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spoofy
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[*] posted on 21-6-2009 at 14:04
fractional distillation



I'm trying to do a basic fractional distillation using water since it is my first time.

The problem is that the water is not coming over to the small flask. In fact it is just condensing in the fractionating column and dripping back down .. before it even seems to make it to my 3 way adapter. Why isn't the water condensing in the condenser instead?


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bfesser
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[*] posted on 21-6-2009 at 14:05


Insulate the column. Water is high boiling, and so will condense relatively easy with only air cooling.
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BromicAcid
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[*] posted on 21-6-2009 at 14:09


Follow above suggestion to help, or add more heat, it can take a lot of energy to force your distilate over through the fractioning collumn. Also, even though you are using a fractioning column, if you're not collecting fractions, is it really a fractional distillation?



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[*] posted on 21-6-2009 at 15:15


Quote: Originally posted by bfesser  
Insulate the column. Water is high boiling, and so will condense relatively easy with only air cooling.


insulation was enough to push it over. thanks
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[*] posted on 21-6-2009 at 15:52


Insulating the column will allow the distillate to come over, but it is not an ideal solution. Insulating the column will decrease it's efficiency, and that is not good for real fractionating.

I think what you are using is too large a column for the amount of liquid that you are distilling. Or perhaps, not enough heat on the flask.

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watson.fawkes
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[*] posted on 21-6-2009 at 16:01


Quote: Originally posted by Paddywhacker  
Insulating the column will allow the distillate to come over, but it is not an ideal solution. Insulating the column will decrease it's efficiency, and that is not good for real fractionating.
Fractionating depends upon the establishment of a thermal gradient between the vaporization end and the condensation end. Insulation will shift the burden of cooling from the walls of the column onto the condenser. If the condenser can reject heat at an adequate rate, you haven't lost efficiency. If your condenser is underpowered (meaning, insufficient rate of heat energy removal), then you will lose efficiency.
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[*] posted on 22-6-2009 at 14:21



Quote: Originally posted by watson.fawkes  
Fractionating depends upon the establishment of a thermal gradient between the vaporization end and the condensation end. Insulation will shift the burden of cooling from the walls of the column onto the condenser.
That's right.
Quote: Originally posted by watson.fawkes  
If the condenser can reject heat at an adequate rate, you haven't lost efficiency. If your condenser is underpowered (meaning, insufficient rate of heat energy removal), then you will lose efficiency.

Nope. Efficiency is a is not a measure of how well you condense the distillate, but of how well you separate the fractions, and that depends on the temperature gradient along the fractionating column, which you will lose by insulating, as you said in your first sentence.
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[*] posted on 22-6-2009 at 14:25


I think I'm missing something here...what fractions are you collecting?
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watson.fawkes
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[*] posted on 22-6-2009 at 16:52


Quote: Originally posted by Paddywhacker  

Quote: Originally posted by watson.fawkes  
If the condenser can reject heat at an adequate rate, you haven't lost efficiency. If your condenser is underpowered (meaning, insufficient rate of heat energy removal), then you will lose efficiency.

Nope. Efficiency is a is not a measure of how well you condense the distillate, but of how well you separate the fractions, and that depends on the temperature gradient along the fractionating column, which you will lose by insulating, as you said in your first sentence.
I wasn't talking about efficiency of condensation at all. You'll lose efficiency if your condenser is underpowered because the temperature at the condenser rises above the condensation point, reducing the temperature difference between the boiling flask and the condenser, and thus lowering the temperature gradient. As long as your condenser is adequately powered, the temperature difference is governed by the temperature of the lowest boiling fraction.

As long as your condenser remains at a fixed temperature, you lose no gradient by insulating the column. You have one temperature at the condenser, another at the boiling flask. There's a continuous function of temperature connecting those two temperatures. The temperature gradient is the first derivative of that temperature function. If you're column is insulated (assume infinitely well), then the only heat exchange is up and down the column. Assuming a column of constant cross section, this yields a linear gradient.

If you have heat leaking out the sides, that's heat that's not available for vaporization, lowering the number of theoretical plates of the column.
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[*] posted on 22-6-2009 at 17:26


I think you are mistaken. The fractionating efficiency ... the number of plates ... depends in part on the temperature gradient between the top and the bottom of the fractionating column.

At the top of the column the take-off vapour spills into the condenser.

The separation is done in the column, not in the condenser, and so depends on the gradient along the column. The condenser is irrelevant except that if its hold-up volume is large then small fractions will mix and smear as they pass through.
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watson.fawkes
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[*] posted on 22-6-2009 at 17:40


Quote: Originally posted by Paddywhacker  
I think you are mistaken.
Nope. I looked it up before replying, just to make sure.
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[*] posted on 22-6-2009 at 19:39


You have got to be yanking my chain. You cannot believe that separation of the fractions takes place in the condenser.

The very best separation of fractions takes place when there is no take-off into the condenser and the lowest-boiling fraction is condensing at the top of the condenser and the lower-boiling fractions are separated out along the column in a refluxing equilibrium.

