PCI-697: DISTILLATION CONTROL Department of Chemical

PCI-697: DISTILLATION CONTROL

Department of Chemical EngineeringKFUPM

Topic: Distillation Principles and Dynamics

Dr. Housam Binous

2

Objectives of distillation control

Operate in Safe, Stable Manner

Operate Within Equipment Constraints

Produce Products at Desired Qualities

Minimize Propagation of Disturbances to Downstream Units

Minimize Energy Consumption

Operating window

3

Steam Condensate Capacity Limit

Boi

lup

Rat

e, V

Flooding

Contours of Constant Separation

Vessel Pressure Limit

20Column Pressure, psia

40 60 80 100 120

Key aspects in distillation control

4

Pressure Control

Material and Energy Balance Controls

Product Quality Control

Product Quality Measurement

Control Techniques and Strategies

Mass and Energy Balance Control

5

L

F

D

SPQC

R

Feed

QCSP

F

Q R

B

SPF

L

QR

SP

SP

L

F

F

QC

Feed QCSP

FR D

L

Energy Balance Control

Material Balance Control

Reflux ratio affects both cutpoint and fractionation

cutpoint and fractionation are clearly separated

Types of Distillation Columns

6

Batch and Continuous Columns

Types of Continuous Columns binary

multi-component

multi-product

extractive

azeotropic

Type of column internals trayed

packed

Non Ideal VLE behavior

7

Slide

Limit of Distillation by Azeotrope

Liquid Mole Fraction of Ethanol0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Vapo

r Mol

e Fr

actio

n of

Eth

anol

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

• Distillation range is restricted by the azeotropic point.

• Binary azeotropic mixtures, such as ethanol/water and IPA/water, can be separated into their pure components by distillation by the addition of a third component, so called the entrainer, which forms a ternary azeotrope with a lower boiling point than any binary azeotrope

Ethanol / Water System

Separation of Azeotropic Mixture

Add the Third Component

Azeotropic Distillation: Volatile Addition

Extractive Distillation: Non-volatile Addition

Shift the Azeotropic Point by Changing Pressure

9

Azeotropic vs. Extractive Distillation

• Azeotropic distillationBy forming a ternary heterogeneous azeotrope lower than any other

binary azeotropic temperatures, nearly pure ethanol can be obtained as a bottom product in an azeotropic distillation column.

Ethanol is obtained as a bottom product from an azeotropic distillation column using an entrainer such as benzene or normal pentane.

• Extractive distillationBy adding a solvent which is exclusively familiar with a wanted

component in a feed mixture, a desired component can be obtained in an extractive distillation column overhead.

Ethanol is obtained as a top product from an extractive distillation with ethylene glycol solvent.

10

Azeotropic vs. Extractive Distillation

FEED

RECYCLE UPPERPHASE

LOWERPHASE

PURE ETHANOL

FEED

SOLVENT

PURE ETHANOL

Azeotropic Distillation Extractive Distillation

11

Pressure-Swing Distillation

12

Distillation Column

13

Reboilers

14

There are a number of designs of reboilers. Their

design principles can be regarded as that of heat-

exchangers that are required to transfer enough

energy to bring the liquid at the bottom of the

column to boiling point.

Reboilers

15

Reboilers

16

Reboilers

17

Reboilers

18

Tube bundle

Condenser & Reflux Drum

19

The vapor moves up the column, and as it exits the top of

the unit, it is cooled by a condenser. The condensed liquid

is stored in a holding vessel known as the reflux drum.

Some of this liquid is recycled back to the top of the column

and this is called the reflux. The condensed liquid that is

removed from the system is known as the distillate.

20

Condenser & Reflux Drum

Trays

21

Random Packing

22

Structured Packing

23

Vapour Flow Conditions

24

Adverse vapor flow conditions can cause : Foaming

Dumping/Weeping

Entrainment

Flooding

25

Foaming

Foaming refers to the expansion of liquid due to passage of

vapor or gas. Although it provides high interfacial liquid-vapor

contact, excessive foaming often leads to liquid buildup on trays.

In some cases, foaming may be so bad that the foam mixes with

liquid on the tray above. Whether foaming will occur depends

primarily on physical properties of the liquid mixtures, but is

sometimes due to tray designs and condition. Whatever the

cause, separation efficiency is always reduced.

Vapor Flow Conditions

Vapor Flow Conditions

26

Flooding

Flooding is brought about by excessive vapor flow, causing liquid

to be entrained in the vapor up the column. The increased

pressure from excessive vapor also backs up the liquid in the

downcomer, causing an increase in liquid holdup on the plate

above. Depending on the degree of flooding, the maximum

capacity of the column may be severely reduced. Flooding is

detected by sharp increases in column differential pressure and

significant decrease in separation efficiency.

