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Studies on simultaneous energy and water minimisation—Part II: Systems with maximum re-use of water
Chemical Engineering Science 60 (2005) 3291 – 3308 www.elsevier.com/locate/ces
Studies on simultaneous energy and water minimisation—Part II: Systems with maximum re-use of water Luciana Savulescu1 , Jin-Kuk Kim∗ , Robin Smith Centre for Process Integration, School of Chemical Engineering and Analytical Science, The University of Manchester, P.O. Box 88, Manchester, M60 1QD, UK Received 26 September 2003; received in revised form 11 November 2004; accepted 23 December 2004 Available online 23 March 2005
Abstract A new systematic design methodology has been developed for the simultaneous management of energy and water systems that also feature maximum re-use of water. A two-dimensional grid diagram is proposed to exploit different options within water systems and also enable reduced complexity of the energy and water network. Isothermal and non-isothermal stream mixing between water streams are introduced to create separate systems between hot and cold water streams in the energy composite curves and provide a design basis for a better structure with fewer units for the heat exchanger network. In addition to allowing re-use of water, issues about heat losses inside unit operations have also been incorporated in the simultaneous management of water and energy. 䉷 2005 Elsevier Ltd. All rights reserved. Keywords: Water minimisation; Energy recovery; Heat loss; Water loss; Separate system
1. Introduction In the previous paper, Studies on Simultaneous Energy and Water Minimisation—Part I, a new systematic design methodology has been presented for simultaneous energy and water management problems in which water re-use was not allowed. A new strategy has been adopted to provide the simplicity in the HEN design with a reduced number of heat transfer units. This is based on the generation of separate systems and non-isothermal stream mixing. However, the re-use of water between water-usingoperations was not considered in Part I. The water and wastewater could be further minimised by allowing water re-use (Wang and Smith, 1994; Kuo and Smith, 1998; Doyle and Smith, 1997; Alva-Argaez, 1999). In this paper (Part II), ∗ Corresponding author. Tel.: +44 161 306 8755; fax: +44 161 236 7439.
E-mail address: [email protected] (J.-K. Kim). 1 Current address: CANMET Energy Diversification Research Labora-
tory, 1615 Lionel-Boulet Blvd., P.O. Box 4800, Varennes, Que., Canada J3X 1S6. 0009-2509/$ - see front matter 䉷 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2004.12.036
the methods presented in Part I will be extended to systems with maximum water re-use where freshwater consumption and wastewater generation are minimised simultaneously with energy consumption. Combinatorial features exist between water re-use, energy recovery, and network design in systems with water re-use. Therefore, new methods are required to investigate the diversity of design options. In this paper, the aim is to propose design methods by incorporating energy and water aspects into the overall design framework for simultaneous energy and water minimisation. Also, several assumptions have initially been made to simplify the analysis in Part I. These assumptions included various operations requiring water at different concentrations and temperatures; each of the operations has a fixed value for temperature supply; no heat losses are allowed in operations; no flowrate loss or gain through an operation; and non-water-using operations were neglected. Later in this paper, the consequences from the relaxation of these assumptions will be discussed, especially the effects of heat losses.
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800
Op 1
Op 3
Op 2
Op 4
C (ppm)
Limiting Composite Curve Water Supply Line
400 Pinch 90 kg/s
100 50 9
41
Op 1
Op 3
Op 2
Op 4
Op 1
Op 4
Op 2
Op 3
M (g/s)
Fig. 1. Different design networks for maximum re-use of water—Example 1.
Table 1 Water-using operation data—Example 1 Operation
CIN (ppm)
COUT (ppm)
TOp,IN (◦ C)
Operation 1 0 100 40 Operation 2 50 100 100 Operation 3 50 800 75 Operation 4 400 800 50 Temperature of fresh water source, TIN = 20 ◦ C Temperature of discharge wastewater, TOUT = 30 ◦ C.
TOp,OUT (◦ C)
Limiting water flowrate (kg/s)
Contaminant mass load (g/s)
40 100 75 50
20 100 40 10
2 5 30 4
2. Energy aspects of water systems
20 kg/s 90 kg/s
Moving now to the analysis of systems with maximum reuse of water, a more elaborate design procedure is required when re-use is considered. Different water networks can achieve the water target (Fig. 1). However, these networks involve different connections between the water-using operations that can require different energy consumptions. If a water system is analysed for energy, the water stream distribution needs to be known. Thus, the water consumption and the water-using network need to be determined before the energy analysis. In order to explore the interactions between the energy and water consumption and gain an understanding of how the systems function, Example 1 illustrated in Part I will be revisited (Table 1). The energy analysis for a water-using operation network will be examined. Different re-use water network structures have been examined for energy consumption. Two different types of heat recovery can be considered: indirect and direct heat recovery. The heat recovery systems have been imposed by the configuration of the water network and therefore by the thermal stream data extracted from the water network. As discussed by Wang and Smith (1994), the water network design can in some cases be evolved, whilst maintaining the freshwater consumption. Fig. 2 shows one of the possible water network structures for Example 1, which can achieve the target for maximum re-use of water, in this case 90 kg/s.
20°C 0 ppm
20 kg/s
50 kg/s
40 kg/s Operation 1 /40°C Operation 2/100°C
Operation 3/75°C 5.7 kg/s
Operation 4/50°C
90 kg/s 30°C
44.3 kg/s
Fig. 2. Water network 1—Example 1. Table 2 Thermal stream data for energy analysis (Water network 1) Stream
TS (◦ C) TT (◦ C) CP (kW/◦ C) Enthalpy (kW)
FW to Op 1 20 FW to Op 2 20 FW to Op 3 20 RW from Op 1 to Op 3 40 RW from Op 2 to Op 4 100 WW from Op 2 100 WW from Op 3 75 WW from Op 4 50
40 100 75 75 50 30 30 30
84 210 84 84 23.94 186.06 168 23.94
1680 16800 4620 2940 −1197 −13 024.2 −7560 −478.8
Once the water network is fixed (Fig. 2) the stream data for the energy recovery problem can be extracted. The thermal stream data for the indirect heat recovery analysis are shown in Table 2. The energy composite curves can be plotted for a minimum temperature difference of 10 ◦ C (Fig. 3) and the potential energy recovery and utility consumption obtained. The system is a threshold problem with
L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308
0 ppm
3780 kW
20 °C
100 Temperature (°C)
Operation 1 / 40 °C
90
50 ppm
80
20 °C
70
Operation 2 / 100 °C
50 ppm
60
20 °C
∆Tmin = 10°C
50
Operation 3 / 75 °C
400 ppm
40
Operation 4 / 50 °C
20 °C
30 20
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100 ppm 30 °C 100 ppm 30 °C 800 ppm 30 °C 800 ppm 30 °C
Fig. 5. Individual operations—Example 1.
5000
10000 15000 20000 Enthalpy (kW)
25000
30000
Fig. 3. Energy composite curves for Water network 1.
a hot utility requirement of 3780 kW. To complete the picture, the energy design method has been carried out and the heat exchanger network determined by applying conventional heat exchanger network design procedures. The heat exchanger network design is shown in Fig. 4. When the energy aspects of water systems are considered there are some distinctive features compared with heat exchanger networks in general. These result from the degree of freedom to mix and split streams in the network compared with classical heat integration where the streams are a variety of fluids and the splitting and mixing options are limited to be restricted to the same stream. The problem addressed here involves only water streams. This allows mixing and splitting of water streams in ways that will allow the overall network of water and heat exchangers to be simplified. However, some restrictions in mixing water streams are imposed by the contaminant concentration constraints in the re-use options. An important observation related to mixing water streams comes from re-use alternatives. First, the individual operations are shown in Fig. 5. Inspection of the limiting concentrations allows some re-use alternatives to be identified. As a result of the analysis of the inlet and outlet concentrations of each of the water-using operations, water from
Operations 1 and 2 could be re-used in Operation 4. There are different possibilities to re-use the water from Operations 1 and 2 to satisfy the requirements of Operation 4. These options are presented in Fig. 6. Option 1: Re-use of water from Operation 1 into Operation 4. In this case, the re-use stream needs heating because the source temperature is lower than the sink temperature. Option 2: Re-use of water from Operation 2 into Operation 4. Here, the re-use water stream has a higher temperature than is required in Operation 4 and therefore a cooling duty is required. Option 3: Re-use of water from Operations 1 and 2 into Operation 4. Two different temperature streams are mixed in order to satisfy the requirements of Operation 4 without the need for either heating or cooling. The concentration constraints allow different re-use options to be exploited, but the energy implications of the options could be very different. Therefore, there are clearly interactions between re-use options and the energy consumption. From Fig. 2, two re-use water sources can be identified. These are the outlet water streams from Operations 1 and 2. These re-use water streams are characterised by different temperature levels, but the same concentration (100 ppm from Table 1). The sink operations for the re-use of water (Operations 3 and 4) are characterised by intermediate temperatures between the re-use water sources
100°C
30°C
Wastewater Streams (hot)
75°C 50°C
30°C
Re-use Stream (hot)
100°C
50°C
30°C
75°C 40°C
40°C H
75°C 100°C
H H
20°C 20°C 20°C
Fig. 4. Heat exchanger network (Water network 1).
Re-use Stream (cold) Fresh water Streams (cold)
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T (°C)
100
2
2
2 Non-isothermal mixing
C 4
50
4
4
H 40
1
1
1 Option 2
Option 1
Option 3
Fig. 6. Energy implications in the re-use options.
20 kg/s
6.91 kg/s
Operation 1 / 40°C 5.7 kg/s
90 kg/s 8.34 kg/s
20°C 0 ppm
20 kg/s
Operation 4 / 50°C
40 kg/s Operation 3 / 75°C
50 kg/s
11.66 kg/s
Operation 2 / 100°C
90 kg/s 30°C
37.39 kg/s
Fig. 7. Water network 2—Example 1.
Table 3 Thermal stream data for energy analysis (Water network 2)
6426 kW 100
TS (◦ C)
TT (◦ C)
CP (kW/◦ C)
Enthalpy (kW)
FW to Op 1 FW to Op 2 FW to Op 3 WW
20 20 20 81
40 100 75 30
84 210 84 378
1680 16800 4620 −19 278
90 Temperature (°C)
Stream
80 70 60 50 40
(TOp1 < TOp4 < TOp3 < TOp2 ). Therefore, the heat from the hot re-use stream (the outlet from Operation 2) could be recovered through direct contact with the cold re-use water stream (the outlet from Operation 1) in order to achieve the temperature requirements for the sink operations. From the above considerations, some modifications to the water network structure should be made. The new network configuration is illustrated in Fig. 7. Within this network there are two non-isothermal mixing points. One non-isothermal mixing point has been introduced for the inlet of Operation 3, such that re-use water from Operations 1 and 2 are mixed with some fresh water. The other non-isothermal mixing point takes place at the inlet of the Operation 4 that uses water from Operations 1 and 2. Apart from these non-isothermal mixing points for the thermal stream data, all the wastewater streams have been mixed together to form one effluent stream. Consequently, the thermal stream data for this new water network configuration are given in Table 3. These stream data do not include the streams involved in the non-isothermal mixing points. Hence, these streams are subject to indirect heat recovery.
30 20 5000 2604 kW
10000 15000 20000 Enthalpy (kW)
25000
30000
Fig. 8. Energy composite curves for Water network 2.
Based on these stream data, the energy composite curves have been constructed for a minimum temperature difference of 10 ◦ C (Fig. 8) and the energy targets determined. The grid representation of the heat exchanger network, including the non-isothermal mixing points for this problem, is presented in Fig. 9. In the case of Water Network 2, the problem is pinched with a large consumption of hot and cold utilities compared with the case of Water Network 1, which is a threshold problem. Summarising, indirect heat transfer can be used to recover the maximum amount of heat subject to temperature difference limitations. Alternatively, direct heat recovery can be used by mixing streams. This second alternative requires fewer heat exchangers, but there is a sacrifice in energy consumption. A comparison of indirect and direct recovery
L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308
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100°C 75°C Wastewater Streams (hot) Re-use Stream (hot)
50°C
81°C
40°C
30°C C
100°C
50°C
75°C
40°C
40°C
20°C
75°C 100°C
H H
20°C 20°C
Re-use Stream (cold) Fresh water Streams (cold)
Fig. 9. Heat exchanger network for Water network 2.
