Vacuum Distillation of Ethanol

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We're in the process of getting our solvent recycler to work with the vacuum distillation of ethanol. The purpose would be to be able to boil the ethanol off at a lower temperature to preserve the quality of the contaminant. The boiling point of ethanol is 79*C and under vacuum (28 inHg) it drops to around 34*C. We typically use a rule of an additional op-temp of 50 degrees Celsius above the boiling point of the solvent is enough to boil the solvent and maintain that boil. In the case of Ethanol, we would use an op-temp of 130*C. Under vacuum we can assume that we would need an op-temp of around 85*C.

What we have found is that we still need to give the ethanol an op-temp of around 130*C to get it to boil under vacuum and it is a slower boil than without vacuum.

Does anyone know why the ethanol requires so much heat to boil under vacuum??

This goes against everything that we know and have proven to work for vacuum distillation and we're at a roadblock.

Any and all insight will be greatly appreciated. Thanks in advance!

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What do you mean by "op temp"?

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Op-Temp = Operational Temp

We have an oil jacketed boiling tank with a 1500 Watt heater. We control the op-temp. If I set the op-temp to 130 degrees the controller will maintain 130 degrees.

Nomally, in a non-vacuum, simple distiller, we would run an op-temp of 130*C to efficiently recover Ethanol.

In this Vacuum distillation version.... for some reason we still need an op-temp of 130*C to recover ethanol efficiently even though we've dropped the boiling point by 50*C.

We tried an op-temp of 90*C yesterday and it took 5 hours to recover 3.3 Liters of ethanol (Less than one gallon). Through experience we should have recovered all 5 gallons over the 5 hour period, probably even within 4 hours.

At the 130*C op-temp we were able to recover all 5 gallons in 6 hours. This is no different than if we were running the same cycle without vacuum. We should run a bit more quickly and at a much lower op-temp, about 80*C-90*C, but for some reason we cannot and we do not understand why.

I hope I'm not being too confusing.

We are using virgin ethanol as well. 96%!! So water is not the issue.

Edited by jm112583

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We tried an op-temp of 90*C yesterday and it took 5 hours to recover 3.3 Liters of ethanol (Less than one gallon). Through experience we should have recovered all 5 gallons over the 5 hour period, probably even within 4 hours.

From my experience it's pretty good rate.

I have hot plate 1200 W. Conical flask 1 L *) (filled usually 800 mL 6-9% ethanol volume). And in water heating bath receiving approximately 100 mL 70% = 70 mL in time 70-90 minutes. No vacuum pomp used.

I am making very detail measurement of either energy usage (watt meter), time (is on watt meter), ethanol concentration before/after. Sometimes it's recorded by digital camera. Drops are going to tube with scale.

*) also experimented with 2 L conical flask, 1 L round-bottom flask, 250 mL round-bottom flask (but these must have 2nd stand).

Let me see my papers.. (from this week distillation):

Vinit = 800 mL

t = 67 min

E=0.77 kWh

Vend = 86 mL

67%

86*0.67 = 57.62 mL

57.62 mL/800 mL=7.2%

Cost 0.77/353*220=0.48 ($0.12896) Vwater=11 L (cost 0.11 ($0.03))

(one of approximately 20-25 distillations made this week)

You're probably wasting time on heating whole setup to reach right temperature from ambient temperature.

I am putting pot (with lid) on gas methane oven, in the mean time conical flask on hot plate (no bath yet), and in 10-15 min reaching 100 C, after that placing pot on hot plate, and putting (hot already) flask inside. The first drops of ethanol are starting flowing around 21-22 minutes later.

I suspect you are using 5 L conical flask?

What is volume of oil?

The higher volume, the longer it will take to reach right temperatures. and harder to maintain it boiling.

How are you protecting setup against cooling down?

(I even cut piece of metal and bend it appropriately to cover pot with everything from above to keep in hot)

Op-Temp = Operational Temp

We have an oil jacketed boiling tank with a 1500 Watt heater. We control the op-temp. If I set the op-temp to 130 degrees the controller will maintain 130 degrees.

These 130 C are on thermometer in distilling setup or measuring oil temperature?

Edited by Sensei

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There's some data here for heats of vaporisation of a few solvents.

http://en.wikipedia.org/wiki/Enthalpy_of_vaporization

Ethanol isn't there, but methanol and water are.

The heat needed is larger at lower temperatures.

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Op-Temp = Operational Temp

We have an oil jacketed boiling tank with a 1500 Watt heater. We control the op-temp. If I set the op-temp to 130 degrees the controller will maintain 130 degrees.

Nomally, in a non-vacuum, simple distiller, we would run an op-temp of 130*C to efficiently recover Ethanol.

In this Vacuum distillation version.... for some reason we still need an op-temp of 130*C to recover ethanol efficiently even though we've dropped the boiling point by 50*C.

We tried an op-temp of 90*C yesterday and it took 5 hours to recover 3.3 Liters of ethanol (Less than one gallon). Through experience we should have recovered all 5 gallons over the 5 hour period, probably even within 4 hours.

At the 130*C op-temp we were able to recover all 5 gallons in 6 hours. This is no different than if we were running the same cycle without vacuum. We should run a bit more quickly and at a much lower op-temp, about 80*C-90*C, but for some reason we cannot and we do not understand why.

I hope I'm not being too confusing.

We are using virgin ethanol as well. 96%!! So water is not the issue.

96% is exactly why water is the problem! You have azeotropic Ethanol. You will need to break the azeotrope, is this for consumption? Because it would affect what I recomend to break the azeotrope.

At 96% Ethanol and water form an azeotrope, no amount of simple distilation without an entrainer will break this.

http://pubs.acs.org/doi/pdf/10.1021/i200028a029

Edited by GM11

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GM11

Firstly, did you see the date of the last post in this thread? They probably have their still running by now.

Secondly, the azeotropic temperature and composition depend on pressure.

Vacuum distillation is one way of breaking an azeotrope.

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GM11

Firstly, did you see the date of the last post in this thread? They probably have their still running by now.

Secondly, the azeotropic temperature and composition depend on pressure.

Vacuum distillation is one way of breaking an azeotrope.

erm............ No I didnt look at the date! I might put a sticky on the screen reminding me to wear glasses before posting. Pressure reduction with Ethanol does not work so well, and depends alot of equipment knowledge etc. Despite the age of the post it might help someone in google world on a search, I know most hate necro posting but some people just google and arrive at a page.

I like to make my own dry ethanol as a reagent, depending on the use I often prefer an increase in pressure.

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Just wanted to say thank you and that I gained from this thread and learned new things despite it being mid way through 2017 !

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Posted (edited)

I think I might have an answer to your question.  I started a topic to answer it and joined this forum as well to that end.  I am now a retired ME but early in my carrer I was the engineering and maintenance manager for two large processing facilities.  We required 8.7 megawatts to run these factories simultaneously but our steam driven generator was only capable of making 5.2 megawatts as I recall.  I assume that you are using a barometric condenser to pull your vacuum.  What I think is happening is that because you have an open system, you are pulling more heat away than is possibly necessary.  Also, once you have established your boiling point, energy is being removed from the boil thus requiring more energy input.  When your solvent or ethyl alcohol is condensed, it indicates an open system.  You might want to look up designing a barometric condenser.  Later in my carrer, I was in upper management.  When I interviewed some engineers for a Vice Presidents job I would ask them why specific volume was important to have in the steam tables by Keenan and Keys provided my spelling of their names is correct the steam tables are amazing pieces of work.

Edited by lsume