Two-phase systems boiling, condensing and distillation

The last example of Chapter 11 dealt with the case where a liquid and a gas were present together, but the gas was inert with respect to the liquid. However, there are a large number of systems on a process plant where a liquid and its own vapour are present together evaporators, condensers, steam drums, deaerators, refrigeration systems, stills and distillation columns. These systems exist in a state of vapour-liquid equilibrium, and their behaviour is significantly different from the gas-liquid system dealt with in Chapter 11, Section 11.6. The liquid and its vapour will have the same temperature, and it will not be possible to decouple the mass and energy balance equations for the liquid from those of the vapour. The way to obtain the necessary time differentials explicitly is to use the Method of Referred Derivatives. 

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Two-phase systems boiling, condensing and distillation 119 where... 

Consider transport across the phase boundary shown in Figure 11.3. We shall denote the two bulk phases by L and V and the interface by I. Though the analysis below is developed for liquid-vapor interphase transport the formalism is generally valid for all two-phase systems. Therefore, what follows applies equally to distillation, stripping, and absorption operations. With a few modifications (to be described later), the analysis below may be used in the determination of rates of condensation, evaporation, vaporization, and boiling. 

An example of heterogeneous azeotropic distillation is the system ethanol and water with benzene as entrainer (Figure 3.3.20). In a first column (without entrainer, column I in Figure 3.3.20b), the binary ethanol-water mixture is separated by normal distillation. An azeotrope with about 90 mol.% ethanol (96wt%) leaves the column on top (A) while water forms the bottom product. The azeotrope is fed to a second column where benzene (recycle of a phase rich in benzene from the separator of the top products of column II and III) is added as entrainer. A new low-boiling heterogeneous azeotrope (B) leaves column II as distillate, and pure ethanol remains as bottom product. After condensation, the heterogeneous azeotrope separates into two phases rich in either benzene (C) or water (D). The phase rich in benzene is recycled back into column II while the phase rich in water is reconditioned in a third column by distillation. The small amount of benzene is separated as top product (azeotrope B), and a mixture of ethanol and water (E) is recycled into column I. 

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. 

One of the pyrometallurgical methods used is the Imperial Smelting Technique. A roasted zinc concentrate is charged together with coke into a blast furnace. At 1000°C zinc is reduced and its vapor passes from the top of the furnace into a condenser. Here the zinc vapor is cooled by molten lead and the two metals form a molten alloy, which is allowed to cool to 44f)°C. At this temperature the metal system has separated into a lead phase and a zinc phase. The lead is circulated for continued cooling purposes. Crude zinc produced by this process contains about 2% Pb, 0.3% Cd and 0.05% Fe. It is refined by distillation in two columns [33.5]. In the first one, zinc is purified from lead and iron. The separation is based on the fact that the boiling point of zinc is 907°C, while lead boils at 1749°C and iron at 2861°C. In the second column, zinc is separated from cadmium (boiHng point 767°C). Zinc metal with 99.9% purity is obtained. 

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