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The Evolution of Distillation: From
Alchemical Art to Modern Science
Distillation: Integration of a Historical
Perspective
The Principle of Distillation
Vapor pressure, also known as vapor tension, is the pressure of a vapor in equilibrium with its liquid. All liquids have a tendency to evaporate (volatility), and all gases have a tendency to condense back into their liquid form. For a liquid i at any given temperature T, there is a pressure Pi at which the gas of that substance is in dynamic equilibrium with its liquid.
This equilibrium can be represented as: Molecules (liquid) ⇌ Molecules (vapor)
As the temperature T increases, the equilibrium is shifted to the right, and the vapor pressure Pi increases exponentially. The dependence of Pi on T is given by the Clausius-Clapeyron equation:
Pi = Pi0 * exp(-ΔHvap / (R * T))
where Pi0 is a known pressure at a known temperature T0, ΔHvap is the heat of vaporization of the liquid, and R is the gas constant. The boiling point Ti is defined as the temperature when Pi = Pext, usually taken as the normal boiling point, so that Pi0 = 1 atm and T0 = Ti.
The goal of distillation is to separate a volatile liquid from a non-volatile substance or, more frequently, the separation of two or more liquids with different vapor tensions. For simplicity, we will refer to the distillation as the separation by means of heat of a mixture of two liquids A and B, where B is more volatile than A (i.e., PB > PA).
Two requirements for distillation can be derived from the previous definition: 1) the process needs a heat source to force the liquid mixture into the gas phase, and 2) the gas phase (enriched in B) has to be condensed back to the liquid phase to obtain a separation of the two components.
The first requirement (evaporation) was already known by ancient Greeks, as Aristotle mentions in his Meteorologica that pure water may be obtained from seawater through evaporation. However, he did not write of any practical method to condense the vapor and take advantage of this phenomenon. This process is known as open evaporation of the mixture.
The second requirement (condensation) was fulfilled around the 1st century A.D. by the Alexandrian chemists, who added to the boiler (main vessel) an alembic (secondary vessel) connected by a side tube. This effect was the
trickling down (destillatio in Latin) of the purified liquid in the alembic. Distillation was officially born.
When the gas in equilibrium with the liquid mixture reaches the vapor pressure, further evaporation is precluded. This limitation was overcome by the introduction of a water jacket surrounding the side tube, which condenses the B-rich gas phase, subtracting it from equilibrium and increasing the efficiency of the process. The condenser, introduced by medieval alchemists around the 13th century, was necessary to obtain reasonable yields of products containing high alcohol contents.
The Theory of Distillation
Alchemists had a very clear idea of what distillation was: "distilling is nothing other than purifying the gross from the subtle." The purification would make the corruptible incorruptible. As with all alchemic processes, a correspondence to the inner self can be drawn; today we still use the verb "to distill" to mean "to extract the essential meaning of something." Distillation was subdivided into ascension (evaporation) and descension (condensation), powerfully symbolized by a bird flying upwards or downwards, or by two dragons, one winged, seizing each other's tails.
Around two centuries ago, a radical shift took place in how man looked at matter. The works of Boyle and Lavoisier were shaping the birth of a new science, chemistry. In the following years, a parallel revolution occurred in the development of distillation, when quantitative rationalizations of the process in mathematical models improved dramatically the efficiency of this separation technique.
Boyle himself is considered to be the first to have performed the first analytical distillation. The origins of the theory of distillation can be traced back to the question François-Marie Raoult was trying to answer at the end of the 19th century: what happens to the vapor pressures of A and B when they are mixed?
Raoult's law states that the total pressure in the case of a mixture is equal to the sum of the partial pressures of the components, where the partial pressure of each component is proportional to its molar fraction in the liquid phase. This effect was the key to understanding the separation of the components in the distillation process.
Practice and Application of Distillation
Although chemical engineers have made distillation more efficient during the last century, the essence of the still apparatus has survived mostly unchanged. The larger the difference in volatility between the two components (the relative volatility α = PB/PA), the easier the separation. If α is close to 1, the separation becomes very difficult, and an azeotrope is formed, where the gas phase has the same composition as the liquid phase.
