Standard Methods for Examination of Water & Wastewater_15 pptx

Ion ExchangeThis chapter discusses the unit process of ion exchange, including topics such as ion exchange reactions, unit operations of ion exchange, sodium and hydrogen cycles, regener

Ion Exchange

This chapter discusses the unit process of ion exchange, including topics such as ion exchange reactions, unit operations of ion exchange, sodium and hydrogen cycles, regeneration, and design of ion exchangers Some of these topics have already been discussed in the various sections of the unit operations in Part II and will only

be incorporated into this chapter by reference

16.1 ION EXCHANGE REACTIONS

Ion exchange is the displacement of one ion by another The displaced ion is originally

a part of an insoluble material, and the displacing ion is originally in solution At the completion of the process, the two ions are in reversed places: the displaced ion moves into solution and the displacing ion becomes a part of the insoluble material Two types of ion exchange materials are used: the cation exchange material and the anion exchange material The cation exchange material exchanges cations, while the anion exchange material exchanges anions The insoluble part of the exchange material is called the host If R−nrepresents the host part and C+m the exchangeable cation, the cation exchange material may be represented by (R−n)r(C+m)rn/m, where r

is the number of active sites in the insoluble material, rn/m is the number of charged exchangeable particles attached to the host material, −n is the charge of the host, and +m is the charge of the exchangeable cation On the other hand, if R+o represents the host part of the anion exchange material and A−p its exchangeable anion, the exchange material may be represented by (R+o)r(A−p)ro/p, where the subscripts and superscripts are similarly defined as those for the cation exchange material Letting

be the displacing cation from solution, the cation exchange reaction is

(16.1)

Also, letting be the displacing anion from solution, the anion exchange reaction may be represented by

(16.2)

As shown by the previous equations, ion exchange reactions are governed by equilibrium For this reason, effluents from ion exchange processes never yield pure water

16

C s

+q

R −n

( )r(C +m)rn/m rn

q

-C s +q

+
(R −n)r C s

+q

( )rn/q

rn m

-C +m

+

A s

−t

R +o

( )r(A − p)ro/ p

ro t

- A s

−t

+
(R +o)r A s

−t

( )ro /t ro

p

- A − p

+

Table 16.1 shows the displacement series for ion exchange materials When an ion species high in the table is in solution, it can displace ion species in the insoluble material below it in the table and, thus, be removed from solution As noted in this table, to remove any cation in solution, the displaceable cation must be the proton

H+; and to remove any anion, the displaceable anion must be the hydroxyl ion OH− Originally, natural and synthetic alumino silicates, called zeolites, were the only ones used as exchange materials Presently, they have been largely replaced

by synthetic resins Synthetic resins are insoluble polymers to which are added,

by certain chemical reactions, acidic and basic groups called functional groups These groups are capable of performing reversible exchange reactions with ions in solution The total number of these groups determines the exchange capacity of the exchange material, while the type of functional group determines ion selectivity When the exchange capacity of the exchange material is exhausted, the exchanger may be regenerated by the reverse reactions above The principles of regeneration are discussed in the section on “Sodium, Hydrogen Cycle, and Regeneration.”

16.2 UNIT OPERATIONS OF ION EXCHANGE

Figure 16.1 shows the schematics of the unit operations of ion exchange Figure 16.1a shows a cation exchanger and Figure 16.1b shows an anion exchanger In both units, the influent is introduced at the top of the vessel The bed of ion exchanger materials would be inside the vessels, where, as the water to be treated passes through, exchange

TABLE 16.1 Displacement Series for Ion Exchange

La3+

Y3+

Ba2+

Pb2+

Sr2+

Ca2+

—

SO 4 −

CrO4−

NO 3 −

AsO4−3

PO 4 −3

MoO4−2

NH4+

of ions takes place This exchange of ions is the chemical reaction of the unit process

of ion exchange; the mere physical passing through of the water with the attendant

head loss and pumping consideration is the unit operation of ion exchange The unit

operations of head losses are similar to those of granular filtration discussed in Part

