeneration in hours


By analogy, the mass bed materials for the anion exchanger in Kiograms is:

24

1000)()(])[(

24

1000)()()]([

int1

int

ult

ts

mi

i i

ult

eq

M

X

tQAt

M

X

tQAnionassAnionTBedM

i

i


The volume in m3 for CatBedVolCatBedVol

24

1000)1()()(])[(

24

1000)1(

)()(])[(

int1

int1

ultp

qs

mi

i i

p

ult

qs

mi

i i

M

X

swelltQCq

swellM

X

tQCq

CatTBedVol

i

i

i

i


the volume in m3 For AnionTBedVolAnionTBedVol

24

1000)1()()(])[( int1

ultp

ts

mi

i i

M

X

swelltQAtolAnionTBedV

i

i

The percentage of swell of the exchanger bed is a very important property

It determines the final size of the tank into which the material is to be put

This value can be obtained through

Experiments

from the manufacturer


Example: Using a bed exchanger, 75 m3 of water per day is to be treated for hardness removal between regenerations having intervals of 8 h. the raw water contains 400 mg/L of hardness as CaCO3. The exchanger is a resin of exchange capacity of 1412.8 geq/m3. Assume that the packed density of the resin is 720 kg/m3. Calculate the mass of exchanger material to be used and the resulting volume when the exchanger is put into operation.


Solution:

Assume cation exchanger:

meq/L 0.004meq/L850

400][

1

mi

i

qsi

i

iCq

24

1000)()(])[( int1

ult

qs

mi

i i

M

X

tQCqsCatTBedMas

i

i

Also, assume that all of the cations are removed

g

meq96.1

g 720(1000)

meq )1000(8.1412

geq8.1412

3

mM

X

ult


Therefore,

nsA kg 02.5124

1000

96.1

)8)(75(004.0

sCatTBedMas

24

1000)1()()(])[( int1

ultp

qs

mi

i i

M

X

swelltQCqCatTBedVol

i

i

Assume swell= 0.8

nsA m 13.0)8.01(720

02.51 3CatTBedVol

Quantity of Regenerant

24

1)()()(][ int

1

tQRCqegenerantCatR i

i

qs

mi

ii

The Kilogram equivalents of regenerant, CatRegenerant, used to regenerate cation exchangers is

The Kilograme equivalents of regenerant, Anion Regenerant, used to regenerate anion exchange is

24

1)()()(][ int

1

tQRAtegenerantAnionR i

i

ts

mi

ii


Example: Using a bed exchanger, 75 m3 of water per day is to be treated for hardness removal between regeneration having intervals of 8 hours. The raw water contains 80 mg/L of Ca+2 and 15 mg/L of Mg2+. The exchanger is a resin of exchange capacity of 1412.8 geq/m3. Assume that the packed density of the resin is 720 kg/m3. Calculate the kilograms of sodium chloride regenerant required assuming R = 2 and that all of the cations were removed


Solution:

24

1)()()(][ int

1

tQRCqegenerantCatR i

i

qs

mi

ii

33

3

1

m

keq)10(22.5

L

geq)10(22.5

)1000)(2/3.24(

15

)1000)(2/1.40(

80][

mi

i

qsi

i

iCq

Therefore,

nsAon regenerati of intervalper kg 0.26

24

1)8)(75)(2)(10(22.5 3

egenerantCatR

Wastewater Production

In the operation of ion exchangers, wastewater are produced. These come form:

Solvent water (used to dissolve the regenerant)

Backwash and rinse requirements

Wastewater production

In the sodium cycle, the concentration of NaCl is about 5 to 10% for an average of 7.5%

If the quantity of regenerant required is 0.26 Kg

The volume of wastewater produced from regeneration can be calculated as follows The total mass of regenerant solution is

0.26 / 0.075 = 3.47 Kg


The corresponding volume is 3.47/ 1000 = 0.0035 m3

For an interval of regeneration of 8 h and assuming a rate of flow for the water treated of 75 m3 / d

The volume of water treated is 75/24(8) = 25m3

Thus, the wastewater produced is 0.0035/25 * 100 = 0.014 % by volume


The quantity of backwash and rinse water requirements should be determined by experiment on the actual exchanger bed to be used in the design and is expressed as a function of bed volume

For the cation exchanger the volume of bed was previously derived as

3int1 m24

1000)()(])[(

ultp

qs

mi

i i

M

X

tQCqCatTBedVol

i

i


With the swelling not being considered. Thus,

24

1000)()()(])[( int1

ultp

qs

mi

i i

M

X

nseVBackwashRitQCqhRinseVolCatBackwas

i

i

For the anion exchangers

24

1000)()()(])[( int1

ultp

ts

mi

i i

M

X

nseVBackwashRitQAtlashRinseVoAnionBackw

i

i


Example: using a bed exchangers, 75 m3 of water per day is to be treated for hardness removal between regeneration having 8 hours. The raw water contains 80 mg/l of Ca and 15 mg/l of Mg. the exchanger is a resin of exchange capacity of 1412.8 geq/m3. Assume that the packed density of the resin is 720 kg/m3. calculate the total volume of rinse and backwash requirement if the backwash and rinse per unit volume of bed is 18m3/m3


Solution:

24

1000)()()(])[( int1

ultp

qs

mi

i i

M

X

nseVBackwashRitQCqhRinseVolCatBackwas

i

i

33

1 m

keq )10(22.5[

mi

i

qsi

i

iCq

g

meq 96.1

g 720(1000)

meq )1000(8.1412

m

meq 8.1412

3

ultM

X

nsA m 66.124

1000

)96.1(720

)18)(8)(75)(10(22.5 33

hRinseVolCatBackwas

ION EXCHANGE. Presentation Outline Ion Exchange Reactions Unit Operations of Ion Exchange Sodium,...

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