Distillation Theory Part VII – Wiped Film Distillation

Up to this point in this series of posts, every description of distillation has described pot or conventional distillation.  The picture has been of a vessel or container that holds the material to be boiled.  The vessel pressure is controlled and it is heated.  Vapor rises and goes through a column and is condensed.  All around the world, many distillations exactly like this are performed.

However, distillation is not confined to this model.  Falling film distillation, forced recirculation, pressure swing columns, spinning cone distillation and wiped film distillation are just a few examples of distillation techniques that add something to this basic model of distillation.

This post will focus on wiped film distillation, because that is my area of expertise.  A wiped film still has the same goal as any distillation apparatus.  It purifies a mixture by producing distillate.

However, it does it in a little bit different way.  The body of a wiped film still is a vertical cylinder.  The feed material is introduced to the inside wall of the still body.  Inside the still is a wiper basket and wiper blades are pressed against the inside wall.  The basket is driven by a motor mounted on top of the cylinder.  The top of the cylinder is sealed so that the system can be pulled under vacuum.  The wipers spread the feed material into a thin film.

As this thin film flows down the still body, it is heated.  The body is jacketed or has electric band heaters.  This heat causes vapors to form.  The vapors are condensed on either an internal condenser or an external condenser and collected as distillate.

This is a simple explanation of wiped film distillation.  It is worth noting a couple of things.  First, this is a very efficient way to quickly heat material to a boiling point.  Only a small amount of material per second enters the still, but all of the heat from the walls of the still enters that small amount of thin film.  This technology can very quickly and efficiently produce a distillate.

Secondly, the heat history is minimal.  The material flows through the still in under a minute, and can be quickly cooled if needed.  It is an excellent technology for distilling heat sensitive materials.

Finally, this is simple distillation.  The majority of wiped film distillation setups are simple distillation machines.  The vapors are formed and are condensed.  It is possible to use a wiped film still as a reboiler (produce vapors) and then connect the vapor arm to a column.  In that case, it can be used for fractional distillation.  But most cases of wiped film distillation are simple distillation and are limited to approximately one theoretical plate.

Final post in theory of distillation.