Understanding Whisky Production · Unit 5: Distillation

Chapter 12: Continuous Column Distillation: The Coffey Still and Grain Whisky

Analyser and rectifier columns, bubble-cap trays, stripper sections, reboilers, and how continuous distillation yields light grain spirit.

While pot stills boil in discrete batches, continuous column distillation treats spirit production as an uninterrupted thermodynamic stream. Patented by former excise officer Aeneas Coffey in the 1830s, the twin-column system revolutionised the economics of distilling by running twenty-four hours a day without needing to be emptied and refilled.

In the analyser column, descending wash meets rising steam, which strips out the volatile alcohol. The alcohol-laden vapour then passes into the rectifier column, where it rises through a series of perforated plates, progressively cooling and concentrating until pure grain spirit is continuously drawn off near the top at around 94% ABV.

The Origin of Continuous Distillation

Continuous distillation processes for spirits production were developed around 200 years ago.

Continuous distillation, in the potable alcohol industry, has its roots in the early 1820s. Robert Stein developed the first truly continuous still in 1827, which was superseded by the apparatus designed by Aeneas Coffey and patented in 1830 (No 5974). Although this type of distillation is used for potable alcohol, you will now find this technology used widely in the chemical process industries, where large volumes of liquid have to be distilled into single components or fractions.

You have already learned that distillation works by the application and removal of heat to exploit differences in the volatility of individual components in a liquid mixture. The heat causes compounds with higher volatility to turn into vapour, leaving the less volatile compounds in the liquid stream.

In the spirits industry, you will remember that our wash feed contains alcohol at around 8% abv (dependent on the input raw material) and a significant number of other flavour compounds (congeners). These compounds will be present at significantly lower concentrations, but are still very important to the final spirit quality and character. This wash may also contain all of the residual cereal solids. The heavy solids, however, may have been removed prior to fermentation - this is actually quite important in the operation of these column stills - more on this later.

This type of column distillation is generally employed where very large volumes of wash need to be distilled into product spirit, with the requisite recovery of flavour compounds. This process produces a lighter distilled product to that produced in the batch pot still process. This type of distillation is thus ideal for grain whisky, grain neutral alcohol, light rums and brandy. Continuous (column) distillation involves column stills, designed to run in a continuous mode of operation. They have a continuous feed of wash, a continuous feed of thermal energy to drive the process, a continual removal of distillate(s) and co-products. The operation and actual physical size of the distillation equipment is determined primarily by the composition of the feed material and the required product distillates.

The following schematic shows the layout of a simple column distillation apparatus:

Key Operating Insight

Welcome back. So what were your thoughts? As you will have recognised, this single column is actually split into component parts - namely a stripping column (below the feed) and a rectifying column (above the feed). The feed liquor is continuously fed into a section around the middle of the stripping column, where it cascades down various tray arrangements. A steam supply provides the thermal energy to drive the column - more on this later. The steam vaporises some liquid as it passes up the column, where it exits the top of the column into an overhead condenser process. Some distillate is returned to the column as reflux, while the remainder leaves as product.

These types of columns can operate either:

• At normal atmospheric pressure
• At higher pressure, where higher temperatures are also found in the distillation system
• In a vacuum distillation mode where operating temperatures are lower

The choice of operating mode is determined by the feed and the product’s composition, as well as an overview of the heat balance within the distillation operations. We must also consider where this distillation mode fits into the overall heat and mass balance within the total distilling operation.

Continuous Distillation Technology - Columns

Several items of plant are common to all types of continuous distillation. However, their scale, materials of construction, capacity, operating conditions, and layout will be different.

The diagram below shows a very simple schematic of a continuous distillation plant. It illustrates the main plant and processes involved:

The above diagram showed this continuous distillation plant with only one column. However, in practice, you will generally see these split into two separate columns. This is because they usually contain well over 30 plates in each column. Due to its height, this makes it impractical to build as a single column. Below are a few key points about a continuous distillation plant.

• This type of distillation equipment is sometimes located in a building, shielding the plant from the elements. However, the reality is that they can also be built to operate in the open, negating the need for additional capital expenditure on a very tall building.
• The two main columns are widely known in the spirits industry as a stripping column and a rectifying column.
• The stripping column strips the incoming wash feed of all the alcohol and highly volatile congeners or flavour compounds. Some of these highly volatile components are much desired in the final product, while some others are not.
• The main objective of the rectifying column is to concentrate the alcohol from that leaving the stripping column to the final product strength.
• The two columns are joined by vapour pipes, which carry the vapours leaving the top of the stripping column to the base of the rectifying column and a pipe returning liquid (hot feints) to the top of the stripping column.
• In a traditional Coffey still the pre-heated wash in the coils of the rectifying column is connected to the top of the stripping column.

