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Author: Subject: Vacuum Distillation
lordmagnus
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[*] posted on 6-2-2006 at 21:28
Vacuum Distillation


Hello,

I am having trouble finding a formula to help me determine what the boiling point of a liquid will be at different atmospheric pressures. I am just guessing, but it's probably a logarythmic based one isn't it? What I am needing is to find out how I can figure by reduceing the pressure inside my distillation rig to a certain level, what is the new boiling temp of the various liquid elements in the boiling flask?




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The_Davster
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[*] posted on 6-2-2006 at 22:07


A nomograph!... In other words a fancy method of connecting the dots to get the result:P

http://www.umsl.edu/~orglab/documents/distillation/dist.htm
http://designer-drugs.com/pte/12.162.180.114/dcd/chemistry/e...


[Edited on 7-2-2006 by rogue chemist]




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sparkgap
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[*] posted on 7-2-2006 at 03:52


...or if you want to do some number crunching, go look up the Clausius-Clapeyron equation. Zubrick, in his "The Organic Chem Lab Survival Manual", details the use of both the nomograph and the equation I mentioned.

sparky (~_~)




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[*] posted on 7-2-2006 at 12:56


That nomograph is very useful, but it lacks precision (ie it gives the wrong answers sometimes). It doesn't have enough parameters to address the question properly. When it comes down to it, you need to do the experiment.
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[*] posted on 16-2-2006 at 10:44


I like to use this java applet on the University of Cambridge's website to calculate boiling points at various pressures.
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vulture
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[*] posted on 17-2-2006 at 06:02


Quote:

go look up the Clausius-Clapeyron equation


Not only is it a pain to work with, its results are mostly of theoretical value, IMHO.




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[*] posted on 17-2-2006 at 10:26


Yes vulture, the Clausius-Clapeyron equation, and the nomograph derived from it, may lack accuracy but what else does one have when planning an experiment into the unknown? Especially if you cannot find experimental data for similar compounds.

I'm glad to have the nomograph and have found it useful. It's theorectical basis, the Clapeyron equation, is rigorous and it is only the approximations and assumptions made while developing and using the Clausius-Clapeyron equation (or the nomograph) that produce the inaccuracies.

At least this is the understanding I get from "Chemical Process Principles," Part I, by Hougen et al, pp. 78-79. ;)




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[*] posted on 18-2-2006 at 04:17


Ofcourse it's useful to some extent. The problem is that you need experimental data (gibbs free energy) for your compound that is likely harder to find than a boiling point curve.



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[*] posted on 18-2-2006 at 10:45


This problem is essentially that of determining the vapor pressure of a substance at a given temperature. The chemical literature is rich with various correlations, etc, for doing this.

Another that I have found to be useful is the Antoine equation. This has the form:

logP = A-B/(C+t) (t in deg C), or in some cases:
logP = -52.23B/T + C (T in deg K)

A, B, and C are constants that are provided in a table for a large number of compounds, inorganic as well as organic.

Ref: "Lange's Handbook of Chemistry," revised 10th edition




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[*] posted on 18-2-2006 at 11:27


Damn, I knew it was a logarithymic formula! Thanks for all the help, this shoudl get me going, now I just gotta get an old window ac so I can build a vacuum pump, and I'll be in buisness



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[*] posted on 19-2-2006 at 09:19
Hydroaspirator


Aspirators: http://www.onlinesciencemall.com/Shop/Control/fp

Why not use 1 of these ? Mine works great ! I believe Magpie uses a similar device.
Besides, mechanical vacuum pumps can be destroyed by corrosive vapours.

[Edited on 19-2-2006 by MadHatter]




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[*] posted on 19-2-2006 at 10:42


Yes, MadHatter, I will always prefer my aspirator as long as it supplies sufficient vacuum. It will give me about 26"HG vacuum in the winter. I expect this will cover most all of my needs.

I was intrigued with the idea of having a vacuum pump and looked into buying one. I had found a Yellow Jacket (used by refrigeration techs) for around $250 that I thought would be pretty sweet. Then when I thought more carefully I realized I would need a cold trap, and worst of all some CO2/acetone for the trap. What a pain in the rear. So I didn't buy it. I would only buy one if I was obsessed with some high vacuum application for which the aspirator wasn't adequate.