Table 4 Summary of indirect and direct heat recovery for Example 1 Heat recovery
QH (kW)
QC (kW)
Number of units
Indirect heat recovery Direct and indirect heat recovery
3780 6426
— 2604
9 6
systems in terms of utility consumption and the number of heat transfer units for Example 1 is given in Table 4. The difference between the energy consumption of the indirect and direct heat recovery schemes is caused by the degradation of driving force when streams are mixed. Mixing at different temperatures acts as a heat transfer unit. So far, there are possibilities to solve the energy minimisation problem using not only indirect heat recovery, but also direct heat recovery. Mixing of water streams can be done at the same temperature (isothermal mixing of streams) and/or at different temperatures (non-isothermal mixing of streams). On the other hand, splitting streams is used in order to achieve the needs of the water and energy network design structure. Although starting from the same water-using operation data, the way to use indirect and direct heat transfer would result in a different energy consumption and design of the heat exchanger network. Thus, a systematic method is required to simultaneously determine the water and energy targets and the design procedure to achieve the heat exchanger network targets. 3. Two-dimensional grid diagram When the energy aspects need to be considered for a water system, new features should be included in the water network design strategy to account for the energy implications in water re-use paths. For a clear picture of the problem a new representation is required. Since both water and energy need to be addressed to solve the problem of simultaneous energy and water management, some modifications to the water network design procedure must be considered.
In this sense, a temperature scale has been incorporated into the water network design grid. A temperature scale has been adopted to account for the energy aspects in the water network and to guide the designer to identify water networks with high energy efficiency. In other words, this new representation will consider re-use options and heat recovery simultaneously in the design. The new tool is called the twodimensional grid diagram. A description of design steps will be explained with Example 1 (Table 1) that was introduced in previous paper (Part I). Step 1: Set up the two-dimensional grid diagram The two-dimensional grid diagram is an extension of the water network design introduced by Kuo and Smith (1998). This new design grid represents a concentration scale on the horizontal axis (according to the water mains concept), and temperature scale on the vertical axis (Fig. 10). In this diagram, the operations are positioned according to their operating temperatures and concentration limiting data. The operations are drawn as horizontal lines as long as isothermal operations are used. For example, Operation 1 needs to be fed with 20 kg/s of fresh water at 40 ◦ C and will discharge 20 kg/s wastewater at 100 ppm and 40 ◦ C. With this new representation, additional details are provided to the designer. Now, the water mains are characterised not only by the inlet and outlet water flowrates and concentrations, but also by information on the temperature of the available/required water. The fresh water source is shown in this representation at the bottom of the first water main according to its supply temperature (20 ◦ C in this case). The other water mains illustrate the level of wastewater discharge temperature (30 ◦ C in this case). Step 2: Connect operations with water mains and water distribution inside mains Since the fresh water is supplied at 20 ◦ C in this case and the water-using operations require different temperature levels, a water distribution needs to be made inside the water main (Fig. 11). This water distribution takes into account the concept of individual water streams introduced earlier. The vertical lines inside the water mains are the result of water distribution inside the mains, and represent the streams for the energy analysis. The streams moving upwards are
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Limiting water flowrate T (°C) (kg/s) 100
100
40
80
Freshwater 90 kg/s 0 ppm
0 kg/s 800 ppm
45.7 kg/s 100 ppm 2
3
3
60 10 20
4 40 20
C (ppm)
1
Water source Wastewater 0 kg/s
Water discharge
Water discharge
Wastewater 44.3 kg/s
Wastewater 45.7 kg/s
Fig. 10. Two-dimensional grid diagram—Step 1.
Limiting water flowrate T (°C) (kg/s) 100
100
40
80
Freshwater 90 kg/s 0 ppm
45.7 kg/s 100 ppm
50 kg/s
20 kg/s
0 kg/s 800 ppm
2
3
40 kg/s
3
60
5.7 kg/s
10 20
40 20
C (ppm)
1
4
20 kg/s
Water source Wastewater 0 kg/s
Water discharge
Water discharge
Wastewater 44.3 kg/s
Wastewater 45.7 kg/s
Fig. 11. Two-dimensional grid diagram—Step 2.
cold streams since their temperatures need to be increased, which means that they require a heat input to satisfy the energy requirements. By supplying fresh water at different temperatures as the operations require, the intermediate water main at 100 ppm receives water at different temperature levels. In this example, the water from Operation 1 reaches the main at 40 ◦ C, while the water from Operation 2 enters at 100 ◦ C, even though both water streams are at 100 ppm. At this stage, the water distribution inside the intermediate water main is not considered. In order to supply Operations 3 and 4 within the second interval (between the second and third water mains), water is extracted from the intermediate main at the required operating temperature levels (75 and 50 ◦ C, respectively). Consequently, these operations will produce
wastewater streams with a level of 800 ppm contaminant concentration, as the limiting data indicates, but at different temperatures according to temperature operating conditions. These wastewater streams need cooling to satisfy the water discharge temperature conditions. Therefore, two hot streams will populate the water main at 800 ppm. Step 3: Merge operations crossing boundaries In the previous step, Operation 3 has been divided into two parts by the 100 ppm water main, and each part has been analysed individually. The result of this split has led to different water flowrate requirements on each side. Therefore, a uniform flowrate across the operation is the objective of this step. This is achieved by changing the point where Operation 3 is fed with the re-use stream (i.e., the water from the middle water main is supplied to the inlet of Operation
L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308
Limiting water flowrate T (°C) (kg/s) 100
100
40
80
Freshwater 90 kg/s 0 ppm
45.7 kg/s 100 ppm
50 kg/s
20 kg/s
3
40 kg/s 75 °C 5.7 kg/s
10 20
20
Water source
C (ppm)
4
20 kg/s
1
40
0 kg/s 800 ppm
2
20 kg/s
60
3297
Wastewater 0 kg/s
Water discharge
Water discharge
Wastewater 44.3 kg/s
Wastewater 45.7 kg/s
Fig. 12. Merge operations crossing intermediate water mains—Step 3.
Limiting water flowrate T (°C) (kg/s) 100
100
40
80
Water source
Freshwater 90 kg/s 0 ppm
45.7 kg/s 100 ppm
0 kg/s 800 ppm
50 kg/s 2 20 kg/s 3
40 kg/s 75 °C
20 kg/s
60
5.7 kg/s 4
10 20
Water sink
20 kg/s 1
40 20
Water source
C (ppm)
Water source
Wastewater 0 kg/s
Water discharge
Water sink
Wastewater 44.3 kg/s
Water discharge Wastewater 45.7 kg/s
Fig. 13. Water sources/sinks for the intermediate water main.
3, not to the middle of Operation 3), which allows the same flowrate for Operation 3 without violating inlet contamination level. The temperature of the re-use water should be 75 ◦ C to satisfy the temperature condition of Operation 3 (Fig. 12). How this temperature is obtained is shown in the next design step of the two-dimensional grid diagram. Step 4: Remove intermediate water mains through water and energy analysis In order to remove the intermediate main at 100 ppm, the connections between the sources and sinks at this level of concentration need to be established. The 100 ppm water main is characterised by two sources and two sinks (Fig. 13). One of the water sources is provided at 40 ◦ C by Operation 1. The other source reaches the 100 ppm water main at 100 ◦ C from Operation 2. The sink operations (Operations
3 and 4) require water at 75 and 50 ◦ C, respectively. These sources and sinks can be matched in different ways to remove the intermediate main. However, different connections will involve a variation in the energy features. Therefore, an evaluation of energy requirements for the re-use water streams is implied. A number of re-use cases with different connections between sources and sinks can be considered from the energy point of view in order to determine the design rules for simultaneous energy and water minimisation. The analysis of four different re-use arrangements inside the pinch water main (100 ppm) will now be examined. Case 1: In this case, the re-use water is distributed starting with the coldest source (Fig. 14). Thus, water from Operation 1 is re-used in Operation 3. However, due to the
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45.7 kg/s 100 ppm
45.7 kg/s 100 ppm
100°C
2
100 °C
2
75 °C
3
75 °C
3
40 °C
4
50 °C
50 °C
4 40 °C
1
30 °C
1
30 °C Water discharge Wastewater 44.3 kg/s QHS = 14,221 kW
Water discharge Wastewater 44.3 kg/s QHS = 11,520 kW QCS = 239 kW
QCS = 2,940 kW Fig. 15. Water re-use inside intermediate water main: Case 2. Fig. 14. Water re-use inside intermediate water main: Case 1.
temperature difference, a 2940 kW heating duty is required for this stream. Since the water source from Operation 1 is used in Operation 3 completely, Operation 4 needs to be fed with water from Operation 2. This new re-use stream is a hot stream with 1197 kW available heat. The remaining water, 44.3 kg/s from Operation 2 is sent to discharge. This wastewater stream can provide 13 024 kW heat. In summary, two re-use streams have been considered. One cold stream resulting from the re-use of water between Operations 1 and 3, and one hot stream resulting from the re-use from Operations 2 to 4. The wastewater from this main is represented by one hot stream. All of these streams are subject to indirect heat recovery. Case 2: A different strategy for re-using water has been considered for this second case (Fig. 15). Each re-use water source has been connected with the nearest sink (in terms of temperature). Water from Operation 1 is re-used in Operation 4, which has an operating temperature closer to the temperature condition of its re-use water source. The resulting re-use stream is a cold stream with a heat requirement of 239 kW. Since Operations 2 and 3 are nearest in terms of temperature, a re-use stream will connect these operations. This new re-use stream is a hot stream with a duty of 2100 kW. The remaining water from each source is collected and discharged. Therefore, two hot wastewater streams are generated. In this situation, the number of streams is increased to four (two re-use streams and two wastewater streams), which will result in a heat exchanger network with a larger number of units. However, the energy involved in the hot streams is
11 520 kW, and the energy required by the cold streams is 239 kW, lower than the previous case. Case 3: In this case, the re-use water is distributed starting with the hottest source, which is provided by Operation 2 (Fig. 16). The amount of water supplied by this source is high enough to satisfy both sinks (Operations 3 and 4). By connecting Operation 2 with the sink operations, two hot streams are created. The remaining wastewater from Operation 2 is sent to discharge. This stream is a hot stream, since its temperature should be reduced from 100 to 30 ◦ C. The other water source from Operation 1 is discharged as a hot stream. In this case, within the intermediate water main four hot streams have been obtained (two re-use streams and two wastewater streams). The energy available through these hot streams is 11 281 kW and is considered for indirect heat recovery. Note that there are no cold streams in this arrangement. Thus, this arrangement involves less energy compared with the previous two cases. Case 4: Since the sink units operate at intermediate temperatures compared with the source operations, the idea of mixing water streams at different temperatures might be considered (Fig. 17). By re-using water from both sources through non-isothermal mixing points the conditions of sink operations are satisfied, not only in terms of concentration, but also temperature. These re-use streams are subject to direct heat recovery. The remaining water for discharge is represented by two hot streams with a heat load of 11 281 kW. These wastewater streams are considered for indirect heat recovery.
L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308
45.7 kg/s 100 ppm 100 °C
2
75 °C
3
50 °C 40 °C
4
The energy analysis inside of the intermediate water mains concerns the level of the energy recovery. Since the waste water should be discharged at a lower temperature than the operating temperatures, it is better to achieve part of this duty by considering hot re-use streams instead of cold re-use streams. Therefore, the unnecessary extra energy in the heat exchanger network should be eliminated by an appropriate stream arrangement. Through the analysis of these four different re-use configurations, some conclusions can be drawn. These conclusions can be formulated as design rules for simultaneous energy and water minimisation.
1
Design Rule 1: Start to distribute the re-use water from the hottest source. Design Rule 2: Connect the re-use water source with the nearest sink (in terms of temperature). Design Rule 3: Introduce non-isothermal mixing points if the temperatures of the sink operations are intermediate to the temperature of source operations.
30 °C Water discharge Wastewater 44.3 kg/s QHS = 11,281 kW QCS = 0 kW Fig. 16. Water re-use inside intermediate water main: Case 3.
45.7 kg/s 100 ppm 100 °C
2
75 °C
3
50 °C 40 °C
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4 1 C
30 °C Water discharge Wastewater 44.3 kg/s QHS = 11,281 kW QCS = 0 kW Fig. 17. Water re-use inside intermediate water main: Case 4.
This arrangement has the advantage of achieving the temperature conditions of the sink operations through direct heat recovery, which means that the number of streams for indirect heat recovery is decreased. Therefore, the number of units in the heat exchanger network will also decrease. Even though the configuration of Case 4 is characterised by the same energy as Case 3 fewer streams are involved, so fewer heat transfer units.
In summary, the two-dimensional grid diagram has been introduced in order to exploit the options within the water system, not only for water minimisation, but also to simultaneously account for energy minimisation. This new design tool provides the configuration of water networks and the stream data for the heat exchanger network through a simultaneous management of water and energy. After the connections between operations have been determined for the most energy efficient system, the thermal stream data are extracted to complete the heat exchanger network design. The individual stream data from each water main are collected. In order to complete the design, the heat exchanger network must be defined. For this purpose, the separate systems procedure developed in Part I of this paper will be used. As described earlier in Part I, the separate systems procedure consists of three steps. In the first step, the streams are selected from the two-dimensional grid diagram without considering any mixing for heat transfer, except the re-use streams involved in the non-isothermal mixing points, and the energy composite curves are generated. In the second step, streams are mixed isothermally within the temperature intervals in order to reduce the number of streams in each interval, and consequently the number of heat transfer units. In the third step, the separate systems generation and non-isothermal stream mixing are used to further reduce the number of heat transfer units.
