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Energy Efficiency
Energy Implications of Water Reduction
Strategies in Kraft Process.
Part II: Results
By E. Mateos-Espejel, M. Marinova, S. Bararpour and J. Paris
Abstract: A new systematic methodology has been developed to study interactions between water and energy
in the kraft pulping process and has been applied to an operating mill. The methodology, which can be used
to find appropriate strategies for water consumption reduction and which also considers their impacts on the
thermal energy efficiency of the process, has been described in Part I of this paper. A case study was subsequently performed and the results are presented in Part II. Four strategies that simultaneously reduce water,
steam and cooling requirements are analyzed. Potential savings are significant.
A
ttempts to reduce the water
consumption in the kraft process have
been made but without analyzing the
global process energy implications of
water closure. These strategies have been primarily based on increased reutilization of bleaching
filtrate, of whitewater, of condensates from the
evaporators and, on the introduction of additional
oxygen washing stages [1-3]. The implementation
of those water savings measures may have serious
effects on the thermal balance of the mill. For
example, the steam consumption may be reduced,
the cooling demand may be increased and, the
effluent temperature may rise. The scope of studies that are usually performed may vary from only
water reduction [4] to process integration where
energy effects are considered. In a two step procedure Towers [5] first applied Pinch Analysis® to
identify opportunities for better energy efficiency,
and proposed complementary measures to reduce
the water used for cooling by increasing the heat
transfer area in the condensers and adding a cooling tower in the water network. Savulescu et al. [3]
used Pinch Analysis® combined with water and
energy analysis in the water network to improve
the energy efficiency. The default (defect???) of
those approaches is that they do not consider water
as a heat source, and this important element of the
thermal problem is often ignored.
The results obtained using the methodology
explained in Part I of this work are presented
below. The proposed strategies for water savings
are analyzed and their energy implications are
examined in order to determine the impacts on
the global heating and cooling demands, on the
minimum energy requirements and, on the pinch
point position. The data used in this study were
taken from a computer simulation of the studied
mill (kraft pulp mill located in Eastern Canada),
38 Pulp & Paper Canada May/June 2010
developed in CADSIM® Plus (Aurel Systems
Inc.).
E. Mateos-Espejel
Department of Chemical Engineering, École
Polytechnique, Montréal,
Canada
Methodology overview
The methodology presented in Part I consists of
5 steps:
Step 1. Analysis of the current water utilization
in the process.
Step 2. Benchmarking the water consumption
to evaluate the opportunities for water savings.
Step 3. Thermal analysis and identification of
restrictions to be considered in the water reduction strategies. These strategies are also computer
simulated to identify the impact on the operating
conditions of the complete process.
Step 4. Analysis of the energy implications of
water closure on the internal heat recovery.
Step 5. Trade-off analysis.
In Part I it has been shown, by means of a
benchmarking assessment, that there is a potential
to reduce the water consumption in the studied
mill. Most of the treated water is used for process
purposes rather than for cooling, which has a
direct effect on the thermal energy balance of the
process. Therefore, it has been decided to focus the
study on the reduction of treated water. Sources
and sinks composite curves have been constructed
in order to identify opportunities for water reutilization. The concentration of the dissolved solids
in the water (DS) has been considered as a process
demand constraint to be satisfied by water sources.
According to the curves obtained the minimum
filtered water consumption is 1000 m3/h and, the
minimum effluent production is of 875 m3/h. The
following strategies for water reduction have been
identified:
• Reuse of condensates from the evaporators;
• Reuse of whitewater from the pulp machine;
• Increased reutilization of bleaching filtrate;
M. Marinova
Department of Chemical Engineering, École
Polytechnique, Montréal,
Canada
S. Bararpour
Department of Chemical Engineering, École
Polytechnique, Montréal,
Canada
J. Paris
Department of Chemical Engineering, École
Polytechnique, Montréal,
Canada
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• Reuse of vacuum pumps sealing water.
