Mixed Bed Water Production Capacity Calculation

In a mixed bed, regenerated H⁺-type cation exchange resin and regenerated OH⁻-type anion exchange resin are mixed together. Around each cation resin bead, there are many anion resin beads, and around each anion resin bead, there are many cation resin beads. Therefore, a mixed bed can be regarded as a multi-stage compound bed composed of many cation and anion exchange resins arranged alternately.

In industries such as semiconductors, integrated circuits, and pharmaceuticals, mixed beds are often used alone for chemical demineralization of water. Since the loading amounts of both cation and anion resins are less than when using separate beds, and the anion resin also needs to remove residual CO₂ and soluble SiO₂ from the water, the operating cycle time is greatly reduced, while regeneration time increases exponentially. In power plants with high-parameter boilers, due to the large volume of boiler make-up water, using a mixed bed alone would result in too frequent regeneration. At the same time, because the water quality requirements are very high, mixed beds are usually used in series after a primary compound bed demineralization system. Only when treating condensate water is a separate mixed bed used.

When water quality requirements are very high, the resins used in the mixed bed must be strong acid cation resin and strong base anion resin. When treating condensate water, cation and anion exchange resins with high strength and a relatively uniform particle size range must be used. Generally speaking, after mixed bed regeneration, the RH-type content per unit volume of cation resin is about 2 to 2.5 times the ROH-type content of anion resin. Therefore, if a mixed bed is used alone for demineralization, the ratio of cation resin to anion resin is usually 1:2.

Normal primary demineralized water (≤10 μS/cm) is used as the feed water for the mixed bed. The effluent conductivity of the mixed bed is controlled to ≤0.2 μS/cm. The empirical value of the periodic water production capacity is 8,000 to 12,000 times. Residual CO₂, soluble SiO₂, and anions in the feed water all need to be removed by anion resin exchange. At the same time, the exchange capacity per unit volume of anion resin in the mixed bed is lower than that of cation resin. The periodic water production capacity of the mixed bed is calculated based on the anion resin. The calculation formula is as follows:

Calculation Formula for Cation/Anion Resin Water Production Capacity

• Resin Volume (m³): Refers to the volume of anion resin in the mixed bed
• Resin Working Exchange Capacity: The working exchange capacity of mixed bed anion resin is generally 400 mol/m³-R
• Total Molar Concentration of Anions in Feed Water (Cl⁻, SO₄²⁻, residual CO₂, soluble SiO₂): Calculated based on the customer's water quality report

Example: Conductivity of 3.2 μS/cm is equivalent to a salt content of 3.2/(molecular weight of salt, generally taken as 100, which is an empirical value) = 0.032 mmol/L. Since the effluent conductivity only needs to be controlled below 0.4 μS/cm, the actual working exchange capacity should be 550 mmol/L. Then the water production capacity per unit volume of resin is:

Now with a 200 mm diameter and 800 mm height, the resin volume is 10.5 L. The ratio of cation to anion resin is 1:2. The estimated water production capacity of the mixed bed is:

The operating flow velocity is 140 m/h, so the flow rate is:

The actual operating time is:

That is approximately 41 hours.