Removal of residual solvent under vacuum

[Adyia]

I have a question regarding removal of a residual solvent from a powder. I want to place an open container of my chemical inside a vacuum chamber with another open container of activated charcoal to absorb the solvent, then maintain a vacuum for some time.

I know you can do this setup using two separate flasks, one with my chemical, the other with charcoal, connected with tubing to each other and a vacuum pump. Will using the vacuum chamber be just as effective?

Thank you.

[scooke]

Dear Adyia,

No, doing it all in a single vessel will not be as effective.  The reason is that the separation is based on relative adsorption.  Even vacuum evaporation will not actually remove 100% of a solvent in a system.  However, the 2-stage process removes as much of the solvent from the system as possible mechanically.  The secondary carbon bed is really to avoid releasing the solvent to the atmosphere, but it does not affect the desorption of the solvent from the powder.

The problem with a "single-pot" desorption is that you are changing the equilibrium to one of simply the vapor pressure of the solvent to the powder to one of the adsorption of the solvent on the carbon.  Basically, IF you need to trap the solvent, the carbon trap MUST be inline with the produced vacuum, or the solvent will simply by-pass the carbon and exit through the vacuum exhaust.  If you don't  need to retain or sequester the desorbed solvent, then no carbon trap is necessary at all - simply use the vacuum.

[Adyia]

Thank you, your reply makes perfect sense to me, as long as the vacuum pump is on and running.

Will the same dynamics apply if I seal the setup and leave it under vacuum pressure for an extended length of time? (the vacuum pump no longer part if the equation)

[scooke]

Dear Adyia,

No.  Whatever the system pressure, keeping all of the components

(powder, solvent, carbon) in the same vessel will just establish an

equilibrium point that results in LESS removal of the solvent from the

powder than with an active removal of vapor by the vacuum system.

yBest regards,

Steven J. Cooke, MChE, FAIC, CQA

President

Process Systems Consulting

drwhocooke@yahoo.com

+63 918 268 3399

[Beight]

Adiyah,

A common way to remove solvents from solids is to pass a dry gas, such as dry nitrogen, over the the sample using a capillary or other small aperature, while under vacuum.  This affects the equilibrium of the solvent by actively 'chasing' it from the solvent space above the solid sample and vastly reducing the amount of time needed to bring the sample to the desired level of dryness.  Many vacuum ovens are equipped with bleed inlets to release vacuum when they need to be opened that can be equipped with a gas line via a rubber hose that allow you to do this.  It's as simple as turning on the vacuum and slightly opening the bleed valve.  For many solvents, just using air as your bleed gas is sufficient to dry your sample.