Introductory Chemistry Concepts & Connections,
Codistillation is distillation which is performed on mixtures in which the two compounds are not miscible.
As a result, simple distillation is usually used only to separate liquids whose boiling points differ greatly (rule of thumb is 25 C),or to separate liquids from involatile solids or oils. For these cases, the vapor pressures of the components are usually sufficiently different that Raoults 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.
Extractive distillation is defined as distillation in the presence of a miscible, high boiling, relatively non-volatile component, the solvent, that forms no azeotrope with the other components in the mixture.
Destructive distillation involves the strong heating of solids (often organic material) in the absence of oxygen (to prevent combustion) to evaporate various high-boiling liquids, as well as thermolysis products. The gases evolved are cooled and condensed as in normal distillation. The destructive distillation of wood to givemethanolis the root of its common name –
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The design of the alembic has served as an inspiration for some modern micro-scale distillation apparatus such as the Hickman stillhead.
Short path distillationis a distillation technique that involves the distillate traveling a short distance, often only a few centimeters. A classic example would be a distillation involving the distillate traveling from one glass bulb to another, without the need for a condenser separating the two chambers. This technique is often used for compounds which are unstable at high temperatures. Advantages are that the temperature of the boiling liquid does not have to be much higher than the boiling point of the distilling substance, and the gases only have to travel a short distance while in the gas-phase before they can be cooled again to a lower temperature.
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As alchemy evolved into the science ofchemistry, vessels called retorts became used for distillations. Both alembics and retorts are forms of glassware with long necks pointing to the side at a downward angle which act as air-cooled condensers to condense the distillate and let it drip downward for collection.
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.
Daltons 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,boilingoccurs 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.
Laboratory scale distillations are almost exclusively run as batch distillations. The device used in distillation, sometimes referred to as astill,consists at a minimum of areboilerorpotin which the source material is heated, acondenserin which the heatedvaporis cooled back to the liquid state, and areceiverin which the concentrated or purified liquid, called thedistillate, is collected. Several laboratory scale techniques for distillation exist (see also distillation types).
Large scaleindustrial distillationapplications include both batch and continuous fractional, vacuum, azeotropic, extractive, and steam distillation. The most widely used industrial applications of continuous, steady-state fractional distillation are in petroleum refineries, petrochemical and chemical plants and natural gas processing plants.
Interactions between the components of the solution create properties unique to the solution, as most processes entail nonideal mixtures, where Raoults law does not hold. Such interactions can result in a constant-boilingazeotropewhich behaves as if it were a pure compound (i.e., boils at a single temperature instead of a range). At an azeotrope, the solution contains the given component in the same proportion as the vapor, so that evaporation does not change the purity, and distillation does not effect separation. For example, ethyl alcohol and water form an azeotrope of 95 percent at 78.2 C.
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 Raoults 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).
Such industrial fractionating towers are also used in air separation, producing liquidoxygen, liquidnitrogen, and high purityargon. Distillation of chlorosilanes also enables the production of high-puritysiliconfor use as asemiconductor.
Perrys Chemical Engineers Handbook,
The idealized model is accurate in the case of chemically similar liquids, such asbenzeneandtoluene. In other cases, severe deviations from Raoults law and Daltons law are observed, most famously in the mixture ofethanoland water. These compounds, when heated together, form anazeotrope, 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 behavior. 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.
In rotary evaporation a vacuum distillation apparatus is used to remove bulksolventsfrom a sample. Typically the vacuum is generated by a water aspirator or a membrane pump.
Some compounds have high boiling points as well as being air sensitive. A simple vacuum distillation system as exemplified above can be used, whereby the vacuum is replaced with an inert gas after the distillation is complete. However, this is a less satisfactory system if one desires to collect fractions under a reduced pressure. To do this a pig adaptor can be added to the end of the condenser, or for better results or for very air sensitive compounds a Perkin triangle apparatus can be used.
Chemistry on its beginnings used retorts as laboratory equipment exclusively for distillation processes.
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Distillation Principles & Practice.
Raoults law assumes that a component contributes to the totalvapor pressureof the mixture in proportion to its percentage of the mixture and its vapor pressure when pure. If one component changes another components vapor pressure, or if the volatility of a component is dependent on its percentage in the mixture, the law will fail.
New York: McGraw-Hill, 1992.ISBN 0070349096
It is a common misconception that in a solution, each component boils at its normalboiling 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 Raoults law and Daltons law.
3rd ed. Upper Saddle River, NJ: Prentice Hall.ISBN 0130874701.
