Multistage Batch Distillation with Azeotrope

This Demonstration simulates how the reflux ratio, reboiler (still) composition and number of equilibrium stages (all of which can be changed with sliders) affect the distillate composition (displayed above the graph) in a multistage batch distillation (or continuous rectification) for a binary mixture. Each black dot on the - equilibrium curve denotes the composition at that equilibrium stage. Hover the mouse over a composition stage label (still, 1, 2, etc.) to display the composition at that stage. Select among a binary mixture with no azeotrope, a minimum-temperature azeotrope or a maximum-temperature azeotrope. Stage 1 is defined as the first stage above the reboiler. Constant molar overflow, 100% stage efficiency and a complete condenser are assumed.
  • Contributed by: Neil Hendren
  • Additional contributions by: John L. Falconer
  • (University of Colorado Boulder, Department of Chemical and Biological Engineering)

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DETAILS

This model assumes a binary mixture with constant molar flow rates between stages in the column for liquid and vapor. The mass balance forms an operating line:
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where is the liquid molar flow rate, is the vapor molar flow rate, is the distillate mole fraction of the more volatile component ("component "), is the vapor mole fraction of at stage and is the liquid mole fraction of at stage .
The reflux ratio is:
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where is the distillate molar flow rate. The mass balance around the condenser is:
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The operating line can be rewritten in terms of reflux ratio:
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Given an equilibrium curve, with vapor composition as a function of the liquid composition for each stage, i.e. , a linear set of equations must be solved to find distillate composition or required reflux ratio , assuming the mole fraction of component in the still () is known. This may be done analytically, numerically or graphically (via the McCabe–Thiele method):
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The mole fraction of component in the still () changes continuously during batch distillation, so process simulators and/or experimental data are commonly used to avoid their calculation.
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