Flowrate Targeting Algorithm for Interplant Resource Conservation

6456

Ind. Eng. Chem. Res. 2010, 49, 6456–6468

Flowrate Targeting Algorithm for Interplant Resource Conservation Network. Part 2: Assisted Integration Scheme Irene M. L. Chew* and Dominic C. Y. Foo* Department of Chemical and EnVironmental Engineering, UniVersity of Nottingham Malaysia, Broga Road, 43500 Semenyih, Selangor, Malaysia

Raymond R. Tan* Center for Engineering and Sustainable DeVelopment Research, De La Salle UniVersity, Manila, 2401 Taft AVenue, 1004 Manila, Philippines

Part 1 of the series (Chew, I. M. L.; Foo, D. C. Y.; Ng, D. K. S.; Tan, R. R. Flowrate Targeting Algorithm for Interplant Resource Conservation Network. Part 1: Unassisted Integration Scheme. Ind. Eng. Chem. Res. DOI: 10.1021/ie901802m.) proposes a systematic three-step targeting algorithm for unassisted integration scheme for interplant resource conserVation network (IPRCN), where cross-plant streams within the pinch region can be used to achieve minimum resource flow rate targets. However, the unassisted scheme does not hold true for all cases. Part 2 of the series explores additional material recovery to be realized by sending cross-plant streams outside the pinch region. This is known as the assisted integration scheme. Appropriate identification of waste streams as the cross-plant streams is an important step in locating the minimum flow rate targets for these cases. The effect of pinch shifting and the generation of new waste streams are also investigated. Introduction 1

In part 1 of the series, a systematic three-step targeting algorithm for the interplant resource conserVation network (IPRCN) is presented. The targeting algorithm determines the minimum fresh resource and waste flow rates for an IPRCN of unassisted integration problem. In essence, the minimum flow rates of the unassisted type IPRCN are achieved by transferring cross-plant streams within the pinch region bounded by the upper and lower pinch points of the participating networks. For four cases shown in part 1 of the series,1 the proposed algorithm is able to locate the actual minimum flow rates of the IPRCN, which is determined by treating all sinks and sources of the individual networks as if they belong to a single network. However, the single-network targets cannot always be achieved. In particular, if a problem is of the assisted type, targeting using the unassisted integration scheme generates overall flow rates that are higher than that of the single network targets. Hence, assisted integration scheme is outlined in this part of the series to determine the actual minimum flow rates for such problems. Finally, the work is also extended to total resource networks, in which source interception and waste treatment are analyzed.

gives nonoptimal material recovery, since cross-plant stream transfer outside the pinch region has not been included. The simplified cascade diagram for an assisted integration scheme is shown in Figure 1, adapted from part 1 of the series.1 As shown, the transfer of cross-plant streams is observed both within (intervals between m ) 1 - 4) and outside the pinch region (interval between m ) 4 - n). In the assisted integration scheme, the cross-plant streams are allowed to be transferred from a network with lower quality pinch (network B) to a network with higher quality pinch (network A), which is not allowed in the unassisted integration scheme.1 As shown in Figure 1a, cross-plant streams which are transferred from the higher purity region (HPR) of the network with a lower purity pinch (network B in this case) are designated as the higher purity assisting flow rate (HPAF); while those from the lower purity region (LPR) are known as the lower purity assisting flow rate (LPAF). As targeting with the assisted integration scheme

Principle of Assisted Integration Scheme As mentioned before, an unassisted integration scheme takes place when maximum material recovery can be achieved by transferring cross-plant streams within the pinch region, with the latter being defined by the upper and lower pinch points of the participating networks. In contrast, an assisted integration scheme involves the transfer of cross-plant streams both within and outside the pinch region to achieve the actual minimum flow rate targets. Therefore, when an IPRCN case is of the assisted type, targeting with an unassisted integration scheme * To whom correspondence should be addressed. Tel.: +60-3-89248130. E-mail: [email protected] (I.M.L.C.); dominic.foo@ nottingham.edu.my (D.C.Y.F.); [email protected] (R.R.T.).

Figure 1. Principles of the assisted integration scheme.

10.1021/ie901804z  2010 American Chemical Society Published on Web 06/28/2010

Ind. Eng. Chem. Res., Vol. 49, No. 14, 2010

6457

Table 1. Limiting Water Data for Example 1 flow rate FSKj (t/h)

concentration CSKj (ppm)

A

SK1 SK2 SK3 SK4 SK5

20.00 66.67 100.00 41.67 10.00

0 50 50 80 400

B

SK6 SK7 SK8 SK9 SK10

20.00 66.67 15.63 42.86 6.67

C

SK11 SK12 SK13 SK14 SK15

20.00 80.00 50.00 40.00 300.00

Σj FSKj

880.17

water network k

sinks j

sources i

flow rate FSRi (t/h)

concentration CSRi (ppm)

SR1 SR2 SR3 SR4 SR5

20.00 66.67 100.00 41.67 10.00

100 80 100 800 800

0 50 80 100 400

SR6 SR7 SR8 SR9 SR10

20.00 66.67 15.63 42.86 6.67

100 80 400 800 1000

0 25 25 50 100

SR11 SR12 SR13 SR14 SR15

20.00 80.00 50.00 40.00 300.00

100 50 125 800 150

Σi FSRi

880.17

Table 2. Limiting Data with Unassisted Integration Scheme for Example 1 water network k

sinks j

flow rate FSKj (t/h)


