Distillation Accessscience from McgrawHill Education
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6/19/2017 Distillation AccessScience from McGrawHill Education (http://www.accessscience.com/) Distillation Article by: King, C. Judson University of California, Berkeley, California. Last updated: 2014 DOI: https://doi.org/10.1036/10978542.201100 (https://doi.org/10.1036/10978542.201100) Content Hide Simple distillations Fractional distillation Vaporliquid equilibria Distillation pressure Molecular distillation Extractive and azeotropic distillation Enhancing energy efficiency Computational methods Stage efficiency Links to Primary Literature Additional Readings A method for separating homogeneous mixtures based upon equilibration of liquid and vapor phases. Substances that differ in volatility appear in different proportions in vapor and liquid phases at equilibrium with one another. Thus, vaporizing part of a volatile liquid produces vapor and liquid products that differ in composition. This outcome constitutes a separation among the components in the original liquid. Through appropriate configurations of repeated vaporliquid contactings, the degree of separation among components differing in volatility can be increased manyfold. See also: Phase equilibrium (/content/phaseequilibrium/505500) Distillation is by far the most common method of separation in the petroleum, natural gas, and petrochemical industries. Its many applications in other industries include air fractionation, solvent recovery and recycling, separation of light isotopes such as hydrogen and deuterium, and production of alcoholic beverages, flavors, fatty acids, and food oils. Simple distillations The two most elementary forms of distillation are a continuous equilibrium distillation and a simple batch distillation (Fig. 1). http://www.accessscience.com/content/distillation/201100 1/10 6/19/2017 Distillation AccessScience from McGrawHill Education Fig. 1 Simple distillations. (a) Continuous equilibrium distillation. (b) Simple batch distillation. In a continuous equilibrium distillation, a continuously flowing liquid feed is heated or reduced in pressure (flashed) so as to cause partial vaporization. The vapor and liquid disengage while flowing through an open drum, and the products emerge as vapor and liquid streams. The vapor product can be condensed to form a liquid distillate. It is also possible to use a vapor feed, subjected to cooling and thereby partial condensation, again followed by disengagement of the resultant vapor and liquid in an open drum. See also: Vapor condenser (/content/vaporcondenser/727300) In a simple batch distillation, an entire batch of liquid is initially charged to a vessel and is then heated, typically by condensation of steam inside a metal coil within the vessel. Vapor is thereby continuously generated, and may be condensed to form a liquid distillate, which is collected. In the batch distillation, increments of vapor are formed in equilibrium with all liquid compositions ranging from the original to the final, whereas the continuous equilibrium distillation gives vapor in equilibrium with only the final liquid composition. Since the distillate consists primarily of the more volatile components and the feed liquid contains more of these substances than does the final liquid, the simple batch distillation gives a more enriched distillate than does the continuous equilibrium distillation. Fractional distillation Unless the vapor pressures of the species being separated are very dissimilar, a simple distillation does not produce highly purified products. Product purities can be increased by repeated partial vaporizations and condensations (Fig. 2a). The liquid from an initial continuous equilibrium distillation (L′ 0) can be partially vaporized by additional heating. The remaining liquid (L′ 1) can again be heated and partially vaporized, forming another liquid (L′ 2); and so forth. Each liquid is progressively enriched in the less volatile substances. Similarly, successive partial condensations of the vapor fraction (V) from the initial continuous equilibrium distillation produce vapor products successively enriched in the more volatile components. See also: Vapor pressure (/content/vaporpressure/727700) http://www.accessscience.com/content/distillation/201100 2/10 6/19/2017 Distillation AccessScience from McGrawHill Education Fig. 2 Distillation by repeated vaporliquid equilibrations. (a) Increasing product purities by successive partial vaporizations and condensations. (b) Recycling intermediate products and elimination of intermediate heating and cooling steps; equivalent to continuous fractional distillation. Symbols are explained in the text. http://www.accessscience.com/content/distillation/201100 3/10 6/19/2017 Distillation AccessScience from