Understanding Whisky Production Unit 5: Distillation
Chapter 10: Principles of Distillation: Thermodynamics, VLE, and Volatility
Raoult's law, vapour-liquid equilibrium, azeotropes, and why ethanol and volatile congener fractions separate during boiling.
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At its simplest, distillation exploits the convenient fact that pure ethanol boils at 78.3°C, whereas water waits politely until 100°C. If you boil a mixture of the two, the vapour that boils off is richer in alcohol than the liquid left behind.
Real distilling, however, is rarely that simple. Alcohol and water molecules exert mutual intermolecular attractions that bend the rules of ideal solutions, producing an azeotropic point at roughly 97.2% ABV beyond which simple boiling cannot separate them. Furthermore, the hundreds of trace congener molecules responsible for flavour do not separate strictly by boiling point; their volatility is heavily influenced by how soluble they are in water versus alcohol at changing pot strengths.
The Terminology of Distillation
The distillation process comes with its own specific vocabulary, which we all need to know. Some of these words may have been introduced to you in earlier lessons. Let us now add to this word bank a little more, in order to equip ourselves fully for discussing distillation theory.
Wash - The fully fermented wash is called beer or fermented mash. This wash will become the feed to the distillation process, whether batch distillation, continuous distillation, or some combination of the two is used. This wash feed is the product of the earlier processing and fermentation of the raw materials (which depending on the final spirit can include a variety of cereals, such as malted barley, maize, wheat, and rye). Each of these washes has its own composition of alcohol, flavour congeners and other potential undesirable compounds. These congeners include other higher alcohols, phenols, esters and aldehydes, all of which give character to the spirits.
Wash has an alcohol concentration expressed as percentage alcohol by volume (this means the percentage by volume of pure alcohol in the liquid). It also contains several different congeners . Alcohol density must be measured at the standard temperature of 20 ° C, or be corrected to that figure, as temperature change can alter its volume by expansion or contraction. The congeners can be grouped into higher alcohols, phenols, esters, aldehydes, and volatile acids etc., which are all organic compounds. They are derived either from the raw materials used or by chemical reactions within various stages of the production processes prior to distillation.
Vapour pressure - This is defined as the pressure exerted by a vapour in thermodynamic equilibrium with its condensed phase, at a given temperature and pressure, within a closed system. The normal boiling point of a liquid is the temperature at which the vapour pressure equals the ambient atmospheric pressure. A substance with a high vapour pressure at normal temperature is defined as volatile e.g. alcohol. Conversely, a substance with a low vapour pressure at normal temperature is described as a low volatile compound. This will become clearer as we discuss the basics of distillation later here, but it is important that we understand volatility now.
Key Operating Insight
Welcome back. So, what were your thoughts? Hopefully, you will have concluded that the puddle of alcohol will be the first to disappear. This is because alcohol releases its vapour more readily than water. Another way of describing this, is to say that alcohol is more volatile than water - a fundamental characteristic that is fully exploited during distillation!
The History of Distillation
The history of distilling can be traced back to ancient Egypt, some 2,300 years ago. Simple pot stills were used for batch distillation; this method continued for many years thereafter.
Distillation involves the separation of volatile components, which is achieved through controlled heat application. The above schematic illustrates the basic stages of batch distillation, as well as the main plant and processes involved. The diagram shows the two inputs to distillation, namely the wash and energy feeds. The stages take the following format:
- Vapour is removed and condensed through a condenser.
- Liquid from the condenser is collected as product.
- Reflux is the condensed liquid returning to a lower level of the still system e.g. to the pot, after condensing on the upper surfaces of the still. The reflux ratio is the ratio of the amount of distillate refluxed to the volume of distillate collected.
- The residue left in the pot after distillation is then removed.
Historically, heat from open fire pits provided the thermal energy needed to boil the feed liquor held in clay, ceramic or glass pots. Vapour condensation may have occurred through air-cooling, or by using very basic methods of water-cooling, with the distillate recovered manually in clay, ceramic or glass containers. Alcohol distillation is likely to have been one of many uses for distillation at that time. Other uses may have included fragrant oils and perfumes from plants and other materials.
When compared with batch distillation, continuous column distillation of spirits is a relatively new concept. It began around 200 years ago when there was a huge drive to produce potable (suitable for human consumption) alcohol at greater output. This was not possible using the batch distillation process, which had a finite capacity (determined by the capacity of the pot) - more on this in later lessons. The continuous stills’ design evolved, based primarily on operation costs, the ability to produce larger volumes of relatively clean spirit and their use in other developing industries at that time. Early continuous distillation plant is likely to have been made from wood frames, with metal plates and pipe work systems. The design, methods and materials of construction have changed over the years, but the same main principles of distillation apply.
