design of discontinuous water-using systems with a graphical method 2011

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Chemical Engineering Journal 172 (2011) 799–810

Contents lists available at ScienceDirect

Chemical Engineering Journal

j ournal homepage : www.elsevier .com/ locate /ce j

Design of discontinuous water-using systems with a graphical method

Jin-Kuk Kim∗

Department of Chemical Engineering, Hanyang University, 17, Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea

a r t i c l e i n f o

Article history:

Received 3 March 2011

Received in revised form 21 June 2011

Accepted 22 June 2011

Keywords:

Water pinch analysis

Water reuse

Water minimisation

Discontinuous water use

a b s t r a c t

A systematic approach using the graphical representation of water use and its system-widemanipulation

is proposed for minimising freshwater and wastewater generation for discontinuous water systems.

Design interactions of time-dependant water reuse and implications of storage tank in the network

design have been fully addressed, and a new targeting and design method has been proposed to provideguidelines for achieving minimum freshwater requirements and to design the configuration of water

re-use networks. Design complexity associated with simultaneous consideration of concentration and

time constraints has been effectively dealt with in the proposed design method, which clearly provides

benefits by reducing water consumptions and maintaining sustainable water usage.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Waterisoneofthekeyresourcesinmanufacturingasitiswidely

used as heat transfer media, for example, cooling water and steam,

and mass transfer agents, for example, water for washing, strip-

ping steam. The efficient use of freshwater with less generation

of wastewater to the environment has been regarded one of the

priorities in process industries, in order to minimise costs associ-

ated with water use and its discharge, as well as to ensure much

reduced environmental burden to the society. Minimising water

consumption and, consequently, wastewater generation from pro-

cess industries can be made through various measures, including

process changes, water reuse and water recycling.

The way to use water is dependent on how the process

is designed and operated, and therefore water requirement for

the process can be reduced by adopting appropriate change of

processing mechanism or design. This might be regarded as a

straightforward option to be considered for water reductions, as

process changes for a particular process canbe made in isolationto

other parts of theplant. However, process changes canbe problem-

atic due to practical and engineering constraints, and may requireheavy capital expenditure to accommodate the change.

On the other hand, water reuse and recycling has been widely

considered and implemented in process industries as a practical

method for water conservation. As long as the reused or repro-

duced water does not degrade the quality of product and process

performance, reusing water with/withoutregenerationcan be seen

as a cost-effective and practical approach. However, it is not an

∗ Tel.: +82 2 2220 2331.

E-mail address: [email protected]

easy task to identify the most appropriate manner of water reuse

andrecycling, especially, in industrial practice,where a large num-

ber of water-using operations are involved with complex design

interactions among them.

In order to investigate water-using systems in a holistic way, a

system-wide analysis has been proposed by Wang and Smith [1]

who introduced the graphical definition of water usage in pro-

cess operations and provided a design procedure to manipulate

graphically the profiles of water usage for obtaining minimum

water requirements. This design framework also includes a design

method of water-using networks which achieve the target. One

of the significant benefits from using their method is to screen

all the available system-wide opportunities for water re-use, as

well as to provide conceptual understanding and insights of water

network design problem. Water regeneration and recycling had

been also investigated by Wang and Smith [1], which allows fur-

ther reductions of freshwater consumption at theexpense of water

regeneration cost. This graphical design method had been further

extended to cover the wide range of design problems in water

systems, including treatment systems using distributed treatment

concepts, design with water-using operations and treatment sys-tems together, water reuse for multi-component systems and

network design with process or engineering constraints [2–7].

There are limitations associatedwith graphical design methods.

For example, it is not straightforward to design large-size indus-

trialwatersystems in which multi-component analysisis necessary

and complex design interactions between various water-using and

water-treating units are to be systematically investigated. These

practical and computational issues have been addressed by adopt-

ing mathematical programming and optimisation techniques, with

which practical constraints canbe readily added, as wellas rigorous

economic trade-off can be effectively carried out [8–12].

1385-8947/$ – seefrontmatter © 2011 Elsevier B.V. All rights reserved.

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J.-K. Kim / Chemical Engineering Journal 172 (2011) 799–810 801

in the design. Recent review papers by Gouws et al. [21] has

provided the detailed overview and summary of contribution

made by process integration and system engineering communities

in the area of water system studies in batch and semi-batch

processes.

Although the automated design method has contributed by

providing a decision-support tool for water and wastewater min-

imisation in discontinuous processes, it is still required to fully

gainthe fundamental understanding of time-dependency on water

reuseand its design implicationsfor water network,whichwill help

engineers to implement water reuse in various industrial practices.

