Short Path Distillation

Positioning the condensor within the evaporator allows a further reduction of operational pressure down to ~ 0.001 mbar. This is due to the fact that a pressure gradient from evaporator to condenser essentially can be avoided. Vaporized particles have a realistic chance to directly arrive at the condenser without colliding with other particles. The geometrical distance between the evaporator and the condenser is in the range of few centimeters which is comparable with the mean free pathlengths in this pressure range.

The essential constructive elements of a shortpath distillation plant are depicted in the adjacent picture. Their interaction and the distillation process are illustrated in the adjacent video.

Typical feed rates for shortpath distillators are ~ 100 kg/m²/h. This may, however, vary depending on the target cut ratio of distillate and residue, the viscosity of the feed material, the film thickness or the geometry of the evaporator (diameter to height ratio). Reliable estimates can only be provided based on piloting trials with authentic feed material.

Operation Pressure

Another physical law to bear in mind is the fact that a continued further reduction of the applied vacuum beyond specific values will not result in a improved process performance. While lower pressure ranges indeed correspond to lower boiling points, the efficiency of the transport mechanisms associated with distillation will suffer.

At a given operating temperature and evaporator surface there exists for each substance with a given molar mass and vapor pressure a maximum evaporation rate (Langmuir-Knudsen). This translates into an ideal operational pressure for each substance.

For practical purposes the dependencies can be captured in a diagram that yields the ideal operation pressure for a given shortpath distillation scenario. A deviation from this ideal pressure may significantly reduce process efficiency. The different curves represent the target percentage of the distillate.

If, e.g., in a KD 600 shortpath distillation unit with 6 m² evaporation surface and a feed rate of 900 kg/h 50 % of the feed should be distilled off, the operation pressure should be ~ 0.045 mbar.

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