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Author: Subject: Insulation on fractional distillation?
Dr.Arz
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[*] posted on 15-10-2014 at 15:58
Insulation on fractional distillation?


Can fractional distillation be improved by insulating the column, or does it do the opposite, I mean, ruin the whole idea? Is it necessary to let the vapors cool when they travel up? I tell straight that I am not sure how the theory goes, so I must ask. When using long fractional tubes, like 1-2 meters high for maximum purity and efficiency.
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[*] posted on 15-10-2014 at 16:16


This depends on how strongly you are heating the flask. If the fractional distillation is going just fine (i.e. all the high b.p. stuff is being condensed) and you insulate the column then the component with the high boiling point will not have time to condense fully, and so it will come over with the low boiling point component and ruin the distillation.

On the other hand if you are distilling two substances with reasonably high boiling points then perhaps everything will be condensed before it reaches the still head, so your distillation won't actually be going anywhere. In this case you should insulate the column to allow the vapors to reach the top of the column and separation to occur.

In general the longer the column the more effective the separation, because there is a longer distance over which the higher b.p. substance can condense. However there is a trade-off because the longer the fractionating column the slower the distillation for a given heat input.
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[*] posted on 15-10-2014 at 16:17


My understanding is that the column needs to be adiabatic, ie, neither losing nor gaining heat.

I have a lab procedure manual that shows how to build a heating jacket for a column to prevent it from losing heat. I have a 600mm column that I use for difficult fractionations. During my last use I insulated it with pipe insulation available from my hardware store.

Quoting from Laboratory Text in Organic Chemistry, by Cason and Rapoport, (1950):

"Since the fractionating zone in a column should be as nearly adiabatic as possible, the column must be either heated or insulated or both."

[Edited on 16-10-2014 by Magpie]




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[*] posted on 15-10-2014 at 17:39


Yes, Magpie is right. An ideal (theoretical) fractional distillation has no heat lost from the column to the outside.



As below, so above.
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[*] posted on 15-10-2014 at 22:32


Does your column have plates or are you planning on packing it?
It's safe to assume that a column you pack, and what you use and how you pack it is going to have an outcome on how it behaves and how you would approach the situation to keep it all functional?
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[*] posted on 16-10-2014 at 00:24


As magpie correctly states, the column should be adiabatic. More expensive columns are vacuum jacketed and silvered for this reason. Some people here believe that this makes the column useless (I think the term used was "like a steam pipe"...). Clearly these people are unaware that for any real fractionation to occur you need a reflux ratio, provided by a splitting head. (Baffcat sells these on ebay for £99, excluding the new sciencemadness 10% discount...) The condensed liquid is returned to the column, and liquid/vapor equilibrium is established. Volatiles in the condensed phase are re-volatilised by the hot vapor, and less volatile components in the vapor phase are condensed by the cooler liquid phase. This establishes a thermal gradient along the length of the column, allowing separation of the components by boiling point. The column should be equilibriated at full reflux for a period prior to collecting distillate to provide this thermal gradient.
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[*] posted on 17-10-2014 at 08:46


DJF90- " The column should be equilibriated at full reflux for a period prior to collecting distillate to provide this thermal gradient." Does the length of time to reach equilibrium depend on the Column length? Diameter? Both? what you are distilling? or any of/all the above?
Its increadibly fascinating how something so ancient and simple as distillation has so many intricacies involved.
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[*] posted on 17-10-2014 at 09:05


Vogel has a section on this, and I direct you to his text for details. From whatI recall the column is operated at full reflux until a stable minimum head temperature is acheived. This corresponds to the best separation possible of the most volatile component (that the column can provide...). Distillate is then collected whilst maintaining a good reflux ratio (9:1 or so...). Clearly there are variables that dictate how the fractionation should be performed, e.g. ease of separation, and these should be treated on an individual basis.

There is a paper in J Chem Ed that instructs how to determine the number of theoretical plates for a column at full reflux, using a mixture of chloroform and benzene. I can't provide the reference right now as its buried somewhere on my pc, but some searching should yield results (and the paper can be requested in the "references" section.
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[*] posted on 17-10-2014 at 11:47


Quote: Originally posted by DJF90  
As magpie correctly states, the column should be adiabatic. More expensive columns are vacuum jacketed and silvered for this reason. Some people here believe that this makes the column useless (I think the term used was "like a steam pipe"...). Clearly these people are unaware that for any real fractionation to occur you need a reflux ratio, provided by a splitting head.


The term 'like a steam pipe' was used by me and trust me, I'm NOT confused about the need for a splitter. But it appears others remain so, despite positive affirmations to the contrary.

