Sciencemadness Discussion Board

Ethanol rates of evaporation with/without stirring.

Tacho - 12-9-2004 at 12:32

I’m doing experiments about the evaporation of ethanol under reduced pressure.
Using the same 300ml flat bottom flask, the same heating bath at 60°C (the ethanol itself remains at about 44°C) and the same vacuum of 160 Torr, two different thing happened in the same 10 minutes:

1) If I use a magnetic stirrer at low speed, say, 150 rpm, 100 ml of ethanol are reduced to about 50ml in a flat bottom flask with constant boiling.

2) If I don’t insert the stirrer in the flask, the same 100 ml are only reduced to about 75ml. No boiling, no bumping.

I knew stirring (or inserting a thin bubble stream) would prevent bumping, but why would it increase evaporation so much? This means that the rates of heat flow have changed. I wonder why...

Is there a theory that makes sense of these results? Or I should do the experiments again?

vulture - 12-9-2004 at 12:41

Better heat transfer and better diffusion at the gas-liquid boundary.

Try stirring your bath too. You will notice it makes quite a difference too.

Tacho - 12-9-2004 at 13:02

Thanks. Would you mind to expand "better diffusion at the gas-liquid boundary" to a paragraph?

Hermes_Trismegistus - 12-9-2004 at 18:46

Tacho, I'll give it a shot.

The ethanol particles that are sitting at the boundry between the solution and the air, bee-bop (a dance from the late 50's) into the air (forming ethanol vapour) and then there is a lack of ethanol particles right at the boundry between air and solution. (I assume that it is mixed with water?)

I also agree with vulture. The stirring must help the heat transfer from hot glass to the mass of solution. That way no thermal gradient can form and there is more heat diffusion than would normally occur from radiance and convection currents alone.

If you put a pot of water on the stove and stir it with a wooden spoon. v.s. just letting it sit and boil I think you will find the same phenomenon occurring. The stirred pot heats up much faster. (as opposed to the watched pot).

H.Trismegistus

vulture - 13-9-2004 at 00:28

I haven't got a sensible explanation for that, I'm beginning to doubt if it even was correct what I said. :(

Anyways, stirring your bath makes one hell of a difference. When I was distilling HNO3 at approx. 100mbar, the bath would be at 90C and the mixture in the flask at 40ish without stirring. With stirring the bath the bath temperature required was only 70C.

thunderfvck - 18-9-2004 at 01:12

How I think of the situation is that the increased stirring simply shifts the molecules all over the place and the molecules have a better chance of absorbing heat from one another/the heat source. Since there are more interactions and absorption of energy the temperature increases faster as opposed to the heat having to work its way through the motionless liquid. It all comes down to what was once there is now not there, and something else is!...haha...so late and so tired.

In an unstirred liquid a molecule called Bob can be sitting on the top of the liquid while Johnny sits on the bottom and absorbs heat. It will take awhile for the heat to reach Bob because that poor bastard is on the top.

Now in a stirred system, Bob and Johnny are free to go where they want at a much quicker pace and now they can, on average, absorb just the same amount of heat in the same time. And so the two of them can live happily and can get heated up much more quickly. And sodomy follows.

I remember having a problem with an oil bath once. I had solution in the flask being magnetically stirred while it was being heated in an oil bath. The temperature of the oil bath greatly varied over time. It would often go from 120C to 140, and sometimes down to 100 or so without changing the temperature at all. Well, maybe an exageration...But I remember setting it at what seemed to be constant at 110 overnight, and I awoke the next morning to find it at 90 or so.

Why does this happen?

So then I decided to turn off the magnetic stirring and the temperature of the oil bath stayed constant for an indefinate period of time.

Could it be my shit hotplate or do the vibrations of the flask due to the magnetic stirring have something to do with such a change in temperature?

[Edited on 18-9-2004 by thunderfvck]

[Edited on 18-9-2004 by thunderfvck]

unionised - 18-9-2004 at 12:58

You shouldn't really use (most) flat bottomed flasks under vacuum.

JohnWW - 18-9-2004 at 13:06

Except for the heavy conical flasks used in laboratories as benchtop vacuum pumps, attached to a running tap.

John W.

unionised - 18-9-2004 at 13:17

I take it that you mean filter flasks and the traps often used with them. Those are specially made from thick glass and are not the "most" flasks that I referred to.

Tacho - 19-9-2004 at 09:51

I think it has all been said here. Although all explanations make sense, I still think this evaporation under vacuum thing is pretty counter-intuitive.

Listen to this: I tested vaporization under vacuum with and without a thin bubble stream (see Vogel's or any other book) in a 10ml setup and it made NO significant difference. I tested the same bubble stream in a 500ml setup and vaporization took place in HALF time! HALF!

You learn something every day...

About vacuum and flasks: Don't listen to me kids, but I'm loosing respect for vacuum. Total vacuum is just 1 atmosfere differential pressure and that's not much... Most flasks seem to handle it fine. Big thin walled PVC pipes, warmed up to 64°C handle 100 torr nice and easy. This is not something you say out loud in public, as it may induce people to be careless about potential hazards, but I though I should mention it.

vulture - 19-9-2004 at 10:14

Quote:

Listen to this: I tested vaporization under vacuum with and without a thin bubble stream (see Vogel's or any other book) in a 10ml setup and it made NO significant difference. I tested the same bubble stream in a 500ml setup and vaporization took place in HALF time! HALF!


That is actually quite logical. For 10ml, the surface area between liquid and vapor is pretty large considering the volume, for 500ml the surface area is pretty small. Hence actions like stirring and bubbling greatly increase importance and why innocent reactions can suddenly explode in large reactors because of small surface area to the coolant.

Quantum - 19-9-2004 at 11:41

I know this may make me sound stupid but what is a bubbler and how does it work?

vulture - 19-9-2004 at 11:58

Small capilary tube going down into your flask (under the fluid surface) and exposed to outside air. Under vacuum it produces a stream of bubbles which contribute to steady boiling and prevent bumping.