Distillation Course Berlin Summer 2008 Sigurd Skogestad Part
Distillation Course Berlin Summer 2008. Sigurd Skogestad. Part 1 Introduction to Distillation: Steady State Design and Operation 1. 2. 3. April 4 -8, 2004 Introduction Steady-state design Steady-state operation KFUPM-Distillation Control Course
BASF Aktiengesellschaft April 4 -8, 2004 KFUPM-Distillation Control Course 2
1. Introduction to distillation n King (Wiley, 1980) on distillation design Shinskey (Mc. Graw-Hill, 1984) on distillation control Kister (Mc. Graw-Hill, 1990) on distillation operation n General info: http: //lorien. ncl. ac. uk/ming/distil 0. htm n 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. n S. Skogestad, Dynamics and control of distillation columns - A tutorial introduction. , Trans IChem. E (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/distillation n n Free steady-state distillation software with thermo package April 4 -8, 2004 KFUPM-Distillation Control Course : http: //www. chemsep. org/ 3
L D F V B April 4 -8, 2004 KFUPM-Distillation Control Course 4
I usually number the stages from the bottom (with reboiler=1), but many do It from the top April 4 -8, 2004 KFUPM-Distillation Control Course 5
Alternative: Packed column April 4 -8, 2004 KFUPM-Distillation Control Course 6
Vapor-liquid equilibrium (VLE) = Equilibrium line y=K(x) Non-ideal Easy sep. Difficult separation (almost az. ) Ideal mixture less common high-boiling az. common low-boiling az. Azeotropes (non-ideal) April 4 -8, 2004 KFUPM-Distillation Control Course 7
Vi+1 yi+1 The equilibrium stage concept Stage i+1 Vi yi Li+1 Xi+1 Equilibrium (VLE): yi = Ki(xi) Stage i Vi-1 yi-1 Material balance stage i (out=in): Li xi + Vi yi = Li+1 xi+1 + Vy-1 yi-1 Li xi Stage i-1 The equlibrium stage concept is used for both tray and packed columns • N = no. of equilibrium stages in column Typical: 0. 7 • Tray column: N = No. trays * Tray-efficiency • Packed columns: N = Height [m] / HETP [m] Typical: 0. 5 m April 4 -8, 2004 KFUPM-Distillation Control Course 8
TOP Simplified energy balance: Vi = Vi+1 (“constant molar flows”) BTM TOP BTM April 4 -8, 2004 KFUPM-Distillation Control Course 9
When use distillation? n n Liquid mixtures (with difference in boiling point) Unbeatable for high-purity separations because n Essentially same energy usage independent of (im)purity! § n Number of stages increases only as log of impurity! § n 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 § n Going from 1% to 0. 0001% (1 ppm) impurity in one product increases energy usage only by about 1% Columns with diameters over 18 m Examples of unlikely uses of distillation: § § April 4 -8, 2004 High-purity silicon for computers (via Si. Cl 3 distillation) Water – heavy-water separation (boiling point difference only 1. 4 C) KFUPM-Distillation Control Course 10
2. Steady-state Design n n Given separation task Find q q q n configuration (column sequence) no. of stages (N) energy usage (V) ”How to design a column in 5 minutes” April 4 -8, 2004 KFUPM-Distillation Control Course 11
Multicomponent and binary mixtures n We will mostly consider separation of binary mixtures n 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 q n The remaining components are almost like “dead-weight” “Composition”: The impurity of key component is the important April 4 -8, 2004 KFUPM-Distillation Control Course 12
Relative volatility, April 4 -8, 2004 KFUPM-Distillation Control Course 13
Ideal mixture: Estimate of relative volatility April 4 -8, 2004 KFUPM-Distillation Control Course 14
IDEAL VLE (constant α) Estimate of relative volatility (2) n Example. iso-pentane (L) – pentane (H) n Example. Nitrogen (L) – Oxygen (H) April 4 -8, 2004 KFUPM-Distillation Control Course 15
Separation factor for column section n Example: Binary separation with purities: 90% light in top and 90% heavy in bottom: n Example: Binary separation with purities: 99. 9% light in top and 98% heavy in bottom: April 4 -8, 2004 KFUPM-Distillation Control Course 16
Minimum no. of stages Total reflux = Infinite energy Total reflux: Vi = Li+1 yi = xi+1 Stage i+1 Li+1 xi+1 Vi yi Stage i Vi-1 yi-1 O Li xi Operating line: April 4 -8, 2004 KFUPM-Distillation Control Course xi+1 = yi (diagonal) 17