So lets try an work this out for once and for all, shall we?

MOST people here, including myself, do not have a splitter head and yet use fractionating columns all the time, with great success.

Case I:

If you don't have a splitter head and run your column adiabatically you are simply NOT FRACTIONATING ANYTING, because all you do is send vapour through your perfectly insulated column, without any downward flow. Without mass exchange between upward vapour flow and downward liquid flow no fractionation can take place.

Consequence: if you don't have a splitter head, rely on NATURAL condensation of vapour, due to heat loss of the column. In plain English: in that case DON'T or POORLY insulate your column.

Case II: you do have a splitter head. Well, in that case, in order to control reflux ratio PERFECTLY also insulate your column PERFECTLY, as in that case the downward liquid flow is purely defined by the splitter head.

Conclusion: running the column adiabatically or not depends on whether you have a splitter head or not. But even with a splitter head not running perfectly adiabatically isn't necessarily a problem: it only means you won't control reflux ratio as well as you would in adiabatic conditions. And most distillers don't obsesses about controlling reflux ratio perfectly: they just want to fractionate well... :P

Quote: Originally posted by DJF90  
Vogel has a section on this, and I direct you to his text for details. From whatI recall the column is operated at full reflux until a stable minimum head temperature is acheived. This corresponds to the best separation possible of the most volatile component (that the column can provide...). Distillate is then collected whilst maintaining a good reflux ratio (9:1 or so...).


Vogel is not the best guide on this although what you quote is largely correct. Reflux ratios of 9:1 or so aren't practical in industry though: you'd hardly produce anything.

Instead do some searches on McCabe Thiele, to better understand how fractionating columns actually work.

A big problem here with those here who quote on adiabaticity is that they quote out of context: an adiabatically run column only works in tandem with an effective reflux controller, i.e. a splitter head.

I refer also to this recent thread where with a simple Vigreux without insulation and without splitter ethanol was rectified from less than 20 % to over 80 %, by a beginner (with my help):

http://www.sciencemadness.org/talk/viewthread.php?tid=33065#...

That result was also compared to an identical distillation but without Vigreux.

[Edited on 17-10-2014 by blogfast25]




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[*] posted on 17-10-2014 at 12:04


Quote: Originally posted by Actinium  
DJF90- " The column should be equilibriated at full reflux for a period prior to collecting distillate to provide this thermal gradient." Does the length of time to reach equilibrium depend on the Column length? Diameter? Both? what you are distilling? or any of/all the above?
Its increadibly fascinating how something so ancient and simple as distillation has so many intricacies involved.


All of the above.

Today's distillation technology has evolved enormously with respect to 'ancient and simple distillations'...

[Edited on 17-10-2014 by blogfast25]




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[*] posted on 17-10-2014 at 12:57


Quote: Originally posted by blogfast25  

A big problem here with those here who quote on adiabaticity is that they quote out of context: an adiabatically run column only works in tandem with an effective reflux controller, i.e. a splitter head.


You are probably referring to me. My quote from Cason & Rapoport indicated that the fractionating zone of the column should be as adiabatic as possible. This did assume that liquid reflux was provided from the top of the column (use of a splitter).

I agree that some condensation has to take place at the top of the column so that there is a liquid flow down through the column.

I think your Case 1/Case 2 analysis is correct - well done.




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[*] posted on 17-10-2014 at 13:07


Quote: Originally posted by Magpie  
[You are probably referring to me. My quote from Cason & Rapoport indicated that the fractionating zone of the column should be as adiabatic as possible. This did assume that liquid reflux was provided from the top of the column (use of a splitter).

I agree that some condensation has to take place at the top of the column so that there is a liquid flow down through the column.

I think your Case 1/Case 2 analysis is correct - well done.


Not only you, Magpie. In the discussion DJF90 refers to and which left a really bad taste in my mouth, others came up with the adiabatic requirement too. As said, without some source of condensate being sent down the column, perfectly insulated columns simply don't actually do anything. It's like sending steam up an insulated pipe. There are quite a few that don't seem to understand that simple fact.

Perfectly insulated columns with perfectly functioning and accurate splitters perform, all other things being equal, better than more primitive systems, I think that goes w/o saying. But look at moonshiners: from 12 % EtOH to 80 - 90 %, not too shabby, I say. Don't talk to these people about "reflux". :D




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[*] posted on 17-10-2014 at 13:41


For normal fractionations which aren't too challenging, say 30°C separation in bp or greater, I use my 200mm fractionating column, uninsulated, w/ broken glass or ss scrub pad packing.

