Don’t Forget the Wipers!

When dialing in a wiped film still, users focus on the big three levers of Feed Rate, Vacuum and Temperature. 

This is the correct approach as these three controls have the majority of the power when controlling a wiped film distillation.

However, it is critical to remember the wipers as well. 

One time, we were starting up a distillation that was supposed to have about 20% distillate.  We went through the whole startup process and had the temperature, feed rate and vacuum at the same conditions as the day before.  But there was very little distillate, it was a tiny stream. 

Panic was starting to set in, setpoints were about to be changed dramatically and wild thoughts were being thrown about as to how the feed went from being 20% volatile to less than 9% volatile overnight.

And then someone noticed that the wipers were running at less than 50 RPM.

In the plant, during the startup process, the wipers were turned on slowly so that they aren’t sitting on the hot dry walls while the first part of the heating up is done.  Generally, the systems were warmed to about 150°C prior to starting the feed pump.  That way, the amount of startup material needed to dial in the distillation was minimized. 

Once the feed pump is started, the wipers were turned up to 300 – 350 RPM.  But this time they were forgotten.

So while the wipers are not considered to be a “big lever”, they are critical to the operation of the unit.  With the feed material not being wiped into a thin film, the still was not efficient at creating distillate. 

The power of the wiped film still comes from the wiped film.  No wipers, no distillation!