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 Raoults law. Each vaporization-condensation cycle (called atheoretical plate) will yield a purer solution of the more volatile component.In reality, each cycle at a given temperature does not occur at exactly the same position in the fractionating column;theoretical plateis thus a concept rather than an accurate description.
Distillation using semi-microscale apparatus. The jointless design eliminates the need to fit pieces together. The pear-shaped flask allows the last drop of residue to be removed, compared with a similarly-sized round-bottom flask The small holdup volume prevents losses. A pig is used to channel the various distillates into three receiving flasks. If necessary the distillation can be carried out under vacuum using the vacuum adapter at the pig.
Carbohydrate-containing plant materials are allowed to ferment, producing a dilute solution ofethanolin the process. Spirits such as whiskey and rum are prepared by distilling these dilute solutions of ethanol. Other components than ethanol are collected in the condensate, including water, esters, and other alcohols which account for the flavor of the beverage.
Thermometer/Boiling point temperature
4th ed. Upper Saddle River, NJ: Prentice Hall.ISBN 0131402080.
For many cases, the boiling points of the components in the mixture will be sufficiently close that Raoults law must be taken into consideration. Thus,fractional distillationmust be used in order to separate the components well by repeated vaporization-condensation cycles within a packed fractionating column.
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.
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 (dishes) which all contain an enriched, boiling liquid mixture, all with their own vapor phase.
Still pot with stirrer bar/anti-bumping granules
at B/R Instrument Corporation (accessed 8 Sep 2006)
Later, copper alembics were invented. Riveted joints were often kept tight by using various mixtures, for instance a dough made of rye flour.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 stills.
Pervaporation is a method for the separation of mixtures of liquids by partial vaporization through a non-porous membrane.
In a kugelrohr a short path distillation apparatus is typically used (generally in combination with a (high) vacuum) to distill high boiling ( 300 C) compounds. The apparatus consists of an oven in which the compound to be distilled is placed, a receiving portion which is outside of the oven, and a means of rotating the sample. The vacuum is normally generated by using a high vacuum pump.
A simple set-up to distill dry and oxygen-freetoluene.
Flash evaporation (or partial evaporation) is the partial vaporization that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a throttling valve or other throttling device. This process is one of the simplest unit operations.
More theoretical plates lead to better separations. A spinning band distillation system uses a spinning band of Teflon or metal to force the rising vapors into close contact with the descending condensate, increasing the number of theoretical plates.
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:
Distillation: Principles and Practices.
In industrial uses, sometimes a packing material is used in the column instead of trays, especially when low pressure drops across the column are required, as when operating under vacuum. This packing material can either be random dumped packing (1-3 wide) such as Raschig rings or structured sheet metal. Liquids tend to wet the surface of the packing and the vapors pass across this wetted surface, where mass transfer takes place. Unlike conventional tray distillation in which every tray represents a separate point of vapor-liquid equilibrium, the vapor-liquid equilibrium curve in a packed column is continuous. However, when modeling packed columns, it is useful to compute a number of theoretical stages to denote the separation efficiency of the packed column with respect to more traditional trays. Differently shaped packings have different surface areas and void space between packings. Both of these factors affect packing performance.
6th Edition. New York: McGraw-Hill, 1984.ISBN 0070494797
Distillation is used for a variety of purposes.Wateris distilled to remove impuritiesfor example, to remove salt from seawater.Airis distilled to separate its components, such asoxygen, which is used for medical applications, andhelium, used for partyballoons. The distillation ofcrude oilproduces fractions useful for specific purposes such astransport, power generation, and heating. The use of distillation onfermentedsolutions to produce distilled beverages with a higher alcohol content is perhaps the oldest form of distillation, known since ancient times.
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.
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Like vacuum distillation,steam distillationis a method for distilling compounds which are heat-sensitive. This process involves using bubbling steam through a heated mixture of the raw material. By Raoults 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.
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.
The process of reactive distillation involves using the reaction vessel as the still. In this process, the product is usually significantly lower-boiling than its reactants. As the product is formed from the reactants, it is vaporized and removed from the reaction mixture. This technique is an example of a continuous vs. a batch process; advantages include less downtime to charge the reaction vessel with starting material, and less workup.
Distillationis a method of separatingchemical substancesbased on differences in their volatilities. It usually forms part of a larger chemical process, and is thus referred to as a unit operation.
Typical industrial distillation towers
Cold finger – bent to direct condensate
Dry distillation, despite its name, is not truly distillation, but rather a chemical reaction known as pyrolysis in which solid substances are heated in a stronglyreducingatmosphere and any volatile fractions are collected.
