In the falling film molecular still method, vaporisation occurs from a film of liquid flowing down a heated surface under a high vacuum. The vapour (molecules) travels a short distance and strikes the condenser nearby. Each molecule is condensed individually. The distillate is subsequently collected.
Construction of Falling Film Molecular Still
The construction of a wiped film molecular still or falling film molecular still is shown in Figure. The vessel has a diameter of one metre. The walls of the vessel are provided with suitable means of heating (jacket). Wipers are provided adjacent to the vessel wall. Wipers are connected to a rotating head through a rotor. The condensers are arranged very close to the wall (evaporating surface) as shown in Figure. The vacuum pump is connected to a large diameter pipe at the centre of the vessel. Provisions are made for collecting the distillate and the undistilled liquid residue at the bottom.
Working on Falling Film Molecular Still
The vessel is heated by suitable means. A vacuum is applied at the centre of the vessel and wipers are allowed to rotate. The feed is entered through the inlet of the vessel. As the liquid flows down the walls, it is spread to form a film by PTFE (polytetrafluoroethylene)) wipers, which are moving at a rate of 3 metres per second. The velocity of the film is 1.5 metres per second. Since the surface is already heated, the liquid film evaporates directly. The vapour (molecules) travels its mean free path and strikes the condenser. The condensate is collected into a vessel. The residue (undistilled or mean free path not travelled) is collected from the bottom of the vessel and re-circulated through the feed port for further distillation. The capacity is about 1000 litres per hour.
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