IDEAL VLE MIXTURE (constant α) Minimum no. of stages, Nmin (with infinite energy) n Infinity energy ) Total reflux. Stage i: n Repeat for all N stages n Fenske’s formula for minimum no. of stages Assumption: Constant relative volatility n April 4 -8, 2004 Applies also to column sections KFUPM-Distillation Control Course 18
Minimum energy )minimum reflux( pinch (a) IDEAL VLE (b) NON-IDEAL VLE Infinite number of stages in pinch region April 4 -8, 2004 KFUPM-Distillation Control Course 19
IDEAL VLE MIXTURE (constant α) Minimum energy, Vmin (with infinite no. of stages) n Feed liquid (King’s formula, assuming pinch at feed): feed vapor: delete the D n NOTE: Almost independent of composition!! For sharp split (r. LD=1, r. HD=0), feed liquid: Assumption: Ideal mixture with constant relative volatility and constant molar flows. April 4 -8, 2004 KFUPM-Distillation Control Course 20
IDEAL VLE MIXTURE (constant α) Examples design April 4 -8, 2004 KFUPM-Distillation Control Course 21
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: 2 Nmin 3 Nmin 4 Nmin April 4 -8, 2004 KFUPM-Distillation Control Course + 25% Vmin + 3 % Vmin + 0. 3 % Vmin 22
Design: How many stages? Conclusion: Select N > 2 Nmin (at least) n 1. 2. Many stages reduce energy costs Many stages is good for control q Can overfractionate (tight control is then not critical) or q April 4 -8, 2004 Get less interactions between top and bottom (because of pinch zone around feed) KFUPM-Distillation Control Course 23
IDEAL VLE MIXTURE (constant α) Real well-designed column n Recall: n 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 q q feed liquid (0 for feed vapor) Important insights: n 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≈ z. F) Design: To improve purity (separation): Increase N N and Vmin both increase sharply as → 1 q q q n q q April 4 -8, 2004 Example. Decrease from 2 to 1. 1: Nmin increases by a factor 7. 3 Vmin increases by a factor 10 ( =ln 2/ln 1. 1) ( =(2 -1)/(1. 1 -1)) KFUPM-Distillation Control Course 24
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 April 4 -8, 2004 KFUPM-Distillation Control Course 25
IDEAL VLE MIXTURE (constant α) Simple formula for feed stage location (Skogestad, 1987) Example. C 3 -splitter. z. FL=0. 65, x. DH= 0. 005, x. BL=0. 1, =1. 12. April 4 -8, 2004 KFUPM-Distillation Control Course 26
IDEAL VLE MIXTURE (constant α) Example: “ 5 min column design” n n Design a column for separating air Feed: 80 mol-% N 2 (L) and 20% O 2 (H) Products: Distillate is 99% N 2 and bottoms is 99. 998% O 2 Component data q q n n Nitrogen: Tb = 77. 4 K, Hvap=5. 57 k. J/mol Oxygen: Tb = 90. 2 K, Hvap=6. 82 k. J/mol Problem: 1) Estimate . 2) Find split D/F. 3) Stages: Find Nmin and 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) April 4 -8, 2004 KFUPM-Distillation Control Course 27
IDEAL VLE MIXTURE (constant α) Solution “ 5 -min design” Also see paper (“Theory of distillation”) April 4 -8, 2004 KFUPM-Distillation Control Course 28
IDEAL VLE MIXTURE (constant α) April 4 -8, 2004 KFUPM-Distillation Control Course 29
IDEAL VLE MIXTURE (constant α) April 4 -8, 2004 KFUPM-Distillation Control Course 30
April 4 -8, 2004 KFUPM-Distillation Control Course 31
Column profiles n 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 BTM April 4 -8, 2004 TOP stage no. KFUPM-Distillation Control Course 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 April 4 -8, 2004 KFUPM-Distillation Control Course 33
“More linear profile with log. compositions”: Proof for infinite reflux and constant relative volatility April 4 -8, 2004 KFUPM-Distillation Control Course 34
Check of feed location n n It is the separation of key components that matters! Plot X = ln(x. L/x. H) 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) April 4 -8, 2004 KFUPM-Distillation Control Course 35
Temperature profiles April 4 -8, 2004 KFUPM-Distillation Control Course 36
Temperature profiles BTM April 4 -8, 2004 TOP KFUPM-Distillation Control Course 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. ! LT ¼ -X Again profile is much more linear in terms of logarithmic temperatures: 342 K Stage no. ! 355 K Pinch: region of little change (no separation) because of “extra” stages April 4 -8, 2004 KFUPM-Distillation Control Course 38
Example using Chemsep n n n 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: n n n 25% n. C 4(1), 25% n. C 5(2), 25% n. C 6(3), 25% n. C 7(4) Key components C 5 (L) and C 6 (H) Relative volatility varies between 2. 5 (bottom) and 3. 5 (top) Assume we want about 99% of C 5 in top and 99% of C 6 in bottom How many stages (N) and approx. L/F? April 4 -8, 2004 KFUPM-Distillation Control Course 39
IDEAL VLE (constant α) Shortcut analysis n 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 n Now run detailed stage-to-stage simulation. . . n n April 4 -8, 2004 KFUPM-Distillation Control Course 40
Data input. . . components April 4 -8, 2004 KFUPM-Distillation Control Course 41
. . . column configuration April 4 -8, 2004 KFUPM-Distillation Control Course 42
. . . thermodynamics Correction: Use Soave-RK also here April 4 -8, 2004 KFUPM-Distillation Control Course 43
. . . feed data April 4 -8, 2004 KFUPM-Distillation Control Course 44
TOP: Specify L/F = 0. 6 BTM: Specify B/F = 0. 5 April 4 -8, 2004 KFUPM-Distillation Control Course 45
L/F = 0. 6 gives 99. 9 % recovery of keys recovery keys = 99. 9 % April 4 -8, 2004 KFUPM-Distillation Control Course 46
Profiles 99. 9% recovery April 4 -8, 2004 KFUPM-Distillation Control Course 47
Liquid phase composition 99. 9 % recovery TOP light non-key (butane) light key (pentane) Stage heavy non-key (heptane) heavy key (hexane) BTM x April 4 -8, 2004 KFUPM-Distillation Control Course 48
Vapor phase composition 99. 9% recovery TOP Stage BTM y April 4 -8, 2004 KFUPM-Distillation Control Course 49
Flow profile 99. 9% recovery TOP L V Stage BTM Flows April 4 -8, 2004 KFUPM-Distillation Control Course 50
Temperature profile 99. 9% recovery TOP Stage BTM Temperature [K] April 4 -8, 2004 KFUPM-Distillation Control Course 51
Turn profile around Temp. TOP BTM Stage April 4 -8, 2004 KFUPM-Distillation Control Course 52
Log (x. L/x. H)-plot (“key ratio profile”): Use to check feed location TOP Stage log(x. L/x. H) straight line: Feed placement OK BTM April 4 -8, 2004 KFUPM-Distillation Control Course 53
With feed moved from stage 10 to 15 TOP 5 Stage 10 15 log(x. L/x. H) has pinch above feed: Too many stages above feed BTM April 4 -8, 2004 KFUPM-Distillation Control Course 54
Relative volatility (Feed back to stage 10) TOP 2. 5 3. 0 3. 5 4. 0 Stage BTM April 4 -8, 2004 KFUPM-Distillation Control Course 55
Mc. Cabe-Thiele diagram 99. 9% recovery TOP y’C 5 BTM April 4 -8, 2004 x’C 5 KFUPM-Distillation Control Course 56
3. Steady-state operation The column is now given! Operational degrees of freedom: n n 1. 2. n n Get right split = cut (“external flows” e. g. D/F) !!! Adjust separation = fractionation (“internal flows” L/V) Column (temperature) profiles Multicomponent mixtures. . . other factors. . . Optimal operation (in a plantwide setting) April 4 -8, 2004 KFUPM-Distillation Control Course 57
Given feed (F) and pressure (p): 2 steady-state degrees of freedom, e. g. L and V. Can use for (for example): Control one composition for each product (x. D, x. B) April 4 -8, 2004 KFUPM-Distillation Control Course 58
Operation conventional column q 2 steady-state degrees of freedom 1. “External flows” (product split D/F). q q q 2. Adjust by changing D/F Moves “profile” up and down Large effect on operation “Internal flows” (L/V). q q q April 4 -8, 2004 Increase L and V with D/F constant Stretches profile Improves separation factor S, but costs energy and limits capacity Small effect Why small effect? Recall design: Purity (separation) mainly influenced by no. of stages (N), which is fixed during operation KFUPM-Distillation Control Course SPLIT (CUT) 59
Operation conventional column 2 steady-state degrees of freedom “External flows” (product split D/F). 1. Adjust by changing D/F Moves “profile” up and down Large effect on operation • • • “Internal flows” (L/V). 2. • • • April 4 -8, 2004 Increase L and V with D/F constant Stretches profile Improves separation factor S, but costs energy and limits capacity Small effect Why small effect? Recall design: Purity (separation) mainly influenced by no. of stages (N), which is fixed during operation KFUPM-Distillation Control Course FRACTIONATION (SEPARATION) 60
n Split D/F (external flows): q q q n Moves entire composition profile up or down. One product gets purer and the other less pure Large effect Internal flows (L/V): q q q “Stretches profile” Both products get purer if we increase internal flows Smaller effect April 4 -8, 2004 Composition profiles for column A (F=1). Change in external flows: D = -0. 02 with V=0 Change in internal flows: V = 1 with D=0 TOP BTM “Less pure”: Breakthrough of light component in bottom KFUPM-Distillation Control Course 61
Implication for control Important to get the right split (D/F) n q q n avoid breakthrough of light components in bottom avoid breakthrough of heavy components in top How can this be done? 1. Measure feed composition (z. F) and adjust D/F ¼ z. F (feedforward control). NO! Does not work in practice because of uncertainty 2. Keep “column profile” in place by measuring and “fixing” it somewhere in the column (feedback control) n n Simplest in practice: Control temperature To minimize movement of profile: Control temperature at most sensitive location April 4 -8, 2004 KFUPM-Distillation Control Course 62
Implication for control LIGHT D TC F HEAVY B Idea: The column is a “tank” filled with heavy and light component Need to adjust the split (D) to keep constant holdups of light and heavy Simplest: “Profile feedback” using sensitive temperature April 4 -8, 2004 KFUPM-Distillation Control Course 63
Temperature profile multicomponent Feed: 25% C 4 25% C 5 (L) 25% C 6 (H) 25% C 7 L/F=0. 6: 99. 9% recovery of L and H Temp. L/F=0. 3: 99% recovery of L and H 20 stages D/F = 0. 5 Vary L/F STEEP PROFILE TOWARDS COLUMN ENDS BECAUSE OF NON-KEYS TOP April 4 -8, 2004 BTM Stage Control: Use temperature about here 64 (large sensitivity) KFUPM-Distillation Control Course
Summary. Steady-state operation of given column n n If split is wrong then one end will be too pure (overpurified), while the other end does not meet spec. (underpurified) Assume now split is right (e. g. control column profile) If column has too few stages, then it may difficult to obtain desired purities (even with maximum heat input): may need to give up one end q You may try lowering the pressure, but usually limited effect q You may consider moving the feed location (look at profile), but usually has limited effect q Normally the only “fix” is to get more stages in your column If it has many stages, then you have two options: q Overpurify one or both ends: Won’t cost much in terms of energy, and makes control easier (no pinch in column) q Keep specifications and save energy: Get pinch in column April 4 -8, 2004 KFUPM-Distillation Control Course 65
Steady-state design and simulation of real columns n Commercial software: Hysys, Aspen, … n Most important: Use right thermodynamics (VLE). SRK or PR works surprisingly well for most mixtures (especially at high pressures and for gases) n Design (given products): Use shortcut method to estimate required no. of stages + feed location. n Operation (given column): First get no. of stages in each section by matching data for composition and temperature profiles. Adjust holdups by matching with dynamic responses April 4 -8, 2004 KFUPM-Distillation Control Course 66
Trays vs. packings n Packings: + Much smaller pressure drop (typically 1/10) + Usually: More stages for given column height - Problems with liquid distribution in larger columns (can use structured packings, but more expensive) n Trays: + More easy to clean + Better for large capacity columns + Larger holdup (typically, 2 times larger): Advantage for control (“have more time”) - Can have inverse response in bottom of column ( - effect - difficult to predict) n Overall: Differences are surprisingly small – also for control April 4 -8, 2004 KFUPM-Distillation Control Course 67
Conclusion steady-state distillation n Understanding the steady-state behavior brings you a very long way towards understanding the control April 4 -8, 2004 KFUPM-Distillation Control Course 68
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