4. Separate systems approach for problems with a pinch The complete network design for simultaneous energy and water minimisation will be generated for the example considering the water distribution from Case 4. This case
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L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308 6.905 kg/s
OP1 / 40 °C
20 kg/s
8.333 kg/s
4.762 kg/s
5.714 kg/s OP4 / 50 °C
0.952 kg/s
20 kg/s 50 kg/s
40 kg/s
OP3 / 75 °C 11.667 kg/s OP2 / 100 °C
37.381 kg/s
Water network Heat exchanger network
Stream Splitting
Stream Mixing
Fig. 18. Network design structure.
Table 5 Thermal stream data of Case 4
FW to Op FW to Op FW to Op WW from WW from WW from WW from
1 2 3 Op Op Op Op
1 2 3 4
TS (◦ C)
TT (◦ C)
CP (kW/◦ C)
Enthalpy (kW)
20 20 20 40 100 75 50
40 100 75 30 30 30 30
84 210 84 29 157 168 24
1680 16800 4620 −290 −10 990 −7560 −480
100 Temperature [°C]
Stream
4265 kW
75 65 Pinch 50 40 30 20
has been selected because fewer streams are involved in the heat recovery analysis. Moreover, in this case, the re-use streams are all involved in direct heat recovery. Therefore, the remaining problem of heat recovery involves only fresh water streams as cold streams and wastewater streams as hot streams, which enables a simple HEN design with fewer heat transfer units. Once the non-isothermal mixing for the water re-use streams is completed, the remaining design is to identify the matching of water streams by generating separate systems and appropriate location of separate systems, as represented in Fig. 18. The energy composite curves with Tmin (10 ◦ C in this case) are generated from the thermal stream data (Table 5). This results in a problem with a pinch at 75 ◦ C for the hot composite curve (Fig. 19). The introduction of non-isothermal stream mixing in the water re-use design might result in higher utility requirement than target. The targeting methods explained in Part I give the maximum attainable heat recovery for the systems, regardless of creating a pinch (i.e., a topology for water network is not considered). However, the new minimum utility consumption needs to be identified with the new water network
485 kW 15395 Enthalpy [kW] Fig. 19. Energy composite curves for Case 4.
Hot utility
Temperature
Splitting water stream Mixing water stream
E
Separate Systems
D
G
C B
A F
Pinch Non-isothermal stream mixing area
Cold utility Enthalpy Fig. 20. Separate systems generation for problems with a pinch (I).
L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308
D
A
C
H
B
Mixing water stream Hot utility
Temperature
..........
3301
G Non-isothermal stream mixing area ..........
F
Pinch B E
C
..........
B
E
D Wastewater discharge A
H Separate Systems Cold utility Enthalpy
Fig. 21. Separate systems generation for problems with a pinch (II).
Modified hot composite curve Hot composite curve
Same flowrate
J
A
C
H
B
Non-isothermal stream mixing area L
.......... Same flowrate
Mixing point ..........
J
L
..........
Cold composite curve
B H
A K
..........
K
Cold utility
Separate Systems Mixing water stream Enthalpy
Fig. 22. Separate systems generation for problems with a pinch (III).
New non-isothermal stream mixing area
-Qc
Hot utility
Temperature
Separate Systems Pinch
Qc Cold utility (a)
Pinch
Cold utility (b)
Pinch
Fig. 23. Stream matching for heat transfer with cold utility.
structure, in which the pinch limits the target minimum utility consumption (3780 kW). The minimum utility target was 3780 kW of hot utility, but now 4265 kW of hot utility and 485 kW of cold utility is required. As separate systems were generated for threshold problems in Part I, an extension of the previous method is required to deal with a problem with a pinch. Two possible ways are (1) fixing the hot composite curve and (2) fixing the cold composite curve. Now these methods will
FC Cold utility
Splitting water stream Mixing water stream Enthalpy
Fig. 24. Separate systems generation for problems with a pinch (IV).
be explored with a set of energy composite curves given in Figs. 20–24. It should be noted that the curves are illustrated for the general situation of a problem with a pinch, not based on the stream data of Example 1.
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First, the separate systems are generated by fixing the hot composite curve and shifting the cold composite curve across, above and below the pinch. The non-isothermal stream mixing area occurs between line ABCDE (modified cold composite curve) and line FCG (original composite curve) in Fig. 20. To satisfy the flowrate requirement overall, the split water stream should be appropriately placed for the modified cold composite curve. At points B, C, D, and E, a certain amount of water is split from the previous separate systems to increase the slope of the composite curve. However, the slope of line AB is greater than that of line FC, which means additional water stream should be added at Point A. This results in an additional freshwater requirement at Point F for the systems. Also the temperature at Point A is higher than the freshwater supply temperature (Point F), which requires additional hot utility to satisfy the temperature of Point A. From inspection of Fig. 20, for the design area below the pinch, shifting the cold composite curve is not favourable in terms of the penalty for freshwater consumption and hot utility. However, shifting the cold composite curve is a feasible design option for the design above the pinch. By contrast, the shape of the hot composite curve can be changed to create separate systems above and below the pinch (Fig. 21). The non-isothermal mixing areas are a quadrilateral BFGC above the pinch and a quadrilateral ADEB below the pinch. Shifting the hot composite curve is a feasible design option for the design above the pinch. Also, the matching between streams in the non-isothermal mixing area below the pinch seems to be straightforward, as illustrated in Fig. 21. However, the non-isothermal mixing may result in infeasible stream matching between mixing streams from the separate systems and streams from the original hot composite curve. This case is explained with Fig. 22, where the freshwater supplied is the same as the wastewater generated. The design region below the pinch is highlighted in Fig. 22, where the wastewater discharge flowrate from original cold composite curve (Point J) is the same as the flowrate from the last separate system (Point A). Following the non-isothermal mixing procedures, streams from the original composite curve (Points K, L, etc.) are distributed and then mixed at the mixing point after the cooler (Point A). But, no streams from the original composite curve are required at the mixing point between Points A and H, because the flowrate of Point A is the same with the wastewater generated from Point J. Therefore, the heat and mass balances around end points of hot composite curves are violated. However, there is a way to make a design below the pinch feasible by fixing the cold composite curve without any penalty. As the hot composite curve is constructed by combining individual hot stream profiles, an individual hot stream can be a candidate to lose its heat to cold utility (Fig. 23a). Part of, or the whole hot stream, is selected to match with cold utility. Then, the remaining problem becomes a system without cold utility requirement (Fig. 23b),
where both end points of hot composite curve and modified hot composite curve are met. This means that the final wastewater discharge conditions become the same as the exit conditions from the last separate system, which allows the feasible stream matching in the non-isothermal mixing area. With the new shape of composite curves, the separate systems can be generated and a feasible new non-isothermal mixing area is created by fixing the cold composite curve below the pinch. Overall, the design region should be considered for separate systems generation when a pinch point exists. In the above pinch part of the design, either fixing the hot composite curve or fixing the cold composite curve can be applied to create feasible separate systems and non-isothermal stream mixing. But below the pinch, matching between cold utility and individual hot streams should be selected and the separate systems be created with remaining streams. Fig. 24 illustrates separate systems generation for the problem with a pinch point when the hot composite curve is fixed above the pinch and the cold composite curve is fixed below the pinch. This design concept is now applied to Case 4 and the method of avoiding infeasible non-isothermal mixing below the pinch will be explained in more detail. Consider Case 4, in which the energy composite curves result in a problem with a pinch. Following the design Temperature [°C]
4265 kW 117.17 °C
100 75
50 28.72 °C
Pinch Non-isothermal stream mixing area (A2)
30 20
Non-isothermal stream mixing area (A1)
Separate Systems 485 kW Enthalpy [kW]
Fig. 25. Separate systems generation for Case 4.
Temperature 117.17˚C 37.381 kg/s
4265 kW 37.381 kg/s
32.619 kg/s 70 kg/s 20 kg/s 90 kg/s
32.619 kg/s 70 kg/s
20 kg/s 90 kg/s 485 kW
Splitting water stream Mixing water stream Enthalpy
Fig. 26. Stream splitting and mixing for separate systems.
L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308
New cold composite curve
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Non-isothermal stream mixing area (A1)
Original cold composite curve
OP3 75 °C
OP1 40 °C 20 kg/s
OP2 100 °C
20kg/s
50 kg/s
Pinch
Cold composite side 32.619 kg/s
20 kg/s 90 kg/s 20 °C
37.381 kg/s H
40 °C
65 °C
90 °C 4265 kW
117.166 °C
Pinch Fig. 27. Design structure for cold composite side.
Original hot composite curve
30 °C
New hot composite curve
Hot composite side Pinch
Pinch 30 °C 90 kg/s
28.717 °C
Non-isothermal stream mixing area (A2)
28.72 °C 30 °C C
50 °C
75 °C
20 kg/s
100 °C
32.619 kg/s
37.381 kg/s
37.381 6.905 kg/s 5.714 kg/s 40 kg/s kg/s OP4 OP3 OP1 OP2 75 °C 50 °C 100 °C 40 °C
Fig. 28. Design structure for hot composite side.
guidelines above, three separate systems are generated, as shown in Fig. 25. Above the pinch, the hot composite curve is fixed, while the cold composite curve is fixed below the pinch. The modified cold composite curve above the pinch should reach 117.17 ◦ C and the modified hot composite curve below the pinch ends at 28.72 ◦ C. Two non-isothermal mixing areas (A1 and A2) are created. The appropriate splitting and mixing of water streams is obtained from the modified composite curves (Fig. 26). The flowrates for mixing and splitting streams is calculated from the differences of enthalpy and temperature for each separate system (assuming the heat capacity is constant). The non-isothermal stream mixing area on the cold composite side (A1) is schematically represented in Fig. 27.
Two streams with 65 and 117.17 ◦ C are split from the modified cold composite curve. These streams are supplied to Operation 3 at 75 ◦ C and Operation 2 at 100 ◦ C. The area with in a circle in Fig. 27 represents the mixing area (A1) in the composite diagram. As indicated previously, modification of stream matching is required for the non-isothermal stream mixing area on the hot composite side (A2), as shown in Fig. 28. Based on the separate systems, the streams are distributed in the non-isothermal stream mixing area from the outlet streams of Operations 1, 4 and 3 to the streams added to the modified hot composite side. The temperature of 30 ◦ C for wastewater generated from the systems cannot be satisfied and the mass balance is also violated in the circle in Fig. 28.
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Pinch
90 kg/s
30 °C
C
50 °C
75 °C
20 kg/s
100 °C
32.619 kg/s
New non-isothermal stream mixing area (A3)
37.381 kg/s
32.619 kg/s 485 kW C 6.905 kg/s
5.714 kg/s
OP1 40 °C
OP4 50 °C
59.355 °C
7.381 kg/s 40 kg/s OP3 75 °C
37.381 kg/s OP2 100 °C
Fig. 29. Shifting the placement of cold utility.
4265 kW 117.17°C Temperature [°C]
100 Pinch 75
New non-isothermal stream mixing area (A3)
Non-isothermal stream mixing area (A1)
50 30 20
Separate Systems Enthalpy [kW]
Fig. 30. New energy composite curves for Case 4.
As explained in Fig. 23, the individual hot streams are searched to match with cold utility. Among the three streams, Operation 3 is selected to decrease the heat exchanger area because the outlet temperature of the stream from Operation 3 is higher than that of the stream from Operations 1 and 4. As presented in Fig. 29, the placement of cold utility is shifted from the modified cold composite side to the individual stream from Operation 3. New energy composite curves are generated (Fig. 30 ). Three separate systems and a new non-isothermal mixing area (A3) are created. No penalty is given to the generation of separate systems and the final network design is shown in Fig. 31. Non-isothermal stream mixing can be observed on the hot and cold composite sides, as well as water re-use between operations. In the identification stage for water re-use, the non-isothermal mixing reduces the number of streams involved in the following HEN design. It may result in an energy penalty, as seen in Case 4, but it enables a simple
HEN design with fewer number of heat transfer units. For the HEN design stage, the non-isothermal stream mixing allows separate systems to be created between a set of energy composite curves. This results in a simpler and better structure for the HEN.
