Graphical Analysis of Process Changes for Water Minimization

Jul 2, 2009 - used to explore the impact of process changes on the pinch point. Process ... such as pinch analysis are widely accepted as promising to...
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Ind. Eng. Chem. Res. 2009, 48, 7145–7151

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Graphical Analysis of Process Changes for Water Minimization Xiao Feng,* Yang Liu, Long Huang, and Chun Deng Department of Chemical Engineering, Xi’an Jiaotong UniVersity, Xi’an 710049, China

The effective changes in a water-using process can reduce freshwater consumption and wastewater generation. The main effects of process changes include the increment of limiting inlet/outlet concentrations of waterusing processes and the reduction of the contaminant load of those processes. Because the water pinch of a water system is the bottleneck to limit the freshwater consumption, in this paper a water pinch diagram is used to explore the impact of process changes on the pinch point. Process changes which can move the pinch point upward and/or leftward can reduce freshwater consumption in the system. To further save freshwater in a water system, a process change which can increase limiting concentrations should be executed to an across-pinch process, and a process change which can reduce contaminant load should be executed below the pinch. 1. Introduction In recent years, stringent emission legislation and the increased cost of freshwater as well as wastewater treatment have motivated the process and manufacturing industries to emphasize water minimization. Process integration techniques such as pinch analysis are widely accepted as promising tools in addressing water minimization problems for the process and manufacturing industries. Generally, there are four water-saving approaches: using water-saving processes, direct wastewater reuse, wastewater regeneration reuse, and wastewater regeneration recycling.1 The last three approaches can be realized through water system integration, such as water pinch technology. Freshwater consumption can be reduced 10-30% by using general water pinch technology. However, after well-planned process changes, another 20% reduction of freshwater consumption can be achieved,2 as shown in Figure 1. Dating back to the 1980s, a change in process has been proposed to enhance the heat recovery for a heat exchange network (HEN) based on the plus/minus principles.3,4 It is also an effective way to further reduce utility targets in the synthesis of a mass exchange network (MEN).5 According to the analogue between a mass exchange network (MEN) or water network (WN) and a heat exchange network (HEN), Mann and Liu2 extended the plus/minus principles for HEN synthesis to synthesize a water network. They deduced that increasing the limiting inlet and limiting outlet concentrations in a water-using operation may cause a change in the freshwater pinch position. The process change that reduces the freshwater flow rate should be across the freshwater pinch in order to remove the contaminant load to a higher concentration interval, and to keep the contaminant-rich water-using streams as rich as possible. However, the process change for mass load lacks analysis in their work.2 In addition, Foo et al.6 applied the process change for property integration.7,8 The property of one discharged source can be modified to be reused further in process sinks.6 Hallale9 pointed out that assessment of the process changes is very valuable. The flow rates or concentrations of water sources or demand can be modified due to the individual waterusing process modifications. Manan et al.10 proposed that the change of water-using process equipment can reduce water consumption further. In the acrylonitrile case study, the steam jet ejector can be replaced by a vacuum pump to eliminate the bulk of freshwater demand.5 However, the research work above

focuses on the analysis of fixed flow rate (FF) operations (e.g., boilers, cooling towers, reactors), and the physical insights lack analysis. The other category operation is fixed contaminant load (FC) operations (e.g., washing, scrubbing, and extraction), which are based on the mass transfer model. The present research on process changes of FC operations is lagging. Feng et al.11 pointed out that the water pinch technology can be utilized to identify the bottleneck of the water system. The approaches for relaxing the bottleneck are discussed, including changing the process and decreasing the demand for water qualities. Hence the freshwater consumption can be further decreased. Wu et al.12 analyzed the effect of process change on the water system in pulp and paper industry. The results showed that the bulk of freshwater can be reduced via process modification. In this paper, the process changes for concentration and mass load are analyzed on the concentration-mass load diagram for FC operations. The heuristic rules for identifying the appropriate process changes are summarized. A practical case is utilized to illustrate the application for the appropriate process changes. 2. Graphical Representation of a Single Process Change There are various operations which consume great amounts of water, such as water cooling, steam heating, steam stripping, desalination, desulfurization, chemical reactions, dehydration, and filtration, as well as washing processes. Some of them can

Figure 1. Freshwater savings when water pinch technology is combined with process changes to further reduce freshwater consumption and wastewater generation.2

10.1021/ie900094m CCC: $40.75  2009 American Chemical Society Published on Web 07/02/2009

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Table 1. Limiting Water Data of Water System 1 process

CIN (ppm)

COUT (ppm)

M (kg/day)