Vapor Flow Conditions

27

Entrainment

Entrainment refers to the liquid carried by vapor up to the tray

above and is again caused by high vapor flow rates. It is

detrimental because tray efficiency is reduced: lower volatile

material is carried to a plate holding liquid of higher volatility. It

could also contaminate high purity distillate. Excessive

entrainment can lead to flooding.

Vapor Flow Conditions

28

Weeping/Dumping

This phenomenon is caused by low vapor flow. The pressure

exerted by the vapor is insufficient to hold up the liquid on the

tray. Therefore, liquid starts to leak through perforations.

Excessive weeping will lead to dumping. That is the liquid on all

trays will crash (dump) through to the base of the column (via a

domino effect) and the column will have to be re-started.

Weeping is indicated by a sharp pressure drop in the column and

reduced separation efficiency.

Column Diameter

29

Most of the above factors that affect column operation is due to vapor

flow conditions: either excessive or too low. Vapor flow velocity is

dependent on column diameter. Weeping determines the minimum

vapor flow required while flooding determines the maximum vapor flow

allowed, hence column capacity. Thus, if the column diameter is not

sized properly, the column will not perform well. Not only will operational

problems occur, the desired separation duties may not be achieved.

Column Diameter

30

Definition of a stage in a process

31

A single stage is a device or a subunit of the process,where two (or more) phases of a different compositioncome in contact with each other, exchange and leavewith new compositions

Lin,xin

Lout,xout

Vout,yout

Vin,yin

- Mass balance• Overall

• Componentsoutoutinin VLVL +=+

outoutoutoutinininin yVxLyVxL +=+

- Energy balanceLin,hin

Lout,hout

Vout,hout

Vin,hin

outoutoutoutinininin hVhLQhVhL +=++Q

• M=Mass• E=Equilibrium• S=summation• H=Enthalpy

32

Distillation Dynamics

MESH

33

Li+1

Vi+2

Plate i + 2

Clear Liquid Froath

Li-1

Li

Vi+1

Vapor streamPlate i + 1

DowncomerVi

Pressure PiTemperature TiPlate i

Perforated plate

Plate i - 1

Distillation Dynamics

34

Distillation DynamicsMass and Energy Balance

Plate k

Plate k + 1

Plate k - 1

Lk – 1

xk-1

Lk

xk

Vk+1

yk+1

Vk

yk

35

Distillation DynamicsMass and Energy Balance

A tray (k)

36

F, xF, hF

Plate k

Plate k + 1

Plate k - 1

Lk – 1

xk-1

Lk

xk

Vk+1

yk+1

Vk

yk

Distillation DynamicsMass and Energy Balance

37

Feed tray (k=f)

Distillation DynamicsMass and Energy Balance

38

Distillation DynamicsMass and Energy Balance

condenser

V1

L1, xD

V2, y2

D, xD

39

Reflux drum (k=1)

Distillation DynamicsMass and Energy Balance

40

Vk

yk

Lk – 1

xk-1

B

xk

reboilerk = n

Distillation DynamicsMass and Energy Balance

41

Reboiler (k=N)

Distillation DynamicsMass and Energy Balance

Z. Nasri and H. Binous, "Applications of the Soave-Redlich-Kwong Equation of State Using Mathematica," Journal of

Chemical Engineering of Japan, 40(6), 2007 pp. 534–538.

Distillation Dynamics (Equations of State)

42

The Soave-Redlich-Kwong Equation of State

( ) ( )( )bVVa

bVTRP

+−

−=

( ) 0223 =−−−+− BABBAZZZ

Note that the Peng-Robinson EOS can also be usedZ. Nasri and H. Binous, Applications of the Peng-Robinson Equation of State using Matlab, Chemical Engineering Education, Spring issue 2009.

Distillation Dynamics

43

Equilibrium relations

Cto1ifor ==i

i

v

liK

φφ

( ) ( )

+

−−−−−=

i

i

iiiv

viiv

ivv Z

BZBB

AA

BABZ

BBZ ln2ln1exp 5.0

5.0

φ

Cor1iwith == iii xKy

SRK EOS

Light hydrocarbon mixtures DePriester charts (1953)

44

Thermodynamic data for mixtures: Simplified models

45

Raoult’s law (Ideal solution/ideal gas):s

iii Pxp = pi is the partial pressure of component i

Dalton’s law (Ideal gas):

Pyp ii =

K-value for ideal gas/ideal solution system:

