Lecture 15. Enhanced Distillation and Supercritical Extraction (1) [Ch. 11] • Enhanced Distillation • Use of Triangular Graphs - Zeotropic mixture -Mixture forming one minimum-boiling azeotrope - Mixture forming two minimum-boiling azeotropes • Residue-Curve Maps • Distillation-Curve Maps • Product-Composition Regions at Total Reflux Enhanced Distillation • Cases when ordinary distillation is not economical - Boiling point difference between components is less than 50℃ - Relative volatility is less than 1.10 - Mixture forms an azeotrope • Enhanced distillation - Extractive distillation : adding a large amount of solvent - Salt distillation : adding a soluble , ionic salt - Pressure-swing distillation : operating at two different pressures - Homogeneous azeotropic distillation : adding an entrainer - Heterogeneous azeotropic distillation : adding an entrainer - Reactive distillation : adding a separating agent to react selectively and reversibly with feed component(s) Zeotropic vs. Azeotropic Systems Binary zeotropic system Binary azeotropic system P = constant AtAzeotrope C T-y T-x Pure B xA, yA Pure A Use of Triangular Graphs • Triangular vapor-liquid diagram for ternary mixture - Too complex to understand • Plot with only equilibr ium liquid composition fofoatenayr a ternary mixture - Easy to understand -Convenient to use - Provide useful information in distillation of ternary components . Residue curve . Distill ati on curve Distillation Curves for Ternary Systems Each curve in each diagram is the locus of possible equilibrium liquid-phase compositions that occur diduring diillidistillation o f a m ixture Mixture forming one min.-boiling azeotrope Mixture not forming an azeotrope Mixture forming two min.-boiling azeotropes Zeotropic Mixture • Product can be either nearly pure methlhanol or near lly pure 1-propanol A Methanol • Nearly pure ethanol (B), the 64.7℃ intermediate-boiling component, cannot be produced either as a distillate or bottoms Dist illate Feed Bottom C B 97.2℃ 78.5℃ 1-Propanol Ethanol Distillation Sequences for Ternary Zeotropic Mixtures Direct sequence Indirect sequence Feed, distillate, and bottoms product compositions lie on a straight, total- material-balance line Mixture Forming One Minimum-Minimum- Boiling Azeotrope A Acetone 56.2℃ 55.7℃ • If the column sppppylit is properly selected - Ternary distillate or bottoms Distillate products can be avoided Feed - Litt le or no ethlihdiillhanol in the distillate - Little or no acetone in the bottoms Bottom C B 78.5℃ 64.7℃ Ethanol Methanol Mixture Forming Two Minimum-Minimum- Boiling Azeotropes A Octane A Region 1 125.8℃ Distillate : A+B Bottoms : A+C 116.1℃ Distillation boundary - cannot be crossed - restricting product composition Region 2 Distillate : A+B Bottoms : B+C C B 136.2℃ 127.1℃ 135.1℃ EthylbenzeneC 2-Ethoxy-ethanolB ResidueResidue--CurveCurve Maps (1) • Rayleigh batch (differential ) distillation - no trays, no packing, no reflux Rate of output : DyD Rate of depletion : d dx dW ()Wx WxW dt W dtW dt dx dW WxW Dy dtWD dt D : instantaneous-distillate rate, mol/h Ddt dW W : moles of liquid left in still dx dW i ()y x dtii Wdt ResidueResidue--CurveCurve Maps (2) • Because W changes with time, it is possible to combine W and t into a single variable, ξ dx dW dx i ()yx i x y dtii Wdt d ii ddW 1 W0 {}t ln dt W dt Wt() dx Nine variables: i PP, T, x1, x2, x3, y1, y2, y3, xyiii ,, 12 • Residue curve:aplot: a plot d of liquid-residue 3 composition with time (from a lower-boiling xi 1 state to a higher- i boiling state) yKxiiii,,,123 • Residue-curve map: 3 a collection of residue Bubble-point- curves for a given Kxii 1 temperature equation ternary system i DistillationDistillation--CurveCurve Maps • Distillation curve : ternary diagram determined for total reflux (infinite reflux ratio) at a constant pressure, usually 1 atm xyij,,1 ij yi,,,j Kxi j i j • Dashed lines: distillation curves • Distillation-curve map • Solid lines: : a collection of distillation curves, residue curves including distillation boundaries • Residue curve vs. distillation curve - Residue curves: continuous curves - Distillation curves: discrete points ProductProduct--CompositionComposition Regions at Total Reflux (1) • Material balance line : a straight line connecting distillate and bottoms compositions must pass through the feed composition • Feasible-product-composition region - two lines limit the possible product range - bow-tie reg ion ProductProduct--CompositionComposition Regions at Total Reflux (2) • For an azeotropic system, a feasible-product-composition regibfdfhdiilliiion can be found for each distillation region Ternary mixture with two minimum- Ternary mixture with three binary boiling binary azeotropes azeotropes and one ternary azeotrope.