Evaporator Crystallizers
Evaporator Crystallizers
Evaporator crystallizers combine evaporation and crystallization in a single unit to produce high-purity solid crystals from solutions while removing solvent (usually water). They are designed to concentrate feed liquids until supersaturation is reached, triggering controlled crystal formation and growth. These systems are essential in processes requiring pure crystalline products, zero liquid discharge (ZLD), or recovery of valuable salts/metals from brines. Common types include forced circulation crystallizers, Oslo (growth-type) crystallizers, draft tube baffle (DTB), and fluidized bed crystallizers, widely used in chemicals, fertilizers, mining, wastewater treatment, and food/pharma salt production.
System Design & Key Components
Evaporator crystallizers integrate an evaporation section (heating tubes or plates) with a crystallization zone (vessel with agitator, draft tube, or fluidized bed). Key components include:
- Heating element (tube bundle, plate pack, or external heater)
- Circulation pump or draft tube for slurry movement
- Crystal growth chamber / elutriation leg
- Solids-liquid separator (cyclone, baffle, or centrifuge interface)
- Slurry discharge pump or valve
- Condenser and vacuum system (for low-temperature operation)
Evaporator Crystallization Process
The continuous or semi-continuous process includes:
- Feed Introduction — Solution enters the evaporator/crystallizer vessel, mixing with existing slurry.
- Evaporation — Heat causes solvent to evaporate, increasing solute concentration and creating supersaturation.
- Crystal Nucleation & Growth — Supersaturation triggers crystal formation; existing crystals grow as solute deposits on their surfaces.
- Slurry Circulation — Pump or draft tube circulates slurry to maintain uniform temperature, prevent scaling, and ensure even crystal growth.
- Solids Separation & Discharge — Slurry is withdrawn; crystals are separated (centrifuge, filter, or elutriation leg) while mother liquor recirculates or is further processed.
- Vapor Condensation — Vapor is condensed (often under vacuum) for solvent recovery or heat reuse; vacuum lowers boiling point and reduces energy use.
Comparisons
| Feature | Forced Circulation Crystallizer | Oslo (Growth-Type) Crystallizer |
|---|---|---|
| Circulation | Pump-forced high-velocity | Draft tube or gravity circulation |
| Crystal Size | Small to medium (high shear) | Large, uniform crystals (low shear) |
| Fouling Resistance | Excellent | Good |
| Best For | High-viscosity, scaling salts | High-purity, large-crystal products |
| Energy Use | Higher (pump) | Lower |
| Feature | Evaporator Crystallizer | Forced Circulation Evaporator |
|---|---|---|
| Primary Output | Crystalline solids + mother liquor | Concentrated liquid/slurry |
| Crystal Growth | Controlled nucleation/growth | Minimal or uncontrolled |
| Supersaturation Control | Precise (for uniform crystals) | General concentration |
| Best For | Pure crystal production, ZLD | Liquid concentration |
| Complexity | Higher (crystal management) | Simpler |
Typical Operating Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Evaporation Rate | 1–50 tons water/h | Scales with size and recirculation |
| Operating Temperature | 50–150 °C | Lower under vacuum for sensitive products |
| Operating Pressure | 0.05–2 bar | Vacuum common |
| Circulation Velocity | 2–5 m/s | High velocity prevents scaling |
| Crystal Size | 100–2000 μm | Controlled by supersaturation and residence |
| Solids Concentration | Up to 60–70% by weight | Depends on solubility and crystal type |
Common Applications & Advantages
| Industry / Application | Typical Product | Primary Goal |
|---|---|---|
| Wastewater / ZLD | Salts (NaCl, Na2SO4, etc.) | Zero liquid discharge, crystal recovery |
| Fertilizers | Ammonium sulfate, potassium nitrate | Pure crystal production |
| Chemicals | Sodium carbonate, citric acid | High-purity solids recovery |
| Mining & Minerals | Lithium, rare earth salts | Metal salt crystallization |
| Food & Pharma | Sugar, citric acid, amino acids | Crystalline product recovery |
Key Advantages of Evaporator Crystallizers
- Produces high-purity, uniform crystals in one unit
- Excellent fouling/scaling resistance
- Handles high-viscosity and crystallizing solutions
- Enables zero liquid discharge (ZLD) systems
- Continuous operation with automated solids management
- High recovery of valuable salts and metals
Also check out, "Plate Evaporator"