An alternative procedure to retrofit an industrial plant. A case study
F.S. Liporace*, F.L.P. Pessoa# and E.M. Queiroz#
*CENPES/PDEAB/EBS - Sala 2114 - Petrobras - Cidade Universitária - Q. 7 - Ilha do Fundão
CEP: 21949-900 - Rio de Janeiro - RJ Tel.: (21) 3865-7027 Fax: (21) 3865-6342
liporace@petrobras.com.br
#Dept. de Eng. Química - Escola de Química - UFRJ - Cidade Universitária - Ilha do Fundão
CEP: 21949-900 - Rio de Janeiro - RJ
mach@eq.ufrj.br
Abstract In this work, an alternative procedure to retrofit an industrial plant is proposed, based on a new heat exchanger network (HEN) synthesis and evolution algorithm. The idea is to consider the retrofitting as a grassroots design, but with match constrains accounted for during the synthesis. First, a lower bound for the HEN total annual cost (TAC) is determined and a new HEN is synthesized. A comparison between this HEN and the actual one is performed to search for structural similarities, forming the set of match constrains. Then, a new HEN keeping those similarities is proposed. The comparison among the three HENs indicate the recommended structural modifications. A Case Study is considered. Results show a reduction on the TAC, indicating the actual HEN is poorly integrated and a retrofit can be performed without many changes on the structure. The automatic HEN synthesis and evolution are performed using the software AtHENS,
Keywords Retrofit; Automatic synthesis; Automatic evolution; Industrial plant.
I. INTRODUCTION
The area of Process Design has evolved a lot
for the last two decades since there was an increase on
the Energetic Integrated Systems research, particularly
on the problem of Heat Exchanger Network (HEN)
synthesis (Gundersen and Naess, 1988). Among the
various methods so far proposed to solve this problem,
the Pinch Design Method, PDM (Linnhoff and
Hindmarsh, 1983), can be highlighted. The PDM
combines a strong thermodynamics basis and an easy
way to synthesize networks near to the pinch point (PP).
On developing processes nowadays, the
concepts of energetic integration are already accounted
for during the synthesis stage, so the total annual cost
(TAC) can be reduced, and hence the energy cost. But,
as almost all the older processes did not do this, they
frequently present a high-energy consumption. In order
to improve it, these processes must go through some
structural changes (retrofitting), which represent an
increase on the capital cost (new heat transfer area, new
piping etc), indicating that there is a trade-off between
the operational and capital costs. Works on this issue
are published since the 80's (Linnhoff and Withrell
(1986), Tjoe and Linnhoff (1986)), the majority of them
are based on the thermodynamic approach. In the 90's,
some works based on the mathematical programming
approach were developed (Ciric and Floudas, 1990),
which use the concept of superstructure and an MINLP
programming to model the problem and solve it
automatically. The great advantage of this approach is
that models can be as exact as the designer may wish,
incorporating all kind of constrains. The disadvantage is
that, in doing so, the problem complexity increases in
the same proportion and so the way to solve it, which
depends on a good initial point to avoid convergence
difficulties and instability. Other works (Asante and
Zhu (1996), Briones and Kokossis (1996)) propose
methods that combine both mathematical and
thermodynamic approaches, decreasing the problem
complexity, but they are still dependent on a good initial
point.
As the major feature of a good retrofit method
is to decrease the energy consumption with the lowest
change on the actual HEN structure, this work proposes
a retrofit procedure, which combines the idea of
incorporating all kind of match constrains from the
retrofit procedures based on the mathematical
programming approach and the fast and easy way to
synthesize grassroots HEN from the design procedures
based on the thermodynamic approach. This
combination was possible due to the new HEN
synthesis and evolution algorithms proposed by
Liporace et al. (1997,1999) to automatically synthesize
and structurally evolve HENs. The performance of
these new algorithms have already been compared to
the ones reported in the literature and good results were
provided (Liporace et al., 2001). Next, some important
features of these algorithms are highlighted.