A

sinks in HPR SK1 SK2 SK3 SK4

20.00 66.67 100.00 41.67

0 50 50 80

B

C


sinks in HPR SK11 SK12 SK13 SK14 SK15

20.00 66.67 15.63 42.86

20.00 80.00 50.00 40.00 300.00

0 50 80 100

0 25 25 50 100

involves the transfer of assisting flow rates (i.e., HPAFs and LPAFs) in the IPRCN, additional cross-plant pipeline(s) are needed in addition to those in the unassisted integration scheme. These assisting flow rates will lead to further resource savings as to achieve the single-network targets in the IPRCN. However, with dependence on case-specific resource scarcity considerations, such additional savings may or may not justify the incremental costs for more piping between networks, which can be determined in the detailed design stage. These additional steps are beyond the scope of this work. There are two subcases where the assisted integration scheme could take place, i.e., characterized by the transfer of LPAFs and HPAFs. A literature example taken from Olesen and Polley2 is used for illustration for the former case. For cases of the HPAF, threshold problems3,4 are analyzed. These are shown in examples 1 and 2, respectively. Assisted Integration Scheme by LPAF As proposed in part 1 of the series,1 flow rate targeting is first carried out for each individual network. Upon the deter-

sources i sources in HPR SR1 SR2 SR3 wastewater in LPR W1 W2 W3 sources in HPR SR6 SR7 SR8 wastewater in LPR W4 W5 W6 sources in HPR SR11 SR12 SR13 SR15 wastewater in LPR W7 W8



20.00 66.67 43.33

100 80 100

50.95 37.38 10

100 800 800

20.00 66.67 3.84

100 80 400

5.12 42.86 6.67

400 800 1000

20.00 80.00 50.00 153.33

100 50 125 150

146.67 40.00

150 800

mination of the minimum fresh resource and waste flow rates, as well as the pinch point of the individual network, a separate flow rate targeting is then conducted in the LPR to identify the individual waste streams as the cross-plant stream candidates for IPRCN in each network. In most cases, the cross-plant streams will originate from the pinch-causing source after it fulfills the sink requirement in the LPR with the minimum pinch flow rate. Thus, the latter will not be extracted as part of the cross-plant streams in the proposed unassisted integration scheme.1 This works well for the IPRCN examples of the unassisted type, as shown in part 1 of the series.1 However, as will be shown in the later section, doing this for the case of assisted type problem leads to higher IPRCN flow rates than the single-network targets. In the newly proposed targeting algorithm for the assisted integration scheme by LPAF, the entire pinch-causing source (including the minimum pinch flow rate) is extracted as the cross-plant stream for the overall flow rates reduction of the IPRCN. However, note that extracting the entire pinch-causing source will cause a source deficit in the LPR. Hence, flow rate

6458


Figure 2. Flow rate targeting with MRPD for IPRCN in example 1 with the unassisted integration scheme.

and impurity load balances in the LPR are restored by the LPAF. This is shown using example 1 that follows. Example 1. The first example is an interplant water integration (IPWI) case study taken from Olesen and Polley.2 It consists of three water networks, with the limiting water data shown in Table 1. In the following section, the three-step targeting algorithm for unassisted integration scheme proposed in part 1 of the series1 is adapted for the assisted integration scheme. In particular, step 1 of the targeting algorithm (i.e., identification of the limiting data for IPRCN) is modified for the use in the assisted integration scheme. Note however that steps 2 and 3 of the targeting algorithm remain unchanged. The material recoVery pinch diagram (MRPD)5,6 is used as the flow rate targeting tool in this example. The original limiting water data for example 1 is shown in Table 1,2 while Table 2 summarizes the limiting water data for unassisted integration scheme, identified using the proposed algorithm in part 1 of the series.1 In principle, all sinks and sources in the HPRs are extracted, along with the individual wastewater streams (W1-W8) in the LPRs for all individual networks. If one were to follow the three-step algorithm of the unassisted integration scheme, the overall minimum fresh water and wastewater flow rates across three networks are both identified as 316.26 t/h, with an overall pinch concentration identified at 150 ppm (see Figure 2). After carrying out IPCRN, the individual minimum fresh water flow rates are identified as 98.33, 48.24, and 169.68 t/h for networks A, B, and C, respectively; while their wastewater flow rates are identified as 47.38, 58.48, and 210.39 t/h, respectively (see Figure 3). The pinch concentrations for these networks are also identified, i.e., 100 and 800 for network A (indicating this to be a doublepinch problem), 400 and 150 ppm for networks B and C, respectively. Figure 4 shows the IPRCN that achieves the overall minimum flow rate targets for the unassisted integration scheme, which is essentially the same as that determined by Foo3 based on an iterative method. Note however that for the single-network targets (when all sinks and sources are taken as a single network), the actual minimum fresh water and the wastewater flow rates for the case are determined as 314.36 t/h, respectively. It is obvious that the unassisted integration scheme fails to locate the single-network targets for this case. Hence, it can be concluded that for example 1, the use of the assisted integration scheme is needed to maximize water savings. The revised algorithm for the assisted integration scheme is given in the following section.

