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Distillation:So simple and yet so complex... and vice versa
Sigurd SkogestadNorwegian University of Science and Technology (NTNU)Trondheim, Norway
Outline
When use distillation Increase in heat input decreases temperature?? Complex model but simple dynamics...
....usually Control: Get rid of some myths! Complex column configurations (Petlyuk/Kaibel)...
... save energy as well as capital
BASF Aktiengesellschaft
F
V
L
B
D
Alternative: Packed column
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 0.0001% (1 ppm) impurity in one product increases required number of stages by factor 3
log(1.e-6)/log(1.e-2)=3 Well suited for scale-up
Columns with diameters over 15 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)
Reflux gives strange effects
Reflux gives strange effects
•INCREASED HEAT INPUT• ) LOWER TEMPERATURE TOP
SO SIMPLE....and yet SO COMPLEX
Simple to model
Stage i
Stage i+1
Stage i-1
Vi
yi
Vi-1
yi-1
Li+1
Xi+1
Li
xi
Equilibrium (VLE): yi = Ki(xi)
Vi+1
yi+1
Material balance stage i (Acc=in-out):dni/dt = Li+1xi+1 + Vy-1yi-1 –Li xi – Vi yi
The equilibrium stage concept
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
Model stage i
Usually most important!
Simple to model... yet difficult to understand SIMPLE TO MODEL
1920’s: Models known. Graphical solution (McCabe-Thiele) 1960’s: Simulation with computer straightforward
No need for more work!?
BEHAVIOR NOT SO SIMPLE TO UNDERSTAND Mathematician:
Large number of coupled equations Nonlinear equations (mainly VLE) Complex behavior expected
Simulation and experience Not so complex Dynamic response: simple
More simulations: Maybe not so simple after all Instability Multiple steady states
Dynamic behavior simple! Example: Composition response of propane-
propylene splitter with 110 stages and large reflux
Propane-propylene with 110 stages. Increase reflux. Simulated composition response with detailed model.
0 200 400 600 800 1000 1200 1400 1600 1800 20000.9945
0.995
0.9955
0.996
0.9965
0.997
0.9975
0.998
0 200 400 600 800 1000 1200 1400 1600 1800 20000.08
0.1
0.12
0.14
0.16
0.18
0.2
0 200 400 600 800 1000 1200 1400 1600 1800 20000.64
0.65
0.66
0.67
0.68
0.69
0.7
0.71
0.72
XD
0 1 2 3 4 5 6 7 8 9 100.0999
0.1
0.1001
0.1002
0.1003
0.1004
0.1005
0.1006
XB
2000 min
Xfeed stage
Observed: “Simple” first-order responses with time constant about 6 h = 400 min
0 200 400 600 800 1000 1200 1400 1600 1800 20000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1Increase in reflux. Mole fraction propylene on all stages
feed stage
All stages have a very similar slow first-order response!Behaves like “a single mixing tank” Why? Reflux gives strong interactions between the stages
SO COMPLEX (model)...and yet SO SIMPLE (response)
Dynamic behavior simple? 1970’s and 1980’s: Mathematical proofs that
dynamics are always stable Based on analyzing dynamic model with L and V
[mol/s] as independent variables In reality, independent variables are
Lw [kg/s] = L [mol/s] ¢ M [kg/mol]
QB [J/s] = V [mol/s] ¢ Hvap [J/mol]
Does it make a difference? YES, in some cases!
Molar and mass refluxt=0: zF is decreased from 0.5 to 0.495.
Lw[kg/s]= L[mol/s]/M where M [kg/mol] is the molecular weight, Data: ML=35, MH=40.
What is happening? Mole wt. depends on composition: more heavy ! M up ! L down ! even more heavy ...)Can even get instability! With MH=40, instability occurs for ML<28 (Jacobsen and Skogestad, 1991)
Instability for “ideal” columns:Many people didn’t believe us when we first presented it in 1991!Likely to happen if the mole weights are sufficiently different
I
IIIII
I
IV
IV
II
Reflux back again....but not composition !?
Reflux
Topcomposition
SO SIMPLE....and yet SO COMPLEX
Multiple steady state solutions IIIIV
I
II
V
I
IIIIV
II
V
V IV
I
Actually notmuch of aproblem withcontrol!
This is why you are not likelyto notice it in practice...unless you look carefully at the reflux....will observe inverse response inan unstable operating point (V)
V
IV
I
SO COMPLEX (no control)....and yet SO SIMPLE (control)
Myth of slow control
Let us get rid of it!!!
Compare manual (“perfect operator”) and automatic control for typical column: 40 stages, Binary mixture with 99% purity both ends, relative volatility = 1.5
First “one-point” control: Control of top composition only Then “two-point” control: Control of both compositions
“Perfect operator”: Steps L directly to correct steady-state value (from 2.70 to 2.74)
Disturbance in V
Want xD constant
Can adjust reflux L
Myth about slow controlOne-point control
“Perfect operator”: Steps L directly Feedback control: Simple PI control Which response is best?
Disturbance in V
CC xDS
Myth about slow controlOne-point control
Myth about slow controlOne-point control
SO SIMILAR (inputs)... and yet SO DIFFERENT (outputs)
Myth about slow controlTwo-point control
“Perfect operator”: Steps L and V directly Feedback control: 2 PI controllers Which response is best?
CC xDS: step up
CC xBS: constant
Myth about slow controlTwo-point control
SO SIMILAR (inputs)... and yet SO DIFFERENT (outputs)
Myth about slow control
Conclusion: Experience operator: Fast control impossible
“takes hours or days before the columns settles”
BUT, with feedback control the response can be fast! Feedback changes the dynamics (eigenvalues) Requires continuous “active” control
Most columns have a single slow mode (without control) Sufficient to close a single loop (typical on temperature) to change
the dynamics for the entire column
Complex columns
Sequence of columns for multicomponent separation
Heat integration Pressure levels Integrated solutions Non-ideal mixtures (azeotropes)
Here: Will consider “Petlyuk” columns
Typical sequence: “Direct split”
A,B,C,D,E,F
A
F
BC
DE
3-product mixture
A+B+C
A+B
A
B
C
1. Direct split
A+B+C
A+B
A
B
C
B+C
A+B+C
A
B
C
B+C
3. Combined(with prefractionator)
2. Indirect split
Towards the Petlyuk column
A+B
A
B
C
B+C
A+B
A
B
C
B+C
A+B
A
B
C
B+C
4. Prefractionator + sidestream column
liquid split
vapor split
5. Petlyuk30-40% less energy
A+B+C A+B+CA+B+C
3.
Implementation of Petlyuk in single shellA
A+B
B
B+C
A+B+C B (pure!)
C
6. DIVIDED-WALL IMPLEMENTATION in single shell!Gives about 40% savings also in capital
thermodynamicallyequivalent
(both about 40%savings in energy)
C
A+B+C
A
SO COMPLEX....and yet SO SIMPLE
5. PREFACTIONATOR IMPLEMENTATION“Thermally coupled” with single reboiler and single condenser
Montz
GC – Chemicals Research and Engineering
Dividing Wall ColumnsOff-center Position of the Dividing Wall
≈≈
Vmin-diagramfor Different Distillation
Arrangements
= DC1/F
VT/F
PA/B
PB/C
PA/C
Vmin(C1)
Vmin (Petlyuk + ISF/ISB)
A B C
A
B
C
A B
B C
C1
C21
C21
SO COMPLEX....and yet SO SIMPLE(to estimate enrgy)
Vmin(A/B)
Vmin(B(C)
Divided wall columns: starting to catch on 1940’s: first patent 1960’s: Thermodynamic analysis (Petlyuk) 1984: First implementation (BASF) 2005: BASF has about 50 divided wall columns
also in Japan, South Africa... Control issues still not quite solved
but I think it should be rather easy
4-product mixture
A,B,C,D
A
BC
D
A – iC5B – nC5C – iC6D – nC6
Direct optimal extension of Petlyuk ideas requires two divided walls.Will look for something simpler
Conventional sequence with 3 columns
4-product mixture: Kaibel column
A+B
A
B
D
C+D
ABCD
C
D
ABCD
A
B (pure!)
C (pure!)
Alternative 3-columnsequence
Kaibel: 1 column!! More then 50% capital savingsAlso saves energy (but maybe not exergy)
A – iC5B – nC5C – iC6D – nC6
Control of Kaibel column
•Prefractionator:
• Close 1 “stabilizing” temperature loop
•Main column
• Close 3 “stabilizing” temperature loops
Close a “stabilizing” temperature
(profile) loop for each split
DSO COMPLEX....and yet SO SIMPLE (to operate)
H=6m
D=5cm
F
S1
S2
B
D
Conclusion
Distillation is important Distillation is unbeatable (in some cases) Distillation is fun Distillation is complex yet simple... and vice versa
column, uses, when use? strange responses... increase heat.. T drops model complex: would expect complicated behavior ... yet simple: show typical response e.g all stages response simple: expect always stable ... yet complex: can be unstable with mss (Lw V) NEW column configurations... “easy first” Petlyuk. Kaibel. make drawing of how it evolves Better. heat-integrated Petlyuk (prefrac). Hidic \item BATCH DISTILLATION (Reflux) \item MODEL, DYNAMICS (Feedback) \item CONTROL (Steady-state misleading) \item MULTIPLE STEADY STATES AND INSTABILITY (Nonlinearity and feedback) \item INTERLINKED COLUMNS (Parallel paths) \item BATCH DISTILLATION AGAIN \item SYSTEMS VIEW \item CONCLUSION
The response is nonlinear....
The response is nonlinear....but this can be corrected by taking log
XD = ln(xDL/xDH)xD
SO SIMPLE....and yet SO COMPLEX
Distillation control
CC
LV
Two-pointLV
TC
Ts
xB
CCxD
Refinery Main Fractionator Gas to Compressor
Heavy Naphtha
Light Cycle Oil
Decant Oil
Feed
HCOPumparound
To Absorber
LCOstripper
Tray# 46-50
Naphthastripper
Tray# 41-45
Steam
Steam
2
10
11
21
22
40
31
36
25
23
Decant Water
4
20
LCOPumparound
HCN Pumparound
Quench
SlurryPumparound
Can make problems...Detuned controllergain
V
V
Multi-Effect PrefractionatorAdditional large energy savings
A B C
A
B
C
A B
B C
HP LP
Forward integrated prefractionator (PF)
Integrated reboiler/condenser
Heat input