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 recycle With Recycle Loop [P] x + r [P] y = (1-r) y x Unit Op y x Unit Op Non-linear model Non-linear matrix manipulation y [P] (x+ry)
Tear Streams in Aspen Hidden Tear Stream is added to Recycle Loop Tear = β*recycle By Component Tear i = β*recycle i β is increased from δ to 1 Issues how fast do you step β? Convergence Tear Unit Op (is mass & energy & P in balance at mixing point ) x Recycle y
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 x 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 γ L i P = i x i P sat
Azeotrope Conditions Conditions on the Activity Coefficient P γ T L j 1 L 1 s 1 = x γ P L + ( 1 x1 ) γ 2 P2 s > 1, j = 1,2... C, Positive Deviations from Raoult's Law γ L j < 1, j = 1,2... C, Negative Deviations from Raoult's Law Minimum Boiling, γ jl > 1 Maximum Boiling, γ jl < 1 Giving Rise to x j =y j, j=,1,2, C
Homogeneous Azeotropes (Cont d) For non-ideal mixtures, the activity coefficients are different from unity: If γ > 1 i yp = xγ P 1 1 1 1 yp = xγ P 2 2 2 2 = γ + ( ) γ s s P x P 1 x P 1 1 1 1 2 2 the mixture has a minimum-boiling azeotrope S S Example Phase diagrams for Isopropyl ether-isopropyl alcohol
Homogeneous Azeotropes (Cont d) For non-ideal mixtures, the activity coefficients are different from unity: If γ < 1 i yp = xγ P 1 1 1 1 yp = xγ P 2 2 2 2 = γ + ( ) γ s s P x P 1 x P 1 1 1 1 2 2 the mixture has a maximum-boiling azeotrope S S Example Phase diagrams for Acetone-Chloroform
Importance of Physical Property In all cases Data Set 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 0.2 DTBP 0.15 H 2 O TBA = Tertiary-butyl alcohol, TBHP =Tertiary-butyl hydroperoxide DTBP = Di-tertiary-butyl peroxide
Basics: 3-Phase Diagrams (Cont d) 0.2 TBA 0.2 DTBP TBA = Tertiary-butyl alcohol DTBP = Di-tertiary-butyl peroxide 0.6 H 2 O
Basics: The Lever Rule
Residue Curves Distillation still Mass balance on species j: Lx = ( L) y + ( L L)( x + x ), j = 1,, C 1 j j j j As L 0: Lx = y dl + Lx + Ldx x dl dldx, j = 1,, C 1 j j j j j j Rearranging: dl dx j / L = x y = x1 ( K{ TPxy,,, }) dx j j j j j = x j dt y j
Multi-component Azeotropes Residue Curve Map dx j /dť = dx j /d ln(l) = x j y j Integrate from various starting points Arrows from low to High Temp Path of the residue composition
dx Sketching Residue Curves (Exercise) x j = j dt y j
Distillation X B, X F and Y D 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 dx dh n+ 1 x n V L n n 1 y n + L D n 1 y D x n y n, for D = 0, L = n-1 V n Distillation Lines x = y, for all n = n n +1 0,1,... x n and y n lie on equilibrium tie lines Tangent to Residue Curve
AspenPlus To Create Residue Maps 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: L x + Dx = Vy n 1 n 1 D n n Approximation for liquid phase: dxn x x n n 1 dh Substituting: dx V D n + dh L L n x y x n n D n 1 n 1 Stripping section of distillation column At total reflux, D = 0 and V n = L n-1 dx n dh x n y n
Nodes
dx x j = j dt Residue Curves (Cont d) y j Residue curves for zeotropic system Residue curves for Azeotropic system
Defining Conditions for Multicomponent 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 S2 M2 D1 S1 M1 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