In general, column stills are tall—and that’s where the generalizations end.
Beyond that common characteristic, there is a wide range of designs and methods of operation, including column stills that make vodka, those that make whiskey or rum, those meant for stripping, and others used for single-pass distillation.
Here’s a brief rundown of the most common designs and uses.
Batch Distillation
For distillers on the smaller end of the spectrum, batch column distillation through a series of plates is the most common.
This type of column requires a chamber—which may be a functional pot still in a hybrid setup—to contain the distillate and a set of plates that can number from just a few up to 20-plus plates.
The number of plates dramatically impacts the product being distilled. On the lower end, a few plates can help to produce a slightly more refined whiskey; more plates can create a higher-ABV distillate for light whiskey or white rum; and the realm of 20-plus plates is the domain of vodka and other neutral spirits.
There are some standard plate designs, and their purpose is always to increase reflux in the column. The two most common styles are the bubble cap and the sieve tray.
Photo by Gabe Toth
In a bubble-cap plate, the vapor travels up into the plate through an opening in the floor of the plate, contacting a cap suspended over the opening, so that the vapor then condenses and collects in a tray. A downcomer pipe allows excess condensate to flow down into the plate below.
In a sieve tray, the vapor passes upward through a series of holes or slots in the tray, condensing as it contacts the distillate already condensed in the tray. The liquid in the tray can flow downward, but the resistance from the vapor flowing upward holds it in a careful equilibrium.
As the distillate gradually moves from one plate to the next, the process is effectively redistilling and refining it at each plate. When conducted slowly, carefully, and with enough plates, column distillation lets you create distinct strata of highly concentrated fractions of the constituent components in the distillate. The heads cut is smaller and highly concentrated, while you can refine the hearts to the required 190 proof for vodka production.
This process relies on the distiller to work carefully to create equilibrium in the column. If the vapor flow is too high, it will flow through the plates without properly condensing and re-evaporating; this results in a less distinct separation of fractions and a lower ABV in the collected distillate. That can occur if you don’t charge the column slowly, or if a surge of heat suddenly increases the vapor flow in the system, disrupting the balance that you worked to create.
Another way to increase reflux in a column still is to use a dephlegmator. This is a condenser at the top of the column, and it cools the vapor that’s poised to run off for collection. Instead, the cooling condenses the vapor and sends it back down into the series of plates for further refinement. As the distiller, you can choose to run the dephlegmator aggressively, preventing any distillate from flowing over—for example, to reach equilibrium at the start of a run or to re-establish it in case of disruption—or at varying levels during the run to keep the system in balance.
Continuous Distillation
Another type of column still is designed for continuous distillation.
This type of still has no pot or chamber; instead, it relies on a steady flow of incoming media, which is distilled as it enters the still. Steady streams of distillate and waste come out of the top and bottom, respectively.
The feed for a continuous still may be previously distilled spirit or a fermented feedstock. There are multiple uses for a continuous column, including:
- a stripping run
- pairing it with a doubler or thumper (see below) to make single-pass whiskey or rum
- using it by itself or in a series with additional columns for single-pass neutral spirits or other high-proof products.
While a continuous still doesn’t rely on a distinct pot or chamber the way a batch still does, it requires a separate vessel—that’s the beer well—which holds the distillate or fermented material to be pumped steadily into the continuous still. You must carefully calibrate both the ABV and the rate of flow to ensure the proper function of the continuous column.
As the distilling medium feeds into the upper half of the continuous still, steam that’s injected near the bottom travels upward through the input flow, stripping the ethanol and other volatile compounds and carrying them upward. The design of the column and its operating specs depend on its intended use, with a desired fraction removed at a defined point farther up the column. At the very top of the column, the heads compounds can vent into the atmosphere, or the distiller can collect them via a separate flow.
Because continuous stills are inefficient to start and stop, distilleries often run them for days, weeks, or even months without stopping. They’ll continually replenish the beer well and, as long as the system continues to operate within the desired specifications, it can keep running except for periodic downtime for cleaning and maintenance.
Industrial-scale producers of neutral spirits might use an array of columns to strip, rectify, purify, and re-rectify distillate, then recover ethanol from the impurity streams (i.e., heads and tails). Very large operations even collect enough heads to sell to adjacent industries, which separate them into constituent compounds, such as isopropyl alcohol, for industrial or commercial use.
Many large American whiskey producers opt to run a continuous still plumbed in-line with a doubler or thumper to provide a second round of distillation on the same run. As the stripped spirit vapor comes off of the continuous column, it enters a secondary chamber. If that vapor condensed before entering the chamber, it’s called a doubler; if it doesn’t condense first, it’s called a thumper because of the sound the vapor creates when condensing and evaporating again. The spirit entering this chamber carries with it the inherent heat energy of its initial distillation; that energy accumulates in the thumper or doubler, and the lightest fraction of the entering distillate passes back out, ready to be proofed down and barreled.
Whiskey distilleries that seek the best of both worlds—the efficiency of column distillation combined with the richness and small-batch character of pot distillation—use a column still for stripping. In that kind of system, the column still can continuously strip whiskey feedstock—the mash or wash—steadily collecting low wines to be pot-distilled separately.
Continuous column stills can be valuable for distillers who have various needs and batch sizes. Understanding how to operate one, and where it might fit into your distillery’s production plan, can help you make the most of your resources or take the next step up.