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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
67
Alexander Drive
P.
0.
Box
12277
Research Triangle Park
North Carolina
27709
United States
of
America
ISBN
0-7131-3551-4
Library
of
Congress Cataloging in Publication Data
Buckley, Page
S.
Design
of
distillation columncontrol 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 designof disullation columncontrol 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 columndesign 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 ofdistillation 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 designofdistillationcontrol systems.
It
is aimed at professional control engineers and any others concerned with
the
numerical definition and specification ofcontrol 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 ofcontrol system studies. Together, these permit quantitative
design of most process controlsystems 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 controlof 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 controlsystems 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 controlsystems
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 Controlof 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 Controlof 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
[...]... 249 255 256 273 279 279 288 289 Part 111 QUANTITATIVE DESIGNOFDISTILLATIONCONTROLSYSTEMS Approaches to Quantitative Design Chapter 12 12.1 12.2 12.3 12.4 12.5 Ways of Designing ControlSystems Kinds of Information Available Functional Layout ofControl Loops Adjustment of Controller Parameters (Controller Tuning) Enhanced ControlofDistillation Columns via On-Line Models 293 295 295 297 299 303 305... Control 16.1 Introduction 16.2 Level Controlof Simple Vessels 16.3 Level Controlof Overhead Condenser Receiver Via TopProduct Withdrawal 16.4 Level Controlof Overhead Condenser Receiver Via Reflux Manipulation 16.5 Column- Base Level Control Via Bottom-Product Manipulation 16.6 Column- Base Level Control Via Feed Flow Manipulation 16.7 Column- Base Level Control Cascaded to Steam FlowControl 16.8 Column- Base... 21.10 Conventional controlof X2 with set-point disturbance 516 2 1.1 Tracking sampled-data controlof X2 with set-point disturbance 1 517 21.12 Conventional control of X2 with feed composition disturbance 518 2 1.13 Tracking sampled-data control of X2 with feed composition disturbance 519 21.14 Comparison of conventional and tracking PL control 520 Strategy for Distillation- ColumnControl i n chemical... 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 distillationcolumn 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... level control via bottom product 162 Material balance control in direction of flow, reflux drum level control via reflux, base level control via bottom product 163 Material balance control in direction of flow, reflux drum level control via dstillate, base level control via boilup 164 6.9 Like Figure 6.8 but with reflux ratioed to dstdlate 165 7.1 Basic control scheme for column with sidestream drawoff... column with sidestream drawoff 171 7.2 7.3 7.4 Controls for liquid sidestream drawoff column 172 Alternate control scheme for column with sidestream drawoff 173 Scheme for control of sidestream composition 175 7.5 Controlof terminal composition 176 7.6 (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... noninteracting controlofcolumn product compositions 1 i DISTILLATIONCONTROL 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... Material-Balance Control C 0 L Q) 3 C 3 0 0 E d 8 s 2 m 8 2 CI z E 3 3 c e 3 - m d E 8 8 c m d - -?$ ' zE= 3 e,o 3 iz&% U Q 9 10 Strategy for Dljtillatwn -Column Control FIGURE 1.5 Distillationcolumn with material balance control in direction of f o lw 1.3 Funahmentnls of Composition Control 11 1.3 FUNDAMENTALS OF COMPOSITION CONTROL Let us consider briefly what must be done to a column to keep terminal... signal flow diagram for column pressure control via manipulation of inert gas and vent valves 412 17.8 Reduction of signal flow diagram of Figure 17.7 412 17.9 Signal flow diagram for column pressure control via manipulation of inert gas and vent valves when reboiler steam is flow or flow ratio controlled 413 17.10 Reduced form of Figure 17.9 414 17.11 Column pressure control via flooded condenser drainnegligible... product 140 5.4 Column feed temperature control with economizer and preheater 141 5.5 Column feed enthalpy control with economizer and preheater 142 5.6 Column with multiple feed trays 144 5.7 Feed system for a split column 146 5.8 Feed system for split vacuum columns 147 5.9 Feed systems for column in parallel 148 5.10 Makeuptpurge feed systems 150 6.1 Bottom product demand, overhead level control via . DISTILLATION CONTROL SYSTEMS Chapter 12 Approaches to Quantitative Design Ways of Designing Control Systems Functional Layout of Control Loops Adjustment of Controller Parameters (Controller. 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. 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