Friday, January 18, 2019

The Physics of Batch Column Stills and Bubble Plates

Until recently the graceful pot stills of Scotland were the most familiar images people had when you talk about distilling. Now batch column stills, also known as hybrid, reflux, or Lomond stills, common in the eau de vie and craft distilling industries, are far more recognizable than they used to be. These occupy an important middle ground between simple pot and more complex continuous stills, representing an evolution of the double retort pot still. In the simplest way of thinking about them, batch column stills are a way to create more refined spirits while using a smaller footprint by increasing the number of times that vapor is condensed back into liquid between the pot and the condenser.

New Deal Distillery's hybrid still
I've previous written about the physics of pot stills, which gives important background explaining concepts such as separation/resolution and reflux. Pot stills come in a bewildering array of shapes that are designed to influence reflux, but columns put all of that complexity inside far more plain-looking tubes. All of them operate by putting some kind of material in the path that the vapor travels from the pot to the condenser to create more opportunities for the vapor to condense and flow back towards the pot. The simplest method involves packing the column with small, high surface area items made of copper or a non-reactive material. In the same way that vapor condenses on the walls and flows back into the pot in a pot still, packing a column with material creates even more surface area for that process. Because the surface area is so high in a packed column, the vapor stream will also exchange material with the wetted surfaces in the column, further enriching the vapor stream with lower boiling constituents and depositing higher boiling constituents in the liquid. One of the significant challenges in setting up a packed column is ensuring that there is still enough void space for vapor and liquid to pass through the material without creating an unsafe amount of pressure. Another downside is that packed columns often have to be disassembled and emptied to clean the packing material thoroughly, which can be a significant challenge with larger columns. While these materials give a very high amount of reflux, care must be taken with the amount and type of packing material. A highly packed column may only be suitable for the production of neutral spirits because the product coming off the still will be almost flavorless. Using copper mesh also creates more opportunities for the metal to catalyze chemical reactions, which may be good for reducing sulfur compounds in the vapor, but can also lead to higher levels of acetaldehyde in general or the carcinogen urethane from fruit mashes with high amounts of cyanide in them.

The art of constructing hybrid stills is figuring out how to create a greater amount of reflux than a simple pot still while still producing flavorful spirits. This generally requires some kind of plate design, which has the advantages of letting distillers more precisely control their reflux ratio.

The simplest setup in this category is the sieve plate, which is just what it sounds like - a perforated plate, usually made from copper. The size of the plate and the ratio of holes to surface determine the amount of reflux each plate generates. Basic sieve plates have two significant advantages - first, they are simple to manufacture and thus cheap and second, they can be installed in the column on pivots that allow them to be turned 90º so that they create a minimal amount of reflux. This allows a distiller to tune the amount of reflux in the column for the type of spirit that they want to create, which is analogous to being able to change the shape and height of a pot still. This flexibility is especially important for craft distillers who want to produce multiple types of spirit on the same equipment. A larger multi-plate column can be fully engaged for making vodka, while most of the plates can be disengaged for more flavorful spirits like whiskey or brandy. Alternatively, creative plumbing can allow multiple columns to be used in series and selectively bypassed, so both a short and a taller column can be used for lighter spirits while the shorter column alone can be used for more flavorful spirits.

Sieve plates with sufficiently small holes operate with a layer of condensed liquid on top of them that is kept from falling back through the holes by vapor pressure. If the vapor pressure is not maintained at an adequate level liquid can 'weep' through the holes, reducing the efficiency of the plate. Some plate column stills have sufficiently high reflux ratios that liquid will collect on the plates passively, others require the plates to be preloaded with wash or water before the run, and many will use a dephlegmator, which is a partial condenser at the top of the column, to build up liquid on the plates. The plates will also need something called a downcomer (see diagram at right), which is a tube that allows liquid to drain from one plate to the plate below it. This pipe is built with a fixed or variable amount of height above the plate to ensure that the liquid level doesn't drop to zero. Similarly the lower end of each downcomer pipe is surrounded by a weir, which prevents vapor from traveling up the downcomer and bypassing the plates. The arrangement of the downcomers forces the liquid to flow across the plate from one side to the other, ensuring good contact between the vapor and the liquid as it proceeds back down to the pot. This setup means that vapor passing through each plate will exchange its heat with the liquid, depositing lower boiling compounds in the liquid phase and vaporizing higher boiling compounds to proceed upwards in the enriched vapor. In essence each plate becomes a small pot still, with the vaporization and condensation processes happening multiple times in miniature. This can be seen as an evolution of single- or double-retort pot stills (primarily found in rum distilleries) and thumpers (primarily used in bourbon distilleries) where the output of one pot still is passed through liquid in a subsequent pot still, then some of the liquid content of the pot is passed back to the previous still.

