design of distillation column control systems

dwm
OF
DISTILLATION
COLUMN
CONTROL
SYSTEMS
PAGE
S.
BUCKLEY
WILLIAM
L.
LUYBEN
JOSEPH
P.
SHUNTA
PRINCIPAL CONSULTANT. ENGINEERING DEPARTMENT
E.I. DU PONT DE NEMOURS
&
CO.
PROFESSOR OF CHEMICAL ENGINEERING
&
CONSULTANT
LEHIGH UNIVERSITY
SENIOR CONSULTANT, ENGINEERING DEPARTMENT
E.I. DU PONT DE NEMOURS
&
CO.
Edward
Arnold
Design
of

Distillation Column
Control
Systems
8
Instrument Society of America
1985
All
rights reserved
Printed in the United States of America
In preparing this work, the author and publisher have not investigated
or
considered patents which may apply
to
the subject matter hereof.
It
is the
responsibility of the readers and
users
of the subject matter
to
protect
themselves against liability for infringement
of
patents. The information
contained herein is
of
a general educational nature. Accordingly, the author
and publisher assume no responsibility and
disclaim
all

liability
of
any kind,
however arising,
as
a result of
using
the subject matter of this work.
The equipment referenced in this work has been selected by the author
as
examples
of
the technology. No endorsement of any product is intended by
the author
or
publisher. In
all
instances, the manufacturer's procedures
should prevad regarding the
use
of specific equipment.
No
representation,
expressed
or
implied, is made with regard
to
the availability of
any
equipment, process, formula,

or
other procedures contained herein.
No
part
of
this publication may be reproduced,
stored in a retrieval system,
or
transmitted, in any form
or
by any means,
electronic, mechanical, photocopying, recording
or
otherwise,
without the prior written permission of the publisher:
Instrument Society of America
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0.
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United States
of
America
ISBN
0-7131-3551-4

Library
of
Congress Cataloging in Publication Data
Buckley, Page
S.
Design
of
distillation column control systems.
Includes indexes.
1.
Distillation apparatus. 2. Chemical process
control.
1.
Luyben, William L.
11.
Shunta, Joseph P.
111.
Title.
TP159D5B83
1985 660.2'8425 84-27813
ISBN
0-7131-3551-4
Book
design by Raymond Solomon
Production by Publishers Creative Services Inc., New York
Preface
t
his
is
a

book
about the design of disullation column control systems.
It
is written primarily fiom the standbint of an engineering design organization,
and is based on years of experience with large design projects
as
well as on
personal plant experience. Most new investment dollars
go
into new or modemized
facilities, and it
is
in the design phase of projects for these facilities that the
most opportunities occur and flexibility exists to influence process control.
Consequently this
book
is aimed primarily
at
design personnel.
It
is our hope,
however, that it will also
be
usell
to
those who have to operate or troubleshoot
existing
plants.
Part
I

is an introduction, including
a
perspective on control and a brief
review of fundamentals of &stillation, with emphasis on topics that will be of
interest
to
the control engneer rather than to the column design engineer. The
distillation review, it is hoped, will
be
particularly
usell
to
nonchemical enpeers.
Part
I1
of the
book,
on concepts and configurations, discusses some practical
aspects of distillation control. Once the requirements for
a
particular column
in
a
particular process are understood, design engineers must make
at
least a
preliminary choice of equipment arrangements and control system configuration.
In
this
section we have mostly avoided the

use
of mathematics and control
theory.
It
is
our
hope that
our
discussions
of
equipment and control system
arrangements will
be
usell
to
process engineers, production supervisors, main-
tenance engineers, and instrument engineers seeking guidelines, alternatives,
and perspectives.
Part
I11
focuses on the quantitative design of distillation control systems.
It
is aimed at professional control engineers and any others concerned with
the
numerical definition and specification of control system performance. Probably
the most important development in process control system design since about
1950
was the evolution of
a
substantial body

of
theory and mathematics, plus
a large catalog of control system studies. Together, these permit quantitative
design of most process control systems with
a
considerable degree of multivariable
control.
It
is
the purpose of this
book
to
indicate the range of
this
technology,
which has been developed for distillation control,
to
the point where it can be
economically and reliably used for design. The ultimate economic advantages
include lower plant investment (particularly in tankage), lower operating costs,
and
closer control of product quality.
For
the most part, we have stayed with
the modest theory of single-input, single-output (SISO) systems presented in
previous
books:
Techniques
of
Process Control

by P.
S.
Buckley (Wiley,
1964)
and
Process
Mohling,
Simulation,
and
Control
fm
Chemical Engineers
by W. L.
Luyben (McGraw-Hill,
1973).
This kind of theory and mathematics not
only
is adequate for noninteracting systems and for simple interacting systems, but
it has the advantages of requiring minimum formal training and of permitting
low design costs. “Modernyy
or
“optimal” control techniques are mentioned
only briefly here because their use on real, industrial-scale distillation columns
has been quite limited
to
date. These techniques are
still
being actively researched
by a number of workers, and
it

