For example, bromobenzene boils at 156 ☌ (at normal atmospheric pressure), but a mixture with water boils at 95 ☌. 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. It suffices that it has significant vapor pressure at the steam's temperature. The substance of interest does not need to be miscible water or soluble in it. The steam carries with it the vapors of the latter. In steam distillation, that positive flow is provided by steam from boiling water, rather than by the boiling of the substances of interest. 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. The vapor would then flow to the condenser only by diffusion, which is an extremely slow process. 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. Principle Įvery 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. The process was also used by al-Dimashqi (1256–1327) to produce rose water on a large scale. 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. 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. The latter variant allows the steam to be heated above the boiling point of water (thus becoming superheated steam), for more efficient extraction. In dry steam distillation, the steam from a boiler is forced to flow through the starting material in a separate 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 the simplest form, water distillation or hydrodistillation, the water is mixed with the starting material in the boiling container. It is however much simpler and economical than those alternatives, and remains important in certain industrial sectors. 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. for example, to extract limonene (boiling point 176 ☌) from orange peels. It is often used to separate volatile essential oils from plant material. It may also be useful when the amount of the desired substance is small compared to that of the non-volatile residues. 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. 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. 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. Steam distillation is a separation process that consists in distilling water together with other volatile and non-volatile components. Method of separation in organic chemistry Steam distillation apparatus, showing aniline steam distillation
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