Understanding Whisky Production Unit 5: Distillation
Chapter 11: Batch Pot Distillation: Copper Stills, Reflux, and Cut Points
Wash still and spirit still operation, foreshots, middle cut, and feints, and how still shape, lyne arm angles, and copper contact govern spirit weight.
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The classic copper pot still is essentially an oversized alembic that has changed surprisingly little over centuries. In traditional Scotch malt distilling, distillation occurs in two distinct stages: the wash still strips all the alcohol from the fermented wash to create low wines of around 20% to 25% ABV, and the spirit still concentrates and refines this liquid into crystal-clear new-make.
Copper is not merely a structural choice; it is an active chemical reagent that scrubs away pungent, foul-smelling sulphur compounds during boiling. The critical operational moment occurs at the spirit safe, where the distiller selects the middle cut: isolating the desirable heart of the run while sending the harsh, pungent foreshots and the heavy, oily feints back for redistillation with the next batch.
The Origin of Batch Pot Still Distillation
The development of the basic pot still has taken place over many hundreds of years. The earliest pot distillation was carried out in ceramic or glass pots, which were heated in open furnaces or on hearths. Air-cooled condensers then recovered the distillate.
Originally, distillers were not aware of the suitability for copper in controlling spirit character. This metal later became material of choice due to its heat conductivity and malleability, which allowed it to be shaped as required. This type of batch pot still distillation plant and process is used globally for producing pot still whisky, more flavoursome rums, brandy products, tequila, gin and vodka to name but a few. Some of these topics will be covered in later lessons in the distillation series.
Modern, mature spirits businesses will already have well-established processes for manufacturing their specific products. The distillers will have a thorough knowledge of their raw materials. They will know how these materials are processed, and the desired character and flavour of their products. The spirits will have a well-defined character. This character is determined by analytical composition and a flavour profile that focuses upon the final sensory experience gained from the product. The distiller will also have a very clear understanding of the impacts of changing distillation conditions upon product quality.
Batch Distillation Process
The following schematic gives a very basic outline of a simple one-stage batch distillation:
This basic distillation aims to raise the temperature of the wash in the pot by using thermal energy until it reaches its boiling temperature. At this point, the volatile vapours are condensed in the condensing system. This recovers all the volatile components as distillate, leaving the less volatile components and any residual solids in the pot. These will then be discarded for further treatment prior to cleaning and restarting the process all over again.
This simple distillation process consists of a copper pot and some form of thermal power e.g. an open fire/steam supply to the pot. A vapour pipe connects the pot to the condensing system, which then recovers the distillate in either a shell and tube, or a worm condenser arrangement. This system has no way of selecting or collecting different condensates, so all vapour is recovered as one distillate.
In this example, we will assume that the feed strength of the wash is around 8% abv. If we use the vapour/liquid equilibrium diagram for an alcohol/water binary mixture, we know that the strength of the collected distillate will be 45% abv initially. It will then reduce down to near 0% abv, with a probable mixed average of around 20-24% abv. The single distillate collected throughout the distillation will include both high volatile and low volatile compounds. These may or may not be desired in the final product.
In these lessons, ethyl alcohol, and ethanol are defined as alcohol. For ease, we will use the word alcohol. We can summarise the three main objectives of a spirits producer when using pot still distillation. These objectives are to:
- Recover as much alcohol from the wash feed as is efficient and economical to do.
- Collect the congeners or flavour compounds that will positively influence the spirits’ character and product flavour.
- Raise the strength of the distillate to above 60% abv for successful maturation.
If we look back at the criteria set for the pot distiller, we know that only one of the three requirements has been met in this simple one stage distillation. That is, they have recovered as much alcohol in the wash feed as is practical and economic to do. This single stage distillation does not have sufficient power or rectification to produce the high strength distillate required for product quality.
This single pot still could have been used to redistill the distillate produced during the first distillation, where a distillate of around 60-70% abv will have been achieved. This very simple form of batch distillation uses the same distillation plant as the first distillation. It reduces the potential process output and still has no way of separating the various distillate component streams.