Then, to achieve practical separation the vapours at the top are bled into the condenser at such a slow rate that the equilibrium established on the column is maintained. Bleeding off at too high a rate will have vapour rushing up the column disturbing the temperature gradient and smearing the fractions.

Read a bit more. And think about it.
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watson.fawkes
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[*] posted on 22-6-2009 at 20:31


Quote: Originally posted by Paddywhacker  
You have got to be yanking my chain. You cannot believe that separation of the fractions takes place in the condenser.
I don't. And I have no idea how you came to believe that. Really, none.

We were talking about insulation, right?
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[*] posted on 23-6-2009 at 04:00


Woo, science battle. I love these

It's agreed, fractionation occurs in the column.

Gentlemen, removing egos from the debate for a minute, is it possible you're talking about two different definitions of "efficiency". I don't understand the fine point that the argument seems to be over.

I was taught (and I may be wrong) that insulating the column is bad, but you must guard against gusts of air. I had a rather large column (40L or so scale), and had to insulate the steel column with a loose sleeve of plastic, so that gusts of wind didn't disrupt the column.

Mossydie, I think spoofy was just doing a trial run with water to get the hang of fractioning. It is quite tricky, as you have to achieve an steady-state system in the column, with a little bit of your distillate coming over into the condenser.

[edit - thanks watson, I replaced the misleading "equilibrate" with "steady-state", I did not mean to imply equilibrium ^_^ ]

[Edited on 24-6-2009 by Ramiel]




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[*] posted on 23-6-2009 at 04:13


Paddywhacker is correct. Separation occurs in the column. Too much heat, and the column will take off (and flood, if its packed), ruining the separation.

Insulation does affect the gradient in the column. Insulation keeps the heat in the column all the way until the top. If you wait for the vapor to reach the condenser without letting it lose heat on the way up there, it is too late, as the heavy keys will have already come over the top. Best separation occurs when equilibrium is established, with the tops at the light key bp and the bottoms at the heavy key bp.

[Edit]: I was also taught that columns rarely need to be insulated on the bench scale. For water, you can definately do without it.

I've only ever seen pilot scale and larger columns being insulated.

[Edited on 6/23/2009 by Saerynide]




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[*] posted on 23-6-2009 at 05:03


This same question and the resulting nonsense about noninsulated columns being more efficient was already answered and discussed in another thread on the same topic.
Most commercial 29/32 wide distillation columns come with a vacuum jacket insulation. I wander why?

The temperature gradient along the column must depend on the vapour/liquid equilibration process if you want the column to work well. Loosing the heat along the column is just deteriorating this equilibrium, causing the vapours to condense at a faster rate as they can evaporate (thus a reflux which can cause column flooding).
The temperature at each point must correspond to the boiling point of the mixture with the composition at that same point. In other words, the temperature gradient must correspond to the composition gradient. And that's why you need good insulation for better efficiency.




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watson.fawkes
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[*] posted on 23-6-2009 at 05:51


Quote: Originally posted by Saerynide  
Insulation does affect the gradient in the column. Insulation keeps the heat in the column all the way until the top.
This is true only in an out-of-steady-state condition. It keeps heat from leaking out of the column, that true. But thermal loss from the walls of the column is not the only way that the column loses heat. The condenser itself is a heat extraction device. Heat goes in the bottom into the boiler and comes out the top from the condenser.

A column in a steady-state heat flow condition is not in a state of thermal equilibrium. Thermal equilibrium is the state where everything is the same temperature. I know it's ordinary usage to say that a column under total reflux is in equilibrium, but it's not true in the physics sense of the word equilibrium. Thermal equilibrium is impossible in a column because operating a column means putting a both a heat source and a cold sink in it. What we have here is modeled thermodynamically in what's called the "quasi-static approximation", where you take all the first derivatives of temperature as constant (and thus all the second derivatives as zero). This isn't the case when the column is taking off product, but the approximation also works when the second derivatives are small.

And there's no magic discontinuity in temperature at the condenser, either. Condensate at the temperature of the condenser drips onto the packing, cooling it. The top of the column, at steady-state, is close to the temperature of the condenser. The bottom of the column, at steady-state, is close to the temperature of the boiler.
Quote:
If you wait for the vapor to reach the condenser without letting it lose heat on the way up there, it is too late, as the heavy keys will have already come over the top. Best separation occurs when equilibrium is established, with the tops at the light key bp and the bottoms at the heavy key bp.
That's right. My point is that there are two sinks to which the vapor can reject heat: the condenser and the walls. You only need one of them. With a packed column, you can do without the heat loss through the walls. In certain cases you can do without the condenser and just use the walls. That's essentially what a Vigreux column does (*). The best fractionation, however, happens with insulated walls.
Quote:
I was also taught that columns rarely need to be insulated on the bench scale.
That's also right. When the bulk of your heat rejection is through the condenser, the heat loss through the walls makes little difference.

(*) Correction: I should have said "That's what a Vigreux column does when it's run without a condenser."

[Edited on 23-6-2009 by watson.fawkes]
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[*] posted on 23-6-2009 at 05:53


I agree with Nicodem. (I may be crazy, but not dumb enough to disagree with him.)

When I worked in industry, all the columns were always insulated. Little columns, big columns, Vigreux columns, packed columns, bubble columns, spinning band columns. As Nicodem says, the best columns come with a vacuum jacket. Sometimes they were heated with heat tapes, for very high boilers.

If you don't believe us, just check Vogel, pg 95 of the 3rd edition:

"Even slight heat losses considerably disturb
the delicate equilibrium of an efficient column, and almost perfect thermal
insulation is required for the separation of components with boiling
points only a few degrees apart. Theoretically, the greatest efficiency
is obtained under adiabatic conditions."

Adiabatic means zero heat loss or gain.

This is not cutting edge science. It's pitiful how many questions that have known answers are debated endlessly back and forth when a quick check of an authoratative reference would settle the issue.
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[*] posted on 23-6-2009 at 11:37


Watson: you seem to be referring to having the condenser mounted vertically above the fractionating column. The usual arrangement for distillation is for there to be something like a 180 degree bend at the top of the column with the condenser mounted in parallel. It isn't possible for the condenser to affect the functioning of the column. We may indeed have been talking about different things.

The information from the other posters gives me pause for thought. It is at odds with my understanding of things, but I can feel a paradigm shift coming on.

For sure, I have insulated columns myself when I wanted just to get the distillation going.

And I have seen research setups with heated columns ... but my understanding was that heating or insulating the column to decrease its temperature differential was for fine-tuning the efficiency of the column.... something I cannot now think of a reason for doing.
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watson.fawkes
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[*] posted on 23-6-2009 at 14:00


Quote: Originally posted by Paddywhacker  
Watson: you seem to be referring to having the condenser mounted vertically above the fractionating column.
I was referring to the situation where there's a fractionating head in any configuration that takes part of the condensate and returns it to the column and takes the rest of it out as product. But my argument is true even if the condenser is only in vapor-phase thermal contact with the top of the column. The only difference is the thermal resistance between the condenser and the top of the column. As a rule, liquid thermal transport yields a much smaller temperature difference between the condenser and the top of the column than does vapor transport.
Quote:

And I have seen research setups with heated columns ... but my understanding was that heating or insulating the column to decrease its temperature differential was for fine-tuning the efficiency of the column.... something I cannot now think of a reason for doing.
Let me ask you a diagnostic question. When you say "temperature differential", what are you specifically referring to? It sounds like there are two temperatures that aren't the same. What two things are those?
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[*] posted on 23-6-2009 at 16:29


temperature differential - the difference in temperature between the bottom of the fractionating column and the top. That which is diminished by insulating the column.

A fractionating column is like a whole series of distillations. If there is no differential then it is if they are all taking place at the same temperature. If there is a differential then it is like a series of distillations at steadily increasing or decreasing temperature. Both will give separation... but which is optimal?
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entropy51
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[*] posted on 23-6-2009 at 16:43


If there's no temperature differential between the bottom of the column and the top of the column then there will be no separation. You will just have a conducting Pyrex pipe instead of a fractionating column.

You two could have figured all this out if you'd been reading a few books instead of slugging it out.

Both of you are half right, but the whole is less than the sum of its parts.:D
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watson.fawkes
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[*] posted on 23-6-2009 at 17:19


Quote: Originally posted by Paddywhacker  
temperature differential - the difference in temperature between the bottom of the fractionating column and the top. That which is diminished by insulating the column.
How is it diminished because the column is insulated? What is the temperature difference between the top of the column and the condenser?
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[*] posted on 23-6-2009 at 18:46


I have to wonder how many CAREFUL fractionations that the people arguing against lagging have done. Higher temperature ones are difficult without actual heating of the column (due to imperfect insulation) in my hands.

Quote: Originally posted by Paddywhacker  
Insulating the column will decrease it's efficiency, and that is not good for real fractionating.


In volume 4 of Techniques of Organic Chemistry (the 835 page distillation volume), the author is so thoroughly full of shit that pages 345-353 are entirely dedicated to the subject of insulating columns for ordinary fractional distillation in the laboratory. From the first paragraph:

"Internal reflux resulting from excessive heat loss may under some conditions bring about a slight increase in fractionating efficiency, but such results are probably fortuitous to a large extent."

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[*] posted on 23-6-2009 at 19:13


Quote: Originally posted by watson.fawkes  
Quote: Originally posted by Paddywhacker  
temperature differential - the difference in temperature between the bottom of the fractionating column and the top. That which is diminished by insulating the column.
How is it diminished because the column is insulated? What is the temperature difference between the top of the column and the condenser?


Watson, if the column is operating at maximum efficiency, with the lightest vapour condensing at the top and with no takeoff into the condenser then the condenser is irrelevent. Heating it a bit more to get takeoff isn't going to suddenly make it relevent.

A fractionating column is like a whole series of distillations. If there is no temperature gradient along its length from it being insulated then it is if they are all taking place at the same temperature, so you will get separation. If there is a temperature gradient then it is like a series of distillations at steadily increasing or decreasing temperature, again, giving separation... but which is optimal?
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