McGrawHill Education The process involved in successive partial vaporizations and condensations (Fig. 2a) would lead to only very small amounts of the most enriched products, along with numerous streams having intermediate compositions. A logical step is to recycle each of these intermediate streams to the prior vessel in the sequence of contactors (Fig. 2b). Recycling of the intermediate vapors and liquids has another highly beneficial effect in that it negates the need for intermediate heaters and coolers. The resultant process (Fig. 2b) is known as continuous fractional distillation. It is usually carried out in a distillation column (Fig. 3), which is a simpler, more compact form of equipment than the cascade of vessels used in the process of recycling intermediate vapors and liquids. However, the processes are functionally equivalent. Fig. 3 Distillation column, carrying out continuous fractional distillation. A distillation column consists of a series of stages, also called trays or plates, on which vapor and liquid are contacted. The liquid and vapor pass through the column in opposite directions, the liquid downward and the vapor upward, a pattern known as countercurrent flow. Liquid is held onto the trays by weirs over which the liquid exits the tray, passing into a vertical downcomer that conveys the liquid to the tray below. Vapor rises up the column through the trays successively. Various tray designs are used. Sieve trays contain a large number of small holes, of the order of 4 mm (0.16 in.) in diameter, and the velocity of the vapor through these holes is sufficient to prevent leakage of liquid through them. Valvecap trays contain much larger holes, of the order of 3 cm (1.2 in.), each fitted with a freefloating bonnet. Bubblecap trays are common in older columns and consist of a fixed bonnet, which forces the vapor out horizontally through slots. Tray design to optimize tray efficiency (see below), tray hydraulics, stability, and workable loading ranges remains an area for innovation in distillation. http://www.accessscience.com/content/distillation/201100 4/10 6/19/2017 Distillation AccessScience from McGrawHill Education Sometimes distillation columns are constructed without discrete stages and simply have vapor and liquid flowing countercurrently through a bed of divided solid material, called packing. The form of the packing is usually chosen to give high surface area per unit bed volume, as well as a high porosity. See also: Distillation column (/content/distillation column/270800) It is necessary for the operation of continuous fractional distillations (Figs. 2b and 3) that there be a cooler at the top and a heater at the bottom of the cascade of countercurrent stages. The overhead cooler is known as the condenser, and the liquid that it generates is known as reflux. The heater is termed a reboiler. The stages above the point at which the feed is supplied are known as the rectifying, or enriching, section, and they serve to remove the less volatile components from the more volatile components that become the overhead product. The stages below the feed point are known as the stripping section, since they serve to strip the more volatile components out of the less volatile components that compose the bottom product. The feed can be either vapor or liquid, or a mixture of the two. There can be multiple feeds, which may enter the column at different locations. There can also be intermediate products, or sidestreams, withdrawn. A batch distillation can also be carried out in staged equipment. The vertical section of the batch still (Fig. 1b) is then equipped with trays and serves as a rectifying section, and an overhead condenser serves to generate liquid reflux, which is returned to the top of the column. Batch distillation with a rectifying section serves to remove the less volatile components to a greater extent from the overhead product; however, the absence of a stripping section means that there cannot be a similar added purification of the remaining liquid. Vaporliquid equilibria The separation accomplished in a distillation relates to the difference in composition of vapor and liquid phases at equilibrium. These relationships are the subject matter of phaseequilibrium thermodynamics. See also: Chemical thermodynamics (/content/chemicalthermodynamics/128200); Gas (/content/gas/280300); Liquid (/content/liquid/386100) Compositions of phases in equilibrium can be determined experimentally. A common device for measuring vaporliquid equilibria is the Othmer still, in which a liquid is heated to form equilibrium vapor, which is then condensed and delivered to a distillate receiver, from which distillate liquid drains back to mix with the original liquid. Sampling and analyzing the distillate and the main liquid gives the compositions