Molecular Distillation
with Short Path Evaporators
Molecular distillation is mostly carried out in short path evaporators. While performing the evaporation the molecules are streaming directly from the heating surface to the internal condenser and there is no flow in the opposite direction.
Essential conditions are:
- The partial pressure of the residual gas must be so low that the mean free path of the residual gas molecules is a multiple value of the distance between the heating surface and the condenser.
- At saturation pressure, the mean free path of the vapour molecules must have the same range as the the distance between the heating surface and the condenser.
Under these ideal conditions molecular distillation takes place without any obstruction from residual gas molecules. All vapour molecules reach the surface of the condenser without colliding with other molecules and thus returning to the liquid.
The operation pressure of 0.001 mbar (a) and the distance between heating surface and condenser in BPSPS short path evaporators provide ideal conditions to allow the molecular distillation.
Basic Operating Principle
Short path distillation is a thermal separation technology operating at process pressures in the range of 1 to 0,001 mbar. It lowers the boiling temperature and is an excellent method for gentle thermal treatment of heat sensitive, high boiling products. The short path evaporator consist basically of a cylindrical body with a heating jacket, a rotor and an internal condenser.
The rotor creates a mechanically agitated, thin product film on the heating surface inside of the body. By means of gravity the product flows down in a spiral path, whereby the volatile portion of the product evaporates. The vapour passes by the shortest route and with practically no pressure drop to the internal condenser. The non-volatile portion reaches the lower part of the evaporator and is discharged through the bottom product outlet. The residual vapours and inert gases flow through the vacuum nozzle to the vacuum system.
Design Features of Short Path Evaporators
SMS Short Path Evaporators are available in a unique variety of rotor designs comprising:
- PTFE or Graphite wiper elements
- Spring loaded PTFE or wiper elements
- Metal wiper elements
- Polymer or Graphite rollers
- Hygienic design
The rotors can be equipped with specially designed splash guards, protecting the internal condenser from droplets in the vapour stream and ensuring highest distillate quality.
The internal condenser can be removed from the bottom of the short path evaporator. The majority of the short path evapoarators is supplied with tubular condenser, but SMS has designed special thermoplate condensers to meet the hygienic requirements of the food and pharma industry.
Typical Applications
The short path evaporator is specifically well suited for distillation, evaporation, concentration and stripping of heat sensitive products as follows:
- Stripping isocyanates from prepolymers
- Stripping of monomers from silicone oils, resins and polymers
- Evaporation of oil and wax fractions from petroleum
- Fractionating of waxes into hard and super hard waxes
- Stripping solvents and dimers form all kind of resins
- Distillation of monoglycerides from di- and triglycerides
- Concentration of Omega-3 fatty acids
- Evaporation of methylester
- Concentration of vitamin E, (tocotrienol & tocopherol)