Bubble cap and valve plates are the next step up, meant to more effectively maintain the liquid level on the plate. While there is significant variation in design, all consist of small pipes with caps or valves on top of them. The liquid on the plate is prevented from passing through the pipe by the pressure on the cap or valve, while vapor can flow up and around to pass through into the liquid. This design allows the still to operate at a lower vapor flow rate than a simple sieve plate because the design reduces or eliminates the chance of weeping.

With all of this careful engineering, what's the point? There are any number of factors that can be pointed at, ranging from smaller footprints (no need for a huge pot still to produce light spirits when you can do the same thing with a more compact column), to efficiency (high reflux columns can be run harder than a pot still without loss of separation), to control and flexibility (pot stills only have two axes of control - heat input and condenser cooling water input). A classic example comes from the world of unaged fruit brandies or eau de vie, which were some of the first major users of batch column stills. This is because they found that the products from single pass distillation in batch columns were significantly different than double distillation in simple pots for certain types of fruit. For instance, one study found that while total ester levels were higher in pot distilled cider brandies, the levels of higher alcohols were elevated in the reflux column distillates.

The utility of batch column stills is even more clear for the craft distilling industry. They are faced by an array of challenges stemming from the huge amounts of capital that are needed to start a commercially viable distillery. Batch column stills present solutions to many of those problems. While they are more expensive than simple pot stills, they are far less expensive than continuous stills or multiple pot stills. They can be configured to produce an array of different spirits from the same system, allowing a new distillery to make lighter unaged spirits that can be sold immediately as well as heavier spirits that are designed for aging in casks. The increased efficiency and smaller footprint both help to save money and maximize the utilization of valuable space, especially for distilleries located in urban areas with higher real estate prices. Last, but not least, they allow the dynamics of distillation runs to be radically altered in comparison to pot stills.

Diagram of New Deal Distillery's hybrid still
The combination of a dephlegmator and a plate column allows for almost unprecedented control over how a distillation run proceeds. With a simple pot still, the only choices available are how to input heat into the pot, the rate that cooling water flows into the condenser, and where the cuts are made. While these tools are obviously sufficient to create some of the best spirits on earth, they require a lot of trial and error to perfect. With a batch column still equipped with a multi-plate column and a dephlegmator, a skilled operator can do something completely impossible with a simple pot still - establish, albeit temporarily, equilibrium. With 100% reflux the pot and column effectively become a closed system. The components of the heads will be compressed into the vapor phase near the top of the column, while the tails are all firmly in the pot. By reducing the flow of the dephlegmator a bit, the heads will leave the column in a comparatively small volume with very little alcohol. With further reduction in dephlemator flow the hearts will then come in behind at a constant ABV, unlike the steadily declining ABV of the hearts fraction from a pot still. Further tweaks will also compress the tails fraction so that very little of the fusel oils contaminate the hearts. This allows the creation of a very clean hearts fraction that can be bottled directly or that needs very little aging to round off its remaining rough edges.

An example of this flexibility is Westland Distillery in Seattle. Though they run a standard double distillation process for the majority of their spirit, their wash still is a batch column still. It is primarily used as a pot still with the dephlegmator turned off and open drains (see below) that empty the plates back into the pot, giving a greater amount of copper contact but with essentially pot still characteristics. Its full capacity is used to redistill the combined heads and tails from previous runs on the spirit still, with the dephlegmator fully engaged and the drains closed to keep the plates flooded to compresses the heads, then slowly reducing the cold water feed into the dephlegmator to extract a cleaner set of hearts, albeit with different character from their standard double distilled spirit.

Westland's wash still - see plate drains on the left side of column
There has been a long-running debate within the spirits community about whether batch column stills count as pot stills. On the one hand, they both operate in batch mode, which influences how the spirit is shaped through cuts. On the other hand, a fully equipped hybrid still with a column and dephlegmator can manipulate the process of distillation in ways that are simply impossible with pot stills, producing very different sorts of spirits. Being able to produce clean, high-proof spirit in a single pass is fundamentally different than simple pot stills, which is demonstrated by their use in the fruit brandy industry. At the same time, the line between the two is blurred by double retort pot stills, which are widely acknowledged to be pot stills, but share characteristics with batch column stills utilizing only a few cross-flow plates. While there is a hard break between batch columns and continuous stills, what the debate really comes down to is mystique - are people willing to pay more for pot still spirits because they expect a certain level of quality and character? I would argue that batch columns can produce spirits of equal quality, as long as the distiller is skilled enough to use their equipment to its maximum potential. If the reputation of batch columns has become tarnished, that is because of the people using them, not because of the technology itself.

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