The following vapour/liquid graph shows the range in composition of the collected distillate from around 75% abv to 0% abv. It has a still feed of around 30% abv.
Batch Distillation Technology - Worm Tub
In our distillation lessons so far, we have discussed the major elements of distillation equipment. We have focused on the pot and its construction materials.
We will pause a moment here to discuss and review the two principle methods used during distillation to recover the vapours as a condensed liquid. The two major condensing systems are either by some form of worm and worm tub arrangement, or now more commonly a shell and tube condenser.
Let us first look at the worm and worm tub, using the following diagram.
A copper coil of diminishing diameter is placed within a tub of water. The vapour from the distillation pot enters at the top of the coil while the tub is filled with cold water. The action of this cooling water cools the surfaces of the coil and causes the vapour to condense as it flows down the internals of the coil and out through the spirit safe or other recovery systems. This is a very traditional method of recovering the distillate. It ensures that the vapour has less exposure to the reactive copper surfaces of the coil.
Advantages of using the copper coil:
- The reduced liquid/vapour contact with the copper surfaces of the worm produces a richer, heavier spirit than a standard shell and tube condenser.
Disadvantages of using the copper coil:
- The coils are exposed to major changes in temperature. This places strain on the plant, which adds to the effects of the normal erosion and corrosion of the copper that takes place.
- Leaks in this type of condensing system (either vapour into cooling liquid or cooling liquid into the product stream) are quite difficult to detect.
- These worms and tubs are very expensive to repair, with significant plant downtime.
- It is not possible to hold a spare worm as a quick replacement part as they are unique and must be manufactured for the specific installation.
- The tubs will be made of wood or metal and will thus require regular monitoring and repair.
As you can guess from the list of disadvantages that outweigh the advantages, the copper coil is not commonly used today. In fact, only a few malt distilleries still use this method. The next page describes the second type of condensing arrangement. This method is now used more commonly. It is called the shell and tube condenser.
Batch Distillation Technology - Shell and Tube Condenser
The principles of this type of condensing arrangement are similar to those of the worm and worm tub process. Its cooling medium involves water passing through a number of bundled tubes, with hot vapour passing across baffle plates on the outside of these tubes. The vapour condenses as it passes along the length of the condenser.
This type of condenser stands in a vertical position. It is typically made of copper, although some distillers use stainless steel to produce different types of new make product. They are generally cheaper to produce, and easier to maintain. This type of condenser uses comparatively less water than the more conventional worm and tub and is more efficient. Some batch spirit distillers will use only worm tub condensers, whilst others may employ shell and tube plant. A very small number use a mixture of both shell and tube and worm and worm tub. The shell and tube condenser is now the preferred method of condensing the still distilled vapours. Some distillers retain the worm and worm tub arrangements to impact on spirit quality and character.
In some distillery operations the vapour condensing processes are linked into more complicated heat recovery systems, where heat from boiling hot still residues is recovered to preheat other process streams. This is particularly important in distillations where the fermented liquor leaving fermentation is at around 34 ° C. This will require significant thermal energy input to bring to the boil. These integrated energy recovery processes help to reduce the raw energy required.
When you have completed this lesson, you should consider the operations in use at your own plant. This will help you to understand more fully the energy and process balances across the batch distillation process.
The Two-Stage Pot Distillation Process and Technology
The two-stage batch pot distillation process is commonly employed for malt and pot still whisky production.
The wash or beer still (the first still) generally has a larger capacity than the second spirit still (sometimes called the low wines still). Separate lessons will cover these products. Generally, gin and vodka only require a single batch pot still distillation, in which they will use alcohol at around 96% abv as a raw material in their production processes.
The schematic shows a two-stage batch distillation process, in this case a malt whisky distillation. It depicts two batch stills (pot stills) with respective lyne arms on the pot carrying the vapour into the respective condensers. It also includes a spirit safe, where the various condensed liquid streams are diverted to respective receivers for further distillation or recovering as final product.