II The unit operation of pumping was also discussed in Part II After ion exchange,

product waters are withdrawn at the respective bottoms of the vessels

16.3 SODIUM, HYDROGEN CYCLE, AND REGENERATION

As shown in Table 16.1, sodium, lithium, and hydrogen are the logical choices

for the exchangeable ions In practice, however, sodium and hydrogen are the

ions of choice The cation exchange resin using sodium may be represented by

(R−n)r(Na+)rn Its exchange reaction with Ca+2 and similar cations is shown below:

(16.3)

As shown, Ca+2 has become embedded in the resin, thus removed from solution,

and Na+ has become solubilized Similar reactions may be formulated for the rest

of the ions in Table 16.1

As soon as the resin is exhausted, it may be regenerated As shown in Equation

(16.3), by the Law of Mass Action, the reaction may be driven to the left by increasing

the concentration of the sodium ion on the right In practice, this is what is actually

done The resin is regenerated by using a concentration of NaCl of about 5 to 10%,

thus, driving the reaction to the left Operations where regeneration is done using

NaCl is said to run on the sodium cycle Regeneration may also be made using acids,

FIGURE 16.1 Unit operations of ion exchange.

Cation waste stream Backwash wasteRegenerant

(Rinsing waste)

Regenerant

waste stream

(cation salts)

Regenerant Backwash waste (Rinsing waste) NaCI (sodium cycle)

or HCI; H 2 SO 4 (acid cycle)

NaOH or

NH 4 OH Anionic waste

Backwash (and rinsing) Sodium salts (sodium cycle)

or Acids (hydrogen cycle) Regenerant

waste

Backwash (and rinsing)

( )r(Na+)rn rn

2 -Ca+2 +
(R −n)r(Ca+2)rn/2+rnNa+

such as H2SO4 When regeneration is through the use of acids, the cycle is called

the hydrogen cycle (from the proton or hydrogen ion content of acids).

Table 16.2 shows approximate exchange capacities and regeneration

require-ments for ion exchangers As shown, the values have great ranges Thus, in practice,

one must have to perform an actual experiment or obtain data from the manufacturer

for a particular ion exchanger to determine the exchange capacity and regeneration

requirement The capacity of an ion exchanger in terms of volume of influent treated

varies with the nature and concentration of ions in solution This is much the same

as the characteristics of activated carbon Hence, the experimental procedure is

practically the same as that of activated carbon

Tables 16.3 and 16.4 show some additional properties of exchangers The acidic

exchangers are cationic exchangers They are called acidic because their exchange

sites are negatively charged to which the H+ ion can attach, hence, acidic The strongly

acidic cation exchangers readily remove cations from solutions, while the weakly

acidic exchangers will remove ions such as calcium and magnesium but have limited

ability to remove sodium and potassium, which are way down the table in the

dis-placement series The basic exchangers, on the other hand, are anionic They have

positively charged exchange sites to which the hydroxyl ion can attach and other

basic species such the quaternary and amine groups The strongly basic exchanger

can readily remove all anions The weakly basic ones remove mainly the anions of

strong acids such as , Cl−, and

16.4 PRODUCTION OF “PURE WATER”

Theoretically, it would seem possible to produce pure water by combining the cation

exchanger operating on the hydrogen cycle and the anion exchanger operating on

the OH cycle This is shown in the following discussions Let Equation (16.1) be

written specifically for the hydrogen cycle The resulting equation is

(16.4)

TABLE 16.2

General Properties of Ion Exchangers

Exchanger, cycle

Exchange Capacity,

Regenerant

Regenerant Requirement ,

Cation exchangers:

Synthetic zeolite, Na 350–700 NaCl 2–3

Anion exchanger:

geq

m 3

- geq

m 3

-SO42− NO3−

R −n

( )r( )H+ rn rn

q

-C s +q
(R −n)r C s

+q

( )rn/q rnH+

From this equation, the number of reference species is rn /q(q), based on the

cation in solution; and the equivalent mass of species is

TABLE 16.3

Some Additional Properties of Cation Exchangers

Material

Exchange Capacity, , Average

Packed Density, , Average

Particle Shape Strongly Acidic:

