Distillation

I

Introduction

Distillation, process of heating a liquid until its more volatile constituents pass into the vapor phase, and then cooling the vapor to recover such constituents in liquid form by condensation. The main purpose of distillation is to separate a mixture of several components by taking advantage of their different volatilities, or the separation of volatile materials from nonvolatile materials. In evaporation and in drying, the purpose usually is to obtain the less volatile constituent; the more volatile constituent, in most cases water, is discarded. In distillation, on the other hand, the principal object of the operation is to obtain the more volatile constituent in pure form. The removal of water from glycerin by vaporizing the water, for example, is called evaporation, but the removal of water from alcohol by vaporizing the alcohol is called distillation, although similar apparatus is used in both cases.

If the difference in volatility (and hence in boiling point) between the two constituents is great, complete separation may be easily accomplished by a single distillation (see Boiling Point). Seawater, for example, which contains about 4 percent dissolved solids (principally common salt), may be readily purified by vaporizing the water, condensing the steam thus formed, and collecting the product, distilled water. This product is, for most purposes, equivalent to pure water, although actually it contains some impurities in the form of dissolved gases, the most important of which is carbon dioxide.

If the boiling points of the constituents of a mixture differ only slightly, complete separation cannot be achieved in a single distillation. An important example is the separation of water, which boils at 100° C (212° F), and alcohol, which boils at 78.5° C (173° F). If a mixture of these two liquids is boiled, the vapor that rises is richer in alcohol and poorer in water than the liquid from which it came, but it is not pure alcohol. In order to concentrate a 10 percent solution of alcohol (such as might be obtained by fermentation) to obtain a 50 percent solution (common for whiskey), the distillate must be redistilled once or twice, and if industrial (95 percent) alcohol is desired, many redistillations are needed.

II

Theory of Distillation

In the simplest mixture of two mutually soluble liquids, the volatility of each is undisturbed by the presence of the other. In such a case, the boiling point of a 50-50 mixture, for example, would be halfway between the boiling points of the pure substances, and the degree of separation produced by a single distillation would depend only on the vapor pressure, or the volatility, of the separate components at this temperature. This simple relationship was first stated by the French chemist François Marie Raoult (1830-1901) and is called Raoult's law. Raoult's law applies only to mixtures of liquids that are very similar in chemical structure, such as benzene and toluene. In most cases wide deviations occur from this law. Thus, one component is only slightly soluble in the other, its volatility is abnormally increased. In the example above, the volatility of alcohol in dilute aqueous solution is several times as great as predicted by Raoult's law. In extremely concentrated alcohol solutions, the deviation is even more striking: The distillation of 99 percent alcohol produces vapor that has less than 99 percent alcohol. For this reason, alcohol cannot be concentrated by distillation beyond 97 percent, even by an infinite number of distillations.

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III

Distillation Apparatus

Technically, the term still is applied only to the vessel in which liquids are boiled during distillation, but the term is sometimes applied to the entire apparatus, including the fractionating column, the condenser, and the receiver in which the distillate is collected. The term still is also extended to cover apparatus for destructive distillation, or cracking. Stills for laboratory work are usually made of glass, but industrial stills are generally made of iron or steel. In cases in which iron would contaminate the product, copper is often employed, and small stills for the distillation of whiskey are frequently made of glass and copper. (The term retort is also sometimes used for a still.)

IV

Fractional Distillation

If a portion of the distillate in the above example is returned from the condenser and made to drip down through a long column onto a series of plates, and if the vapor as it rises on its way to the condenser is made to bubble through this liquid at each plate, the vapor and liquid will interact so that some of the water in the vapor condenses and some of the alcohol in the liquid vaporizes. The interaction at each plate is thus equivalent to a redistillation, and by building a column with a sufficient number of plates, 95 percent alcohol can be obtained in a single operation. Moreover, by feeding the original 10 percent alcohol solution gradually at a point in the middle of the column, virtually all the alcohol may be stripped from the water as it descends to the lowest plate, so that no alcohol need be wasted.

This process, known as rectification, fractionation, or fractional distillation, is common in industrial usage, not only for simple mixtures of two components (such as alcohol and water in fermentation products, or oxygen and nitrogen in liquid air) but also for highly complex mixtures such as those found in coal tar and petroleum. The fractionating column most often used is the so-called bubble tower, in which the plates are arranged horizontally a few centimeters apart and the ascending vapors are forced to rise through bubble caps in each plate and then bubble through the liquid. The plates are baffled so that the liquid flows from left to right on one plate, then overflows onto the plate below, and there flows from right to left. If the interaction between liquid and vapor is incomplete, or if frothing and entrainment occur so that some of the liquid is carried up by the vapor to the plate above, five actual plates might be required to do the work of four theoretical plates, producing four redistillations. An inexpensive equivalent of a bubble tower is the so-called packed column, in which the liquid flows down over a packing of earthenware rings or bits of glass tubing.

The only disadvantage of fractional distillation is that a large fraction (as much as one-half) of the condensed distillate must be refluxed, or returned to the top of the tower and eventually boiled again, and more heat must therefore be supplied. On the other hand, the continuous operation made possible by fractionation allows great heating economies, because the outgoing distillate may be used to preheat the incoming feed.

When the mixture consists of many components, they are drawn off at different points along the tower. Industrial distillation towers for petroleum often have over 100 plates, with as many as ten different fractions being drawn off at suitable points. Towers with more than 500 plates have been used for the separation of isotopes (see Isotope) by distillation.