TOTM, 3/07: Column Stills

[jeff]

The first installment of Straightbourbon.com's "Topic of the Month" series will deal with column still design and function. Though the theory behind distillation is relatively simple to understand, in practice it is a complex process to ensure quality, safety and maximum yield. Let's use this month to discuss how column stills work, how they can be manipulated, their general construction and maintenance. So, start typing and tell us what you know about column stills.

Sound off

[TNbourbon]

...So, start typing and tell us what you know about column stills.

Sound off

They're built in a column (I think ). Okay, take it away, guys (and gals).

[Gillman]

On the plate of each column, a distillation occurs and reoccurs as the distillate condenses and falls back through the column. Steam pushing up through holes in the plates vaporises the alcohol. The more plates and higher the column, the more complete the separation of the ethanol will be. In effect, each chamber of a column still operates like a pot still. Originally the column stills had only a few plates and were primitive, e.g. the three-chamber beer still (still used to distill Armagnac and some other foreign liquors). Later they became sophisticated and larger. Chemical engineers work out the computations to design column stills and it is a question of how much throughput you want at what energy cost at what purity.

These calculations can be very complex.

Generally one run through a modern "beer" still is not enough to cleanse the liquor of undesirable congeners (impurities which can flavor the liquor). So they run it through another still, a thumper still or doubler still in U.S. practice. This increases proof by about 5-10 points and achieves the required congener level.

Column stills also can be accompanied by a rectification tower in which purity is increased through condensing the vaporised liquid from the column still and drawing off (or "purging") the undesirable secondary elements such as fusels and aldehydes.

An extraction still sometimes is used to achieve this.

Remember this: most of the undesirable congeners come off above the boiling point of alcohol. So the more often you boil the alcohol off first, the more the bad fusels stay back with the water. However, some bad fusels including methanol lift off before the alcohol. So you must separate that part out and re-distill it. That is the foreshots, the former are the aftershots. You want broadly a "middle cut", where the ethanol is.

You could link 20 pot stills to achieve what a column still does but the loss of ethanol and enormous energy output make that inefficient.

In a different life I want to be a chemical engineer.

Gary

[T47]

Are all Bourbons Column Still products? I recall from some of the Irish whiskey threads the topic of Column vs. Pot Still comes up again and again.

When I tried to find out some information searching the net, it seemed that the prevailing opinion that I could find was that the Column still was created as a less maintenance intensive process and that the Pot Still offered a fuller flavored product? I recall there was some educated disagreement with that theory.

Is there some pairing of Column vs. Pot Still bottles that one could try that might best reflect the process's influence on the final product or in reality does the process not effect the end product significantly?

I don't want to get off topic and start a Pot vs. Column Still debate, just curious about the influence of the process in general.

[TNbourbon]

...Is there some pairing of Column vs. Pot Still bottles that one could try that might best reflect the process's influence on the final product..?..

Woodford Reserve Four Grain (100% pot still) and early-batch standard Woodford Reserve (100% column still, as Old Forester 'honey barrels').

[jeff]

You want broadly a "middle cut", where the ethanol is.

I understand why you would want to remove the "heads" and "tails", but how much is that as a percent of the whole run? And I am really curious as to how this is accomplished when the still is being run continuously?

[Gillman]

I am not sure about the percentages of material held back as heads and tails. But heads and tails there are definitely, even in a column still. It does not really run continuously. When mash goes through the first vapours condensed are re-distilled as being too high in methanol. Ditto for the last vapours before shut-down and cleaning. The cycle is longer than for pot stills but column stills are cleaned out too.

Gary

[barturtle]

I understand why you would want to remove the "heads" and "tails", but how much is that as a percent of the whole run? And I am really curious as to how this is accomplished when the still is being run continuously?

I think what Gary is saying (and it seems to work in my head, though that is no guarantee of accuracy) that by distilling to the proper temperature, the tails are always left behind (being too heavy to be distilled at this low temp). Then by running it through the doubler at a much lower temperatures the heads are taken off (i.e. they don't actually redistil the whiskey, but run the still-a doubler is a still, basically-at just high enough of a temp to lift off the heads.)

I had never thought about how they take the heads and tails off in a continuous process, but this seems to be an appropriate way to get it done...whether this is how it is actually achieved, someone else will have to confirm.

[jeff]

Here is a link

to a PDF file on the distillation and denaturing of alcohol. On Page 63 begins a discussion of modern distilling apparatus. A few pages in you will find several diagrams of distilling plates and column still design. Page 73 has an interior diagram that is helpful.

The page numbers refer to those printed on the document, not the actual PDF page number

[jeff]

Gary made me think about something I thought was relatively obvious, but now I'm not sure:

Is a continuous still really continuous? I assumed this thing could run for hours on end with new beer being added all along. Is this the case, or is it simply run in batches and cleaned in between? If so, how much is a batch and how long does it take to run?