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Design of Water Network with Internal Mains for Multi-contaminant Wastewater Regeneration Recycle
0263–8762/04/$30.00+0.00 # 2004 Institution of Chemical Engineers Trans IChemE, Part A, October 2004 Chemical Engineering Research and Design, 82(A10): 1331–1336
DESIGN OF WATER NETWORK WITH INTERNAL MAINS FOR MULTI-CONTAMINANT WASTEWATER REGENERATION RECYCLE D. CAO, X. FENG and X. DUAN Department of Chemical Engineering, State Key Laboratory of Multi-Phase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China
W
ater networks incorporating a regeneration recycle can minimize freshwater consumption and wastewater discharge, to the extent of approaching the inherent minimum value. Based on the water network structure provided with internal water mains, this paper addresses the design methodology for water recycling with regeneration, and extends prior work to cases involving multi-contaminant systems. In such a structure water is supplied to each water-using process from freshwater main, regenerated water main, or regeneration water main according to water quality. How to determine the contaminant concentrations in the regeneration water main is the key in such a water network design. The regeneration concentrations should be the lowest compatible with the freshwater consumption of the system equal to its inherent minimum value. The corresponding design methodology focuses on how to determine which main to use as the water source and how much the water flow rate is to be for each water-using process, on the basis of mass balance and the inlet contaminant concentration condition of that process. Finally, an example is given to illustrate the design method. Keywords: water network; regeneration recycle; water mains; water saving; multiple contaminants.
INTRODUCTION Water scarcity and stricter environmental regulations on industrial effluents underlie the growing emphasis on freshwater minimization in industry, which corresponds to wastewater minimization. Reducing freshwater usage and wastewater discharge has become one of the main targets for design and optimization of process systems. At present, research to save freshwater and reduce wastewater mainly focuses on WAP (water allocation planning) technology. WAP technology treats the water utilization processes in an enterprise as an organic whole, and considers how to allocate the water quantity and quality to each water-using unit, so that water reuse of the system is maximized and at the same time the wastewater discharge is minimized (Feng and Seider, 2001; Wang et al., 2003; Wang and Smith, 1994; Bagajewicz et al., 2001; Huang et al., 1999; Mann and Liu, 1999; Kuo and Smith, 1998a; Alva-Argaez et al., 1999; Kuo and Smith, 1998b) When WAP technology is used to design water networks, traditionally new pipes will be installed to connect directly water-using units in the water network. If the Correspondence to: Professor X. Feng, Department of Chemical Engineering, State Key Laboratory of Multi-Phase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China. E-mail: [email protected]
entire plant involves only a few processes, the network is fairly simple and water savings at or near the maximum are achieved. However, for a large petrochemical or chemical complex, with many unit processes, the piping network becomes very complicated and difficult to control and operate. When the quantity or quality of unit processes changes, the operation of other processes will be affected. To make the design, operation and control of water utilization systems easier, Feng and Seider (2001) presented a new water network structure and proposed the design methodology for single contaminant systems. This new structure simplified the water network design and operation by introducing one or more internal water mains. Then this design methodology was extended for multiple contaminant systems (Wang et al., 2003). However, until now, the design methodology for such a proposed water network has only considered water reuse. Using water reuse and water regeneration recycle at the same time can minimize freshwater consumption and wastewater discharge to the maximum extent. In particular, when the freshwater target concerning only water reuse is much higher than the inherent minimum freshwater usage (which will be defined later), in order to further reduce freshwater consumption and wastewater discharge, a water regeneration recycle should be considered. Therefore, in this paper, the design methodology of the water network
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with water mains concerning a water regeneration recycle is developed. Emphasis is placed on positioning of the regeneration water main.
PROCESS MODEL AND MASS BALANCE Water-using units can be classified into three types: (1) water is a reactant—in such processes, water may generate or lose, e.g. water and coal are reacted to generate synthesis gas; (2) water is a mass transfer medium—in such processes, water is used to remove contaminants from process streams in the given units, for example, steam stripping, liquid – liquid extraction and washing operations. In this paper, only the water used in such processes is studied; (3) water is an energy medium in cooling or heating, for instance, steam or circulating cooling water in process industries—in such processes, the contaminant concentration in water normally remains unchanged. Here we use U and S to represent respectively the number of water-using processes and the number of contaminants: U ¼ {iji is the number of water-using processes, S ¼ {kjk is the number of contaminants; k ¼ 1, 2, 3, . . . , S}
(1) (2)
Mass Balance of Process For each water-using process, the mass balance of water and the mass balance of contaminant are the most fundamental formulae. The water balance is: Fi,in ¼ Fi,out þ vi
i[U
(3)
where F denotes the water flow rate, subscript i means process i, U is the set of all processes, subscripts in and out represent inlet and outlet of the process, respectively, and v represents the loss flow rate of water. The contaminant balance is as follows: Fi,in Ci,k,in þ Mi,k ¼ Fi,out Ci,k,out þ vi Ci,k,L (4) i [ U, k [ S where C denotes contaminant concentration, subscript k means contaminant k, S is the set of all contaminants and subscript L means loss. Normally, vi and Ci,k,L are given before design. The constraint conditions are the maximum inlet and outlet concentrations. Max Ci,k,in Ci,k,in
i [ U, k [ S
(5)
and Max Ci,k,out Ci,k,out
i [ U, k [ S
where superscript Max denotes maximum.
