Understanding Whisky Production Unit 5: Distillation
Chapter 13: Whisky Distillation Traditions: Double, Triple, and Grain Systems
Malt Scotch double distillation, Irish triple distillation, grain whisky production, material balances, and condenser metallurgy.
On this page
Distillation traditions across the whisky world represent different historical solutions to the same underlying thermodynamic puzzle. Scottish malt distilleries standardise on two distillations; traditional Irish distilleries often employ three pot stills, recycling intermediate fractions through an intermediate still to achieve an exceptionally light, refined spirit.
Condenser design exerts an equally decisive influence on spirit style. Traditional worm tubs (coiled copper pipes submerged in cold water tanks) provide modest copper exposure and rapid condensation, retaining heavy, savoury, and sulphury notes. Modern shell-and-tube condensers, packed with hundreds of narrow vertical tubes, provide expansive copper surface area, producing delicate, ester-driven new-make.
Malt Whisky
Malt whisky is produced from 100% malted barley, and in some cases with added non-endogenous enzymes.
It is generally distilled in a two-stage batch pot still process. The input raw material (e.g. peated or non-peated malt) contributes to the flavour. As will the processes of mashing, fermentation and (most importantly for this lesson) the distillation itself. This type of distillation produces a range of distillates with different character and quality, depending on all of the above. This gives the blender a wide choice of final product for sale e.g. as blended product or as single (or even blended) malt.
Irish pot still whiskey uses a similar distillation process to this two-stage batch pot distillation. The main difference is the raw material input: both malted and unmalted barley are often used in the mash bill.
Grain whisky
Grain whisky is produced from a range of different mash bills (wheat, maize, rye, malted barley) and in some cases, with other non-endogenous enzymes.
Grain whisky will generally be distilled in a two-column continuous distillation process with greater rectification than that in the batch pot still process. This produces a lighter, cleaner product. This is important as the production of this lighter whisky allows the blender greater flexibility in adding character and quality from the more flavoursome malt distillates.
Other grain whiskies, such as Irish and Canadian, can be produced in a three-stage process of stripping column (also known as analyser or beer column), extractive column and rectifying column processes.
Malt Whisky Batch Pot Distillation Process
A two-stage batch pot distillation process is the method of choice in producing malt whisky across the globe. By exploring this process, we can gain a deeper understanding of other non-Scotch malt whisky production processes.
We have already discussed much of the theory and fundamentals of this two-stage batch pot distillation process in the Basics of Distillation and Batch Pot Distillation lessons. Take some time to review these again if needed.
In summary, this two-stage batch pot still process involves a first wash distillation, followed by a second spirit distillation. These two-stage distillation processes can recover a final product distillate at a range of between 63-70% abv, from an initial wash feed strength of around 7-11% abv (remember the lesson on the basics of distillation and the alcohol / water liquid / vapour equilibrium diagram). This distillate will also include a significant number of other flavour compounds (congeners), which give the distillate the traditional aroma of new make malt whisky. It is important to reach a final distillate strength of above 63.5%, as this is found to be the most optimal strength for maturation.
The feed to the first distillation is wash at a temperature of around 30-34 ° C, (usually this temperature is raised by preheating with other hot process streams) with an alcohol concentration of around 7-11% abv. It also contains a variable concentration of grain-derived solids, depending on the type of mashing process involved. The role of this first distillation is to recover all the volatile compounds, including alcohol and a significant number of flavour compounds (congeners) in the collected distillate (known as low wines).
The feed to the second distillation (also called spirit distillation) will be a mixture of the collected low wines from the first wash distillation, along with feints and foreshots from a previous spirit distillation. This second distillation recovers three separate distillate streams: the foreshots (or early high volatiles), the spirit cut, and finally the lower volatile feints. The spirit cut is the heart of the distillation. It delivers the quality of your desired product. Changes in character are made by altering spirit cut points, the rate of distillation, vapour condensing conditions, and the composition of feints or foreshots. The residue left in the spirit still is primarily water. These spent lees are discharged for additional treatment.
The diagram below shows the material balance over a two-stage batch pot distillation process, with 100 units of incoming wash. This is a typical material balance, which you should update for your own distillation process.
Of the 100 units of wash feed, 65 units will become pot ale (residues after first distillation), 10.5-11.5 units will become spirit, and 23.5-24.5 will become spent lees (residues after spirit distillation). The feints and foreshots represent some 20.5-23.0 units of each original 100 units of feed fermented wash. However, as they are recycled within the process, they do not feature as products or outputs in the material balance.