Figure 3. Flow rate targeting with MRPD for the individual networks in example 1 with the unassisted integration scheme: (a) network A, (b) network B, (c) network C.

Step 1: Identification of Limiting Data for IPRCN. In the newly proposed algorithm for the assisted integration scheme by LPAF, one will have to identify the pinch-causing source of the overall network, i.e., at the overall pinch point of the IPRCN. The main reason for this step is that the HPR indicates water deficits of the overall network. Hence, all sources in the HPR should be exploited to satisfy the flow rate and load requirement of all sinks in the same region. As mentioned earlier, the pinch


6459

Figure 4. IPRCN with the unassisted integration scheme for example 1 (flow rate in tons/hour; quality in parts per million, given in parentheses).

concentration for the overall network is identified at 150 ppm. Therefore, all pinch-causing sources with concentration lower than 150 ppm shall be extracted as the cross-plant stream candidates for IPRCN. This corresponds to the pinch-causing source of network A, i.e., SR1 and SR3 at 100 ppm (120 t/h). Note that the pinch-causing sources of networks B and C are omitted, as both possess pinch concentrations that are either equal to or of lower quality than the overall pinch point of the IPRCN. As shown in Table 2, step 1 of the unassisted integration scheme1 identifies only part of source SR3 being extracted as the limiting data for IPRCN, i.e., 43.33 t/h in the HPR and 50.95 t/h as W1. The rest of this source has been utilized as the minimum pinch flow rate to fulfill the requirement of SK5 (400 ppm), since it is the only sink in the LPR of network A. If the entire SR1 and SR3 flow rates are extracted as the cross-plant stream candidate, this means that the pinch-causing source that is originally used to satisfy the requirement of SK5 in the LPR of network A is no longer available, which leads to a flow rate deficit in the LPR of network A. Following the algorithm in part 1 of the series,1 the minimum pinch flow rate for network A corresponds to 5.71 t/h. Hence, a LPAF is needed to restore the flow rate balance in the LPR of network A. The identification of the LPAF is illustrated next. Identification of LPAF. The LPAF performs as a supplementary source to restore the flow rate balance in the LPR of network A, after the entire pinch-causing source SR1 and SR3 (120 t/h, 100 ppm) is extracted as the cross-plant stream candidate for IPRCN. Note that the LPAF originates from the LPR of the overall network. In essence, water is in excess in the LPR. Hence, extracting the LPAF from this region will not

lead to a flow rate deficit in the overall network. It is proposed that the LPAF be taken from the pinch-causing source of the overall network; however, its flow rate should be lower than that of the allocated flow rate of the pinch-causing source to the LPR of the overall network. For example 1, SR15 from network C is identified as the LPAF, since it coincides with the overall pinch of the IPRCN at 150 ppm (see Table 1). After identification of the LPAF, its minimum flow rate needed in the LPR of network A (to restore the flow rate balance) can then be targeted using the multiple-source targeting procedure.7 In principle, the LPAF is treated as the cleanest source in the LPR of network A. The targeting procedure7 then identifies that the LPAF should have a minimum flow rate of 6.15 t/h. After the incorporation of LPAF as new cross-plant stream in network A, the revised wastewater streams are identified. The description of the targeting procedure is omitted here for brevity. Readers may refer to the original source for the detailed waste targeting procedure.8 Table 3 shows the revised limiting data for the assisted integration scheme. It is observed that wastewater W1 experiences a slight increase in its flow rate, i.e., to 37.82 t/h in the assisted integration scheme (from 37.38 t/h in the unassisted scheme in Table 2). This is because SR15 is a lower quality source (as compared to SR3 of 100 ppm). Hence, a higher flow rate is needed to satisfy the requirement of LPR in network A. Note also that the wastewater W2 (800 ppm) flow rate remains unchanged at 10 t/h. In network C, since a portion of W6 is being utilized as LPAF for network A, its flow rate is reduced from 146.67 to 140.52 t/h () 146.67 - 6.15 t/h). Finally, note that the limiting water data for network

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Table 3. Revised Limiting Data with Assisted Integration Scheme for Example 1 water network k

A

B

C

sinks j




20.00 66.67 100.00 41.67

0 50 50 80


sinks in HPR SK11 SK12 SK13 SK14 SK15

20.00 66.67 15.63 42.86

20.00 80.00 50.00 40.00 300.00

0 50 80 100

0 25 25 50 100

sources i sources in HPR SR1 SR2 SR3 wastewater in LPR W1 W2 sources in HPR SR6 SR7 SR8 wastewater in LPR W3 W4 W5 sources in HPR SR11 SR12 SR13 SR15 wastewater in LPR W6 W7

B in the assisted integration scheme remains identical with those extracted through the unassisted integration scheme (see Tables 2 and 3). Steps 2 and 3: Targeting for Minimum Resource Flow Rates for IPRCN and Individual Networks. Steps 2 and 3 of the assisted integration scheme are essentially the same as the original steps in the unassisted scheme. An MRPD for IPRCN is next plotted in Figure 5 using the revised limiting data identified in Table 3. The overall minimum fresh water (FFW) and wastewater (FWW) flow rates are both targeted as 314.36 t/h (Figure 5), which are identical to the single-network targets. The overall pinch concentration is identified at 150 ppm, identical to that of the result in the unassisted case. After identification of the overall minimum flow rates for the IPRCN, the minimum flow rates for each individual network are then identified. Readers may refer to part 1 of the series1 for the detailed flow rate targeting step for individual networks. Note that the cross-plant streams should be identified from the revised limiting data in Table 3 instead of Table 2. Figure 6 shows one of the possible networks that achieves the flow rate

Figure 5. Flow rate targeting with MRPD for IPRCN in example 1 with the assisted integration scheme by LPAF.



