fibonacci search for optimal feed location
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FIBONACCI SEARCH FOR OPTIMAL FEED LOCATIONJ.C. WANG
a
a Engineering Department , Union Carbide Chemicals and Plastics , P.O. Box 8361, South
Charleston, WV, 25303
Published online: 24 Apr 2007.
To cite this article: J.C. WANG (1980) FIBONACCI SEARCH FOR OPTIMAL FEED LOCATION, Chemical EngineeringCommunications, 4:6, 651-663, DOI: 10.1080/00986448008935937
To link to this article: http://dx.doi.org/10.1080/00986448008935937
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Chern.Eng. Commun. Vol.4, pp. 651-663
0098-6445/80/0406-0651S04.50/0
< lGordon and Breach, Science Publishers Inc., 1980
Printed in the U.S.A.
FIBONACCI SEARCH FOR OPTIMAL
FEED LOCATION
J e WANG*
Union Carbide Chemicals and Plastics,
Engineering Department,
P.o. ox 8361, South Charleston, WV 25303
Received
uly
23,1979; infinal
form
October 11,1979)
A Fibonacci search technique is used in conjunction with a rigorous multicomponent distillation computer
module to find the opt ima l feed location within a sect ion
o
a disti llat ion column. The funct ion to be
minimized can be one of the following:
keycomponentratiodifference
reflux
ratio or reboiler ratio
condenser
dutyor reboilerduty
his technique has been used successfully in the relocation
o
feed stages
o
many existing distillation
columns and thus saved energy consumption. We shall describe this technique with a sample problem.
INTRODUCTION
Feed location is an important factor in the design of a new disti llation column or in
evaluat ing the performance of an existing column. A mislocated feed not only can
seriously impair separation efficiency but also affect the energy consumption. The
subject of optimal feed location has been studied by many investigators.
McCabe and
Thiele, ' Gil li land,' and others define the optimum feed stage based on matching the
ratio of the light and heavy key components in the feed and on the feed stage. Scheibel
and Montross, Floyd and Hipkin;'
and
Maas,' have developed procedures to find
the
opt imum feed stage that produces the maximum separations. Shipman ' gave a
criterion of the optimal feed stage based on the minimization of the entropy
production. Although his approach is thermodynamica lly sound, the method is
difficult to apply and has not much practical value. Recently, Waller and Gustafsson
t reated the feed stage problem in their opt imizat ion of the steady state distiIJation
operation.
Distillation processes are the most energy intensive operation in the petroleum and
petrochemical industries. Prengle, et at, reported that up to 40 of the energy
consumed in a petrochemical plant is taken by the distillation columns. is therefore
more logical to define the optimal feed location based on the energy consumption.
Luyben has given such a cri ter ion by stating The minimum energy consumption is
achieved in a column with a fixed number of total t rays when not only
X
D
and
X
B
are
held at their specified values but also the feed is introduced onto the optimum feed
Dr.
Wang isnowwith Simulation Science, Inc., 1400 N. Harbor Blvd., Fullerton, CA 92635.
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652
J.C.
WANG
tray. In this
paper
we shall show
that the
energy consumption of a distillation column
with a fixed
number of
stages for a specified
separation
is an unimodal function or the
feed
stage
location. This function can be
evaluated
by using a
steady
state
computer
simulation program.
The
problem of finding
the
optimal feed location is then to search
for
the min imum
point of this function. A Fibonacci search procedure is then
developed for solving this problem.
The
procedure is simple
and
can be readily
incorporated into any distillation simulation program.
THE
SIMULATION
MODULE
Ever since the invention of the digital computer, numerous methods have been
proposed for
the computer
solution of
the
distillation problem. is beyond
our
scope to
review
the
various methods in this paper. By selecting some
of the
methods we have
developed a sophisticated
computer
program not only for disti llat ion
bu t
also for
absorption (with or without reboiler)
and
liquid-liquid extraction.
