Welcome To Our Presentation

Myself SHOSHI PRASAD SHILStd. ID. – 20121207020

DEPARTMENT OF APPLIED CHEMISTRY & CHEMICAL ENGINEERING.

BANGABANDHU SHEIKH MUJIBUR RAHMAN SCIENCE & TECHNOLOGY UNIVERSITY.

Our Presentation Topic is

Properties of Ion-exchangers

ObjectivesChemistry of Ion-

ExchangerThe Examined

Properties

Ion-Exchanger Characteristics

Characteristics in Aqueous Medium

Ion-Exchange Resin Making

Characteristics in Non-aqueous Medium

The chemistry of ion exchange

Ion-exchangers’ CharacteristicsAn ion exchanger is a framework or a matrix which carries a positive or a negative electric charge. The composition of ion- exchanger is of ions of opposite sign called counter ions and ions of same sign called co-ions. For example, (an ion-exchanger)

RSO4H Here, if ‘R’ is the matrix then ‘H’ is the counter ion and ‘SO4’ is co-ion.

CROSSLINKING TO MAKE ION EXCHANGE RESINS

The examined Properties of ion-exchangers

1. Ionic FormIn expressing the results of most resin properties, the ionic

form should always be mentioned, as the values are different according to the ions in the resin beads. This applies in particular to the following properties: 1. Capacity 2. Moisture 3. Density 2. Ion exchange

capacityThis is the number of sites where exchange actually takes place during one cycle

3. Moisture Holding CapacityMoisture (water retention, or water content) is related to

porosity and ionic form. The water content is expressed as a percentage of the moist resin weight in a given ionic form. Here is the influence of porosity on resin performance: High moisture : fast exchange, good adsorption

properties, low total capacity. Low moisture: high total capacity, difficult to

regenerate, no removal of big ions, tendency to fouling. 4. Specific GravitySpecific Gravity is important for 1. Mixed bed separation,

2. Stratified beds (Strata bed), 3. Floating beds, 4. Backwashing.

5. Bulk Density And Shipping WeightThe bulk density of a resin is usually expressed as the

mass of one liter of the resin. As there are small variations of bulk density from batch to batch, a standard value, called shipping weight, is used for packaging the resin in the production plant.6. Ionic Form And Volume

ChangeResins change volume according to the ions loaded on them. For instance, a gel type strongly basic resin may swell by more than 25 % if it is totally converted from the chloride form (as delivered) to the hydroxide form. The volume change is due to the different hydration state of the ions in the resin: for instance, weak resins are almost not dissociated in regenerated form, so there are no free ions in the resin beads.

7. Particle Size DistributionTraditionally, the particle size distribution of a resin was

measured using a set of sieves. The volume on the resin fraction remaining on each sieve is measured in a glass cylinder. It was found practical to express the results in cumulative percentage through a given sieve opening. The components of a particle size distribution are: Mean diameter Uniformity coefficient Effective size Harmonic mean size Quantity of fine beads Quantity of big beads

8. Stability9. Selectivity

10. Optical Aspect

On the other view, characteristics of ion-exchanger depends on the medium ion-exchangers used; Mainly these are enclosed as – A. Aqueous Media & B. Non Aqueous Media.

Ionization and association of inorganic groups: Strong

acid or strong base resins are completely ionized but weak acid and bases are not. Weak acid groups are not ionized at low pH while weak bases are not ionized at high pH.

Exchange capacity: The maximum capacity of a resin is the number of exchangeable counter ions per specific amount of resin.

In Aqueous Media

The effective capacity for non-ionized resins depends on- i. pH of the external solution ii. Concentration of the external solution.

Swelling: When a dry ion exchange resin is immersed in water, a considerable amount is taken up and as a result the resin swells. A resin tends to swell for the following reasons-

a. The fixed and counter ion tend to solvate. b. Neighboring fixed ionic groups repel one another and thus tend to stretch the main matrix.

Selectivity: An anion exchanger exchanges its counter ions in preference for the other ions in solution for certain physical and chemical reasons. The selectivity enhanced by – a. Decreasing the concentration of the external solution and b. Increasing the cross linking of the resin.

Effect of complexion agent: The value of distribution co-efficient changes greatly with increasing acid concentration.

a) In dilute acids, the resin swells more than in concentrated acid and therefore ion exchange is enhanced. b) Increase acid concentration calls complexation. Poisoning: Resin should be capable of re generating when

poisioned and this reduce the elution capacity by the resin.

The behavior of ion exchanger in non aqueous media can be summarized as follows:

Resins swell more strongly in polar then in non polar solvents. in pure methanol and ethanol, the resins usually swell less then in water, and in nonpolar solvents such as benzene and kerosene there is no at all.

Non polar solvents may be enhance swelling by interaction with the organic constituents of the matrix, if there is similarity in structure. For example, weak acid resins in H+ form swell more strongly in methanol then water.

Ion Exchanger In Non-aqueous Media

The less polar the solvent the weaker is the solvation of the ions. In water, cation exchangers in Li+ form swell more strongly then the other alkali-ion forms, since Li+ has the higher solvation tendency.

The lower in the dielectric constant of the solvent, the stronger is the electrostatic between ions of opposite charges, so that ion pair formation and association increased, and the osmotic activity of the resin reduced, these effects tend to reduce swelling.

Electrostatic repulsion between fixed charges becomes stronger when dielectric constant is lowered, and it tends to increase the swelling. The alkali ion forms of the weak acid resin swells more strongly in water then in alcohols. Strong base resin swells more strongly in alcohols then in water.

Ion Exchanger In Non-aqueous Media

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