For the challenging fractionations like carbon tet/chloroform (16°C of bp separation) I use a packed 600mm column, insulated, with a splitter head. I would likely also use this setup for separating fatty acids from a natural product like saponified coconut oil.




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[*] posted on 17-10-2014 at 14:04


IDK what this splitter of which you speak needs to be, but the implication is that standard student glassware sets are incapable of fractionation if the column is insulated, period...Well, above the packing is a little more column and an adapter, a thermometer...and if the packing is insulated and even heated, and the bp's not particularly close, quite much of this reflux is in the column regardless of no lost heat, depending on how much of what you've got going.

It should be enough to say with ordinary glassware to maintain boiling and go for the reflux ratio dictated by the HETP of the column, insulated from the top of the flask to the top of the packing. It works. Really.




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[*] posted on 18-10-2014 at 02:21


In practice, insulation is often needed with compounds that boil above 100C, since heat loss prevents them from reaching the still head.
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[*] posted on 18-10-2014 at 05:33


Quote: Originally posted by S.C. Wack  
IDK what this splitter of which you speak needs to be, [...]


That explains rather a lot. Example:

http://www.sigmaaldrich.com/catalog/product/aldrich/z413097?...

I'm beginning to wonder if you know what reflux ratio actually means (see below).

Quote: Originally posted by S.C. Wack  
but the implication is that standard student glassware sets are incapable of fractionation if the column is insulated, period...[...]


True. If nothing forms a condensate, there no liquid phase and no fractionating.

Quote: Originally posted by S.C. Wack  
Well, above the packing is a little more column and an adapter, a thermometer...and if the packing is insulated and even heated, and the bp's not particularly close, quite much of this reflux is in the column regardless of no lost heat, depending on how much of what you've got going.


If that 'little more column' is not insulated it will act as a primitive condenser but it won't allow you to control reflux ratio with any degree of accuracy.

Quote: Originally posted by S.C. Wack  
It should be enough to say with ordinary glassware to maintain boiling and go for the reflux ratio dictated by the HETP of the column, insulated from the top of the flask to the top of the packing. It works. Really.


Yes, if you have a splitter to send condensate back down, see top of this column:



V is the vapour flux (say mol/s) flowing through the (insulated) column. C is a condenser/splitter which splits the condensate into D (mol/s, distillate) and L (mol/s) the liquid flux you're sending down the column to cause fractionating.

R is the reflux ratio, V = D + L the mass balance at the top of the column.

The splitter is often a timed valve: for 'x' seconds it will send all condensate down the column, for 'y' seconds all condensate is collected as distillate. R = y/x.


Quote: Originally posted by forgottenpassword  
In practice, insulation is often needed with compounds that boil above 100C, since heat loss prevents them from reaching the still head.


Yes but that doesn't fundamentally change anything.

[Edited on 18-10-2014 by blogfast25]




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[*] posted on 18-10-2014 at 06:07


I dunno - it changes whether the column is insulated or not!
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[*] posted on 18-10-2014 at 06:48


A minor correction:

R=L/D




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[*] posted on 18-10-2014 at 07:04


Quote: Originally posted by blogfast25  
if you have a splitter to send condensate back down


In the simple glassware world, this is known as a thermometer bulb. I'm not saying this is perfect or wise, it is what can be done without whichever thingy you want to attach. The heat can be adjusted so that there is boiling and the reflux occurs, much of this at the top of the column and which you can see, which does not resemble what you have in mind or ever attain a reflux as you would like it. I'm not saying it's faster or as perfect as anyone would like, or how an engineer would design things.

[Edited on 18-10-2014 by S.C. Wack]




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[*] posted on 18-10-2014 at 11:24


Quote: Originally posted by Magpie  
A minor correction:

R=L/D


Correct, but I've seen the inverse being used too. V = D + L is the more important equation here.




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[*] posted on 18-10-2014 at 11:26


Quote: Originally posted by forgottenpassword  
I dunno - it changes whether the column is insulated or not!


True but it doesn't change the reasoning here used.




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[*] posted on 18-10-2014 at 14:57


Quote: Originally posted by blogfast25  
Quote: Originally posted by S.C. Wack  
IDK what this splitter of which you speak needs to be, [...]


That explains rather a lot.