These
operations
can be classif ied as the Multicomponent
Multistage
Separation
Processes and there
fore we have des ignated
our
program as
the MMSP
program. The program is
modular
and
it consists of various subprograms.
These
include
the thermodynamics
subprograms, the
mathematical
subprograms, the initiation subprograms,
and the
separation subprograms. It
can
also be readily incorporated to
any
process flowsheet
s imulator such as Union
Carbide s IPES (Integrated ro ess Engineering System)
program. The
input can
either
come from
the
standard
8
1
2
x
II data
sheets or from
a stored file.
The output can either
be printed or s tored in a file to be used for column
design
programs
such as
our APT ll
Purpose ray program. Figure I shows
the
information flow
of
such a program system.
The
methods used in
the MMSP program
are:
The Wang-Hcnkc
method
(DSTLWH)
for distillation;
the
Sujata s Sum
Rate '
method
(DSTLSR)
for absorb
ers or liquid-liquid extractors; and
the
Block Tridiagonal
matrix
method (DSTLBT)
developed by
Naphthali and
Sandholrn
and
modified by
Tang.
In
the
last method,
the
MESH (Material
Balance, Equilibrium,
Summation,
and Heat
balance)
equa-.
tions are solved simultaneously using the
Newton
Raphson technique. The MESH
equations
are arranged
in such a way
t ha t t he Jacobian matrix
is in a simple block
tridiagonal form. Using this method, var ious top
and
bottom specifications can be
made.
These
include condenser
and
reboiler
temperature,
condenser
and
reboiler
heat
duty, reflux
and
reboiler ratio,
and
most importantly,
the product
purities in
terms of
mole fraction, moles, or percent recovery
of
the key components.
For
the
design problems of
determining the
stage
and
reflux
requirements
in a
disti llat ion column for a specified key
component
separation, we employed
VanWin
kle s
shortcut
method
(SHTCUT). The number
of equil ibrium stages are first
computed from
SHTCUT
and
the
reflux
requirement and
energy consumption
are
computed by use of the Block Tridiagonal matrix method (DSTLBT).
The MMSP program
has been used within Union Carbide for the simulat ion,
design, control studies,
and
energy
audit
of
many
distillation columns, absorbers,
and
liquid-liquid extractors. The column
can
be a simple one with one feed
and
two
product s or a complex one with multiple feeds, multip le s idedraws,
and
multiple
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Ul
8
~
«
z
~
o
0::
J:
I
z
«
J:
u
OPTIMAL
FEED
LOCATION
PHYSICAL
PROPERTY
rFEED AND COLUMN
INPUT DATA FORMS
INPUT DATA FORMS
•
•
HYSICAL
PROPERTY
~ D T
FEED AND
COLUMN
DATASET
PP-COMMON
BANK
INPUT DATA SET
~
-.
THERMODYNAMICS
-
COLUMN INITIATION
SUBPROGRAMS
-
SUBPROGRAMS
MATHEMATICAL
I
UBPROGRAMS
•
EPARATION SUBPROGRAMS
,
GFLASH
XFLASH
SHTCUT
DSTLBT
DSTLSR
DSTLWH
•
ROCESS FLOWSHEET
STAND ALONE PROGRAM
\ S
S
~
PROBLEMSL
OR PROP. \
FILES
•
•
t
P R I N T R
TRAY DESIGN
PROGRAM
O U T P U T
P T ~ L L PURPOSE TRAY
FIGURE
I I nfo rma ti on flow for MMSP simulation and design.
653
Ul
m
s
i
>
Ul
Ul
tl
o
0::
D.
z
J:
U
interstage heaters or coolers).