is hoped that they eventually will be developed
into practical design methods.
As
of the date of the writing of this
book,
however, these mathematically elegant methods
are
little
used in industry because
of their complexity, high engineering cost, and limitation
to
relatively low-
order systems. Simulation techniques also are not covered since there are several
texts
that
treat this topic extensively.
In the past five years, we have witnessed the introduction and proliferation
of microprocessor-based digital controls of various
sorts
that are intended
to
replace analog controls. In fact, most of the newly installed control systems are
of this
type.
In addition, we are seeing more control being implemented in
process control computers. Sampled-data control theory has taken on new
importance because of these developments and
so
we have included a chapter
on previous work we have done in this area

as
it
relates specifically to distillation
columns. The concepts we present are quite basic as opposed
to
the recent
advances in adaptative, multivariable, and predictive control, but we hope they
will benefit those interested in synthesizing single-loop sampled-data controllers.
Many thanks are due our associates in the Du Pont Company, particularly
R.
K.
Cox, and throughout the industrial and academic communities for helpful
comments and suggestions. Many of the concepts presented in this book have
been vigorously debated (over untold cans
of
beer) during the Distillation
Control Short Courses held at Lehgh University every other spring since
1968.
We also wish
to
thank Leigh Kelleher for major assistance in formatting
and editing, Arlene Little and Elaine Camper for typing, and Ned Beard and
his
Art
Group for preparing the illustrations.
Pade
S.
Buckley
William
L. Luyben

Joseph P. Shunta
Nomenclature
i
n
this
work an effort has been made:
(1)
to
use
symbols
and
units
commonly
employed by chemical engineers,
(2)
to
define each symbol
in
a chapter when
the need for that
symbol
arises, and
(3)
to
keep
symbols
and
units
as
consistent

as
possible from chapter
to
chapter.
A
few
symbols,
however, have different
meanings
in
different parts
of
the text. The list that follows contains the major
symbols
and their usual meanings:
a
A
B
C
Cll
D
E
F
Bc
BL
h
H
i
K
1

Lo
LR
M
Mw
P
P
C
f
transportation lag or dead
time,
usually seconds or minutes
area,
ft2
bottom-product flow, mols/min
specific heat, pcu/lbm
"C
acoustic capacitance, fi5/lbf
control-valve flow coefficient, gallons per minute of water flow
when valve pressure drop
is
1
psi
diameter, feet, or top-product flow rate fi-om condenser
or
condensate receiver, mols/min
Murphree tray efficiency
cycles/minute or cps
feed rate
to
column, mols/min

ft
Ibmass
mass-force conversion factor,
32.2
-
-
sec' Ib force
local
acceleration due
to
gravity, ft/sec2
heat-transfer
film
c&cient,
head of liquid or liquid level, feet
fl
(has different meaning when used
as
subscript)
static gain
distance, feet
external reflux, mols/min
liquid downflow
in
column, mols/min
liquid holdup, mols
molecular weight
pressure, psi
pressure, lbf/fi?, or atmosphere,
or

mm
Hg
pcu/sec
"C
fi?
523
524
Pcu
5
Q
4
R
5
t
T
U
V
VI-
W
Y
ZF
Z
Y
29
X
z
a
Ml-
5
6,

60
h
Am
CL
P
E
7
+
0
Nomenclature
pound
centigrade
units
(heat required
to
heat one
pound
of water
1°C)
vapor pressure of pure component, speciesj
heat flow,
pcu/sec,
or
fraction of feed that is liquid (molar basis)
flow rate, ft3/sec or
ft3/min
reflux ratio,
LJD
Laplace transform variable
time,

seconds or minutes
temperature, degrees Celsius or Kelvin, or
sampling
time
interval in sampled-data control systems
pcu/sec
f?
"C
overall heat-transfer coefficient,
~
vapor flow, mols/min, or
volume,
ft3
volume in tank corresponding
to
level transmitter span,
AHT
weight rate of flow, usually Ibm/sec
weight, lbm
mol fraction more volatile component in
a
liquid
mol fiaction more volatile component in a vapor
z-transform variable, or
mol fraction more volatile component in feed
acoustic or hydraulic impedance, Ibf sec/fi5
relative volatility
specific heat ratio, or
activity coefficient
liquid-level transmitter span, feet, corresponding

to
full-scale output
difkrence between set-point signal and signal from measurement device
damping ratio in a quadratic expression
arbitrary input signal
arbitrary output signal
latent heat of vaporization, pcu/lbm
molar latent heat of vaporization, pcu/mol
viscosity, lbmlft sec
=
centipoise/
1488
density, lbm/ft3
time
constant, usually seconds or minutes
enthalpy, pcu/lbm
fi-equency, radians/unit
time
Subscripts
Q
quadratic
B
bottom of tower
R
reset, or
reflux
525
L
H
f

f
i
i
SP
0
S
st
C
D
OL
light component or key
heavy component or
key
feed
feedforward
inlet
arbitrary tray location
or
component
outlet
stripping section
set point
steam
controller
distillate (top product)
open loop (used outside of brackets)
Symbols
on
Illustrations
CC

or
xc
FC
LC
PC
TC
HS
LS
HL
LL
cw
composition control
flow control
liquid level control
pressure control
temperature control
high signal selector
low signal selector
high signal limiter
low signal limiter
cooling water