5. Heat losses from the operations So far, each water-using operation has been considered to be isothermal. However, heat losses might take place within an operation. The consequences of heat losses to the targeting and design procedure will now be evaluated. In order to incorporate heat losses, non-isothermal operations need to be introduced. These operations can be characterised through a difference between the water operation inlet and outlet temperatures. The difference in temperature between the inlet and outlet water streams could result from heat transfer. Hence, mass and energy transfer could take place in the same unit. To understand the effect of heat losses, a comparison between a simple operation without heat loss and an operation with heat loss is necessary. This comparison is illustrated in Fig. 32 . In the absence of heat loss (Fig. 32a), the amount of energy that can be recovered from the hot water stream (outlet stream) is higher than in the case where there is an energy loss (Fig. 32b). Once the amount of energy recovered is decreased due to a lower outlet temperature of the water stream (TOp,OUT < TOp,IN ), the utility consumption increases. The temperature difference between the inlet and outlet water operation streams will be considered as a temperature loss difference (Tloss ). Thus, each operation with heat loss will be characterised by its temperature loss difference. The above representation gives a clear picture of the difference between the isothermal and non-isothermal unit
L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308 6.905 kg/s
OP1 / 40 °C 8.333 kg/s
4.762 kg/s
5.714 kg/s
OP4 / 50 °C
0.952 kg/s
7.381 kg/s
16.166 kg/s
OP3 / 75 °C
3.834 kg/s
C 485 kW
11.667 kg/s
OP2 / 100 °C 20 kg/s
32.619 16.453 kg/s kg/s
20 °C 30 °C
40 °C 50 °C
59.355 °C
33.547 kg/s 4265 kW
90 kg/s
3305
65 °C
90 °C
75 °C
100 °C
H
37.381 kg/s
117.166 °C
37.381 kg/s
90 kg/s
32.619 kg/s 20 kg/s
Fig. 31. Final network design for Case 4.
No Heat Loss T
Heat Loss T
Outlet water stream
TOp,IN TOp,OUT
TOp
Outlet water stream
Inlet water stream
Inlet water stream
TOUT
TOUT
TIN
TIN H
(a)
QH
(b)
Qloss
H QH
Fig. 32. Consequence of heat loss from a water-using operation.
operations. The consequence of heat losses from waterusing operations can be translated into larger values of utility consumption. Since the heat losses lead to high utility consumption, a new way to target the utility consumption for water systems with non-isothermal operations is needed. 5.1. Targeting utility consumption In the previous paper (Studies on Simultaneous Energy and Water Minimisation—Part I), a targeting analysis for simultaneous energy and water minimisation problems was introduced. The idea is to target the water and energy consumption before design. When there are heat losses from the water-using operations, the water target is unchanged but the minimum utility consumption changes. The temperature of the water supply is assumed to be fixed. Heat can be recovered from the effluent, subject to Tmin (10 ◦ C in this case). If there are no heat losses or flowrate changes, then the utility required is directly related to the flowrate of water through the overall system and the water supply and discharge temperatures, as given in the
equation below (Eq. (1)). This relation can be used to determine the utility consumption for the no re-use situation, as well as for re-use cases QH = F Cp(TOUT − TIN ).
(1)
Once heat losses are considered, the formula for utility targeting needs to be changed. Equations need to be defined as a function of the water usage, inlet and outlet water operation temperatures (defined according to Tmin ) and the temperature difference that defines the heat losses (Tloss ). For the no re-use of water policy, two formulae can be established according to the value of temperature loss difference. Eq. (2) gives the formula for systems with water-using operations characterised by the same temperature loss difference. While, for a system with operations with different values of Tloss,i (where i is the operation number) the utility target is determined from Eq. (3). In this second case of different Tloss,i , the heat loss is calculated for each operation and added to the utility consumption. The heat loss from each operation is a function of the water flowrate through that particular unit (Fi ) and the temperature loss difference
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Table 6 Water-using operation data—Example 2 CIN (ppm)
Operation
COUT (ppm)
TOp,IN (◦ C)
Operation 1 0 100 40 Operation 2 50 100 100 Operation 3 50 800 75 Operation 4 400 800 50 Temperature of fresh water source, TIN = 20 ◦ C. Temperature of discharge wastewater, TOUT = 30 ◦ C.
TOp,OUT (◦ C)
Limiting water flowrate (kg/s)
Contaminant mass load (g/s)
30 90 65 40
20 100 40 10
2 5 30 4
Table 7 Energy loss data Operation
Operation Operation Operation Operation
1 2 3 4
TOp,IN (◦ C)
TOp,OUT (◦ C)
40 100 75 50
30 90 65 40
Total
Flowrate (kg/s)
Energy loss (kW)
20 50 40 5.7
840 2100 1680 240
115.7
4860
that characterises the respective unit (Tloss,i ).
Then, the utility target could be determined using the targeting relations previously formulated through a heat balance for the overall water system . For this example, the water target is determined from the slope of the water supply line to be 90 kg/s (Fig. 1). The heat losses from the water-using operations are shown in Table 7, and the total energy loss from the whole four unit operations is 4860 kW. The utility consumption for this example can be calculated using the formula in Eq. (4) that characterises systems with the same value of the temperature loss difference for each operation. QH = F Cp Tmin + (Fi )Cp Tloss = 8640 kW.
QH = F Cp(Tmin + Tloss ),
(2)
QH = F Cp Tmin + (Fi Cp Tloss,i ).
(3)
5.2. Network design of a system with heat losses
In the case when water can be re-used from one operation to another for water minimisation, the utility consumption can be determined with the following formulae, according to the values of temperature loss differences between operations. Eq. (4) presents the formula for utility calculation for systems characterised by the same Tloss for each unit operation. (Fi ) represents the summation of the water flowrate passing through each unit operation i. When Tloss,i differs in the water system, the utility consumption can be calculated by the relation given in Eq. (5). QH = F Cp Tmin + (Fi )Cp Tloss ,
(4)
QH = F Cp Tmin + (Fi Cp Tloss,i ).
(5)
Using these equations, the target for utility consumption in a water system can be easily determined once the target for water consumption has been established. In order to evaluate the above targeting procedure for utility consumption in systems with heat losses and evaluate the applicability of the design procedure introduced earlier for simultaneous energy and water minimisation, an example will be considered with Example 2. The water-using operation data (inlet and outlet limiting concentrations, inlet and outlet operating temperatures, limiting water flowrates and contaminant mass loads) are given in Table 6. This set of data is characterised by the same temperature loss difference of 10 ◦ C for each unit operation. Based on the water-using operation data, the limiting water profile can be plotted and the water target identified.
After targeting the water and energy consumption, the design procedure follows. The design tool for simultaneous energy and water minimisation introduced in Part I will be applied to cases with heat losses from the water-using operations. Previously, water-using operations have been considered to be isothermal, and therefore have been represented as horizontal lines in the two-dimensional grid diagram. However, when an operation has the outlet water temperature different from the inlet water temperature due to heat loss, its representation in the two-dimensional grid diagram is a line between two temperature levels. Thus, a non-isothermal operation is illustrated through a sloping line. For the example, Operation 1 is illustrated in the grid diagram as a line from 40 to 30 ◦ C, Operation 2 has the starting point on the 100 ◦ C temperature level and end point on the 90 ◦ C temperature level, and so on (Fig. 33). In this case, all operations have heat losses, hence, the operation lines point to lower temperature levels. In the case when water is heated when passing through an operation, then its outlet water temperature is higher than its inlet water temperature. Thus, the water stream gains heat inside the operation and its temperature is raised. Such operations can be represented through lines going up from a low temperature level (inlet water temperature) to a high temperature level (outlet water temperature). Fig. 33 shows the results for the first part of the design giving connections between operations for the most efficient energy usage. This design includes non-isothermal mixing
L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308 Limiting water flowrate (kg/s)
Freshwater 90 kg/s 0 ppm
T (°C)
100
40
80
0 kg/s 800 ppm
45.7 kg/s 100 ppm 50 kg/s
100
3307
100 °C 2
20 kg/s
75 °C 3
40 kg/s 65 °C
20 kg/s
60
5.7 kg/s
4 50 °C
10 20
20 kg/s
1
40
20
Water source
C (ppm)
Wastewater 0 kg/s
40 °C
Water discharge
Water discharge
Wastewater 44.3 kg/s
Wastewater 45.7 kg/s
Fig. 33. Two-dimensional grid diagram including operations with heat losses.
Table 8 Thermal stream data Stream
FW to Op FW to Op FW to Op WW from WW from WW from
1 2 3 Op 2 Op 3 Op 4
TS (◦ C)
TT (◦ C)
Flowrate CP (kW/◦ C) (kg/s)
Enthalpy (kW)
20 20 20 90 65 40
40 100 75 30 30 30
20 50 20 33.1 40 5.7
1680 16 800 4620 −8340 −5880 −239.4
84 210 84 139 168 24
Temperature[°C]
8640 kW
142.16 °C
100 90
the profile of the composite curves after the separate systems have been generated. The design procedure for the heat exchanger network based on the separate systems approach has led to a network design shown in Fig. 35. It is noted that the separate systems can be also generated by shifting down the hot composite curve. For this example, the wastewater stream from Operation 1 has its temperature equal to the discharge temperature. Therefore, this stream will not be involved in the heat exchanger network, and consequently a smaller number of units results. The design procedure for simultaneous energy and water minimisation based on the two-dimensional grid diagram and separate systems approach can be applied for systems with water-using operations involving heat losses. 6. Conclusions
Separate Systems 65 Non-isothermal stream mixing area 40 30 20 Enthalpy [kW]
Fig. 34. Energy composite curves and separate systems generation.
of re-use water streams in order to achieve the temperature conditions for the inlets of Operation 3 and Operation 4. The remaining cold/hot streams within each water main (Table 8) will be analysed using the separate systems procedure. Based on the above thermal data (Table 8) and, the energy composite curves have been plotted and then the separate systems generated by shifting the cold composite curve up. Fig. 34 illustrates the initial composite curves and also
A new design procedure has been developed to achieve both water and energy targets for systems using water at different temperatures and re-use of water. This procedure involves two stages. In the first stage, re-use options within the water system are exploited not only from the point of view of contaminant concentration, but also considering energy. A new grid representation (the two-dimensional grid diagram) has been introduced. After identifying the configuration of the water-using network considering energy, in the second stage a separate systems approach is used to ensure the simplicity in the HEN. This new design procedure also addresses the complexity issue involved in the overall design of water and energy networks. The simultaneous energy and water management analysis of systems with heat losses indicates a penalty in the energy consumption resulting from the loss. The tools introduced (the two-dimensional grid diagram and separate systems procedure) can also be applied to cases with heat loss from the operations.
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OP1 (40 °C→30 °C)
11.190 kg/s 5 kg/s
3.801 kg/s
5.714 kg/s
OP4 (50 °C→40 °C) 1.905 kg/s
OP3 (75 °C→65°C) 15 kg/s
OP2 (100 °C→90 °C)
90 kg/s 20 °C
30 °C
55 °C
30 °C
40 °C
65 °C
H
142.16 °C 80 °C 8640 kW 90 °C
90 kg/s
33.095 kg/s 40 kg/s
Fig. 35. Overall design of water and heat exchanger network—System including heat losses from operations.
Notation C C
CIN COUT CP Cp CR CV Hvap Tmin Tloss Tloss,i F Fi Floss FLS FS,DU FS,IU Fw H H
LS M MW
concentration, ppm cooler limiting inlet concentration, ppm limiting outlet concentration, ppm flowrate capacity, kW/◦ C heat capacity at constant pressure, kJ/kg ◦ C condensate return heat capacity at constant volume, kJ/kg ◦ C vaporisation heat, kJ/kg minimum temperature approach, ◦ C loss of water temperature in overall systems, ◦ C loss of water temperature in each operation, ◦ C water flowrate, kg/s water flowrate in each operation, kg/s flowrate loss, kg/s flowrate of live steam, kg/s flowrate of live steam with direct use, kg/s flowrate of live steam with indirect use, kg/s freshwater flowrate, kg/s enthalpy, kW heater live steam mass load, g/s make-up
OP QC QCS Qloss QH QHS T Tin TLS TOp TOp,IN TOp,OUT TS TT Tout
operation cold utility, kW available heat of cold stream, kW heat loss, kW hot utility,kW available heat of hot stream, kW temperature, ◦ C temperature of freshwater, ◦ C temperature of live steam, ◦ C water temperature of operation, ◦ C inlet water temperature of operation, ◦ C outlet water temperature of operation, ◦ C supply temperature of stream, ◦ C terminal temperature of stream, ◦ C discharge temperature of wastewater, ◦ C
References Alva-Argaez, A., 1999. Integrated design of water systems. Ph.D. Thesis, UMIST, Manchester, UK. Doyle, S.J., Smith, R., 1997. Targeting water reuse with multiple constraints. Transaction of IChemE 75 (Part B), 181–189. Kuo, W.J., Smith, R., 1998. Designing for the interactions between water-use and effluent treatment. Transaction of IChemE 76 (Part A), 287–301. Wang, Y.P., Smith, R., 1994. Wastewater minimisation. Chemical Engineering Science 49 (7), 981–1006.