The above diagram shows the basic layout of a wash and spirit still. It gives an overview of the ancillary equipment installed to maintain safe and efficient operation. The diagram also shows several different shapes to the shoulders and neck of the stills - this is not a fully inclusive list.
Key Operating Insight
Welcome back. So what were your thoughts? The diagram shows the two pot stills and the associated thermal energy supply to the process. In addition to what is shown, your list should include: Wash feed tank (wash charger) Wash preheating arrangements Lyne arms for both stills Shell and tube condensers (or worm tubs) for distillate recovery Low wines Foreshots and feints receivers Spirit receiver Spirit safe Spirit still feed preheating arrangements Pot ale tanks Spent lees tanks Pipe work, pumps and valve arrangements, with temperature, pressure and flow measurement on key processes (to ensure safe and consistent operation), connect all parts of the plant.
Pot stills have been made from copper for many years, even before its use as a catalyst in the control of spirit quality was fully understood. We have already learned that this metal is malleable and has excellent heat transfer characteristics. We also know that it is an excellent catalyst for many of the reactions that affect the final product’s quality. These pot stills come in a variety of shapes and capacities.
The distillation process is a major energy consumer. Great efforts have been made to recover energy in order to make it more efficient. Historically, this energy was supplied via direct firing by coal, gas or fuel oil. In most distilleries this has now been replaced by the use of indirect sources (although some still use this method). High-pressure steam is fed into the stills, via either steam coils or steam pans, which allows greater control of thermal heat and the rate of heating. It also gives more consistent heat application to the process.
Both the direct and indirect application of thermal energy has the potential to create char on the inside of the copper surfaces during the wash distillation. This can cause poor heat transfer and efficiency loss but more importantly can also impact quality and flavour profile. More energy is then required for complete distillation. This situation requires intense cleaning, by either CIP (cleaning in place) or manual intervention, to remove this char and re-energise the internal copper surfaces.
Historically the wash stills were fitted with a rummager (a mechanically driven arm that pulls a copper chain along the base of the still) which was used to remove heavy fouling from the base of the pot. The base of the copper pot was also thicker in this type of installation.
Recent developments have provided alternative external heating systems to the wash still. Liquor is removed constantly throughout distillation, raised to boiling temperature, and then returned to the pot. These systems are easier to service and maintain and avoid the unsafe practise of confined space entry to the still. Fouling is generally not an issue in the spirit distillation, as there are little or no solids present.
Pot Design Implications - Shape of the Body
The design and build of the pot affect product quality and process operations:
Shape of the Body
There are wide variations in the size and shape of pot stills. Stills designed for steam heating have a wide flat bottom. The flues, or side sheets, tend to be vertical. This makes them suitable for steam pans and coils. The capacity of the still’s body is designed to ensure that the level of liquid retained after distillation does not fall below the top of the heaters. Some stills in use today will still have high domed bottoms, and outwardly sloping flue or side sheets, specifically for the now outdated direct heating from coal fires.
The ratio of wash feed to overall still capacity in the wash still, and the low wines, feints and foreshots in the spirit still, are all important. The larger the still charge, the less heating area per litre. The reduced space above the liquid level reduces the effective separation of liquid and vapours. The reduced available surface area also reduces the amount of reflux. In the wash still, high wash levels and low surface area can lead to frothing and foaming, with entrainment of wash solids into the recovered low wines. This also increases the probability of fouling in the lyne arm and condensers. This fouling could cause inefficient condensation and longer distillation runs. We need to avoid entrainment through foaming and frothing, as it alters the flavour of the final spirit. In distillation, entrainment is the carry-over of liquid or solids with the spirit vapours, which contaminate the distilled product. This can affect vapour passing from a pot still to its condenser, or from the stripping column to the rectifier in continuous distillation systems.
Pot Design Implications - Shape of the Still Head and Reflux
Different reflux rates are a major factor in differentiating distillates between distilleries. Reflux returns some of the least volatile components back into the still, producing a slightly lighter spirit and more rectification.
This explains why the shape and size of the still head has a greater influence on spirit character, compared to any other part of the distillation plant. The size and shape of the still head affects reflux due to the amount of copper surface available, and the temperature difference between the vapour and copper surfaces whilst condensation is occurring. Please refer back to the previous figure showing examples of still shapes.