Therefore, the development of a graphical design methodology has

been aimed at providing systematic targeting and design tools for

the use and reuse of water when concentration and timing infor-

mation has to be simultaneously considered in a single design

framework. Drawbacks identified from previous studies of Wang

and Smith [13] and Majozi et al. [14] have been addressed as a

new design method proposed in this paper which considers those

issues systematically. This includes that the requirement of stor-

age capacity and its interactions in the water network design is

fully considered; water flowrate for each operation is notfixed, but

varied when targeting and designing of water-using operations are

made; water reuse is considered for the operation as long as the

concentration constraints are met.The paper is organised to explain a new targeting method

first, which is then followed by a novel design procedure for

the water network. Three case studies are illustrated to demon-

strate the effectiveness and robustness of the developed design

method, and the limitation of the developed method has been also

discussed.

2. New designmethod: targeting

The proposed design methodology has two steps: targeting

and network design. The targeting is to represent time-dependant

water-using operations and their water use in a unified and inte-

grated format, to collectively investigate overall characteristics of

time-dependant water use and re-use, and to obtain minimumwater requirement for the water systems.

The newdesignmethodpresentedin thispaper adopts thelimit-

ingwater flowprofile conceptused in Kim andSmith’soptimisation

study [19] f or discontinuous water systems, as shown in Fig. 3.

For continuous water processes, the limiting water conditions are

defined with the rate of mass load to be transferred to the water

stream, and maximum allowable water concentration for inlet and

outlet, while the limiting water flow profile, for time-dependant

water systems, is based on overall mass load to be transferred to

the water stream, which inherently includes timing information by

multiplying time duration with the rate of mass load. It should be

Fig. 3. Limiting water flow profile [19].

noted that any profiles below and at the limiting water flow pro-

file will be considered, which ensures that water flowrate to be

achieved in the design is not fixed, but varied, subject to targeting

and design procedure. Another point to be made is that the limit-

ing water flow profile is separately defined for each time interval

for all the water-using operations, which allows the identification

of reuse potentials within the same time interval as well as across

different time intervals.

In this study, the minimum water requirement or maximum

water re-use has been targeted in two parts, which provides

the boundary of target. A single figure for the target would be

ideal, however, identifying a unique target for minimum water

requirement using graphical representation and its manipulation

is not realistic, because both concentration and timing information

should be simultaneously taken into account. Using mathematical

programming and optimisation techniques is clearly an attractivealternative option for targeting purposes. But, this automated way

had been previously attempted [19] andit is aimed in this study at

creating conceptual understanding of design problems and provid-

ing fundamental knowledge for how time-dependant water reuse

should be considered in the design.

First, the minimum water requirement without considering

time constraints is identified, which is termed as “lower bound

targeting (LBT)”. LBT provides a theoretical minimum water

requirement, and the water flowrate obtained from LBT may not

always be achieved through the network design, due to time-

constraints. However, this LBT procedure provides valuable insight

200

4

400

40.5

100

C[ppm]

M[kg]

20.5

200

P1

8

400

30

100

C[ppm]

M[kg]

2.5

P2

P3 Pinch

Composite curve

Minimum flow

Lower boundtarget: 102.5 t

(a) (b)

Fig. 4. Example 1 – lower bound targeting.



802 J.-K. Kim / Chemical Engineering Journal 172 (2011) 799–810

Table 1

Example 1 – limiting water data [13].

Process Limiting

concentration

(ppm)

Limiting

flowrate (t/h)

Time (h)

C in C out T s T f

P1 100 400 100 0.5 1.5

P2 0 200 80 0 0.5

P3 100 200 50 0.5 1.0

of discontinuous water systems as how good the final designis can

be checked against the lower bound target.

The second part of the targeting approach is to consider time

constraints into the investigation of water re-use potential, but the

maximum reuse of water is only considered within the same time

interval, and the reuse possibility from previous time interval to

next through storage tank is not permitted. This procedure pro-

vides another target which canbe further reduced by implementing

water re-use between different time intervals. The identifiedupper

bound target results from conservative application of water re-use

as the multi-period nature of water re-use has not been fully con-

sidered. This upper bound targeting (UBT) method allows users toavoid possible poor design withmore freshwater flowrate required

than target from UBT.