The design, build, and operation of a continuous distillation process depend on the composition of the feed and desired products. The distiller can use a range of computerised simulation model tools to establish both design and operational criteria.

One of the most common methods used for designing these stills is the McCabe-Thiele method. First used in the early 1920s and it remains popular today. These design models calculate the number of theoretical plates needed for both the vapour stripping and rectifying sections within any continuous distillation plant. Thankfully, you will not be asked to design a continuous still, but it is important to recognise how this type of plant is designed and built.

These columns are either rectangular or circular in cross-section. They are manufactured from either copper or stainless steel, or in some cases a combination of the two. You will later learn more about copper’s role in this type of distillation plant, similar to that in the batch pot still process.

Continuous Distillation Technology - Trays or Plates

To create a more efficient process, we can add perforated trays to the columns.

• This ensures liquid is retained on the tray for a period, whilst the rising vapours pass through the retained liquor.
• It ensures a much improved vapour/liquid contact, thus improving separation.
• The design of the trays or plates in the stripping and rectifying columns is governed by the feed’s composition.
• Trays and plates encourage vapour liquid contact, so that the falling liquid partially vaporises.
• Rising vapour partially condenses with the heat being transferred from the condensing vapour to vaporising liquid in equal quantity - an equilibrium phase.
• The equilibrium phase involves heat transfer, which takes time to achieve. The plates or trays facilitate this.
• A key feature of any tray design is to minimise any pressure drop from one to the other.

The design will include the following types:

• Bubble cap trays
• Sieve trays
• Valve trays
• Disc and donut

Continuous Distillation Technology - Thermal Energy Supply and Heat Recovery

Thermal energy heats the base of the distillation columns.

Thermal Energy

Heat is supplied either through live steam injection into the column or via a reboiler. If live steam injection is used, it will occur via a thermo-compressor type arrangement, where live steam supplements steam recovered from the boiling liquid leaving the base of the stripping column. Alternatively, a reboiler can be used. It takes the form of a heat exchanger between liquid from the bottom of the column and live steam.

The following points discuss the operation of a reboiler in more detail.

• A liquid level is maintained in the base of the column, which overflows into the reboiler.
• Heat transfer from the live steam converts this liquid to a mixture of liquid and vapour.
• This vapour/liquid stream then feeds back into the column above the liquid level, providing the thermal energy to drive the distillation column.
• The reboiler methodology is prone to choking if not maintained and cleaned regularly.

Both forms of thermal energy provision have potential issues.

• Live steam injection into the distillation adds an additional volume to the liquid leaving the stripping column base through condensation.
• Reboilers are prone to fouling.

Important points to note about the thermo compressor:

• It passes the boiling liquid from the base of the stripping column into a vessel, where steam is flashed off from the liquid by creating a partial vacuum.
• This vacuum is generated by high-pressure steam passing through a restriction in a pipe (Venturi) connected to the flash vessel.
• The reduced pressure steam from the Venturi, along with the steam created in the flash vessel, is then injected into the base of the stripping column.
• This process cools the spent liquor, but also reduces the overall steam requirements by recovering this heat from the boiling liquid.

Stripping Column Feed (Wash)

The wash leaves the fermentation process at a temperature of around 34 ° C. It needs considerable live steam (thermal energy) input to the column, to bring it to its boiling point. This is very inefficient, so other forms of thermal energy are used to pre-heat the fermented wash to near its boiling point, prior to addition to the stripping column. These streams include hot vapours leaving the top of the rectifying column. We can potentially use more of the heat from the waste liquor leaving the base of the stripping column. The key here is to design a heat recovery process from all available hot streams, which raises the temperature of the fermented liquor feed cost effectively.

Distillate Recovery

The vapours leaving the top of the rectifying column require cooling to recover as distillate. This takes place in either a single or a two-stage shell and tube heat exchange process. The recovered distillate is either recovered in the spirit receiver as product or returned to the top of the rectifying column as reflux. This separates components more efficiently.

Reflux refers to the portion of the condensed overhead liquid that returns to the rectifying column. This downflowing reflux liquid provides cooling and partial condensation of the ascending vapours. This increases the efficacy of this process. A higher reflux level improves the separation of high volatile compounds from low volatile compounds in the feed.

Feed, Product and Intermediate Product Tanks

This column distillation process relies on a continuous supply of wash to the stripping column, and a continuous removal of product distillate and other intermediate liquid streams. It also needs a continuous supply of thermal energy (steam). To ensure this is possible, the process needs a number of large tanks of sufficient capacity to deliver these operations. All product tanks and intermediate product tanks are designed to allow for regular accounting of all alcohol products. This facilitates accurate accounting.

Continuous (Column) Distillation Process

We can carry out continuous distillation in stills of a variety of types and shapes.

As we have previously learned, distillation uses evaporation and condensation to separate component substances in a liquid mixture.

