Presentation on theme: "Introduction to Distillation: Steady State Design and Operation"— Presentation transcript:
1 Introduction to Distillation: Steady State Design and Operation Distillation Course Berlin Summer 2008.Sigurd Skogestad. Part 1Introduction to Distillation: Steady State Design and OperationIntroductionSteady-state designSteady-state operation
3 1. Introduction to distillation King (Wiley, 1980) on distillation designShinskey (McGraw-Hill, 1984) on distillation controlKister (McGraw-Hill, 1990) on distillation operationGeneral info:I.J. Halvorsen and S. Skogestad, ``Distillation Theory'', In: Encyclopedia of Separation Science. Ian D. Wilson (Editor-in-chief), Academic Press, 2000, ppS. Skogestad, Dynamics and control of distillation columns - A tutorial introduction., Trans IChemE (UK), Vol. 75, Part A, Sept. 1997, (Presented at Distillation and Absorbtion 97, Maastricht, Netherlands, 8-10 Sept. 1997).More: see home page Sigurd SkogestadFree steady-state distillation software with thermo package :
7 Vapor-liquid equilibrium (VLE) = Equilibrium line y=K(x)Non-idealDifficult separation(almost az.)Easy sep.Ideal mixturecommon low-boiling az.less common high-boiling az.Azeotropes(non-ideal)
8 The equilibrium stage concept Vi+1yi+1Stage i+1Material balance stage i (out=in):Li xi + Vi yi = Li+1xi+1 + Vy-1yi-1Li+1Xi+1ViyiStage iEquilibrium (VLE): yi = Ki(xi)LixiVi-1yi-1Stage i-1The equlibrium stage concept is used for both tray and packed columnsN = no. of equilibrium stages in columnTray column: N = No.trays * Tray-efficiencyPacked columns: N = Height [m] / HETP [m]Typical: 0.7Typical: 0.5 m
9 TOP BTM TOP BTM Simplified energy balance: Vi = Vi+1 (“constant molar flows”)BTMTOPBTM
10 When use distillation?Liquid mixtures (with difference in boiling point)Unbeatable for high-purity separations becauseEssentially same energy usage independent of (im)purity!Going from 1% to % (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 2Well suited for scale-upColumns with diameters over 18 mExamples 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 mixturesMulticomponent mixtures: For relatively ideal mixtures this is almost the same as binary - if we consider the “pseudo-binary” separation between the key componentsL = light key componentH = heavy key componentThe remaining components are almost like “dead-weight”“Composition”: The impurity of key component is the important
16 Separation factor for column or column section 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:
17 Minimum no. of stages Total reflux = Infinite energy Stage i+1 ViyiVi-1yi-1Li+1xi+1LixiTotal reflux:Vi = Li+1yi = xi+1OOperating line: xi+1 = yi (diagonal)
18 Minimum no. of stages, Nmin (with infinite energy) IDEAL VLE (constant α)IDEAL MIXTUREMinimum no. of stages, Nmin (with infinite energy)Infinity energy ) Total reflux. Stage i:Repeat for all N stagesFenske’s formula for minimum no. of stagesAssumption: Constant relative volatilityApplies also to column sections
19 Minimum energy (minimum reflux) pinch(a) IDEAL VLE(b) NON-IDEAL VLEInfinite number of stages in pinch region
20 Minimum energy, Vmin (with infinite no. of stages) IDEAL MIXTUREIDEAL 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
22 Design: How many stages? Energy (V) vs. number of stages (N)Trade-off between number of stages and energyActual V approaches Vmin for N approximately 2 x Nmin or larger, typically:2Nmin + 25% Vmin3Nmin + 3 % Vmin4Nmin % Vmin
23 Design: How many stages? Conclusion: Select N > 2 Nmin (at least)Many stages reduce energy costsMany stages is good for controlCan overfractionate (tight control is then not critical)orGet less interactions between top and bottom (because of pinch zone around feed)
24 Real well-designed column IDEAL VLE (constant α)IDEAL MIXTUREReal well-designed columnRecall:Choose N ≈ 2 Nmin:Get V ≈ 1.25 Vmin and Q ≈ 1.25 ¢ Vmin ¢ HvapN = 3-4 Nmin gives V very close to VminImportant insights:Vmin is a good measure of energy usage QVmin is almost independent of purityVmin is weakly dependent on feed comp. (feed liquid: get vaporization term D/F≈ zF)Design: To improve purity (separation): Increase NN and Vmin both increase sharply as → 1Example. Decrease from 2 to 1.1:Nmin increases by a factor ( =ln 2/ln1.1)Vmin increases by a factor ( =(2-1)/(1.1-1))feed liquid(0 for feed vapor)
25 NON-OPTIMALFeed stage locationwith “extra” stages in top:“Pinch” above feed stage(mixture on feed stage is “heavier” than feed)OPTIMAL:No pinchor: pinch on bothsides of feed stage(mixture on feed stage hassame composition as feed)feed line (q-line):vertical for liquid feed;horizontal for vapor feedNON-OPTIMALwith “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 Simple formula for feed stage location (Skogestad, 1987) IDEAL VLE (constant α)IDEAL MIXTURESimple formula for feed stage location (Skogestad, 1987)Example. C3-splitter. zFL=0.65, xDH= 0.005, xBL=0.1, =1.12.
27 Example: “5 min column design” IDEAL VLE (constant α)IDEAL MIXTUREExample: “5 min column design”Design a column for separating airFeed: 80 mol-% N2 (L) and 20% O2 (H)Products: Distillate is 99% N2 and bottoms is % O2Component dataNitrogen: Tb = 77.4 K, Hvap=5.57 kJ/molOxygen: Tb = 90.2 K, Hvap=6.82 kJ/molProblem: 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)
28 Solution “5-min design” IDEAL VLE (constant α)IDEAL MIXTURESolution “5-min design”Also see paper (“Theory of distillation”)
32 Column profiles Binary separation. Typical composition profile Example column A(binary, 41 stages,99% purities, =1.5)Typical: Flat profile at column endsxi =mole fractionof lightcomponentHere: No pinch (flat profile) around feedbecause we have “few” stagescompared to required separationTOPBTMstage no.
33 Binary distillation: Typical column profiles pinch below feed(have extra stages inbottom compared torequired separation)Note: here with composition on x-axis
34 “More linear profile with log. compositions”: Proof for infinite reflux and constant relative volatility
35 Check of feed locationIt 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 stageFeed is OK if no pinch or pinch on both sides of feedIf misplaced feed location: May get better purity or save energy by moving it (if possible)
38 Typical temperature profiles T Binary distillation:Typical temperature profilesTFlat 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 -XAgain profile is much more linearin terms of logarithmic temperatures:342KStage no. !355KPinch: region of little change (no separation) because of “extra” stages
39 Example using Chemsep http://www.chemsep.org/ Written by Ross Taylor, Clarkson UniversityLite version: max 50 stages and 5 componentsLite version is free and extremely simple to useExample: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 bottomHow many stages (N) and approx. L/F?
40 Shortcut analysis Nmin = ln S / ln = ln (1/(0.01*0.01)) / ln 3 = 8.4 IDEAL VLE (constant α)Shortcut analysisNmin = 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=0.6Now run detailed stage-to-stage simulation...
57 3. Steady-state operation The column is now given!Operational degrees of freedom:Get right split = cut (“external flows” e.g. D/F) !!!Adjust separation = fractionation (“internal flows” L/V)Column (temperature) profilesMulticomponent mixtures...other factors...Optimal operation (in a plantwide setting)
58 Given feed (F) and pressure (p): 2 steady-state degrees of freedom, e 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 (xD, xB)
59 Operation conventional column 2 steady-state degrees of freedom“External flows” (product split D/F).Adjust by changing D/FMoves “profile” up and downLarge effect on operation“Internal flows” (L/V).Increase L and V with D/F constantStretches profileImproves separation factor S, but costs energy and limits capacitySmall effectWhy small effect? Recall design: Purity (separation) mainly influenced by no. of stages (N), which is fixed during operationSPLIT (CUT)
60 Operation conventional column 2 steady-state degrees of freedom“External flows” (product split D/F).Adjust by changing D/FMoves “profile” up and downLarge effect on operation“Internal flows” (L/V).Increase L and V with D/F constantStretches profileImproves separation factor S, but costs energy and limits capacitySmall effectWhy small effect? Recall design: Purity (separation) mainly influenced by no. of stages (N), which is fixed during operationFRACTIONATION(SEPARATION)
61 Split D/F (external flows): Moves entire composition profile up or down.One product gets purer and the other less pureLarge effectInternal flows (L/V):“Stretches profile”Both products get purer if we increase internal flowsSmaller effectComposition profiles for column A (F=1).Change in external flows: D = with V=0Change in internal flows: V = with D=0TOPBTM“Less pure”:Breakthrough of light component in bottom
62 Implication for control Important to get the right split (D/F)avoid breakthrough of light components in bottomavoid breakthrough of heavy components in topHow can this be done?Measure feed composition (zF) and adjust D/F ¼ zF (feedforward control).2. Keep “column profile” in place by measuring and “fixing” it somewhere in the column (feedback control)Simplest in practice: Control temperatureTo minimize movement of profile:NO! Does not work in practice because of uncertaintyControl temperature at most sensitive location
63 Implication for control LIGHTHEAVYFDBTCIdea: The column is a “tank” filled with heavy and light componentNeed to adjust the split (D) to keep constant holdups of light and heavySimplest: “Profile feedback” using sensitive temperature
64 Temperature profile multicomponent Feed:25% C425% C5 (L)25% C6 (H)25% C720 stagesD/F = 0.5Vary L/FL/F=0.6:99.9% recoveryof L and HTemp.L/F=0.3:99% recoveryof L and HSTEEP PROFILE TOWARDS COLUMN ENDSBECAUSE OF NON-KEYSBTMTOPStageControl: Use temperature about here(large sensitivity)
65 Summary. Steady-state operation of given column 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 endYou may try lowering the pressure, but usually limited effectYou may consider moving the feed location (look at profile), but usually has limited effectNormally the only “fix” is to get more stages in your columnIf it has many stages, then you have two options:Overpurify one or both ends: Won’t cost much in terms of energy, and makes control easier (no pinch in column)Keep specifications and save energy: Get pinch in column
66 Steady-state design and simulation of real columns Commercial software: Hysys, Aspen, …Most important: Use right thermodynamics (VLE). SRK or PR works surprisingly well for most mixtures (especially at high pressures and for gases)Design (given products): Use shortcut method to estimate required no. of stages + feed location.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
67 Trays vs. packings Packings: Trays: + 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)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)Overall: Differences are surprisingly small – also for control
68 Conclusion steady-state distillation Understanding the steady-state behavior brings you a very long way towards understanding the control
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