The Theory of Distillation
Distillation can be defined as the process that separates the component substances within a liquid mixture, using evaporation and condensation.
Distillation uses the differences in volatility between each of the mixture’s components to produce either complete or partial separation. This in turn increases the concentration of selected components. It is a common process, employed across many industries throughout the world. It is, of course, vitally important within the spirits industry.
Many chemical reactions occur during distillation but if we were to consider an ideal mixture of alcohol and water then distillation is a physical separation process.
Key Operating Insight
Welcome back. So, what were your thoughts? When you boil your kettle, the electrical energy increases the water’s temperature until it boils and the liquid water vaporises. Conversely, when you chill the water in your freezer, the temperature drops, and the water turns to solid ice. The above scenarios demonstrate the three states of matter: solid, liquid and vapour. When matter changes from one state to another, the change is known as phase transition. A solid melting, a liquid boiling, a vapour condensing to a liquid, and a liquid solidifying, are all examples of phase transition.
A pure substance melts, freezes, boils or condenses at specific temperatures. For example, water exists as a solid (ice) at any temperature below 0 ° C, as liquid in the range 0-100 ° C and as water vapour (steam) above 100 ° C (all at atmospheric pressure). These temperatures can be used to gauge the purity of substances, as any contamination alters their values. For a solid to be melted into a liquid, the particles must gain sufficient energy to escape the bonds of their rigid structure. They must then acquire even more energy to become gaseous. In each case, the amount of heat required to change the state (but without changing the temperature) is called the latent heat. It varies from substance to substance.
Distillation involves partial evaporation of the liquid, followed by condensation of the vapour. The resulting condensate is enriched in the more volatile components. The starting mixture is thus separated into two fractions, each with different concentrations. Pure alcohol boils at 78.37 o C (rounded to 78.4), compared with 100 ° C for water. When a mixture of the two boils, it does so at a temperature between their individual boiling points. Both the alcohol and water release vapours. Alcohol is more volatile than water. It thus releases its vapour more readily from the mixture than water does. The mixed vapour will contain a higher percentage of alcohol than the liquid from which it came. This key factor allows the distiller to increase the alcohol content of the spirit in a controlled manner.
The same principles of boiling point and volatility apply to the many congeners present in the distillation feed, having been produced during the previous process operations. Some congeners are very volatile and vaporise at lower temperatures than the less volatile components. The least volatile congeners remain in the pot towards the end of distillation. This range in volatility allows the distiller to decide which congeners to recover in the final spirit - more of this in later lessons.
To help you more fully understand distillation theory, we will look at the following diagram. It shows a very simple two-component mixture of alcohol and water.
The relationship between alcohol and water is very important to all distillers. It forms the basis on which all distillations are carried out. It is known exactly how much alcohol there will be in the vapour from a given concentration of alcohol in any mix of water and alcohol.
- The X-axis shows the alcohol concentration from zero to 100%.
- The Y-axis shows the temperature range from 75 ° C to 100 ° C.
- The lower curved line (blue) shows the boiling temperature of the alcohol / water mixture at various alcohol concentrations.
- The upper curved line (orange) shows the amount of alcohol in the vapour.
- In this example the dew and bubble points are considered to be the same.
- The bubble point is the temperature at which a solution starts to evaporate.
- The dew point is the temperature at which the vapour starts to condense.
Using the above:
Starting at A, draw a vertical line from this point where an 8% abv (alcohol by volume) feed is brought to the boil.
Follow a vertical line to its intersection at point B where its temperature is measured at 94 ° C.
The alcohol concentration in the corresponding vapour is found by drawing a horizontal line until it meets the vapour line at point C.
Draw a vertical line to intersect the X-axis at point D, where the percentage (% abv) is found to be 45%. This is the strength of the distillate produced.
The remaining liquid in the still now has a reduced alcohol concentration. As this liquid boiled at a higher temperature, the recovered distillate will be of a lower strength. It follows that by repeating this theory over the length of the distillation run, we will get to a point when the still temperature rises towards 100 ° C, indicating that most (if not all) alcohol will have been recovered as distillate. This is the point at which distillation is stopped. However, distillers may choose to stop marginally earlier, allowing for a small efficiency loss, which is outweighed by the cost of the energy required. We will revisit this diagram in subsequent lessons on batch and continuous distillation lessons.