When a liquid mixture is heated to boiling point, the composition of the vapour above the liquid is different to the liquid’s composition. When this vapour is separated and condensed to a liquid, it becomes richer in high volatile compounds from the original liquid. Please review the vapour/liquid equilibrium diagram for the binary ethyl alcohol/water mixture.

This is the basis for continuous distillation:

• The wash feed is heated up and then fed into the stripping column.
• The feed flows down the column.
• The highly volatile compounds vaporise and ascend the column.
• This rising vapour will eventually cool, however some of the vapour continues to rise as vapour.
• The liquid enriched in the less volatile components descends the column.
• The high volatile products exit the top of the rectification column, whilst compounds of low volatility exit from the base of the column.
• Steam provided to the base of the stripping column provides the thermal energy to drive the process.

The schematic below gives a very basic overview of the steps involved:

This schematic shows a two-column continuous distillation process, depicting a stripping column and a rectifying column. It also shows overhead condensing arrangements and a fusel oil decanter for removal of iso-amyl alcohol (fusel oil). We will explain fusel oil removal later here.

High volume producers need a distillation process that makes product efficiently and quickly, and in much higher volumes than the batch system allows. These types of stills will operate at a steady state for days, weeks and sometimes months on end (so long as the internal surfaces of the columns, condensers and ancillary equipment are not fouled).

Steady state conditions will be maintained over many days and weeks as long as there are no material changes to wash feed rate, composition and temperature. This places an even greater importance in ensuring that all pre-distillation processes are robust and produce a consistent feed of fermented wash at the end of fermentation. Once a steady supply of feed and energy is achieved, the continuous still will reach a balanced operation. This balanced operation allows the constant removal of alcohol from the product distillate, as well as other congeners including fusel oils and the non-volatile still residues from the system.

In a perfectly balanced still, if such a thing exists, the composition of the liquid and vapour in the columns remains constant at any given point in the process. The distiller will aim to achieve steady state conditions as soon as possible into the still run. During start up and shut down of the distillation process, out of specification distillates are recovered for recycling during normal operations.

The above schematic shows a typical layout employed in high volume spirits production across the world. As we have learned in previous lessons, not one design, shape or method of operation will fit the requirements of all distillers.

The following section will give a generic overview of a two-column continuous still operation. It will not include all the variants that are in operation. We will instead cover these variants in supporting lessons for specific distilled product types.

The Stripping Column

The stripping column varies in capacity, depending on the specification of the incoming fermented liquor. Its capacity also depends on the still’s overall capacity. However, it will usually have around 20-30 plates or trays.

In some parts of the world the stripping column will be referred to as the beer column or analyser.

The wash will have been collected in the wash charger (a still feed tank) of sufficient capacity to maintain a steady rate of feed. This bulk feed tank is topped up at regular intervals as each subsequent fermentation is completed. It provides a constant and consistent feed of wash. It is vitally important that all downstream production processes are completed effectively and efficiently to ensure consistency of feed. The use of this large capacity feed tank will even out slight variations in alcohol concentrations and temperature of wash feed. This will ensure that once reached, steady state conditions are maintained.

• The wash feed temperature at end of fermentation depends on the type of fermentation. Generally, it will be anywhere between 30-34 ° C, with a strength of 7-14% abv. This will be dependent on the raw materials used.
• The wash, with or without heavy solids, is preheated through heat exchanging with hot overhead rectifying column vapours. It will reach a temperature of around 85 ° C before feeding into the stripping column near to the top of this column.
• The wash cascades down the stripping column, passing from plate to plate, meeting and mixing with the ascending vapour.
• Alcohol free spent wash or stillage (we will now refer to this as spent wash) will eventually reach the base of the stripping column.
• This spent wash contains water, varying quantities of raw materials, and yeast debris.
• The spent wash then passes through a reboiler or thermo compressor (depending on which type of thermal energy delivery is in operation) and is finally pumped to a bulk stillage tank for further processing.
• Steam, or hot vapours, from the reboiler or thermo compressor will enter the bottom of the stripping column. Here, it will meet the descending wash and then progressively strip it of its volatile components as it ascends the stripping column.
• We will need to provide sufficient thermal energy to ensure that all volatiles are stripped from the wash. This will avoid alcohol loss (which would occur when the spent wash leaves the base of the stripping column).
• The highly volatile rich vapours leaving the top of the stripping column will be at an alcohol concentration of between 10-25% abv, depending on the wash composition.

The Rectifying Column

This column varies in shape, size and the number of plates, according to the overall capacity of the plant.

Hot vapours are carried along the vapour pipes connecting the top of the stripping column to the base of the rectifying column. They will enter the rectifying column base, condensing and evaporating the liquid on the plates as it passes from plate to plate up the column. These hot vapours will be the only thermal energy source to the rectifying column. They will act in the same manner as steam in the stripping column - remember the earlier images of the two columns as one single column.