PPK sii /=

Relative volatility for ideal gas/ideal solution system:sj

siji PPKK // =

Antoine equation:

i

ii

si CT

BAP+

−=ln

T, P

V

L

Thermodynamic calculations using K-values

46

Bubble point

Procedure:

a) Select T

b) Ki(T)

c) calculate:

d) if T is too high

e) Adjusting T

g) Final composition can be corrected using

∑i

ii xK

1>∑i

ii xK

∑=

iii

iii xK

xKy

Distillation Dynamics

47

Enthalpy departure from ideal behavior

SRK EOS

( ) ( )

−

+

+−=P

RTAP

RTAdTdT

ZBZLog

PRTB

ZRTH D221)1(

Liquid and vapor phase enthalpiesideal case

48

)()(),( 2,21,1 refLPref

LPL TTCxTTCxTxh −+−=

[ ] [ ])()(),( 2,221,11 refVPref

VPV TTCxTTCyTyH −++−+= λλ

49

d

hw

active areaAa

Liquid arrivingfrom plate above

perforated area

Downcomer

Distillation Dynamics

column diameter

heir height

50

d

Distillation Dynamics

Distillation Dynamics

51

Francis Weir Formula – Hydrodynamics consideration

52

Distillation Dynamics

Summation rules

5% positive step in the reflux ratio at t=10 min

53

0 5 10 15 203.05

3.10

3.15

3.20

3.25

3.30

3.35

3.40

time

reflux

ratio

R=3.07

R=3.30

Dynamic responses of distillate mole fractions(positive step in the reflux ratio at t=100 minutes)

(multi-component distillation 27 plates column ; ethane/propane/n-C4and n-C5 mixture)

54

Nasri, Z. and H. Binous, “Rigorous distillation dynamics simulations using a computer algebra,” Computer

Applications in Engineering Education, DOI: 10.1002/cae.20385, 2009.

55

Dynamic responses of distillate mole fractions(positive/negative steps in the reflux ratio at t=100 minutes)

(binary distillation of benzene-toluene mixture)

Binous, H., E. Al-Mutairi and N. Faqir, “Study of the separation of simple binary and ternary mixtures of aromatic

compounds,” Computer Applications in Engineering Education, DOI: 10.1002/cae.20533, 2011.

56

Dynamic responses of distillate mole fractions(positive/negative steps in the reboil ratio at t=100 minutes)

(binary distillation of benzene-toluene mixture)

Binous, H., E. Al-Mutairi and N. Faqir, “Study of the separation of simple binary and ternary mixtures of aromatic

compounds,” Computer Applications in Engineering Education, DOI: 10.1002/cae.20533, 2011.

Challenges for controlBinary distillation of benzene-toluene mixture

57

Dual product control of a binary distillation column, using

reflux and reboil ratios as manipulated variables, is difficult

because the two control loops interact. For example, if

distillate purity needs to be adjusted, the reflux ratio is

increased, which affects in a negative way the bottom

purity. Thus, the reboil ratio is manipulated by the bottom

control loop, which increases the overhead vapor flow rate

and affects the top control loop.

58

First-order transfer functions with dead time, obtained using Mathematica, are

given below:

Transfer functions(binary distillation of benzene-toluene mixture)

sesG

s

+=

−

237.0040.0)(

02.0

11 sesG

s

+=

−

170.0034.0)(

3.0

21

sesG

s

+−

=−

178.0022.0)(

3.0

12 sesG

s

+−

=−

243.0058.0)(

1.0

22

sesG

s

d +=

−

196.0069.0)(

3.0

1 sesG

s

d +=

−

179.0114.0)(

3.0

2

[ ])()()(

)()(

)()()()(

)()(

2

1

2221

1211 sxsGsG

sSsR

sGsGsGsG

sxsx

fd

d

b

d

+

=

where the input-output model is:

59

Wood and Berry devised a non-interacting control(here we use SIMULINK)

R. K. Wood and M. W. Berry, Terminal composition control of a binary distillation column, Chem Eng Sci 28 (1973), 1707–1717.

60

Wood and Berry devised a non-interacting control(both bottom and distillate purity in controlled)

0 10 20 30 40 50 600.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

0.055

time

botto

m c

ompo

sitio

n

0 10 20 30 40 50 600.95

0.955

0.96

0.965

0.97

0.975

0.98

time

dist

illat

e co

mpo

sitio

n

distillate

bottom

Binous, H., E. Al-Mutairi and N. Faqir, “Study of the separation of simple binary and ternary mixtures of aromatic

compounds,” Computer Applications in Engineering Education, DOI: 10.1002/cae.20533, 2011.