Studies on simultaneous energy and water minimisation—Part II: Systems with maximum re-use of water Luciana Savulescu1 , Jin-Kuk Kim∗ , Robin Smith Centre for Process Integration, School of Chemical Engineering and Analytical Science, The University of Manchester, P.O. Box 88, Manchester, M60 1QD, UK Received 26 September 2003; received in revised form 11 November 2004; accepted 23 December 2004 Available online 23 March 2005
Abstract A new systematic design methodology has been developed for the simultaneous management of energy and water systems that also feature maximum re-use of water. A two-dimensional grid diagram is proposed to exploit different options within water systems and also enable reduced complexity of the energy and water network. Isothermal and non-isothermal stream mixing between water streams are introduced to create separate systems between hot and cold water streams in the energy composite curves and provide a design basis for a better structure with fewer units for the heat exchanger network. In addition to allowing re-use of water, issues about heat losses inside unit operations have also been incorporated in the simultaneous management of water and energy. 䉷 2005 Elsevier Ltd. All rights reserved. Keywords: Water minimisation; Energy recovery; Heat loss; Water loss; Separate system
1. Introduction In the previous paper, Studies on Simultaneous Energy and Water Minimisation—Part I, a new systematic design methodology has been presented for simultaneous energy and water management problems in which water re-use was not allowed. A new strategy has been adopted to provide the simplicity in the HEN design with a reduced number of heat transfer units. This is based on the generation of separate systems and non-isothermal stream mixing. However, the re-use of water between water-usingoperations was not considered in Part I. The water and wastewater could be further minimised by allowing water re-use (Wang and Smith, 1994; Kuo and Smith, 1998; Doyle and Smith, 1997; Alva-Argaez, 1999). In this paper (Part II), ∗ Corresponding author. Tel.: +44 161 306 8755; fax: +44 161 236 7439.
E-mail address: [email protected] (J.-K. Kim). 1 Current address: CANMET Energy Diversification Research Labora-
tory, 1615 Lionel-Boulet Blvd., P.O. Box 4800, Varennes, Que., Canada J3X 1S6. 0009-2509/$ - see front matter 䉷 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2004.12.036
the methods presented in Part I will be extended to systems with maximum water re-use where freshwater consumption and wastewater generation are minimised simultaneously with energy consumption. Combinatorial features exist between water re-use, energy recovery, and network design in systems with water re-use. Therefore, new methods are required to investigate the diversity of design options. In this paper, the aim is to propose design methods by incorporating energy and water aspects into the overall design framework for simultaneous energy and water minimisation. Also, several assumptions have initially been made to simplify the analysis in Part I. These assumptions included various operations requiring water at different concentrations and temperatures; each of the operations has a fixed value for temperature supply; no heat losses are allowed in operations; no flowrate loss or gain through an operation; and non-water-using operations were neglected. Later in this paper, the consequences from the relaxation of these assumptions will be discussed, especially the effects of heat losses.
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800
Op 1
Op 3
Op 2
Op 4
C (ppm)
Limiting Composite Curve Water Supply Line
400 Pinch 90 kg/s
100 50 9
41
Op 1
Op 3
Op 2
Op 4
Op 1
Op 4
Op 2
Op 3
M (g/s)
Fig. 1. Different design networks for maximum re-use of water—Example 1.
Table 1 Water-using operation data—Example 1 Operation
CIN (ppm)
COUT (ppm)
TOp,IN (◦ C)
Operation 1 0 100 40 Operation 2 50 100 100 Operation 3 50 800 75 Operation 4 400 800 50 Temperature of fresh water source, TIN = 20 ◦ C Temperature of discharge wastewater, TOUT = 30 ◦ C.
TOp,OUT (◦ C)
Limiting water flowrate (kg/s)
Contaminant mass load (g/s)
40 100 75 50
20 100 40 10
2 5 30 4
2. Energy aspects of water systems
20 kg/s 90 kg/s
Moving now to the analysis of systems with maximum reuse of water, a more elaborate design procedure is required when re-use is considered. Different water networks can achieve the water target (Fig. 1). However, these networks involve different connections between the water-using operations that can require different energy consumptions. If a water system is analysed for energy, the water stream distribution needs to be known. Thus, the water consumption and the water-using network need to be determined before the energy analysis. In order to explore the interactions between the energy and water consumption and gain an understanding of how the systems function, Example 1 illustrated in Part I will be revisited (Table 1). The energy analysis for a water-using operation network will be examined. Different re-use water network structures have been examined for energy consumption. Two different types of heat recovery can be considered: indirect and direct heat recovery. The heat recovery systems have been imposed by the configuration of the water network and therefore by the thermal stream data extracted from the water network. As discussed by Wang and Smith (1994), the water network design can in some cases be evolved, whilst maintaining the freshwater consumption. Fig. 2 shows one of the possible water network structures for Example 1, which can achieve the target for maximum re-use of water, in this case 90 kg/s.
20°C 0 ppm
20 kg/s
50 kg/s
40 kg/s Operation 1 /40°C Operation 2/100°C
Operation 3/75°C 5.7 kg/s
Operation 4/50°C
90 kg/s 30°C
44.3 kg/s
Fig. 2. Water network 1—Example 1. Table 2 Thermal stream data for energy analysis (Water network 1) Stream
TS (◦ C) TT (◦ C) CP (kW/◦ C) Enthalpy (kW)
FW to Op 1 20 FW to Op 2 20 FW to Op 3 20 RW from Op 1 to Op 3 40 RW from Op 2 to Op 4 100 WW from Op 2 100 WW from Op 3 75 WW from Op 4 50
40 100 75 75 50 30 30 30
84 210 84 84 23.94 186.06 168 23.94
1680 16800 4620 2940 −1197 −13 024.2 −7560 −478.8
Once the water network is fixed (Fig. 2) the stream data for the energy recovery problem can be extracted. The thermal stream data for the indirect heat recovery analysis are shown in Table 2. The energy composite curves can be plotted for a minimum temperature difference of 10 ◦ C (Fig. 3) and the potential energy recovery and utility consumption obtained. The system is a threshold problem with
L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308
0 ppm
3780 kW
20 °C
100 Temperature (°C)
Operation 1 / 40 °C
90
50 ppm
80
20 °C
70
Operation 2 / 100 °C
50 ppm
60
20 °C
∆Tmin = 10°C
50
Operation 3 / 75 °C
400 ppm
40
Operation 4 / 50 °C
20 °C
30 20
3293
100 ppm 30 °C 100 ppm 30 °C 800 ppm 30 °C 800 ppm 30 °C
Fig. 5. Individual operations—Example 1.
5000
10000 15000 20000 Enthalpy (kW)
25000
30000
Fig. 3. Energy composite curves for Water network 1.
a hot utility requirement of 3780 kW. To complete the picture, the energy design method has been carried out and the heat exchanger network determined by applying conventional heat exchanger network design procedures. The heat exchanger network design is shown in Fig. 4. When the energy aspects of water systems are considered there are some distinctive features compared with heat exchanger networks in general. These result from the degree of freedom to mix and split streams in the network compared with classical heat integration where the streams are a variety of fluids and the splitting and mixing options are limited to be restricted to the same stream. The problem addressed here involves only water streams. This allows mixing and splitting of water streams in ways that will allow the overall network of water and heat exchangers to be simplified. However, some restrictions in mixing water streams are imposed by the contaminant concentration constraints in the re-use options. An important observation related to mixing water streams comes from re-use alternatives. First, the individual operations are shown in Fig. 5. Inspection of the limiting concentrations allows some re-use alternatives to be identified. As a result of the analysis of the inlet and outlet concentrations of each of the water-using operations, water from
Operations 1 and 2 could be re-used in Operation 4. There are different possibilities to re-use the water from Operations 1 and 2 to satisfy the requirements of Operation 4. These options are presented in Fig. 6. Option 1: Re-use of water from Operation 1 into Operation 4. In this case, the re-use stream needs heating because the source temperature is lower than the sink temperature. Option 2: Re-use of water from Operation 2 into Operation 4. Here, the re-use water stream has a higher temperature than is required in Operation 4 and therefore a cooling duty is required. Option 3: Re-use of water from Operations 1 and 2 into Operation 4. Two different temperature streams are mixed in order to satisfy the requirements of Operation 4 without the need for either heating or cooling. The concentration constraints allow different re-use options to be exploited, but the energy implications of the options could be very different. Therefore, there are clearly interactions between re-use options and the energy consumption. From Fig. 2, two re-use water sources can be identified. These are the outlet water streams from Operations 1 and 2. These re-use water streams are characterised by different temperature levels, but the same concentration (100 ppm from Table 1). The sink operations for the re-use of water (Operations 3 and 4) are characterised by intermediate temperatures between the re-use water sources
100°C
30°C
Wastewater Streams (hot)
75°C 50°C
30°C
Re-use Stream (hot)
100°C
50°C
30°C
75°C 40°C
40°C H
75°C 100°C
H H
20°C 20°C 20°C
Fig. 4. Heat exchanger network (Water network 1).
Re-use Stream (cold) Fresh water Streams (cold)
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T (°C)
100
2
2
2 Non-isothermal mixing
C 4
50
4
4
H 40
1
1
1 Option 2
Option 1
Option 3
Fig. 6. Energy implications in the re-use options.
20 kg/s
6.91 kg/s
Operation 1 / 40°C 5.7 kg/s
90 kg/s 8.34 kg/s
20°C 0 ppm
20 kg/s
Operation 4 / 50°C
40 kg/s Operation 3 / 75°C
50 kg/s
11.66 kg/s
Operation 2 / 100°C
90 kg/s 30°C
37.39 kg/s
Fig. 7. Water network 2—Example 1.
Table 3 Thermal stream data for energy analysis (Water network 2)
6426 kW 100
TS (◦ C)
TT (◦ C)
CP (kW/◦ C)
Enthalpy (kW)
FW to Op 1 FW to Op 2 FW to Op 3 WW
20 20 20 81
40 100 75 30
84 210 84 378
1680 16800 4620 −19 278
90 Temperature (°C)
Stream
80 70 60 50 40
(TOp1 < TOp4 < TOp3 < TOp2 ). Therefore, the heat from the hot re-use stream (the outlet from Operation 2) could be recovered through direct contact with the cold re-use water stream (the outlet from Operation 1) in order to achieve the temperature requirements for the sink operations. From the above considerations, some modifications to the water network structure should be made. The new network configuration is illustrated in Fig. 7. Within this network there are two non-isothermal mixing points. One non-isothermal mixing point has been introduced for the inlet of Operation 3, such that re-use water from Operations 1 and 2 are mixed with some fresh water. The other non-isothermal mixing point takes place at the inlet of the Operation 4 that uses water from Operations 1 and 2. Apart from these non-isothermal mixing points for the thermal stream data, all the wastewater streams have been mixed together to form one effluent stream. Consequently, the thermal stream data for this new water network configuration are given in Table 3. These stream data do not include the streams involved in the non-isothermal mixing points. Hence, these streams are subject to indirect heat recovery.
30 20 5000 2604 kW
10000 15000 20000 Enthalpy (kW)
25000
30000
Fig. 8. Energy composite curves for Water network 2.
Based on these stream data, the energy composite curves have been constructed for a minimum temperature difference of 10 ◦ C (Fig. 8) and the energy targets determined. The grid representation of the heat exchanger network, including the non-isothermal mixing points for this problem, is presented in Fig. 9. In the case of Water Network 2, the problem is pinched with a large consumption of hot and cold utilities compared with the case of Water Network 1, which is a threshold problem. Summarising, indirect heat transfer can be used to recover the maximum amount of heat subject to temperature difference limitations. Alternatively, direct heat recovery can be used by mixing streams. This second alternative requires fewer heat exchangers, but there is a sacrifice in energy consumption. A comparison of indirect and direct recovery
L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308
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100°C 75°C Wastewater Streams (hot) Re-use Stream (hot)
50°C
81°C
40°C
30°C C
100°C
50°C
75°C
40°C
40°C
20°C
75°C 100°C
H H
20°C 20°C
Re-use Stream (cold) Fresh water Streams (cold)
Fig. 9. Heat exchanger network for Water network 2.
