I am distilling a lquid at approx 250C and am left with tar at the end of the process.
I would rather avoid this horrible looking black mass so was wondering if the adding of say silicon oil with a bp of approx 300C to the liquid that is
being distilled would help avoid this or at least make clean up easier.
If this was doable i suppose the next question would be how much oil would one add to say 500ml or 1000ml of liquid that is to be distilled?
Any help would be so appreciated.
Thanks.not_important - 29-4-2007 at 07:18
It's common to add a chaser for distillations that otherwise would go to near dryness. I normally use kerosene fractions, previously distilled
through a short column to avoid including lower boiling cuts. Hot kerosene is a decent solvent as well, I often empty the hot flask inside a metal
drum flushed with CO2, and if needed reflux some fresh kerosene in the flask and dump that hot.
I generally add about 10% chaser unless the distillation is pretty clean without it, then I'll go as low as 5%.
300 C might be a little close to the BP of what you're after, unless you are fractionating. The higher the BP, the larger the separation between two
compounds must be to get the same amount of separation for N theoretical plates. This is less important if later processing would make it easy to get
ride of any silicone oil that came over.
[Edited on 29-4-2007 by not_important]turd - 29-4-2007 at 12:42
Quote:
It's common to add a chaser for distillations that otherwise would go to near dryness.
For certain values of common, maybe. But I have never heard of this and it seems illogical to add another impurity to the mixture.
Common measures against scorching are better vacuum and/or smaller distillation apparati (microdistillation/kugelrohr).Tsjerk - 29-4-2007 at 12:48
Why can't you add the silicon oil or an other solvent AFTER distilling your product and reflux that for a while? So you can dissolve/losen up the tar
you produced and get rid of it easier. That way you would use the same amount of solvent, but don't have the risk of contaminations in your product.not_important - 29-4-2007 at 13:05
A chaser is intended to prevent overheating in the flask when attempting to distill all of a material; this can reduce tar formation. It also forces
the last of the desired substance out of the distillation apparatus, increasing effective yield.Dr.3vil - 29-4-2007 at 13:27
so basically the idea here is to add a higher boiling point liquid in order to force out the product you want if it happens to be the last thing left
in the pot??Nerro - 29-4-2007 at 13:38
It doesn't force anything out, it just helps keep the temperature down when the last fraction is evaporating so that the last fraction is not
partially decomposed by the heat. Hence the yield is going to be higher (duh! more of it comes over!).
Vacuum is another great way to avoid scorching your stuff because you won't have to heat so strongly as it lowers boiling points. It does however add
work/trouble to the distillation as your glassware has to be up to the task, I have broken numerous "spiders" (those adapters that go from NS29 to
three of four NS14,5 necks) while distilling under vacuum as the vacuum puts stress on the adapters. When I tried to turn them they just snapped
leaving me with a latex glove full of glass shards...
If your product isn't going to be in the last bit of the distilate you can of course just stop distilling once the boiling point goes up from the
fraction you desire to the next fraction. If however you do end up with a flask full of tar you can put a mixture of all available solvents (water,
methylene chloride, acetone, ethanol, methanol, ether etc.) in it and leave it to stand for a long time or heat/stir to dissolve the crap in the
flask. I've done it a couple of times and it seems to be pretty effective.
[Edited on 29-4-2007 by Nerro]not_important - 30-4-2007 at 05:57
Nerro has it right. A chaser doesn't 'force' in some special way, it does displace lower boiling components from the pot and column by allowing those
to be heated above the lower boiling materials boiling points, and by replacing the layer of those on packing (not too important for most packings).
When using a chaser you will see takeoff of the lower boiling stuff decline and go to zero, even though you are slowly increasing the temperature of
the pot. You'll see reflux of the chaser working its way up the column (or flask neck for simple distillation). At the point it reaches the
thermometer the temperature will jump to the boiling point of t he chaser, or of its lowest boiling components if it is a mix as with kerosene.
And, as Nerro said, if you're not interested in t he higher boiling parts of the reaction mix you are distilling, those will function as a chaser
provided they are at least a few percent of the total. You need enough high boilers to wet the column, based on its holdup volume, the flask walls,
and still have enough to form a small pool in the flask.
I use hot kerosene or similar very cheap solvent for cleanup, as it is low cost and not likely to be regulated or banned. I can't see throwing away
useful room temperature solvents dissolving tar that is just going to get burned or thrown away.demax - 30-4-2007 at 18:20
i dont have a vacuum pump to reduce the temp so for the time being this is not an option.
The reason i posed this question is becuase i am over going scruba scruba on tarred up g-ware and would rather go a once off rinse.
Right now i have some HCL stirring away hope this does the trick.
Hmmm i love inhaling HCL vapour in the morning!!!!
Thanks allMatchheads - 11-5-2007 at 19:19
demax, you can't specify so far as to propose a chaser until you say whether the two liquids are miscible or immiscible. This condition is what
distillation is basically about.
Why doesn't anybody call it silicone oil? Silicon is something else entirely.not_important - 11-5-2007 at 19:40
Quote:
Originally posted by Matchheads
Why doesn't anybody call it silicone oil? Silicon is something else entirely.
And it's so easy to remember the difference - silicon is rocks, silicone is boobs.
