Sciencemadness Discussion Board

Why bokakob style columns are not used?

Refinery - 29-4-2020 at 23:57

While introducing myself to alcohol distillation, I came up with the well known bokakob design. It is very simple and effective method of reflux fracional distillation. See picture attached.

Why similar devices are not used in chemistry? Only column distillations without any reflux are employed, and it appears that without reflux it is either impossible or extremely time consuming to separate pure products.

I came up with an idea to use claisen type still head, put small condenser on top of the second head and a packed column under it, above the reaction flask, and apply a valve before the collection tube. This would allow the condensate to flow back to the packed column and cause reflux, and the enriched condensate would collect in the extraction head, which of it flow rate can be adjusted with the valve. I suppose a vacuum could be pulled via condenser and rec flask as per usual due to valve?

BOKA-LAB.png - 43kB

Metacelsus - 30-4-2020 at 05:34

Quote: Originally posted by Refinery  

Why similar devices are not used in chemistry? Only column distillations without any reflux are employed, and it appears that without reflux it is either impossible or extremely time consuming to separate pure products.


No, reflux splitter heads on fractional distillation columns are commonly used. I think there was a post here about them; I'll try to find it.

Edit: See DFJ90's post from this thread: http://www.sciencemadness.org/talk/viewthread.php?tid=38826

[Edited on 2020-4-30 by Metacelsus]

Texium - 30-4-2020 at 05:37

Here you go, a very expensive lab version of what you described!

41C20576-73A8-4586-9479-51731FF94390.jpeg - 28kB

Fery - 30-4-2020 at 23:14

There is a lab glass producer (Kavalier, glass product name Simax) in my country producing these very expensive glass parts:

distillation head approx 400 EUR including VAT
https://www.verkon.cz/hlava-destilacni-simax/


distillation column, jacketed, filled with Rashig rings
https://www.verkon.cz/kolona-destilacni-dle-hempla-s-naplni-...




Recently I ordered 500, 750, 1000 mm long columns without rings as I've already bought 1 kg of Raschig rings quite cheaply.
The distillation head is still on my wish list of what to buy in future.

Fulmen - 30-4-2020 at 23:47

Don't forget that most setups employ an air cooled column, this provides the reflux. It will however suffer from the same limitations as the "bokakob", it's hard to get a specific reflux ratio.

About 25 years ago I had the opportunity to test the coolest still ever. It had a steam powered boiler, a plate column appr 15cm in diameter and maybe 3m tall, and a variable reflux head. 100% glass.

G-Coupled - 1-5-2020 at 01:16

Quote: Originally posted by Fulmen  
About 25 years ago I had the opportunity to test the coolest still ever. It had a steam powered boiler, a plate column appr 15cm in diameter and maybe 3m tall, and a variable reflux head. 100% glass.


In what context did you come across that cool piece of kit?:cool:

Refinery - 1-5-2020 at 01:32

Correct me if I'm wrong, but reflux ratio and efficiency can be improved by insulating the column, preferably with vacuum. I actually planned on tigging an insulating vacuum mantle on my still and boiler to boost efficiency. Thermal radiation and conductive losses increase fuel consumption greatly during long reflux runs.

I thought I would be reinventing the wheel, but I find it fancy that I actually already have the frac setup on hand as I described it. Next, I should test it out. On the other hand, I don't have anything specific to fraction with. For alcohol, I have my full-sized boka, which I can easily use to produce azeo EtOH in much, much higher capacity than any tabletop still. On the other hand, just distilling OTC denatured spirits is the easiest and cheapest way around.

I suppose they use these purpose-built 3m glass columns in pharmaceutical and other specialty industries. My 100cm packing, 75mm dim boka can deliver about a liter of azeo per hour, so I suppose 3000/150 still could do that 5-6 times over.

Fulmen - 1-5-2020 at 02:43

G-Coupled: At a company training facility. You should have seen the look on our faces when we saw that thing :D The reflux head was a moveable baffle that directed the condensation either back to the column or to a takeoff spigot. This was put on a timer to control the ratio. Come to think of it I saw that setup at a local school a few years ago, so it's still around.


Refinery: An overhead condenser and insulated column will of course be more efficient, but a cooled column does work OK. It will of course be harder to maintain a constant reflux ratio, and the column will not be as efficient or have the same capacity.