Basics of Vacuum Distillation

With Thin Film Distillation, a substantial decrease of boiling temperature is obtained by reducing the operating pressure. This allows thermal separation of products that would be destroyed by conventional vacuum distillation (pot still or distillation column) because of the necessary high temperatures and long residence time.

In a Thin Film Evaporator the raw material is heated up on the internal surface of a heated tube until the lower boiling component starts to evaporate. These vapors are then liquefied on the cold tubes of a condenser. In simple thin film evaporator design the condenser is located outside but as close to the evaporator as possible. 

In a Short Path Evaporator the condenser is located inside the evaporator body. Due to the different design details, short path evaporators can be used down to 0.001 mbar as compared with conventional thin film evaporators which can be used down to approximately 1 mbar.  Residence time in Thin Film and Short Path Evaporators is only few seconds, which helps to avoid thermal decomposition of the product.

Short-Path Distillation is a thermal separation process for thermal sensitive products. Short residence time and low evaporation temperature will cause a minimum thermal stress to the distilled product. Typical applications are high molecular organic compounds particularly from the fields of chemistry, pharmaceutical and food industry. Distillation is one of the most important thermal separation methods. Short-Path Distillation is a continuous separation process working under vacuum conditions. Evaporation takes place from a heated wiped film. Caused by the pressure drop between the place of vaporization and the vacuum system the operating pressure in typical wiped film evaporators with external condenser is limited to some millibars. The considerably lower pressure in the Short Path Evaporator is obtained by the short distance for the vapors on their way from the evaporator surface to the condenser. In addition, the cross section area of flow is equal to the evaporator surface, so that there is only a minor pressure drop between evaporator and condenser - a detail which is absolutely necessary for this process!

Typical pressure range is the "fine vacuum range", e.g. between 1 and 0.001 mbar. At this pressure the required evaporation temperature will decrease so far that during the short residence time no thermal decomposition of the product can occur.

Different from the traditional Thin Film Evaporator design the condenser is located inside the Short Path Evaporator body. There is no vapor line between evaporator and condenser. The large volume of the vapors at very low pressure would lead to high vapor speed even with large diameters. Because of the large pressure drop the design with external condenser cannot be used below 1 mbar operating pressure.

A comparison with conventional pot still distillation demonstrates the special features of the process.  The raw product is heated in the evaporator vessel until evaporation temperature, vapors of the low boiling component are transferred to the cooler where they will be condensed. Distillate from the condenser will be collected in a vessel. Boiling temperature can be reduced according to the vapor pressure curve by connecting a vacuum pump system.

In a still pot distillation there are some disadvantages for heat sensitive products:

  • long residence time caused by the batch process
  • poor vacuum at the place of evaporation, because drops in pressure and static height of the liquid have to be added to the suction pressure of the vacuum pump set

The ultimate pressure is not determined by the size of the vacuum pump set, but is limited by the conductivity of the piping and the static height of the liquid in the evaporator vessel.

Evaporation from a thin film will eliminate the influence of the static height of liquid. In a Wiped-Film Evaporator (WFE) the product runs down on a heated cylindrical surface on a cylindrical pipe. The liquid film is mixed continuously by a rotating wiper system. Condensation takes place in a condenser located outside, which is connected to a vacuum system. The obtainable vacuum is limited to some mbar due to the pressure drop of the connection between evaporator and condenser.

Using Short-Path Distillation will eliminate this disadvantage. The condenser is located inside a cylindrical evaporator. The distance for the vapors between evaporator and condenser is extremely short. In fact, there is no pressure drop. If the distance between evaporator and condenser is in the order of the average free path length of the molecules, this process is also called "molecular distillation".

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