The water streams considered for reuse are currently sewered.
The next section presents the description of the proposed strategies for their advantageous utilization.
Strategies for water closure
Reuse of condensates from the evaporators
This first strategy deals with the reutilization of the condensate
from the evaporators in the re-caustification unit, washing unit
and pre-washing step of the bleaching (Fig. 1). These condensates have a zero level of DS but to avoid odour problems the
methanol concentration must also be considered as a constraint
for reuse. In the current process configuration, the various condensates are mixed. In order to implement the identified strategy
they must be segregated in 2 types [6]. The condensates produced
in the 2nd to 6th effect of the evaporator trains and the stripped
condensate have a low concentration of methanol, therefore,
they can be reused in washing-type operations. The condensate
produced in the 7th effect has a high concentration of methanol,
consequently it is acceptable for reuse in the re-caustification loop
but not in washing operations. This strategy saves 350 m3/h (15%
of the total) of water and thermal energy savings are also expected.
As the condensate temperature is higher than the temperature of
the hot and warm water, the injection of steam in the mixer following the pre-washer could be reduced. Similarly, the temperature increase of the washing filtrate, which is partly reused in the
digester, could decrease the digester steam consumption.
Reuse of whitewater
Whitewater reutilization in the last stage of the bleaching section is a common practice, but for some washing sequences it
can also be done in the other stages [1]. The strategy proposed
here (Fig. 2) is to reuse whitewater in the washer of the second
bleaching stage, saving 110 m3/h (5% of the total). The reduction of hot water consumption is directly proportional to the low
pressure steam utilization, as steam is used to attain the hot water
temperature (71°C).
Reuse of bleaching filtrate
The studied mill has already accomplished a certain level of
closure in the bleaching plant. However, its effluent production of 32 m3/adt is above the Canadian median (28.4 m3/adt)
[1], which leaves room for improvement. For example, part of
the filtrate from stage 5 is actually reused in stage 3. The new
strategy (Fig. 3) considers the further increase the reutilization by
15 m3/h (1% of the total). It would be necessary to relax the mill
constraints to enhance even more its reutilization. This would
decrease the minimum water consumption and effluent production. However, it would be essential to consider the technical
problems which may arise.
Vacuum pumps sealing water
In the current process configuration the water from the vacuum
pump sealing is used to cool mixed effluents from the process. As
a result of the implementation of the strategies previously proposed in this section, the quantity of effluent will be substantially
reduced, therefore it will be possible to decrease the consumption
of the vacuum pump sealing water. The temperature of the water,
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Fig. 1. Strategy for reutilization of evaporators
condensates.
Fig. 2. Strategy for whitewater reutilization.
Fig. 3. Strategy for bleaching filtrate reutilization.
which must not be above 40°C, is the constraint for the implementation of this strategy. Houle et al. [7] have shown that it is
Fig. 4. Strategy for vacuum pump sealing water
reutilization.
May/June 2010 Pulp & Paper Canada 39
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Energy Efficiency
which must not be above 40°C, is the constraint for the implementaion of this strategy. Houle et al. [7] have shown that it is
possible to reuse up to 64 % of the sealing water effluent, without
affecting the performance of the equipment. Therefore, 70 m3/h
(3% of the total) of water is saved.
Energy implications
The water savings strategies have been introduced in the computer simulation of the studied mill. Their implementation involves
modifications in the treated water network of the process which
can be noted by comparison of Fig. 5a (which is identical to Fig.
2 of Part I) and Fig. 5b.
The use of make up water in the warm water tank and of LP
steam in the hot water tank is no longer needed because of the
condensate reuse. The fresh water preheated and used in the prewashing is now replaced by a direct utilization of condensate from
the evaporators. The strategies for water reuse also reduce the
quantity of warm water required for pulp bleaching and eliminate
the warm water use in the re-caustification loop.