The LBT and UBT will be explained with the example 1, as given

in Table 1, which includes limiting water data and time informa-

tionfor three water-using operations. It is assumed thatfreshwater

is available at the concentration of 0 ppm. The limiting water data

conditions are expressed in terms of limiting water flow profile,

based on limiting concentration and limiting flow. In LBT, the first

step of theprocedure is to draw individual limiting water flowpro-

files in the graph as shown in Fig. 4(a), and the single profile of

composite curve is created by combining three individual profiles

for each concentration interval as shown in Fig. 4(b). The manipu-

lation for creatinga single composite curve from individual profiles

follows the same method developed by Wang and Smith [1]. How-ever, the method in this paper is based on limiting water flow

profiles with which time duration of water-using operation has

been embedded, and, consequently, used for targeting of discon-

tinuous water-usingprocesses. The minimum flowtarget from LBT

is calculated to be 102.5t, which can be obtained from a simple

mass balance for the pinch point.

There are two steps for UBT, as illustrated in Figs. 5 and 6. The

first step of UBT is to divide time intervals based on the limiting

data, and to put individual water-using elements specific to the

particular time interval in the separate concentration and mass

load diagram. In Fig. 5, three time intervals (four boundaries based

on timing information) are first made, and individual operations

are represented in the each time interval, for example, the limiting

water flow profile of P2 is drawn in the first interval [0–0.5h], and

profiles of P3 and P1 are given in the second interval [0.5–1h]. The

water use of P1 is splitto twoparts, according to timing information,

which are put in the second and third time interval, respectively.

The second step of UBT is to analyse water re-use opportuni-

ties in each time interval and to find minimum freshwater flowrate

required for each interval separately. To screen maximum water

re-use potentials, it is required to create the limiting water flow

profiles and to obtain minimum freshwater flow separately by fol-

lowing the same approach used in LBT. The minimum freshwater

needed for each time interval are 40t for the first interval, 43.75 t

for the second interval and 37.5 t for the third interval, which gives

121.25t of freshwater flow as a upper bound target.

Upper bound target from UBT and lower bound target from

LBT provides the range of possible minimum flow which can be

achieved through network design explained in the following sec-

tion. Water reuse is only considered within a single batch orproduction campaign, which means that water re-use from pre-

vious batch to next batch is not allowed.

3. New designmethod: network design

The designof the network of water-using operations is made by

following a few design rules as below:

• Design rule 1: the network design is carried out in sequence from

the first time interval to the following interval, which allows full

utilisation of available water sources from the previous interval

to the next interval.

• Designrule2: reusing as much water from available watersourcesfrom previous time interval as possible is preferred as long as the

intake of freshwater can be reduced. Also, reusing wastewater

source(s) with the lowest concentration, other than freshwater,

is tobe fullyutilisedfirst. Thesewillminimisethe overallintake of

freshwater for the whole network. However, this strategy results

in the introduction of storage tanks for enabling water reuse in

Concentration

P2

40 t

200 ppm

8 kg

Mass load(M)

100 ppm

Mass load(M)

P1P3

Concentration

2.5 kg

200 ppm

400 ppm

25 t 50 t

17.5 kg

Concentration

Mass load(M)

P1

100 ppm

400 ppm

50 t

15 kg

h5.0h0 h5.1h1

Fig. 5. Example 1 – upper bound targeting (I).




Concentration

P2

40 t

200 ppm

8 kg

Mass load(M)

100 ppm

Mass load(M)

Concentration

200 ppm

400 ppm

43.75 t

17.5 kg

Designregion

Concentration

Mass load(M)

P1

100 ppm

400 ppm

50 t

15 kg

0.5 h h5.1h10 h

Compositecurve(P1 + P3)

Designregion

75 t

37.5 t

Upper bound target: 121.25 t (= 40 t + 43.75 t + 37.5 t)

Fig. 6. Example 1 – upper bound targeting (II).

different time intervals. As demonstrated by Kim and Smith [19],

the cost of storage tank is relatively cheap, and it is beneficial to

save further water requirements.• Design rule 3: once the selection of water source(s) to be used in

the particular timeinterval, the maximum re-use between opera-

tions is sought and the corresponding water network is designed

by following water main method developed by Kuo and Smith

[5]. The details of water main method are not explained in this

paper, butcan be found elsewhere, including Smith [22], asithad

been well established and widely applied.• Design rule 4: when more than one re-use possibilities exist, the

matching between water sources and sinks should be made such

that, first, the design to minimise freshwater consumption for

the particular time interval is selected. If there is no differencein freshwater requirements, then, the number of connections

between sources and sinks should be minimised. This strategy

helps to reduce piping cost and design complexities, although

piping cost is heavily dependent on hydraulic design of piping

systems and physical distances between water sources and sinks.