I have 2 aspirators, a chrome-plated brass one ($15), and a Nalgene plastic one. I bought the plastic one for vacuum distillation of HNO3, and then found out through this forum that I don't need vacuum for making 68% HNO3. :D

The only other case where I can see that a mechanical pump might be preferable is where the vacuum must be maintained for a long time. In this case it might be cost benevolent to pay for the electricity vs the water. But considering the capital cost, maintenance, and general hassle of the mechanical pump, I doubt if it would be worth it.




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[*] posted on 19-2-2006 at 10:58


Hummm,

Taht's a good idea. I didn't think of the cold trap and acetone to prevent vapor from reaching the pump. When I did do som A/C work, there is a device you can get, it's basically a venturi/aspirator, except you hook it to a tank of compresed air, and it pull like a 27" vacuum in a cars aircoditioning system. Although I'll probably get the aspirator for now whenI get to that point. Anyone have a diagram of how to setup the mech. pump to a closed circuit distillation setup with the cold trap and such, incase I am missing anything?




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[*] posted on 19-2-2006 at 17:10


I have a question relating to this topic; Do most vacuum pumps have regulators on them to adjust how much of a vacuum it will pull? More specifically, does the more expensive model at cynmar have this feature?. The only reason I'm looking at getting a vacuum pump is because I don't have a water source out in my lab and I don't feel like jerry rigging a hydroaspirator with a pond pump in a plastic bucket.
Here is the vacuum pump from cynmar:

http://www.cynmar.com/product_info.php?products_id=8524

Does anyone have any experience with this particular vacuum pump?




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[*] posted on 9-3-2006 at 17:55


Quote:
I had found a Yellow Jacket (used by refrigeration techs) for around $250 that I thought would be pretty sweet.
I can testify to the quality of the Ritchie Yellowjacket. I have the 1.5 CFM model and it has met all of my needs. The vac oil wreaks of whatever i've been working with, and I'd love to know how to clean out that little compartment without draining it.

Douchermann, most direct drive pumps have a threaded cap that can be loosened to regulate or release the vacuum. Normally there is a gauge in the front to determine roughly what you're pulling.



Anti-suck back in case of power failure.
Built in vacuum indicator gauge.
Heavy duty handle.
Heavy duty casting body.
Large fitting for easier oil filling.
Accommodates oil exhaust filter #93386.
Large replaceable sight glass. Easy to read.
Large oil drain.
Large oil capacity to absorb moisture.
Metal with rubber overlay base for stability.
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If you don't have running water where you're working, this is the best alternative.
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[*] posted on 9-3-2006 at 20:56


this vacuum pump is insane. I have the 4cfm model and the vacuum is almost _too_ strong.
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[*] posted on 10-3-2006 at 13:53


I have always found the nomograph useful to explain to some one who wants to strip of an organic solvent using a 2 mm Hg oil pump where the solvent went.
mick
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[*] posted on 28-3-2006 at 22:21
Oiless


Yeah, my Pneumotive continuous vane pump pulls down over 29 ". It's an oiless system but I
still restrict it to air conditioning work. BTW, EPA regulations only require 27" for the recycling
tanks. 27" is about 74 torr - great for HNO3 distillation where the H2SO4/nitrate in the
flask can be kept below 45C(113F).




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MadHatter
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[*] posted on 12-4-2006 at 19:49
Clausius-Clayperon


I hope this is some use to the members. In the CRC Handbook Of Chemistry and
Physics(85th Edition) there exists some data for the delta-H in the C-C equation for about
850 compounds. It's referenced at the boiling point and 25C. The pages are 6-107 to
6-122 which corresponds to pages 1096 to 1111 in Adobe Acrobat. This book is on my FTP
in the CHEMISTRY - OTHER folder. Hope this helps somebody.

Magpie, Vulture is correct in stating that C-C is a pain to work with. Antoine's is certainly
easier but I have a harder time finding the A,B,C constants although I'll check out Lange's
book from the university I work for. BTW, I went to the website for Yellow Jacket and they
claim to pull a vacuum of 15 microns - more than sufficient for most chemistry.

The C-C equation is more rigorous than Antoine's, but what the hell, I really do enjoy crunching
numbers !


[Edited on 2006/4/13 by MadHatter]




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