Steam distillation

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Steam distillation apparatus, showing aniline steam distillation Steam distilation.jpg
Steam distillation apparatus, showing aniline steam distillation

Steam distillation is a separation process that consists of distilling water together with other volatile and non-volatile components. The steam from the boiling water carries the vapor of the volatiles to a condenser; both are cooled and return to the liquid or solid state, while the non-volatile residues remain behind in the boiling container.

Contents

If, as is usually the case, the volatiles are not miscible with water, they will spontaneously form a distinct phase after condensation, allowing them to be separated by decantation or with a separatory funnel. [1]

Steam distillation can be used when the boiling point of the substance to be extracted is higher than that of water, and the starting material cannot be heated to that temperature because of decomposition or other unwanted reactions. It may also be useful when the amount of the desired substance is small compared to that of the non-volatile residues. It is often used to separate volatile essential oils from plant material. [2] for example, to extract limonene (boiling point 176 °C) from orange peels.

Steam distillation once was a popular laboratory method for purification of organic compounds, but it has been replaced in many such uses by vacuum distillation and supercritical fluid extraction. It is however much simpler and economical than those alternatives, and remains important in certain industrial sectors. [3]

In the simplest form, water distillation or hydrodistillation, the water is mixed with the starting material in the boiling container. In direct steam distillation, the starting material is suspended above the water in the boiling flask, supported by a metal mesh or perforated screen. In dry steam distillation, the steam from a boiler is forced to flow through the starting material in a separate container. The latter variant allows the steam to be heated above the boiling point of water (thus becoming superheated steam), for more efficient extraction. [4]

History

Steam distillation apparatus Alambic bussard.jpg
Steam distillation apparatus

Steam distillation is used in many of the recipes given in the Kitāb al-Taraffuq fī al-ʿiṭr ('Book of Gentleness on Perfume'), also known as the Kitāb Kīmiyāʾ al-ʿiṭr wa-l-taṣʿīdāt ('Book of the Chemistry of Perfume and Distillations'), attributed to the early Arabic philosopher al-Kindi (c.801–873). [5] Steam distillation was also used by the Persian philosopher and physician Avicenna (980–1037) to produce essential oils by adding water to rose petals and distilling the mixture. [6] The process was also used by al-Dimashqi (1256–1327) to produce rose water on a large scale. [7]

Principle

Every substance has some vapor pressure even below its boiling point, so in theory it could be distilled at any temperature by collecting and condensing its vapors. However, ordinary distillation below the boiling point is not practical because a layer of vapor-rich air would form over the liquid, and evaporation would stop as soon as the partial pressure of the vapor in that layer reached the vapor pressure. The vapor would then flow to the condenser only by diffusion, which is an extremely slow process.

Simple distillation is generally done by boiling the starting material, because, once its vapor pressure exceeds atmospheric pressure, that still vapor-rich layer of air will be disrupted, and there will be a significant and steady flow of vapor from the boiling flask to the condenser.

In steam distillation, that positive flow is provided by steam from boiling water, rather than by the boiling of the substances of interest. The steam carries with it the vapors of the latter.

The substance of interest does not need to be miscible water or soluble in it. It suffices that it has significant vapor pressure at the steam's temperature.

If the water forms an azeotrope with the substances of interest, the boiling point of the mixture may be lower than the boiling point of water. For example, bromobenzene boils at 156 °C (at normal atmospheric pressure), but a mixture with water boils at 95 °C. [8] However, the formation of an azeotrope is not necessary for steam distillation to work.

Applications

A boiling water distiller. Boiling tank on top and holding tank on the bottom. Steam water distiller.JPG
A boiling water distiller. Boiling tank on top and holding tank on the bottom.

Steam distillation is often employed in the isolation of essential oils, for use in perfumes, for example. In this method, steam is passed through the plant material containing the desired oils. Eucalyptus oil, camphor oil and orange oil are obtained by this method on an industrial scale. [2] Steam distillation is a means of purifying fatty acids, e.g. from tall oils. [9]

Steam distillation is sometimes used in the chemical laboratory. Illustrative is a classic preparation of bromobiphenyl where steam distillation is used to first remove the excess benzene and subsequently to purifiy the brominated product. [10] In one preparation of benzophenone, steam is employed to first recover unreacted carbon tetrachloride and subsequently to hydrolyze the intermediate benzophenone dichloride into benzophenone, which is in fact not steam distilled. [11] It one preparation of a purine, steam distillation is used to remove volatile benzaldehyde from nonvolatile product. [12]

Equipment

Hydrodistillation using the Steam distillation apparatus, Clevenger-type apparatus. (A) Power regulator; (B) Heating mantle with round bottom flask containing water and aromatic leaves; (C) Clevenger-type apparatus that returns the hydrosol to the still and maintains the essential oil phase, but only for essential oils that are less dense than water and therefore float; (D) The condenser. Hydrodistillation using the Clevenger-type apparatus - N. Sadgrove and G. Jones, Agriculture 2015, 5(1), 48-102.png
Hydrodistillation using the Steam distillation apparatus, Clevenger-type apparatus. (A) Power regulator; (B) Heating mantle with round bottom flask containing water and aromatic leaves; (C) Clevenger-type apparatus that returns the hydrosol to the still and maintains the essential oil phase, but only for essential oils that are less dense than water and therefore float; (D) The condenser.

