Oil Baths (II)

bigwhisky · Wed Jun 01, 2005 4:38 pm

hotplate stirrer- check
pyrex bowl- check
dow silicone oil- check
rbf- check

Okay it seems that swib has all that is needed for a bath.
Swib has never used a hot oil bath b4 and is hesitant but brave.
Questions swib has:

how much of the rbf needs to be covered by oil?
swib imagines that this oil will expand when heated y/n?
what sort of temp diff can one expect b/w bath and flask contents?
emrgency proceedures require what equipment on hand?

any help is much appreciated.

Bigwhisky

zub · Wed Jun 01, 2005 7:22 pm

a decent stand with clamps is essential, on account of the rbf wanting to float out of the bath.
the flask gets slippery as well, so moving it around is awkward.
rxn temps of 100c shouldn't cause much concern for the oil overheating.
frying potatoes is spookier, though, in both cases, a way to smother a fire is a good plan.

loki · guinea pig

ya, immerse the rbf to the height of the liquid in it is the way to do it i'd say. having the bath and heat source on a jack would probably be very helpful too, but jacks are expensive of course.

to avoid problems with heat, use peanut or grapeseed oil, these two are very tolerant to temps up to about 300 degrees and don't decompose very fast.

it probably kinda goes without saying that any lab should have a fire blanket, a bucket of sand and a CO2 extinguisher for fires... but it should be emphasised. fires suck.

bio · Working Bee

!0-15 degrees temp diff above the BP will give a very slow takeoff or reflux with a normal insulated rig a little more with a column. Near the end of distilling you will have to raise the delta T to 30-50 degrees to get it all out using vigorous stirring.

Strong secure clamping is needed for the floating problem and helps to support the bottom of the flask with something such as a stainless steel strainer turned upside down and pushed in w/ the flask to conform to the bottom. Hanging the flask from the clamp is not so good with setups bigger than about 500ml and is inviting disaster IMHO. You will find out about this when bumping occurs without a doubt.

bigwhisky · Thu Jun 02, 2005 5:00 pm

Yep got a good clamp, forgot to mention this.

like the idea of the strainer, very clever.

flashpoint of silicone oil is above 320c so feel comfy wit dat.

Q- loki says

bio · Working Bee

Yep, any oil expands proportional to temp. Never used the silicone oil afraid I'd spill it.

It is easy to overfill and not so easy to get it out while 200deg., so just leave a couple inches from the top of the bowl and run a test with the flask in it.

I like the superhydrogenated veg oil as you can leave the support ring and heater in and after it solidifies just put it away with no spill worries. Using 2 or 3 different sizes simplifies things. When the oil begins to darken and smell funny just change it. You will get many hours use from it just remember that the smoke point of veg oil lowers as it degrades

An oil bath is really much better than other methods when fractionating mixtures because you can just monitor the oil temp to know when the next fraction should come over or when to raise the heat when it slows down.

Sektor · Sun Jun 12, 2005 10:30 am

wondering if sand would be ok to use inside say a pot on a hotplate? then place rb flask in sand for a HI+pfed reduction? whats the downside? does the sand have unbalanced heating? can it support the rb flask with condenser attached? suggestions? thanks heaps

bio · Working Bee

..............does the sand have unbalanced heating? ......................

Yes, it is useable with fine silica sand however sand has a very large thermal lag and monitoring the temp is not so easy as it might seem due to this and potential steam or air pockets.

Although this is quite an old standby heating method for higher temps an air bath is much better. I wonder if anyone uses them these days? I even read an article once that said a sand bath had exploded due to moist sand that somehow had trapped enough steam, I quess, to erupt sand and glassware thoughout the lab.

If you need high temps build a simple air bath (see Vogel3) substituting the burner with a dimmer controlled NiChrome coil or just use a heating mantle. Commercially available mantles that have the ceramic variable sized inserts work on the "air bath" principle and have a lot of advantages IMHO.

lemuralia · Sun Jun 12, 2005 2:41 pm

would'nt it safer to use polyethylene glycol? PEG doesnt smoke or stink either.

thenewrunne · Chemically Balanced

Where would one find PEG OTC? or just order it I guess?

bio · Working Bee

............would'nt it safer to use polyethylene glycol? PEG doesnt smoke or stink either..........

By high temps what was meant in relation to a sand bath was >250deg which is generally when sand baths would be used. For boiling hydriodic acid cooking oil is fine or PEG, maybe, although it has low vapor pressure and is combustible

Here is a little info on PEGs........

..................Physical characteristics of the polyglycols are given in Table (9). As in the case of polyisobutenes, prolonged heating to 150 – 175 °C leads to depolymerization. Aldehydes are formed that can react further to give acidic substances. Traces of alkali or alkaline-earth metals enhance this degradation. Thermal decomposition can be prevented by antioxidants such as phenyl-a-naphthylamine, so that the oils can be used as heat-transfer fluids up to 250 °C. The relatively poor oxidation stability can be improved by inhibitors to achieve the stability of petroleum products. .......................

Never used the stuff myself but Therminol heat transfer fluid good for 300deg has to be used in a closed system . The low vapor pressure causes a lot of fumes. Anyone who has tried brake fluid for example has also found this out.

bigwhisky · Tue Jun 14, 2005 4:00 pm

Sand baths can be cool but bigwhisky would give them a miss. have heard that granules can fuck up the joints in your glasswear coz they are really difficult to clean out and whats more there are some issues with sand baths shorting out hotplates although i am not to sure on this one. would you take your expensive hotplate stirrer down the beach? i wouldn't

bio · Working Bee

........some issues with sand baths shorting out hotplates although i am not to sure.......

I think perhaps what you mean is element burned out or ceramic tops breaking. Dry sand itself is not an electrical conductor.

Sand baths concentrate the heat similar to metal containers, on the surface of the plate and the ceramic ones will crack sooner or later.

Corning says IIRC metal can be used up to Heat 3 on the Pyroceram otherwise you are on your own. I contacted Thermoline when I got a new 12" ceramic high wattage unit about this as their manual prohibits metal or any type of insulation. When I asked the product engineer if it was OK to use metal vessels with a wire ring underneath and then only on the lower settings all I could get out of the guy was "we don't reccomend it". Would probably be OK but I ain't chancing it.

This one has the dial where the first click is either 1 or 10. The first time I accidently turned it to 10 from cold after a couple minutes I heard this horrible popping loud expansion noise3 or 4 times and thought for sure it had cracked. Luckily it survived but it is better to slowly heat the top from the lowest setting for a couple minutes if you expect them to last.