Individual barred terms
(e.g.,
V,
P)
indicate average values.
Combined barred terms
[e.g.,
HG(z)]

have special meaning in sampled-data
control systems (see Chapter
21).
K,G,(s)
measurement transfer function
K,G,(s)
controller transfer function
K,,G,(s)
control valve transfer function
KpGp(s)
process transfer function
Contents
Preface
Part
I
INTRODUCTION
Chapter
1
Strategy
for
Distillation-Column
Control
1.1
Distillation Control Objectives
1.2
Arrangements for Maw-id-Balance Control
1.3
Fundamentals of Composition Control
1.4
Compensation for Various Disturbances

1.5
Startup and Shutdown
1.6
Control System Design Philosophy
1.7
Procedure for Overall Control System Design
1.8
Column
Design Philosophy and Control System Design
1.9
Existing Columns-Typical Practices and Troubleshooting
1.10
Conventions Followed in This Book
1.11
Literature
Chapter
2
Fundamentals
of
Distillation
2.1
Introduction
2.2
Tray Hydraulics
2.3
Vapor-Liquid Equilibrium Fundamentals
2.4
Graphical Solution Techniques
2.5
Effects of Variables

Part
II
CONCEPTS AND CONFIGURATIONS
Chapter
3
Overhead System Arrangements
3.1
Introduction
3.2
Types of Condensers
3.3
Atmospheric Columns
3.4
3.5
Pressure Columns-Vapor Product
3.6
Miscellaneous Pressure-Control Techniques
3.7
3.8
Vacuum and Pressure Columns-Liquid Product
Gravity-Return Reflux Versus Pumped-Back Reflux
Control Techniques with Air-Cooled Condensers
pwe
mtt
1
3
3
6
11
12

13
14
19
19
20
21
22
25
25
28
30
49
65
67
69
69
70
72
80
84
86
90
99
V
vi
contents
3.9
‘Tempered” Versus Once-Through Coolant
3.10
Level Control of Condensate Receiver and Required

Holdup
Chapter
4
Column-Base and Reboiler Arrangements
4.1
Introduction
4.2
Vertical Thermosyphon Reboilers
4.3
Flooded Thermosyphon (Steam-Side) Reboilers
4.4
Forced-Circulation Reboilers
4.5
Flooded-Bundle
Kettle
Reboilers
4.6
Internal Reboilers
4.7
Steam Supply and Condensate Removal
4.8
Required Holdup
for
Level Control
4.9
Miscellaneous Column-Base Designs
4.10
Miscellaneous Reboiler Designs
Chapter
5

Feed System Arrangements
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
General Comments
Feed Flow Control
Feed Temperature Control
Feed Enthalpy Control
Feed Tray Location
Feed Tank Sizing
Feed System for Double-Column Systems
Feeds with Makeup/Purge to Tankage
Feed Systems in Sequences of
Columns
With and Without
Recycles
Chapter
6
Level
Control and Feedforward
Options
6.1
Introduction
6.2

6.3
6.4
Unfavorable Control Schemes
6.5
Unreasonable Control Schemes
Material-Balance Control in Direction Opposite
to
Flow
Material-Balance
Control
in Direction
of
Flow
Chapter
7
Control
of
Sidestream Drawoff Columns
Side-Draw
Columns
with
Large
Sidestreams
7.1
Introduction
7.2
100
100
109
109

110
114
116
117
119
122
126
130
133
137
137
137
140
141
143
144
145
149
151
153
153
154
157
166
166
169
169
169
contents
7.3

7.4
7.5
7.6
7.7
Chapter
8
8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.8
Chapter
9
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
9.9
9.10
9.11
9.12
9.13
Side-Draw Columns with Small Sidestreams

Composition Control of Side-Draw Columns
An Improved Approach
to
Composition Control
of
Side-
Prefiactionator Plus Sidestream Drawoff
Column
Other Schemes
Draw Columns
Minimizing Energy Requirements
Introduction
Cons e
rv
a
ti
o
n
Design Considerations
in
Heat-Recovery Schemes
Multiple Loads Supplied by
a
Single Source
Single
Source, Single Load
Split Feed Columns
Combined Sensible and Latent Heat Recovery
Energy Recovery by Vapor Recompression
Application

of
Protective
Controls
to
Distillation Columns
Introduction
Overrides and Interlocks
Implementation
of
Overrides
Controllers
in
Override Circuits
Anti Reset-Windup
Feedforward Compensation with Overrides
Overrides
for
Column Overhead System
Overrides for Column-Base System
Automatic Stamp and Shutdown
“Idle”
or
Total Reflux
Miscellaneous Overrides
Design Considerations
Overrides for Side-Draw Columns
Chapter
IO
Indirect Composition Measurements
10.1