Quote: Originally posted by S.C. Wack  
In the simple glassware world, this is known as a thermometer bulb


I'm going to have to conditionally retract this, since instead of watching football like I should be doing, I'm writing and looking for a distillation "splitter" and still having no idea WTF. So I'm not calling anything a "splitter". Sigma-Aldrich may be the only manufacturer of what you're talking about calling it that or something, because what Ace sells as a splitter...IDK what to do with that...What I'm saying is, how much does the desired item cost? If you're running a 500 mm Vigreux, anything over a what, 5:1 ratio is useless...PS hmmm actually the reflux ratio can be changed using the top of the column and the adapter as a condenser, yes maybe not quite to the point of total reflux if you want your distillate today: a bag of water, just like how condensers used to be.

[Edited on 18-10-2014 by S.C. Wack]




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[*] posted on 18-10-2014 at 18:14


As blogfast shows in his sketch a splitter is nothing more than a condenser that can route part of the condensate to the top of the column. Splitters generally have the ability to vary the amount returned thereby allowing one to set the reflux ratio.

There are many different splitter configurations. I have the Hennion type as shown below:

Hennion head.jpg - 81kB

The male end at the lower left connects to the column head. The two tublatures at the top are for the cold-finger type condenser cooling water. The male end at the lower right connects to the condensate receiver. The lower stopcock is used to set the reflux ratio. The other valves and tublatures are for use in isolation during vacuum distillations, I believe.

As to cost, yes, they are very expensive. I would not have purchased it except for the deep discount offered by Dr. Bob.

One certainly doesn't have to have one of these, as you say. But for difficult separations I would wager that most professional chemists do.



[Edited on 19-10-2014 by Magpie]

[Edited on 19-10-2014 by Magpie]

[Edited on 19-10-2014 by Magpie]



[Edited on 19-10-2014 by Magpie]

[Edited on 19-10-2014 by Magpie]

[Edited on 19-10-2014 by Magpie]

[Edited on 19-10-2014 by Magpie]




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[*] posted on 19-10-2014 at 04:39


Quote: Originally posted by Magpie  
Splitters generally have the ability to vary the amount returned thereby allowing one to set the reflux ratio.

There are many different splitter configurations. I have the Hennion type as shown below:


That's correct and lucky you to have one of those. I agree that for difficult separations of significant amounts of stuff it's more or less indispensable.

In the case of your Hennion gadget, clearly you can set reflux with it but do you actually know the value of the reflux ratio? Methinks not but please correct me.

Clearly if that bottom valve is closed you run at total reflux but if that bottom valve is fully open, does all condensate flow in to the distillate receiver? It's not clear from the photo.

As pointed out a much simpler geometry uses a timed valve. An overhead condenser on top of the column condenses all of V, then directs the condensate for 'x' seconds back into the top of the column and for 'y' seconds in to the distillate receiver. I've seen this used on a pilot plant rectifying column for high grade methanol I used to work on some 30 years ago, as a student.

The cost of these splitters is a problem (although the Sigma gadget probably isn't very representative). Someone above mentioned that eBay (UK) seller 'baffcat' sells a splitter for £99 but I could not find the item.

I'm also interested in SC's alternative that he wrote about a couple of posts ago but haven't really found anything. What he called a 'thermometer bulb'...


[Edited on 19-10-2014 by blogfast25]




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[*] posted on 19-10-2014 at 06:16


I looked at vogel 3rd ed and the whole setup with variacs etc would be outrageously expensive to set up,
I did try to make my own with nichrome wire in sections wrapped around a column and temp sensors for each section. the whole thing microcontroller controlled.
The problem I had was this gave me a max temp of around 120c, also it melted parts of the insulation.
I have no idea if my other solution is considered any good or not but I have settled for the following.....
I have a clasian adapter at the top of the column, in the first port I have a small cold finger that I can lower or raise around 5cm each way, I get reflux on this and it seems to work ok.
Obviously if I could control the temp of the fluid used in the cold finger it would work alot better, but so far I havnt bothered. When I do any of my distillation I mess about until I get a fixed temp coming over, if it starts wondering slowly up now and then I lower the finger until it stays at a fixed temp.
All I can really offer is I made chloroform with IPA, I got chloroform come over at bang on its BP, then it climbed 2-3C after around 20 mins, I returned this to the flask and lowered he finger a bit, I did this several times and ended up with chloroform with a bang on BP.
I also ended up with 4 other distillates that I have no idea what they are! One day with better equipment I will analyze the buggers and see how good my method was.
A clasien adapter is pretty cheap and at a push I would use two thermometers one to reflux (ok a bit piss poor) the other to show the temp going into the condenser.

I am expecting to be told this is a bad way, but I did what I could with what I have. I have some ethanol screen wash to distill that should sort the men from the boys lol Lets see if I can get ethanol using that lol
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