“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 Introduction to multicomponent distillation

Problem Solving Exercise

Problem Solving Exercise

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

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

McCabe Thiele: Partial Condenser Why consider using a partial condenser? yD D Desire vapor distillate? y1 x1 x0 y2 y1 V1 LO y3 x3 x2 y4 1 3 2 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) xO Running a partial condenser lowers the column operating pressure (i.e. don’t have to fully condense the distillate product

McCabe Thiele: Partial Condenser Seader & Henley, 2006

McCabe Thiele: Reboilers Seader & Henley, 2006

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

Effect of Different Reflux Ratios Increase R --- cost effect??

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

Suggests there is a relation between N, Nmin, and Rmin. 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.

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 Molokanov Eqn: Seader & Henley, 2006

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

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

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

Binary Shortcut Methods

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

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

AspenPlus: DSTWU