Introduction
10.2
Single-Tray Temperature
10.3
Differential Temperature
10.4
Differential Vapor Pressure
vii
170
170
174
176
180
181
181
181
182
183
186
189
189
189
193
193
194
195
199
200
202
205

208
21
1
21 3
214
21 7
220
229
229
229
230
231
viii
10.5
Pressure-Compensated Temperature
10.6
Multicomponent Compositions Computed from
10.7
Double-Differential Temperature
10.8
Average Temperature
10.9
Composition Estimators
Temperature and Pressure Measurements
Chapter
11
Miscellaneous Measurements and Controls
1
1.1
Introduction

11.2
11.3
Meters
11.4
Heat-Flow Computations
11.5
Column-Base Level Measurements
11.6
Control Valves
11.7
Column
AP
Measurement
11.8
Temperature-Measurement Dynamics
11.9
Flow and Flow-Ratio Conventions
1 1.10
Control-Valve Split Ranging
Calculation of Distillation-Column Internal Reflux
Temperature and Pressure Compensation of
Gas
Flow
Part
111
QUANTITATIVE DESIGN
OF
DISTILLATION
CONTROL SYSTEMS
Chapter

12
Approaches
to
Quantitative Design
Ways of Designing Control Systems
Functional Layout of Control Loops
Adjustment
of
Controller Parameters (Controller Tuning)
Enhanced Control of Distillation
Columns
via On-Line
12.1
12.2
Kinds of Information Available
12.3
12.4
12.5
Models
Chapter
13
Tray Dynamics-Material Balance
13.1
Introduction
13.2
Tray Hydraulics
13.3
Derivation of Overall Tray Equation
13.4
Mathematical Model for Combined Trays

Chapter
14
Distillation-Column Material-Balance Control
14.1
Mathematical Model-Open Loop
14.2
Control in
the
Direction of
Flow
GmtenB
234
239
240
241
241
243
243
243
249
255
256
273
279
279
288
289
293
295
295

297
299
303
305
313
313
314
320
323
327
327
333
contents
ix
14.3
Control in the Direction Opposite
to
Flow
14.4
Material-Balance Control in Sidestream Drawoff Columns
14.5
Top
and Bottom Level Control Combinations
Chapter
15
Condenser and
Reboiler
Dynamics
15.1
15.2

Flooded Condensers-Open-Loop Dynamics
15.3
Reboilers-Open-Loop Dynamics
15.4
Partially Flooded Reboilers
15.5
Liquid-Cooled Condensers with
No
Condensate Holdup
Partially Flooded Reboilers for Low-Boiling Materials
Chapter
16
Liquid
Level
Control
16.1
Introduction
16.2
Level Control
of
Simple Vessels
16.3
Level Control
of
Overhead Condenser Receiver Via Top-
16.4
Level Control
of
Overhead Condenser Receiver Via Reflux
16.5

Column-Base Level Control Via Bottom-Product
16.6
Column-Base Level Control Via Feed Flow Manipulation
16.7
Column-Base Level Control Cascaded to
Steam
Flow-
16.8
Column-Base Level Control Via Condensate Throttling
Product Withdrawal
Manipulation
Manipulation
Control
from a Flooded Reboiler (Cascade Level-Flow
Control)
Chapter
17
Pressure and
A
P
Control
17.1
Introduction
17.2
Heat-Storage Effect on
Column
Pressure
17.3
Pressure Control Via Vent and Inert
Gas

Valves
17.4
Pressure Control Via Flooded Condenser
17.5
Pressure Control Via Condenser
Cooling
Water
17.6
Column
AI'
Control Via Heat
to
Reboiler
Chapter
1
8
Composition Dynamics-Binary Distillation
18.1
Introduction
18.2
Basic Tray Dynamics
18.3
Feed Tray Dynamics
337
342
343
347
347
349
357

366
371
375
375
375
386
386
389
389
390
399
405
405
405
408
415
420
420
427
427
427
432
X
contents
18.4
Top-Tray and Overhead System Composition Dynamics
18.5
Reboiler and Column-Base Composition Dynamics
18.6
Inverse Response

18.7
Overall Composition Dynamics
Chapter
19
Calculation
of
Steady-State Gains
19.1
Introduction
19.2
Design Procedure
19.3
Exact
R
Procedure
19.4
Column Operation Procedure
19.5
Examples
Chapter
20
Composition Control-Binary Distillation
20.1
Introduction
20.2
Feedback Control
of
Composition
20.3
Interaction Compensation

