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Why is glycerin so hard to distill?
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I understand that you can't simply distill glycerin, but I don't comprehend why that is.

I understand that glycerin will decompose if heated before it will boil at normal pressures.

So one would have to use a vacuum still. At low pressures its boiling point approaches 170 C, and it shouldn't decompose at those temps Even 190 C is not crazy high as far as boiling points go.

So why couldn't I raise the temperature of glycerin at near vacuum to 190 C and let it boil off just like water would?
 
Registered: March 16, 2008Reply With QuoteReport This Post
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You can. Thats how they used to purify glycerol in the old days when soap makers sold their glycerol off to the dynamite factories. However nowadays they use expensive high tech stuff like wiped film evaporators to distil it, which are much more efficient and gives a purer glycerol.
 
Location: Scotland | Registered: March 19, 2006Reply With QuoteReport This Post
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That makes sense.

I assume that the impurities that carry over with this form of distillation (as opposed to the modern high-tech form) are mostly water and other substances with boiling points lower than glycerin.

Thanks for the response.
 
Registered: March 16, 2008Reply With QuoteReport This Post
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The water would boil off pretty quickly, then youd get quite a good purity of glycerol - I think about 90-95%. You can then distil it again if you want purer stuff. You can buy equipment - at a price - to get high grade stuff. I'm thinking of companies like EET (Eetcorp)who use some kind of membrane technology, and Artisan who make a thing called a rototherm which uses evaporation/distillation.
 
Location: Scotland | Registered: March 19, 2006Reply With QuoteReport This Post
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quote:
Originally posted by Jehu:
The water would boil off pretty quickly, then youd get quite a good purity of glycerol - I think about 90-95%.


If it's not water or MeOH, what is the other 5-10%? Just curious.
 
Registered: March 16, 2008Reply With QuoteReport This Post
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quote:
Originally posted by FuriousGeorge:
quote:
Originally posted by Jehu:
The water would boil off pretty quickly, then youd get quite a good purity of glycerol - I think about 90-95%.


If it's not water or MeOH, what is the other 5-10%? Just curious.

Salts, Methyl Esters, Mono, Di & Triglycerides, Soap and any other organic impurities that come through the transesterification reaction. It all depends how the byproduct stream is pre-treated before entering distillation. Using vacuum distillation you can get a lot higher than 95% but there is a lot of energy used.
 
Location: East Yorkshire | Registered: January 14, 2006Reply With QuoteReport This Post



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quote:
Originally posted by Fuzznag:
quote:
Originally posted by FuriousGeorge:
quote:
Originally posted by Jehu:
The water would boil off pretty quickly, then youd get quite a good purity of glycerol - I think about 90-95%.


If it's not water or MeOH, what is the other 5-10%? Just curious.

Salts, Methyl Esters, Mono, Di & Triglycerides, Soap and any other organic impurities that come through the transesterification reaction. It all depends how the byproduct stream is pre-treated before entering distillation. Using vacuum distillation you can get a lot higher than 95% but there is a lot of energy used.


You said 'salts' which confuses me. Salts like NaCL or K3PO4 or Na3PO4 have a much higher boiling point than glycerin. I think I see the mix-up, and it's my fault for being unclear.

I'm talking about the glycerin condensate from and old-school still.

Let's say the glycerol byproduct is acidified and separated, and only the layer which is mostly Glycerin/Methanol/Water/MONG/Soluble Salts goes to the still (perhaps you would want to remove at least the MeOH first, but bear with me):

Assuming you had a low-tech still as I described above, what kind of purity would you expect the glycerin condensate to be at?

This message has been edited. Last edited by: FuriousGeorge,
 
Registered: March 16, 2008Reply With QuoteReport This Post
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The reason you can't distill glycerol normally is because its boiling point 290C is higher than its decomposition temperature 280C.

You either have to distill with steam or use a vacuum.
Vacuum is probably easier than steam in the backyard or kitchen, and you don't have the dilution problem.

I tried using vacuum to dry glycerol recently. The glycerol was only at 80C with a 15 inch vacuum and air bubbling in from the bottom. A lot of soap fell out which surprised me, before I started I had according to SG about 60% glycerol after I am guessing about 80% because the hydrometer was off the scale. Dissolved stuff would obviously give false readings.