Pot Design Implications - The Lyne Arm (Lye Pipe)
The lyne arm, or lye pipe, is a large diameter pipe connecting the swan neck to the top of the worm or condenser.
It affects the quality and character of the distillate, depending on the length and orientation of the pipe, as well as whether the pipe is sloping upwards or downwards to the condenser. The diameter of the lyne arm is also important as it can determine vapour velocity and therefore copper contact.
Some stills are additionally fitted with a purifier (an intermediate vapour condenser) at a point between the lyne arm and the condenser. The condensate from this intermediate vapour condenser is returned to the pot to provide additional reflux. This increases the rectifying ability of the still and influences the congener content of the final distillate.
Other forms of forced reflux can include the use of chilled water jackets installed to the head of the stills. In this situation cooled water within the jacket forces additional vapour condensation, copper contact and increased rectification. Other strategies could include cooled pipes within the still. This approach creates additional vapour condensation, increased copper contact and increased reflux.
Despite the variability of design and installation between distillers, all designs are required to have adequate copper contact with the boiling vapours to provide reflux. They will also all need reactive copper surfaces as the main drivers for delivery of the desired product quality and character. The factors to consider include:
- Design, build and capacity of the pot
- Design and build of still shoulders
- Design and build of swan neck
- Design and build of lyne arm
- Potential installation and use of purifier
In earlier lessons, we learned that the low pH of the incoming wash and organic acids in the feed liquor creates considerable erosion and corrosion in certain parts of the copper plant. Other chemical reactions take place on the internal copper surfaces which increases the rate of corrosion/erosion. Generally, the areas of the still that wear out the fastest, in order of shortest lifespan are the condenser, lyne arm, swan neck, head and finally the pot.
These parts will need to be removed and replaced with new copper. The sections of these pot stills will have been brazed together during manufacture and build. The worn areas will thus be cut from the pot, repaired or replaced. This will take considerable time and incur loss of production. A modular stills design, using bolted flanged sections, is now in use. This makes removal and repair easier, quicker and more cost effective.
A range of safety systems are installed in this process, including anti-vacuum, pressure relief and overfill protection. These will avoid any major incident, and vent any potential alcohol externally, rather than internally in the still house.
Plants are designed to account for all of the raw material inputs, and any liquid stream containing alcohol, for both efficiency and regulatory purposes. This accounting process may be on a weekly basis for efficiency assessment. However, it is more likely to occur on a monthly basis for regulatory reasons. This ensures compliance with all local customs and excise legislation. In the United Kingdom, this is the HMRC (Her Majesty’s Revenue and Customs). Legislation will vary depending on where in the world your distillery is located.
Condensate Recovery
The two main condensate recovery processes use either a shell and tube condenser, or a worm and worm tub.
We discussed these two methods of recovering condensed vapours in the Batch Pot Still Distillation lesson. Please review these lessons if you want to refresh your knowledge of this very important process.
Key Operating Insight
Welcome back. So what were your thoughts? These condenser arrangements act as heat exchangers, using a flow of coolant water to condense vapour and then cool it. These systems have two inputs, namely hot vapour from the stills, and coolant in the form of water. They have two outputs, namely the cool condensed spirit, and warm or hot water. Most condensers are of the shell and tube type. Hot water can be reused in the process.
Take the opportunity to review these operations at your own plant, in order to deepen your understanding of the energy and process balances across the batch distillation process.
Grain Whisky Continuous Coffey Still Distillation Process
Historically, Scottish grain distillers most commonly used the Coffey still continuous distillation process.
The more common two-column process, with wash being heated externally to the still, is now more frequently used. We discussed much of the theory and fundamentals of this two-stage batch distillation process in the Basics of Distillation and Continuous Distillation lessons.
This is also the main type of distillation used in the production of Irish grain whiskey. Similar column processes are also used in American and Canadian whisky production. These will be explained later here.
Key Operating Insight
Welcome back. So what were your thoughts? Hopefully, you have remembered the importance of the stripping (analyser) column, and the rectification and condensing arrangements that produce a much higher strength and lighter distillate to that achieved in the batch pot still processes. You should also remember how the wash feed is preheated prior to entry to the stripping column - this process is very different in the Coffey still process.
The original Coffey stills were designed and built in the 1820s. They were complex pieces of plant.
These early stills were viewed as a series of progressive distillations. They were designed for the following reasons:
- To produce a higher output
- To use steam instead of solid fuel
- To produce a lighter spirit
- Reduced production costs per L of alcohol
These requirements are as relevant today as they were at the time of the original design.