20.4
Feedforward Compensation
20.5
Relative-Gain Matrix
20.6
Composition Measurement Location
Chapter
21
Sampled-Data Control
of
Disti1lat.m Columns
2
1.1
Introduction
21.2
Control Algorithms
21.3
Servo
and Regulator Control
21.4
Feedforward Control
2
1.5
Interaction Compensation
21.6
Sampled-Data Control for Loops with Overrides
433
439
439
441

445
445
446
448
449
462
465
465
466
468
475
478
489
493
493
494
496
502
508
51
0
Nomenclature
Subject Index
Author Index
523
527
530
Figures
1.1
1.2

1.3
1.4
1.5
Material balance control in direction opposite
to
flow
Material balance control in dlrection of flow
Overall material balance control in direction opposite
to
flow
Overall material balance control with intermediate material
Distillation column with material balance control
in
direction of
balance control in direction of flow
flow
2.1
2.2
2.3
2.4
2.5
2.6
Nomenclature and conventions for typical dlstillation column
Control variables for distillation column
Schematic of typical sieve tray
Vapor pressure and temperature measurement
Temperature vs. pressure for pure component
Typical method of plotting vapor pressure vs. temperature
2.7
2.8

2.9
2.10
2.11
2.12
2.13
2.14
2.15
2.16
2.17
2.18
2.19
Temperature vs. composition
of
binary mixture at constant
Pressure vs. composition of binary mixture at constant
x vs y for binary mixture
Bubble point
and
dew point at constant temperature
Bubble
point
and
dew point at constant pressure
Isothermal flash
Graphical representation
of
equation
(2.13)
Relative volatility on x-y diagram
Typical activity coefficients as functions of light component

Typical homogeneous “maximum boiling” azeotrope
Homogeneous “minimum
boiling”
azeotrope
Heterogeneous azeotropes
Simple distillation column
pressure
temperature
composition
7
8
9
10
26
27
29
31
32
33
34
35
36
37
38
41
42
44
46
47
48

49
50
xi
xii
2.20
2.21
2.22
2.23
2.24
2.25
2.26
2.27
2.28
2.29
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
3.12
3.13
3.14
3.15
3.16A

3.16B
3.17
3.18
Material balance on stripping section
Operating line of stripping section
Material balance on rectifjring section
x-y diagram showing both stripping and rerufying operating
McCabe-Thiele dagram-stepping between
VLE
curve and
q-line
on
x-y dagram
McCabe-Thele diagram for rating problem
Column operation
at
minimum reflux ratio
Costs vs. reflux ratio
Minimum number of trays required at total
rdw
lines
operating lines
to
estimate number of trays required
Horizontal condenser, vapor in shell
Vertical condenser, vapor in
tubes
Alternative overhead system for pressure column
Air-cooled condenser
Spray condenser

Preferred overhead system
for
atmospheric column
Alternative overhead system
for
atmospheric column
Thermowell installation under vertical condenser
Tempered coolant system
Overhead system
for
vacuum
or
pressure column-large amount
Overhead system for vacuum column-small amount of inerts
Alternative overhead system for pressure
or
vacuum column-
Alternative pressure control system
Overhead system
for
pressure column-vapor product
Alternative overhead system
for
pressure column-vapor
Column pressure control by hot
gas
bypass
Column pressure control by hot
gas
bypass

Column pressure control with flooded condenser
Gravity flow reflux (flow controlled) and distillate (level
of inerts
small
amount of inerts
product
controlled)
F&ures
52
53
54
55
56
59
61
62
63
64
71
72
73
74
75
76
77
78
79
81
82
83

84
85
86
88
89
90
92
F4qures
3.19
3.20
3.21
3.22
3.23
3.24
3.25
3.26
3.27
4.1A
4.1B
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
4.10
4.11
4.12

4.13
4.14
4.15
4.16
4.17
Liquid-vapor disengagement space built into condenser
Gravity flow reflux system with ground-located
surge
tank for
Control of gravity reflux by throttling top product flow
Control of gravity reflux flow rate by overflowing through
Gravity-flow reflux, surge
tank
with Sutro weir,
T~
>
3-5
Undesirable piping arrangements for returning reflux to
column
Preferred piping arrangement for returning reflux
to
column
Condensate receiver level control via distillate
Proportional-only condenser seal pot level control via reflux
distillate
Sutro weir and by throttling distillate flow
minutes
flow
Vertical thermosyphon-heat flux vs. supply side liquid level
Distillation column base with thermosyphon reboiler

