Azeotropic Distillation Methods. Dr. Stathis Skouras, Gas Processing and LNG RDI Centre Trondheim, Statoil, Norway

Transcription

1 Azeotropic Distillation Methods Dr. Stathis Skouras, Gas Processing and LNG RDI Centre Trondheim, Statoil, Norway

2 Schedule Tuesday 1/12/2015: : Lecture - Natural Gas Processing : Available for questions/discussion at TTPL Wednesday 2/2/ : Available for questions/discussion at TTPL Thursday 3/12/ : Lecture - Azeotropic distillation methods : Available for questions/discussion at TTPL Friday 4/12/ : : Available for questions/discussion at TTPL 2

3 Outline Introduction Importance and industrial relevance of azeotropic distillation Main part Theory: residue curve maps and distillation curve maps How to make residue curve maps at Aspen Plus Feasibility analysis of azeotropic distillation Azeotropic distillation methods Examples from the Oil & Gas Industry Summary 3

4 Importance and industrial relevance of azeotropic distillation Need for efficient recovery and recycle of organic solvents in chemical industry Distillation is the most common unit operation in recovery processes because of its ability to produce high purity products Most liquid mixtures of organic solvents form azeotropes that complicate the design of recovery processes Azeotropes make separation impossible by normal distillation but can be also utilised to separate mixtures not ordinarily separable by normal distillation Azeotropic mixtures may often be effectively separated by distillation by adding a third component, called entrainer Knowledge of the limitations and possibilities in azeotropic distillation is a topic of great practical and industrial interest 4

5 Terminology The methods and tools presented in this lecture also appply for: Azeotropic mixtures, close boiling systems, low relative volatility systems Original components A and B: The components that form the azeotrope and need to be separated Entrainer: A third component (E or C) added to enhance separation Binary azeotrope: Azeotrope formed by two components Ternary azeotrope: Azeotrope formed by three components Homogeneous azeotrope: Azeotrope where the forming components are miscible Heterogeneous azeotrope: Azeotrope where the forming components are immiscible Minimum boiling azeotrope: Azeotrope with lower boiling point than its constituent components (most common) Maximum boiling azeotrope: Azeotrope with higher boiling point than its constituent components (less common) 5

6 Theory: Residue curve maps (RCM) and distillation curve maps (DCM) For ordinary multicomponent distillation determination of feasible schemes and column design is straightforward McCabe-Thiele method and Fenske-Underwood- Gilliland equations are powerful tools Azeotropic phase equilibrium diagrams such as residue curve maps (RCM) or distillation curve maps (DCM) are sometimes nicknamed the McCabe-Thiele of azeotropic distillation and provide insight and understanding RCM or DCM sketched together with material balance lines and operating lines are used to identify feasible distillation schemes and products 6

7 Residue curves Consider the process of differential (open) distillation (Rayleigh distillation) The component mass balance is written: dx dt i dw ( yi xi) Wdt and by considering the dimensionless time variable ξ (dξ=dv/w) dxi xi yi d Integrating the above equation from any initial composition (x w0 ) will generate a residue curve A (T A ) x W 0 T A < T B < T C The residue curve describes the change of the still pot composition with time (trajectory) C (T C ) Still pot composition trajectory B (T B ) 7

8 Distillation curves Consider the process of continuous distillation at total reflux (45 line at McCabe-Thiele diagram) Starting with a liquid composition at stage n (x i,n ) and by doing repeated phase equilibrium calculations (E-mapping) upwards we get: x y x y E n y i, n i, n x i, n i, n 1 E n 1 y i, n 1 i, n 1 x i, n 1 i, n 2 Total reflux (V = L = R) Y i,n-1 Condenser V, y D L, x D x i,n-1 x i,n Stage n-1 Stage n y i,n... By doing this from any initial composition (x 0 ) the distillation curve can be constructed The distillation curve describes the change of the component composition along the column (trajectory) Reboiler x B 8

9 Singular points in RCM and DCM Pure component vertices and azeotropes are singular points in the RCM and DCM dxi xi yi 0 d The behaviour at the vicinity of singular points depends on the two eigenvalues a) Stable node ( ): Point with the highest boiling point Bottom product in distillation. All residue curves end at this point - Both eigenvalues negative b) Unstable node ( ): Point with the lowest boiling point Top product in distillation. All residue curves start at this point - Both eigenvalues positive c) Saddles ( ): Point with an intermediate boiling point Residue curves move towards and then away from these points One positive and one negative eigenvalue 9

10 Relationship between residue curves and distillation curves Both are pure representations of the VLE and no other information needed to construct them Have the same topological structure and singular points Distillation boundaries exist and split the composition space into distillation regions DO NOT completely coincide to each other BUT provide the same information and can be equally used for feasibility analysis Residue curve Distillation curve 10

11 «Distillation synthesis» in Aspen Plus 11

12 Find azeotropes 12

13 Continue to Aspen Plus Residue Curves 13

14 Residue curves 14

15 Feasibility analysis based on RCM and DCM For a feasible separation the material balances should be fulfilled: D, x D F = D + B F, z F F z F = D x D + B x B Feasibility rules a) The top (x D ) and bottom (x B ) compositions must lie in a straight line through feed (z F ) B, x B x B b) The top (x D ) and bottom (x B ) compositions must lie on the same residue (distillation) curve x D Products x D and x B must lie on the same distillation region 15

16 Feasibility analysis based on RCM and DCM Zeotropic mixture No distillation boundaries Only one distillation region exists No limitations regarding possible products independently of feed location Direct split: The most volatile is taken at the first column Indirect split: The less volatile is taken at the first column F F 16

17 Feasibility analysis based on RCM and DCM Azeotropic mixtures One boundary exists (AzAC AzAB) A Two distillation regions (I and II) Different products for feed in regions I and II AzAC F 1 AzAB Feed F 1 in Area I o AzAC as top product o Component A as bottom product C F 2 B Feed F 2 in Area II AzAC as top product Component B as bottom product 17

18 Azeotropic distillation methods 1) Pressure swing distillation No entrainer required 2) Homogeneous azeotropic (homoazeotropic) distillation 3) Heterogeneous azeotropic (heteroazeotropic) distillation 4) Extractive distillation Entrainer enhanced methods 18

19 1) Pressure swing distillation Principle: Overcome the azeotropic composition by changing the system pressure Key factors: Azeotrope sensitive to pressure changes, recycle ratio which increases costs Application: Tetrahydrofuran/water * * Stichlmair and Fair, Distillation: Principles and practice, Wiley-VCH, (1998) 19

20 2) Homogeneous azeotropic distillation Definition: o Entrainer completely miscible with the original components o Entrainer may (or not) form additional azeotropes with the original components o The distillation is carried out in a sequence of columns F + E E 2 Principle: o The addition of the entrainer results in a residue curve map promising for separation o Both original components must belong to the same distillation region 20

21 Feasibility for homogeneous azeotropic distillation Example Use of intermediate entrainer Original components A and B form a min. Az AB Components A and B belong to the same distillation region F D 1 Original feed (F) is close to the azeotrope Az AB Total feed (F ) is a mix of fresh feed (F) and entrainer (E) Component A is taken as bottom product in Column 1 A AzAB=F D 1 B B Component B is taken as top product in Column 2 Entrainer (E) is recovered as bottom product in Column 2 F F 1 2 Entrainer (E) is recycled to Column 1 Applicability of homoazeotropic distillation is limited Restrictive feasibility rules (intermediate entrainers are rare) Other distillation methods are preferably applied A E 21

22 3) Heterogeneous azeotropic distillation Definition: o Entrainer is immiscible and forms azeotrope with at least one of the original azeotropic components o The distillation is carried out in a combined columndecanter column o Entrainer is recovered and recycled to the first column Principle: o Liquid-liquid immiscibilities are used to overcome azeotropic compositions o Distillation boundaries can be crossed by immiscibility Applicability: o Widely used in the industry o One of the oldest methods of azeotropic distillation 22

23 Classic example: Ethanol/water + benzene (E) 1) Preconcentrator Aqueous feed dilute in EtOH (F1) EtOH-Water azeotrope at top (D1) Pure water at bottom (B1) 2) Azeotropic column Ternary heterogeneous azeotrope (A12E) at top Splits in two liquid phases in a decanter Benzene-rich phase is recycled at the top Pure EtOH is taken at bottom (B2) Benzene 3) Entrainer recovery column Aqueous phase from decanter is column feed Pure water is taken at the bottom (B3) Top product (D3) is close to the EtOH-water azeotrope + some benzene left A12E H 2 O EtOH 23

24 4) Extractive distillation Definition: o Heavy entrainer is used with high boiling point o Distillation is carried out in a two-feed column with a heavy entrainer added continously at the top o Entrainer is recovered in a second column Principle: o The entrainer alters the relative volatility of the original components o The entrainer has a substantial higher affinity to one of the original components and extracts it downwards the azeotropic column Applicability: o Most widely used method in the industry 24

25 Feasibility and synthesis for extractive distillation Pure component 1 (D1) is taken as top product from extractive column Entrainer extracts component 2 at the bottom (B1) Entrainer recovery column separates entrainer from component 2 Pure entrainer (E) is recovered at bottom (B2) and used as reflux in extractive column Rectifying section Rectifying section Binary feed (1 & 2) F Bottom section Bottom section 25

26 Examples from Oil & Gas CRAIER plant at Kårstø A common azeotrope in oil and gas industry is the ethane / CO 2 azeotrope CO 2 has a volatility between methane and ethane CO 2 distributes between natural gas and NGL in a gas processing plant CO 2 C1 C2 C3 * Anette Kornberg, Equation of State for the System CO2 and Ethane, Project report, NTNU, Dec

27 CO 2 Removal and Increased Ethane Recovery (CRAIER) 27

28 CRAIER column Az C 2 /CO 2 ~ 50 mol% CO 2 C 2 + CO 2 (+ C1) ~ 20 mol% CO 2 C 2 product (pure) 28

29 Examples from Oil & Gas Ryan Holmes process Invented by Jim Ryan and Art Holmes* Cryogenic distillation process for the removal of CO 2 from natural gas Suitable for removing high CO 2 contents from natural gas Uses extractive distillation to break the CO 2 / C2 azeotrope Uses Natural Gas Liquid (NGL) as entrainer, which is an internal product of the process Various configurations with 2, 3 and 4 columns * A. S. Holmes, J. M. Ryan, Cryogenic distillation separation of acid gases from methane, US patent,

30 Added entrainer De-C1 column Extractive column Entrainer recovery column CO 2 /C 2+ C 2+ C 4+ Entrainer (C4+) recycle 30

31 MTBE Production and Separation Unit Azeo C4-MeOH + water (E) Feed C3 C5+ MTBE methanol MeOHwater 31

32 Process description Feed to 1 st separation column C3 C4 (with excess isobutylene) C5+ Water Methanol First Column (Distillation Column) Bottom: MTBE Top: C4 methanol azeotrope Second Column (Extraction Column) Addition of water countercurrent to flow Methanol has more affinity for water pass to aqueous phase Top: Raffinate (C3,C4,C5+) Bottom: Methanol/Water Third Column (Distillation Column) Top: methanol Bottom: Water 32

33 Summary Separation of azeotropic mixtures is a topic of great practical and industrial interest Azeotropic mixtures are impossible to separate by ordinary distillation, but may be effectively be separated by adding a third component, called entrainer Residue curve maps (RCM) and distillation curve maps (DCM) are representations of the thermodynamic behavior (VLE and VLLE) of azeotropic mixtures RCM and DCM are used to identify feasible distillation schemes 33

34 Summary Homogeneous azeotropic distillation o Only few RCM and DCM lead to feasible schemes o Limiting use in the industry Heteroazeotropic distillation o Ordinary distillation combined with a decanter is used o Liquid-liquid immiscibilities are used to overcome azeotropic compositions o Method widely used in the industry Extractive distillation o Heavy entrainer used that extracts one of the original components and enhances separation o Broad range of feasible entrainers (no liquid-liquid immiscibility required) o The most widely used method in the industry 34

35 Presenters name: Dr. Stathis Skouras Presenters title: Principal Researcher Tel: