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CBE 417 “Chemical Engineering Equilibrium Separations”

CBE 417 “Chemical Engineering Equilibrium Separations”. Lecture: 9. 28 Sep 2012. Overview. McCabe-Thiele graphical technique (binary systems) Condensers Reboilers Binary Shortcut Methods AspenPlus : Shortcut methods: DSTWU Rigorous method: RADFRAC Efficiencies

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CBE 417 “Chemical Engineering Equilibrium Separations”

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  1. CBE 417 “Chemical Engineering Equilibrium Separations” Lecture: 9 28 Sep 2012

  2. Overview • McCabe-Thiele graphical technique (binary systems) • Condensers • Reboilers • Binary Shortcut Methods • AspenPlus: • Shortcut methods: DSTWU • Rigorous method: RADFRAC • Efficiencies • Introduction to multicomponent distillation

  3. Problem Solving Exercise

  4. Problem Solving Exercise

  5. McCabe Thiele: Total Condenser y1 V1 LO D y1 xD xD 1 x2 y3 x1 y2 2 x3 y4 3

  6. McCabe Thiele: Partial Condenser yD D y1 V1 LO xO y1 x0 1 x2 y3 x1 y2 2 x3 y4 3

  7. McCabe Thiele: Partial Condenser Why consider using a partial condenser? yD D • Desire vapor distillate? y1 V1 • Need to control temperature in the column? • Concept: • control distillation column temperatures by controlling the operating pressure. • Condenser design / column opn. P • Minimum desirable condenser temperature is ~ 49 oC (120 oF). • Construction of column more expensive at higher pressures (i.e. > 14.8 bar [200 psig]) • Bottoms T too high (decomposition or Tc approached) LO xO y1 x0 1 x2 y3 x1 y2 2 x3 y4 3 • Running a partial condenser lowers the column operating pressure (i.e. don’t have to fully condense the distillate product

  8. McCabe Thiele: Partial Condenser Seader & Henley, 2006

  9. McCabe Thiele: Reboilers Seader & Henley, 2006

  10. Effect of Different Reflux Ratios • Before we saw that.. • increasing reflux resulted in an enrichment of vapor stream in the “lighter” component

  11. Effect of Different Reflux Ratios • Increase R --- cost effect??

  12. Effect of Different Reflux Ratios R = factor* Rmin Seader & Henley, 2006

  13. Tie Together • Found using McCabe-Thiele (knowing xD , xB , & feed condition): • Nmin (at total reflux) • Rmin(at infinite number of stages) • If given actual R (or choose by R = factor* Rmin ) can find: • N • NF (optimum feed stage number) Suggests there is a relation between N, Nmin, and Rmin.

  14. Gilliland Correlation 61 Data points over ranges: No. components: 2 to 11 q : 0.28 to 1.42 P : vacuum to 42.4 bar  : 1.11 to 4.05 Rmin: 0.53 to 9.09 Nmin: 3.4 to 60.3 MolokanovEqn: Seader & Henley, 2006

  15. Binary Shortcut Methods McCabe-Thiele is good at showing procedure and helping explain concepts for binary distillation. It can be extended to multiple feeds and side draws (Wankat). When consider multicomponent mixtures, McCabe-Thiele is not quite as useful. However, having some shortcut methods to aid in final column design is useful. Minimum number of stages: Fenske Equation - binary; total condenser;  ~ constant

  16. Binary Shortcut Methods Minimum reflux ratio: Underwood Equation - binary; assumes rectifying line intersects eq. line at feed line;  ~ constant R=factor* Rmin Approximate number of equilibrium stages (N): Gilliland correlation

  17. Binary Shortcut Methods Optimum feed stage location (NF): Kirkbride Equation - binary; approximate result

  18. Binary Shortcut Methods

  19. Summary Binary Shortcut Methods (FUG) Fenske Equation Underwood Equation R=factor* Rmin Gilliland correlation (Nequil) Kirkbride Equation (NF(opt))

  20. Overview • Questions on homework?? • McCabe-Thiele graphical technique (binary systems) • Condensers • Reboilers • Binary Shortcut Methods • AspenPlus: • Shortcut methods: DSTWU • Rigorous method: RADFRAC • Efficiencies • Introduction to multicomponent distillation

  21. AspenPlus: DSTWU

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