| Pages:
1
2
3 |
Dr.Arz
Harmless
Posts: 26
Registered: 14-10-2014
Member Is Offline
Mood: No Mood
|
|
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.
|
|
|
Oscilllator
National Hazard
Posts: 659
Registered: 8-10-2012
Location: The aqueous layer
Member Is Offline
Mood: No Mood
|
|
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.
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
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]
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
Metacelsus
International Hazard
Posts: 2539
Registered: 26-12-2012
Location: Boston, MA
Member Is Offline
Mood: Double, double, toil and trouble
|
|
Yes, Magpie is right. An ideal (theoretical) fractional distillation has no heat lost from the column to the outside.
|
|
|
Actinium
Hazard to Self
Posts: 67
Registered: 2-9-2014
Member Is Offline
Mood: synthetic
|
|
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?
|
|
|
DJF90
International Hazard
Posts: 2266
Registered: 15-12-2007
Location: At the bench
Member Is Offline
Mood: No Mood
|
|
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.
|
|
|
Actinium
Hazard to Self
Posts: 67
Registered: 2-9-2014
Member Is Offline
Mood: synthetic
|
|
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.
|
|
|
DJF90
International Hazard
Posts: 2266
Registered: 15-12-2007
Location: At the bench
Member Is Offline
Mood: No Mood
|
|
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.
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
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... 
| 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]
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
| 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]
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
| 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.
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
| 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". 
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
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.
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
S.C. Wack
bibliomaster
Posts: 2419
Registered: 7-5-2004
Location: Cornworld, Central USA
Member Is Offline
Mood: Enhanced
|
|
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.
|
|
|
forgottenpassword
Hazard to Others
Posts: 374
Registered: 12-12-2013
Member Is Offline
Mood: No Mood
|
|
In practice, insulation is often needed with compounds that boil above 100C, since heat loss prevents them from reaching the still head.
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
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.
Yes but that doesn't fundamentally change anything.
[Edited on 18-10-2014 by blogfast25]
|
|
|
forgottenpassword
Hazard to Others
Posts: 374
Registered: 12-12-2013
Member Is Offline
Mood: No Mood
|
|
I dunno - it changes whether the column is insulated or not!
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
A minor correction:
R=L/D
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
S.C. Wack
bibliomaster
Posts: 2419
Registered: 7-5-2004
Location: Cornworld, Central USA
Member Is Offline
Mood: Enhanced
|
|
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]
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
Correct, but I've seen the inverse being used too. V = D + L is the more important equation here.
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
True but it doesn't change the reasoning here used.
|
|
|
S.C. Wack
bibliomaster
Posts: 2419
Registered: 7-5-2004
Location: Cornworld, Central USA
Member Is Offline
Mood: Enhanced
|
|
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]
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
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:
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]
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
| 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]
|
|
|
Little_Ghost_again
National Hazard
Posts: 985
Registered: 16-9-2014
Member Is Offline
Mood: Baffled
|
|
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
|
|
|
| Pages:
1
2
3 |
![[*]](images/xpblue/default_icon.gif){kind=icon}
