Vacuum vs. boiling point!

  • Thread starter Pjwt
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Now this may seem a stupid question, but as my old dad said there are no stupid questions only stupid answers!

I am distilling some liquid which has a boiling point of about 190 C, I have put the liquid under vacuum of -1bar and the liquid now boils at about 160C, I would like the liquid to boil at about 140C, so is it possible to put the liquid under a vacuum of say 1.5 Bar? The reason I ask is that when I look at vacuum pumps available off the shelf they seem to stop at – 1bar is this something to do with the fact that the surface pressure at the surface of the earth is about 1 Bar? Or am I seeing complications where there are no complications.

Oh and this is my first posting so be gentle with me.

Thanks pjwt
 

Answers and Replies

  • #2
S_Happens
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You are mixing up pressure scales in your post. Pressure is typically measured with reference to either the local environment ( here it's atmospheric pressure at the earth's surface or something very close) or absolute 0 pressure.

Your first quote of "-1 bar vacuum" is what I could consider to be unconventional. Typically vacuum is positive, no matter which direction you choose to go. If you start from absolute that "-1 bar vacuum" would be very close to 0 bar, while if you started from atmospheric reference it would really be +1 bar vacuum (using "vacuum" to denote that it's positive value away from 0 barg or atmospheric reference).

Your next pressure (1.5 bar vacuum) is what I would consider to be conventional use, but that much vacuum simply does not exist. Read up on absolute pressure.

Your last statement using pressures tries to compare two different pressure scales. Your -1 bar is in reference to atmospheric (gauge) pressure, while the scale where the surface of the earth at 1 bar is absolute. Apples to Oranges.

To be clear, when measuring pressure you must choose where you want your zero pressure reference to be. For absolute readings, 0 starts at 0 absolute and there are no pressures below it. Therefore pressure is greater than or equal to 0. For atmospheric pressure (gauge), 0 is set at the pressure at the surface of the earth and you can have pressures both above and below (vacuum) zero. Technically this scale could mean that you can have negative pressure (vacuum), but convention in my industry is to use positive value and the notation of vacuum.

To answer your question again, NO, you cannot reach 1.5 bar of vacuum. That said, I'm a little skeptical of your liquid boiling at 160C at near absolute 0 pressure. What is this liquid, and are you actually measuring the pressure or just assuming that you are reaching it because of the rating on a vacuum pump?
 
  • #3
sophiecentaur
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Totally agreed.
You can't do better than zero pressure (0Bar). In fact, you can't reach it; the best you can hope for is about 1millionth of a Bar.
@Pjwt
Your 1.5Bar is a Positive Pressure compared to Atmospheric Pressure. That will elevate the boiling point (as in a pressure cooker).
 
  • #4
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at 0BAR the "boiling point" is about 0K not 140C
 
  • #5
sophiecentaur
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I don't think that is the conclusion to draw. That view would imply that nothing would remain solid in deep space. In fact, there will be blocks of ice out there - very cold but above @0K but solid - not boiling.
 
  • #6
K^2
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at 0BAR the "boiling point" is about 0K not 140C
The only fluid that remains liquid at 0K is Helium, so at 0 bar, it's the only fluid that has a boiling point.

There are fluids that you can prevent from boiling at room temperatures at 0 bar. Ionic fluids have almost zero vapor pressure and high surface tension at fairly high temperatures due to ionic bonds. However, at zero pressure, they are technically superheated fluids. The energy in vapor pressure is insufficient to overcome surface tension, preventing boiling, unless there is a nucleation bubble that is sufficiently large. In most applications, that minimum size is larger than the container making boiling impossible.

I don't think that is the conclusion to draw. That view would imply that nothing would remain solid in deep space. In fact, there will be blocks of ice out there - very cold but above @0K but solid - not boiling.
It's easier to explain with an image.

512px-Water_phase_diagram.svg.png


Phase diagram for water. At 1bar = 10^5 Pa, you have two transition temperatures. At 273K - melting, and 373K - boiling. At roughly 10 mbar = 10^3 Pa, there is a triple point (TP). At that point, liquid, solid, and gaseous phases can coexist. Bellow that pressure, there is only one transition temperature, from solid directly to gas. The liquid phase cannot exist, so there is no boiling.

Solids still have a vapor pressure, though. So they will evaporate over time. It simply takes a very, very, very long time if the temperature is low enough.
 
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Maybe "boiling point" isn't the right word. I was just saying that vapor pressure is not zero ever.

Particles leave solids too unless there are not enough energy quanta in the entire solid to remove even 1 particle from the lattice. Then you're stuck. But by then, the Temperature is ~0k for any solid made of a large number of particles.
 
  • #8
S_Happens
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Oh and this is my first posting so be gentle with me.

Thanks pjwt

Are you considering a second post? Distillation is actually how I make my living, so I am interested in what it is that you are doing, regardless of whether you care about others talking about sublimation and such.
 
  • #9
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Are you considering a second post? Distillation is actually how I make my living, so I am interested in what it is that you are doing, regardless of whether you care about others talking about sublimation and such.



Unfortunately it got a little complicated for me; I did get a very succinct replay from “S_Happens” which was most helpful.
I am self employed and I work in the industrial sector and have a selection of work (problems) one being a Still for distilling “cleaning solution”.
I was informed that this Still was faulty and in need of repair!
On checking the operation of this very well made, no doubt very expensive item! It could: - achieve a maximum temp of 160C. I could achieve a vacuum of about -0.9 Bar (you guys don’t like this -0.9 bar as a description of vacuum do you ) when I could not find any obvious faults with this item, I enquired when was the last time it did actually work satisfactory, apparently it has not worked since the solution that was being distilled was changed to a different solution, now the original solution distilled was Methylene Chloride and this has a boiling point of well below 50C now the new solution has a boiling point of about 193C (both at atmosphere) so if I was unable to increase the temperature of the Still I would just increase the vacuum hence the original post.
Now when I observe the solution in the still when at 160C and a vacuum of -0.8 to -0.9 bar it is bubbling (maybe boiling) but it hardly distils any liquid.
Now I understand that if the Still was loaded with Methylene Chloride which boils at say 50C would certainly be very effective compared to a solution which boils at 193C.
The heating elements used to heat the solution are not far off 24 Kw and I ran this Still all day (8hrs) and didn’t decant more than 5 Lts ! I’m sure it would not be viable to run this still with such a poor output.
There is much I don’t know about the solution they are trying to distils and the company providing the solution only tell me that the main bulk of the product boils at 193C they are unable to tell me what the evaporation is in KJ/Mol
And finally I should say that I have 3000lts to distil at some point!
 
  • #10
S_Happens
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I was, am, and will continue to be S_Happens.

Ok, so you're distilling a mixture to produce a solvent as your distillate. Do you know what your new mixture is? You cannot simply rely on the boiling point of the distillate you want. Mixtures can create interesting properties, azeotropes, surprising distillates due to activity coefficients, etc.

Like I said, I work in the chemical industry, doing distillation (no personal vacuum distialltion). If you can provide more details about the still itself, such as how the vacuum is produced, how many stages (if it's packed I don't really care about how many theoretical stages), approximate size, condensing medium, I'm sure we can figure it out.

Based on what you've said so far, my concentration would be 1) condensing 2) how the vacuum is produced/where it is measured 3) what the mixture is and whether it might have any unexpected properties.
 

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