Table 4 Summary of indirect and direct heat recovery for Example 1 Heat recovery
QH (kW)
QC (kW)
Number of units
Indirect heat recovery Direct and indirect heat recovery
3780 6426
— 2604
9 6
systems in terms of utility consumption and the number of heat transfer units for Example 1 is given in Table 4. The difference between the energy consumption of the indirect and direct heat recovery schemes is caused by the degradation of driving force when streams are mixed. Mixing at different temperatures acts as a heat transfer unit. So far, there are possibilities to solve the energy minimisation problem using not only indirect heat recovery, but also direct heat recovery. Mixing of water streams can be done at the same temperature (isothermal mixing of streams) and/or at different temperatures (non-isothermal mixing of streams). On the other hand, splitting streams is used in order to achieve the needs of the water and energy network design structure. Although starting from the same water-using operation data, the way to use indirect and direct heat transfer would result in a different energy consumption and design of the heat exchanger network. Thus, a systematic method is required to simultaneously determine the water and energy targets and the design procedure to achieve the heat exchanger network targets. 3. Two-dimensional grid diagram When the energy aspects need to be considered for a water system, new features should be included in the water network design strategy to account for the energy implications in water re-use paths. For a clear picture of the problem a new representation is required. Since both water and energy need to be addressed to solve the problem of simultaneous energy and water management, some modifications to the water network design procedure must be considered.
In this sense, a temperature scale has been incorporated into the water network design grid. A temperature scale has been adopted to account for the energy aspects in the water network and to guide the designer to identify water networks with high energy efficiency. In other words, this new representation will consider re-use options and heat recovery simultaneously in the design. The new tool is called the twodimensional grid diagram. A description of design steps will be explained with Example 1 (Table 1) that was introduced in previous paper (Part I). Step 1: Set up the two-dimensional grid diagram The two-dimensional grid diagram is an extension of the water network design introduced by Kuo and Smith (1998). This new design grid represents a concentration scale on the horizontal axis (according to the water mains concept), and temperature scale on the vertical axis (Fig. 10). In this diagram, the operations are positioned according to their operating temperatures and concentration limiting data. The operations are drawn as horizontal lines as long as isothermal operations are used. For example, Operation 1 needs to be fed with 20 kg/s of fresh water at 40 ◦ C and will discharge 20 kg/s wastewater at 100 ppm and 40 ◦ C. With this new representation, additional details are provided to the designer. Now, the water mains are characterised not only by the inlet and outlet water flowrates and concentrations, but also by information on the temperature of the available/required water. The fresh water source is shown in this representation at the bottom of the first water main according to its supply temperature (20 ◦ C in this case). The other water mains illustrate the level of wastewater discharge temperature (30 ◦ C in this case). Step 2: Connect operations with water mains and water distribution inside mains Since the fresh water is supplied at 20 ◦ C in this case and the water-using operations require different temperature levels, a water distribution needs to be made inside the water main (Fig. 11). This water distribution takes into account the concept of individual water streams introduced earlier. The vertical lines inside the water mains are the result of water distribution inside the mains, and represent the streams for the energy analysis. The streams moving upwards are
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Limiting water flowrate T (°C) (kg/s) 100
100
40
80
Freshwater 90 kg/s 0 ppm
0 kg/s 800 ppm
45.7 kg/s 100 ppm 2
3
3
60 10 20
4 40 20
C (ppm)
1
Water source Wastewater 0 kg/s
Water discharge
Water discharge
Wastewater 44.3 kg/s
Wastewater 45.7 kg/s
Fig. 10. Two-dimensional grid diagram—Step 1.
Limiting water flowrate T (°C) (kg/s) 100
100
40
80
Freshwater 90 kg/s 0 ppm
45.7 kg/s 100 ppm
50 kg/s
20 kg/s
0 kg/s 800 ppm
2
3
40 kg/s
3
60
5.7 kg/s
10 20
40 20
C (ppm)
1
4
20 kg/s
Water source Wastewater 0 kg/s
Water discharge
Water discharge
Wastewater 44.3 kg/s
Wastewater 45.7 kg/s
Fig. 11. Two-dimensional grid diagram—Step 2.
cold streams since their temperatures need to be increased, which means that they require a heat input to satisfy the energy requirements. By supplying fresh water at different temperatures as the operations require, the intermediate water main at 100 ppm receives water at different temperature levels. In this example, the water from Operation 1 reaches the main at 40 ◦ C, while the water from Operation 2 enters at 100 ◦ C, even though both water streams are at 100 ppm. At this stage, the water distribution inside the intermediate water main is not considered. In order to supply Operations 3 and 4 within the second interval (between the second and third water mains), water is extracted from the intermediate main at the required operating temperature levels (75 and 50 ◦ C, respectively). Consequently, these operations will produce
wastewater streams with a level of 800 ppm contaminant concentration, as the limiting data indicates, but at different temperatures according to temperature operating conditions. These wastewater streams need cooling to satisfy the water discharge temperature conditions. Therefore, two hot streams will populate the water main at 800 ppm. Step 3: Merge operations crossing boundaries In the previous step, Operation 3 has been divided into two parts by the 100 ppm water main, and each part has been analysed individually. The result of this split has led to different water flowrate requirements on each side. Therefore, a uniform flowrate across the operation is the objective of this step. This is achieved by changing the point where Operation 3 is fed with the re-use stream (i.e., the water from the middle water main is supplied to the inlet of Operation
L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308
Limiting water flowrate T (°C) (kg/s) 100
100
40
80
Freshwater 90 kg/s 0 ppm
45.7 kg/s 100 ppm
50 kg/s
20 kg/s
3
40 kg/s 75 °C 5.7 kg/s
10 20
20
Water source
C (ppm)
4
20 kg/s
1
40
0 kg/s 800 ppm
2
20 kg/s
60
3297
Wastewater 0 kg/s
Water discharge
Water discharge
Wastewater 44.3 kg/s
Wastewater 45.7 kg/s
Fig. 12. Merge operations crossing intermediate water mains—Step 3.
Limiting water flowrate T (°C) (kg/s) 100
100
40
80
Water source
Freshwater 90 kg/s 0 ppm
45.7 kg/s 100 ppm
0 kg/s 800 ppm
50 kg/s 2 20 kg/s 3
40 kg/s 75 °C
20 kg/s
60
5.7 kg/s 4
10 20
Water sink
20 kg/s 1
40 20
Water source
C (ppm)
Water source
Wastewater 0 kg/s
Water discharge
Water sink
Wastewater 44.3 kg/s
Water discharge Wastewater 45.7 kg/s
Fig. 13. Water sources/sinks for the intermediate water main.
3, not to the middle of Operation 3), which allows the same flowrate for Operation 3 without violating inlet contamination level. The temperature of the re-use water should be 75 ◦ C to satisfy the temperature condition of Operation 3 (Fig. 12). How this temperature is obtained is shown in the next design step of the two-dimensional grid diagram. Step 4: Remove intermediate water mains through water and energy analysis In order to remove the intermediate main at 100 ppm, the connections between the sources and sinks at this level of concentration need to be established. The 100 ppm water main is characterised by two sources and two sinks (Fig. 13). One of the water sources is provided at 40 ◦ C by Operation 1. The other source reaches the 100 ppm water main at 100 ◦ C from Operation 2. The sink operations (Operations
3 and 4) require water at 75 and 50 ◦ C, respectively. These sources and sinks can be matched in different ways to remove the intermediate main. However, different connections will involve a variation in the energy features. Therefore, an evaluation of energy requirements for the re-use water streams is implied. A number of re-use cases with different connections between sources and sinks can be considered from the energy point of view in order to determine the design rules for simultaneous energy and water minimisation. The analysis of four different re-use arrangements inside the pinch water main (100 ppm) will now be examined. Case 1: In this case, the re-use water is distributed starting with the coldest source (Fig. 14). Thus, water from Operation 1 is re-used in Operation 3. However, due to the
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45.7 kg/s 100 ppm
45.7 kg/s 100 ppm
100°C
2
100 °C
2
75 °C
3
75 °C
3
40 °C
4
50 °C
50 °C
4 40 °C
1
30 °C
1
30 °C Water discharge Wastewater 44.3 kg/s QHS = 14,221 kW
Water discharge Wastewater 44.3 kg/s QHS = 11,520 kW QCS = 239 kW
QCS = 2,940 kW Fig. 15. Water re-use inside intermediate water main: Case 2. Fig. 14. Water re-use inside intermediate water main: Case 1.
temperature difference, a 2940 kW heating duty is required for this stream. Since the water source from Operation 1 is used in Operation 3 completely, Operation 4 needs to be fed with water from Operation 2. This new re-use stream is a hot stream with 1197 kW available heat. The remaining water, 44.3 kg/s from Operation 2 is sent to discharge. This wastewater stream can provide 13 024 kW heat. In summary, two re-use streams have been considered. One cold stream resulting from the re-use of water between Operations 1 and 3, and one hot stream resulting from the re-use from Operations 2 to 4. The wastewater from this main is represented by one hot stream. All of these streams are subject to indirect heat recovery. Case 2: A different strategy for re-using water has been considered for this second case (Fig. 15). Each re-use water source has been connected with the nearest sink (in terms of temperature). Water from Operation 1 is re-used in Operation 4, which has an operating temperature closer to the temperature condition of its re-use water source. The resulting re-use stream is a cold stream with a heat requirement of 239 kW. Since Operations 2 and 3 are nearest in terms of temperature, a re-use stream will connect these operations. This new re-use stream is a hot stream with a duty of 2100 kW. The remaining water from each source is collected and discharged. Therefore, two hot wastewater streams are generated. In this situation, the number of streams is increased to four (two re-use streams and two wastewater streams), which will result in a heat exchanger network with a larger number of units. However, the energy involved in the hot streams is
11 520 kW, and the energy required by the cold streams is 239 kW, lower than the previous case. Case 3: In this case, the re-use water is distributed starting with the hottest source, which is provided by Operation 2 (Fig. 16). The amount of water supplied by this source is high enough to satisfy both sinks (Operations 3 and 4). By connecting Operation 2 with the sink operations, two hot streams are created. The remaining wastewater from Operation 2 is sent to discharge. This stream is a hot stream, since its temperature should be reduced from 100 to 30 ◦ C. The other water source from Operation 1 is discharged as a hot stream. In this case, within the intermediate water main four hot streams have been obtained (two re-use streams and two wastewater streams). The energy available through these hot streams is 11 281 kW and is considered for indirect heat recovery. Note that there are no cold streams in this arrangement. Thus, this arrangement involves less energy compared with the previous two cases. Case 4: Since the sink units operate at intermediate temperatures compared with the source operations, the idea of mixing water streams at different temperatures might be considered (Fig. 17). By re-using water from both sources through non-isothermal mixing points the conditions of sink operations are satisfied, not only in terms of concentration, but also temperature. These re-use streams are subject to direct heat recovery. The remaining water for discharge is represented by two hot streams with a heat load of 11 281 kW. These wastewater streams are considered for indirect heat recovery.
L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308
45.7 kg/s 100 ppm 100 °C
2
75 °C
3
50 °C 40 °C
4
The energy analysis inside of the intermediate water mains concerns the level of the energy recovery. Since the waste water should be discharged at a lower temperature than the operating temperatures, it is better to achieve part of this duty by considering hot re-use streams instead of cold re-use streams. Therefore, the unnecessary extra energy in the heat exchanger network should be eliminated by an appropriate stream arrangement. Through the analysis of these four different re-use configurations, some conclusions can be drawn. These conclusions can be formulated as design rules for simultaneous energy and water minimisation.
1
Design Rule 1: Start to distribute the re-use water from the hottest source. Design Rule 2: Connect the re-use water source with the nearest sink (in terms of temperature). Design Rule 3: Introduce non-isothermal mixing points if the temperatures of the sink operations are intermediate to the temperature of source operations.
30 °C Water discharge Wastewater 44.3 kg/s QHS = 11,281 kW QCS = 0 kW Fig. 16. Water re-use inside intermediate water main: Case 3.
45.7 kg/s 100 ppm 100 °C
2
75 °C
3
50 °C 40 °C
3299
4 1 C
30 °C Water discharge Wastewater 44.3 kg/s QHS = 11,281 kW QCS = 0 kW Fig. 17. Water re-use inside intermediate water main: Case 4.
This arrangement has the advantage of achieving the temperature conditions of the sink operations through direct heat recovery, which means that the number of streams for indirect heat recovery is decreased. Therefore, the number of units in the heat exchanger network will also decrease. Even though the configuration of Case 4 is characterised by the same energy as Case 3 fewer streams are involved, so fewer heat transfer units.
In summary, the two-dimensional grid diagram has been introduced in order to exploit the options within the water system, not only for water minimisation, but also to simultaneously account for energy minimisation. This new design tool provides the configuration of water networks and the stream data for the heat exchanger network through a simultaneous management of water and energy. After the connections between operations have been determined for the most energy efficient system, the thermal stream data are extracted to complete the heat exchanger network design. The individual stream data from each water main are collected. In order to complete the design, the heat exchanger network must be defined. For this purpose, the separate systems procedure developed in Part I of this paper will be used. As described earlier in Part I, the separate systems procedure consists of three steps. In the first step, the streams are selected from the two-dimensional grid diagram without considering any mixing for heat transfer, except the re-use streams involved in the non-isothermal mixing points, and the energy composite curves are generated. In the second step, streams are mixed isothermally within the temperature intervals in order to reduce the number of streams in each interval, and consequently the number of heat transfer units. In the third step, the separate systems generation and non-isothermal stream mixing are used to further reduce the number of heat transfer units.
