Agitated Thin Film Evaporator (ATFE) Calculator
Wiped-Film Rotor Hydrodynamics, Residence Time Distribution & Heat Flux Sizing
1 Feed Rate & Thermodynamic State
2 Temperatures & Heating Jacket
3 Rotor & Barrel Geometry
Interactive Rotor Bow-Wave & Thin Film Annulus Visualizer
ATFE Thermal & Hydrodynamic Diagnostic Report
First-Principles Wiped-Film Mechanics: ATFE Hydrodynamics
The Agitated Thin Film Evaporator (ATFE), also termed wiped-film or scraped-surface evaporator, is designed to distill, concentrate, and devolatilize highly viscous, sticky, or heat-sensitive materials (such as pharmaceutical active ingredients, vitamins, polymers, and hazardous wastes) that would otherwise burn, decompose, or foul in standard shell-and-tube or plate evaporators.
1. Thermal Evaporation Duty ((Q_{tot}))
The required heat transfer rate combines sensible preheating to boiling temperature and latent solvent vaporization:
(Q_{sens} = rac{dot{m}_{feed}}{3600} cdot c_p cdot (T_{boil} - T_{in})) (kW)
(Q_{evap} = rac{dot{m}_{evap}}{3600} cdot Delta h_{vap}) (kW)
(Q_{tot} = Q_{sens} + Q_{evap}) (kW)
2. Wiped-Film Heat Transfer Coefficient ((h_{film}))
Unlike falling film evaporators where film flow is laminar and gravity-governed, the rotating blades in an ATFE generate a turbulent bow wave directly in front of each blade tip. McKelvey and Sharples developed the core empirical correlation for agitated thin-film thermal coefficients:
Where (Gamma = rac{dot{m}_{feed}}{pi D_i}) is the peripheral mass loading rate (kg/m-s). The overall heat transfer coefficient (U) follows standard resistance summation across jacket condensation, stainless steel shell wall conduction, and process thin film:
3. Ultra-Short Mean Residence Time (( au))
The mean residence time represents the time fluid particles spend exposed to heated barrel wall before discharging as concentrated bottoms:
Because ( au) is under 30 seconds (compared to 1 to 2 hours in batch kettles), heat-sensitive organic compounds undergo virtually zero thermal degradation or caramelization.
5 Fatal Engineering Traps in Thin Film Evaporator Design
1. Film Breakup & Dry Spot Burnout (Critical Heat Flux)
If feed rate drops or evaporation percentage exceeds the wetting threshold ((Gamma < 0.05 ext{ kg/m-s})), surface tension gradients (Marangoni effect) rupture the liquid film into rivulets. Exposed dry patches on the heated wall reach jacket temperature rapidly, causing instantaneous thermal degradation, black carbonaceous polymer baking, and complete loss of heat transfer. Minimum wetting rate interlocks are mandatory.
2. Thermal Expansion Galling & Rotor Blade Clash
Close-clearance rigid rotor blades operate with 0.5 to 1.5 mm clearance from the honed inner cylinder. When hot steam or thermal oil (160°C - 220°C) is admitted to the jacket during commissioning before rotor thermal equilibrium is reached, differential thermal expansion between the heated outer barrel and cooler internal rotor causes metal-to-metal contact, catastrophic rotational seizing, and shaft shearing.
3. High-Viscosity Motor Stalling at Bottom Discharge
As solvent boils off along the length of the vertical cylinder, concentrate viscosity can skyrocket exponentially (from 50 cP at inlet to over 25,000 cP at outlet). Rotor viscous torque scales directly with dynamic viscosity ((T propto mu rac{v_{tip}}{delta})). Undersizing the drive gearbox causes thermal overload tripping, locking the rotor and allowing the sticky residue to solidify rock-hard inside the unit.
4. Vapor Entrainment & Distillate Contamination
Under deep vacuum, vapor volumetric flow expands dramatically. If internal vapor upward superficial velocity exceeds 5 m/s, violent liquid boiling droplets from the bow wave are swept directly into the overhead vapor nozzle, contaminating the purified solvent distillate with heavy non-volatile concentrate. Internal rotor-mounted centrifugal entrainment separators (demister blades) are essential.
5. Mechanical Seal Thermal Runaway Under Deep Vacuum
The top rotor driveshaft penetration requires a double-acting mechanical seal with pressurized barrier fluid (Plan 53A or 54) to isolate deep vacuum (< 1 mbar abs) from the atmosphere. If barrier fluid circulation fails or cooling is lost, high frictional heat at 400 RPM causes the silicon carbide seal faces to warp, breaching vacuum, pulling atmospheric air into hot solvent vapors, and creating a flammable vapor cloud hazard.
Frequently Asked Questions (FAQ)
How does an agitated thin film evaporator differ from a falling film evaporator?
A falling film evaporator relies strictly on gravity to pull fluid down vertical tubes and cannot handle fluids with viscosities above 100 to 200 cP or fouling sludges. An ATFE incorporates high-speed rotating wiper blades that mechanically force the liquid into a continuous, turbulent 0.5 - 2 mm film against the jacketed wall, successfully handling viscosities up to 50,000 cP and crystallizing slurries with zero scaling.
What is the difference between rigid fixed-clearance and hinged wiper blades?
Rigid blades are welded or machined directly to the rotor with a fixed 0.5 to 1.5 mm radial gap from the wall, making them ideal for clean polymers, devolatilization, and high-purity distills. Hinged scraper blades swing outward under centrifugal force, physically contacting the inner wall with carbon, PTFE, or PEEK shoes, which continuously scrapes away baking scale in fouling or crystallizing slurries.
What typical overall heat transfer coefficients (U-values) are achieved in an ATFE?
For aqueous solutions and light organic solvents (viscosity < 5 cP), ATFE overall U-values reach 1,200 to 2,000 W/m²-K (210 - 350 Btu/hr-ft²-°F). For moderately viscous fluids (100 - 1,000 cP), U-values range from 600 to 1,000 W/m²-K. For extreme polymer devolatilization (> 10,000 cP), U drops to 200 to 450 W/m²-K due to high boundary layer thermal conduction resistance.
Why are ATFEs operated under deep vacuum?
Operating under vacuum (typically 0.5 mbar to 50 mbar absolute) substantially depresses solvent boiling temperatures. For example, water boils at 45°C under 95 mbar, and high-boiling organics like botanicals, essential oils, and pharmaceutical vitamins boil 100°C to 150°C below atmospheric boiling points, preventing thermal degradation and isomerization.