Air bubbles in crystallization solution ??

[wati]

Dear all,

I'm doing a experiment with cooling crystalization in a small baffled vessel (stirred). From my probe i notice air bubbles (trap) in my solution which is not desired. How can i easily remove these bubbles.Cooling from 55 to 50deg.C.

Any suggestion /advice is welcome!

Thanks

wati

 

[m777182]

You should optimize your aggitation: more homogenious suspension should be achieved with less power input; no vortex should be created. In a 200 ml or 1000ml beaker it is usually not easy because the aggitator (or multi step aggitators) should be hand tailored for your vessel and experiments are usually performed on batch mode. It means that the volume is changing by time and therefore the aggitator works at different regimes.
When you trap bubbles it won't be easy to get rid of them-by slow aggitation your cristalls will settle down and they will agglomerate, recrystallize and change their original form.
m777182

 

[wati]

Thanks,
But its a vessel with about 5L saturated solution. First I tried diffent speed on agitating, hovewer without any specific results of reducing bubbles.Then I tried to insert the probe after I filled the vessel with sat. solution and adjust the for probe about 40-45 degrees, while shaking off the bubbles trapped in the probe.This relief the problem a little bit. However as you mentioned is not easy to get rid of all bubbles, since these bubbles may effect the CSD measurments.

Another problem is that I have less concentration(sample)for CSD measurement after cooling. Any idea how to optimize this?

Thanks

 

[m777182]

OK, 5L vessel is still a small unit; with buffles your stirrer has about 100 mm dia. You should carefully select the type of your stirrer: maybe a sort of Ekato's MIG or INTERMIG or a marine propeler with a draft tube will minimize vortex. Draft tube works fine with contoured bottom, whish is not a case by a 5L beaker.Try to increase the local velocity around the probe. In a large vessel you could fix it into a Venturi, however in 5L it is not easy. I do not understand what you mean with "I have less concentration(sample)for CSD measurement after cooling". Whart sort of probe you actually use and what do you measure? Is it a CSD?
m777182

 

[wati]

I use marine propeller stirrer and a draft tube in my vessel.After cooling (@ 50oC steady state) I take sample out for a CSD measurement in a laser diffraction analyser also @ 50oC. During measurements I encounter 2 problems: 1) some bubbles and impurities in my sample beaker give bad CSD results. 2) Concentration too low for optical.

Results: large CSD and width -> not desired

thanks.

 

[m777182]

Avoid impurities by filtrating the solutions before crystallizing if you are not using dirty solutions on purpose.
Some particle size analyzers do use ultra sonic pretreatment of the suspension to get rid of the bubbles; however it wil destroy the agglomerates as well and it may change your CSD. You may try a short vacuum treatment with the same purpose.
To concentrate the solids you can use a small hidrocyclon (difficult to make in such a small scale and CSD bias again) or maybe filtrate the solids and redisperse it in a small amount of mother liquor. It will give you a lot of work and a nasty mass ballance.
m777182

 

[unclesyd]

One thing I've done in our pilot plant is on some small units is reverse the agitator direction on some highly concentrated salt solutions that had N2 bubble problems. This wasn't a cure all but had a large affect on some solutions. This was on crystallization study units.