Sulfonated polystyrene:

Homogeneous resin:

Resins from phenol methylene

sulfuric acid

Weakly Acidic:

Acrylic or meta acrylic:

Homogeneous resin:

Phenolic and related

condensation products

Inorganic materials:

Celluloses:

dry meq gm - m - kg 3

C s

+q rn

q

-C s

+q rn q - q( ) - C s

+q

q

-=

Letting the molar concentration of be gmol/L, the corresponding con-centration in geq/L is

Note: From , the units of q is equivalents per mole.

Therefore, the total concentration in gram equivalents per liter of removable cations

in solution, [CatT ] eq , is the sum of all the cations Let there be a total of i cations.

Thus,

(16.5)

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

TABLE 16.4

Some Additional Properties of Anion Exchangers

Material

Exchange Capacity,

, Average

Packed Density, , Average Particle

Shape Strongly Basic:

Polystyrene matrix:

Trimethyl benzene ammonium:

Dimethyl hydroxyethyl benzyl ammonium

Condensation products with pyridium

quaternary amine

Weakly Basic:

Mixed aliphatic amine and quaternary

ammonium

dry meq gm - m - kg 3

C s

+q

C s

+q

C s

+q

[ ]C s

+q

C s +q q

- q C s

+q

[ ]

=

q C s

+q

[ ]

CatT

[ ]eq q i C s i

+q i

i=1

i =m

∑

=

Let [AnionT] eq and [HT ] eq be the total anions and the hydrogen ions displaced, respectively Since the number of equivalents of one substance in a reaction is equal

to the number of equivalents of all the other substances participating in the reaction,

(16.6)

Let the [AnionT ] eq from the effluent of the cation exchanger be introduced into

an anion exchanger For the anion exchanger operating under the OH cycle, the total equivalents of OH− released from the anion bed is equal to that of the anions,

[AnionT] eq , removed from solution Let [OHT ] eq be this total OH− Since [AnionT ] eq

is equal to [HT] eq , [OHT ] eq must be equal to [HT ] eq This means that all the acids produced in the cation exchanger are neutralized in the anion exchanger, and all ions in the water have been removed by using the combination of cation exchanger followed by anion exchanger

On the surface, the combination of cation exchanger and anion exchanger would mean that pure water is produced As shown in Equations (16.1) and (16.2), however, the unit process of ion exchange is governed by equilibrium constants The values

of these constants depend upon how tightly the removed ions from solution are bound to the bed exchanger sites In general, however, by the nature of equilibrium constants, the concentrations of the affected solutes in solution are extremely small Practically, then, we may say that “pure water” has been produced

By analogy with Equation (16.5),

(16.7)

As with q i , the units of t i are equivalents per mole

Cu2+= 30 mg/L, Zn2+= 15 mg/L, and Calculate the total equiv-alents of cations and anions, assuming the volume of the wastewater is 450 m3

Solution:

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

Total equivalents of cations = 2.069(450) = 931.05 Ans

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

∑ = 2.395 ∑ = 2.069

Note: The ions are not balanced, meaning error in analysis.

a

Equiv mass b120 /58 = 2.069

AnionT

[ ]eq = [HT]eq = [CatT]eq

AnionT

[ ]eq t i A s i

−t i

i=1

i =m

∑

=

CrO42−

Ni2+ = 20 mg/L

CrO4− = 120

Cu2+ = 30

Zn2+ = 15

Ni2+ = 20

CrO4− = [ 52 + 4 16 ( ) ]/2 = 58

16.5 ACTIVE OR EXCHANGE ZONE

Figure 16.2 is the same figure illustrated in a previous chapter under carbon adsorp-tion The length of the active zone was derived in that chapter and is reproduced next

(16.8)

where

δ = length of active zone

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

= volume treated at breakthrough

[C o] = influent concentration to δ

= total volume treated at time t n+1

= total volume treated at time t n

[C n+1] = concentration of solute at effluent of δ at time tn+1

[C n] = concentration of solute at effluent of δ at time tn

A s = surficial area of exchanger bed

FIGURE 16.2 Active zones at various times during adsorption and the breakthrough curve.