The HEN synthesis stage uses a modified PDM
rule to synthesize the HEN near to the PP, namely the relation between the heat capacity flowrates of the
incoming and outgoing streams. The traditional PDM
rule (Linnhoff and Hindmarsh, 1983) states that the heat
capacity flowrate of the incoming stream must be lower
than or equal to the heat capacity flowrate of the
outgoing stream. For some examples, it was verified
that if this rule was blindly used in the synthesis near to
the PP, the synthesis away from the PP could not be
performed with minima consumption of utilities
(MCU). This was overcome by a slightly modification
on that rule, i.e., when the heat capacity flowrate of the
incoming stream is lower than the heat capacity
flowrate of the outgoing, this one is split in order to
match the heat capacity flowrate. In doing so, streams
with adequate temperature levels remain on the set of
streams that is used in the synthesis away from the PP
(Liporace et al., 1997). This algorithm generates HEN
with more splitters than the normal, but these may
disappear during the evolution stage as long as the
controllability analysis says so. In the synthesis away
from the PP, it is used the heuristic rule of Ponton &
Donaldson (Ponton and Donaldson, 1974)).
In the structural optimization stage, the loop
identification procedure is based on the determination
of the rank of the Incidence Matrix (Pethe et al., 1989)
and to restore the minimum temperature difference
(MTD) and/or the target temperatures, if both are
violated when the loop is broken, it is used the
Simulation Matrix (Liporace et al., 1999). The
traditional restoration procedure (Linnhoff et al., 1982)
finds a path between a hot and a cold utility that passes
through the heat exchanger where the MTD violation is
taking place. Such search is not always easy to be
conducted automatically and, sometimes, the path does
not even exist. So, Liporace et al. (1999) proposed to
represent the HEN structure, with its heat exchangers,
heaters, coolers, splitters and mixers, in a form of a
matrix (Simulation Matrix) in order to avoid the need
for locating the path. Every time a loop is broken, this
matrix allows the performance of a steady-state
simulation of the HEN in order to verify in which unit
the MTD violation is occurring, restore it by decreasing
its heat load, and also check if all the process streams
reach their target temperature. The Simulation Matrix
allowed the automation of the loop-breaking procedure
without the user interference. In our point of view, a
loop should only be broken if the TAC is decreased,
different from the traditional procedures (Trivedi et al.,
1990; Zhu,J.Y. et al., 1999), which try to reach the
minimum number of units with the lowest energy
penalty without accounting for the HEN TAC.
The proposed new procedure to perform a
retrofit analysis on an operating industrial site, based
only on the thermodynamic approach, consists of 5
steps:
1) identify the process streams and the actual
HEN;
2) "supertarget" the process;
3) synthesize a HEN that accomplishes the
energy targets and evolve it by identifying and breaking
the loops when economically indicated;
4) compare the HEN from (3) with the actual
one in order to identify structural similarities and so
define a set of match constrains (similarities);
5) synthesize and evolve a HEN containing
those similarities, if they exist, obtaining the proposed
new HEN.
The first characteristic of this alternative
procedure is that a lower bound to the TAC is
calculated and, assuming a grassroots design, a
completely new HEN is generated (step 3), which
would be the best HEN for that set of process streams.
This synthesis is preceded by the "supertargetting"
stage, which do not account for the existing heat
transfer area, since it is considered that the HEN does
not exist. The comparison between this new HEN and
the actual one indicates how far the later is from the
optimum HEN (indicating the need to retrofit or not)
and shows structural similarities between them that
would be used as constrains to generate a new HEN. Of
course, it is up to the designer to select and use those
constrains or any other he may wish. The HEN
synthesis and evolution algorithms developed by the
authors are based only on the thermodynamic approach,
so there are no convergence and instability difficulties
and, of course, they do not depend on an initial HEN
structure. These facts contribute to reduce significantly
the computing time to reach a solution, especially when
dealing with big size problems, even when the match
constrains are accounted for. This lower computing time
allows the designer to use his experience without taking
too much of his time and he may try several match
constrains.
To illustrate the procedure, a Case Study using
operational data extracted from an industrial site is
considered. It is an Oximation Unit of a Caprolactma
Production Plant.
The HEN synthesis and evolution is performed
using the software AtHENS (Automatic Heat
Exchanger Network Synthesis), developed at
Departamento de Engenharia Química from Escola de
Química of Universidade Federal do Rio de Janeiro,
which is based on the algorithms previously presented.
II. PROCEDURE AND CASE STUDY
A. Identification of the process streams, of the actual HEN and supertargetting the process
The Case Study unit produces oxime, an
intermediate reactant for the synthesis of caprolactama.