According to Dow Chemicals Glycerol has a vapor pressure of 64mmHg at 100C, that's about 29" of vacuum. I think that by raising the temp I can get it to boil at a more achievable vacuum. If I ever buy a new deep fryer I will steal the old one and us it as an oil bath to increase the heat and give it a try.
 
Location: Nimbin Australia | Registered: December 04, 2007Reply With QuoteReport This Post
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quote:
Originally posted by FuriousGeorge:
quote:
Originally posted by Fuzznag:
quote:
Originally posted by FuriousGeorge:
quote:
Originally posted by Jehu:
The water would boil off pretty quickly, then youd get quite a good purity of glycerol - I think about 90-95%.


If it's not water or MeOH, what is the other 5-10%? Just curious.

Salts, Methyl Esters, Mono, Di & Triglycerides, Soap and any other organic impurities that come through the transesterification reaction. It all depends how the byproduct stream is pre-treated before entering distillation. Using vacuum distillation you can get a lot higher than 95% but there is a lot of energy used.


You said 'salts' which confuses me. Salts like NaCL or K3PO4 or Na3PO4 have a much higher boiling point than glycerin. I think I see the mix-up, and it's my fault for being unclear.

I'm talking about the glycerin condensate from and old-school still.

Let's say the glycerol byproduct is acidified and separated, and only the layer which is mostly Glycerin/Methanol/Water/MONG/Soluble Salts goes to the still (perhaps you would want to remove at least the MeOH first, but bear with me):

Assuming you had a low-tech still as I described above, what kind of purity would you expect the glycerin condensate to be at?

At best I would say 95% probably nearer to 90% in reality.
 
Location: East Yorkshire | Registered: January 14, 2006Reply With QuoteReport This Post
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quote:
Originally posted by Fuzznag:
Salts, Methyl Esters, Mono, Di & Triglycerides, Soap and any other organic impurities that come through the transesterification reaction.



quote:
Originally posted by FuriousGeorge:
You said 'salts' which confuses me. Salts like NaCL or K3PO4 or Na3PO4 have a much higher boiling point than glycerin. I think I see the mix-up, and it's my fault for being unclear.

I'm talking about the glycerin condensate from and old-school still.


quote:
Originally posted by Fuzznag:
At best I would say 95% probably nearer to 90% in reality.


Sorry to keep going around in circles, but I'm wondering what it is about distillation that I don't understand:

I'm trying to guess what is in the byproduct, after acidification and separation of insoluble salts and FFA:

Compound - Boiling Point
K3PO4 - 2000ºC (?)
K2SO4 - 1680ºC
NaCl - 1400ºC
NaOH - 1388ºC
KOH - 1370ºC
Triglycerides - 400º to 800ºC (depending on source)
Stearic Acid - 383ºC
Palmetic Acid - 351ºC
H2SO4 - 337ºC
FAME - 330º to 357ºC
Glycerin - 290ºC
Linolenic Acid - 230ºC
Linoleic Acid - 229ºC
Oleic Acid - 194ºC
H3PO4 - 158ºC
Water - 100ºC
Methanol - 65ºC

If you figure what goes into the old-school still is 85% glycerin and the other 15% are the chemicals I just listed, why would you only get 90-95% from distillation? I believe you, but I don't understand.

I guess I can see how any FFA suspended in the glycerin could be carried over, but at the same time FFA isn't soluble in glycerin, so it wouldn't be much, and I would assume you could mitigate it by carefully controlling temp, and using fractional distillation.

Same goes for FAME.

I would think you would have to be pretty sloppy about it to get much water, MeOH, or H3PO4 in the glycerin condensate.

Sulfuric Acid is the only one I see as really problematic, since its BP is pretty close to glycerin's, but if you pretreated with a little KOH you could be sure to have very little of that (or Phosphoric Acid) going into the still.

Still, both of you guys say 90-95%, and you would know better than me. I'm just trying to say that I don't understand how salts and triglycerides could get into your condensate.
 
Registered: March 16, 2008Reply With QuoteReport This Post
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Besides the usual impurities there are substances like diglycerol and polyglycerol, and sodium salts of glycerol also exist (eg CH2OHCHOHCH2O-Na and related compounds - ie methoxide versions of glycerol).
 
