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Distillation is a method of Separating Chemical Substance s based on differences in their Volatilities in a boiling liquid mixture. Distillation usually forms part of a larger chemical process, and is thus referred to as a Unit Operation . Commercially, distillation has a number of uses. It is used to separate Crude Oil into more fractions for specific uses such as Transport , Power Generation and heating. Water is distilled to remove impurities, such as salt from sea water. Air is distilled to separate its components - notably Oxygen , Nitrogen and Argon - for industrial use. Distillation of Fermented Solutions has been used since ancient times to produce Distilled Beverages with a higher alcohol content. HISTORY Early forms of distillation were known to alchemists from the 1st Century AD,123 and the later development of large-scale distillation apparatus occurred in response to demands for spirits. Hypathia Of Alexandria is credited with having invented the distillation apparatus, Biology, Joan Solomon, Pat O'Brien, Peter Horsfall, Nelson Thornes, p.41 and the first exact description of apparatus for distillation is given by Zosimos of Alexandria , in the fourth century. Distillation was further advanced by was first distilled by another Muslim chemist Al-Razi (Rhazes) in the 9th century, producing Kerosene ,4 while Steam Distillation was invented by Avicenna in the early 11th century.5 As Alchemy evolved into the science of Chemistry , vessels called Retort s became used for distillations. Both alembics and retorts are forms of Glassware with long necks pointing to the side at a downward angle which acted as air-cooled condensers to condense the distillate and let it drip downward for collection. Later, copper alembics were invented. Riveted joints were often kept tight by using various mixtures, for instance a dough made of rye flour. Sealing Technique , accessed 16 November 2006 . These alembics often featured a cooling system around the beak, using cold water for instance, which made the condensation of alcohol more efficient. These were called Pot Still s. Today, the retorts and pot stills have been largely supplanted by more efficient distillation methods in most industrial processes. However, the pot still is still widely used for the elaboration of some fine alcohols, such as 2006 . of flower water or Essential Oils . APPLICATIONS OF DISTILLATION The application of distillation can roughly be divided in four groups: Laboratory Scale , Industrial Distillation , distillation of herbs for perfumery and medicinals ( Herbal Distillate ) and Food Processing . The latter two are distinct from the former two, in that in the distillation is not used as a true purification method, but more to transfer all volatiles from the source materials to the distillate. The main difference between laboratory scale distillation and industrial distillation is that laboratory scale distillation is often performed batch-wise, whereas industrial distillation often occurs continuously. In Batch Distillation , the composition of the source material, the vapors of the distilling compounds and the distillate change during the distillation. In batch distillation, a still is charged (supplied) with a batch of feed mixture, which is then separated into its component fractions which are collected sequentially from most volatile to less volatile, with the bottoms (remaining least or non-volatile fraction) removed at the end. The still can then be recharged and the process repeated. In Continuous Distillation , the source materials, vapors and distillate are kept at a constant composition by carefully replenishing the source material and removing fractions from both vapor and liquid in the system. This results in a better control of the separation process. IDEALIZED DISTILLATION MODEL It is a common misconception that in a solution, each component boils at its normal Boiling Point - the vapors of each component will collect separately and purely. This, however, does not occur even in an idealized system. Idealized models of distillation are essentially governed by Raoult's Law and Dalton's Law . Raoult's law assumes that a component contributes to the total Vapor Pressure of the mixture in proportion to its percentage of the mixture and its vapor pressure when pure. If one component changes another component's vapor pressure, or if the volatility of a component is dependent on its percentage in the mixture, the law will fail. Dalton's law states that the total vapor pressure is the sum of the vapor pressures of each individual component in the mixture. When a multi-component system is heated, the vapor pressure of each component will rise, thus causing the total vapor pressure to rise. When the total vapor pressure reaches the ambient pressure, Boiling occurs and liquid turns to gas throughout the bulk of the solution. Note that a given mixture has one boiling point, when the components are mutually soluble. The idealized model is accurate in the case of chemically similar liquids, such as Benzene and Toluene . In other cases, severe deviations from Raoult's law and Dalton's law are observed, most famously in the mixture of Ethanol and water. These compounds, when heated together, form an Azeotrope , in which the boiling temperature of the mixture is lower than the boiling temperature of each separate liquid. Virtually all liquids, when mixed and heated, will display azeotropic behaviour. Although there are Computational Methods that can be used to estimate the behavior of a mixture of arbitrary components, the only way to obtain accurate Vapor-liquid Equilibrium data is by measurement. It is not possible to ''completely'' purify a mixture of components by distillation, as this would require each component in the mixture to have a zero Partial Pressure . If ultra-pure products are the goal, then further Chemical Separation must be applied. Batch distillation See Also: Batch distillation Heating an ideal mixture of two volatile substances A and B (with A having the higher volatility, or lower boiling point) in a batch distillation setup (such as in an apparatus depicted in the opening figure) until the mixture is boiling results in a vapor above the liquid which contains a mixture of A and B. The ratio between A and B in the vapor