The particle size and related physical properties of ion exchange resin have a great influence on its work and performance.
(1) Resin particle size
Ion exchange resins are usually made into small beads in the shape of beads, and its size is also important. The resin particles are finer, the reaction speed is greater, but the resistance of the fine particles to the passage of the liquid is greater, and a higher working pressure is required; especially the high viscosity of the concentrated sugar liquid, this effect is more significant. Therefore, the size of the resin particles should be selected appropriately. If the resin particle size is below 0.2mm (approximately 70 mesh), it will significantly increase the resistance of the fluid to pass, reducing the flow rate and production capacity.
The resin particle size is usually measured by the wet sieving method. The resin is sieved after fully absorbing water and swelled, and the remaining amount on the 20, 30, 40, 50… mesh screen is accumulated, and 90% of the particles can pass through it. The diameter of the mesh is called the “effective particle size” of the resin. The effective particle size of most common resin products is between 0.4 and 0.6 mm.
Whether the resin particles are uniform is expressed by the uniformity coefficient. It is the ratio of the corresponding sieve hole diameter to the effective particle size by taking the cumulative retained amount of 40% particles on the coordinate chart of the “effective particle size” of the resin. For example, the effective particle size of a resin (IR-120) is 0.4～0.6mm, and the remaining particles on the 20 mesh sieve, 30 mesh sieve and 40 mesh sieve are 18.3%, 41.1%, and 31.3% respectively, then the calculation is The uniformity coefficient is 2.0.
(2) Density of resin
The density of the resin when it is dry is called the true density. The weight of the wet resin per unit volume (including the gap between particles) is called the apparent density. The density of the resin is related to its degree of crosslinking and the nature of the exchange group. Generally, a resin with a high degree of crosslinking has a higher density, a strong acid or a strong basic resin has a higher density than a weak acid or a weak basic resin, and a macroporous resin has a lower density. For example, the true density of the styrene-based gel-type strong acid cation resin is 1.26g/mL, and the apparent density is 0.85g/mL; and the true density of the acrylic gel-type weak acid cation resin is 1.19g/mL, the apparent density is 0.75g/mL.
(3) Solubility of resin
The ion exchange resin should be an insoluble substance. However, the substances with a low degree of polymerization mixed in the resin synthesis process and the substances generated by the decomposition of the resin will dissolve out during operation. Resins with a lower degree of cross-linking and more active groups have a greater tendency to dissolve.
ion exchange resin contains a large number of hydrophilic groups, which will swell when it comes in contact with water. When the ions in the resin change, for example, the cationic resin changes from H+ to Na+, and the anionic resin changes from Cl- to OH-, both will expand due to the increase in ion diameter, increasing the volume of the resin. Generally, a resin with a low degree of crosslinking has a larger degree of expansion. When designing the ion exchange device, the swelling degree of the resin must be considered to adapt to the resin volume change caused by the ion conversion in the resin during production operation.
The resin particles have changes such as transfer, friction, expansion and contraction during use, and there will be a small amount of loss and breakage after long-term use, so the resin must have higher mechanical strength and wear resistance. Generally, resins with a low degree of cross-linking are more easily broken, but the durability of the resin is more mainly determined by the uniformity of the cross-linked structure and its strength. Such as macroporous resin, with a higher degree of cross-linking, the structure is stable, and can withstand repeated regeneration.