Location: Scotland | Registered: March 19, 2006Reply With QuoteReport This Post
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somebody, maybe me, sould try this out in a old pressure cooker and pull vacuum on it. Add a reflux on top too. It gets more difficult though, you would need to know the vapor pressure curve of each inpurity to make sure it stays out of the gylcerin and a really accurate vacuum gauge because substances' vapor pressures change a lot between 29-30'' HG. I would also really like to try distilling biodiesel, it is supposed to help cold flow but i cant seem to get any awnerrs on how much it helps CP, PP, CCFP.
 
Registered: April 13, 2009Reply With QuoteReport This Post



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There are easier and cheaper ways to winterise biodiesel. Even if you distilled it there's no guarantee that the first fraction will have a low enough CP without further treatment.
 
Location: Scotland | Registered: March 19, 2006Reply With QuoteReport This Post
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What would these easier and cheaper ways be? Besides mixing with diesel/karosene?
 
Registered: April 13, 2009Reply With QuoteReport This Post
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quote:
Originally posted by 2000psdbio:
What would these easier and cheaper ways be? Besides mixing with diesel/karosene?

There are cloud point depressants specifically for biodiesel on the market, such as Wintron and Coldflow. Some people find RUG works well up to a point. I used xylene at one time which I found very good.
 
Location: Scotland | Registered: March 19, 2006Reply With QuoteReport This Post
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First I would like to say this is a interesting read. And I think I am learning something.
Jehu when you used the xylene do you have any info on how will it worked and how much you use per liter?
Thanks
 
Registered: April 03, 2006Reply With QuoteReport This Post
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Astro, I used to get waste xylene from work for nothing but I dont anymore, otherwise I'd still use it. I used about 5%. It did a great job, and it also gave a terrific power boost to the fuel, and of course it was free. I think printers and automotive painters use xylene, so if you use RUG I'd go for xylene instead if you can get it cheaper.
 
Location: Scotland | Registered: March 19, 2006Reply With QuoteReport This Post
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Furious,

I might see where concepts aren't coming together for you.

The problem with impurities in distillation is that evaporation does not happen immediately, completely, at the boiling point of the distillate. Think about how you can see clouds of water vapor rising from a cup of coffee, or sweat evaporates from your skin, both of which are at a temp way below 100c. From another perspective, when you distill wine you don't get pure alcohol, but brandy.

At the boundary between liquid and gas phase of a substance ("the surface") there are random molecules in the liquid phase which have enough heat energy to break their attraction to the other liquid phase molecules and fly off into the gas phase above. (btw, this is why decreased pressure -vacuum- lowers the boiling point of a liquid; there's less "crowding" of the gas molecules). There are also molecules in the gas phase which lose energy, by cooling, and are grabbed by those intermolecular forces and return to the liquid phase. (this is why increased pressure increases boiling point; the gas phase molecules are being "pushed" into closer contact with the liquid.)

This equilibrium, which varies by temperature, is the "vapor pressure" of a liquid and it is a CURVE, not a definite point. So while you're boiling off your methanol at around 65c, there is also water evaporating off as well. probably a bit of glycerin too.

Further complicating the issue is that "systems" of solvents or solutions have different boiling points than their components. For example, adding table salt (sodium chloride) to water lowers the point at which the water boils out of the solution; adding antifreeze to water raises the boiling point.

You can increase the effectiveness of the process of purification by distillation by raising the temperature slowly, and by repeated distillation to decrease the quantity and variety of impurities-- the first distillation should get rid of all the salts, for example, which should make the fractional distillation a little more "textbook".

Hope this helps.
 
Registered: November 03, 2009Reply With QuoteReport This Post



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Hi
Yesterday I tried to distillate the glycerol. I put a 4KW heater at the bottom of a 50 liters stainless still barrel and I put only 15 liters of glycerol. Also I put a vacuum pump and I pump 0,5 bar. The problem was that the glycerol blowed up and it goes to the condenser. Do anyone knows how I can the glycerol not to blow? Thanks.
 
Location: Greece | Registered: August 01, 2010Reply With QuoteReport This Post
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Cycle the 4KW heater, bring the glycerin up to temp much, much more slowly. You want to approach the boil point cautiously, not blow past it and flash-boil everything.
 
Location: Southern WI, USA | Registered: May 18, 2006Reply With QuoteReport This Post
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