As was expected, the implementation of the strategies also
impacts the energy requirement of the process. The steam consumption is reduced in the bleaching plant, in the digester, in the
de-aerator and in the hot water production. Therefore, less steam
needs to be produced and this represents a reduction of 14 MW
of the total heat consumption of the mill. The water-thermal
composite curves [8] show the reduction of the steam used for
water heating and for de-aeration (Fig. 6). The cooling demand
is also reduced by 13.1 MW, despite an increase of temperature
of the bleaching effluents (+8°C).
The implementation of the water closure strategies has an
effect on the thermal composite curves, as can be seen in Fig. 7.
The minimum heating requirement is decreased by 1.1 MW and
the minimum cooling requirement is increased by 6.9 MW. The
reason for those effects is that in the current configuration all the
cooling water used in the condensers of the evaporator trains is
reused in the process. This is no longer the case after the implementation of these strategies.
The reduction of the process heating demand results from
the elimination of the pinch rules violations. These violations are
due to the use of steam below the pinch point for the hot water
production, in the pre-washer steam mixer, and in the de-aerator.
After the implementation of the water reduction strategies the
pinch point is lowered from 71 to 57 °C because the energy content of the effluents at a low temperature below the pinch point
is significantly reduced.
Consequently, if the implementation of energy upgrading or
conversion devices (heat pumps, trigeneration units) is envisaged,
their installation may be affected, as the temperature of the energy
available will be reduced.
After the implementation of the proposed strategies less
energy will be utilized in the process. However, other options
for improved energy efficiency may not be feasible anymore,
therefore, a trade-off analysis is needed in order to evaluate all the
thermal effects of the water closure.
Trade-off analysis
A global water-energy scenario which includes all the strategies
identified is compared with the results of an optimized heat
40 Pulp & Paper Canada May/June 2010
Fig. 5a. Treated water network before implementation of
water saving strategies.
Fig. 5b. Treated water network after implementation of
water saving strategies.
exchanger network (HEN) designed to improve the internal heat
recovery within the process [9]. The HX-NET software was used
to develop the HEN. The two possibilities have been developed
independently and economically assessed using the simple pay
back time (PBT) and the following prices:
• Steam produced with bunker oil: 25 $/t;
• Fresh water: 0.065$/m3;
• Effluent treatment: 0.1$/m3.
The water-energy scenario saves water and reduces the efflupulpandpapercanada.com
PEER REVIEWED
ent production. As a result the low temperature energy available
in the process effluents could be reused. The HEN scenario saves
more energy as it enhances the internal heat recovery within the
process. The disadvantage of both scenarios is mainly linked to
the process modifications to be introduced. On the other hand,
water system closure is not compatible with those options in the
HEN design that are associated with the hot water production.
The economic analysis shows that both scenarios are feasible,
although the water-energy scenario has a lower investment and
payback time; the HEN scenario is more interesting in a longterm perspective.
It is evident that both scenarios are compatible, once their
interactions have been elucidated. It would be possible to implement them at the same time, but further studies are needed to
define the best possible design of water and energy systems in
the mill.
Fig. 6. Effect of the water strategies in water-thermal composite curves.
CONCLUSIONS
Four strategies for water reutilization in a kraft pulp mill have
been identified and their energy implications have been studied.
The proposed strategies save 540 m3/h (24% of the total) of
water, 14 MW of steam, and 13.1 MW of cooling demand. Their
energy implications go beyond effluent temperature increase and
energy savings, affecting the complete thermal balance of the
process, the MER and the pinch point temperature. The water
closure strategies must be the core of any process energy optimization project in order to evaluate all the aspects that could be
affected and, must be considered in decisions for the implementation in a mill of an optimal energy efficiency strategy.
ACKNOWLEDGEMENTS
This work was supported by a grant from the R&D Cooperative program of the National Science and Engineering Research
Council of Canada. The industrial partners to this project and
most specially the mill which supplied the data are gratefully
thanked. E. Mateos-Espejel receives financial support from the
Mexican Council of Science and Technology as a PhD candidate.