The
mi xtu re to be s ep ar at ed can be of any de gre e of
nonideality ranging from those obeying Raoult s law to highly nonideal systems such
as azeotropes. We have incorporated in
our
Thermodynamics Subprogram Library a
spectrum of equilibrium models. These include Chao Seader correlation, Robinson
Chao correlation, Poynting correction, and others for liquid phase fugacity; Redlich
Kister, Regulat Solution, NRTL Wilson equation, Margules, and Uniquac for liquid
activity coefficients; Redlich-Kwong, Second Virial, Prausnitz-Chueh modified
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654 J.C. WANG
Redlich-Kwong and other equa tions of state to compute vapor phase fugacity
coefficient and vapor
enthalpy departure
We also
can
use the BWR Starling BWR
and
Soave modified Redlich-Kwong equation to
calculate
K-values
and
enthalpy
departures for hydrocarbon mixtures. The data required for all these models can be
retrieved from a comprehensive Data Bank in which the pure component physical
properties of more
than
1000 compounds
are
stored.
The
MMSP
program is used for the evaluat ion of the energy consumption of the
dist il la tion column as a function of the location of the main feed stream. The input
data
to the program include:
• total
number
of stages including condenser and reboiler
• feed conditions, rates and compositions
• sided raw locations and rates, if any
• purity specifications in terms of mole fraction or percent recovery of the
specified components in the distillate and bottom products
• the codes to indicate the models selected for calcu lat ing the K-values
and
the
enthalpies. The pure component data can be retr ieved from the Data Bank
bu t
binary interaction
parameters
must be supplied in the input sheets.
Most of the input data items listed above are provided with the item numbers in the
data sheets. Any of these items can be changed by supplying its item number and its
new value. At the end of each case, the computed resul ts are stored on the computer.
The
program will then proceed to read the
change
cards, change the values of the
specified items, and perform the computation for the new case. Since the new case
starts with a converged solution of the previous case, the new case converges very fast.
Finally, the resul ts of all the cases
are
printed in a single output.
This
change case
procedure can be overseen by a control subrou tine such as the one described in this
paper for the optimal feed location.
The
output
from the program includes the column profiles of stage temperatures
internal vapor
and
liquid rates , the stage vapor
and
liquid compositions
and
the
following items which are related to the energy consumption of the distillation
column:
• the reflux rat io or reflux rate,
• the reboi ler rat io or reboiler vapor rate
• the condenser
duty
• the reboiler duty.
addition the following two items related to
matching
the compositions in the feed
and
on the feed
stage can
also be computed:
• the absolute value of the difference between the key components
ratio
in the feed
and that on the feed stage, i.e.,
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OPTIM L
FEED LO TION
655
• the sum of the absolute values of the difference between the feed composition and
the feed stage composition for all the components.
where Z - feed composition, m.f.;
feed stage composition, m.f. Subscripts i for the
ith component, l for the light key, for the heavy key.
EFFE T
OF
FEED LO TION
ON
DISTILL TION
To show the effect of the feed stage location on the performance of the distillation
column, let us consider the problem of benzene-toluene-xylene separation. As shown in
Fig. 2, 100 Kg molesjhr of the mixture is fed to a distillation column to be separated to
a disti llate containing 0.01 mole of toluene and a bottom product containing 0.01
mole
of benzene. A shortcut calculation indicated
that
40 stages are required at a
reflux ratio equal to 1.2 times the minimum. Therefore, 42 stages including condenser
and reboiler are used in the rigorous calculation. The stages are numbered from top to
bottom with the condenser as the first s tage and the reboiler as the 42nd stage. The
effect of the feed stage in the middle section of the column (Stage 16 to
Stage
29) is to
be studied by using the change case procedure.
The simulation was done by using the Block Tridiagonal matrix method with the
purity specifications for
the
distillate (0.0001 rn.f. of toluene) and the bottom product
(0.0001 m.f. of benzene). The Wang-Henke method was used for initiating the column
profiles to be used in the base case. The Chao-Seader correlation' was used to
calculate the K-values and the Yen-Alexander correlation was used for calculating
the vapor and liquid enthalpies. All the physical properties required were retrieved
from the Data Bank. The computed results are plotted in Figure 3. The smoothed
curves for the fourteen case studies with feed stage at 16 to 29 include the values of l
k
and l the reflux ratio, the reboiler ratio, the condenser duty, and the reboiler duty.