Relationship between vapor volume in tubes of thermosyphon
Flooded reboiler
Flooded reboiler for boiling point materials
Column base with forced-circulation reboiler
Kettle-type reboiler with internal weir
Protective circuits for tube bundle chamber in kettle-type
Column base with internal reboiler-no baffles
or
weirs
Column base with isolated internal reboiler
Steam header configuration
Improved steam supply and
flow
control system
Level control of column base via bottom product throttling,
Proportional-only level control system for column base
Column base level control by steam flow manipulation, cascade
Column
base design and arrangement for minimum holdup
Reboiler piping arrangement for preferential
boiling
of
reflux
Arrangement for column base overflow into intermediate surge
reboiler, heat load, and base liquid level
reboiler
cascade arrangement
arrangement
from lowest downcomer
vessel

xiii
93
94
95
96
98
99
99
101
103
111
111
113
115
116
117
118
119
121
123
124
125
127
128
131
132
133
134
XiV
Fgures

5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
5.10
Makeuptpurge feed systems
Feed system for distillation column
Column
feed systems with positive displacement pumps
Column
feed preheat via exchange with bottom product
Column feed temperature control with economizer and
Column feed enthalpy control with economizer and preheater
Column with multiple feed trays
Feed system for a split column
Feed system for split vacuum columns
Feed systems for column in parallel
preheater
6.1
6.2
6.3
6.4
6.5
6.6
6.7

6.8
6.9
Bottom product demand, overhead level control via
top
product, base level via feed
Bottom product demand, overhead level control via reflux, base
level control via feed
Bottom product demand, overhead level control via
boil
up,
base level control via feed
Distillate demand, reflux
drum
level control via reflux, base
level control via feed
Distillate demand, reflux
drum
level control via base level
control via feed
Material balance control
in
direction of flow, reflux drum level
control via distillate, base level control via bottom product
Material balance control
in
direction of flow, reflux
drum
level
control via reflux, base level control via bottom product
Material balance control

in
direction of flow, reflux
drum
level
control via dstillate, base level control via boilup
Like
Figure
6.8
but with reflux ratioed
to
dstdlate
7.1
7.2
7.3
7.4
7.5
Control of terminal composition
7.6
Basic control scheme for column with sidestream drawoff
Controls for liquid sidestream drawoff column
Alternate control scheme for column with sidestream drawoff
Scheme for control of sidestream composition
(A)
In the control system
finally
chosen, the toluene impurity
content
in
the dlstillate producer is controlled by the reflux
ratio.

(B)
The five alternative sidestream tray positions and
138
139
140
141
142
144
146
147
148
150
155
156
158
159
160
162
163
164
165
171
172
173
175
176
Fipres
xv
7.7
7.8

8.1
8.2
8.3
8.4
8.5
8.6
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
9.9
9.10
9.11
9.12
9.13
9.14
9.15
9.16
9.17
9.18
9.19
9.20
9.21
their controls, which regulate the benzene
and
xylene

impurities in the sidestream drawoff, are shown
in
this
blowup.
D-scheme
L-scheme
Heat recovery scheme-single source, multiple
loads
Scheme for establishing heat load priorities
Heat recovery-single source, single
load
Heat recovery-single source, single load-scheme
2
Heat recovery-split feed, single load
Heat recovery via vapor recompression
Median selector
(J.P.
Shunta design)
High hter
Low limiter
High limiter schematic
Low limiter schematic
Column
base temperature control with
AP
override
Anti
reset-windup for cascade loops
Impulse feedfonvard with
PI

controller and overrides
Overrides for column overhead system
High cooling water exit temperature override on condensate
Overrides for column base system
Scheme for protecting centrifugal pump against dead heading
Effect of entrainment
on
overhead composition
Entrainment override
Limited utility override on feed
Steam header pressure protective override
Control scheme for balancing condenser and reboiler heat loads
Hard
and
soft
constraints
Flow rate controls for composition control
Feed flow system
Low temperature overrides for drawoff valves
temperature control
177
178
179
184
185
187
188
190
191
196

197
197
198
198
203
204
205
206
208
209
214
216
216
218
219
219
220
221
222
223
9.22
Steam valve overrides
9.23
9.24
Override for
minimum
vapor flow up column
Override for
minimm
liquid flow down column

10.1
DVP
cell schematic
10.2
10.3
10.3
Pressure compensation of temperature measurement
Composition vs.
t
and
p
Composition vs.
t
and
p
11.1
Measurements needed for internal reflux computation
11.2
11.3
11.4
11.5
11.6
11.7
11.8
11.9
11.10
11.11
11.12
11.13
11.14

