Degassing Deep Dive, Part 4
Degassing Deep Dive: Part 4
How do I know when I need to degas?
A lot of times, it isn't clear that a degassing pass is necessary. The operator might think that the feed material contains no lights, or they may suspect that there are lights, but they don't want to waste a pass on degassing if they don't have to. There are many ways to determine if a degas pass is necessary. Several are listed here.
1. Spitting in the inlet.
If your unit is glass or if it has inlet site glasses, observing the stream of feed material can easily tell you if a degas pass is necessary. When attempting to pull vacuum levels of your product pass, if the feed is spitting, foaming or aggressively bubbling, a degas pass is needed. This behavior is caused by the super-volatile components vaporizing the instant they feel the vacuum of the still.
2. Obvious feed material in the distillate
Sometimes a colorless distillate is expected from a dark feed. But the observed distillate is colored - indicating that spitting is occurring from the walls of the still onto the internal condenser. This is caused by the super-volatile components violently vaporizing off of the wall of the still and slinging droplets of feed onto the condenser. This is an indication that a degas pass is needed prior to distilling the final product.
3. Rising pressure in the pressure gauge.
Sometimes operators of wiped film stills will state that there isn't a way to know if a feed needs degassing if you are running in stainless steel. However, if your manufacturer correctly configured your still, there is a VERY obvious tell.
The pressure of the still should be monitored at a point BETWEEN the still and the cold trap. Some manufacturers place the gauge between the cold trap and the vacuum pump. All this does is let you know the pressure applied to the still body, but not the pressure that is actually present IN the still body. When the super-volatiles form, the pressure of the still rises (see the ideal gas law). But, the cold trap removes them when they try to travel to the vacuum pump, so a pressure gauge after the cold trap won't see the pressure rise.
A pressure gauge correctly located will see the pressure rise. For example, sometimes I have pulled a system down to 0.02 Torr. As soon as feed enters the still, the pressure will rise to 1-5 Torr, because of the super-volatile that has not been removed. This would not be observed with the wrong placement. What would be observed is a higher temperature needed to drive the distillation, or a failure to distill altogether. The vacuum level is 100 times higher than desired and it does affect the boiling points dramatically.
These three indicators can tell you if a degas pass is necessary before continuing with your distillation.