The design procedure for water network design is explained

with example 1. Based on the design rule 1, the design of first time

intervalis carried outfirst, whichthen followed bysecond and third

time interval. Fig. 7 shows the first time interval in which only one

water-using operation is employed. The freshwater flow required

for the first interval is 40t, and correspondingly, wastewater gen-

erated in the end of time interval is 40t with the concentration of

200ppm.For the second time interval, there are two water sources,

namely, 40t of wastewater generatedfrom the previoustime inter-

val at 200ppm (shown as intermediate water (I) in Fig. 8) and

freshwater at 0 ppm. By following design rule 2, wastewater from

intermediate water (I) is re-used, but the addition of freshwater

is required, because maximum inlet concentration allowed in this

time interval is 100 ppm. By mixing two sources of water equally,

100 ppm of water is generated and supplied to this time interval,

and the required freshwater is 37.5 t. From intermediate water (I),

37.5t of wastewater rare used, while 2.5 t of unused water will be

remained to be used for the next available time interval. The net-

work configuration of this time interval is shown in Fig. 8 suchthat

no re-use of water between operations are envisaged, due to the

same inlet maximum concentration allowed, by following design

rule 3. Water consumed in P1 and P3 can be used for the next time

interval as water sources at 200 ppm and 400ppm, respectively.

For the time interval between 1 h and 1.5 h, there are four water

sources available which include one from the first time interval,

two from the second time interval and freshwater (Fig. 9). The

200 ppm water source is selected, rather than 400 ppm the water

source, by following design rule 2, which will minimise the use of

freshwater. As the maximum inlet concentration acceptable for P1

is 100ppm, equal amounts of water from freshwater source and

200 ppm of water sources are combined to supply 50t to P1. There

are two possibilities for taking 25t of 200 ppmwater: one option is

to useintermediate water (II-b) only;the other optionis to useboth

sources of intermediate water (I-a) and (II-b). By following design

rule4, the designforthistimeintervalinclude only onewater re-usefrom intermediate water (II-b) as shown in Fig. 9.

Final water network design is obtained by simply assembling

the design of each time intervals and representing it in a single

network, as shown in Fig. 10. Overall freshwater requirement is

102.5, which achieves the lower bound target through the design,

Fig. 7. Example 1 – design: time interval [0–0.5h].




Table 2

Example 1 – results comparison.

Method Water

consumption (t)

Storage tank (t)

No water reuse 127.5 0

New conceptual design 102.5 62.5

Mathematical optimization [19] 102.5 37.5

while62.5t of storage capacityis necessaryto support water re-usebetween timeintervals. The result is compared withother methods

as shown in Table 2.

When a conventional method based on once-through water

usage is applied, 127.5 t of freshwater is consumed without inter-

mediate storage required. With the new design method, water

consumption is reduced by 19.6% at the expense of storage capac-

ity. Compared to the results obtained from an automated design

method [19], as well as to the design obtained with the aid of WCA

(Water Cascade Analysis) [15], the requirement of storage tanks

that results from the developed design method is bigger than opti-

mal although the required freshwater flowrate is the same.

4. Application of new designmethod

The new design methodology presented in this paper is applied

to a second example. Details of example 2 are given in Table 3,

which had been used in the study of Kim and Smith [16], and it

is assumed that the freshwater source is available at 0 ppm. The

procedure of LBT, gives 185t of minimum freshwater requirement,

while Fig. 11 shows the UBT, resulting in 211.25t of upper bound

target.

The first step of the design is to achieve the target between the

two boundaries obtainedfromUBT andLBT,so thedesignof the first

time interval [0–1h] is carried out. 20t of freshwater is consumed

in P1 andits effluent is availablefor re-use in thenext time interval.

In thenext time interval,the reuse of water from theintermedi-

ate water (I)does not contribute reducing theamount of freshwater

required. This is because water source for the reuse is available at100ppm, andthe maximum inletconcentration to the liming water

flow composite curve is also 100 ppm (Fig. 12). For example, if 20t

at 100ppm is fully reused in this time interval,freshwater required

is 100t and 120t of the water is supplied to this time interval at

16.7ppm. When no water from the intermediate water (I) is re-

used, 100 t of freshwater is still required. Therefore, by followingdesign rule 4, it is decided not to re-use water from the intermedi-

ate water (I). The rest of the designis based on design rule 3, which

identifies water re-use between P2 and P4 within this time inter-

val. Twoadditional water sources at 100 ppmand800 ppm, become

available from the result of water main method.

