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Author: Subject: Lab-scale continuous fractional distillation
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[*] posted on 19-4-2018 at 17:45
Lab-scale continuous fractional distillation

Has anyone had any success using a continuous fractional distillation process at a lab scale?

I want to distill a hundred litres or so of a crude/dilute product. Aside from buying/building a bigger distillation setup, or doing countless small batch runs, a continuous process, like used in industry, seems like a good option.

I tried cobbling together a crude setup yesterday (using what limited glassware I have) but honestly it didn't work very well at all. Probably not surprising considering the setup:

Essentially I had a 500ml 2-neck round bottom flask atop my hotplate on its side, with a tubing adapter in the lower neck (to draw off the bottoms) and a 300mm air condenser in the other, extending at an incline. The end of the air condenser was attached to an angled vacuum adapter, and a 300mm vigreux column was attached to this, mounted vertically, followed by a still head w/thermocouple probe, another angled adapter, and a condenser mounted vertically. A dropping funnel was connected with a hose to supply the feedstock, and a siphon was used to withdraw the bottoms.

With this setup I found it very difficult to control the rate of addition of feedstock and takeoff of bottoms. Also, the vacuum adapter in between the two reflux 'columns' tended to flood. I didn't use a completely vertical column setup because I ran out of room, and also it's obviously not desirable to have the feedstock just drip down into the flask rather than slowly go down the walls of the column, and the only thing I had on hand to use was an air condenser. I suppose I could possibly have packed it with something.
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[*] posted on 19-4-2018 at 22:19

Easiest would be to rig a rotavap with two collection flasks which can be removed from the vacuum system without removing vacuum from the main system. This would allow continuous take off. Next would be to connect a straw to the relief valve so that it can be used to suck up more crude product and inject it down the vapor duct without taking the vacuum off.
If that isn't fast enough consider something truly continuous such as agitated thin film or falling film distillation. It all depends on what you want to do (collect the distillate or the residue) and how often you'll be going to do it.
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[*] posted on 19-4-2018 at 23:58

I have 20 litres of kerosene waiting to be distilled,
so I considered continuous distillation and decided that it is too difficult for now,
based mainly on that thinking;

. your 'stripping' section is equivalent to very few theoretical 'plates' - I guess one
. your 'rectification' section is not much better - I guess two 'plates' equivalent

Are three or less 'plates' enough to separate your mixture ?

. ideally the feedstock should enter the junction of the stripping and rectification sections at the temperature that would exist at that point under steady-state full reflux with no feedstock or product flow.
Your setup is introducing cold feedstock - completely upsetting efficient separation.

. most difficult of all is maintaining a state of dynamic equilibrium between feedstock rate and takeoff rate,
if not automatically regulated then a continuous distillation rig will require more continuous attention than a batch system.

. IF I made a continuous distillation setup I would want it to be semi-permanent
to justify the time required to set up and fine-tune - and the cost.

Is your product higher or lower b.p. than the solvent ?

If lower then I suggest that you just get the largest pot that you can
and use a column of sufficient(i) theoretical plates in a batch system.

If higher then maybe a continuous distillation setup would be quicker,
but unless your product is quite valuable,
I doubt that the extra cost would be easy to justify.

(i) Decide on the allowable concentration of product in the pot at the END of the batch run - that will be discarded.
Decide what concentration/purity of product is required.
Use sufficient theoretical plates.

Can you divulge solvent and product b.p. plus initial and desired concentrations ?

When doing chemistry, don't forget basic physics.
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[*] posted on 20-4-2018 at 00:09

Actually the feedstock should probably be added to the bottom. IIRC in industrial setups the feed is often in the middle of the column somewhere, but at a temperature that's in equilibrium with the column. Adding cold, dilute feed to the top would throw everything out of whack, reducing the concentration of the product.

But your biggest problem is probably how to regulate this properly. How did you regulate the takeoff, ideally it should be self-leveling (like a side-takeoff, don't know if a siphon would work with the boiling). Trying to balance feed and takeoff manually sounds like a very difficult task. You also want to measure the takeoff concentration somehow, the simplest would probably be the temperature in the boiler.
If you get this sorted it might be possible to get some useful results. I assume you don't have any automatic regulation, this will of course limit the results somewhat. What's your goal, maximum recovery or maximum purity?

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