Short path vacuum distillation apparatus with vertical condenser (cold finger), to minimize the distillation path;
Large-scale industrial towers use reflux to achieve a more complete separation of products. Reflux refers to the portion of the condensed overhead liquid product from a distillation or fractionation tower that is returned to the upper part of the tower as shown in the schematic diagram of a typical, large-scale industrial distillation tower. Inside the tower, the downflowing reflux liquid provides cooling and condensation of the upflowing vapors thereby increasing the efficacy of the distillation tower. The more reflux is provided for a given number of theoretical plates, the better is the towers separation of lower boiling materials from higher boiling materials. Alternatively, the more reflux is provided for a given desired separation, the fewer theoretical plates are required.
Freeze distillation is an analogous method of purification usingfreezinginstead of evaporation. It is not truly distillation, and does not produce products equivalent to distillation. This process is used in the production of ice beer and ice wine to increaseethanolandsugarcontent, respectively.
Steam distillation of various aromatic herbs and flowers can result in two products; anessential oilas 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 processingand skin care.
Perrys Chemical Engineers Handbook,
1:Stirrer bar/anti-bumping granules2:Still pot3:Fractionating column4:Thermometer/Boiling point temperature5:Teflon tap 16:Cold finger7:Cooling water out8:Cooling water in9:Teflon tap 210:Vacuum/gas inlet11:Teflon tap 312:Still receiver
6th Edition (McGraw-Hill, 1984.ISBN 0070494797)
The Perkin triangle has means via a series ofglassor teflon taps to allow fractions to be isolated from the rest of the still, without the main body of the distillation being removed from either the vacuum or heat source, and thus can remain in a state of reflux. To do this, the sample is first isolated from the vacuum by means of the taps, the vacuum over the sample is then replaced with an inert gas (such asnitrogenorargon) and can then be stoppered and removed. A fresh collection vessel can then be added to the system, evacuated and linked back into the distillation system via the taps to collect a second fraction, and so on, until all fractions have been collected.
A rotary evaporator is able to distill solvents more quickly at lower temperatures through the use of avacuum.
Seader, J. D. and Ernest J. Henley.
Design and operation of a distillation tower depends on the feed and desired products. Given a simple, binary component feed, analytical methods such as the McCabe-Thiele methodor the Fenske equationcan be used. For a multi-component feed, simulation models are used both for design and operation. Moreover, the efficiencies of the vapor-liquid contact devices (referred to as plates or trays) used in distillation towers are typically lower than that of a theoretical 100 percent efficient equilibrium stage. Hence, a distillation tower needs more trays than the number of theoretical vapor-liquid equilibrium stages.
J. D. Seader, and Ernest J. Henley.
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.
If the azeotrope is not considered sufficiently pure for use, there exist some techniques to break the azeotrope to give a pure distillate. This set of techniques are known asazeotropic distillation. Some techniques achieve this by jumping over the azeotropic composition (by adding an additional component to create a new azeotrope, or by varying the pressure). Others work by chemically or physically remove or sequester the impurity. For example, to purify ethanol beyond 95 percent, a drying agent or a desiccant such as potassium carbonate can be added to convert the soluble water into insoluble water of crystallization. Molecular sieves are often used for this purpose as well.
Wexford College Press.ISBN 1929148186.
Distillation was developed into its modern form with the invention of the alembic by Islamic alchemistJabir ibn Hayyanin around 800C.E.He is also credited with the invention of numerous other chemical apparatus and processes that are still in use today.
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 asvacuum distillationand it is commonly found in the laboratory in the form of the rotary evaporator.
Insimple 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 Raoults law.
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 cognac and Scotch whisky. The unique shape of each pot still is said to give the alcohol a distinctive taste. Pot stills made of various materials (wood, clay, stainless steel) are also used by bootleggers in various countries. Small pot stills are also sold for the domestic productionof flower water or essential oils.
Industrial distillationis typically performed in large, vertical cylindrical columns known asdistillation towersordistillation columnswith diameters ranging from about 65 centimeters to 6 meters and heights ranging from about 6 meters to 60 meters or more. When the process feed has a diverse composition, as in distillingcrude oil, liquid outlets at intervals up the column allow for the withdrawal of differentfractionsor products having different boiling points or boiling ranges. The lightest products (those with the lowest boiling point) exit from the top of the columns and the heaviest products (those with the highest boiling point) exit from the bottom of the column and are often called thebottoms.
Stichlmair, Johann G., and James R. Fair. 2001.
The application of distillation can roughly be divided in four groups:laboratory scaleindustrial distillation, distillation of herbs for perfumery and medicinals (herbal distillate) andfood 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.
It is not possible tocompletelypurify 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.