Mixed Bed

Definition of Mixed Bed

Mixed bed is a column or tank that contains a mixture of cation resins and anion resins in one container. These resins function to exchange ions contained in water, thus producing water with a very high level of purity (usually used in industries such as power plants, pharmaceuticals, or electronics).

• Cation resins: Exchanges positive ions such as Ca²⁺, Mg²⁺, Na⁺ with H⁺ ions.

• Anion resin: Exchanges negative ions such as Cl-, SO₄²-, NO₃- with OH- ions.

• When H⁺ and OH- combine, pure water (H₂O) is formed.

Working Principle

1. Water that has gone through the cation exchanger and anion exchanger process (usually in a two-stage system) enters the mixed bed.

2. In the mixed bed column, the water contacts a mixture of cation and anion resins evenly.

3. These resins complete the deionization process, removing any residual ions that may still be present.

4. The output of the mixed bed usually has a conductivity of <0.1 µS/cm, which means it is very pure.

Mixed Bed Resin Regeneration

Once saturated, the resin can no longer exchange ions and must be regenerated:

• Cation resins are regenerated with strong acids (usually HCl or H₂SO₄).

• Anion resins are regenerated with a strong base (usually NaOH).
  However, because the resins are already mixed, regeneration is carried out after the resins are separated first (usually by specific gravity difference and water flow in a certain direction).

Mixed Bed Applications

• Steam power plants - boiler feed water.

• Semiconductor and electronics industry - ultrapure water.

• Pharmaceutical and laboratory - water for sensitive processes.

• Reverse osmosis (RO) system polishing - final stage of water quality refinement.