Based on the mass conservation of water, the total freshwater consumption flow rate of the whole system can be expressed as: X f Fi,in i[U (7) Ff ¼ i
Process Model
i ¼ 1, 2, 3, . . . , U}
Mass Balance of System
(6)
where the superscript f denotes freshwater. The total water loss of the whole system is: X vi i [ U FL ¼
(8)
i
where superscript L denotes loss. The total flow rate of the wastewater discharge of the whole system is: X w Fw ¼ Fi,out i[U (9) i
where the superscript w denotes wastewater. To the whole water utilization system, the water balance should be Ff ¼ Fw þ FL
(10)
WATER NETWORK STRUCTURE In traditional water networks, water reuse is realized by connecting units directly by pipes. To make water networks easier to design, operate and control, a new water network structure with installing internal water mains was proposed (Feng and Seider, 2001). All plants always contain a freshwater main and a wastewater main, which are external water mains. The ‘internal water mains’ are similar mains possessing uniform contaminant concentrations between freshwater and wastewater. They receive water from some water-using processes at outlet contaminant concentrations less than or equal to their contaminant concentrations, and supply water to unit processes at inlet concentrations greater than or equal to their contaminant concentrations. In this way, all waterusing units are only connected to water mains, including external and internal water mains. Figure 1 gives an example of such a water network, in which the left vertical line is the freshwater main, the right vertical line the wastewater main, and the two vertical lines in the middle the internal water mains. The numbers above each main and in parentheses are the contaminant concentrations (the water system in Figure 1 involves three contaminants), and below each main is marked its flow rate. The numbered boxes represent water-using units which are connected by water mains, that is, they receive water from the main on the left, and discharge water to the main on the right. Obviously, as a water source, an internal water main should be positioned at a contaminant concentration less than or equal to that needed by the water sinks. Furthermore, as a water sink, an internal water main must receive water from at least one unit process. When a water regeneration recycle is considered, except for the processes that must use freshwater, all other waterusing processes can use regenerated water or wastewater
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DESIGN OF WATER NETWORK WITH INTERNAL MAINS
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Figure 2. Qualitative relationship between freshwater consumption F f and regeneration concentration Ci.
DETERMINING CONTAMINANT CONCENTRATIONS OF REGENERATION WATER MAIN Figure 1. Water network of example 1.
from other processes. In this paper, we only consider the water network with two internal water mains, one of which is the regeneration water main, the other, the regenerated water main, as has already been shown in Figure 1. This is a design decision that simplifies the water network. To save on the costs of regeneration, it may be preferable to use two or more regenerated water mains. So in the future, perhaps a synthesis strategy can be developed that positions additional regenerated water mains on the basis of regeneration cost estimates as a function of concentrations. In such a structure, the regeneration water main receives wastewater from some water-using units (in Figure 1, from process units 1, 2, 3, 4 and 5), supplies part of the water to some other units at concentrations higher than or equal to its concentrations (in Figure 1, to process units 6 and 7), and also sends the other part of the water to the wastewater regeneration process. The regenerated water from the water regeneration process goes to the regenerated water main and is used as a water source to supply water to some water-using units (in Figure 1, to process units 3, 4 and 5). Positioning the internal water mains properly is very important, as it determines the freshwater consumption and the regeneration cost. For the water system with regeneration recycle, normally the freshwater consumption changes with the contaminant concentration of the regeneration water, having the relationship shown qualitatively in Figure 2. If the concentrations are high enough, the freshwater consumption will remain unchanged. However, if the contaminant concentrations of the regeneration water main reduce to below certain limits (Ck,R1 in Figure 2), the freshwater consumption will increase with the decrease of the contaminant concentrations of the regeneration water main. The contaminant concentrations of the regeneration water main should therefore not be lower than this limiting value of Ck,R1.
