Separation Trains Azeotropes S,S&L Chapter 9.5 Terry A. Ring Chemical Engineering University of Utah

Distillation Recycle Loops

Closing Recycle Loops Matrix Mathematics Without Recycle Loop [P] x = y Straight forward martix multiplication gives y With Recycle Loop [P] x + r [P] y = (1-r) y Non-linear model Non-linear matrix manipulation Unit Op y x recycle Unit Op x y [P] (x+ry)

Tear Streams in Aspen Hidden Tear Stream is added to Recycle Loop Tear = β*Recycle By Component Teari = β*Recyclei β is increased from δ to 1 Issues how fast do you step β? Convergence (is mass & energy & P in balance at mixing point ) Tear Recycle Unit Op y x

Aspen Help Diagnosing Tear Stream Convergence Most of the time the problem is the inside loop U1 U2

What is an Azeotrope?

Introduction Separation sequences are complicated by the presence of azeotropes, often involving mixtures of oxygenated organic compounds: Alcohols Ketones Ethers Acids Water In these cases, distillation boundaries limit the product compositions of a column to lie within a bounded region Prevents the removal of certain species in high concentrations

Binary Distillation IPA/IPE x Binary Distillation IPA/IPE IPA-IPE x Mininum-boiling Azeotropes x

Binary Distillation IPA/IPE Acetone/Chloroform Maximum-boiling Azeotropes x

Can multi-component Distillations have Azeotropes? Yes! Possibly Multiple!

Raoult’s Law γLi

Azeotrope Conditions Conditions on the Activity Coefficient Minimum Boiling, γjL> 1 Maximum Boiling, γjL< 1 Giving Rise to xj=yj, j=,1,2,…C

Homogeneous Azeotropes (Cont’d) For non-ideal mixtures, the activity coefficients are different from unity: If the mixture has a minimum-boiling azeotrope Example – Phase diagrams for Isopropyl ether-Isopropyl alcohol

Homogeneous Azeotropes (Cont’d) For non-ideal mixtures, the activity coefficients are different from unity: If the mixture has a maximum-boiling azeotrope Example – Phase diagrams for Acetone-Chloroform

Importance of Physical Property Data Set In all cases Need sophisticated liquid phase model to accurately predict the activity coefficient for the liquid. For High Pressure (> 10 bar) Cases Only Also need sophisticated (non-ideal) gas phase fugacity model

Two Types of Min. Boiling Azeotropes Homogeneous Azeotrope Heterogeneous Azeotrope A B A B Overlay with Liquid/Liquid Separation which is sometimes best separation method (costs much less)

Instructional Objectives When you have finished studying this unit, you should: Be able to sketch the residue curves on a tertiary phase diagram Be able to define the range of possible product compositions using distillation, given the feed composition and the tertiary phase diagram Be able to define the PFD for a heterogeneous azeotropic distillation system Be able to define the PFD for a pressure swing distillation system

Concepts Needed Phase Diagram for 3 phases Lever Rule on Phase Diagram Residue Curves

Basics: 3-Phase Diagrams 0.2 TBA 0.65 DTBP TBA = Tertiary-butyl alcohol DTBP = Di-tertiary-butyl peroxide TBHP =Tertiary-butyl hydroperoxide Rxn TBA + TBHP ->DTBP 0.2 DTBP 0.15 H2O TBA = Tertiary-butyl alcohol, TBHP =Tertiary-butyl hydroperoxide DTBP = Di-tertiary-butyl peroxide

Basics: 3-Phase Diagrams (Cont’d) 0.2 TBA TBA = Tertiary-butyl alcohol DTBP = Di-tertiary-butyl peroxide 0.2 DTBP 0.6 H2O TBA = Tertiary-butyl alcohol DTBP = Di-tertiary-butyl peroxide

Basics: The Lever Rule

Residue Curves Mass balance on species j: Rearranging: Distillation still Mass balance on species j: Rearranging:

Multi-component Azeotropes Residue Curve Map dxj /dť = dxj /d ln(L) = xj – yj Integrate from various starting points Arrows from low to High Temp Path of the residue composition

Sketching Residue Curves (Exercise)

Distillation XB, XF and YD form a line for a Distillation Column Line can not cross Feasible Region line For Partial Condenser For Total Condenser

Distillation Boundaries Equilibrium Trays in Total Reflux Distillation Lines xn and yn lie on equilibrium tie lines Tangent to Residue Curve

To Create Residue Maps AspenPlus After putting in the components and selecting the physical property method Choose In Properties Choose Residue Curves In Simulation Choose Distillation Search

Residue Curves  Liquid Compositions at Total Reflux Species balance on top n-1 trays: Stripping section of distillation column Approximation for liquid phase: Substituting: At total reflux, D = 0 and Vn = Ln-1

Nodes

Residue Curves (Cont’d) Residue curves for zeotropic system Residue curves for Azeotropic system

Defining Conditions for Multi-component Azeotrope t goes from 0 to 1, ideal to non-ideal to find Azeotrope

Product Composition Regions for Zeotropic Systems

Product Composition Regions for Azeotropic Systems

Heterogeneous Azeotropic Distillation Example: Dehydration of Ethanol Try toluene as an entrainer What are the zones of exclusion?

Ethanol/Water Distillation with Toluene to Break Azeotrope M1 M2 D1 Distillation Line Tie Line

Ethanol/Water Distillation with Benzene To Break Azeotrope

How To Break Azeotropes with Entrainer Separation Train Synthesis Identify Azeotropes Some distillations are not Azeotropic and can be accomplished relatively easily Identify alternative separators Select Mass Separating Agent or Entrainer Identify feasible distillate and bottoms product compositions Residue Curve Analysis

Pressure Swing to Break Azeotrope Temp. of Azeotrope vs. Pressure Mole Fraction of Azeotrope

Pressure-swing Distillation (Cont’d) Example: Dehydration of Tetrahydrofuran (THF) T-x-y diagrams for THF and water

Other Multi-component Distillation Problems Multiple Steady States Run same distillation column with same set points but different computational starting point Get Two or More Different Results Top or bottom compositions This is real in that the column will have two different operating conditions! Happens most often with multi component distillation