Rising Falling Film Evaporator
Rising Film Evaporators
Rising film evaporators — also known as long-tube vertical evaporators — are efficient thermal separators that concentrate solutions by boiling the liquid inside vertical tubes. The feed enters at the bottom, vapor generated during boiling lifts the liquid upward as a rising film along the tube walls. This vapor-lift action creates high turbulence and excellent heat transfer. Rising film evaporators are best suited for clean, low-to-moderate viscosity liquids with low fouling tendency. They are commonly used in food processing, dairy, chemicals, pharmaceuticals, and wastewater concentration where moderate residence time and good economy are needed.
System Design & Key Components
Rising film evaporators feature a vertical shell-and-tube heat exchanger with long tubes (typically 4–10 m long, 25–50 mm diameter). Key components include:
- Bottom feed inlet and liquid distribution zone
- Vertical tube bundle (rising film inside tubes)
- Shell-side heating jacket with steam/hot water circulation
- Top vapor-liquid separator (cyclone or baffle type)
- Concentrate discharge from separator bottom
- Condenser and vacuum system (for low-temperature operation)
Signature feature: Vapor lift creates natural circulation and high turbulence inside the tubes, delivering good heat transfer (1,500–3,000 W/m²·K) without mechanical pumping.
Rising Film Evaporation Process
The continuous process occurs as follows:
- Feed Entry — Pre-heated liquid feed enters the bottom of the tubes and begins to heat.
- Boiling Initiation — As the liquid absorbs heat from the tube walls, boiling starts; vapor bubbles form and rise.
- Rising Film Formation — Vapor bubbles lift the liquid upward, forming a rising annular film along the tube walls with high turbulence.
- Evaporation & Two-Phase Flow — Rapid evaporation continues as the film rises; vapor volume increases, accelerating flow and enhancing heat transfer.
- Separation — At the top of the tubes, the vapor-liquid mixture enters a separator where vapor is disengaged and routed to the condenser.
- Concentrate Discharge — Concentrated liquid collects in the separator and is discharged; vapor is condensed for solvent recovery or heat reuse.
Comparisons
Rising Film vs. Falling Film Evaporator
Rising Film vs. Forced Circulation Evaporator
| Feature | Rising Film Evaporator | Falling Film Evaporator |
|---|---|---|
| Liquid Flow Direction | Upward (vapor lift) | Downward (gravity-driven thin film) |
| Residence Time | Moderate (minutes) | Very short (seconds to minutes) |
| Heat Transfer Coefficient | Good (vapor-induced turbulence) | Very high (thin, gravity film) |
| Fouling Tendency | Moderate to high (longer tube residence) | Low (self-cleaning film flow) |
| Best For | Clean, low-viscosity, moderate-volume liquids | Heat-sensitive, low-to-medium viscosity products |
| Energy Efficiency | Moderate | High |
| Feature | Rising Film Evaporator | Forced Circulation Evaporator |
|---|---|---|
| Circulation Method | Vapor lift (no pump) | Pump-forced high-velocity recirculation |
| Fouling Resistance | Moderate | Excellent (high velocity prevents scaling) |
| Viscosity Limit | Low to moderate | High (up to 50,000 cP+) |
| Energy Use | Lower (no pump) | Higher (pump energy) |
| Best For | Clean, low-viscosity liquids | High-viscosity, scaling, crystallizing solutions |
| Residence Time | Moderate | Longer (recirculation) |
Typical Operating Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Evaporation Rate | 1–50 tons water/h | Scales with tube bundle size |
| Operating Temperature | 60–130 °C | Higher than falling film due to longer residence |
| Operating Pressure | 0.1–2 bar (vacuum to atmospheric) | Vacuum used for heat-sensitive feeds |
| Residence Time | Minutes | Longer than falling film |
| Heat Transfer Coefficient | 1,500–3,000 W/m²·K | Good, but lower than falling film |
| Viscosity Limit | Low to moderate (up to ~2,000 cP) | Less tolerant than forced circulation |
Common Applications & Advantages
| Industry / Application | Typical Process | Primary Goal |
|---|---|---|
| Food & Beverage | Fruit juice, sugar syrup, coffee extract | Concentration with moderate heat exposure |
| Chemicals | Caustic solutions, organic acids | Concentration of clean liquids |
| Pharmaceuticals | API solutions, herbal extracts | Concentration without excessive degradation |
| Wastewater | Brine concentration | Volume reduction |
| Desalination | Pre-concentration before further processing | Water recovery |
Key Advantages of Rising Film Evaporators
- No recirculation pump needed — vapor lift provides natural circulation
- Good heat transfer and reasonable evaporation rates
- Simple design with lower capital cost than forced circulation
- Effective for clean, low-viscosity liquids
- Can operate under vacuum for lower-temperature processing
- Proven technology in food and chemical concentration
Also check out, "Evaporator Crystallizers"
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