Pages: 1

Author Topic: Question about boiling temperature at reduced pressure in relation to flask size  (Read 103 times)

myhero

  • Pupae
  • **
  • Posts: 80
Question about boiling temperature at reduced pressure in relation to flask size
« on: December 13, 2012, 02:32:16 PM »
I have encountered this issue and I am not sure why this happens.

Sometime ago I distilled 100ml of an oil in a 250ml RBF at reduced pressure, deep vacuum. The main fraction (95%+ of oil) came over at around 30°C. Recently I distilled 800ml of the same exact oil in a 2L 3 neck RBF and the main fraction came over at around 50°C.

I suppose the volume to be evacuated has some influence on the resulting internal pressure of the distillation setup. Am I right or is there another reason?

lugh

  • Global Moderator
  • Foundress Queen
  • *****
  • Posts: 876
Re: Question about boiling temperature at reduced pressure in relation to flask size
« Reply #1 on: December 13, 2012, 03:13:42 PM »
Quote
Am I right or is there another reason?

You might want to check for leaks  8)
Chemistry is our Covalent Bond

myhero

  • Pupae
  • **
  • Posts: 80
Re: Question about boiling temperature at reduced pressure in relation to flask size
« Reply #2 on: December 13, 2012, 04:51:04 PM »
wouldn't it be more unstable if there were leaks?

I distilled the whole thing and the temp range was quite tight

Hawkwind

  • Larvae
  • *
  • Posts: 18
Re: Question about boiling temperature at reduced pressure in relation to flask size
« Reply #3 on: December 14, 2012, 12:42:15 AM »
A larger mantle or oil bath might heat up the distillation apparatus and thermometer more.  And a "stable" leak might simply make the vacuum less effective (higher absolute pressure) and raise the boiling point.
« Last Edit: December 14, 2012, 12:55:35 AM by Hawkwind »

Wizard X

  • Lord of the Realms
  • Foundress Queen
  • *****
  • Posts: 1,224
Re: Question about boiling temperature at reduced pressure in relation to flask size
« Reply #4 on: December 14, 2012, 04:33:27 AM »
A Bleed Valve (aka, needle flow valve) is used on most vacuum pumps to regulate the vacuum required by providing a controllable LEAK. Notice the Bleed Valve with the black knob.



Albert Einstein - "Great ideas often receive violent opposition from mediocre minds."

myhero

  • Pupae
  • **
  • Posts: 80
Re: Question about boiling temperature at reduced pressure in relation to flask size
« Reply #5 on: December 14, 2012, 09:22:29 AM »
Thanks guys, sounds like there was a small leak then..... I ll find out when I try again to distill something with the same equipment

myhero

  • Pupae
  • **
  • Posts: 80
Re: Question about boiling temperature at reduced pressure in relation to flask size
« Reply #6 on: December 14, 2012, 09:51:28 AM »
Well a vacuum gauge and needle valve would definitely help. I ll see if I can manage to add them to the shopping list anytime soon :)

myhero

  • Pupae
  • **
  • Posts: 80
Re: Question about boiling temperature at reduced pressure in relation to flask size
« Reply #7 on: December 14, 2012, 03:38:47 PM »
could be it....but last time i used it ( i use it not so often) it was ok... then i put it in the basement (cellar) which is humid. is perhaps where it got the moisture from. Could it penetrate inside the oil just because it's a humid environment where i store it.

I normally run it for half an hour with the gas ballast open to get rid of said moisture before starting to distill....

lugh

  • Global Moderator
  • Foundress Queen
  • *****
  • Posts: 876
Re: Question about boiling temperature at reduced pressure in relation to flask size
« Reply #8 on: December 14, 2012, 05:51:41 PM »
Chemistry is our Covalent Bond

sassa

  • Subordinate Wasp
  • ***
  • Posts: 120
Re: Question about boiling temperature at reduced pressure in relation to flask size
« Reply #9 on: December 14, 2012, 09:58:42 PM »
What the hell kind of pump do you use to boil "oil" at 30-50 ºC? :o...i thought my last pump was strong as hell when bought...but seems have to be a toy in comparison to yours! ;)
.where are you measuring the temp?i suposse you are checking it adjusting the mercury ball at few mm from the condenser
are you using fractionating column?the same vacuum grease,if used?
are you using exactly the same distilling set-up except flask dimensions?
   I saw several times...of course  putting the thermometer ball at same position ever(few mm besides first portion of condenser) that if one use a fractionating column ,the temp at oil comes over was 10-20ºC more than on simple distillation set-up without fractionating column,..and too...position of thermometer ball upwarsd or downwards only some mm can read diferent distilling temperatures of about few grades at same vacuum.
  I think the empty space of that 2 liter flask is working as a semi-fractionating column,making necesary that 20ºC more to let the oil begins to came over....
  That was some of the reasons i used to distill ketone without column because that incresing on distillation temp...that made often distillation harder and slower...Less purity sure...but less tar formation and ketone stress when vacuum it´s not as strong as prefearable.
  Anyway,..50 º C it´s a way low temp so don´t care at all...
  i doubt too any leak on distillation set-up can work with that consistency...And talking about a so strong vacuum..i think only a minimal leak will increase distillation temp way more than 20ºC!
  Because that,on that kind distillations,...i used to choose ever the smaller flask needed to contain the mix to minimize that issues.

myhero

  • Pupae
  • **
  • Posts: 80
Re: Question about boiling temperature at reduced pressure in relation to flask size
« Reply #10 on: December 17, 2012, 09:50:57 AM »
I have a Leybold D2A pump. bought it used a few years ago, paid 60 EUR. We changed the oil, took it to some place where they checked it and they said it was fine. It sure is fine :)

I ll try reading more of that material you suggested lugh. Some of it have already studied, some not.

myhero

  • Pupae
  • **
  • Posts: 80
Re: Question about boiling temperature at reduced pressure in relation to flask size
« Reply #11 on: December 17, 2012, 09:57:22 AM »
Yes I am using a column. I am placing the thermometer bulb (i am using a thermocouple) just below the point where the distillation head opens towards the side. same vacuum grease. Anyway next distillation I will tell you more.
Pages: 1