ExothermicReaction
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Effect of boil rate on distillation?
After last nights Sandenmeyer I extracted the result with a little over a 1L of DCM (220mlx5). So I've got the DCM and ~100 mL of gamma-butyrolactone
in a 2L boiling flask. DCM comes off at about 40C and GBL requires around 200C. Given the wide difference in temp, is there any reason not to crank
the heat and get the dcm to a rolling boil? Is there any benefit to bringing it over slowly?
I plan on distilling at atmosphere until I get to about 150C then I'll add vacuum and pull the GBLs boiling point down as close to that as I can.
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Sauron
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Just the fact that you have 160 C difference in the bp'd does not mean you will get no GBL in your DCM, particularly as the DCM gets less abundant.
NEVER rush ANY distillation.
If you interpose a short but efficient column between the pot and still head you will get a very good seperation with a SLOW boilup.
Watch your vapor temperature at the still head. When you get to about 80 C you can figure almost all the DCM is over and very little GBL.
FRACTIONATE the rest with a 10:1 reflux ration, after equilibrating a better column, and take off anything that comes over below 50 C.
Reject that fraction as mostly DCM. Now take off everything from 50-100 C, reject it too.
Not Take off everything from 100-175 C.
What is left in the pot is almost pure GBL.
Let is cool to RT. Now fractionate carefully under vacuum. DCM will come off without any heating.
Calculate the bp of GBL at your system's pressure, and collect GBL at +/- 5 C of that temperature. Upon a careful redistillation you should get the
product boiling over only a 2-3 C range.
A rotavap would be a good way to strip DCM from GBL, so would a Kugelrohr.
[Edited on 1-12-2007 by Sauron]
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ExothermicReaction
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Thanks! Thats a better summary of the general principles of distillation than I've read from most of the books I've been reading lately.
I'm assuming the extra zero on the threshold temperature difference it a typo?
My present concern is not with distillation in general but for a specific distillation which embodies a particular case of distillation where there
are only two components in the solution and where their boling points are separated by a wide amount.(>100C in this case). In such a situation, is
there any benefit to a lengthy column equilibrium or even to a slow boil for that matter? Considering that total yield is not a factor since
everything will come out, the measure of effectiveness is.
1. Speed. I'm looking for the shortest time to the desired product.
2. Purity of the two components after distillation.
Why not use a procedure like :
Determine the minimum boiling point of the higher bp fraction. (200C for GBL)
Determine the max boiling point for your lower bp fraction (~44 for DCM)
Heat the solution to somewhere safely below the min bp of the higher fraction and let the bottom fraction boil off rapidly knowing that as long as the
temp stays below your safety margin you must be getting completely pure distillate of your lower bp fraction since there are only two compounds in the
solution.
The only issue I can think of, with what I know of distillation so far, is that by greatly exceeding the bp of the lower bp fraction, we could cause
the lower bp compound to break down due to excessive heat.
Are there principles I'm not accounting for in this model?
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Sauron
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You can plot the vapor-liquid equilibrium for a two hase system like this and see exactly that a single theoretical plate gets you.
If you dislike theory, then run the fast distillation you want then test the dcm for presence of GBL and you can then calculate how much GBL you are
losing. If acceptable fine, if not then do it right.
If the bp of DCM is 40 C and the bp of GBL is 200 C then the difference is 160 C. The threshold for a decent 1-plate seperation (simple distillation)
is a differtence of 100 C. My point was and is, the fact that yours is a difference of 160 C. is not a license to toss good procedure out the window.
The use of a column without a reflux control head is not really fractionation. It is just slightly enhanced simple distillation.
Fractionation is so slow and tedious not only because of the time required to equilibrate the column, but more significantly, the reflux ratio. For
example a RR of 10 means you return 10 drops of distillate to the column for every drop you take off. Therefore, this takes 10 times longer than 100%
takeoff and 2X longer than a 1:1 RR. Right? But, the overall seperation is akin to ten seperations of a single theoretical plate.
There are some column types that allow very fast boilups while achieving very good seperations, but they tend to be expensive pieces of glassware.
With any kind of column you have to avoid flooding of the column, and this speaks to not only the boilup rate but the amount of free volume in the
column. This why for example, Raschig rings are far better than glass beads as packing. Berl saddles are better than either, the smaller the better.
[Edited on 1-12-2007 by Sauron]
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ExothermicReaction
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Ok, I'm shutting things down for the night. I finally got a good chunk to come over at 158C which is inline with the 21 mm/HG I'm pulling. I've got 3
different flasks with the results of 3 temp ranges. The final one is the result of that 158C run. (After which the the dripping stopped and the temp
dropped to 120C. The other 2 are the upper and lower range from about 70C to 150C. I figure tomorrow I can do another distillation with these. Think a
vigurex will do the trick? Also, tonight I used a 400mm West . Think I'd have better success tomorrow with a 200mm? I've been wondering how condenser
length effects things. Tonight, I had to add vacuum to even get off all of the dcm. My best guess is that its because I was using a large 2l flask and
there was so much heat dissipation on the top that it just refluxed for a long time. The last bit of dcm just kept condensing on the flask and
dropping back in. I tried to insulate it with aluminum foil, but finally just added vacuum and did the math to figure where I should stop. Any
thoughts?