The batch pot still distiller will now have a way of meeting all criteria. All alcohol is recovered, whilst attaining all product quality, flavour and strength requirements.
First Distillation
Let us now consider the generic batch pot still process. The first stage involves a distillation to remove all alcohol from the incoming wash, along with all the wanted and unwanted volatile components (congeners) produced in earlier parts of the process. This wash contains a varying amount of solids, which will remain in the pot at the end of distillation. These solids are removed for further treatment, and then potentially used as animal feed. The distillation is powered by thermal energy in the form of steam in the base of the pot still. Distillers in remote areas may continue to use more basic means of direct heat e.g. by coal or gas fired external means. The distillation feed will be at around 8% abv, depending on the raw materials being processed. Distillation stops when the cost of recovering the last of the alcohol is more than the value of the product. This is likely to be when abv in the vapour is less than 1%. This distillate will be called low wines or intermediate spirit; it will have a strength of around 22-24% abv.
Key Operating Insight
Welcome back. So what were your thoughts? The residue left in the pot at the end of this first distillation will contain water, suspended and soluble solids from the remains of the raw materials, and yeast debris. Dissolved salts will also be present in this stream. These salts include copper salts removed by erosion and/or corrosion, and from the raw materials. Water, yeast and non-volatile compounds will also be found. This topic will be covered in a later lesson on co-products.
Second Distillation
Having collected all the low wines from the first distillation, the distiller can now initiate the second distillation.
Systems and processes have been developed, in which the distiller has the opportunity to collect various cuts of the distillate from this second distillation. They do this by splitting the streams through a spirit safe, or by some other form of valve arrangement. Low volatility compounds from the previous distillation may coat the surfaces of the still, lyne arm and condenser. These compounds can be recovered in early running of this distillation. During the early stages of this second distillation, the very volatile components will feature at high concentration in the strongly alcoholic distillate. Conversely, the less volatile congeners will be more dominant in the low alcohol stream at the end of this batch distillation. Both the early and late running of these distillates will contain unpleasant compounds, which the distiller will not want to collect.
- The first fraction is known as heads, first running, or foreshots.
- The latter running is known as tails or feints.
- The distiller only looks at the middle cut to deliver the quality distillate for his product. The middle cut may also be referred to as the centre cut, the heart or simply the spirit cut.
- The heads and tails both contain significant concentrations of alcohol. This alcohol will thus be retained for future distillation with subsequent low wines.
- Distillers use two methods to monitor the actual point of cutting from foreshots to spirit. Some do this on a strictly time basis, while others use a ‘mist’ test where water is added on a 50:50 ratio to the actual sample. If this mixture goes hazy, then there are still heavy components to be taken off in the foreshots.
- The cut point is the time at which the distiller starts to recover the required quality of the distillate into a separate spirit receiver. The fraction collected is known as the middle cut, the centre cut, the heart, or simply the spirit cut.
- The remaining liquor in the pot still is mainly water. It is most commonly known as spent lees. Having completed this cycle of wash and spirit distillation, the plant is cleaned ready for the next two-stage process.
All distillers have a responsibility to account for all the alcohol and raw materials used in their respective processes. This process is called accounting. Both the distiller and regulatory authorities use it. Accurate accounting processes almost totally depend on the distiller’s ability to accurately account for all streams of alcohol in this two-stage process.
Cut Points
In this section, we will explore the cut point procedures in the second distillation of a two-stage batch pot still operation.
The liquid/vapour diagram below, for the two-component water/alcohol mixture, shows how changes in alcohol feed composition affect the concentration of alcohol in the distillate recovered from that vapour. We will examine this relationship between liquid and vapour composition in the spirit still throughout a generic spirit distillation.
In this very important schematic, we can see the distillation time (expressed in hours) along the X-axis from the start at zero time, to the end some seven hours later.
- The Y-axis gives us the alcohol concentration in both liquid and vapour extrapolated from the original liquid/vapour equilibrium graph.