The Coffey still is the traditional method of grain whisky production. It is a two-column system, composed of an analyser and rectifier, which contains sieve plates. As in previous lessons we will call the analyser the stripping column and the rectifier the rectifying column. It is designed to operate continuously under steady conditions. A much smaller packed column (or fusel oil decanter system) separates, recovers, and removes the highly pungent fusel oil (iso-amyl alcohol). Traditionally, these columns, trays, ancillary condensers, and pipe work were made of copper. They were also rectangular in cross-section. New designs have seen stripping columns replaced with either circular or rectangular stainless-steel columns. Stainless steel wash coils in the rectifying column have also replaced the traditional copper wash coils, as they were prone to erosion from the low pH, high acidity and solids in the wash feed.
The Stripping and Rectifying Columns
The two main columns of a Coffey still are the stripping column and the rectifying column.
The two feeds to this distillation process are:
- Wash at approximately 8-9% abv, entering through the wash coils at the top of the rectifying column at around 30-34 ° C.
- Thermal energy in the form of steam. This enters the base of the stripping column, providing the thermal energy to drive the distillation process.
The following diagram shows this process. We will ‘walk through’ all relevant operations.
This diagram shows the layout of the Coffey still, but does not include all the plates required to complete rectification to less than 94.8% abv.
The two columns are divided into a number of sections, separated by perforated trays on which there is a counter flow of liquid and vapour. Typically, the stripping column has around 30 trays, while the rectifying column has around 40 trays. The wash flows down the rectifying column within the stainless steel or copper wash coils that wind their way between the trays. They are connected from one section to the other by external bends protruding from the frame. As the wash descends the rectifying column, the temperature is increased by the contact of hot vapours passing from the stripping column into the rectifying column and up through the various plates. By the time it reaches the base of the rectifying column, its temperature will have increased to around 90-94 ° C, the boiling point of an ~ 8.5% abv solution.
Key Operating Insight
Welcome back. So what were your thoughts? In the modern two-column distillation process, other hot vapours and/or liquids in external heat exchangers preheat the wash feed to the still. These are generally shell and tube heat exchangers, which utilise heat from hot overhead vapours, or from the boiling spent wash leaving the base of the stripping column. The rectifying column does not have wash coils.
This hot wash is then redirected to the top of the stripping column via the re-ascending wash pipe, where it then enters the top of the column. The wash cascades down the stripping column through a system of weirs, plates and down comers. The liquid passing from tray to tray on a downward path meets vapour forcing itself upward through the plate perforations. It pushes through the liquid layer, driven by the thermal energy input at the base of the column.
Gradually, the wash is stripped of its more volatile components as it descends the stripping column. Conversely, the vapour becomes richer in alcohol and volatile congeners as it rises. Spent wash contains little (or a very low concentration of) alcohol as it leaves the base of the stripping column at its boiling point.
Flash steam is then recovered through a thermo-compressor to augment the live steam feed. The addition and condensation of pure, clean, live steam adds around an additional 10% to the liquid loading in the still as it condenses within the feed liquor.
The alcohol enriched vapour at around 15-30% abv, transfers via the vapour pipes to the base of the rectifying column. Here, it rises through the trays, heating the wash within the descending wash coils. This vapour is condensed as it rises, providing downflow of liquid as reflux on each tray, where equilibrium will be reached for the particular temperature. This can be viewed as a series of batch distillations progressing up the rectifying column. The higher the tray, the higher the concentration of alcohol, and the more volatile the components will be.
The alcohol concentration on the lower plates of the rectifying column is around 10-15% abv. The vapour is in equilibrium with the liquid. This liquid stream, known as hot feints, is recycled within the distillation process to the top of the stripping column for further distillation. (We will review feints handling later here.)
Vapour passes upwards, condensing in both the liquid on the plate and the exterior of the wash coils. This releases latent heat to the incoming wash coils, and also to the liquid on each plate. The incoming wash receives pre-heating, whilst condensation in the liquid on the plate generates more vapour for fractional distillation.
The most volatile components will rise to the top of the column, and out into the overhead condenser. Here, the distillate is recycled back to the rectifying column as reflux, or after cooling into the cold feints receiver (prior to returning to the stripping column for further distillation). The latter would probably occur through the hot feints return system.