20.00 66.67 100.00

100 80 100

37.82 10.00

800 800

20.00 66.67 3.84

100 80 400

5.12 42.86 6.67

400 800 1000

20.00 80.00 50.00 153.33

100 50 125 150

140.52 () 146.67 - 6.15) 40.00

150 800

targets, designed using the nearest neighbor algorithm.6 With comparison of the IPRCNs in Figures 4 and 6, it is observed that the assisted integration scheme (Figure 6) needs three crossplant pipelines to achieve the minimum flow rate targets, whereas only two cross-plant pipelines are required for the unassisted integration scheme (Figure 4). As shown in Figure 6, the additional cross-plant pipeline is used to transfer the LPAF from SR15 in network C to SK5 in network A; while two other cross-plant pipelines remain the same as in the unassisted integration scheme (see Figure 4). In practice, there exists a trade-off between the benefits of additional water savings and the costs of additional piping between the networks, which depends on case-specific cost parameters. Assisted Integration Scheme by HPAF This section focuses on the assisted integration scheme with HPAF, which normally takes place when the zero discharge network(s) is involved in the IPRCN. Note that a zero discharge network always possesses a threshold point at the lowest purity level of the network, indicating that all sinks and sources are found in the HPR.3 In other words, there is no LPR for such a network. When a zero discharge network is involved in an IPRCN, it may receive cross-plant stream(s) from other networks in order to reduce its fresh water flow rate. In this case, its threshold point will disappear and a new pinch point forms at a higher purity level. The pinch-causing source at the newly formed pinch point can then be extracted as a cross-plant stream to assist further material recovery in other networks within the IPRCN. Note that this is similar to the pinch shifting principle in an unassisted integration scheme, reported in part 1 of the series.1 In the proposed assisted integration scheme, the new crossplant stream (as the HPAF) is extracted from the newly formed pinch point (with quality higher than the original threshold point). The identified HPAF will assist in the generation of additional cross-plant flow rate in other networks, which then reduce the fresh water consumption in the IPRCN. As a result of the new pinch point, a new pinch region is also generated in the IPRCN. This is shown using


6461

Figure 6. IPRCN with the assisted integration scheme for example 1 (flow rate in tons/hour; quality in part per million, given in parentheses). Table 4. Limiting Water Data for Example 2 water network k 4

A

B2



SK1 SK2 SK3

1200 800 500

120 105 80

SK4 SK5 SK6

50 20 100

20 50 400

sinks j

ΣjFSKj

sources i

2670



SR1 SR2 SR3 SR4

500 2000 400 300

100 110 110 60

SR5 SR6 SR7

20 50 40

20 100 250

ΣiFSRi

3310

Table 5. Limiting Water Data with Unassisted Integration Scheme for Example 2 water network k

sinks j



A

B

sinks in HPR SK4 SK5 SK6

50 20 100

20 50 400

example 2 that follows. Note further that the three-step targeting algorithm for the assisted integration scheme is modified here to demonstrate the use of HPAF in assisting further material recovery. The targeting algorithm is similar to the case of the assisted scheme with LPAF, where modification is only needed for step 1 of the procedure, i.e., identification of limiting data for IPRCN; while steps 2 and 3 remain unchanged. For this example, water cascade analysis (WCA)9,10 is utilized as the flow rate targeting tool.

sources i



wastewater in LPR W1 W2

20 680

60 110

20 50 40

20 100 250

sources in HPR SR5 SR6 SR7

Example 2 (Threshold Problems). The second example demonstrates the assisted integration scheme by HPAF using two threshold IPWI problems taken from Jacob et al.4 and Foo.3 Network A4 is a zero fresh water network with excess water sources; while network B2 is a zero discharge network that requires fresh water feed. The limiting water data is shown in Table 4. Note that before IPWI is carried out, the minimum wastewater and fresh water flow rates for reuse/recycle are determined as 700 and 60 t/h for networks A and B, respectively

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Table 6. Flow Rates Targeting with WCA for IPRCN in Example 2 with Unassisted Integration Scheme Cm (ppm)

ΣjFSKj (t/h)

ΣiFSri (t/h)

ΣiFSRi - ΣjFSKj (t/h)

FC,m (t/h)

∆Lm (kg/h)

cumulative ∆Lm (kg/h)

FFW ) 26.00 0

0

20

50

50

20

20

-30

0.52

-4.00

-0.12

-24.00

-0.24

-4.00

-0.16

46.00

0.46

-20

60

20

20

100

50

50

110

680

680

250

40

40

400

26.00

101.64

766.00

114.90

FWW ) 666.00

1 000 000

0.40 0.16 0.00 (PINCH) 0.46 102.10

-100

100

726.00

0.52

217.00 665 733.60

0

665 950.60

Table 7. Flow Rates Targeting with WCA for Network A in Example 2 with Unassisted Integration Scheme Cm (ppm)

ΣjFSKj (t/h)

ΣiFSRi (t/h)


FC,m (t/h)

∆Lm (kg/h)


FFW ) 0.00 0

0 280 ) (300 - 20)

60 80 100 105

0.00

280.00

5.60

-220.00 500

-4.40

500 -800

800 2386 ) (2400 - 14)

110 120

-500

500

0.00 280

1.20 280.00

1.40

-520.00

-2.60

1866.00

18.66

2386 -1200

1200

0.00 (limiting pinch) 5.60

FWW ) 666.00

2.60 0.00 (secondary pinch) 18.66

665 920.08

1 000 000

665 938.74

Table 8. Flow Rates Targeting with WCA for Network B in Example 2 with Unassisted Integration Scheme Cm (ppm)

ΣjFSKj (t/h)

ΣiFSRi (t/h)


FC,m (t/h)

∆Lm (kg/h)


FFW ) 26.00 0

0

20

50

50

20

20

-30

20(W1)

20

100

50

50

110

14(W2)

14

250

40

40

1 000 000

100

0.52

-4.00

-0.12

-24.00

-0.24

-4.00

-0.16

46.00

0.46

60.00

8.40

100.00

15.00

-20

60

400

26.00

-100 0

0.40 0.16 0.00 (new pinch) 0.46 8.86

FWW ) 0.00

(WCAs for the individual networks are not shown for brevity). The pinch region for the IPRCN is then identified, which is bounded by the pinch and threshold points of the individual networks, i.e., 60 and 1 000 000 ppm. If one were to use the unassisted integration scheme1 for this example, the identified limiting water data for the IPRCN is shown in Table 5. Following the proposed algorithm outlined in part 1 of the series,1 the minimum fresh water and wastewater flow rates for the IPRCN are targeted as 26 and 666 t/h, respectively, as shown in the cascade table in Table 6, with the overall pinch concentration of 100 ppm. Note that since both networks are of the threshold type, the minimum flow rates for

0.52

23.86 0.00 23.86

the individual networks are also equal to that of the overall network, i.e., 666 t/h of wastewater for network A with zero fresh water (Table 7), and 26 t/h of fresh water flow rate for network B with zero wastewater (Table 8). As shown in the tables, two pinch concentrations (i.e., 60 and 110 ppm) in network A and a new pinch (100 pm) in network B are formed upon the use of the unassisted integration scheme. These flow rate targets are consistent with the earlier work, with Figure 7 showing a possible IPRCN for the unassisted integration scheme.3 However, the single-network targets for this example are determined as 23.33 and 663.33 t/h for fresh water and


6463

Figure 7. IPRCN with unassisted integration scheme for example 2 (flow rate in tons/hour; quality in parts per million, given in parentheses).

wastewater flow rates respectively. These flow rates are obviously lower than those determined by the unassisted targeting algorithm.1 This resembles the same situation as in example 1, where the assisted integration scheme is needed to achieve the single-network targets. As the earlier proposed assisted integration with LPAF did not consider the formation of the new pinch point (e.g., for network B in this case) in the identification of the new limiting data, a new assisted integration scheme by HPAF is hence proposed. A simplified cascade diagram in Figure 8 is used to illustrate the proposed targeting procedure for this example. The diagram summarizes the WCA for flow rates targeting in networks A and B, which are reported in Tables 7 and 8. Note that the values in the boxes represent the net water flow rate at each concentration level (column 4 in Tables 7 and 8), with positive values indicating net flow rate surplus and vice versa. Besides, the cumulative water flow rates (column 5 in Tables 7 and 8) are also shown in the cascade diagram. As shown in Figure 8, the dotted area indicates the original pinch region (i.e., 60 -1 000 000 ppm) before IPRCN is carried out. On the other hand, the shaded area indicates the newly formed pinch region (i.e., 60 and 100 ppm) when the unassisted integration scheme is performed. Note also that cross-plant streams (W1 and W2) of the unassisted integration scheme are actually found within the old pinch region. However, since the unassisted integration scheme does not achieve the single-network targets, additional steps are needed. This involves the use of HPAF(s) via the assisted integration scheme, which is sent from a network with lower quality pinch (network B) to that of higher quality pinch (network A). In principle, any of the sources between the new pinch (at 100 ppm) and the original threshold point (1 000 000 ppm) may be extracted as the HPAF, so long as it has higher purity than the secondary pinch of network A.

Figure 8. Assisted integration scheme by HPAF for example 2 (flow rate in tons/hour).

