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Unformatted text preview: an enthalpy-concentration diagram, using the supplied data. The result is shown below, where the dashed lines are tie lines for the vapor-liquid equilibrium compositions at the temperatures indicated and a pressure of 1 atm. Exercise 7.56 (continued)
Analysis: (continued) (b) and (c) For the condition of total reflux, passing vapor and liquid streams have the same compositions and are saturated vapor and liquid, respectively. Therefore, while vapor and liquid streams at equilibrium are related by tie lines extending from the saturated liquid curve to the saturated vapor curve, passing vapor and liquid streams are related by vertical operating lines extending from the saturated liquid curve to the saturated vapor curve. By alternating between the equilibrium and operating lines, from the bottoms composition to the distillate composition, the minimum number of stages is determined by the number of tie lines used, as shown in the diagram below for the example of a methanol bottoms mole fraction of 0.04 and a methanol distillate mole fraction of 0.98, where the construction lines are solid lines and the equilibrium lines are numbered from the top. The minimum number of stages is seen to be 6. The value is independent of the feed condition because at total reflux, there is no feed. The temperature of the bottoms is 93.5oC. The temperature of the distillate is 64.7oC...
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