Continuous distillation, as opposed to the more commonly known batch distillation, is an ongoing process that allows for the constant production of spirits.
Typically it’s a process associated for bulk production and neutral spirits. But it doesn’t have to be!
Continuous distillation as a method has the potential to elevate a craft distillery’s output, without compromising the artisanal quality that defines the spirit of the craft movement.
Whether you’re a seasoned distiller looking to expand your knowledge, or you’re at the starting line of planning and research to launch a distillery, understanding the differences between batch distillation vs continuous distillation is crucial. You may be surprised about what you think once you go beyond the stereotype…
This article aims to demystify continuous distillation and guide you through its complexities. It will also help you comprehend how to leverage it to create exceptional spirits at any scale.
Understanding continuous distillation
Continuous distillation is characterised by a constant feed of wash into the still, with alcohol being separated and collected simultaneously. Unlike batch distillation, which operates in stages, continuous distillation is a relentless process that doesn’t stop. The wash is automatically fed in, the power constantly pumping. Nothing stops for liquid to be cleared, added or for the still to be cleaned.
The primary advantage of continuous distillation is efficiency.
Distillers can process a larger volume of wash in a shorter time frame, translating to increased production capacity. This efficiency doesn’t have to come at the cost of quality though. If managed correctly, continuous distillation can produce spirits of high purity and consistent character. Look at some of the great Calvados makers as an example of this!
That said, the method is particularly beneficial for spirits that require a high level of rectification, and to be made in large volumes like vodka and certain types of grain whisky.
How does continuous distillation work?
Most continuous stills have two main columns: the stripping column and the rectifying column. Here’s a simplified explanation of how these components work together:
Stripping column
Wash is continuously fed into the top of the stripping column. It cascades down various tray arrangements, while steam provides the necessary thermal energy from below.
This steam vaporises some of the liquid as it passes up the column, stripping it of its volatile components. Meanwhile, the less volatile compounds remain in the liquid stream and descend.
An easy way to think of stage one is that wash goes in the top, and somewhere in the column it meets steam. Ethanol evaporates back up and moves to the next column, the rest of the liquid is taken from the base and dispensed of (the same as stillage).
The primary role of the stripping column is to separate alcohol and highly volatile congeners from the incoming wash feed.
Rectifying column
The rectifying column concentrates the alcohol from the vapour leaving the stripping column to the final product strength.
Hot vapours from the stripping column enter the base of the rectifying column. The rising vapours, now rich in alcohol, are progressively purified as they ascend through the trays. By the time the vapour reaches the top of the rectifying column, it has a high alcohol concentration, often around 92-96% ABV.
The least volatile components accumulate at the base of this column as feints, which are then recycled back to the stripping column. The desirable liquid is collected from a plate near the top of the rectifying column at the desired spirit strength.
The rectifying column works just like any column (more on plates and design below). The only difference is that because the process is continuous, there needs to be a way of removing the feints, fusel oils etc, as there’s no way of making cuts as you would in a batch process. This happens half way up (more on that below).
Continuous distillation equipment considerations
Mastering continuous distillation requires a thorough understanding of the equipment involved.
Distillers must consider the size and scale of their operations. Column stills come in various capacities, designed to accommodate different production volumes. It’s not all gargantuan towers. For example, some Armagnac production is continuous, rustic and small.
The material of the still is also a crucial consideration, with copper being a popular choice due to its sulphur-removing properties, which can result in a smoother spirit.
Key to any column is the trays, sieves etc. inside. Each type of tray in a column still serves the purpose of improving the contact between rising vapour and descending liquid, enhancing the efficiency of the distillation process.
Sieve trays
These trays are essentially perforated plates with many holes. The vapour passes directly through these holes once they are volatile enough. Any vapour that refluxes (returns to liquid state) falls back down. Sometimes despite having holes perforated everywhere, the rate of reflux can be so high that a small amount of liquid sits on the plate – with liquid wanting to go down, but being pushed up / held up / reheated from the vapours from underneath.
Sieve trays are simpler and less expensive than bubble cap trays. They offer low resistance to flow, which can be good for those looking for a good amount of rectification, but don’t need to reach 95% + ABV.
Bubble cap trays
Bubble cap trays contain small domes or caps with slits. Vapour rises through a riser and is forced through the slits in the bubble cap, percolating through the liquid on the tray.