1 2 3 4 5

0.00 25.00 25.00 50.00 75.00

30.00 50.00 75.00 100.00 120.00

1.00 1.00 1.00 1.00 0.80

be changed so that water consumption would be reduced. For example, a water-cooling process can be replaced by an aircooling process, a steam-jet ejector can be replaced by a vacuum pump, single-stage washing may be modified into multistage washing, and parallel-current washing can be changed into countercurrent washing. However, the cost for process modification would increase while a certain amount of freshwater is reduced, e.g., energy consumption or investment expenditures. At this stage, freshwater minimization is the first objective. Once the process changes are implemented into practice, several alternative process changes will be screened based on economic and controllable considerations. Generally, process changes could have two effects on water systems: increase the limiting inlet or outlet concentrations of water-consuming processes2 as shown in Figure 2b, or reduce contaminant loads of some processes as shown in Figure 2c. For example, using multistage washing to replace single-stage washing, or using countercurrent washing to replace parallelcurrent washing, the driving force is increased and it can be viewed as increasing the limiting concentrations of waterconsuming processes. In this case, because the limiting water profile changes, the minimum freshwater supply line of the process will be steeper, as shown in Figure 2b, which implies less freshwater demand of the process. On the other hand, using air cooling instead of water cooling, or replacing a steam-jet ejector by a vacuum pump,10 is reducing contaminant mass loads of the processes. In this case, the minimum freshwater supply line of the process will be also steeper, as the result of the shifted limiting water profile of the process, as shown in Figure 2c. Note that decreasing the limiting inlet or outlet concentration will need more freshwater to be consumed to meet the process requirements. In addition, increasing the mass load of a process will need more freshwater to remove the extra mass load. Both decreasing the limiting concentrations and increasing the mass load are not profitable for saving freshwater and will not be explored in this paper.

Figure 3. Limiting composite curve of water system.

Figure 4. Pinch moves leftward via the increment of limiting inlet concentration of an across-pinch process.

Note that process changes to save freshwater at different locations in the water system will have different effects, some of which can save freshwater, but some cannot. Therefore, appropriate process changes in a water system which could reduce freshwater consumption are of great importance. The process change in concentration or mass load will be analyzed in the following sections. 3. Analysis of Limiting Concentration Change Figure 2. Water supply lines before and after process changes.

A process change that can increase the limiting concentrations of a process can be set in the following three possible locations

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Figure 5. Pinch moves leftward via the increment of the limiting outlet concentration of an across-pinch process.

Figure 8. Pinch moves upward when the limiting outlet concentration of process 1 increases to a concentration higher than the original pinch.

Figure 6. Pinch moves leftward via the increment of both the limiting inlet and outlet concentrations of an across-pinch process.

Figure 9. Pinch remains unchanged when increasing the limiting inlet concentration of an above-pinch process.

Figure 7. Pinch remains unchanged when the limiting outlet concentration of a below-pinch process is less than the pinch concentration.

Figure 10. Pinch moves upward via the elimination of contaminant mass load below the pinch.

in a system: across the pinch, below the pinch, or above the pinch. The effect of process changes in the above three locations will be analyzed in subsections 3.1, 3.2, and 3.3. A water system containing five water-using processes, water system 1, is used as an example to analyze these possibilities. The limiting water data are shown in Table 1, and the limiting composite curve of the water-using system is represented in Figure 3. The water pinch locates at 50 ppm concentration and 2.5 kg/day load. The freshwater target of the system is 50 tons/

day if optimal water reuse is ensured. To further reduce the consumption of freshwater, process changes can be considered. 3.1. Increasing the Limiting Concentrations of an AcrossPinch Process. First, increase the limiting inlet and/or outlet concentrations of an across-pinch process. In the water system in Figure 3, process 3 is an across-pinch process. The influence on freshwater consumption is identified when the limiting inlet or outlet concentration of process 3 is increased. If the limiting inlet concentration of process 3 is increased from 25 to 30 ppm,

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Figure 11. Pinch remains unchanged when reducing contaminant mass load above the pinch.

Figure 12. Pinch moves leftward via the reduction of contaminant mass load of an across-pinch process.

and the outlet concentration remains unchanged, the pinch of the water system is shifted leftward from 2.5 kg/day and 50 ppm to 2.44 kg/day and 50 ppm, as shown in Figure 4. Correspondingly, the slope of the water supply line is increased, which results in reduction on freshwater consumption. As for process 3, its contaminant mass load remains unchanged, and its water flow rate is increased. Next, the limiting outlet concentration of process 3 is increased from 75 to 90 ppm, and the inlet concentration remains unchanged. The pinch of the water system is changed, as shown in Figure 5. The new pinch locates at 2.38 kg/day and 50 ppm; it moves leftward which causes the same result as increasing the limiting inlet concentration of process 3. As for process 3, its contaminant mass load is unchanged, and its water flow rate is decreased. If both inlet and outlet concentrations are increased, the pinch also moves leftward, as shown in Figure 6. The inlet concentration is increased from 25 to 30 ppm, and the outlet concentration is increased from 75 to 90 ppm. The new pinch locates at 2.33 kg/day and 50 ppm.

