One of the simplest choices distillers have to make in their initial planning process is also one of the most impactful: what type of still to acquire and use.
With some high-tech exceptions, still types can be broken down into a few primary types. Here, we compare pot and column stills, as well as the difference between batch and continuous distillation.
Pot Distillation
The simplest version of a still is a pot still. It includes a vessel that’s heated to drive off vapor, a way to allow that vapor to escape and be collected, and a way to condense that vapor back into liquid form.
Because of the simplicity of the method—vaporize and condense—products made using a pot still tend to have more varied constituents in them. Pot-distilled spirits tend to be a rougher mix containing a higher level of congeners—that is, all the nonethanol compounds. They can include very light fractions, such as methanol, acetone, and ethyl acetate, as well as heavy, richly flavored compounds, such as fusel oils and heavier alcohols.
Pot distilling is a coarser approach to distilling, and the distiller is less able to separate out very light, low-boiling compounds from the hearts—which consist primarily of ethanol—or to easily separate the hearts from the heavy, high-boiling tails. Separate fractions tend to bleed into each other, with less-defined boundaries.
This is because the amount of reflux that the vapor experiences in a pot still is relatively low. Reflux is the phenomenon where distillate vapor condenses, falls back into the liquid mass, and vaporizes again. Reflux allows for additional separation in the distillate because the heaviest compounds condense more readily, while portions of the lightest compounds remain in the vapor phase and continue to pass through the still.
Pot stills often have a helmet attached to the top of the chamber, and the helmet increases reflux to varying degrees. The helmet may be straight, ball-shaped, or in a lantern style that tapers from a wide base to a narrower top. After the vapor leaves the pot—or, if present, after it passes through the helmet—it enters the lyne arm. The lyne arm collects the vapor to be condensed, and it can also impact reflux. Lyne arms on different stills may be horizontal, sloped upward to increase reflux, or sloped downward to minimize reflux.
Pot-distilled spirits often require multiple distillations to clean up adequately, generally with a stripping run that collects all the heads, hearts, and tails, followed by a finishing run that lets the distiller make heads and tails cuts. These spirits are often barrel-aged to allow for the medley of complex—and, in some cases, undesirable—flavors to mature and evolve.
Pot distillation also can be used with neutral or very light spirits that are being redistilled with botanicals and other ingredients, for products such as gin or genever, aquavit, and absinthe.
Pot distillation is a type of batch distillation. In batch distilling, the distiller fills the still with a charge of spirit or fermented media, conducts the distillation, and empties the still before filling it again. The distilled batches have distinct starts and stops.
Column Distillation
Another type of batch distillation is based on that approach, but it’s designed to maximize reflux.
Batch column stills can produce lighter or neutral spirits using a series of plates that increase the rate of reflux. A batch column still can be built in combination with a pot still, producing a hybrid pot-column still—a design that’s popular among craft distillers who want flexibility in their production operations.
Batch column stills begin, as do pot stills, with a chamber that contains the still’s charge. Vapor may or may not pass through a helmet before entering the bottom of the column. As the vapor travels upward, it passes through a series of plates—as few as three or four and potentially up to 20 or even 40—that are designed to increase reflux.
The most common plate designs include the sieve tray and bubble cap. On a sieve tray, a series of holes allows vapor to pass upward, while the condensed liquid can flow downward—although that downward flow is restricted by upward vapor pressure. In a bubble-cap plate, the vapor passes up through openings around the perimeter of a metal cap, which forces the incoming vapor to condense and collect in the tray; a pipe known as a downcomer allows excess liquid in the tray to travel down to the previous tray.
As the vapor passes through each plate, it becomes more refined and more separated by component, according to their individual boiling points. This lets the distiller separate out high proportions of congeners and impurities and to take very specific fractions of the distillate. A column still with enough plates, managed carefully, can be used to produce neutral spirit at 95 percent or higher ABV.
Column distillation can be conflated with continuous distillation because continuous stills rely on a column design. However, continuous stills operate on different principles and rely on a steady stream of incoming media that is distilled as it comes in, with a steady waste stream flowing out of the bottom and a flow of distillate coming from the top. Because of the inefficiencies of starting and stopping continuous stills, they generally run for days or weeks on end. (While not a part of the still itself, in many instances, continuous distillation relies on a separate beer well or other buffer tank; fermented media go into it after fermentation, waiting to be used for distillation.)
As the spirit or other fermented media initially enters the upper half of the continuous still, it falls downward and meets the steam being injected near the bottom of the still. As the steam travels upward through the alcoholic input, it strips the volatile compounds and carries them upward.
Continuous distillation can be used for multiple purposes. Whiskey or rum distilleries that aim to preserve the pot-distilled character of their products may use a continuous stripping still for their initial distillation, then redistill those low wines again in a finishing still. Vodka distilleries or those that focus on producing neutral spirit, on the other hand, can use a continuous still—or a set of continuous stills in series—to take fermentation media all the way to 95 percent ABV or higher in a single pass.
In some continuous vodka columns at the craft scale, distillers also collect a separate flow of heads. In other cases, they allow its vapor to vent to the atmosphere. Very large, industrial ethanol plants sometimes have distillation arranged in sets of three or more column stills, designed to produce highly refined ethanol and specific side-streams of fermentation by-products, such as acetone, that are used in other industries.
Ultimately, the products distillers plan to make will help determine the type of still that would be the best fit—and, likewise, that chosen still will impact which spirits distillers produce and how they’re made.