4. Separate systems approach for problems with a pinch The complete network design for simultaneous energy and water minimisation will be generated for the example considering the water distribution from Case 4. This case
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L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308 6.905 kg/s
OP1 / 40 °C
20 kg/s
8.333 kg/s
4.762 kg/s
5.714 kg/s OP4 / 50 °C
0.952 kg/s
20 kg/s 50 kg/s
40 kg/s
OP3 / 75 °C 11.667 kg/s OP2 / 100 °C
37.381 kg/s
Water network Heat exchanger network
Stream Splitting
Stream Mixing
Fig. 18. Network design structure.
Table 5 Thermal stream data of Case 4
FW to Op FW to Op FW to Op WW from WW from WW from WW from
1 2 3 Op Op Op Op
1 2 3 4
TS (◦ C)
TT (◦ C)
CP (kW/◦ C)
Enthalpy (kW)
20 20 20 40 100 75 50
40 100 75 30 30 30 30
84 210 84 29 157 168 24
1680 16800 4620 −290 −10 990 −7560 −480
100 Temperature [°C]
Stream
4265 kW
75 65 Pinch 50 40 30 20
has been selected because fewer streams are involved in the heat recovery analysis. Moreover, in this case, the re-use streams are all involved in direct heat recovery. Therefore, the remaining problem of heat recovery involves only fresh water streams as cold streams and wastewater streams as hot streams, which enables a simple HEN design with fewer heat transfer units. Once the non-isothermal mixing for the water re-use streams is completed, the remaining design is to identify the matching of water streams by generating separate systems and appropriate location of separate systems, as represented in Fig. 18. The energy composite curves with Tmin (10 ◦ C in this case) are generated from the thermal stream data (Table 5). This results in a problem with a pinch at 75 ◦ C for the hot composite curve (Fig. 19). The introduction of non-isothermal stream mixing in the water re-use design might result in higher utility requirement than target. The targeting methods explained in Part I give the maximum attainable heat recovery for the systems, regardless of creating a pinch (i.e., a topology for water network is not considered). However, the new minimum utility consumption needs to be identified with the new water network
485 kW 15395 Enthalpy [kW] Fig. 19. Energy composite curves for Case 4.
Hot utility
Temperature
Splitting water stream Mixing water stream
E
Separate Systems
D
G
C B
A F
Pinch Non-isothermal stream mixing area
Cold utility Enthalpy Fig. 20. Separate systems generation for problems with a pinch (I).
L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308
D
A
C
H
B
Mixing water stream Hot utility
Temperature
..........
3301
G Non-isothermal stream mixing area ..........
F
Pinch B E
C
..........
B
E
D Wastewater discharge A
H Separate Systems Cold utility Enthalpy
Fig. 21. Separate systems generation for problems with a pinch (II).
Modified hot composite curve Hot composite curve
Same flowrate
J
A
C
H
B
Non-isothermal stream mixing area L
.......... Same flowrate
Mixing point ..........
J
L
..........
Cold composite curve
B H
A K
..........
K
Cold utility
Separate Systems Mixing water stream Enthalpy
Fig. 22. Separate systems generation for problems with a pinch (III).
New non-isothermal stream mixing area
-Qc
Hot utility
Temperature
Separate Systems Pinch
Qc Cold utility (a)
Pinch
Cold utility (b)
Pinch
Fig. 23. Stream matching for heat transfer with cold utility.
structure, in which the pinch limits the target minimum utility consumption (3780 kW). The minimum utility target was 3780 kW of hot utility, but now 4265 kW of hot utility and 485 kW of cold utility is required. As separate systems were generated for threshold problems in Part I, an extension of the previous method is required to deal with a problem with a pinch. Two possible ways are (1) fixing the hot composite curve and (2) fixing the cold composite curve. Now these methods will
FC Cold utility
Splitting water stream Mixing water stream Enthalpy
Fig. 24. Separate systems generation for problems with a pinch (IV).
be explored with a set of energy composite curves given in Figs. 20–24. It should be noted that the curves are illustrated for the general situation of a problem with a pinch, not based on the stream data of Example 1.
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First, the separate systems are generated by fixing the hot composite curve and shifting the cold composite curve across, above and below the pinch. The non-isothermal stream mixing area occurs between line ABCDE (modified cold composite curve) and line FCG (original composite curve) in Fig. 20. To satisfy the flowrate requirement overall, the split water stream should be appropriately placed for the modified cold composite curve. At points B, C, D, and E, a certain amount of water is split from the previous separate systems to increase the slope of the composite curve. However, the slope of line AB is greater than that of line FC, which means additional water stream should be added at Point A. This results in an additional freshwater requirement at Point F for the systems. Also the temperature at Point A is higher than the freshwater supply temperature (Point F), which requires additional hot utility to satisfy the temperature of Point A. From inspection of Fig. 20, for the design area below the pinch, shifting the cold composite curve is not favourable in terms of the penalty for freshwater consumption and hot utility. However, shifting the cold composite curve is a feasible design option for the design above the pinch. By contrast, the shape of the hot composite curve can be changed to create separate systems above and below the pinch (Fig. 21). The non-isothermal mixing areas are a quadrilateral BFGC above the pinch and a quadrilateral ADEB below the pinch. Shifting the hot composite curve is a feasible design option for the design above the pinch. Also, the matching between streams in the non-isothermal mixing area below the pinch seems to be straightforward, as illustrated in Fig. 21. However, the non-isothermal mixing may result in infeasible stream matching between mixing streams from the separate systems and streams from the original hot composite curve. This case is explained with Fig. 22, where the freshwater supplied is the same as the wastewater generated. The design region below the pinch is highlighted in Fig. 22, where the wastewater discharge flowrate from original cold composite curve (Point J) is the same as the flowrate from the last separate system (Point A). Following the non-isothermal mixing procedures, streams from the original composite curve (Points K, L, etc.) are distributed and then mixed at the mixing point after the cooler (Point A). But, no streams from the original composite curve are required at the mixing point between Points A and H, because the flowrate of Point A is the same with the wastewater generated from Point J. Therefore, the heat and mass balances around end points of hot composite curves are violated. However, there is a way to make a design below the pinch feasible by fixing the cold composite curve without any penalty. As the hot composite curve is constructed by combining individual hot stream profiles, an individual hot stream can be a candidate to lose its heat to cold utility (Fig. 23a). Part of, or the whole hot stream, is selected to match with cold utility. Then, the remaining problem becomes a system without cold utility requirement (Fig. 23b),
where both end points of hot composite curve and modified hot composite curve are met. This means that the final wastewater discharge conditions become the same as the exit conditions from the last separate system, which allows the feasible stream matching in the non-isothermal mixing area. With the new shape of composite curves, the separate systems can be generated and a feasible new non-isothermal mixing area is created by fixing the cold composite curve below the pinch. Overall, the design region should be considered for separate systems generation when a pinch point exists. In the above pinch part of the design, either fixing the hot composite curve or fixing the cold composite curve can be applied to create feasible separate systems and non-isothermal stream mixing. But below the pinch, matching between cold utility and individual hot streams should be selected and the separate systems be created with remaining streams. Fig. 24 illustrates separate systems generation for the problem with a pinch point when the hot composite curve is fixed above the pinch and the cold composite curve is fixed below the pinch. This design concept is now applied to Case 4 and the method of avoiding infeasible non-isothermal mixing below the pinch will be explained in more detail. Consider Case 4, in which the energy composite curves result in a problem with a pinch. Following the design Temperature [°C]
4265 kW 117.17 °C
100 75
50 28.72 °C
Pinch Non-isothermal stream mixing area (A2)
30 20
Non-isothermal stream mixing area (A1)
Separate Systems 485 kW Enthalpy [kW]
Fig. 25. Separate systems generation for Case 4.
Temperature 117.17˚C 37.381 kg/s
4265 kW 37.381 kg/s
32.619 kg/s 70 kg/s 20 kg/s 90 kg/s
32.619 kg/s 70 kg/s
20 kg/s 90 kg/s 485 kW
Splitting water stream Mixing water stream Enthalpy
Fig. 26. Stream splitting and mixing for separate systems.
L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308
New cold composite curve
3303
Non-isothermal stream mixing area (A1)
Original cold composite curve
OP3 75 °C
OP1 40 °C 20 kg/s
OP2 100 °C
20kg/s
50 kg/s
Pinch
Cold composite side 32.619 kg/s
20 kg/s 90 kg/s 20 °C
37.381 kg/s H
40 °C
65 °C
90 °C 4265 kW
117.166 °C
Pinch Fig. 27. Design structure for cold composite side.
Original hot composite curve
30 °C
New hot composite curve
Hot composite side Pinch
Pinch 30 °C 90 kg/s
28.717 °C
Non-isothermal stream mixing area (A2)
28.72 °C 30 °C C
50 °C
75 °C
20 kg/s
100 °C
32.619 kg/s
37.381 kg/s
37.381 6.905 kg/s 5.714 kg/s 40 kg/s kg/s OP4 OP3 OP1 OP2 75 °C 50 °C 100 °C 40 °C
Fig. 28. Design structure for hot composite side.
guidelines above, three separate systems are generated, as shown in Fig. 25. Above the pinch, the hot composite curve is fixed, while the cold composite curve is fixed below the pinch. The modified cold composite curve above the pinch should reach 117.17 ◦ C and the modified hot composite curve below the pinch ends at 28.72 ◦ C. Two non-isothermal mixing areas (A1 and A2) are created. The appropriate splitting and mixing of water streams is obtained from the modified composite curves (Fig. 26). The flowrates for mixing and splitting streams is calculated from the differences of enthalpy and temperature for each separate system (assuming the heat capacity is constant). The non-isothermal stream mixing area on the cold composite side (A1) is schematically represented in Fig. 27.
Two streams with 65 and 117.17 ◦ C are split from the modified cold composite curve. These streams are supplied to Operation 3 at 75 ◦ C and Operation 2 at 100 ◦ C. The area with in a circle in Fig. 27 represents the mixing area (A1) in the composite diagram. As indicated previously, modification of stream matching is required for the non-isothermal stream mixing area on the hot composite side (A2), as shown in Fig. 28. Based on the separate systems, the streams are distributed in the non-isothermal stream mixing area from the outlet streams of Operations 1, 4 and 3 to the streams added to the modified hot composite side. The temperature of 30 ◦ C for wastewater generated from the systems cannot be satisfied and the mass balance is also violated in the circle in Fig. 28.
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Pinch
90 kg/s
30 °C
C
50 °C
75 °C
20 kg/s
100 °C
32.619 kg/s
New non-isothermal stream mixing area (A3)
37.381 kg/s
32.619 kg/s 485 kW C 6.905 kg/s
5.714 kg/s
OP1 40 °C
OP4 50 °C
59.355 °C
7.381 kg/s 40 kg/s OP3 75 °C
37.381 kg/s OP2 100 °C
Fig. 29. Shifting the placement of cold utility.
4265 kW 117.17°C Temperature [°C]
100 Pinch 75
New non-isothermal stream mixing area (A3)
Non-isothermal stream mixing area (A1)
50 30 20
Separate Systems Enthalpy [kW]
Fig. 30. New energy composite curves for Case 4.
As explained in Fig. 23, the individual hot streams are searched to match with cold utility. Among the three streams, Operation 3 is selected to decrease the heat exchanger area because the outlet temperature of the stream from Operation 3 is higher than that of the stream from Operations 1 and 4. As presented in Fig. 29, the placement of cold utility is shifted from the modified cold composite side to the individual stream from Operation 3. New energy composite curves are generated (Fig. 30 ). Three separate systems and a new non-isothermal mixing area (A3) are created. No penalty is given to the generation of separate systems and the final network design is shown in Fig. 31. Non-isothermal stream mixing can be observed on the hot and cold composite sides, as well as water re-use between operations. In the identification stage for water re-use, the non-isothermal mixing reduces the number of streams involved in the following HEN design. It may result in an energy penalty, as seen in Case 4, but it enables a simple
HEN design with fewer number of heat transfer units. For the HEN design stage, the non-isothermal stream mixing allows separate systems to be created between a set of energy composite curves. This results in a simpler and better structure for the HEN.