11.15
11.16
11.17
11.18
11.19
11.20
Pneumatic hardware configuration for internal reflux
Typical compensated
gas
flow metering scheme
Heat flow computer for heat transfer
Head-level relationship in a vessel
Column base-reboiler manometer
Schematic diagram
of
Isplacement-type level transmitter
Internal damping chamber
External damping for
AP
level
measurement
Velocity
error
in
head measurement
Spechc gravity compensation of head measurement
of
liquid
Insufficient purge: transmitter lag in
response

to
rapid rise
in
InsufFcient purge: level transmitter erroneous response to rapid
Typical
gas
flow purge system
Improved
gas
flow purge system
Best
gas
flow purge system
Angled
nozzle
with dip
tube
Level measurement with
AI'
transmitter with double remote
Level measurement with
two
flush diaphragm transmitters and
Level measurement with flush diaphragm AP transmitter and
computation
level
level
rise
in
pressure

seals
a
summing
relay
1.1
repeater
224
225
226
232
232
235
237
245
245
251
255
257
259
260
261
262
244
265
267
268
269
270
270
271

272
273
274
F&%WJ
11.21
11.22
11.23
11.24
11.25
11.26
11.27
11.28
12.1
12.2
12.3
13.1
13.2
13.3
14.1
14.2
14.3
14.4
14.5
14.6
14.7
14.8
14.9
15.1
15.2
15.3

15.4
xvii
Effect of velacity on step response
Effect on step response of various annular
fills
Effect on step response of annular clearance
Effect of fluid velocity on step response
Effect of
annular
fill on step response;
v
=
lO/sec
Effect of annular clearance on step response
Split ranging of large and small valve
Split ranging reflux and distillate valves
Single loop with feedforward compensation, derivative,
PI
controller, overrides, and predictor
Single-loop with feedforward compensation
Primary controller with optional enhanced control fatures-
cascade control system
Distillation tray schematic for flows and liquid elevations
Preliminary signal flow diagram for tray material balance
Material balance coupling with vapor and liquid flow
dynamics
Distillation column material balance
Signal flow diagram for column material balance
Signal flow diagram-condensate receiver
Signal

flow
diagram column base
Signal flow diagram-material balance control in direction of
Signal flow diagram-material balance control
in
direction
Rearranged version of Figure
14.6
Material balance signal flow-vapor sidestream drawoff
Material balance signal flow diagram-liquid sidestream drawoff
flow
opposite
to
flow
Horizontal condenser with coolant in
tubes
and partially
First signal
flow
diagram for
P,
of
flooded condenser
First reduction
of
signal flow diagram of Figure
15.2
Final signal flow diagram for P, of flded condenser
flded on shell side
281

282
284
285
286
287
290
291
300
302
304
315
322
324
328
334
335
337
338
340
341
344
345
350
353
354
355
xviii
15.5
15.6
15.7

15.8
15.9
15.10
15.11
15.12
15.13
15.14
15.15
15.16
16.1
16.2
16.3
16.4
16.5
16.6
16.7
16.8
16.9
16.10
16.11
17.1
17.2
First signal flow diagram for w, of flooded condenser
Reduced signal flow diagram for w, of flooded condenser
Schematic representation of column base and reboiler holdup
Preliminary signal flow diagram for heat transfer dynamics
Partial reduction of Figure
15.8
Signal flow diagram for base level control cascaded
to

steam
Final signal flow diagram
for
base level control cascaded
to
Signal flow diagram for base level control by direct
Preliminary signal flow diagram for flooded reboiler
Reduced signal flow diagram for flooded reboiler
Signal flow dagram for flooded reboiler for low bohng point
Reduced signal flow diagram for flooded reboiler for low
flow control
steam flow control
manipulation of steam valve
materials
boiling
point materials
Level control of simple vessel
Signal flow diagram
for
simple level control system
PI
level control cascaded
to
flow control
Signal flow dagram for proportional-only condensate seal
pot
Signal flow diagram for proportional-only column base level
Responses of
PI
averaging level control system with dead time

Base level control via steam flow control with inverse response
Partial reduction
of
Figure
16.7
Inverse response predictor for base level control via steam flow
Preliminary signal flow diagram for column base level control
Reduced version of Figure
16.10
level control via reflux flow manipulation
control via feed flow manipulation
to
step change
in
outflow
and reboiler swell
via condensate throttling from
a
flooded
reboiler
Simplified treatment of heat storage effect on column pressure
Preliminary signal flow diagram for column heat storage
dynamics
dynamics
FiguVes
358
359
360
362
363

364
366
367
368
369
372
372
376
378
384
388
391
392
396
397
398
402
403
407
408
Fkures
17.3
17.4
17.5
17.6
17.7
17.8
17.9
17.10
17.11

17.12
17.13
17.14
17.15
17.16
17.17
17.18
18.1
18.2
18.3
18.4
18.5
18.6
18.7
19.1
19.2
Reduced signal flow diagram of preliminary lagram
Partial signal flow diagram for reboiler dynamics
Reduced form of signal diagram of Figure
17.4
Combined signal flow diagram for Figures
17.3
and
17.5
Partial signal flow diagram for column pressure control via
manipulation of inert gas and vent valves
Reduction
of
signal flow diagram of Figure
17.7

