Particle Size:

The significance of particle size for proper column performance within an ion exchange unit is quite obvious. A rate of exchange, pressure drop and back wash expansions are all dependent on particle size. The resin beads or particles might be created along with diameters ranging from 1mm to less than 0.04 mm. For most of the ion exchange operations, an effective size of 0.4 - 0.6 mm diameter is preferred. This corresponds to particle size distribution falling among the 20- and 50-mesh screens. The ion exchange reactions are typically conducted in the aqueous media in that the particles have fully hydrated diameter. That is the value which is to be taken within consideration. The size of the water swollen resin will depend on the categories of functional group and the amount of cross linking of the polymer.

The size of the particle is one of the parameters affecting the rate of ion exchange reaction. Besides this, the other parameters affecting rate are size and charge of the ion included, degree of cross linking and the temperature. As a matter of fact, decreasing the size of the particle materially reduces the time needs for the equilibrium to be attained with the contacting solution. Because the time required to achieve the equilibration is decreased the efficiency of a given volume of resin increases. Instead, the volume of the resin needed to perform a specific operation decreases.

The physical aspects of operation are also considerably altered through the change within the particle size. Along with the decreasing particle size, a friction loss or pressure drop of a liquid flowing by the column increases. This means that for a given flow rate, along with decreasing particle size, the pressure drop in a column rises.

An ion exchange column is commonly backwashed at the end of an operating cycle to erase the foreign material and reclassify the particles. The back washing step expand the bed to various extents depending upon the specific gravity of the resin. A finer the mesh size and the lower the density, a greater will be the bed expansion.

Generally, the smaller resin particles (~ 50 meshes) are physically more stable. This is important when the resin is mechanically moved or it goes through large volume changes.