These results show clearly that all the quantities listed are unimodal functions of the
feed stage location with only one minimum point within
the
section of the column. It
was also noticed
that
the minimum points for all the quantit ies with the exception of
l
k
fall on the same point. This has been true for all the problems we have encountered.
Therefore, the criterion of optimal feed stage based on the matching of the key
component ratio is not correct. The criterion based on the energy consumption should
be used. Since the energy consumption is related to the reflux rate, the boil-up rate,
the condenser duty, arid the reboiler duty, the optimal feed location can also be defined
as the one that minimized one of these quantities (or the weighted sum of these
quantities). The optimal feed location problem can then be determined by applying a
unidimensional search technique.
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656
p 101
KPA
F, 100 Kc;MoL.ES /HR
S TU RED L IQ U ID I
T 364
0
K
BENzENE, 0.6 M.F.]
TOL.UENE, 0.3
_XVL.ENE, 0. 1
e WANG
___
3 _
16
8
~ I t K G M o L E S
HR
0,0001
MIF.j
TOL.UENE,
0.7499
VL.ENE
1
0.2500
FIGURE 2 The sample problem.
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1. 4
i.a
OPTIMAL
FEEO
LOCATION
5. 5 t \ \ 4 ~
4 4
. ~ I _ _ _ I _ l
0
5
4 t - - t - ~ - ; - -
...
0
0(
III
J:
5 •3 t l l
II:
III
5 • 2 r t t \ ~ I _ _
III
o
Z
~
5. 1 r - - ~ t - \ - - t _ _ - _ : ~ - I _ _ ~ r _ _ -
c
5.0
+-+----11--+-++
II:
III
: 4.
9 t I l 1 ~
o
lD
III
0::4.
8
r \ f \ ~
III
U
1. 2
4.6
II:
III
1.0
4. 5
c
0.8
4.4
0(
l:t:
0
i
0.4
z
o
E
0.2
rJl
o
L
0.0
29 28 27
NUMBER
FIGURE 3 Effect of feed stage on distillatiion (benzene-toluene-xylene separation).
THE SEARCH ALGORITHM
657
0
i=
0(
0::
3.7
II:
III
J
3. 6
0
lD
III
0::
3.5
1.8
1.7
~
0::
1. 6
s
i o
5
,
0::
x
::I
J
II
III
0::
There are various methods for the search of the minimum point of a function. One of
these is the graphical method that we just described in the previous section. Beveridge
and Schechter,' Wilde,19 and others have reviewed one dimensional search techniques.
Himmelblau' and Beveridge and Schechter ' compare the efficiency of the various
unidimensional search techniques. They show that the Golden Search and the
Fibonacci Search are the two most efficient methods. Since the Fibonacci search
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658
J.e. WANG
t echn ique is based on the Fibonacci
numbers
it is especia lly convenient for integer
search problems such as our optimal feed stage problem. this case, the Fibonacci
number
F
is the
number
of s tages within a sect ion of columns in which
the
optimal
feed stage lies and n is the required number of function evaluat ions column
simulations .
Using the Fibonacci search technique, we have developed a computational proce
dure for locating the
optimum
feed stage with a section of the distillation column. The
function to be minimized can be one of the following:
1 = s, I Zlk
-
X
lk
· the key ratio difference
.
Zhk X
I
z, - X; iF I The
composition difference
R
/
D the reflux ratio
1 V
/
B the boilup ratio
1
Qc the condenser
heat duty
1 = QR, the reboiler heat
duty
1
2
3
4
5
6
where
J
F indicates the feed stage number.