The raw materials used on the synthesis of oxime are
ammonia, air, hydrogen and ciclehexanone. The process
is called HPO and has 4 stages:
- hydroxilamine (HYAM) synthesis;
- oxime synthesis by reacting HYAM with
ciclehexanone using toluene as solvent;
- oxime-toluene separation;
- nitrate ions synthesis (used on the HYAM
synthesis).
Based on the Process Sheets, 8 cold streams
and 23 hot streams were identified. Table 1 shows the
necessary data to perform the energetic integration
study: inlet (Ti) and outlet temperatures (To) of the
process streams and their heat capacity flowrate
(M.Cp). In this Table, cu means cold utility and hu, hot
utility. Besides, in order to optimize the MTD, the heat
transfer coefficient (h) for each process stream must be
known. For the streams which data on density, viscosity
and thermal conductivity were known, the heat transfer
coefficient was estimated using the well known Dittus-
Boelter correlation, assuming a 3/4" pipe and a mean
fluid flow velocity of 1.5 m/s. For the others streams,
this parameter was arbitrated according to the literature
(Sinnot, 1993).
Table 1 - Data on process streams
Table 2 presents the data on operational (hot and cold utilities) and capital costs. A heat exchanger lifetime of 10.0 years and a linear depreciation (0.0% as the rate of interest) were used (Wells and Rose, 1986).
Table 2 - Data on operational and capital cost
Using the data presented in Tables 1 and 2, the optimum MTD is 5.0ºC. Table 3 presents the "supertargetting" results and it also shows a comparison among the target utilities consumption and the actual one, pointing a maximum reduction of 76% on the hot utility consumption and of 24% on the cold utility consumption. The great reduction on the hot utility consumption indicates that it is worthwhile to perform some structural and/or parametric changes on the actual HEN.
Table 3 - "Supertargetting" results
The actual HEN structure is shown in Fig. 1. Horizontal lines represent the process streams, with hot streams being directed from the left to the right and cold streams from the right to the left. The process-process heat exchangers are the vertical lines. Circles with uparrows and down-arrows represent the heaters and coolers, respectively. Based on the data presented in Table 2, the actual HEN TAC is $7.58 x 105/year.
Fig. 1 - Actual HEN (HEN 1) TAC: $7.58 x 105/year
Two facts (the first is the most important) are
responsible for the high-energy consumption, especially
hot utility:
- use of hot utility below the PP (represented
by a full vertical line on Fig. 1) on cold streams 10, 16,
17, 18, 28 and 30;
- poor integration among the process streams
(only 3 process-process heat exchangers exist on the
actual HEN: hot stream 26 with cold stream 27; cold
stream 28 with hot stream 29 and cold stream 30 with
hot stream 31).
B. Synthesis and structural evolution of an HEN with MCU - Lower bound
It has already been shown that if only the energy consumption is accounted for, it is worthwhile to care for some structural changes on the actual HEN. In order to verify their amplitude, a lower bound for the HEN should be generated, i.e., a completely new HEN with the lowest TAC. Figure 2 and Table 4 present the initial HEN with MCU. In this Table, ITH is the inlet temperature of hot stream, OTH is the outlet temperature of hot stream, ITC is the inlet temperature of cold stream, OTC is the outlet temperature of cold stream, A is the heat transfer area. .Splitting of cold stream 15 and hot stream 26 is a consequence of the algorithm proposed by Liporace et al. (1997), as already presented. The heat transfer areas were calculated using the heat transfer coefficient from Table 1 and assuming countercurrent configuration and, because of that, they are preliminary results. Of course, in the match where there is a process stream changing phase, the area calculated, as well as the intermediate temperatures, must be seen as rough estimations due to the assumption of constant specific heat made to characterize those streams.
Fig. 2 - HEN with MCU (HEN 2) TAC: $3.98 x 105/year
Table 4 - Data on the HEN presented in Fig. 2 (HEN 2)
Comparing Figs. 1 and 2, it can be seen an
increase on energetic integration among the process
streams in the HEN 2 and that the actual processprocess
heat exchangers do not appear on HEN 2.
Besides, it must be noted that, using the same cost
parameters from Table 2, the TAC is reduced 47%.
