A rising film evaporator introduces feed at the base of long vertical tubes — partial vaporisation within the tube generates vapour bubbles that accelerate the remaining liquid upward through thermosiphon action. Processing 200–50,000 kg/h at temperature differences of 10–30°C, it is preferred for low-viscosity, non-fouling feeds requiring high throughput with simple mechanical design and no moving parts.
Feed liquid enters the tube base below the steam-heated shell — sensible heat raises liquid to boiling. Nucleate boiling generates vapour bubbles that expand rapidly, accelerating liquid upward through slug and annular flow regimes. Tube exit velocity of 15–30 m/s drives vapour-liquid separation in the top vapour head. The thermosiphon requires no pump — natural density difference drives recirculation continuously without mechanical assistance.
Challenges
Superheat Required at Tube Entry (No Boiling in Lower Tube)
Lower tube acts as preheater — requires additional heat load and longer tube length vs. falling film.
Limited Concentration Ratio per Pass (<3:1 Typical)
Rising film becomes unstable as viscosity or solids increase — requires multiple effects.
Sensitivity to Feed Composition Variation
Changes in boiling point elevation alter two-phase flow regime — unstable operation with variable feeds.
Higher ΔT Required vs. Falling Film (Minimum 10–12°C)
Onset of rising film action requires minimum ΔT — inefficient at low steam pressure.
Tube-Side Fouling from Suspended Solids at Tube Base
Solids accumulate at tube base before thermosiphon action begins — progressive blockage.
Solutions
- Pre-Evaporation Flash Vessel at Tube Inlet: Flash vessel pre-vaporises 5–10% of feed — initiates two-phase flow immediately at tube entry.
- Recirculation Ratio Control (4:1 to 8:1 per Pass): Controlled recirculation dilutes product below viscosity and solids limits for stable film.
- Continuous Feed Monitoring and Flow Trim (PID Feed Controller): Feed composition monitoring adjusts tube-side flow rate to maintain stable two-phase regime.
- Mechanical Vapour Recompression (MVR) to Maintain Minimum ΔT: MVR compressor boosts vapour pressure — maintains minimum ΔT at low steam conditions.
- Coarse Screen (100 µm) at Feed Inlet + Periodic Back-Flush: Inlet screen removes solids above minimum settling diameter — prevents base blockage.
Applications
- Chemical: Caustic soda, ammonium nitrate, urea solution evaporation — high-throughput simple feeds.
- Food & Beverage: Sugar liquor, thin juice concentration, corn steep liquor evaporation.
- Pharmaceutical: Low-viscosity fermentation broth, dilute API solution concentration.
- Pulp & Paper: Black liquor thin evaporation before multiple-effect downstream stages.
- Fertiliser: Ammonium sulphate and nitrate solution concentration before crystallisation.
Models & Capacities
Model | Scale | Evaporation Capacity [kg/h] | Heating Area [m²] | Tube Length [mm] | Tube Count | Tube Ø [mm] | Steam Temp [°C] | Min ΔT [°C] | Recirculation Ratio | Installed Power [kW] | Overall H [mm] |
|---|---|---|---|---|---|---|---|---|---|---|---|
RFE-LAB-01 | Lab | 10–100 | 0.1 | 2,000 | 3 | Ø25 | 80–130 | 10 | 4:1–8:1 | 0.37 | 4,500 |
RFE-LAB-05 | Lab | 50–500 | 0.5 | 3,000 | 6 | Ø25 | 80–140 | 10 | 4:1–8:1 | 0.75 | 6,500 |
RFE-PIL-2 | Pilot | 200–2,000 | 2 | 4,000 | 9 | Ø38 | 80–140 | 10 | 4:1–6:1 | 2.2 | 8,500 |
RFE-PIL-5 | Pilot | 500–5,000 | 5 | 5,000 | 18 | Ø38 | 80–150 | 10 | 4:1–6:1 | 4.0 | 10,500 |
RFE-P-010 | Production | 1,000–10,000 | 10 | 5,000 | 28 | Ø50 | 80–150 | 12 | 4:1–6:1 | 7.5 | 11,500 |
RFE-P-025 | Production | 2,500–25,000 | 25 | 6,000 | 50 | Ø50 | 80–160 | 12 | 3:1–5:1 | 11 | 13,500 |
RFE-P-050 | Production | 5,000–50,000 | 50 | 7,000 | 75 | Ø50 | 80–170 | 12 | 3:1–5:1 | 22 | 16,000 |
RFE-LP-100 | Large Prod | 10,000–100,000 | 100 | 7,000 | 128 | Ø63 | 80–170 | 15 | 3:1–4:1 | 37 | 18,500 |