The computational procedure for the
optimal
feed stage
search
algorithm includes
the following steps:
t p
I
Supply the input
data
for
the
column simulation except
the
feed
stage
location.
The
data requirements are outlined in the previous section. Also specify the function to be
minimized.
t p
2
Specify the section of the co lumn to be
searched
by giving the
stage
number J F near
the column top and the stage number
J
F
b
near the bottom.
The
difference between
J
F
b
and J F
must be a Fibonacci
number F• Then
find
the
number
of
function evaluations
n corresponding to F from the table of Fibonacci number Table I .
t p
3
Determine the first two stage numbers to be checked as follows:
JF - JF + F _
2
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Step 4
OPTIM L
FEED LOC TION
T LE I
The First Fibonacci Numbers
n
0
I I
2 2
3
3
4 5
5 8
6 13
7
21
8
34
9
55
10 89
11
144
12
233
13 377
659
Eva lu at e the function to be minimiz ed as
I
and
12
c orr esp on di ng to the feed s ta ge
numbers
JF
and
JF
2
by performing the column simulation with the purity specifica-
tions for the top and bottom product streams. Also set the search number I 2.
t p 5
Checkj vs./2 If
II
< 2 go to Step 6 otherwise set:
and evaluate a new
12
corresponding to the new
JF
2
by p erf or min g the column
simulation then go to Step 7.
t p 6
Set
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660
J.C. WANG
and
evaluate
a new corresponding to
the
new
I
by performing
t he column
simulation.
t p 7
Update
the search
counter
I as
I
=
I
I
t p
8
Cheek
I vs. n If I = n stop the i teration and go to
output.
Otherwise
repeat Step
5
through
Step
8.
This procedure has been programmed into
the
MMSP
program
as
the fBSRCH
subprogram. The
results from all the column simulations
are
stored and printed in
the
final
output
along with
the
column profiles corresponding to
the
optimal feed
stage
location.
These
results including
reflux ratio, reboiler
rate
condenser
duty
reboiler
duty number
of iterations used in
the
column simulation,
and the
convergence
of the column simulations are printed in
tabular
form.
APPLICATION
THE
SEARCH PROCEDURE
We shall i llustrate
the f ibonacci
search procedure by applying it to
the
sample
problem
of
benzene-toluene-xylene
separation
discussed previously.
The
section
of the
column between stage 16
and
29 was
searched
for
the
optimal feed
stage
location
that
produces the
minimum
reboiler heat duty.
The
total
number
of stages within
the
section is
the fibonacci number
13.
from
Table
I we found
that
n
-
6
and
therefore a
maximum
of
six 6 column simulations were required.
The search
procedure was
started
as follows.
figure
4 is
the
graphical representation of this procedure.
The
first two feed stages to be checked were
I
JF F
n
_
=
16
5 = 21
JF
2
= JF F
n
_
I 16
8 - 24
Column
simulations using
DSTLBT
were performed with these two feed stages.
The
corresponding reboiler duties were
obtained
as:
4.7702
GJ/hr
= 4.5997
GJ/hr
Since
>
the minimum
point
must
lie in
the
section between
stage
21 to
stage
29.
Therefore
we eliminated
the
section from
stage
16 to
stage
21 from
further
consideration.
We then
followed
Step
5 to set
the
top limit
of the
new section as
JF
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OPTIMAL F EE D L OC AT IO N
661
1\
\
\
I
\
--_.
II
\
_. -
-- _
..-
i
..
-.-
_ _
--- -
f-- -
/
I
1\
i
z
J
Q
\
;
/
I
,
\
.J
{
lJJ
a
1/
t:
i:
I-
/
\
6
r
V
.
-
SECOND
,
5 ELIMINATION J
-
1 IRST
ELIMINATION
_
JF .
JFz JF.
JF ,
4
(MIN)
I
3
I
29
28
27
26 25
24
23
22 21
20
19 18
I
5. 4
4 .