After the synthesis of the HEN with MCU, its
structural optimization must be carried out by
identifying and breaking the loops that might exist in
order to reduce, not the operational cost, but the TAC.
Figure 3 presents the first loop identified, which
involves the process-process heat exchangers 1-7-8-9
(counted from the left to the right and represented by
bold lines). The dashed line represents the heat
exchanger with the lowest heat load. Figure 4 shows the
HEN after breaking this loop. The appearance of a
cooler on hot stream 14 is a consequence of the
restoration of the MTD/temperature target. Three
additional loops can be identified, but only one is
broken, since the others would increase the TAC.
Figure 5 presents the final HEN with the lowest TAC. It
represents the lower bound for the HEN of this case
(HEN 3).
Fig. 3 - First loop identification
Fig. 4 - HEN after breaking the first loop TAC: $3.75 x 105/year
Fig. 5 - Lower bound for HEN structure (HEN 3) TAC: $3.74 x 105/year
Here, an important fact must be noted. The
second loop is composed by 2 process-process heat
exchangers and by 2 coolers (heat exchangers 1 and 2
and coolers on hot streams 14 and 26B). Among them,
the cooler on hot stream 14 has the lowest heat load. As
the loop is broken, eliminating this cooler, and a steadystate
simulation is performed to correct any MTD/target temperatures violation, there appears the need to include
a new cooler in the same place of the eliminated one,
but with a different heat load (Fig. 5). Nevertheless,
there was a decrease on the HEN TAC. This is because,
even keeping the same HEN structure as before and a
higher utility consumption, the new HEN has a better
heat exchanger heat load / driving force relation, which
contributes to decrease the heat transfer areas and,
consequently, the capital cost, overcoming the increase
on the operational cost. As the structure does not
change, the third loop is equal to the second one and the
same fact happens. However, now the increase on the
operational cost will not be overcome by the decreased
on the capital cost, so the loop is not broken. These
slightly modifications on the heat exchanger heat load /
driving force relation are easily and automatically
identified by steady-state simulations performed with
the Simulation Matrix (Liporace et al., 1999).
Comparing HEN 3 (Fig. 5) and HEN 1 (actual
HEN, Fig. 1), it can be seen a great structure similarity
in terms of the coolers, but none in terms of the processprocess
heat exchangers. So the actual HEN is really far
from the ideal HEN structure and its TAC. As it was
identified a possibility of a great reduction on the TAC,
it is worthwhile to proceed with the retrofit analysis,
even with poor structural similarities. Also note that
there could be a case where, even with different HEN
structures, the actual TAC is near to the lower bound
and there is no advantage in going on with the retrofit
analysis.
C. Proposal of an HEN keeping the existing processprocess units and its structural evolution
In this stage, in order to minimize the proposal
of structural modifications on the actual HEN structure,
it will be designed an HEN that keeps the process
streams already integrated, namely: 26-27, 28-29 and
30-31, i.e., in the synthesis stage, it is allowed to the hot
stream 26 combine just with the cold stream 27 and/or
with cold utility, to the cold stream 27 combine just
with the hot stream 26 and/or with hot utility, to the cold
stream 28 combine just with the hot stream 29 and/or
with hot utility and so on. These define the set of match
constrains to be used in the synthesis stage. As the set
of process streams did not change, the MTD, the
utilities target, as well as the PP temperature, also did
not.
Figure 6 and Table 5 present the HEN with the
specified match constrains. Note that, because of them,
the HEN structure becomes a little bit less complex than
the HEN with MCU (HEN 2), with a lower integration
among the process streams and, therefore, a higher
utility cost. But the reduction on the TAC, compared
with the actual one, is the same (a coincidence) as the
one obtained with the HEN with no match constrains
and MCU, as shown in Table 6. They are the same
because the increase on the operational cost was
counterbalanced by the decrease on the capital cost as a
consequence of a better heat exchanger heat load /
driving force relation among the heat exchangers of the
HEN 4.
Fig. 6 - HEN with match constrains (HEN 4) TAC: $3.98 x 105/year
Table 5 - Data on the HEN presented in Fig. 6 (HEN 4)
Table 6 - Comparison between HEN 1, HEN 2 and HEN
The loops in HEN 4 are not broken since their
elimination do not lead to a decrease on the TAC.
Hence, the proposed design of the case with match
constrains is HEN 4.
Table 7 shows the comparison between the
actual HEN and the two final optimized HENs proposed
in this work, one obtained without match constrains
(HEN 3) and the other obtained from the HEN with
match constrains (HEN 4). Of course, HEN 3 presents
better results in terms of reduction on the utility
consumption and on the TAC since its initial point is the
best one. But, accounting only for the reduction on TAC
(this is what really matters to the industry), both HENs
show similar performances, with HEN 4 presenting a
less complex structure than HEN 3 and more structural
similarities, allowing the possibility of using the
existing heat exchangers.
Table 7 - Comparison between HEN 1, HEN 3 and HEN 4
Table 5 presents the heat transfer areas of the units proposed in HEN 4 and the respective actual
values. Based on the data on capital cost from Table 2,
an investment of $7.90 x 105 (only on heat transfer area)
must be done to achieve the desired structural changes.
As these changes will allow a reduction of $3.60 x
105/year on the TAC, the estimated return of investment
is 2.2 years.
The main concern of this procedure is to
generate a HEN showing, as much as possible,
structural similarities with the actual HEN in order to
minimize the proposed changes. On the other hand, the
existing retrofit procedures assume the possibility of
available heat transfer area redistribution. However, the
actual heat transfer area can not be simply redistributed.
This goal, in our point of view, should be left to a later
step, since, depending on the streams involved and on
the changes on the operational conditions of the actual
HEN, only a detailed heat exchanger design can
effectively indicate the possibility of its use and
redistribution.
III. CONCLUSIONS
In this work, an alternative procedure to retrofit
HEN is proposed. It is based on a new, fast and
automatic grassroots design and evolution algorithms
and its main concern is to propose a new HEN with as
much structural similarities with the actual HEN as
possible in order to minimize the investment on new
heat transfer area. These similarities are treated as a set
of match constrains, which is used in the initial HEN
synthesis stage.
The advantage of this procedure over the
existing mathematical programming ones is that it
allows the designer to interfere in the synthesis stage
using his experience to forbid and/or keep a match
depending on the results obtained along the study
without taking too much of his time. In the
mathematical programming procedures, those constrains
must be stated at the beginning and, as said before, the
more the constrains the more the problem complexity.
Difficulties on convergence and instability are also
absent.
To illustrate it, a Case Study, based on an
existing and operating site, was considered. According
to the proposed procedure, the solution is HEN 4. It
repeats the existing match (process-process and
coolers), needs few new heat exchangers and presents
approximately the same reduction on the TAC as HEN
3, which would be the optimal solution if only the TAC
value were taken into account.
This procedure was used on another retrofit
study (PETROBRAS refinery) and the results obtained
were considered well satisfactory.
At last, AtHENS was fast in solving a big size
problem in the HEN synthesis stage (with or without
match constrains) as well as in the structural
optimization stage, by identifying and breaking the
loops. It took AtHENS less than one minute of average
computing time (Pentium 166 MHz and 32 MB RAM)
to complete the HEN synthesis and evolution.
IV. NOMENCLATURE
A | heat transfer area (m2) |
A* | existing heat transfer area (m2) |
AtHENS | automatic heat exchanger network synthesis |
cu | cold utility |
h | heat transfer coefficient (W/m2 ºC) |
HEN | heat exchanger network |
hu | hot utility |
HYAM | hydroxilamine |
ITC | inlet temperature of cold stream (ºC) |
ITH | inlet temperature of hot stream (ºC) |
M.Cp | heat capacity flowrate (W/ºC) |
MCU | minima consumption of utilities |
MTD | minimum temperature difference (ºC) |
OTC | outlet temperature of cold stream (ºC) |
OTH | outlet temperature of hot stream (ºC) |
PDM | pinch design method |
PP | pinch point (ºC) |
Ti | inlet temperature (ºC) |
To | outlet temperature (ºC) |
ACKNOWLEDGEMENT
The authors would like to acknowledge CAPES (Coordenadoria de Aperfeiçoamento de Pessoal de Nível Superior) for their financial support. Special acknowledgement to Nitrocarbono, for their financial and logistic support.
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Received: October 13, 1999.
Accepted for publication: July 9, 2001.
Recommended by Subject Editor P.Toledo.