The vapours reaching the overhead condenser will contain some highly volatile, obnoxious, unwanted compounds. The system’s temperature will be set to facilitate the release of these unwanted components, while maximising alcohol recovery.
Handling of Feints in a Coffey Still
Feints processing is an important recycle and recovery process for alcohol and other higher alcohols in the Coffey still process.
We will use the following diagram to discuss the production and handling of feints in the Coffey still distillation process.
The production of hot and cold feints is illustrated above. Hot feints, removed from the base of the rectifying column, are pumped to the hot feints tank and back to the top of the stripping column with a portion of cold feints (if they are available) for further distillation.
At the top of the rectifying column, the heads are partially condensed. Unwanted volatiles are vented from the system to the atmosphere; the main flow of condensate is returned as reflux. Some of this condensate is returned to the cold feints receiver, along with off-specification spirits produced during start up and shutdown of the process.
It is essential to return hot feints at a controlled steady rate. This maintains balanced distillation conditions; the hot feints tank provides the mechanism for this. The process of feints return is used to maintain steady distillation conditions. With cold feints at > 95% abv, and hot feints at around 10% abv, these streams can be returned at varying ratios to deliver this.
Fusel Oil Removal and Recovery
The removal and recovery of fusel oil is a key part of the Coffey still distillation process.
The following diagram shows the distribution of congeners under three different conditions within the rectifying column. You will see that how the still operates affects the distribution within the column.
These three distribution diagrams show the importance of maintaining a balanced operation. They depict significant differences in the manner and area in which these congeners concentrate in the rectifying column.
In graph one, the level of amyl alcohol is concentrating at the very base of the rectifying column and the top of the stripping column. In graph three, the amyl alcohol level peaks much further up the rectifying column; it will probably spill over into product spirit. Graph two shows the peak of iso-amyl alcohol at around plates 4-6, under good average running conditions. The following schematic shows the process of fusel oil removal and recovery, via a small packed column:
Here, we can see that a side stream is withdrawn from around plates 4-6 in the rectifying column. It feeds into the centre of a small packed column and is driven by steam entering the base. The fusel oil is separated from the higher volatile alcohol and other congeners. It is removed from the base of the column for sale as a co-product. Hot feints from the base of the rectifying column are fed into the hot feints tank and recycled back to the top of the stripping column. The vapour leaving this small column is condensed with some returning as reflux to the fusel oil column while the remainder is returned to the hot feints tank via the cold feints system for recycling back to the stripping column. This simple schematic does not show the detailed layout of the reflux divider or condensing and cold feints systems.
A fusel oil decanter system is also used in the continuous distillation of rum and whisky production. This simple separation of fusel oil uses a water decanting process in conjunction with the principle of the insolubility of fusel oil in alcohol concentrations less than 30% abv
Continuous Distillation in North America
Scottish and Irish immigrants took their knowledge of whisky distillation to America.
In the late 18th century, these early settlers adapted the process to allow them to use indigenous cereals of corn and rye. By the middle of the 19th century, the larger distillers were using the continuous process, rather than the volume limiting batch process.
Nearly all pot stills were phased out by the early 1900s. The majority of all major Bourbon and Tennessee whiskey distilleries were by now using column distillation.
There is one clear difference between the products from Coffey (or continuous) distillation in Scotland, to those from America: Scottish grain whisky is distilled at high strength (< 94.8% abv by law), producing a cleaner, lighter spirit, which is primarily used in blending with the more flavoursome batch pot still malt whisky. Bourbon distillers require a more flavour intense product, which can be matured and bottled. This is achieved using a smaller single column still, which allows them to collect the distillate at around 56-60% abv before this strength is increased by boiling in a doubler. This doubler is a cylindrical vessel containing a steam coil, which is used to produce a distillate of increased strength. By law, this strength cannot exceed 80% abv. This is shown diagrammatically in the following schematic:
This is a very basic schematic, which shows the key elements of this type of distillation. It gives you a great comparison with the other forms of continuous distillation featured in this and other distillation lessons.
Older Bourbon columns are made from copper. More modern columns are made from stainless steel. Similar to the stainless-steel stills used to produce Scotch and Irish whiskies, the system includes sacrificial copper to help control spirit character and quality. Canadian distilleries and other non-cereal processing plants use this type of process.
Key Takeaways
Every mechanical choice in the distillation house, from the number of still passes to the configuration of condensers, shifts the delicate balance between copper contact and congener retention, setting the physical limits of the spirit’s flavour profile.
About the author
