distillation
distillation
[‚dis·tə′lā·shən]distillation
Distillation
the separation of liquid mixtures into various fractions, which differ by composition; the process is based on the differences in boiling point of the components.
Different methods of distillation are used, depending on the physical properties of the components of the liquid mixtures. Simple distillation is accomplished by partial vaporization of the boiling liquid, with continuous removal and subsequent condensation of the vapors formed. Since the vapors above a boiling liquid mixture contain more of the low-boiling components than the liquid, the condensate (called the distillate) is enriched and the unevaporated liquid (residue) is impoverished in them. The initial liquid mixture boils in a still. The vapors formed are led off continuously to a condenser where the distillate forms and then drops into a receptacle. In simple distillation there is a continuous drop in the amount of low-boiling components in the vapor and liquid phases. Hence, the formation of the distillate is a function of time. Simple distillation as carried out in this way is a batch process. Semicontinuous distillation is used to accelerate the process; here the still is fed continuously with an amount of starting mixture equal to the vapor drawn off.
Fractional distillation is one of the varieties of simple distillation, used to separate a liquid mixture into fractions boiling over narrow temperature ranges. Distillates of different composition are led off successively in a number of collectors. The first distillate fraction is sent to one collector; it is richest in the low-boiling components. A second collector receives a less rich portion, a third receives one that is still poorer, and so on. In each distillate (fraction) one or more components of the starting mixture with similar boiling points predominate. To improve mixture separation, simple distillation is often combined with countercurrent fractionation; here the vapors formed in the still are partly condensed in a fractionating column, the condensate (reflux) is continuously returned to the still, and the vapors remaining after the fractionating column go to a condenser, from which the distillate drops into the collector. By this means the distillate is enriched to a greater extent in low-boiling components, since the partial condensation (fractionation) of vapors leads primarily to the condensation of high-boiling vapors.
Equilibrium distillation (flash evaporation) is characterized by the evaporation of part of the liquid and prolonged contact between the vapors and the unevaporated liquid, until phase equilibrium is reached. The mixture for separation passes through tubes that are heated externally by fuel gases. The resultant liquid-vapor mixture, a composition close to a state of equilibrium, passes to a separator, where the liquid is separated mechanically from the vapor. From the separator the vapors pass to the condenser, where the condensate drops into a receiver and the liquid remaining in the separator passes to a collection tank. In this process the liquid-vapor equilibrium is determined by the materials balance and the phase equilibrium conditions. Equilibrium distillation is rarely used for two-component mixtures, but good results are usual with multicomponent mixtures, from which it is possible to obtain fractions that differ considerably in composition.
Distillation in a current of steam or inert gases is used when it is necessary to lower the distillation temperature, when separating components that are not heat-resistant, and when separating substances with low vaporization temperature from components with high vaporization temperature. Steam or inert gas is bubbled through the liquid layer. In steam distillation the mixture of steam and the vapors of volatile components that form are led off from the apparatus, condensed, and cooled. The composition of the vapors formed in the still is independent of the composition of the liquid, and the boiling point of the mixture is always lower than that of each component at any given pressure. In inert gas distillation the components of the solution are evaporated in a current of the gas, even if the solution does not boil; the generation of vapor during evaporation can take place at any temperature, independent of the external pressure, making it possible to run the process at low temperatures.
Molecular distillation is based on the separation of liquid mixtures by their free evaporation in a high vacuum—133-13.3 millinewtons per sq m (10-3-10-4 mm mercury stokes)—at temperatures below their boiling points. The process is effected with a distance between evaporation and condensation surfaces less than the mean free path of the molecules of the substance being distilled. Because of the vacuum, the molecules of the vapor move from the evaporating to the condensing surface with a minimum number of collisions. In molecular distillation, the change in vapor composition compared with that for the liquid is determined by the difference in the evaporation rates of the various components. Hence the method can be used to separate mixtures whose components have the same vapor pressure. At a given liquid temperature and vapor pressures corresponding to it, the rate of molecular distillation rises with a decrease in pressure in the apparatus.
In order to cut the time required for diffusion of the molecules of a volatile component from a deep liquid layer to the evaporation surface, distillation is, in modern molecular stills, carried out with very thin liquid films; this also makes it possible to cut the time that the substance remains on the vapor-releasing surface and reduces the danger of thermal decomposition. Apparatus with horizontal and vertical vapor-releasing surfaces are used for molecular distillation; centrifugal plants have been used most extensively industrially, where the process is characterized by the minimum possible liquid film thickness (mean 0.05 mm) and time on the heating surface (0.03-1.2 sec). In the centrifugal apparatus the mixture for separation is fed to an evaporator, which is a rapidly rotating cone (sometimes a disk). Centrifugal force carries the liquid from the center to the periphery (or top). The vapors of the substance being distilled are collected on a fixed condenser, which is placed parallel to the vapor-releasing surface, and the distillate is drawn off continuously. The residue after distillation is shed into an annular gutter and removed from the still. The separation effect is increased by placing a number of the stills in series.
Molecular distillation is used for separating and purifying high-molecular and thermally unstable organic substances— for example, to purify the esters of sebacic, stearic, oleic, and other acids; to extract vitamins from fish and various vegetable oils; and to manufacture drugs and vacuum oils.
In metallurgy, distillation is used in pyrometallurgical processes, which are based on the conversion of reducible metals to a vapor-like state, followed by condensation. Metallurgical distillation represents a combination of chemical (oxidation-reduction reactions) and physical (evaporation-condensation) processes. Reduction is effected either with carbonaceous reducing agents or metallothermically. It is possible to separate free metal in the dead roasting of sulfide concentrates. In distillation the extent of separation is determined by the difference in composition between distilland mixture and its vapor. During distillation the complete passage of a metal into the gaseous phase is accomplished by reducing the metals at temperatures and pressures that ensure that the reduced metal will be formed in a vapor-like state of aggregation.
REFERENCES
Kasatkin, A. G. Osnovnye protsessy i apparaty khimicheskoi tekhnologii, 8th ed. Moscow, 1971.Gel’perin, N. I. Distilliatsiia i rektifikatsiia. Moscow-Leningrad, 1947.
Bagaturov, S. A. Teoriia i raschet peregonki i rektifikatsii. Moscow, 1961.
Tsiborovskii, la. Protsessy khimicheskoi tekhnologii. Leningrad, 1958. (Translated from Polish.)
Matrozov, V. I. Apparatura dlia molekuliarnoi distilliatsii. Moscow, 1954.
Chizhikov, D. M. Metallurgiia tiazhelykh tsvetnykh metallov. Moscow-Leningrad, 1948.
Lakernik, M. M. Elektrotermiia v metallurgii medi, svintsa, i tsinka. Moscow, 1964.
V. L. PEBALK