Pressure Vs. Vacuum Filtration


#1

Hey friendos!

I’m certain the google will give me some answers, but figure I’d run it by the community first. Couple of quick questions. I’ve been using vacuum filtration pretty much exclusively; dcvc, celite, bentonite etc. with fairly decent results. I have noticed that pressure seems to be much more commonly used on scale processes. Just curious as to the benefits/drawbacks for pressure vs vacuum for filtration.

I saw a couple hints that pressure is better for slower moving things like bentonite, is that true?

How does pressure effect a column (obviously I know HPLC exists so it mustn’t be a huge issue) in terms of channeling? Is there anything else I need to be wary of?

I"m guessing inert gas is ideal for this scenario? Nitrogen being the most obvious choice?

Can I take this a step further and make a filter/dcvc inside a co2 setup with a solvent gradient?

Is there common equipment people are using for pressure assisted filtration as opposed to vacuum? Buchner/hochstrom equivalent?

Thanks in advance!


#2

I believe more pressure will lead to more channeling effects, but I’m not sure what the determining factors are for that.

Nitrogen would be ideal, though I don’t personally see any real reason to not just use compressed air.

I’ve seen some medium-ish scale systems that use both pressure and vacuum. The advantage I see for pressure is you can keep increasing it forever (technically) - vacuum only gets so deep.

Not sure if there’s any real advantage to having say 10 bar on the pressure side and -1 bar on the output side vs just 11 bar on the pressure side, but maybe someone else has some insight into that.

I know beaker and wrench make a pressurized filtration system, as do evolved extraction. I’m sure there are others, those are just the two I know off of the top of my head.


#3


One atmosphere is 14.696 psi. Pulling a vacuum and removing the air gives you around a 14.5 psi differental. If you think about it you can see that its all pressurized filtration, you just get a larger differental with positve pressure rather than negitive.The easy way to whip up a pressurized filter is a sanatary spool with filterplates, feed it with a diaghapragm pump or inert gas to push the liquid. You wont have channling if your filtercakes are packed correctally, just like buchner filtration.


#4

A pressurized system can theoretically be pressurized to any degree to force the mobile phase through any given column no matter how large. So very long columns or very big columns are no obstacle because the mobile phase will not be impacted by the pressure from above other than as a motive force.

A vacuum system is limited to only those vacuum levels that do not boil the organic solvents in the column. You simply cannot turn the vacuum up beyond a certain point before boiling solvents like alcohol or hexane when in a dense volume of stationary phase material. This is the limitation because in most respects I find that vacuum type set ups seem very well suited otherwise to cannabis separations and are easier to work with as well.


#5

and @beaker for the win again :wink: