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