4.
5. 5
5. 6
5. 7
5. 8
5. 9
4
(JF. i
It: 4 .
g 4. 7
lJJ
.It: 4.
e
5. 3
~ 5
FEED ST GE NUM ER
FIGURE 4 Graphical representation of the Fibonacci search procedure.
JF - 21. We also
s tf
-
f2
- 4.5997 GJ/hr, and JF - J
2
24. A new
J
2
was
obtained as
J
2
J
t
+
J
b
- JF
=
21 + 29 - 24 - 26
Another column simulation with feed stage at 26 was performed. The reboiler duty
was obtained as
f
4.7046 GJ
/hr
Since
f
<
f2
the optimal feed location must lie above stage 26. The column section
between stage 26 and stage 29 was eliminated from further consideration. Following
Step 6 we set J
b
- 26 /2 f - 4.5997
GJ/hr,
and J - JF 24. A new JF was
obtained as
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662 J.e. WANG
J
I
= JF +
J
b
-
J
-
21 + 26 - 24 23
The
column simulation with
J
I
23 yielded
the
reboiler
duty
II
-
4.6242
GJ/hr
Since f
>
I the minimum point must lie in the section from stage 24 to
stage
26. The
section to be searched was finally reduced to three stages. The reboiler duties
corresponding to two of these stages as feed
stage
24 and 26 were already obtained.
The
final column simulation performed with feed
stage J
F
I
- 25 yielded a reboi ler
duty II = 4.6168 GJ /hr
This
was larger
than
the reboiler duty I 4.5997 obtained
for feed stage JF = 24. Therefore the opt imal feed location was found to be
stage
24
from the top.
Table
II lists the results obta ined from the MMSP program for the above search
procedure. The total
number
of column simulations used in this search procedure was
5 as compared to 13 used in the case study. In
Table
we compared the
computer
times used and the
computer
costs for these two procedures. An IBM
370/168
computer
was used for the simulations.
The advantage
of using the Fibonacci
Search
procedure becomes more obvious when larger numbers of stages are in the section to
be searched.
TABLE II
Results of Fibonacci Search Applied to the Sample Problem
Feed Reflux Boilup Condenser Duty Reboiler Duty
Stage No.
a,
a, Ratio tio GJjhr GJjhr
2\ 0 31\6 0 \024
1.4771
3.5292
4.6349 4.7702
26 0 6\80
0 \789 1.4417 3 48\4 4.5703 4.7056
23
0.0695 0.0610
1.3983
3.42\ 0 4.4887 4.6242
25
0.3422 0.0986 1.3942 3.4156
4.4818
4.6\68
24 0.\088
0.0589
\.3838 3 40\3
4.4625 4.5977
TABLE III
Comparison etween Fibonacci Search and Case Study
ibon cci
Search
Case Study
No of Column Simulations 5
\3
Total No. of Iterations
DSTLBT 20
50
Total CPU Time. Sec.
IBM 370/168
3.27 8.78
Total Cost.
Printing CPU
4.36 20.00
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8/19/2019 Fibonacci Search for Optimal Feed Location
14/14
CONCLUSION
OPTIMAL FEED LOCATION
663
A computational procedure using the Fibonacci search technique has been developed
for the location of the opt imal feed
stage
within a section of the column such that the
min imum energy consumption of the distillation column can be achieved.
The
procedure is simple
and
can be readily incorpora ted into
any
rigorous distillation
simulat ion module. This procedure has been used in Union
Carbide
for relocation of
the feed stage in many existing columns. During 1978 over 25,000 column simulations
have been performed by use of the MMSP
computer
program. Eight percent of these
simulat ions used the Fibonacci search procedure for opt imal feed location. Most of
them were for feed
stage
relocation in existing disti llat ion columns to reduce energy
consumption in the plants. The saving of energy cost was estimated to be over a million
dollars per year.
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