For the third time interval [3–3.5h] there are many potential

options for water reuse, due to three available water sources that

result from the design of previous two time intervals, as shown in

Fig. 13. One of the possibilities is to mix equal amounts of 35t from

water sources at 100 ppm and freshwater, which is then supplied

at 50ppm. From design rule 4, it is not beneficial to mix freshwa-

ter with the contaminated water in this case, as the same 35t of

freshwater is needed even if only freshwater is supplied. Another

possibility is to utilise water from the intermediate (II-b) source,

which requires more than 35t of freshwater. Therefore, freshwa-

ter is taken as a single water source for time interval [3–3.5h]. The

wastewater from P2 is reused for P3 after mixing with freshwater,

by following design rule 3. It should be noted that consideration

made in the second and third time intervals also contributes to

avoid unnecessary introduction of storage tanks.

Thefinaltimeinterval between 3.5h and5 h introduces themix-

ing between freshwater and intermediate (II-a) (Fig. 14). The four

sub-networks of water systems are nowcombined, as illustrated in

Fig. 15. The overall freshwater required for the final design is 185 t

with 30t of storage capacity, which satisfies the lower bound tar-

get. Table 4 illustrates the benefits of using the developed design

method, which compares with optimal results published by Kim

and Smith [19]. Against a design with no water reuse, considerable

cost saving with the reduction of freshwater demand is achieved.

The design identified from the developed graphical method coin-

cides with the design obtained from mathematical optimisation

when costs of freshwater and storage tanks are considered in theobjective function. When compared the result from the proposed

method in this paper to the network design in which piping cost is

simultaneously considered withfreshwater and storage tank costs,

it gives conceptual insightsthat pipingcostplaysan importantpart

in the water network design and the developed method is limited

to deal with more rigorous economic trade-off. However, Table 4

clearly illustrates the proposed design method is able to provide

significant savings for discontinuous water systems by systemati-

cally implementing water re-use under time constraints, although

optimal or near-optimal solutions may not be achieved.

5. Discussion onnew designmethod

Two examples illustrated in the previous sectionsdemonstrated

the applicability and usefulness of the new design method. How-

ever, there are a few issues to be addressed in terms of the

methodological limitation associated with the targeting concept

and design procedure presented in this paper.

As commented in the previous sections,the developedtargeting

provides the feasible range of possible target freshwater flowrate,

and it has been attempted to achieve as close to the lower bound

target as possible during the design. In examples 1 and 2, the net-

work design is based on the lower bound target flowrate. This may

not be the case for other design problems, which will be explained

with example 3. Table 5 shows the water data of four water-using

operations, of which water concentration and flow condition is

Table 3

Example 2 – limiting water data [19].

Process Limiting c oncentration

(ppm)

Limiting flowrate (t/h) Limiting flow (t) Time (h)

C in C out T s T f

P1 0 100 20 20 0.0 1.0

P2 50 100 100 2001.0 3.0

P4 400 800 10 20

P2 50 100 100 503.0 3.5

P3 50 800 40 20

P3 50 800 40 60 3.5 5.0

P2andP2 are the sameoperation; P3 and P3 arethe same operation.




100

3.5

400

800

35 t

70 t

17.5

50

C[ppm]

M[kg]

Compositecurve

(P2 + P3)

100

Designregion

10

400

800

18

50

C[ppm]

M[kg]

Compositecurve

(P2 + P4)

200 t

100 t

0 h 1 h 3 h

C[ppm]

P120 t

100

2M

[kg]

3.5 h 5 h

P3

800

56.25 t

45

50

C[ppm]

M[kg]

60 t

Upper bound target: 211.25 t (= 20 t + 100 t + 35 t + 56.25 t)

Fig. 11. Example 2 – upper bound targeting.

Table 4

Example 2 – results comparison.

Method No water reuse New graphical

design method

Mathematical optimization [19]a

Optimal

design 1

Optimal

design 2

Water Consumption (t) 230 185 185 190.6

Cost (k£ year−1) 368.0 296.0 296.0 305.0

Storage tank Capacity (t) – 30 30 45

Cost (k£ year−1) – 7.3 7.3 13.6

Piping cost (t) 150.1 170.8 170.8 123.6

Overall cost (k£ year−1) 518.1 (100%) 474.1 (91.5%) 474.1 (91.5%) 442.2 (85.3%)

a Design 1: objective function considersfreshwater and storage tank costs only.