δ

2 (V x–V b ) C [ ] ∑ V o ( n+1–V n) [C n+1 ] + [ ]C n

2

–

A sρp

X M

- 

 

ult

-=

V x

V b

V n+1

V n

Feed

Effluent

d

d

d < d

C1

Cb

Co

Exhaustion

Volume of water treated, V

0

L

C l e a n w a t e r

ρp = pack density of ion exchange material

(X /M) ult = ultimate exchange capacity of the bed or simply, the exchange capacity

of the bed

It should be emphasized that to use the equation [C o ], [C n+1], and [C n] should

be the total concentration of ions For example, if the influent is composed of the ions Ca2+= 50 mg/L, Mg2+= 60 mg/L, and Zn2+= 2 mg/L, then [C o]meq in meq/L is

50/(Ca/2) + 60/(Mg/2) + 2/(Zn/2)

pro-ducing the results below Determine the length δ 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 [C o]

is equal to 2.2 meq/L (X/M) ult= 6.5 meq/g

Solution:

0.06 1.0 0.08 1.20 0.09 1.30 0.10 1.40 0.20 1.48 0.46 1.58 1.30 1.70 1.80 1.85 2.10 2.00

C, meq/L , L ( )

V

δ

2 (V x–V b ) C [ ] ∑ V o ( n+1–V n) [C n+1]+[ ]C n

2

–

A sρp

X M

- 

 

ult

-=

C n+1

[ ] + [ ]C n

2

  (V n+1–V n) [C n++++1] + [ ]C n

2

A s π 0.025( )2

4 - 0.00049 m2

16.6 DESIGN OF ION EXCHANGERS

Generally, designs of ion exchangers should include the following: quantity of exchange materials and regenerants; dimension of the bed (volume of bed); interval

of bed regeneration, backwash, and rinse water requirements The amount of exchange materials determines the dimension of the bed The interval of regeneration may be arbitrarily set from which the quantity of exchange bed material may be calculated Regeneration, backwash, and rinse waters may pose pollution problems

16.6.1 Q UANTITY OF E XCHANGE M ATERIALS

Before discussing quantities of exchange materials, a method of expressing exchange capacity in terms of calcium carbonate is addressed This method of expressing capacity is very troublesome, and it should not have been adopted; nonetheless, it

is used and we must know it As shown in Tables 16.2, 16.3, and 16.4, equivalents, among other units, are used to express exchange capacities This is appropriate because reactants react in equivalent amounts; but to express this in terms of calcium carbonate is a bit unusual As addressed in previous chapters, however, arbitrarily adopt CaCO3/2 = 50 as the equivalent mass of calcium carbonate From this, the exchange capacity, expressed in equivalents, may be obtained by dividing the exchange capacity expressed in calcium carbonate by 50

Let us first derive the formula for the exchange materials for the cation bed The amount of exchange bed materials required can be determined by calculating first the amount of displacing ions in solution to be removed Let the exchange capacity

of the bed be (X /M ) ult meq/g of bed The equivalents of ion displaced from the bed

is equal to the equivalents of displacing ion in solution; therefore, the mass of bed

material CatTBedMass in kilograms is

(16.9)

Q is the m3/d of flow and t int is the interval of regeneration in hours In concept, the interval of regeneration may be arbitrarily set A value of 8 hours is not unreasonable The factor (1000/24) is used so that the unit of CatTBedMass will be in kilograms.

δ 2{(2–1) 2.2[ ] 0.7076– } 0.00049

( ) 750( ) 1000( ) 6.5( ) - 0.0012 m 1.2 mm

CatTBedMass ([CatT]eq ) Q ( ) t( )int

X M

- 

 

ult

- 1000

24

=

∑i =m

i=1q i C s i

+q i

( ) Q ( ) t( )int

X M

- 

 

ult

- 1000

24

=