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.