Figure 13. Pinch diagram of the case study.

From the above analysis, it is clear that increasing the limiting inlet concentration and/or outlet concentration of an across-pinch process can reduce freshwater consumption. 3.2. Increasing the Limiting Concentration of a BelowPinch Process. Process 1 in Figure 3 is a below-pinch process. If its outlet concentration is increased from 30 to 40 ppm, the pinch of the water system is unchanged, so that the freshwater consumption also remains unchanged, as shown in Figure 7. Next, the limiting outlet concentration of process 1 is further increased from 30 to 50 ppm, that is, to the pinch concentration. Similarly, the pinch remains unchanged, too.

Table 2. Water Consumption Data operation data

limiting data

process

F (tons/day)

CIN(ppm)

COUT(ppm)

F (tons/day)

CIN(ppm)

COUT(ppm)

M (kg/day)

DW CTA CTB WS

300 400 100 60

0 0 15 0

200 10 25 250

200 400 100 60

5 5 15 0

305 15 25 250

60 4 1 15

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Figure 14. Water network after water system integration. Table 3. Limiting Water Data after Process Changes process

F (tons/day)

CIN (ppm)

COUT (ppm)

M (kg/day)

DW CTA CTB WS1 WS2

200.0 100.0 40 57.6

5.0 15.0 0.0 15.0

305.0 25.0 15.0 265

60.0 1.0 0.6 14.4

Furthermore, the limiting outlet concentration of process 1 is increased from 30 to 60 ppm, which is higher than the pinch concentration. At this time, the pinch moves upward, as shown in Figure 8. Comparing Figures 7 and 8, it can be found that the effects of process changes in Figure 7 just increase the space between the limiting composite curve and the water supply line; in other words, it only increases the mass transfer driving force, but the pinch position and the freshwater consumption remain unchanged. However, the process change in Figure 8 can achieve a water-saving effect. The reason for the pinch position to remain unchanged in Figure 7 is a fact that the increased concentrations are still not higher than the pinch one. It can be explained as there is no mass transfer across the pinch, so the pinch remains unchanged. However, when the increased outlet concentration is higher than the pinch concentration, as in Figure 8, the process change causes a part of the contaminant load below the pinch to be shifted above the pinch. That part of the contaminant load, which needs to consume freshwater originally, can be removed by reusing water. Therefore, the change as in Figure 8 shown can move the pinch location and reduce the freshwater consumption. Obviously, increasing the limiting inlet concentration of a below-pinch process is similar to that as shown in Figure 7; there is also no mass transfer across the pinch, so it cannot change the pinch position or save freshwater. The following conclusions can be drawn from the above analysis: 1. Increasing the limiting outlet concentration of a belowpinch process to above the pinch concentration can reduce freshwater consumption. 2. Increasing the limiting outlet concentration of a belowpinch process to a concentration still below or even equal to the pinch concentration, and increasing the limiting inlet concentration of a below-pinch process, cannot change the pinch position and hence cannot save freshwater. 3.3. Increasing the Limiting Concentration of an AbovePinch Process. The water pinch position will not be changed by increasing the limiting concentration of an above-pinch process, so the freshwater consumption will be not reduced. For example, the limiting inlet concentration of process 4, which

Figure 15. New pinch after process changes.

Figure 16. Water network after process changes.

is an above-pinch process, is increased from 50 to 60 ppm, as shown in Figure 9. There is a similar result if increasing the outlet concentration of an above-pinch process: the pinch position cannot be changed and freshwater cannot be saved. 4. Analysis of Contaminant Mass Load Change A process change that can reduce the contaminant mass load can be executed in the following three possible ways: reducing the contaminant mass load below the pinch, above the pinch, and across the pinch.

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Figure 17. Flowchart for identifying the appropriate process changes.