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Table 9. Minimum Pinch Flow Rate Targeting with WCA for Network B in Example 2 (with Maximum Flow Rate of W2) Cm (ppm)

ΣjFSKj (t/h)

ΣiFSRi (t/h)


FC,m (t/h)

∆Lm (kg/h)


FPINCH) 0.00 100

0 0.00

110

680(W2)

250

40

400

0.00

680 680.00

95.20

720.00

108.00

620.00

619 752.00

40 -100

100

1 000 000

0.00 (PINCH) 95.20 203.20

0

619 955.20

Table 10. Revised Limiting Data for Assisted Integration Scheme (Example 2) water network k

sinks j



sources i

20 50 400

wastewater in LPR W1 W2 sources in HPR SR5 SR6 SR7

A B

sinks in HPR SK4 SK5 SK6

50 20 100



29.20 716.80

60 110

20 4 () 50 - 46) 40

20 100 250

Table 11. Flow Rates Targeting with WCA for IPRCN in Example 2 with Assisted Integration Scheme Cm (ppm)

ΣjFSKj (t/h)

ΣiFSRi (t/h)


FC,m (t/h)

∆Lm (kg/h)


FFW ) 23.33 0

0

20

50

50

20

60

20

29.2

0.47

-6.67

-0.20

-26.67

-0.27

29.2 2.53

0.05

2.53

0.05

6.53

0.03

6.53

0.03

723.33

7.23

723.33

94.03

763.33

114.50

0

100

4

105

4

716.8

120 40 100

716.8

7.40 101.43 FWW ) 663.33

0

With the use of HPAF, larger flow rates of cross-plant streams (found within the limiting and secondary pinch of network A) may be sent from network A to network B for the reduction of overall flow rates. In this example, the latter corresponds to the cross-plant stream W1 of network A (since it is an existing cross-plant stream in the region, see Figure 8). The flow rates for this stream as well as the HPAF are to be determined in the revised step 1 for the assisted integration scheme, which is described as follows. Step 1: Identification of Limiting Data for IPRCN. As mentioned earlier, HPAFs may be extracted from any sources between the new pinch (100 ppm) and the original threshold point (1 000 000 ppm) of network B but should have purity higher than that of the secondary pinch of network A. In this example, HPAF is extracted from the allocated flow rate of the pinch-causing source to the LPR of network B, i.e., 46 t/h (observed from the interval of 100-105 ppm in Figure 8). Note that even though this HPAF from network B has a higher concentration (100 ppm) than the limiting pinch (60 ppm) of network A, it may still be used in the LPR of the latter, since it is still purer than the secondary pinch (110

0.00 (PINCH) 0.46

0.17

40 -100

0.27

0.13

0

250

0.47

0.10

0

110

1 000 000

23.33

-20

80

400

-30

215.93 663 068.00 663 283.93

ppm). As a result, the consumption of other sources within network A is reduced, and larger cross-plant streams flow rate may be extracted for fresh water flow rate reduction in network B. A close inspection of Table 8 reveals that the entire pinchcausing source allocated to the LPR (46 t/h, 100 ppm, originating from SR6), along with sources of W2 (14 t/h, 110 ppm) and SR7 (40 t/h, 250 ppm) are all consumed by SK6 of much lower quality (100 t/h, 400 ppm). Besides, it is also observed that a large flow rate of cross-plant stream W2 from network A is left unutilized (Table 5). Since the flow rate of cross-plant source W2 is in excess, its flow rate can be maximized in order to reduce the consumption of the higher purity pinch-causing source (SR6). The unused SR6 can then be extracted as HPAF. Note that the additional crossplant flow rate of W2 functions as a supplementary source to restore the flow rate balance of the extracted HPAF. This is similar to the extraction of the cross-plant stream (from the newly formed pinch) in the pinch shifting scenario as illustrated in part 1 of the series.1 To determine the minimum pinch flow rate needed in the LPR (i.e., by maximizing the


Figure 9. Simplified cascade diagram after the assisted integration scheme for example 2 (flow rate in tons/hour).

cross-plant stream flow rate of W2), flow rate targeting is to be conducted in the LPR, similar to the waste targeting procedure of Ng et al.8 This step is described next. Table 9 shows the WCA for targeting the minimum pinch flow rate for LPR in network B (below the newly formed pinch).

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The flow rate of W2 is set at its maximum flow rate (680 t/h). It is then determined that no pinch flow rate is needed (FPINCH ) 0) in the LPR of this network. This indicates that the entire allocated flow rate of SR6 (46 t/h) can be extracted as the HPAF. The identified HPAF will become a new source in network A to reduce flow rate consumption of other sources in the network. Note that the minimum cross-plant flow rate for W2 will be determined in the third step of the targeting algorithm, which will be discussed later. Network A now experiences excess source stream flow rate upon receiving 46 t/h of HPAF from network B. The excess source stream flow rate will then be used as the cross-plant flow rate in network B. Similar to the earlier case, these crossplant flow rates are identified using the waste targeting procedure.8 Table 10 summarizes the revised limiting data for the assisted integration scheme after HPAF (46 t/h) is utilized as a new source in network A. Note that both crossplant streams have increased their flow rate, i.e., 29.20 t/h for W1 (from 20.00 t/h) and 716.80 t/h for W2 (from 680.00 t/h), as compared to the case of unassisted integration scheme (see Table 5). In other words, the 46 t/h of HPAF from network B “frees up” other sources in network A and allows additional flow rate of cross-plant streams to be generated for further water recovery in the IPRCN. Steps 2 and 3: Targeting for Minimum Resource Flow Rates for IPRCN and Individual Networks. Targeting is next conducted for IPRCN using the revised limiting data in Table 10. As shown in the WCA in Table 11, the overall minimum fresh water and wastewater are determined as 23.33 and 663.33 t/h, respectively. The overall pinch concentration is identified as 60 ppm, which coincides with the original pinch of network A. The targeted overall minimum flow rates are identical to those of the single-network targets. This shows that the new

Figure 10. IPRCN with the assisted integration scheme for example 2 (flow rate in tons/hour; quality in parts per million, given in parentheses).

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Table 12. Regeneration Flow Rate Targeting with WCA for Example 1 Cm (ppm) 0

ΣjFSKj (t/h)

ΣiFSRi (t/h)

-60.00

60.00

Cout ) 10

FReg ) 272.53 130.00

50

273.33

80.00

80

57.29

133.33

342.86

160.00

-193.33

-182.86

50.00

50.00

150

108.26

108.26

800 1000

16.67

∆Lm (kg/h)

15.63

-1.04

53.73

53.73

6.67

6.67

1 000 000

algorithm is applicable for the IPRCN problem of the assisted integration type. In the third step of the targeting algorithm, flow rate targeting is carried out for the individual networks. Note again that the cross-plant streams should be identified from the revised limiting data in Table 10 instead of Table 4. Since both networks are of the threshold type, this step determines that the individual networks A and B achieve the same flow rates as the overall network, i.e., 663.33 t/h wastewater for network A (zero fresh water) and 23.33 t/h fresh water for network B (zero wastewater). Figure 9 shows the simplified cascade diagram for example 2. As shown, the flow rate of W1 (60 ppm) is increased from 20 to 26.67 t/h due to the use of 46 t/h of HPAF (100 ppm). Besides, 42 t/h of supplementary source is extracted along with the original cross-plant stream W2 of 14 t/h (110 ppm). For a detailed flow rate targeting step for individual networks, readers are referred to part 1 of this series.1 Note that in Figure 9, after IPWI has been carried out (with supplementary source and HPAF), the HPAF is no longer seen to be transferred from the HPR of network B. This is due to the reallocation of the threshold concentration (1 000 000 ppm) to the new pinches at 60 and 100 ppm. Figure 10 shows one of the possible IPRCN for the example. As shown, there are three cross-plant streams in this example, i.e., with two of them sending from networks A to B while the other from networks B to A. Note that with the assisted integration scheme, additional pipeline(s) are normally needed for the HPAF. Once again, it depends on case-specific circumstances whether the added resource savings make the incremental investment in piping worthwhile. As shown in Figure 10, an additional cross-plant pipeline is observed to connect between SR6 and SK2; while two other cross-plant pipelines remain the same as in the unassisted integration scheme (see Figure 7).


FFW ) 60.00 0.00

0.00

272.53

4.09

142.53

3.56

-50.81

-1.52

76.04

125

400

FC,m (t/h)

272.53 -130.00

25

100


0.00 (limiting pinch) 4.09 7.65 6.13

25.24

0.50

-157.62

-3.94

-107.62

-2.69

0.64

0.16

-0.40

-0.16

53.33

10.67

FWW ) 60.00

6.63 2.69 0.00 (secondary pinch) 0.16 0.00 (tertiary pinch) 10.67

59 940.00 59 950.67

to part 1 of the series,1 the targeting procedure shall follow the work that is presented by Ng and co-workers.11–13 However, rather than having separate interception units as in part 1 of the series,1 a single centralized unit is used here, where water sources may be intercepted for further reuse/recycle in the network and/or for final discharge. Because of space limitations, only a brief description of the procedure is outlined here. Readers are referred to the original works for the detailed steps. For this example, a centralized interception unit with fixed outlet concentration type is assumed, with an outlet concentration (Cout) of 10 ppm. Following the targeting algorithm of Ng et al.,11,12 the total minimum regeneration flow rate (FReg) required in the IPRCN is determined as 272.53 t/h, with the total minimum fresh water (FFW) and wastewater (FWW) flow rates of 60 t/h, respectively. These are shown in the cascade table in Table 12. Note that the targeting algorithm makes use of the revised limiting data in Table 3. To determine the minimum flow rate targets for fresh water, regeneration, and

Source Interception and Waste Treatment (Example 1 Revisited) In this section, example 1 is revisited to illustrate the incorporation of source interception unit in the IPRCN. Similar

Figure 11. Minimum treatment flow rate targeting with fixed Cout type interception unit for example 1.


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Figure 12. Total water network with fixed outlet quality type interception unit for example 1 (flow rate in tons/hour; quality in parts per million, given in parentheses).

