Thin film evaporation differentiates between plants for thin-film, short-path or molecular distillation. All systems are recommended for separation of volatile from less volatile components, for organic products, which are characterised by thermic instability, high molecular weight or high viscosity. The most important characteristics are pressures of 1 to 0,001 mbar as well as short retention times of the products in the heated evaporator zone to avoid overheating or damage of the product. The plants are made of glass or of glass-metal-combination according to the requirements of the products.
Thin-Film (Wiped Film) Distillation
Evaporator and condenser are separated components which are connected by nozzles. The vacuum pump is directly connected to the condenser. Using this type of equipment the ultimate pressure is limited to some mbars. The medium retention time of the liquid within the thin-film evaporator is depending on viscosity and throughput.
Short Path Distillation
This is a thin-film evaporation under reduced pressure down to 0,001 mbar. In this type of distillation system the condenser is integrated in the centre of the evaporator. The distance between evaporation surface and condenser is extremely short and pressure losses are eliminated.
Film evaporation at reduced pressures of 0,001 mbar is also called “molecular distillation”.
Design and Performance of the Different Evaporation Systems
Thin-film evaporators are the most part made of glass. Larger systems for pilot plant operation are available in stainless steel.
Fig.2: Rotor Inserts
a) Wiper elements made of glass resp. of glass-fibre reinforced PTFE.
b) Wiper elements made of PTFE or graphite with spiral gearing
c) Rolling wiper elements made of PTFE
The liquid film on the evaporator surface is generated by means of rotors according to fig. 2. The thickness of the liquid film is between approx. 0,1-0,5 mm and depends on viscosity, rotary speed as well as the type of rotor system itself.
The wiper elements according to fig. 2a consists of glass panes running on bearings, which
allow a free motion of the panes. Due to the centrifugal force the wings will be pressed to the outside, will scrape along the wall and will ensure a thin liquid film. These wiper elements are only installed in model SP 200.
The rotor according to fig. 2b consists of stainless steel with wiper elements made of PTFE or electrographite. Due to the spiral gearing the product will be conveyed through the evaporator. Fig. 2c shows a universal rotor with rolling wiper elements made of PTFE. The application of this rotor ranges from laboratory apparatus to pilot plants.
The extremely gentle heating of the evaporator is achieved by a heated liquid (oil) from an external circulation thermostat.
Of late years, the demand for test and pilot plants for gentle distillation has considerably increased in the chemical, pharmaceutical and petrochemical industry. The constantly rising number of new complicated products and the rising world market price for raw materials are responsible for this demand.
For delicate distillation of thermally sensitive products, we have developed efficient plants laboratory and test operation which are preferably used for the following applications: