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Introduction to Distillation: Steady State Design and Operation

Introduction to Distillation: Steady State Design and Operation. Distillation Course Berlin Summer 2008. Sigurd Skogestad. Part 1 . Introduction Steady-state design Steady-state operation. BASF Aktiengesellschaft. 1. Introduction to distillation. King (Wiley, 1980) on distillation design

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Introduction to Distillation: Steady State Design and Operation

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  1. Introduction to Distillation: Steady State Designand Operation Distillation Course Berlin Summer 2008. Sigurd Skogestad. Part 1 • Introduction • Steady-state design • Steady-state operation

  2. BASF Aktiengesellschaft

  3. 1. Introduction to distillation • King (Wiley, 1980) on distillation design • Shinskey (McGraw-Hill, 1984) on distillation control • Kister (McGraw-Hill, 1990) on distillation operation • General info: http://lorien.ncl.ac.uk/ming/distil/distil0.htm • I.J. Halvorsen and S. Skogestad, ``Distillation Theory'', In: Encyclopedia of Separation Science. Ian D. Wilson (Editor-in-chief), Academic Press, 2000, pp. 1117-1134. • S. Skogestad, Dynamics and control of distillation columns - A tutorial introduction., Trans IChemE (UK), Vol. 75, Part A, Sept. 1997, 539-562 (Presented at Distillation and Absorbtion 97, Maastricht, Netherlands, 8-10 Sept. 1997). • More: see home page Sigurd Skogestad http://www.nt.ntnu.no/users/skoge/ http://www.nt.ntnu.no/users/skoge/distillation • Free steady-state distillation software with thermo package : http://www.chemsep.org/

  4. L D F V B

  5. I usually number the stages from the bottom (with reboiler=1), but many do It from the top

  6. Alternative: Packed column

  7. Vapor-liquid equilibrium (VLE) = Equilibrium line y=K(x) Non-ideal Difficult separation (almost az.) Easy sep. Ideal mixture common low-boiling az. less common high-boiling az. Azeotropes (non-ideal)

  8. The equilibrium stage concept Vi+1 yi+1 Stage i+1 Material balance stage i (out=in): Li xi + Vi yi = Li+1xi+1 + Vy-1yi-1 Li+1 Xi+1 Vi yi Equilibrium (VLE): yi = Ki(xi) Stage i Li xi Vi-1 yi-1 Stage i-1 • The equlibrium stage concept is used for both tray and packed columns • N = no. of equilibrium stages in column • Tray column: N = No.trays * Tray-efficiency • Packed columns: N = Height [m] / HETP [m] Typical: 0.7 Typical: 0.5 m

  9. TOP Simplified energy balance: Vi = Vi+1 (“constant molar flows”) BTM TOP BTM

  10. When use distillation? • Liquid mixtures (with difference in boiling point) • Unbeatable for high-purity separations because • Essentially same energy usage independent of (im)purity! • Going from 1% to 0.0001% (1 ppm) impurity in one product increases energy usage only by about 1% • Number of stages increases only as log of impurity! • Going from 1% to 0.001% (1 ppm) impurity in one product increases required number of stages only by factor 2 • Well suited for scale-up • Columns with diameters over 18 m • Examples of unlikely uses of distillation: • High-purity silicon for computers (via SiCl3 distillation) • Water – heavy-water separation (boiling point difference only 1.4C)

  11. 2. Steady-state Design • Given separation task • Find • configuration (column sequence) • no. of stages (N) • energy usage (V) • ”How to design a column in 5 minutes”

  12. Multicomponent and binary mixtures • We will mostly consider separation of binary mixtures • Multicomponent mixtures: For relatively ideal mixtures this is almost the same as binary - if we consider the “pseudo-binary” separation between the key components L = light key component H = heavy key component • The remaining components are almost like “dead-weight” • “Composition”: The impurity of key component is the important

  13. Relative volatility, 

  14. Ideal mixture:Estimate of relative volatility

  15. IDEAL VLE (constant α) Example. iso-pentane (L) – pentane (H) Example. Nitrogen (L) – Oxygen (H) Estimate of relative volatility (2)

  16. Example: Binary separation with purities: 90% light in top and 90% heavy in bottom: Example: Binary separation with purities: 99.9% light in top and 98% heavy in bottom: Separation factor for column or column section

  17. Stage i+1 Total reflux: Vi = Li+1 yi = xi+1 Li+1 xi+1 Vi yi Stage i Li xi Vi-1 yi-1 Minimum no. of stages Total reflux = Infinite energy O Operating line: xi+1 = yi (diagonal)

  18. IDEAL MIXTURE IDEAL VLE (constant α) Infinity energy ) Total reflux. Stage i: Repeat for all N stages Fenske’s formula for minimum no. of stages Assumption: Constant relative volatility Applies also to column sections Minimum no. of stages, Nmin(with infinite energy)

  19. Minimum energy (minimum reflux) pinch (a) IDEAL VLE (b) NON-IDEAL VLE Infinite number of stages in pinch region

  20. IDEAL MIXTURE IDEAL VLE (constant α) Minimum energy, Vmin(with infinite no. of stages) • Feed liquid (King’s formula, assuming pinch at feed): • NOTE: Almost independent of composition!! For sharp split (rLD=1, rHD=0), feed liquid: Assumption: Ideal mixture with constant relative volatility and constant molar flows. feed vapor: delete the D

  21. IDEAL MIXTURE IDEAL VLE (constant α) Examples design

  22. Design: How many stages? • Energy (V) vs. number of stages (N) • Trade-off between number of stages and energy • Actual V approaches Vmin for N approximately 2 x Nmin or larger, typically: • 2Nmin  + 25% Vmin • 3Nmin  + 3 % Vmin • 4Nmin  + 0.3 % Vmin

  23. Design: How many stages? • Conclusion: Select N > 2 Nmin (at least) • Many stages reduce energy costs • Many stages is good for control • Can overfractionate (tight control is then not critical) or • Get less interactions between top and bottom (because of pinch zone around feed)

  24. IDEAL MIXTURE IDEAL VLE (constant α) Real well-designed column • Recall: • Choose N ≈ 2 Nmin: • Get V ≈ 1.25 Vmin and Q ≈ 1.25 ¢ Vmin¢ Hvap • N = 3-4 Nmin gives V very close to Vmin • Important insights: • Vmin is a good measure of energy usage Q • Vmin is almost independent of purity • Vmin is weakly dependent on feed comp. (feed liquid: get vaporization term D/F≈ zF) • Design: To improve purity (separation): Increase N • N and Vmin both increase sharply as  → 1 • Example. Decrease  from 2 to 1.1: • Nmin increases by a factor 7.3 ( =ln 2/ln1.1) • Vmin increases by a factor 10 ( =(2-1)/(1.1-1)) feed liquid (0 for feed vapor)

  25. NON-OPTIMAL Feed stage location with “extra” stages in top: “Pinch” above feed stage (mixture on feed stage is “heavier” than feed) OPTIMAL: • No pinch • or: pinch on both • sides of feed stage • (mixture on feed stage has • same composition as feed) feed line (q-line): vertical for liquid feed; horizontal for vapor feed NON-OPTIMAL with “extra” stages in bottom: “Pinch” below feed stage (mixture on feed stage is “lighter” than feed) Note: Extra stages (and pinch) is NOT a problem, because it implies lower energy usage. Preferably, the pinch should be on both side of the feed. “Pinch”: Section of column where little separation occurs

  26. IDEAL MIXTURE IDEAL VLE (constant α) Simple formula for feed stage location (Skogestad, 1987) Example. C3-splitter. zFL=0.65, xDH= 0.005, xBL=0.1, =1.12.

  27. IDEAL MIXTURE IDEAL VLE (constant α) Example: “5 min column design” • Design a column for separating air • Feed: 80 mol-% N2 (L) and 20% O2 (H) • Products: Distillate is 99% N2 and bottoms is 99.998% O2 • Component data • Nitrogen: Tb = 77.4 K,  Hvap=5.57 kJ/mol • Oxygen: Tb = 90.2 K,  Hvap=6.82 kJ/mol • Problem: 1) Estimate . 2) Find split D/F. 3) Stages: Find Nminand 4) suggest values for N and NF. 5) Energy usage:Find Vmin/F for a) vapor feed and b) liquid feed. • Given: For vapor feed and sharp sep. of binary mixture: Vmin/F = 1/(-1)

  28. IDEAL MIXTURE IDEAL VLE (constant α) Solution “5-min design” Also see paper (“Theory of distillation”)

  29. IDEAL MIXTURE IDEAL VLE (constant α)

  30. IDEAL MIXTURE IDEAL VLE (constant α)

  31. Column profiles • Binary separation. Typical composition profile Example column A (binary, 41 stages, 99% purities, =1.5) Typical: Flat profile at column ends xi = mole fraction of light component Here: No pinch (flat profile) around feed because we have “few” stages compared to required separation TOP BTM stage no.

  32. Binary distillation: Typical column profiles pinch below feed (have extra stages in bottom compared to required separation) Note: here with composition on x-axis

  33. “More linear profile with log. compositions”: Proof for infinite reflux and constant relative volatility

  34. Check of feed location • It is the separation of key components that matters! • Plot X = ln(xL/xH) versus stage no. • Feed is misplaced if “pinch” (no change in X) only on one side of feed stage • Feed is OK if no pinch or pinch on both sides of feed • If misplaced feed location: May get better purity or save energy by moving it (if possible)

  35. Temperature profiles

  36. Temperature profiles BTM TOP

  37. Binary distillation: Typical temperature profiles T Flat around feed when pinch (turned around with T on y-axis) Flat temperature profile toward column end (because of high purity) Stage no. ! LT¼ -X Again profile is much more linear in terms of logarithmic temperatures: 342K Stage no. ! 355K Pinch: region of little change (no separation) because of “extra” stages

  38. Example using Chemsep • http://www.chemsep.org/ • Written by Ross Taylor, Clarkson University • Lite version: max 50 stages and 5 components • Lite version is free and extremely simple to use • Example: • 25% nC4(1), 25% nC5(2), 25% nC6(3), 25% nC7(4) • Key components C5 (L) and C6 (H) • Relative volatility varies between 2.5 (bottom) and 3.5 (top) • Assume we want about 99% of C5 in top and 99% of C6 in bottom • How many stages (N) and approx. L/F?

  39. IDEAL VLE (constant α) Shortcut analysis • Nmin = ln S / ln  = ln (1/(0.01*0.01)) / ln 3 = 8.4 (this no. does not depend on neon-keys) • Lmin/F ¼ 1/(-1) = 1/(3-1) = 0.5 (but non-keys change this...) • Let us try N = 20 and L/F=0.6 • Now run detailed stage-to-stage simulation...

  40. Data input... components

  41. ... column configuration

  42. ... thermodynamics Correction: Use Soave-RK also here

  43. ... feed data

  44. TOP: Specify L/F = 0.6 BTM: Specify B/F = 0.5

  45. L/F = 0.6 gives 99.9 % recovery of keys recovery keys = 99.9 %

  46. Profiles 99.9% recovery

  47. Liquid phase composition99.9 % recovery TOP light non-key (butane) light key (pentane) Stage heavy non-key (heptane) heavy key (hexane) BTM x

  48. Vapor phase composition99.9% recovery TOP Stage BTM y

  49. Flow profile99.9% recovery TOP V L Stage BTM Flows

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