cross-plant streams for each individual network, step 3 of the proposed targeting scheme is to be followed, using the revised limiting data in Table 3. Detailed targeting steps may be found in an example in part 1 of the series.1 Following the waste targeting procedure by Ng et al.,8 two wastewater streams are identified (by performing flow rate targeting for the region below the tertiary pinch of 800 ppm in Table 12), i.e., WW1 with 53.33 t/h at 800 ppm from networks A and B and WW2 with 6.67 t/h at 1000 ppm from network B. Next, targeting is performed for the waste treatment system following the procedure outlined by Ng et al.13 As shown in Figure 11, a wastewater composite curve is plotted on a cumulative impurity load versus cumulative flow rate diagram, with the slope of its segments representing the impurity concentration of the individual wastewater streams, arranged in ascending order.13 Assuming an allowable discharge limit (CD) of 30 ppm, the allowable impurity load discharge to the environment (∆mD) is determined as 1.80 kg/h () 60 t/h × 30 ppm). Since the wastewater streams have a total load of 49.33 kg/h () 53.33 t/h × 800 ppm + 6.67 t/h × 1000 ppm), the minimum impurity load removal (∆mR) from the wastewater is determined as 47.53 kg/h () 49.33 - 1.80 kg/h). Note that the interception between the treatment line and wastewater composite curve indicates that wastewater WW1 (at 800 ppm) has a bypass flow rate (FB) of 1.52 t/h. Hence, a total wastewater flow rate of 58.48 t/h is to be treated (FT) prior to environmental discharge. Figure 12 shows one of the possible IPRCN for the problem. Note that the interception unit treats a total of 331 t/h of water. A

big portion (272.53 t/h) of the treated water is reused/recycled in the individual networks; while the rest (58.48 t/h) is sent for environmental discharge by mixing with a small flow rate of bypassed water stream (1.52 t/h) from network A. These flowrates match the targets obtained in Table 12 and Figure 11. Conclusions A revised targeting algorithm for the assisted integration scheme is proposed to determine the single network targets for an IPRCN when the unassisted integration scheme1 fails to do so. In principle, the minimum flow rate targets are achieved with the use of LPAF or HPAF in the assisted integration scheme. However, this normally leads to additional cross-plant streams to achieve the minimum flow rate targets for the IPRCN. Hence, performing both unassisted and assisted integration schemes for an IPRCN gives better insights on the trade-off between the benefits of additional resource savings and the cost of more cross-plant piping. Acknowledgment The financial support from University of Nottingham through New Researcher Fund (Grant NRF 3822/A2RBR9) and Research Studentship is gratefully acknowledged. Funding from the Ministry of Science, Technology and Innovation (MOSTI) Malaysia through Science Fund (Grant 03-02-12-SF0018) is deeply appreciated.

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Literature Cited (1) Chew, I. M. L.; Foo, D. C. Y.; Ng, D. K. S.; Tan, R. R. Flowrate Targeting Algorithm for Interplant Resource Conservation Network. Part 1: Unassisted Integration Scheme. Ind. Eng. Chem. Res. DOI: 10.1021/ ie901802m. (2) Olesen, S. G.; Polley, G. T. Dealing with plant geography and piping constraints in water network design. Trans. IChemE, Part B 1996, 74, 273– 276. (3) Foo, D. C. Y. Flowrate targeting for threshold problems and plantwide integration for water network synthesis. J. EnViron. Manage. 2008, 88 (2), 253–274. (4) Jacob, J.; Kaipe, H.; Couderc, F.; Paris, J. Water network analysis in pulp and paper processes by pinch and linear programming techniques. Chem. Eng. Commun. 2002, 189 (2), 184–206. (5) El-Halwagi, M. M.; Gabriel, F.; Harell, D. Rigorous graphical targeting for resource conservation via material recycle/reuse networks. Ind. Eng. Chem. Res. 2003, 42 (19), 4319–4328. (6) Prakash, R.; Shenoy, U. V. Targeting and design of water networks for fixed flow rate and fixed contaminant load operations. Chem. Eng. Sci. 2005, 60 (1), 255–268. (7) Foo, D. C. Y. Water Cascade Analysis for Single and Multiple Impure Fresh Water Feed. Chem. Eng. Res. Des. 2007, 85 (8), 1169–1177.

(8) Ng, D. K. S.; Foo, D. C. Y.; Tan, R. R. Targeting for total water networkPart 1: Waste stream identification. Ind. Eng. Chem. Res. 2007, 46, 9107–9113. (9) Manan, Z. A.; Tan, Y. L.; Foo, D. C. Y. Targeting the minimum water flowrate using water cascade analysis technique. AIChE J. 2004, 50 (12), 3169– 3183. (10) Foo, D. C. Y.; Manan, Z. A.; Tan, Y. L. Use cascade analysis to optimize water networks. Chem. Eng. Prog. 2006, 102 (7), 45–52. (11) Ng, D. K. S.; Foo, D. C. Y.; Tan, R. R.; Tan, Y. L. Ultimate flowrate targeting with regeneration placement. Trans. IChemE, Part A 2007, 85 (A9), 1253–1267. (12) Ng, D. K. S.; Foo, D. C. Y.; Tan, R. R.; Tan, Y. L. Extension of Targeting Procedure for “Ultimate Flowrate Targeting with Regeneration Placement” by Ng et al., Chem. Eng. Res. Des., 85 (A9), 1253-1267. Chem. Eng. Res. Des. 2008, 86 (10), 1182–1186. (13) Ng, D. K. S.; Foo, D. C. Y.; Tan, R. R. Targeting for total water network-Part 2: Waste treatment targeting and interactions with water system elements. Ind. Eng. Chem. Res. 2007, 46, 9114–9125.

ReceiVed for reView November 13, 2009 ReVised manuscript receiVed May 7, 2010 Accepted May 25, 2010 IE901804Z

Flowrate Targeting Algorithm for Interplant Resource Conservation

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