BTW, in this case, I would gladly give up 5-10% yield if I can significantly speed up the process.
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not_important
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Sauron got it - if you don't know the vapor-liquid equilibrium curves for your mixture then you can't safely say that a rapid simple distillation
won't bring over a significant portion of your product. Unless the bubble and dew curves are nearly right angles, some amount will.
Those curves are related to co-distillation, of which steam distillation is a subset. The classic example is the separation of mixed nitro-phenols by
steam distillation. The para and meta compounds interact with other molecules of themselves and with water, the ortho form likes to play with itself
rather than interacting with water and co-distills even though its boiling point is 217 C. Another example the the co-distillation of caffeine with
DMC. Kerosene hydrocarbons and metallic mercury have both been used to co-distill very high boiling compounds.
Another thing that can happen with very vigorous boiling is the carryover of droplets of the pot liquid, contaminating the distillate.
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stoichiometric_steve
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about rapid distillation...i had to painfully learn that you cant crank up the heat to speed up distillations infinitely.
i distilled an amine (about 50ml) and didnt have much time, so i decided to speed things up by setting the heating mantle to its highest power level.
things worked fine for 2 minutes (after cranking up the heat), then the head temperature dropped suddenly and nothing came over. a few minutes later,
a ,much higher boiling water-white, very viscous liquid came over.
this was the most expensive demonstration of the fact that bonding energy can be very close to vaporisation energy, sometimes so close that the
product polymerizes. this is what happens in the fractionation of crude oil, and you end up with something indistillable that is sold as "vaseline".
so take my advice, instead of rushing things, shut em off and continue another time...it will save you money and grey hair!
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Sauron
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You might consider chromatography to determine if and when fractions are pure.
GC if you have it and TLC if you don't.
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Magpie
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From ExothermicReaction:
| Quote: |
Are there principles I'm not accounting for in this model?
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Some reading up on theory might indeed be helpful here. Specifically take a look at Raout's law. For ideal solutions it will tell you what the vapor
composition is that is in equilibrium with a given liquid composition. Even if the components have bp's that are more than 100C apart the vapor will
still contain some of the higher boiling component. That is why fractional distillation with a good reflux ratio is so helpful. And a good
reflux ratio necessarily slows things down. Even with simple distillation (1 equilibrium stage) you can get some reflux if you just slow everything
down. The extra time and effort you put into this really just depends on your expected product purity and yield.
[Edited on by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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ExothermicReaction
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Thanks for all the insights! This helps a a lot. I ended up redistilling 3 of the 4 fractions collected the other night - this time much more slowly
and I recovered ~ 70% more product than I had the first time. A few things that helped things progress more smoothly this time :
1. I brought the temperature of my room up.
2. I wrapped my flask and take-off adapter in a couple layers of aluminum foil.
3. I used a 200mm condenser rather than a 400. (Not positive of the effect, but it seemed to work really well)
The first time, the temp pegged at 40C for a long time. I think this is because my flask was giving off heat way too quickly and creating a total
reflux. Heating the room and wrapping the flask helped. Also, I kept the rate down to 1 or 2 drops a second, sometimes less. I decided against using
my Vigreux column since I wanted to get a simple distillation pegged as a baseline before complicating the procedure.
The resulting distillate was further reacted with NaHCO3 under reflux and then evaporated yielding a crystalline solid that has been proven to be
quite pure by qualitative bioassay. :-)
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smuv
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I can't see how a condenser could have an effect on a fractionation.
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ExothermicReaction
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smuv : Yeah, I'm not saying that it did. I should have phrased the last post more clearly. I'm not sure of the effect of condenser path length on
distillation, and its been something I want to understand but haven't come across it in my research. Any idea what the effect of short vs. long
condenser may be?
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Sauron
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As long as the condenser is adequate to condense all the vapor, additional length is not required.
In technical terms, the capacity of the cooling system has to equal the heat input to the pot. Usually this is expressed in calories (kcal),
With a low boiling component like DCM it would be relatively easy to lose some through an inadequate condenser.
Rule of thumb is that your condenser water should be at least 20 C cooler than the lowest boiling component. So, assuming yoiu are using tap water,
just make sure it is always <20 C and you will be OK.
A West condenser is better than a Liebig, but you probably know that.
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ExothermicReaction
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Thanks Sauron. I just finished a second reaction and tomorrow I'm going to try and pull out more product than the first time. After reading this
thread, I'm quite sure that I dumped a decent amount of GBL back into my DCM bottle since I pulled the DCM over too rapid. This time, i did a 2 mole
reaction (up from 1) so I'm going to have a fairly large amount of DCM to deal with (~2.2L). It will take a really long time to distill out the dcm at
a 'proper' rate of 1 or 2 drops a second. The Faraday in me wants to be meticulous and go for the highest yield, but the Edison is looking for
efficient, economical production and time is expensive. I imagine there is curve of product loss as the distillation rate goes up. I'm wondering if
this is linear or logarithmic, or something else? I learned last time that if I pull overf the DCM by boiling it rather rapidly, I will lose 35%-40%
off of the baseline 70% yield that others have reported. How about if I incrementally speed up the distillation. Do you think if I were to bring it
over at 3 or 4 drops a second I would lose more than 10%?
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