• The least volatile components collect in the base of the rectifying column as a liquid commonly known as feints (or hot feints).
• The feints are recycled back to the top of the stripping column for further distillation - remember the two columns as one scenario.
• Liquid product is collected from a single plate near the top of the rectifying column at the required spirit strength of around 92-96% abv. For grain whisky in Scotland, this will be below 94.8% abv by regulation.
• The final product’s strength will also significantly affect the concentration of other flavour compounds (congeners).
• The lower strength distillates will contain significantly more congeners than the higher strength distillates.
• Any off-specification product produced during start up or shut down operations (or from the main distillation itself) is recovered to storage for recycling back to the stripping column.
• There is also usually an option to recycle any off-specification product back for re-distillation, should analytical or organoleptic assessment deem the product unsuitable.
• Vapours pass out the top of the rectifying column. They will contain the very highly volatile components, including some alcohol and a very small portion of water.
• These vapours are then condensed in the overhead condenser arrangements, possibly providing heat to the incoming wash feed.
• Some condensed vapours are returned as reflux to the still.
• The remaining vapours are recovered as cold feints and returned to the cold feints receiver. They are then recycled via the hot feints to the stripping column. The temperature of this overhead condensing system is controlled in order to vent unwanted highly volatile components from the system. This will also maximise alcohol recovery as reflux or cold feints.

Congener Distribution within the Rectifying Column and Fusel Oil Removal

The higher alcohols are separated and concentrated at various points within the rectifying column.

Congener Distribution

The graph below shows the potential distribution of the main congeners within the rectifying column. These congeners include ethyl alcohol, iso-butyl alcohol and the heavy fusel oils (e.g. iso-amyl alcohol).

Generally, this type of light distillation will recover all the ethyl, butyl and n-propyl alcohols. It will also remove the majority of the iso-amyl alcohol. This graph shows only ethyl, butyl and iso-amyl alcohols and their distribution. This data will vary for each distillation process and fermented wash feed. The separation of compounds is the key point of interest here, not the absolute values indicated.

The X-axis gives the concentration of the various compounds, while the Y-axis shows the rectifying column plate number. Samples are drawn from each of these plates and sent to the laboratory for gas chromatographic analysis. This will establish the concentrations of these analytes. This graph shows iso-butanol (butanol) levels peaking around plate 14. The iso-amyl alcohol (fusel oil) level peaks at around plates 4-6, with alcohol (spirit) increasing in strength until it reaches the offtake at the spirit plate. The spirit plate is one of the solid plates installed near the top of the rectifying column.

From this graph, we can clearly see the separation of iso-butanol and fusel oil. By removing a side stream of liquid from around plate 4-6, we can remove fusel oil from the system, process this side stream, and return any remaining liquid back to the stripping column via the hot feints system.

Fusel Oil Removal

Failure to remove iso-amyl alcohol from the rectifying column will result in a significant accumulation of this compound. This would eventually overflow into the recovered spirit, affecting spirit character and quality.

Iso-amyl alcohol has a very pungent, feints odour, which is not removed during maturation. We can remove it by withdrawing a portion of the liquid from plates 4-6 to a decantation tank, where the alcohol strength is diluted by water addition. This separates out the fusel oil and forms an oil layer, which is then decanted off to a storage tank.

An alternative process to using the decanter is to use a separate, smaller, packed column in a continuous mode. This will separate the component parts, recover the iso-amyl alcohol, and return the remaining liquid back through the hot feints system back to the stripping column. This is another much smaller form of continuous distillation, which is used to separate component parts of a liquid mixture. However, it utilises a packed column rather than individual plates.

The fusel oil removal processes are covered in detail in subsequent lessons on whisky and neutral spirit.

The Role of Copper

As previously mentioned, copper was historically the metal of choice used to produce the original continuous distillation Coffey stills. These Scottish Coffey stills were used for producing grain whisky. Today, copper, stainless steel, or a combination of the two metals, is used to make continuous distillation plant across the world.

Greater understanding of the distillation processes showed that copper had several properties that made it an ideal material of construction for many stills. To achieve the desired effect in continuous distillation, copper could be included in the vapour pipes, in the hot feints tanks, or as copper plates at the top of the stripping column (or at the base of the rectifying column). Sacrificial copper could also be added in these areas (or combinations of all of the above could be employed). To recap the significance of copper, please review the Basics of Distillation lesson and refresh your understanding.

Key Takeaways

Column stills achieve high rectification efficiency, stripping away heavy congeners and leaving a light, versatile spirit. This grain whisky forms the essential foundation for blended Scotch, providing a consistent canvas that showcases blended single malts.

About the author

John Fegan · Editor

John has run Uisge Beatha since 2015 and Whiskipedia.com since 2020. He owns whisky casks and writes from direct experience of their storage, insurance and bottling decisions.

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