Distillation Practice
A variety of stills are used throughout the spirits industry. Let us return to our earlier diagram, which gives a basic overview of the steps involved.
Although you are unlikely to encounter such a simple layout in practice, this diagram clearly identifies the key elements of distillation. We must consider the following points:
- The main distillation plant will consist of a vessel holding the feed material to the still, along with a feed of thermal energy to drive the process.
- The still may comprise several batch operations, or a continuous distillation process along with pipe work, valves, condensing systems and various receivers to collect the distillate or distillates.
- Condensed vapour is returned as reflux, which aids separation efficiency. In a batch pot distillation this happens within the pot itself.
- In continuous distillation this happens both within the rectifying column and as a liquid return from the condensing system.
- Modern systems are likely to include energy/heat recovery systems that preheat the still feed and recover heat from the vapour condensing.
- The control of all these processes could be manual, fully automated or a combination of the two methods.
- This is an energy intensive process. The pressure and temperature of distillation operations is continually increasing. If not carefully controlled and monitored, this could result in very unsafe conditions.
- To avoid and prevent dangerous events, several safety systems are employed. They cover overpressure, underpressure (vacuum), overfilling and over temperature scenarios - more of this in other lessons.
Needless to say, there is no singular/specific scale or design of plant used to meet everyone’s individual needs, to meet production requirements for the style of whisky you are producing. Each distillation plant, in theory, is performing the same task. Yet they will be very different in practice and operation.
The two main types of distillation used extensively throughout spirits production will be either a batch (pot still) distillation or continuous (column) distillation. Some processes may also employ variations of these, where a column-like process is installed to aid additional separation.
Batch Distillation
Batch distillation, as its name suggests, is a process where a still, (more commonly called a pot still in the spirits industry), is charged with a specific volume of wash. The following points are characteristic of batch distillation:
- Thermal energy distils the liquid to vapour and then condense the product as distillate in a condenser.
- Several portions (cuts) are recovered from the distillate.
- This type of batch distillation may consist of one, two or even three stages of batch distillation.
- Each stage of these distillations allows recovery of specific liquid streams as product that can be recovered or recycled to other stages of this batch process.
- In general, any residual liquor remaining in the still will be discarded although in the rum and sour mash processes this is retained for further processing.
- At the end of the process, the still is often cleaned then recharged with a new batch of wash for the next distillation.
Batch distillation is generally completed within an eight-hour window of operation, although some may take longer depending on the size and capacity of the stills and ancillary equipment. The batch distillation process is commonly used to produce malt whisky and pot still whiskey. It is also associated with rums in fairly small production plants where overall annual capacity can vary anywhere between 20,000 L to 15 mLa (million litres alcohol per annum). Batch distillation is also found in very small capacity operations, as this type of distillation is readily scaleable.
Batch distillation does not have the same type of rectification (the process of increasing alcohol concentration through repeated distillations) and separation of components when compared with the much larger continuous type distilling operations. It also produces a much more flavoursome product.
Batch still capacities range from as little as 500 L to over 25,000 L.
The above shows a simple schematic of a two-stage batch distillation process. Two batch stills (pot stills), with respective lyne arms on the pot, are carrying the vapour into the respective condensers.
The spirit safe is where the various condensed liquid streams are diverted to respective receivers for further distillation or recovering as final product (more of this in the batch distillation lesson).
Continuous Distillation
As we previously learned, continuous distillation is a fairly recent discovery. It was established some two hundred years ago, in response to the need to produce significantly larger volumes of lighter spirits. We should note the following points:
- Continuous stills operate with a constant, consistent feed of fermented wash and a constant introduction of thermal energy to drive the process.
- Product spirit and co-product streams are also continuously removed.
- These types of column distillation processes have feed rates ranging from 2,000 to 100,000 L of wash per hour.
- These processes are used to produce very large volumes of grain whisky, grain neutral alcohol, light rum and brandy. They are always used within large fuel alcohol plants.
- The early continuous stills were rectangular. As manufacturing methods have evolved, they now come in both rectangular and circular column form.
High volume producers need a distillation process that produces product efficiently, quickly and in much higher volume than the batch system will allow.
These stills operate at a steady state for days, weeks and sometimes months on end, provided that 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 fermented liquor feed rate, composition and temperature. This places an even greater importance in ensuring all pre-distillation processes are consistent and produce a consistent feed of wash at the end of fermentation.