They provide excellent contact between vapour and liquid, which is great for handling foaming liquids and ensuring a consistent distribution of vapour, especially under varying operation conditions.
Valve trays
In valve trays, the perforations come with movable caps. As the vapour rises, it lifts these caps, creating its own area for vapour passage. This mechanism causes the vapour to be directed sideways into any liquid pooling, resulting in more effective mixing compared to sieve trays.
Valve trays are designed to enhance the mixing of liquid and gas, optimising vapour-liquid contact within the unit. They are offered in various designs, ranging from single pass to multiple downcomers, and can be tailored to suit specific operational requirements.
Disc and donut trays
These trays have a central disc (the “donut”) surrounded by a ring (the “disc”). Vapour flows up around the disc, through the liquid, and then passes through the central donut.
They provide good vapour-liquid contact and are particularly effective in reducing pressure drop across the tray. These trays are less common but can be advantageous in specific applications where pressure drop is a concern.
The choice of which tray depends on various factors such as the type of liquid being distilled, the required purity of the product, operation conditions, cost considerations, and maintenance requirements.
For example, sieve trays are ideal for applications where cost and simplicity are priorities, and where liquids are non-foaming. Meanwhile, valve trays are suitable for a wide range of conditions, offering a balance between efficiency and operational flexibility.
Operational aspects
Temperature control is the trickiest part of continuous distillation. Often, finding the system’s equilibrium of temp, flow rate and feed is the most complicated part of the process.
For example, the heat must be applied in a way that vaporises alcohol efficiently, but still allows for the wash to cascade down the rectifying column. Meanwhile the flow rates of both the wash and the cooling water must be carefully managed to maintain a balance between the input and output of the still.
Consistency is the hallmark of a well-run continuous distillation process. By maintaining stable conditions within the still, distillers can ensure that each drop of spirit is as good as the last.
Suitable spirits for continuous distillation
Continuous distillation isn’t a one-size-fits-all solution, but it excels in the production of certain spirits. High-proof, clean spirits such as vodka benefit immensely from this method. The reason lies in the ability of continuous stills to perform multiple distillations in one go – they are designed to produce spirit that is pure and high in alcohol content.
Whiskies and brandy, which are typically rich in flavour and require a certain level of congeners (flavour compounds), can also be produced via continuous distillation. However, craft distillers must carefully manage the process to retain the desired flavour profile. The precision / efficiency of continuous distillation can be a double-edged sword. If not handled with care, the spirit may lose the complexity that defines it.
In whisky, continuous stills are predominantly used for grain whiskies and performed at scale. In brandy smaller craft operations can be found. There are many Calvados and Armagnac producers who use continuous systems. In both cases, the spirit has rich, pronounced flavours of the base material, be it apple or grape.
They show that continuous distillation does not have to be used in towering columns, nor that it must be for neutral (or almost neutral) spirits.
Cleaning and maintenance
The backbone of any successful distillation process, particularly continuous, is rigorous cleaning and maintenance. A clean still is non-negotiable for producing quality spirits.
Residues from the wash can cause off-flavours and contaminate subsequent batches. Therefore, a strict shutdown and cleaning protocol must be in place, involving rinsing and the use of distillery specific cleaning agents.
See our Cleanliness article for more on CIP systems and how they are used.
Maintenance is equally important. Continuous stills are complex systems with moving parts that are prone to wear and tear. Regular check-ups and replacement of parts like seals and gaskets ensure that the still operates at peak efficiency.
This preventive approach to maintenance avoids costly downtime and extends the life of the still.
For new distillers and those operating a continuous system, the idea of stopping everything to do cleaning and maintenance can seem daunting. The whole point is that you don’t stop! Yet, embrace it as part of the distillation ritual – a commitment to excellence – and you’ll reap the rewards.
That extends even for those where distilling wine is on a time critical deadline to avoid spoilage. The same is true for those under pressure to turnover tanks and maximise the season for apples or other fruit.
As we’ve explored, continuous distilling suits certain spirits more than others. The systems are complex. The parts are intricate. Distillers and new distillery startups considering it should take the time to gain a deep understanding of both the equipment and the operational process.
The focus area is always about striking a balance between maintaining flavour and using technology to process liquid in an incredibly efficient manner. Where do you fall on this spectrum and what do you want for your spirit…
