Mixed-Bed Deionization (Mixed-Bed IX)
The most powerful single-step ion exchange process for producing ultra-pure water (resistivity > 15–18.4 MΩ·cm)
What is Mixed-Bed Deionization?
A mixed-bed unit contains an intimate blend of strong-acid cation (SAC) and strong-base anion (SBA) exchange resins in the same vessel vessel. The two resins are physically mixed (typically 40:60 or 50:50 by volume). Water passes through thousands of alternating cation/anion micro-reactors, achieving extremely high purity in a single pass.
Chemical Reactions Inside the Bed
Cation resin (H⁺ form):
R–SO3⁻ H⁺ + Na⁺ → R–SO3⁻ Na⁺ + H⁺
Anion resin (OH⁻ form):
R–N⁺(CH3)3 OH⁻ + Cl⁻ → R–N⁺(CH3)3 Cl⁻ + OH⁻
Instantly in the same micro-zone:
H⁺ + OH⁻OH → H2O
All dissolved salts are converted to water → effluent is theoretically pure H2O.
Typical Performance (2025 resins)
| Parameter | Typical Value |
|---|---|
| Effluent resistivity | 16–18.4 MΩ·cm |
| Silica (SiO2) | < 5–20 ppb (often < 2 ppb with premium Type-I) |
| Sodium | < 5 ppb |
| TOC | 1–10 ppb (depends on feed) |
| Run length (RO permeate feed) | 10,000 – 30,000 bed volumes |
| Operating capacity | 0.45 – 0.80 eq/L of mixed resin |
Process Flow Options
| Type | Description | Use Case |
|---|---|---|
| Service mixed bed | Non-regenerable or external regeneration | Small systems, laboratories, final polishing |
| Regenerable mixed bed | Resins separated, regenerated, remixed in same vessel | Power plants, large industry |
| External regeneration (off-site) regeneration | Spent resin replaced with fresh/regenerated resin | Most common for < 50 m³/h plants |
| Polisher after two-bed | Final cleanup after separate cation + anion units | Ultra-pure water, electronics, pharma |
Regeneration Sequence (In-Place Regenerable Mixed Bed)
- Backwash – fluidize and classify resins (cation sinks, anion floats)
- Separation – air mix or hydraulic to create clean interface
- Caustic introduction – NaOH upward through anion layer only
- Acid introduction – HCl/H2SO4 downward through cation layer only
- Displacement rinses
- Air mix – remix the two regenerated resins
- Final rinse to < 1 µS/cm
Advantages vs Separate Two-Bed System
| Parameter | Two-Bed (Cation + Anion) | Mixed Bed |
|---|---|---|
| Effluent quality | 10–100 µS/cm | > 10 MΩ·cm |
| Silica removal | Poor to moderate | Excellent |
| Chemical consumption | Lower | Higher (but offset by longer runs) |
| Footprint | Larger (two vessels) | Single vessel |
| Typical use | General demin | Ultra-pure polishing |
Typical Applications (2025)
- Electronics & semiconductor rinse water
- Power plant boiler make-up & condensate polishing
- Pharmaceutical pure water (USP, WFI)
- Laboratory ultra-pure water systems
- Microelectronics, solar panel manufacturing
- Nuclear reactor water chemistry
Quick Specification Table (Modern Gel Resins)
| Component | Typical Choice |
|---|---|
| Cation resin | Nuclear-grade SAC, 8–10 % DVB, ≥ 2.0 eq/L |
| Anion resin | Type-I SBA gel or macroporous, ≥ 1.3 eq/L |
| Mixing ratio | 40:60 or 50:50 (cation:anion by volume) |
| Service flow | 20–50 BV/h |
| Endpoint | Conductivity > 0.06–0.1 µS/cm or SiO2 > 10–20 ppb |
Related Topics
Determination of Anion Exchange Resin Capacity
Determination of Cation Exchange Resin Capacity
Mixed Bed Ion Exchange Resin Capacity