4.1. Reducing the Contaminant Mass Load Below the Pinch. If all the contaminant mass load of process 1 in Figure 3, which is below the pinch, is eliminated, then the pinch position changes, as shown in Figure 10. The pinch moves upward and the freshwater consumption decreases. Therefore, reducing the contaminant mass load below the pinch can reduce freshwater consumption. 4.2. Reducing the Contaminant Mass Load Above the Pinch. If the contaminant mass load of process 4, which is an above-pinch process, is reduced from 1.0 to 0.5 kg/day, then the pinch remains unchanged, and the freshwater consumption cannot be reduced, as shown in Figure 11. However, the total contaminant load of the system is reduced, and the final concentration of the discharged wastewater can be decreased, which is useful for the reduction of environmental problems. 4.3. Reducing the Contaminant Mass Load of an AcrossPinch Process. If the contaminant mass load of process 3, which is an across-pinch process, is reduced from 1.0 to 0.4 kg/day, then the pinch moves, as shown in Figure 12. The pinch moves leftward and the freshwater consumption decreases. In fact, this

case can be regarded as the combination of the cases presented in sections 4.1 and 4.2. An across-pinch process can be divided into two parts: a below-pinch part and an above-pinch part. The water-saving effect is determined by the below-pinch part. 5. Case Study A styrene monomer (SM) plant is utilized as the case study to illustrate the application for process changes. The suspended solids (SS) concentration is the chief factor preventing water reuse, and it is selected as the key contaminant. The water system of the plant is composed of the following four processes. The dehydration filter (DW) is used to separate materiel and water. Cooling tower A (CTA) is used to cool the circulation water, and soft water is utilized to complement water loss for the circulation water system. However, common water is utilized in cooling tower B (CTB) to complement water loss for the circulation water system. The washer (WS) is used to wash materiel.

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Currently, all processes use freshwater, and the total consumption is 860 tons/day. The water consumption data for each process are shown in Table 2. The pinch concentration of the water system is targeted as 15.0 ppm, as shown in Figure 13. The minimum freshwater consumption for direct water reuse is determined as 460 tons/ day, which is 400 tons/day less than the actual consumption. The water-saving effect is significant from water system integration. The water network is constructed as shown in Figure 14. On the basis of water system integration, process changes can be considered to reduce freshwater consumption further. Analyzing each process in the water system, it can be found that cooling tower A (CTA) is below the pinch, so it can be considered to reduce its contaminant mass load. The washer (WS) is across the pinch, so it can be considered to increase its limit concentrations. Cooling tower B (CTB) is above the pinch, so it remains unchanged. Therefore, air cooling is used to replace water cooling for the heat load in cooling tower A (CTA), and its contaminant mass load is reduced to zero. In order to reuse more contaminated water and save more high-quality water, the washer (WS) can be replace by a two-stage washer, WS1 and WS2. WS1 is below the pinch and uses freshwater. WS2 is above the pinch and uses wastewater. The new limiting data after process changes are shown in Table 3. The new pinch is targeted as 265 ppm according to the data in Table 3, as shown in Figure 15. The corresponding minimum freshwater consumption is determined as 256.6 tons/day, which is decreased about 200 tons/day compared with 460 tons/day. Figure 16 shows the water network after process changes. 6. Conclusions In this paper, process changes have been analyzed on the pinch diagram. Adopting a water-saving process in an appropriate place can change the water-using process with these effects, either increasing the limiting concentrations and/or reducing the contaminant load removed. It can move the pinch position upward and/or leftward and reduce freshwater consumption of a water system. Figure 17 represents the flowchart for adopting a water-saving process in an appropriate place. The following summarized rules can be used to judge a process change. 1. A process change which can increase limiting concentrations of the process should be adopted in an across-pinch process to save freshwater. If a below-pinch process becomes an acrosspinch process by increasing the outlet concentration from below the pinch to above the pinch, the process change can also reduce freshwater consumption. The process changes, other than the above two cases with increasing the limiting concentrations, cannot reduce freshwater consumption. 2. A process change which can reduce contaminant load should be adopted below or across the pinch to save water. Such process changes used above the pinch cannot save freshwater.

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Although the above rules are summarized from the analysis for the FC problem, they can be extended to the FF problem. Acknowledgment Financial support provided by the National Natural Science Foundation of China under Grant 50876079 is gratefully acknowledged. Notation CIN ) inlet concentration, ppm COUT ) outlet concentration, ppm Cpinch ) pinch concentration, ppm F ) water flow rate, tons/day FFW ) freshwater flow rate, tons/day M ) mass load, kg/day FC ) fixed contaminant mass load FF ) fixed flow rate HEN ) heat exchange network MEN ) mass exchange network WN ) water network

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ReceiVed for reView January 19, 2009 ReVised manuscript receiVed May 23, 2009 Accepted June 10, 2009 IE900094M