Fractional distillation
Chemical compounds are separated by heating them to a temperature at which one or more fractions of the mixture will vaporize.
The fractional distillation of organic substances played an important role in the 9th-century works attributed to the Islamic alchemist Jabir ibn Hayyan, as for example in the Kitāb al-Sabʿīn ('The Book of Seventy'), translated into Latin by Gerard of Cremona (c. 1114–1187) under the title Liber de septuaginta.
[1] The Jabirian experiments with fractional distillation of animal and vegetable substances, and to a lesser degree also of mineral substances, formed the main topic of the De anima in arte alkimiae, an originally Arabic work falsely attributed to Avicenna that was translated into Latin and would go on to form the most important alchemical source for Roger Bacon (c. 1220–1292).
So, by heating the mixture, the most volatile component (ethanol) will concentrate to a greater degree in the vapor leaving the liquid.
For this reason, ethanol cannot be completely purified by direct fractional distillation of ethanol–water mixtures.
As the distance from the still pot increases, a temperature gradient is formed in the column; it is coolest at the top and hottest at the bottom.
The vapor condenses on the glass platforms, known as trays, inside the column, and runs back down into the liquid below, refluxing distillate.
Vacuum distillation systems operate at reduced pressure, thereby lowering the boiling points of the materials.
The crude oil fractions with higher boiling points: Large-scale industrial towers use reflux to achieve a more complete separation of products.
Alternatively, the more reflux provided for a given desired separation, the fewer theoretical plates are required.
Hence, a distillation column needs more plates than the number of theoretical vapor-liquid equilibrium stages.
Fractional distillation towers or columns are designed to achieve the required separation efficiently.
The purpose of the mechanical design, on the other hand, is to select the tower internals, column diameter, and height.
For the efficient selection of tower internals and the accurate calculation of column height and diameter, many factors must be taken into account.
Some of the factors involved in design calculations include feed load size and properties and the type of distillation column used.
Packing columns are normally used for smaller towers and loads that are corrosive or temperature-sensitive or for vacuum service where pressure drop is important.
In the oil refining industry, the design and operation of fractionation towers is still largely accomplished on an empirical basis.
The calculations involved in the design of petroleum fractionation columns require in the usual practice the use of numerable charts, tables, and complex empirical equations.
In recent years, however, a considerable amount of work has been done to develop efficient and reliable computer-aided design procedures for fractional distillation.
