1 Heteroazeotropic Batch Distillation Feasibility and Operation Stathis Skouras 7. May 2004 Department of Chemical Engineering, NTNU NTNU

2 Introduction & Overview Introduction: Distillation, azeotrope, heterogeneous azeotrope (heteroazeotropic), heteroazeotropic distillation - what are they actually? Motivation - industrial relevance Batch distillation - Background Overview of talk: Time requirements for zeotropic mixtures Separation of heteroazeotropic mixtures in the multivessel column Time requirements for heteroazeotropic mixtures Heteroazeotropic batch distillation: A systematic approach –Process description and column operation –Feasibility and entrainer selection Main contributions

3 Introduction Distillation: A technique for separating mixtures into their constituent components by exploiting differences in vapour- and liquid phase compositions arising from partial vaporisation of the liquid phase and partial condensation of the vapour phase Perry et al., Perry’s Chemical Engineer’s Handbook, (1997)

4 Introduction Distillation, azeotrope: An azeotrope occurs for a boiling mixture of two or more species when the vapour and liquid phases in equilibrium have the same composition. As a consequence, we cannot separate such a mixture by boiling or condensing it and enhanced distillation techniques have to be applied Biegler et al., Systematic Methods of Chemical Process Design, (1997)

5 Introduction Distillation, azeotrope, heterogeneous azeotrope (heteroazeotrope): Heterogeneous behaviour means that the liquid phase partitions into two or more liquid phases at equilibrium. Two-liquid phase formation provides a means of breaking this azeotrope. Biegler et al., Systematic Methods of Chemical Process Design, (1997) Perry et al., Perry’s Chemical Engineer’s Handbook, (1997)

6 Introduction Distillation, azeotrope, heterogeneous azeotrope (heteroazeotrope), heteroazeotropic distillation: An enhanced distillation technique which uses minimum-boiling azeotropes and liquid-liquid immiscibilities in combination to defeat the presence of other azeotropes or tangent pinches that would otherwise prevent the desired separation Doherty and Malone, Conceptual Design of Distillation Systems, (2001)

7 Introduction Distillation, azeotrope, heterogeneous azeotrope (heteroazeotropic), heteroazeotropic distillation - what are they actually? Motivation – industrial relevance Heteroazeotropic distillation is a very common enhanced distillation technique: –Ethanol/water separation by using benzene, cyclohexane, toluene, etc –First successful application (patent) in 1902 in Germany by Young Heteroazeotropic distillation is a very powerful and flexible process: –Exploits several physical phenomena (enhanced vapour-liquid behaviour and liquid-liquid immiscibilities) –More possibilities for the separation of azeotropic mixtures than homoazeotropic distillation –Simplified distillation sequences (decantation + distillation)

8 Introduction Distillation, azeotrope, heterogeneous azeotrope (heteroazeotropic), heteroazeotropic distillation - what are they actually? Motivation – industrial relevance Batch distillation - Background Well suited for small-scale production (pharmaceutical, fine/specialty chemical industry) Separation of multicomponent mixtures in one single column. Various mixtures of different feeds can be processed More labour and energy intensive Heteroazeotropic distillation in batch columns not well understood. The presence of azeotropes complicates the design and synthesis of the process (what is feasible, how to operate the columns,)

9 Batch Distillation Arrangements Rectifier (two-vessel column) Conventional multivessel (with vapour bypass) Modified multivessel (without vapour bypass)

10 Time Requirements in Batch Columns Zeotropic mixture: Methanol/Ethanol/1-Propanol The modified multivessel (without vapour bypass) is the best WHY? Specification Conventional multivessel (with vapour bypass) [h] Modified multivessel (no vapour bypass) [%] Two-vessel column [%] Base case-Equimolar x F =[1/3,1/3,1/3] [0.99,0.97,0.99] [0.99,0.99,0.99] [0.995,0.995,0.995] Rich in light x F =[0.7,0.15,0.15] [0.99,0.97,0.99] [0.99,0.99,0.99] [0.995,0.995,0.995] Rich in intermediate x F =[0.15,0.7,0.15] [0.99,0.97,0.99] [0.99,0.99,0.99] [0.995,0.995,0.995] Rich in heavy x F =[0.15,0.15,0.7] [0.99,0.97,0.99] [0.99,0.99,0.99] [0.995,0.995,0.995]

11 Time Requirements in Various Batch Columns Zeotropic mixture: Methanol/Ethanol/1-Propanol Conventional multivessel (+) The vapour stream entering the middle vessel improves the composition dynamics of the light component (-) Practical difficulties with a vapour stream entering the middle vessel

12 Separation of Ternary Heteroazeotropic Mixtures in the Multivessel Column Is it feasible? –No study in the literature for a multivessel column How should we perform the separation? –Operation –Control

13 Separation of Ternary Heteroazeotropic Mixtures in the Multivessel Column The mixtureThe column

14 Separation of Ternary Heteroazeotropic Mixtures in the Multivessel Column Operation Build-up step Decantation step

15 (+) Multivessel configurations perform better than the rectifier column (-) Modified multivessel less attractive for heteroazeotropic mixtures (-) Practical difficulties with vapour streams entering a decanter Time Requirements in Various Batch Columns Ternary heteroazeotropic mixtures Specification Conventional multivessel-decanter hybrid [h] Modified multivessel-decanter hybrid [%] Rectifier- decanter hybrid [%] Class x F =[1/3,1/3,1/3] [0.99,0.97,0.99] [0.99,0.98,0.99] Class 1.0-1a x F =[0.6,0.2,0.2] [0.97,0.97,0.99] [0.98,0.99,0.99] Class 2.0-2b x F =[0.45,0.05,0.5] [0.97,0.97,0.99] [0.999,0.999,0.999]

16 Heteroazeotropic Batch Distillation The story so far 1.Time requirements for zeotropic mixtures –Multivessel configurations perform better –Modified multivessel better than conventional multivessel –Practical considerations regarding the modified multivessel 2.Separation of heteroazeotropic mixtures in the multivessel column –It is feasible –Showed how to separate the mixtures (operation, control, etc) 3. Time requirements for heteroazeotropic mixtures –Multivessel configurations better than the rectifier column –Practical considerations regarding the modified multivessel –Use the conventional multivessel for such mixtures UNTIL NOW THE MIXTURES WERE TERNARY AND ALREADY CONTAINED A HETEROAZEOTROPE

17 Heteroazeotropic Batch Distillation A systematic approach Formulation of the problem –The original mixture is binary (AB) azeotropic or close-boiling –The separation by simple distillation is impossible (AB is azeotropic) or uneconomical (AB is close-boiling) –An entrainer (E) is added that forms heteroazeotrope with at least one (preferably) of the original components The tasks –What has to be done? (process description) –How to operate the columns in a simple way? (operation) –Which separations are feasible? (feasibility) –How to choose entrainers for the process? (entrainer selection)

18 Process Description How to do Strategy A: Do the steps sequentially (+): Recovery of pure E (-): Time consuming Strategy B: Do the steps simultaneously (+): Less time consuming (-): Cannot recover pure E Example Close-boiling (AB) + Entrainer (E) What has to be done Step 1: Product recovery (L A ) Step 2: Entrainer recovery (E or L E ) Pure B in the still at steady state

19 Operation Rectifier column Use a T-controller to indirectly adjust the holdup of the entrainer-lean phase (L E ) No need to predetermine holdups of the immiscible phases in the decanter Simple realisation of the desired steady state results Both strategies A and B can be realised by adjusting the temperature setpoint

20 Operation Multivessel column Use a L-controller to reflux all of the entrainer-lean phase (L E ) Use a T-controller to indirectly adjust the holdup in the middle vessel No need to predetermine holdups in the vessels Simple realisation of the desired steady state results Strategy A is implemented. Both process steps are performed simultaneously in the same column

21 An Example Water (A) / Dioxane (B) is azeotropic Benzene (E) forms binary heteroazeotrope with water Two distillation boundaries and limit the products under distillation Three distillation regions complicate the synthesis of the process Water (A) / Dioxane (B) + Benzene (E)

22 Simulations for the Rectifier Column Column profile restored during the process Still path crosses distillation boundaries These results cannot be obtained by homoazeotropic distillation Pure and anhydrous ethanol recovered in the still at steady state and water recovered with the aqueous phase in the decanter

23 Feasibility and Entrainer Selection Which separations are feasible with the proposed processes? –Develop a method to check feasibility without doing simulations –Use only the distillation lines map of the mixture and the binodal curve (VLLE) How to choose entrainers for the processes? –propose simple rules for “screening” feasible entrainers

24 Feasibility Conditions Operation Place (A+B+E) in the still Start the process Collect some of the heteroazeotrope in the decanter Feasibility condition 1: It should exist a feed region where the heteroazeotrope is the unstable node so as it will boil overhead and start accumulated in the decanter Feasibility Same for rectifier and multivessel

25 Feasibility Conditions Operation The heteroazeotrope splits in two phases Reflux the entrainer-rich phase (L E ) Accumulate (remove) the entrainer-lean phase (L A ) Pure B in the still Feasibility condition 2: It should, at steady state, exist a distillation line connecting the reflux composition L E with the still product composition B in the direction of increasing temperature from L E to B

26 Checking Feasibility: An example Example Azeotropic (AB) + Light entrainer (E) Steady State Products L A and L E in the decanter B in the still Feasibility conditions  1) It exists a feed region where the heteroazeotrope is the unstable node  2) It exists, at steady state, a distillation line connecting the reflux composition L E with the still product composition B in the direction of increasing temperature from L E to B

27 Checking Feasibility Three general cases for the original mixture (AB): a) Close-boiling (low relative volatility) mixture (10 cases, 5 feasible) b) Minimum-boiling (min) homoazeotropic mixtures (9 cases, 4 feasible) c) Maximum-boiling (max) homoazeotropic mixtures (7 cases, 2 feasible) The results for all cases helped us to formulate: Two entrainer selection rules Two guidelines for avoiding infeasible entrainers

28 Entrainer Selection Simple rules for entrainer selection: 1) The entrainer (E) should form a heteroazeotrope (AzEA or AzEB) with one of the original components (A or B) and/or a ternary heteroazeotrope (AzEAB) 2) The vertex of the original component to be obtained in the still at steady state (A or B) should be connected with the steady state reflux point of the entrainer-rich phase (L E ) with a distillation line (residue curve) in the direction of increasing temperature from the top of the column to the bottom (L E  A or L E  B) Guidelines for avoiding infeasible entrainers: 1) The entrainer (E) must not form a max. azeotrope with any of the original components (A or B) 2) The entrainer (E) should preferably not form a ternary saddle homoazeotrope

29 Main Contributions Comparison of different batch column configurations, in terms of time requirements, for zeotropic and heteroazeotropic mixtures –The vapour stream configuration in the middle vessel plays significant role –Practical considerations for eliminating the vapour bypass Addressing separation of ternary heteroazeotropic mixtures in the multivessel column –Showing how to perform the separation (control, operation) Systematic and comprehensive study of the heteroazeotropic batch distillation process –Detailed analysis of the process –Proposing control schemes for simple column operation –Addressing feasibility issues –Proposing rules for entrainer selection

30 Thank you for your attention…