Thanks are also given to Dr. L. Savulescu (NRCan-CTECVarennes) for her guidance in the water-energy analysis.
LITERATURE
1. Towers, M., Turner, P.A., Survey of bleach plant washing practices in Canadian mills,
Pulp & Paper Canada 99(7):44-49 (1998).
2. Syberg, O., Swaney, J., Vice, K., Russell, W., Water reduction strategies for existing
bleach plants, Pulp & Paper Canada 99(7):80-83 (1998).
3. Savulescu, L., Poulin, B., Hammache, A., Bedard, S., Gennaoui, S., Water and energy
savings at a kraft paperboard mill using process integration, Pulp and Paper Canada
106(9): 29-31 (2005).
4. Syberg, O., Barynin, J., Impact of water reduction on kraft mill heat balance, in Proceed.
TAPPI Int. Eng. Conf., Part 3. Miami, FL. TAPPI Press, Norcross, GA. (1998).
5. Towers, M., Energy reduction at a kraft mill: Examining the effects of process integration,
benchmarking, and water reduction, in Proceed. TAPPI Fall Tech. Conf. Atlanta, GA.
TAPPI Press, Norcross, GA. (2004).
6. Gullichsen, J., Fogelholm, C.-J., Papermaking Science and Technology, Book 6. Published by the Finnish Paper Eng. Assoc. and TAPPI (1999).
7. Houle, J.F., Brousseau, Y., Dorica, J., Paris, J., Reduction of fresh water consumption for
process and non-process uses in an integrated newsprint mill, in Proceed. of the 84th Annual
Meeting of the Technical Section of CPPA, Part A. Montreal, QC (1998).
8. Alva-Argaez, A., Savulescu, L., Poulin, B., A process integration-based decision support
system for the identification of water and energy efficiency improvements in the pulp and paper
industry, in Reprints 93rd Annual Meet. of PAPTAC, Book C. Montreal, QC (2007).
9. Lutz, E., Identification and analysis of energy saving projects in a Kraft mill, in Reprints
94rd Annual Meet. of PAPTAC, Book C. Montreal, QC (2008).
pulpandpapercanada.com
Fig. 7. Energy implications in the composite curves of the
complete process.
Table I. Summary of cost analysis for water-energy and
HEN scenarios
Scenario
Water-energy
HEN
Heating
saved (MW)
Cooling
saved (MW)
Invest. (M$)
PBT
(a)
14
30
13.1
23
0.2
4
0.1
0.4
Résumé : Une nouvelle méthodologie systématique a été développée pour étudier les interactions entre l’eau et l’énergie dans le procédé
kraft et elle a été appliquée à une usine en opération. La méthodologie
qui peut être utilisée pour identifier des stratégies appropriées afin de
réduire la consommation de l’eau et qui considère aussi leurs impacts
sur l’efficacité thermique du procédé est décrite dans la partie I de ce
travail. Une étude de cas a été développée et les résultats sont présentés dans la partie II. Quatre stratégies qui réduisent simultanément la
consommation d’eau et de vapeur, ainsi que les besoins de refroidissement sont analysées. Le potentiel des économies est significatif.
Keywords: Kraft process, energy efficiency, water
system closure, Pinch Analysis, Water-Energy
Reference: Mateos-Espejel, E., Marinova, M., Bararpour,
S., Paris, J. Energy Implications of Water Reduction Strategies in Kraft
Process. Part II: Results, Pulp & Paper Canada 111(3): T56-T59 (May/June
2010). Paper presented at the 94th Annual Meeting in Montreal, February
5-7, 2008. Not to be reproduced without permission of PAPTAC. Manuscript received December 17, 2007. Revised manuscript approved for publication by the Review Panel March 29, 2010.
May/June 2010 Pulp & Paper Canada 41
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