Evaporation (the separation of one, two, or several liquids)

EVAPORATION, the separation of one, two, or several liquids from solid bodies or from each other by means of heating, whereby the separated liquid is converted into a vaporous state and volatilizes, while non-volatile or sparingly volatile substances remain in the vessel where the taken mixture was originally placed. Evaporation differs from distillation, in which the evaporated liquid is collected anew for further use. The sources of heat for evaporation are the sun and fuel. Natural solar heat is utilized in southern latitudes when obtaining salt from sea or lake water. In the temperate zone, so-called graduation towers are used for this purpose—structures in which the saline solution trickles down through bundles of brushwood, whereby a significant increase in the evaporation surface of the liquid is achieved. Evaporation is used in obtaining many chemical-pharmaceutical and galenic preparations, such as Glauber's salt, Epsom salt, and other salts, opium and licorice extracts, sabraz extract, and others. When obtaining pharmaceutical preparations by means of evaporation, it is important to prevent overheating and decomposition (for example, plant extracts), which can sometimes completely deprive them of their therapeutic action. In evaporation by means of fuel combustion, the following cases are distinguished: 1) direct heating of the vessel with the evaporated liquid by a flame (burning furnace gases); 2) evaporation by heated air; 3) heating by steam, liquid, or by means of a solid mass (steam, water, oil, sand, and other baths). When heating by steam, a distinction is made between: a) live steam, when the latter is released through an open tube directly into the evaporated liquid (used exclusively for separating low-boiling liquids with a boiling point below 100°); b) closed steam, supplied to the evaporated liquid in closed pipes (usually coils) or in so-called steam jackets enclosing the heated vessel, whereby 1 kg of saturated steam yields 530 calories of heat upon condensation. This method of heating is standard in pharmaceutical production. In this case, the temperature of the evaporated liquid does not rise above the temperature of the steam used for heating, which eliminates the danger of scorching or decomposition of the dissolved substances. For the evaporation of solutions that decompose at the boiling point of the solvent under ordinary atmospheric pressure (for example, plant extracts containing alkaloids or glucosides), vacuum apparatuses are used, in which evaporation is carried out in a rarefied space.