At the Tales of the Cocktail convention this year, I moderated a panel with three distillers who run column stills; one 5-column rum still, one continuous bourbon still, and one pot-column hybrid.
Kevin Curtis is the Distillery Operations Manager for Michter's Whiskey. This company is building a new distillery with a continuous column still. He filled me (and the audience) in on how bourbon stills work. The below post is a combination of information gleaned from distillery visits, books, and Kevin's information at Tales.
The Purpose of a Bourbon Still
As opposed to some other types of stills, for bourbon the focus is not on getting a pure alcohol out of the still. By law, bourbon must not be distilled to above 80% alcohol by volume. The focus seems to be more on the continuous and stripping nature of the column still, as opposed to a discontinuous (batch) pot still.
Scotch Vs. Bourbon Distillation
More on that: If you know about single-malt scotch whisky, you'll note that it goes through a copper pot still. However, only liquids go into the still. Grains are fermented into beer, just like in bourbon, but in scotch the solids are separated from the liquid beer before going into the pot stills. In bourbon, the solids and liquids go into the still together (in practice; there is no law about this). In a pot still, one would have to be careful that the solids didn't burn against the still, making it even harder to clean than just scooping out the leftover solids.
A bourbon column still thus performs two functions: It separates the solids from the liquids, and it separates the liquids into mostly-alcohol (to keep) and mostly-water (to recycle).
How Liquid and Steam Moves Inside a Column Still
As you probably know, a column still is filled with perforated plates. The beer to be distilled is pumped in near the top of the column (but not at the very top), and steam is pumped up from the bottom of the column. While the plates are perforated, this lets the steam come up through the still, but the beer does not drip down through these holes. Rather, the beer runs across each plate to the other side, flows down to the next plate and flow across it to the other side.
The steam coming up through the column vaporizes the alcohol from the beer (which then flows to the top of the column), while leaving the water and grain solids to keep dripping down to the bottom. (As in every still, it is tuned so that the alcohol that has a lower boiling temperature evaporates off, leaving the water with its higher boiling point behind.)
The Top Part of the Still
As I mentioned, the beer initially doesn't enter the very top of the still but near the top. Above that point in the still, the steam is being rectified on those plates. It is up there where copper is crucial, and where you find "bubble caps" in stills. More on those:
Column stills can be built from stainless steel but there needs to be interaction with copper at some point. In other stills (as we'll see in a future post), the part of the still where beer is being separated and distilled is stainless, and where the steam is being rectified there is copper.
Bubble caps provide additional refinement of the spirit and increase contact with copper. Some other stills (I learned this at Absolut) are filled at the top with bits of shredded copper or copper pieces that look like jacks for similar reasons.
This also makes it easier to replace the copper at the top of the still or inside the top when needed, rather than the whole still column that can last for decades.
The Doubler or Thumper in Bourbon Distillation
Most bourbon undergoes a second distillation in a continuous pot still called a doubler of a thumper. Sometimes it looks just like a regular pot still. In other distilleries, it looks just like a flat-topped metal container- you wouldn't know it's doing anything.
In a doubler, the vapor off the column still is condensed back into liquid and this is run through the pot still. In a thumper, the vapor itself goes into the still to be redistilled (and makes a thumping sound that I've never heard but I associate with the sound of radiator pipes clanging in East Coast apartment buildings).
This second distillation is needed to raise the proof of the distilled spirit a little further, and this can be done in a pot still because there are no longer any solids to worry about. The waste product of the doubler/thumper is additional water.
The only other place I can recall seeing continuous pot stills was in Jamaica for Appleton rum. According to my tour guide at Jack Daniel's, they do not run a second distillation through a thumber/doubler at all. (see comments for a dispute on this)
Update: I was given permission to post this bourbon still schematic. You'll see that the beer goes into the still at Plate 15, the new-make spirit is condensed and sent to the pot still (doubler), then recondensed before entering the High Wine Tank at the end.
Anyway, that's what I think I know about column distillation in bourbon. If you have any additional questions (or corrections!) please let me know. In future posts, we'll look at other types of column stills and see how they work.