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chemrox
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[*] posted on 26-8-2015 at 12:39
Heat excgange problem


I have a 5 L flask I'm heating with a sand bath. The solvent I'm using boils at 85*C. The sand bath is up to 150*C after 3 hours! The flask is connect to a soxhlet and condenser and is a one neck flask. There's no opportunity to measure the solvent temp directly. It has started boiling. I assume the equilibrium will eventually be reached where the sand bath is closer to the solvent temperature but I have no idea what the delta will be. The solvent is naptha. This setup worked great with ether but we need to try extracting with a different solvent and naptha was reported by another worker who used it in a different way.



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Little_Ghost_again
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[*] posted on 26-8-2015 at 12:47


have you insulated the flask? 5L seems a pretty big area to loose heat from as solvent condenses etc, but then again what the fuck does a noob like me know?

[Edited on 26-8-2015 by Little_Ghost_again]




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chemrox
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[*] posted on 26-8-2015 at 13:20


Yes the glass is all covered with Al foil that extends over the sand container which is a stainless bowl. The issue is ultimately what temp to set the temp control? Right now I have two thermometers in the sand; one for the temp controller and another Hg as a check. I have them set to agree, whatever that may mean. The drag with so much lag is the H2O pressure often drops at night enough to kick off the water cutoff controller. That guy cuts off the power to everything. It's a protection against losing solvent through evaporation through the top. Glad you're still among us; how's it going? I just had a walnut sized tumor cut out of my liver.

Shut down after 6.5 hrs. I'll try a cruder method in the hood.

[Edited on 26-8-2015 by chemrox]




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Magpie
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[*] posted on 26-8-2015 at 13:33


The rule of thumb that I have seen is a delta T of 20°C between the liquid and the oil in an oil bath. But it seems you have a delta T of ≥65° when using sand. Sand is a good insulator and it does not provide convection currents as does oil.

I think you will just have to find the sand temperature by trial and error.

Can you use oil (silicone oil would be best) or do you consider it too dangerous?




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[*] posted on 26-8-2015 at 13:39


Sand is horrible.

Heats up and cools down far too slowly.

If you only need 85 C what's wrong with a water bath ?

Max you'll be out is 15 C instead of 65 C with sand.




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[*] posted on 26-8-2015 at 15:14


Yeah now why didn't I think of that? Maybe because it worked so well with low boiling solvents. Water indeed. I'll try it tomorrow and if it loses too much heat I'll use a phthalate bath. Water sounds about right. Suppose I need to push it a little to get the vapor up the column to reflux conditions. I could salt the water and get 105 for the bath.

What if I kept the sand and added water? That way I wouldn't have so much bloody steam.

[Edited on 26-8-2015 by chemrox]




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[*] posted on 27-8-2015 at 11:35


Water in sand did not work as I'd hoped. I was thinking that the sand particles would have an affinity toward water and this would serve to increase the entropy of the system thus raising the bp. Wrong! The sand simply provided more surface area for the water to evaporate. Silicon oil would be more expensive than getting a used mantle from ebay. I have some nbutyl pthalate which I use in my oil bath. (That is too small for the 5L flask. I think I shall try the nbutyl pthalate in the basin. It will take ~1.5 L but the stuff will rise to 125-130 without smoking. bp = 340.



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[*] posted on 27-8-2015 at 11:37


You can use wet sand, which will lower the heat capacity over just water, which takes a long time to heat also. Same for water and some BBs or other filler. If you could find a bath vessel that is shaped more conically or spherically, then it would be even better, or maybe put a large lead donut in the bottom of a cylidiner to displace some water and make it heat faster.

That is why heating mantles are ideal for larger flasks, as they are designed to fit them, so you are not heating up lots of extra stuff.
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[*] posted on 27-8-2015 at 12:51


Quote: Originally posted by aga  
what's wrong with a water bath ?




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chemrox
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[*] posted on 27-8-2015 at 13:42


Not enough temperature gain for a push i.e. superheating. Also evaporation and steam. Tired of experimenting with heating methods too. Waiter might do it might not but would have to be replenished periodically.



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[*] posted on 27-8-2015 at 14:34


Heat up a large pan of water on the apparatus known as a Gas Stove.

IUPAC version: Inthermalate an unsmall isolatory containing Oxidane using oxidation of Tetrahydridocarbon.

This pre-heated oxidane resevoir can be used to replenish your heating bath when needed.




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[*] posted on 27-8-2015 at 15:25


Cottonseed oil used to be standard for hot oil baths. Other vegetable oils might suffice also.



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[*] posted on 28-8-2015 at 11:55


Rapaport set us up with hydrolized cottonseed oil. It smoked at low tempertures and gave everyone headaches. I hated the undergraduate labs at Berkeley. Corks and tubing, flame heating, really primative and to no point. Cason like to brag about that shit saying we wasted our grant money on special equipment. He did show me a Podbilienak column that was made with concentric tubing and nichrome wire. I'd like to get a glass blower to make one with joints. Anyway, Mag, why would I use veggie oil when I've got dibutyl phthalate?

It turns out I should have tested the bp with a capillary tube. It turned out to be 136. More than my poor hot plate could deliver. I'm switching to acetone.




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[*] posted on 28-8-2015 at 12:20


I have Cason & Rapaport's book, which I like, especially for distillation ideas. Maybe that's where I read about cottonseed oil.

I've never used a vegetable oil or sand - only water, steam, DOT 5 silicone oil, a Bunsen burner, or a mantle. I prefer a mantle if feasible. Water and steam are good for low boilers like diethyl ether and dichloromethane. Silicone oil is good for solvents with higher bps. A bath is nice if I want to hold it at temperature for some time as I can use it with a TC and a PID controller. What I use all depends on the situation at hand.




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smile.gif posted on 28-8-2015 at 12:42


You might find it funny, but alternatively (if heat control isn't EXTREMELY CRUCIAL), you can use a steel/aluminium pan, place your flask in the middle of it (just slightly hovering) and wrap the whole thing with aluminium foil so that you can still see a small part of the flask. I've used this method to synthetize diethylether, and it works just as a charm. It was however only with a 1L three neck RBF, I don't know how this method will work with bigger aparatus.



[Edited on 28-8-2015 by Firmware21]

[Edited on 28-8-2015 by Firmware21]
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[*] posted on 28-8-2015 at 13:04


I was am trying for even heating over and throughout the flask. Your methos would heat the flask but would also make heat spots. In you case at the bottom. The reason some workers like Rapaport favor oil baths is to get the most even possible heating. With high bp liquids metal alloys are sometimes used to get even heating. In my case it was possible to change the solvent to accommodate the apparatus. If the solvent doesn't give the best extraction and naptha seems more promising I've got two ways to go: static extraction e.g. boil the stuff in the solvent for awhile and separate the solids. An alternative would be to use a stronger heater. I don't like open flame with flammable liquids so it would have to be an appropriate mantle; used on ebay, $80-200.



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[*] posted on 28-8-2015 at 15:17


A bath of copper coated 4.5mm steel spheres conducts better than sand, doesn't scratch glassware, doesn't evaporate, stink or (usually!) catch fire.

They can be sourced from your finer sporting foods purveyors, who are under the misapprehension that this heating bath media is actually air gun ammunition...

(Edit)

Might I ask what you are extracting?

[Edited on 28-8-2015 by Bert]




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