Signal flow diagram for column pressure control via
manipulation of inert gas and vent valves when reboiler
steam is flow or flow ratio controlled
Reduced
form
of Figure
17.9
Column
pressure control via flooded condenser drain-
negligible inerts and reboiler steam flow or flow-ratio
controlled
Partially reduced version of Figure
17.1
I
Column pressure control via flooded condenser, reboiler steam
Partially reduced version
of
Figure
17.13
Signal flow diagram for column pressure control via
Equivalent network for
vapor
flow and pressures in column
Column
AP
(base pressure) control cascaded
to
steam flow
Column
P

(base pressure) control via direct manipulation of
not flow or flow-ratio controlled, significant inerts
manipulation of condenser cooling water
control
steam valve
Flows
to
and from basic tray
Signal flow diagram for basic tray
Signal flow diagram for feed tray
Signal flow diagram for top tray and overhead system
Partly reduced signal flow diagram for top tray and overhead
Signal flow diagram for reboiler composition dynamics
Signal flow lagram for ,Rippin-Lamb model for binary
system
distillation column dynamics
Effect on calculation
of
rectifying section when:
A.
Guess for
R
is
too
small or
B.
Guess for
xD
is
too

close
to
1.00000
Reflux via reflux drum level control: bottom product via base
level control
xix
409
410
410
410
412
412
413
414
416
417
418
419
421
423
424
425
428
431
434
436
437
440
443
450

452
xx
19.3
19.4
Distillate via reflux
drum
level control: bottom product via base
Distillate via
reflux
drum
level control:
boil
up via base level
level control
control
20.1
20.2
20.3
20.4
20.5
20.6
20.7
20.8
20.9
20.10
20.11
Distillate via reflux
drum
level control; bottom product via base
Partial signal flow diagram for Figure 20.1

Partial signal flow diagram for system with reflux manipulated
Signal
flow
diagram for system of Figure 20.2 with decouplers
Partly reduced signal flow diagram of Figure 20.4
Partly reduced signaVflow diagram of Figure 20.5
Composition control of distillation column with feedforward
yTvs.
R
qvs.
R
x,
vs.
v,
level control
by reflux
drum
level
compensation and decouplers
YT
vs.
vs
21.1 Sampled-data control
2 1.2
21.3 ccDual” sampled-data control
21.4a “Dual” and set-point control
21.4b
21.4~
2 1.5
21.6 Interaction compensation

21.7a Set-point change without compensators
2 1.7b
21.7~ Disturbance in feed cornposition
21.7d Disturbance in feed composition
21.8
21.9
21.10
2
1.1
1
Discrete PID sampled-data control
“Dual”
and PID control for feed composition disturbance
“Dual” and PID control for feed rate disturbance
Sampled-data feedforward/feedback control loop
Set-point change with compensators
Convential “dual” control in loop with overrides
Tracking “dual” control
in
loop with overrides
Conventional control
of
X2
with set-point disturbance
Tracking sampled-data control
of
X2
with set-point disturbance
454
456

466
467
469
470
472
473
476
485
486
487
488
496
498
499
503
504
505
507
509
511
511
511
511
512
513
516
517
21.12 Conventional control of
X2
with feed composition disturbance

518
2 1.13
disturbance
51
9
21.14 Comparison of conventional and tracking
PL
control
520
Tracking sampled-data control of
X2
with feed composition
Strategy
for
Distillation-Column
Control
i
n chemical plants and petroleum refineries, there
are,
today,
many distillation
columns that
are
working well. There are also many others
that
are not working
well,
and
at least a few that function very poorly, or not
at

all.
Failure
to
obtain
performance specified by the column design engineer is due, in many cases,
to
faulty or inadequate control system design. Troubleshooting of columns that
are already in operation is frequently necessary, but practical considerations
usually
limit
corrective measures
to
relatively minor items. Proper original
design is by far the best way
to
guarantee satisfactory operation and control.
Therefore, in this book we will approach the design of integrated distillation-
column control systems as
a
systems problem in process design. The application
of feedforward, feedback, and protective controls
wdl
be coordinated with the
sizing and proper location of process holdups
to
achieve both automatic start-
up and shutdown
and
smooth, noninteracting control of column product
compositions.

1
.i
DISTILLATION CONTROL OBJECTIVES
The starring point of any design project is
a
definition of objectives. For
distillation there
are
many possible approaches, but the one chosen here is one
the authors have found broadly
useful
in virtually
all
kinds
of processes.’
It
has three main facets:
(1)
material-balance control,
(2)
product quality control,
and
(3)
satisfaction of constraints.
As
applied specifically
to
distillation columns,
this philosophy suggests the following:
1.

Material-balance control”
*
This term is sometimes used by others”
to
mean
a
control system in which reflux
is
set
by
reflux drum
level control, and distillatelfeed
ratio
is
set
manually
or
by
a
composition (temperature)
controller. The authors
of
this
book
have been unable
to
find any special merit
for
this scheme
except

for
some high reflux ratio
columns.
3