From an older post by Kilomentor, a discussion of emulsions:
I don't know what it is about a chemist, but there's a real temptation to try things on full scale first. No matter how big the emulsion is (500 mL, 2 L, 40 L), there's always a temptation to experiment on the actual situation instead of pulling a little off and proving it on a small scale first. (And it's not like I've not been there myself...)
If one examines closely a portion of the emulsion, one can sometimes get a useful clue to the action that will work. Sometimes the more vigorous stirring in the plant setting has suspended small gas bubbles in the droplets of one phase causing them to float rather than settle. These gas droplets can also be associated with some sediment that is suspended therein as well. Application of vacuum to a gently stirred mixture of the emulsion and separated phases can cause these bubbles to break followed by a separation of the phases. This attempt is particularly easy to try in the lab on a 500 ml sample of emulsion from the plant. Just place the filled flask on the rotovap; rotate gently and apply a water aspirator vacuum. Gentle warming is also easy to try out in this configuration. A note of caution should be registered here. You may see a clearing of the emulsion and there is a temptation to take the clarified two-phase mixture and for added safety filter it under vacuum through a pad of Celite. This filtration can undo all the good you have done. Sucking the last of the solvent through the Celite can put gas right back into the phases!The technique of putting a slight bit of vacuum and/gentle stirring and warming on an emulsion can be really helpful. But here's something that Kilomentor mentions that's really key for the beginner -- trying it on a sample.
I don't know what it is about a chemist, but there's a real temptation to try things on full scale first. No matter how big the emulsion is (500 mL, 2 L, 40 L), there's always a temptation to experiment on the actual situation instead of pulling a little off and proving it on a small scale first. (And it's not like I've not been there myself...)
@CJ - But it's 5:00PM, I'm ready for dinner, and I'll just try this one thing on my 25-L reactor...
ReplyDeleteAnd how many times has THAT worked out in a positive way? :)
This points to a case where the customary scales in school can be good experience for process problem solving (sort of inverse training). Don't have to be skimpy as typical "samples" from a plant are 100-1000 mL which provides material to explore multiple options in small amounts, akin to familiar lab scales, and then still run a demo of the chosen route at reasonable level (which likewise can be similar to larger runs done in school) before attempt to move back to the plant.
ReplyDeleteSee Arr Oh- indeed does seem Murphy's law always get enforced at the most inconvenient moments.
CMCguy
what works even better on small scale is filtering the emulsion through a sintered glass Buchner under vacuum. It degasses the filtrates and it also removes fine particulates that often form interphase and stabilise emulsions
ReplyDelete@ milkshake
ReplyDeleteDo you filter it straight through or you also use celite pad before you filter. I did POCl3 reaction yesterday and ended up with lot of emulsion and fine particulates. Suggestions are welcome on that sucker reaction as I had to scale up since it worked. If you neutralize the same reaction with ammonia it exasperates the problem.
1. problems usually get exacerbated - its the chemists who get exasperated. 2. I am not a big fan of Celite but you are encouraged to try it. 3. You may want to evaporate as much POCl3 as possible before the quench, and after the quench to make the reaction mildly acidic again - it helps to break the emulsions.
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