Design 2: objective function considersfreshwater,piping and storage tank costs.

Fig. 12. Example 2 – design: time interval [1–3 h].




Fig. 15. Example 2 – final design.

Step 1: the design of water network is first carried out, assumingthat no water can be reused between different batches as it has

been done in the three previous examples.

Step 2: all the possible water sources which are to be discharged

from the current batch production, but can be reused in the next

batch production, are identified.

Step 3: the water network design for the next batch production is

repeated, but water sources identified from step 2 are included in

thenetworkdesign.Design rulesexplained in Section 3 are applied

to screen all the possible water sources and sinks, and select addi-

tional match which can further reduce freshwater consumption

for the next batch production.

Step 4: steps 2 and 3 are repeated until no improvements have

been achieved.

The design method extended for designing repeated batch pro-

cesses is now explained with example 3. The result of step 1 is

Fig. 17, which is the design for a single batch production and water

reuse is not considered beyond the single batch cycle. From step

2, water sources available for the next batch are identified, whichinclude discharged water of P3 in the first time interval, that of P2

and P3 in the secondtimeinterval, that of P1, P2 and P4 inthe third

time interval and that of P1 in the fourth interval. When additional

water sources are included in the design step 3, wastewater dis-

charged from P3 and P4 is not chosen for the water reuse from the

previous batch to the next batch, as 800ppm of wastewater does

notcontributefor reducingthe freshwater consumption.Water dis-

charged fromP1 or P2can be reused fromthe previous batch to the

next batch by mixingit with freshwater. Water reuse from P1 in the

third time interval [2.5–4.5h] of the previous batch to P3 in the first

time interval [0–1.5h] of the next batch is selected, as this match

reduces design complexities, compared to otherpotential matches.

IfP2inthethirdtimeintervalisselectedforthewaterreuseforP3in

thenext batch, water stream from P3 is split to three streams, lead-ingto a complex splittingjunction.Afterone iterationof steps 2 and

3, no improvement wasobservedand thewater network is finalised

as illustrated in Fig. 18. The final water network obtained for the

repeated batch production consumes 250 t of freshwater, which

Compositecurve

(P1 + P2 + P4)

100

2

400

800

140 t

22

50

C[ppm]

M[kg]

14

0 h 1.5 h 2.5 h 4.5 h 5 h

P3

800

56.25 t

45

50

C[ppm]

M[kg]

60 t

100

Designregion

10

400

800

18

50

C[ppm]

M[kg]

Compositecurve

(P2 + P3)

70 t

140 t

C[ppm]

P1

10 t

100

1M

[kg]

Uper bound target: 276.25 t (= 56.25 t + 70 t + 140 t + 10 t)

Fig. 16. Example 3 – upper bound targeting.




Fig. 17. Example 3 – final design.

corresponds to a lower bound target. This illustrates that thedesign

obtained in Fig. 18 achieves theoretical minimum water require-

ment for the discontinuous water systems by allowing repeated

water reuse between different batch productions.

The methodologypresented in this paper is based on the graph-

ical representation of water usage of water-using operation and

its manipulation for identifying the target and designing the net-

work configuration, and therefore, all the common drawbacks

documented for pinch analysis in the process engineering commu-

nities are also observed. This includes difficulties related to when

the methodology is applied for complex water network problems,

for example, a large number of water-using operations and multi-

contaminant systems, as well as when it is required to address

rigorous economic trade-offs.

However, it should be noted that the proposed method clearly

enables to reduce water consumptions considerably in the discon-

tinuous water systems by giving systematic design guidelines for

engineers, as well as provides methodological advantages by gain-

ing fundamental understanding of time-dependant water use and

reuse. Also, the method explained here helps users to consolidate

their confidence in applying process integration technologies to

discontinuous systems in practice.

Fig. 18. Example 3 – final design forrepeatedbatch processes.




6. Conclusions

A new systematic targeting and design method for the max-

imising water re-use in the discontinuous water systems has been

developed. First, the time-dependant reusability of water has been

screened by obtaining upper bound target and lower bound target.

Next, the design is carried out to achieve the lower bound target,

by systematically identifying water reuse within and beyond the

particular time interval, and the requirement of storage tank for

reusing water in the different time intervals. Three examples had

been used for illustrating the design methodology developed and

demonstrating the robustness and applicability of the method.

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design of discontinuous water-using systems with a graphical method 2011

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