Wiped Film Evaporator & Short Path Distillation Calculator
Perform engineering sizing and process modeling for industrial Wiped Film Evaporators (WFE / ATFE) and short-path molecular stills. Calculate rotor tip speed, dynamic thin-film thickness, residence time, heat transfer duty, evaporation flux, and check minimum wetting limits.
1. Process Feed & Evaporator Geometry
2. Thin-Film, Thermal & Kinetic Sizing
Engineering Principles & Agitated Thin-Film Derivations
Wiped Film Evaporators (WFE) and Short-Path Molecular Stills (SPE) are the definitive separation tools for thermolabile (heat-sensitive) active pharmaceutical ingredients, high-potency cannabis extracts, monoglycerides, and polymers that degrade under boiling times longer than one minute.
1. Wiper Rotor Kinematics & Bow Wave Hydrodynamics
Rotor peripheral tip velocity (v_{tip}) is determined from cylinder inside diameter (D) and rotational speed (N):
Operating tip speed between 7.0 and 11.0 m/s generates intense hydrodynamic bow waves ahead of each wiper blade, renewing the wall surface 20 to 50 times per second.
2. Dynamic Thin Film Thickness & Mean Residence Time
Under gravity drainage and blade wiping, the mean agitated film thickness (delta) is correlated via the modified Nusselt/Kern-Karpath formulation:
Where (Gamma = rac{dot{M}_{feed}}{pi D}) is the peripheral wetting rate. Total liquid hold-up inside the active heated cylinder is (V_{film} = pi D L delta). The mean thermal residence time ( au) is:
3. Heat Transfer Coefficient & Evaporative Flux
Because the agitated film is ultra-thin and turbulent, the convective film coefficient is extraordinarily high, yielding overall heat transfer coefficients (U approx 1,000 ext{ to }2,200, ext{W/m}^2cdot ext{K}):
4. Minimum Wetting Rate ((Gamma_{min})) Safety Boundary
To guarantee complete surface coverage without dry burn-on, the residue mass flow per perimeter at the bottom cone exit must satisfy:
5 Fatal Engineering Traps & Industrial Operating Hazards
1. Dry Spot Film Rupture & Wall Burn-on Crust
Pushing evaporation cut past 90% starves the bottom perimeter of liquid below the minimum wetting rate ((Gamma < 90, ext{kg/m}cdot ext{h})). The liquid film tears into rivulets, exposing bare heated 316L stainless steel. Thermally sensitive organic oil bakes onto the dry wall, producing a charred, insoluble black crust that degrades heat transfer by 70% and ruins batch color purity.
2. Entrainment Carryover to Internal Molecular Condenser
Operating with excessive rotor speed (>12 m/s) or flashing high-volatility solvent creates violent droplet splattering. Liquid droplets are thrown across the 35 mm internal vacuum gap directly onto the pure distillate condenser surface. This mechanical splash bypasses thermal vaporization entirely, instantly contaminating pharmaceutical-grade distillate with crude feed residue.
3. PTFE / Carbon Wiper Blade Thermal Binding & Seizure
Virgin PTFE has a thermal expansion coefficient nearly 10 times higher than 316L stainless steel. If replacement wiper blades are machined with cold clearances under 1.5 mm, heating thermal oil to 220°C expands the blades until they lock tightly against the heated cylinder. The drive motor stalls or snaps the mechanical shear pin on the rotor shaft within minutes.
4. Cold Condenser Crystallization Freeze-up
In high-purity molecular distillation of high-melting-point compounds (e.g. cannabinoids, fatty acids, stearates), chilling the internal condenser coolant below the distillate solidification point freezes crystalline solid directly on the condenser surface. The solid wax layer bridges across the vacuum gap to the rotating blades, destroying wiper assemblies.
5. Viscous Hydrodynamic Drag Motor Overload
Concentrating high molecular weight polymers or resins causes liquid viscosity to skyrocket from 200 cP up to 15,000 cP at the residue outlet. Agitating a thick viscous layer demands huge rotor torque ((P_{motor} propto mu v_{tip}^2)). Sizing with standard fractional horsepower gearmotors causes thermal overload tripouts under steady-state distillation.