Steam distillation is aseparation process that consists ofdistilling water together with othervolatile and non-volatile components. Thesteam from the boiling water carries the vapor of the volatiles to acondenser; both are cooled and return to the liquid or solid state, while the non-volatile residues remain behind in the boiling container.
If, as is usually the case, the volatiles are not miscible with water, they will spontaneously form a distinctphase after condensation, allowing them to be separated bydecantation or with aseparatory funnel.[1]
Steam distillation can be used when theboiling point of the substance to be extracted is higher than that of water, and the starting material cannot be heated to that temperature because ofdecomposition 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 volatileessential oils from plant material.[2] for example, to extractlimonene (boiling point 176 °C) fromorange peels.
Steam distillation once was a popular laboratory method for purification of organic compounds, but it has been replaced in many such uses byvacuum distillation andsupercritical 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 orhydrodistillation, the water is mixed with the starting material in the boiling container. Indirect steam distillation, the starting material is suspended above the water in the boiling flask, supported by a metalmesh or perforated screen. Indry steam distillation, the steam from aboiler 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 becomingsuperheated steam), for more efficient extraction.[4]
Steam distillation is used in many of the recipes given in theKitāb al-Taraffuq fī al-ʿiṭr ('Book of Gentleness on Perfume'), also known as theKitāb Kīmiyāʾ al-ʿiṭr wa-l-taṣʿīdāt ('Book of the Chemistry of Perfume and Distillations'), attributed to the early Arabic philosopheral-Kindi (c. 801–873).[5] Steam distillation was also used by the Persian philosopher and physicianAvicenna (980–1037) to produceessential oils by adding water to rose petals and distilling the mixture.[6] The process was also used byal-Dimashqi (1256–1327) to producerose water on a large scale.[7]
Every substance has somevapor 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 thepartial 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 anazeotrope 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.
Steam distillation is often employed in the isolation ofessential oils, for use inperfumes, for example. In this method, steam is passed through the plant material containing the desired oils.Eucalyptus oil,camphor oil andorange oil are obtained by this method on an industrial scale.[2] Steam distillation is a means of purifying fatty acids, e.g. fromtall oils.[9]
Steam distillation is sometimes used in thechemical 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 ofbenzophenone, steam is employed to first recover unreactedcarbon tetrachloride and subsequently to hydrolyze the intermediate benzophenone dichloride into benzophenone, which is in fact not steam distilled.[11] In one preparation of apurine, steam distillation is used to remove volatilebenzaldehyde from nonvolatile product.[12]
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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]
TheLikens-Nickerson apparatus simultaneously performs steam distillation and extraction. It is typically used to isolate targetorganic compounds for further analysis.[16]
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