Once a steady supply of feed and energy is achieved, the continuous still will reach a balanced operation. Alcohol and other congeners including fusel oils and non-volatile still residues will be consistently removed from the system. We will look more closely at this in further distillation lessons.
The above is a simplified schematic of a two-column continuous distillation process. More specifically, it depicts a Coffey still, used in the continuous production of grain whisky. In this process, steam and inlet wash are continuously added to the system. Spirit product is continuously removed and sent to the receiver. Spent wash (residues left after distillation) is continuously removed for further treatment.
Key Operating Insight
Welcome back. So, what were your thoughts? You will have noted there are several main differences. These include: distillation volumes and times the type and shape of the distillation equipment the mode of distillation the maximum alcohol concentration for each type of distillation the end products
We will review these two modes of distillation in more depth in subsequent lessons.
The Role of Copper
Copper is currently the metal of choice for construction of most, if not all, pot stills. Both copper and stainless steel are the preferred materials used to make column stills.
We have already learned that the first pot stills are likely to have been made from glass, clay or porcelain. Whereas the very early continuous stills had wooden frames with copper trays and pipe work. These materials of construction will have been chosen by trial and error, as distilling theory, technology and analytical measurement developed and evolved.
Copper soon became the material of choice for the manufacture of both batch and continuous stills, as manufacture and design methods developed and evolved.
As understanding of distillation processes increased, it became apparent that copper possessed several properties that made it an ideal material for still construction.
- Copper is malleable and durable. This made it easier to hammer, roll and form into the now traditional spherical like pots used in the batch processes, as well as the earlier rectangular design of the continuous stills.
- Copper is a very good conductor of heat.
- Almost all parts of the distillation plant and ancillary equipment such as condensers, trays in the continuous stills and pipe work systems are likely to be made from copper.
Our understanding of copper’s role in controlling spirit quality has only developed over the last 40-50 years, as analytical science, measurement and assessment of organoleptic (nosing of spirit in controlled conditions) quality has evolved. We now understand the significant role copper plays in the removal of certain unwanted sulphur compounds during distillation. Copper can also play a major role in catalysing other very important reactions. These include the many complex reactions in the control of ethyl carbamate, a known carcinogen described in a previous lesson.
We must also consider the following points when exploring the use of copper.
- As it is a malleable, relatively soft metal, copper (when clean) provides a very reactive surface for a number of reactions during distillation. This includes the removal of unwanted pungent sulphur compounds.
- Vigorous heat application, along with the relatively low pH (high acidity) of the wash, makes copper susceptible to both erosion and corrosion. This can thin certain parts of the distillation equipment, which is costly to repair and replace. It can also lead to significant loss in production volume due to plant unavailability when undergoing the subsequent repairs.
- The internals of the stills, pipe work and other plant can often become fouled during distillation. They will need to be physically cleaned or chemically cleaned (or a combination of both).
- These cleaning processes deliver a fresh clean reactive copper surface, which is exposed to heat and low pH, and so the cycle will start again.
- This exposure to high temperature, low pH and regular cleaning leads to an increased copper content of the liquid and solid residues remaining after distillation.
- These streams will have to be treated carefully; as much copper as possible should be removed prior to discharge to the environment. We will discuss more of this in co-products in later lessons.
As previously mentioned, continuous stills were manufactured from both stainless steel and copper. Historically, the traditional Coffey still used in the production of grain whisky in Scotland was made from copper, as were its ancillary pipe work and condensers.
These stills operated on a continuous basis for several days at a time before requiring shutdown for cleaning. The vigorous flow of low pH feed wash, high distillation temperature, and abrasion from the feed’s solids, led to considerable erosion and corrosion.
Technological advancement, alongside analysis of the various liquid streams, has led to the conclusion that copper is only required in certain key reaction areas within this form of continuous distillation. This has instigated significant changes in method and materials used for construction. These older, traditional, continuous Coffey stills are more likely to be made from stainless steel; copper will only be used for certain parts of the still. Copper plays a significant role in the quality control of both product streams from either the batch or continuous process. This will be explored in more detail in subsequent lessons.
Key Takeaways
Distillation relies on differential volatility. Hydrophobic compounds evaporate early in the run even if they have high boiling points, whereas hydrophilic compounds remain dissolved in the water-rich pot liquid until late in the distillation cycle.
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