To overcome this limitation, the concept of fractional distillation or rectification was developed. By continuously repeating the evaporation-
The rule of thumb is to use this method when the boiling temperatures of the two components (A and B) differ by at least 60 K (TA > TB + 60 K). A thermometer is used to monitor the temperature of the boiling mixture: the temperature will stay constant while the mixture is boiling at the mixture's boiling point (TM), and then it will rise towards the boiling point of the pure component A (TA).
When the mixture boils, the mole fraction of the liquid component A (xA) and the mole fraction of the vapor component B (YB) can be calculated using the following equations:
(8) xA = (PA/PM)exp[(ΔHvapA/R)*(1/TB - 1/TM)] (9) YB = (αxB)/(1 + (α-1)xB)
Equation 8 represents the bottom line of the temperature-mole fraction diagram (Fig. 6), while Eq. 9 represents the upper line.
Fractional Distillation
One simple distillation cycle is represented by the path 1-2-3-4 in Fig. 6. Starting from a mixture of B = 20% and A = 80% at room temperature (1), if the mixture is heated, it will reach its boiling point TM (2) with the same composition. The vapor has a different composition (3), and when the vapor is cooled, it condenses to a liquid (4) that is richer in the lower-boiling component B.
However, the required number of distillation steps (n) is usually greater than
- To achieve a high-efficiency distillation, an appropriate fractionating column is necessary, which includes n-1 theoretical plates in addition to the final distillation plate.
The fractionating column can take different forms, such as an open tube column, a Vigreux column, or a packed column. These columns increase the surface area for contact between the vapor and liquid phases, allowing for more efficient separation. Vigreux columns can be used if TA > TB + 25 K, and packed columns can be used even if TA > TB + 1 K.
The Heating Bath and Vacuum Distillation
The heating bath allows for the uniform distribution of heat to the liquid, usually using oil or water. This is important because the boiling point of the mixture, denoted as TM, must be less than 420 K. If this condition is not satisfied, the components A or B may decompose before reaching the boiling point, so TM must somehow be decreased.
Lavoisier had shown the spontaneous evaporation of ether in a vacuum, and Philippe Lebon worked on this idea in 1796 to conceive vacuum distillation. Applying a partial vacuum to the apparatus decreases the temperature at which TM = Pext, and therefore the boiling point of the mixture. In other words, in these conditions, molecules require less energy to leave the surface of the liquid.
Steam Distillation
In 1800, Sir Benjamin Thomson, Count Rumford used open steam as a heating agent, creating steam distillation. This process is still very useful if one wants to separate essences or flavor oils from plant material (leaves or seeds). In this case, component B can be distilled from the mixture at a temperature well below its normal boiling point TB, since it is not soluble in A (water) and can exert its own full vapor pressure TB (Raoult's law cannot apply). B does not have to be a liquid at room temperature: caffeine (a solid) can be steam distilled from green tea, for example.
Continuous and Batch Distillation
Furthermore, all the various distillation methods discussed can be performed in continuous mode (ongoing separation in which a mixture is continuously fed into the process without interruption) or batch mode (where the mixture is added at the beginning, distillate fractions are taken out sequentially, and the remaining bottom fraction is removed at the end).
Main Classes of Distillation
In this section, we learned that there are 4 main classes of distillations: simple, fractional, vacuum, and steam, which can be performed in two modes: continuous and batch.
Applications of Distillation
Historical Uses
The first uses of distillation were alchemic: mercury, nitric acid, sal- ammoniac, acetic acid, and sulfur, together with many other substances (including urine!), were purified through distillation. Today, the "purification" concept is still applied to solvents, after they have been used in laboratory or industrial extractions.
Around the 11th century, distillation was first used in Italy to increase the alcohol percentage of aqueous mixtures derived from the fermentation of plant extracts rich in carbohydrates. Fermentation usually stops when the yeast dies in its own 15% ethanol solution, so the concentration had to be increased by distillation. The results were the spirits, or waters of life, which had different names depending on where and from what they were produced.
Separation of Air
Pure nitrogen, oxygen, and argon are obtained by rectification of air. To separate air by rectification, part of it must be liquefied, which can only be achieved below its critical point (Tc = 132.5K, Pc = 37.7 bar). N2 is then used for ammonia synthesis, as an inert gas, and as a cryogenic liquid, while