- Two vertical cut point lines are marked at approximately 25 min and 3 h 15 min into the spirit run. The distiller will make the cut during these times.
- Distillate recovered before the first cut will be heads whilst that recovered after the second cut is tails or feints.
- The distiller will have established the timing of these cuts over time, likely by trial and error, to ensure the distillate in this cut meets product requirements.
- This diagram shows the effect of the above cut, but also shows the changes in composition of the distillate recovered if the decision is made to take an earlier or later cut.
- Taking an earlier cut will give a higher strength product, containing more highly volatile compounds.
- Taking a later cut will give a lower strength product, containing more low volatile compounds.
These changes in composition will produce spirit of very different character and quality. The liquid composition in the pot will not change unless there is a change in initial feed composition, or changes to the rate of distillation (caused by changes to thermal energy provision).
In this diagram, we can see the distillation time along the X-axis.
- The Y-axis represents the concentration of high, medium and low volatility compounds or congeners. These lines on the above figure represent only one compound in each of the high, medium and low volatility category.
There are many compounds, and each will display a different profile based on their volatility. This schematic builds on the previous one. It gives a representation and comparison of how, and when, compounds with significantly differing volatilities will be recovered in the distillate throughout the spirit run.
If you look at the two schematics together, you will see that recovery of the very volatile components comes early in the running of high concentrations of alcohol. However, those of low volatility are recovered at low alcohol concentration, generally towards the end of the spirit run.
We must understand that if we change the timings of the original cut point, to something either slightly earlier or slightly later, we will not only change the alcohol concentration of the distillate but also the flavour profile. This is because different compounds will be recovered due to the change in volatility.
These are two generic overviews of the vapour/liquid composition and volatility data throughout a batch spirit distillation. These types of diagrams can be drawn up for any spirits producer who has their actual process and analytical data. Their own processes and raw materials will drive these. The distiller, whether producing whisky, rum, brandy, tequila, gin etc. (or any other distilled product for that matter), will have a full understanding of which compounds feature in the high, medium and low volatility range. They will already have made a decision on which cut point delivers the distillate of the required quality and flavour. Their key task will be to make sure that they can repeat all aspects of wash and spirit distillations, cut point timings and composition of materials in both batch distillations, to ensure a consistent product of the required quality and specification.
Other Styles of Batch (Pot) Distillation
Triple Distillation
The two-stage batch pot style distillation is the preferred choice for most distillers. This process delivers a distillate of a suitable strength for maturation. However, several distillers, including some Scottish and Irish malt or pot whisky distillers, employ a three-stage distillation process.
A triple batch distillation involves a three-stage distillation process. It has the added complication of recovering a second batch of heads and tails from the second spirit distillation.
The distillation of low wines and feints and foreshots from previous distillations in the second stage of this three-stage process is commonly called the intermediate distillation. However, it mirrors the spirit distillation in a two-still batch process in many ways.
This type of distillation has a greater rectification power than the two-stage process. It should also have more control over spirit quality and character if standard distillation procedures are followed.
Batch Distillation with Plates
Another alternative method of achieving a greater level of rectification is to use a pot still with a small number of plates fitted both above the shoulder of the pot and below the vapour pipe. The diagram below shows this form of pot still:
- This type of batch pot still has between three and eight plates installed above the shoulders.
- A reflux condenser installed above the plates (also known as a dephlegmator) provides the downflow (reflux) of liquid needed for good contact between the liquid and vapour. These plates operate in a manner that we will describe when exploring the operation of a continuous still.
- Overall, the additional plates produce a higher strength stronger distillate and recover fewer low volatility (heavy) congeners than would be the case in a standard batch pot distillation.
- This type of still is often referred to as the Carter Head or Lomond still in the whisky industry.
Key Takeaways
Still geometry directly controls reflux and copper contact. Tall necks and upward-sloping lyne arms encourage heavier vapours to condense and fall back into the pot, yielding light, floral spirits; squat stills with downward arms carry heavier, meaty congeners over to the condenser.
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