On a lab scale, steam distillations are carried out using steam generated outside the system and piped through the mixture to be purified. [14] [1] Steam can also be generated in-situ using a Clevenger-type apparatus. [15]

See also

Related Research Articles

<span class="mw-page-title-main">Distillation</span> Method of separating mixtures

Distillation, also classical distillation, is the process of separating the component substances of a liquid mixture of two or more chemically discrete substances; the separation process is realized by way of the selective boiling of the mixture and the condensation of the vapors in a still.

<span class="mw-page-title-main">Azeotrope</span> Mixture of two or more liquids whose proportions do not change when the mixture is distilled

An azeotrope or a constant heating point mixture is a mixture of two or more components in fluidic states whose proportions cannot be altered or changed by simple distillation. This happens because when an azeotrope is boiled, the vapour has the same proportions of constituents as the unboiled mixture. Azeotropic mixture behavior is important for fluid separation processes.

Fractional distillation is the separation of a mixture into its component parts, or fractions. Chemical compounds are separated by heating them to a temperature at which one or more fractions of the mixture will vaporize. It uses distillation to fractionate. Generally the component parts have boiling points that differ by less than 25 °C (45 °F) from each other under a pressure of one atmosphere. If the difference in boiling points is greater than 25 °C, a simple distillation is typically used.

<span class="mw-page-title-main">Vacuum distillation</span> Low-pressure and low-temperature distillation method

Vacuum distillation or distillation under reduced pressure is a type of distillation performed under reduced pressure, which allows the purification of compounds not readily distilled at ambient pressures or simply to save time or energy. This technique separates compounds based on differences in their boiling points. This technique is used when the boiling point of the desired compound is difficult to achieve or will cause the compound to decompose. Reduced pressures decrease the boiling point of compounds. The reduction in boiling point can be calculated using a temperature-pressure nomograph using the Clausius–Clapeyron relation.

<span class="mw-page-title-main">Rotary evaporator</span> Device used in chemical laboratories

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<span class="mw-page-title-main">Azeotropic distillation</span> Any of a range of techniques used to break an azeotrope in distillation

In chemistry, azeotropic distillation is any of a range of techniques used to break an azeotrope in distillation. In chemical engineering, azeotropic distillation usually refers to the specific technique of adding another component to generate a new, lower-boiling azeotrope that is heterogeneous, such as the example below with the addition of benzene to water and ethanol.

<span class="mw-page-title-main">Geraniol</span> Monoterpenoid and alcohol that is the primary component of citronella oil

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<span class="mw-page-title-main">Aluminium chloride</span> Chemical compound

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<i>tert</i>-Butyl alcohol Chemical compound

tert-Butyl alcohol is the simplest tertiary alcohol, with a formula of (CH3)3COH (sometimes represented as t-BuOH). Its isomers are 1-butanol, isobutanol, and butan-2-ol. tert-Butyl alcohol is a colorless solid, which melts near room temperature and has a camphor-like odor. It is miscible with water, ethanol and diethyl ether.

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<span class="mw-page-title-main">Volatility (chemistry)</span> Tendency of a substance to vaporize

In chemistry, volatility is a material quality which describes how readily a substance vaporizes. At a given temperature and pressure, a substance with high volatility is more likely to exist as a vapour, while a substance with low volatility is more likely to be a liquid or solid. Volatility can also describe the tendency of a vapor to condense into a liquid or solid; less volatile substances will more readily condense from a vapor than highly volatile ones. Differences in volatility can be observed by comparing how fast substances within a group evaporate when exposed to the atmosphere. A highly volatile substance such as rubbing alcohol will quickly evaporate, while a substance with low volatility such as vegetable oil will remain condensed. In general, solids are much less volatile than liquids, but there are some exceptions. Solids that sublimate such as dry ice or iodine can vaporize at a similar rate as some liquids under standard conditions.

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The Marcusson apparatus, Dean-Stark apparatus, Dean–Stark receiver, distilling trap, or Dean–Stark Head is a piece of laboratory glassware used in synthetic chemistry to collect water from a reactor. It is used in combination with a reflux condenser and a distillation flask for the separation of water from liquids. This may be a continuous removal of the water that is produced during a chemical reaction performed at reflux temperature, such as in esterification reactions. The original setup by Julius Marcusson was refined by the American chemists Ernest Woodward Dean (1888–1959) and David Dewey Stark (1893–1979) in 1920 for determination of the water content in petroleum.

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References

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