5. Heat losses from the operations So far, each water-using operation has been considered to be isothermal. However, heat losses might take place within an operation. The consequences of heat losses to the targeting and design procedure will now be evaluated. In order to incorporate heat losses, non-isothermal operations need to be introduced. These operations can be characterised through a difference between the water operation inlet and outlet temperatures. The difference in temperature between the inlet and outlet water streams could result from heat transfer. Hence, mass and energy transfer could take place in the same unit. To understand the effect of heat losses, a comparison between a simple operation without heat loss and an operation with heat loss is necessary. This comparison is illustrated in Fig. 32 . In the absence of heat loss (Fig. 32a), the amount of energy that can be recovered from the hot water stream (outlet stream) is higher than in the case where there is an energy loss (Fig. 32b). Once the amount of energy recovered is decreased due to a lower outlet temperature of the water stream (TOp,OUT < TOp,IN ), the utility consumption increases. The temperature difference between the inlet and outlet water operation streams will be considered as a temperature loss difference (Tloss ). Thus, each operation with heat loss will be characterised by its temperature loss difference. The above representation gives a clear picture of the difference between the isothermal and non-isothermal unit
L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308 6.905 kg/s
OP1 / 40 °C 8.333 kg/s
4.762 kg/s
5.714 kg/s
OP4 / 50 °C
0.952 kg/s
7.381 kg/s
16.166 kg/s
OP3 / 75 °C
3.834 kg/s
C 485 kW
11.667 kg/s
OP2 / 100 °C 20 kg/s
32.619 16.453 kg/s kg/s
20 °C 30 °C
40 °C 50 °C
59.355 °C
33.547 kg/s 4265 kW
90 kg/s
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65 °C
90 °C
75 °C
100 °C
H
37.381 kg/s
117.166 °C
37.381 kg/s
90 kg/s
32.619 kg/s 20 kg/s
Fig. 31. Final network design for Case 4.
No Heat Loss T
Heat Loss T
Outlet water stream
TOp,IN TOp,OUT
TOp
Outlet water stream
Inlet water stream
Inlet water stream
TOUT
TOUT
TIN
TIN H
(a)
QH
(b)
Qloss
H QH
Fig. 32. Consequence of heat loss from a water-using operation.
operations. The consequence of heat losses from waterusing operations can be translated into larger values of utility consumption. Since the heat losses lead to high utility consumption, a new way to target the utility consumption for water systems with non-isothermal operations is needed. 5.1. Targeting utility consumption In the previous paper (Studies on Simultaneous Energy and Water Minimisation—Part I), a targeting analysis for simultaneous energy and water minimisation problems was introduced. The idea is to target the water and energy consumption before design. When there are heat losses from the water-using operations, the water target is unchanged but the minimum utility consumption changes. The temperature of the water supply is assumed to be fixed. Heat can be recovered from the effluent, subject to Tmin (10 ◦ C in this case). If there are no heat losses or flowrate changes, then the utility required is directly related to the flowrate of water through the overall system and the water supply and discharge temperatures, as given in the
equation below (Eq. (1)). This relation can be used to determine the utility consumption for the no re-use situation, as well as for re-use cases QH = F Cp(TOUT − TIN ).
(1)
Once heat losses are considered, the formula for utility targeting needs to be changed. Equations need to be defined as a function of the water usage, inlet and outlet water operation temperatures (defined according to Tmin ) and the temperature difference that defines the heat losses (Tloss ). For the no re-use of water policy, two formulae can be established according to the value of temperature loss difference. Eq. (2) gives the formula for systems with water-using operations characterised by the same temperature loss difference. While, for a system with operations with different values of Tloss,i (where i is the operation number) the utility target is determined from Eq. (3). In this second case of different Tloss,i , the heat loss is calculated for each operation and added to the utility consumption. The heat loss from each operation is a function of the water flowrate through that particular unit (Fi ) and the temperature loss difference
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Table 6 Water-using operation data—Example 2 CIN (ppm)
Operation
COUT (ppm)
TOp,IN (◦ C)
Operation 1 0 100 40 Operation 2 50 100 100 Operation 3 50 800 75 Operation 4 400 800 50 Temperature of fresh water source, TIN = 20 ◦ C. Temperature of discharge wastewater, TOUT = 30 ◦ C.
TOp,OUT (◦ C)
Limiting water flowrate (kg/s)
Contaminant mass load (g/s)
30 90 65 40
20 100 40 10
2 5 30 4
Table 7 Energy loss data Operation
Operation Operation Operation Operation
1 2 3 4
TOp,IN (◦ C)
TOp,OUT (◦ C)
40 100 75 50
30 90 65 40
Total
Flowrate (kg/s)
Energy loss (kW)
20 50 40 5.7
840 2100 1680 240
115.7
4860
that characterises the respective unit (Tloss,i ).
Then, the utility target could be determined using the targeting relations previously formulated through a heat balance for the overall water system . For this example, the water target is determined from the slope of the water supply line to be 90 kg/s (Fig. 1). The heat losses from the water-using operations are shown in Table 7, and the total energy loss from the whole four unit operations is 4860 kW. The utility consumption for this example can be calculated using the formula in Eq. (4) that characterises systems with the same value of the temperature loss difference for each operation. QH = F Cp Tmin + (Fi )Cp Tloss = 8640 kW.
QH = F Cp(Tmin + Tloss ),
(2)
QH = F Cp Tmin + (Fi Cp Tloss,i ).
(3)
5.2. Network design of a system with heat losses
In the case when water can be re-used from one operation to another for water minimisation, the utility consumption can be determined with the following formulae, according to the values of temperature loss differences between operations. Eq. (4) presents the formula for utility calculation for systems characterised by the same Tloss for each unit operation. (Fi ) represents the summation of the water flowrate passing through each unit operation i. When Tloss,i differs in the water system, the utility consumption can be calculated by the relation given in Eq. (5). QH = F Cp Tmin + (Fi )Cp Tloss ,
(4)
QH = F Cp Tmin + (Fi Cp Tloss,i ).
(5)
Using these equations, the target for utility consumption in a water system can be easily determined once the target for water consumption has been established. In order to evaluate the above targeting procedure for utility consumption in systems with heat losses and evaluate the applicability of the design procedure introduced earlier for simultaneous energy and water minimisation, an example will be considered with Example 2. The water-using operation data (inlet and outlet limiting concentrations, inlet and outlet operating temperatures, limiting water flowrates and contaminant mass loads) are given in Table 6. This set of data is characterised by the same temperature loss difference of 10 ◦ C for each unit operation. Based on the water-using operation data, the limiting water profile can be plotted and the water target identified.
After targeting the water and energy consumption, the design procedure follows. The design tool for simultaneous energy and water minimisation introduced in Part I will be applied to cases with heat losses from the water-using operations. Previously, water-using operations have been considered to be isothermal, and therefore have been represented as horizontal lines in the two-dimensional grid diagram. However, when an operation has the outlet water temperature different from the inlet water temperature due to heat loss, its representation in the two-dimensional grid diagram is a line between two temperature levels. Thus, a non-isothermal operation is illustrated through a sloping line. For the example, Operation 1 is illustrated in the grid diagram as a line from 40 to 30 ◦ C, Operation 2 has the starting point on the 100 ◦ C temperature level and end point on the 90 ◦ C temperature level, and so on (Fig. 33). In this case, all operations have heat losses, hence, the operation lines point to lower temperature levels. In the case when water is heated when passing through an operation, then its outlet water temperature is higher than its inlet water temperature. Thus, the water stream gains heat inside the operation and its temperature is raised. Such operations can be represented through lines going up from a low temperature level (inlet water temperature) to a high temperature level (outlet water temperature). Fig. 33 shows the results for the first part of the design giving connections between operations for the most efficient energy usage. This design includes non-isothermal mixing
L. Savulescu et al. / Chemical Engineering Science 60 (2005) 3291 – 3308 Limiting water flowrate (kg/s)
Freshwater 90 kg/s 0 ppm
T (°C)
100
40
80
0 kg/s 800 ppm
45.7 kg/s 100 ppm 50 kg/s
100
3307
100 °C 2
20 kg/s
75 °C 3
40 kg/s 65 °C
20 kg/s
60
5.7 kg/s
4 50 °C
10 20
20 kg/s
1
40
20
Water source
C (ppm)
Wastewater 0 kg/s
40 °C
Water discharge
Water discharge
Wastewater 44.3 kg/s
Wastewater 45.7 kg/s
Fig. 33. Two-dimensional grid diagram including operations with heat losses.
Table 8 Thermal stream data Stream
FW to Op FW to Op FW to Op WW from WW from WW from
1 2 3 Op 2 Op 3 Op 4
TS (◦ C)
TT (◦ C)
Flowrate CP (kW/◦ C) (kg/s)
Enthalpy (kW)
20 20 20 90 65 40
40 100 75 30 30 30
20 50 20 33.1 40 5.7
1680 16 800 4620 −8340 −5880 −239.4
84 210 84 139 168 24
Temperature[°C]
8640 kW
142.16 °C
100 90
the profile of the composite curves after the separate systems have been generated. The design procedure for the heat exchanger network based on the separate systems approach has led to a network design shown in Fig. 35. It is noted that the separate systems can be also generated by shifting down the hot composite curve. For this example, the wastewater stream from Operation 1 has its temperature equal to the discharge temperature. Therefore, this stream will not be involved in the heat exchanger network, and consequently a smaller number of units results. The design procedure for simultaneous energy and water minimisation based on the two-dimensional grid diagram and separate systems approach can be applied for systems with water-using operations involving heat losses. 6. Conclusions
Separate Systems 65 Non-isothermal stream mixing area 40 30 20 Enthalpy [kW]
Fig. 34. Energy composite curves and separate systems generation.
of re-use water streams in order to achieve the temperature conditions for the inlets of Operation 3 and Operation 4. The remaining cold/hot streams within each water main (Table 8) will be analysed using the separate systems procedure. Based on the above thermal data (Table 8) and, the energy composite curves have been plotted and then the separate systems generated by shifting the cold composite curve up. Fig. 34 illustrates the initial composite curves and also
A new design procedure has been developed to achieve both water and energy targets for systems using water at different temperatures and re-use of water. This procedure involves two stages. In the first stage, re-use options within the water system are exploited not only from the point of view of contaminant concentration, but also considering energy. A new grid representation (the two-dimensional grid diagram) has been introduced. After identifying the configuration of the water-using network considering energy, in the second stage a separate systems approach is used to ensure the simplicity in the HEN. This new design procedure also addresses the complexity issue involved in the overall design of water and energy networks. The simultaneous energy and water management analysis of systems with heat losses indicates a penalty in the energy consumption resulting from the loss. The tools introduced (the two-dimensional grid diagram and separate systems procedure) can also be applied to cases with heat loss from the operations.
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OP1 (40 °C→30 °C)
11.190 kg/s 5 kg/s
3.801 kg/s
5.714 kg/s
OP4 (50 °C→40 °C) 1.905 kg/s
OP3 (75 °C→65°C) 15 kg/s
OP2 (100 °C→90 °C)
90 kg/s 20 °C
30 °C
55 °C
30 °C
40 °C
65 °C
H
142.16 °C 80 °C 8640 kW 90 °C
90 kg/s
33.095 kg/s 40 kg/s
Fig. 35. Overall design of water and heat exchanger network—System including heat losses from operations.
Notation C C
CIN COUT CP Cp CR CV Hvap Tmin Tloss Tloss,i F Fi Floss FLS FS,DU FS,IU Fw H H
LS M MW
concentration, ppm cooler limiting inlet concentration, ppm limiting outlet concentration, ppm flowrate capacity, kW/◦ C heat capacity at constant pressure, kJ/kg ◦ C condensate return heat capacity at constant volume, kJ/kg ◦ C vaporisation heat, kJ/kg minimum temperature approach, ◦ C loss of water temperature in overall systems, ◦ C loss of water temperature in each operation, ◦ C water flowrate, kg/s water flowrate in each operation, kg/s flowrate loss, kg/s flowrate of live steam, kg/s flowrate of live steam with direct use, kg/s flowrate of live steam with indirect use, kg/s freshwater flowrate, kg/s enthalpy, kW heater live steam mass load, g/s make-up
OP QC QCS Qloss QH QHS T Tin TLS TOp TOp,IN TOp,OUT TS TT Tout
operation cold utility, kW available heat of cold stream, kW heat loss, kW hot utility,kW available heat of hot stream, kW temperature, ◦ C temperature of freshwater, ◦ C temperature of live steam, ◦ C water temperature of operation, ◦ C inlet water temperature of operation, ◦ C outlet water temperature of operation, ◦ C supply temperature of stream, ◦ C terminal temperature of stream, ◦ C discharge temperature of wastewater, ◦ C
References Alva-Argaez, A., 1999. Integrated design of water systems. Ph.D. Thesis, UMIST, Manchester, UK. Doyle, S.J., Smith, R., 1997. Targeting water reuse with multiple constraints. Transaction of IChemE 75 (Part B), 181–189. Kuo, W.J., Smith, R., 1998. Designing for the interactions between water-use and effluent treatment. Transaction of IChemE 76 (Part A), 287–301. Wang, Y.P., Smith, R., 1994. Wastewater minimisation. Chemical Engineering Science 49 (7), 981–1006.