will be different from the ratio in the liquid: the ratio in the liquid will be determined by how the original mixture was prepared, while the ratio in the vapor will be enriched in the more volatile compound, A (due to Raoult's Law, see above). The vapor goes through the condenser and is removed from the system. This in turn means that the ratio of compounds in the remaining liquid is now different from the initial ratio (i.e. more enriched in B than the starting liquid). The result is that the ratio in the liquid mixture is changing, becoming richer in component B. This causes the boiling point of the mixture to rise, which in turn results in a rise in the temperature in the vapor, which results in a changing ratio of A : B in the gas phase (as distillation continues, there is an increasing proportion of B in the gas phase). This results in a slowly changing ratio A : B in the distillate. If the difference in vapor pressure between the two components A and B is large (generally expressed as the difference in boiling points), the mixture in the beginning of the distillation is highly enriched in component A, and when component A has distilled off, the boiling liquid is enriched in component B. Continuous distillation In continuous distillation, the process is different from the above in that fractions are withdrawn from both the vapor and the liquid at such a speed that the combined ratio of the two fractions is exactly the same as the ratio in the starting mixture. In this way a stream of enriched component A and a stream of enriched component B is obtained. Moreover, a stream of crude mixture (which has the same ratio of A and B as the mixture in the still) can be added to the distilling mixture to replenish the liquid, meaning that the system can be run continuously. General improvements Both batch and continuous distillations can be improved by making use of a Fractionating Column on top of the distillation flask. The column improves separation by providing a larger surface area for the vapor and condensate to come into contact. This helps it remain at equilibrium for as long as possible. The column can even exist of small subsystems ('trays' or 'dishes') which all contain an enriched, boiling liquid mixture, all with their own vapor-liquid equilibrium. There are differences between laboratory-scale and industrial-scale fractionating columns, but the principles are the same. Examples of fractionating columns (in increasing efficacy) include:
LABORATORY SCALE DISTILLATION Laboratory scale distillations are almost exclusively run as batch distillations. The device used in distillation, sometimes referred to as a ''). Simple distillation In simple distillation, all the hot vapors produced are immediately channeled into a condenser which cools and condenses the vapors. Thus, the distillate will not be pure - its composition will be identical to the composition of the vapors at the given temperature and pressure, and can be computed from Raoult's law. As a result, simple distillation is usually used only to separate liquids whose boiling points differ greatly (rule of thumb is 25 °C), ST07 Separation of liquid - liquid mixtures (solutions) , DIDAC by IUPAC or to separate liquids from involatile solids or oils. For these cases, the vapor pressures of the components are usually sufficiently different that Raoult's law may be neglected due to the insignificant contribution of the less volatile component. In this case, the distillate may be sufficiently pure for its intended purpose. Fractional distillation See Also: Fractional distillation For many cases, the boiling points of the components in the mixture will be sufficiently close that Raoult's law must be taken into consideration. Thus, fractional distillation must be used in order to separate the components well by repeated vaporization-condensation cycles within a packed fractionating column. As the solution to be purified is heated, its vapors rise to the Fractionating Column . As it rises, it cools, condensing on the condenser walls and the surfaces of the packing material. Here, the condensate continues to be heated by the rising hot vapors; it vaporizes once more. However, the composition of the fresh vapors are determined once again by Raoult's law. Each vaporization-condensation cycle (called a '' Theoretical Plate '') will yield a purer solution of the more volatile component. Fractional Distillation In reality, each cycle at a given temperature does not occur at exactly the same position in the fractionating column; ''theoretical plate'' is thus a concept rather than an accurate description. More theoretical plates lead to better separations. A 2006 ) Steam distillation See Also: Steam distillation Like vacuum distillation, steam distillation is a method for distilling compounds which are heat-sensitive. This process involves using bubbling steam through a heated mixture of the raw material. By Raoult's law, some of the target compound will vaporize (in accordance with its partial pressure). The vapor mixture is cooled and condensed, usually yielding a layer of oil and a layer of water. Steam distillation of various Aromatic herbs and flowers can result in two products; an Essential Oil as well as a watery Herbal Distillate . The Essential Oils are often used in perfumery and Aromatherapy while the watery distillates have many applications in Aromatherapy , Food Processing and Skin Care . usually boils at 189 °C. Under a vacuum, it distills off into the receiver at only 70 °C.]] Vacuum distillation See Also: Vacuum distillation Some compounds have very high boiling points. To boil such compounds, it is often better to lower the pressure at which such compounds are boiled instead of increasing the temperature. Once the pressure is lowered to the vapor pressure of the compound (at the given temperature), boiling and the rest of the distillation process can commence. This technique is referred to as vacuum distillation and it is commonly found in the laboratory in the form of the Rotary Evaporator . This technique is also very useful for compounds which boil beyond their Decomposition Temperature at atmospheric pressure and which would therefore be decomposed by any attempt to boil them under atmospheric pressure. Air-sensitive vacuum distillation | ||
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