Simple distillation phase diagram

Liquid-vapor phase diagrams, and boiling-point diagrams in particular, are of importance in connection with distillation, which usually has as its object the partial or complete separation of a liquid solution into its components. Distillation consists basically of boiUng the solution and condensing the vapor into a separate receiver. A simple one-plate distillation of a binary system having no maximum or minimum in its boiling-point curve can be understood by reference to Fig. 3. Let the mole fraction of B in the initial solution be represented by... 

Short-Term Exposure Limit (STEL), 12 silver mirror test, 643 silver nitrate test, 249, 645-646 simple crystallization, 87 simple distillation. See also distillation defined, 61-63 experiment, 130-132 improvement on, 64 phase diagrams, 63 setup, 62... 

Figure 13.10 on the next page summarizes the general appearance of some relatively simple temperature-composition phase diagrams of binary systems. If the system does not form an azeotrope (zeotropic behavior), the equilibrated gas phase is richer in one component than the liquid phase at all liquid compositions, and the liquid mixture can be separated into its two components by fractional distillation. The gas in equilibrium with an azeotropic mixture, however, is not enriched in either component. Fractional distillation of a system with an azeotrope leads to separation into one pure component and the azeotropic mixture. 

Several additional features of these systems are to be noted. First and foremost is the fact that mixtures that lead to azeotrope formation cannot be separated into their constituent components by simple distillation. This is best seen from the boiling-point diagram shown in Figure 6.20a, where we indicate the pathway that results from a process of repeated vaporization and condensation. Starting with a liquid feed at F, the mixture is brought to a boil at K and the first vapor (L) collected and condensed. The cycle of vaporization and condensation is then repeated until the azeotropic composition at A is reached. At this point no further enrichment by vaporization is obtained, as the compositions in the two phases are the same. The mixture continues to boil at a constant temperature, yielding a mixture of constant composition, until the liquid charge is exhausted. 

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