Ion Exchange.pdf

8/14/2019 Ion Exchange.pdf

1/26

Ion Exchange


2/26

Ion exchange

Ion exchange is an adsorption phenomenon where the

mechanism of adsorption is electrostatic. Electrostatic forces

hold ions to charged functional groups on the surface of the

ion exchange resin. The adsorbed ions replace ions that are

on the resin surface on a 1:1 charge basis. For example:


3/26


4/26

Applications of ion exchange in water & wastewater

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

Fe, Mn removal from groundwater.

Recovery of valuable waste products Ag, Au, U Demineralization (exchange all cations for H all anions

for OH)

Removal of NO3, NH4, PO4(nutrient removal).


5/26

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).


6/26

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.


7/26


8/26

These resins are generally manufactured by polymerizing

neutral organic molecules such as sytrene (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.


9/26


10/26

Divinylbenzene


11/26

Ion Exchange Resin:


12/26

Resin classification:

Resins are classified based on the type of functional group

they contain and their % of cross-linkages

Cationic Exchangers:

- Strongly acidic functional groups derived from strong acids

e.g., R-SO3H (sulfonic).

- Weakly acidic functional groups derived from weak acids,

e.g., R-COOH (carboxylic).


13/26

Anionic Exchangers

- Strongly basic functional groups derived from

quaternary ammonia compounds, R-N-OH.

- Weakly basic - functional groups derived from primary

and secondary amines, R-NH3OH or R-R-NH2OH.


14/26

Ion Exchange

Ion Exchange Process exchanges undesirable ions contained in

the raw water with more desirable ones that produces

acceptable BFW (BoilerFeed Water).

For example, in the sof teni ng pr ocess, calcium and

magnesium ions are exchanged for sodium ions. In

dealkalization, the ions contributing to alkalinity (carbonates,

bicarbonates, etc) are removed and replaced with chloride

ions. Demineralizationin this process replaces all cations with

hydrogen ions (H+), and all anions with hydroxyl ions (OH)

making pure water (H+ +OH).


15/26

The ion exchange material needs to be regenerated after a

period of operation.

The operating period will differ from process to process andwill depend to some extent on the amount of impurities in the

water and the required purity of the treated water.


16/26

Regeneration

Regeneration is accomplished in three steps

1. Back washing

2. Regenerating the resin bed with regenerating chemicals

3. Rinsing


17/26

Ion exchange unit


18/26


19/26


20/26


21/26

Under operating conditions the raw water is introduced

through the top connection and distributor. The water flows

through the resin bed where ion exchange takes place.

The treated water is removed via the bottom connection.

Under regeneration operation, raw water as backwash is

introduced through the bottom connection and removed from

the top connection.


22/26


23/26

This is done by introducing a stream of raw water at the top

connection and removing it from the bottom connection. This water

is also disposed to waste.

Normally ion exchange units are installed in pairs. When one is

operating the other is being regenerated.

An automatic switch over of electronically controlled valves takes

the pair of units through the correct cycles at the prescribed time

intervals, without disrupting the treating process


24/26

Ion Exchange

The interior is generally treated to protect the tank against corrosion

from the salt.

The units are normally of the downflow type, and the size and

volume of the units are dictated by the hardness of the water and the

volume of treated water needed to be produced between each

regeneration cycle.

Resin is supported by an underdrain system that removes the treated

water and distributes brine evenly during regeneration. Minimum

depth of resin should be no less than 24 inches above the underdrain.


25/26

Ion Exchange

Ion-exchange does not alter thewaterspH or alkalinity.

The stability of the water is altered due to the removal of calcium

and magnesium and an increase in dissolved solids.

For each ppm(parts permillion) of calcium removed and replaced

by sodium, total dissolved solids increase by 0.15 ppm.

For each ppm of magnesium removed and replaced by sodium, total

dissolved solids increase by 0.88 ppm.


26/26

Special Ion Exchangers - Commercially Available

Documents

Ion Exchange.pdf