It should be pointed out that, different from Feng and Seider (2001), the contaminant concentrations of the regeneration water main in this paper are the actual concentrations after full blending. A single-contaminant system is a specific case of multiple-contaminant systems and is much easier to deal with than multiple-contaminant systems. Therefore, in this paper, only multiple-contaminant systems are considered. Concentrations of the regenerated water main can be simply fixed at certain values (Wang and Smith, 1994; Mann and Liu, 1999; Kuo and Smith, 1998a). Normally the concentrations are set very low (Wang and Smith, 1994; Mann and Liu, 1999), for example, 10 ppm (Wang and Smith, 1994). However, actually the concentrations can be set slightly higher. Except for the processes that can only use freshwater, all other processes can use regenerated water. Consequently, the post-regeneration concentrations can be set as the lowest concentrations of all the maximum inlet concentrations of the processes that use regenerated water (Mann and Liu, 1999). Higher postregeneration concentrations can reduce the regeneration cost. Therefore, in this paper, the post-regeneration concentrations will be set in this way. The contaminant concentrations of the regeneration water main are the lowest concentrations with minimum freshwater consumption. If the concentrations are higher, the regeneration cost will increase, but the freshwater consumption cannot be reduced anymore. If the concentrations are lower, the freshwater consumption will increase unavoidably. The procedure to determine the regeneration concentrations comprises of the following six steps. Step 1: Determine the Inherent Minimum Freshwater Usage and the Inherent Minimum Wastewater Discharge of the System The inherent minimum freshwater usage of a system is defined as the sum of the minimum freshwater consumed by the processes that can only use freshwater. For example,
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if the maximum inlet concentration of at least one contaminant in a process is 0 ppm, this process has to consume freshwater, and if the wastewater discharges from the process at its maximum outlet contaminant concentrations, the freshwater consumption of the process has reached its minimum. The other processes in the system can use water from the regenerated water main or from the regeneration water main. Because water can be recycled, there is always sufficient water in the regenerated water main. When the system consumes freshwater equal to the inherent minimum freshwater usage, the corresponding wastewater discharge is defined as the inherent minimum wastewater discharge, which can be calculated through the water balance, equation (10).
Step 2: Arrange all Unit Processes in the Order of their Maximum Outlet Concentrations For single contaminant systems, unit processes can be arranged directly in the order of their maximum outlet concentrations. However, for multiple contaminant systems, the following rule is proposed to arrange processes in a multiple contaminant system. For each contaminant, k, rank-order its maximum outlet concentration from the unit processes in increasing order. Let the serial numbers, Ni,k, contain an integer ranking, where subscript i refers to process i and k refers to contaminant k. Then, the composite maximum outlet concentration, Ri, is defined as the repeated product of Ni,k, k ¼ 1, . . ., S, for each unit process i as given by: Y Ni,k i ¼ 1, . . . , U; k ¼ 1, . . . , S (11) Ri ¼ As a simple demonstration, for the system specified in the first five columns of Table 1, the calculated result is shown in columns 6 and 7. For example, N1,2 means the serial number of contaminant 2 in process 1. Because the maximum outlet concentration of contaminant 2 in process 1 is 400, which is greater than that in process 3, i.e. 45, and less than that in process 1, i.e. 12,500, N1,2 ¼ 2. In a similar manner, contaminant 1 for process 1 is the lowest of all the three processes, and therefore, N1,1 ¼ 1. Likewise, N1,3 ¼ 1. According to equation (11), R1 ¼ N1,1 . N1,2 . N1,3 ¼ 1 . 2 . 1 ¼ 2. Then list unit processes according to the rising order of their values of Ri, that is, in the order of R1 ¼ 2, R3 ¼ 9 and R2 ¼ 12, or process 1, 3 and 2, as shown in Table 1. Note that if the Ri of one process is equal to another, their relative list sequence should be arranged in the order of their maximum inlet concentrations. Table 1. Limiting data for a three-process system and computed results for their R. Process
F Lim (t h21)
1
45
2
34
3
56
Contaminant
CMax in (ppm)
CMax out (ppm)
Ni,k
Ri
1 2 3 1 2 3 1 2 3
0 0 0 20 300 45 120 20 300
15 400 35 120 12,500 180 220 45 9500
1 2 1 2 3 2 3 1 3
2 12 9
Step 3: Determine the Preliminary Position of Regeneration Water Main Start with the process with the highest relative composite maximum outlet concentrations (that is, the highest value of Ri), that is, the last one in the above sort, or process U. Let this process use the water from the regenerated water main (or freshwater main, depending on its maximum inlet concentrations) and discharge wastewater at its maximum outlet concentrations. Then calculate the water usage and discharge of this process by performing mass balances of contaminants and water [equations (3) and (4)]. In the same way, calculate the water usage and discharge of the next lower-ordered process, U 2 1, in the sort, one by one, and sum the discharged wastewater until the sum is just greater than the inherent minimum wastewater discharge that is determined in step 1. At this time, the processes taking part in summing up the discharged wastewater are from process U to a certain process U 2 i. This last process is called the critical process P, serially related to U by the relation P ¼ U 2 i. The preliminary position of the regeneration water main is set between the critical process P and the process P þ 1. In this way, the wastewater discharge of the system will be the inherent minimum wastewater discharge. If the preliminary concentrations of the regeneration water main are the same as those of process P, the wastewater discharge will be greater than the inherent minimum wastewater discharge, while, if they are the same as those of process P þ 1, the wastewater discharge will be less than the inherent minimum wastewater discharge. That is to say that the preliminary concentrations of the regeneration water main should lie between process P and process P þ 1. Step 4: Calculate the Actual Concentrations of the Regeneration Water Main The actual concentrations of the regeneration water main are determined by the following equations by considering full blending: X Fj!Rl j ¼ 1, . . . , P (12) FRl,in ¼ j
Mk,Rl ¼
X
Fj!Rl Cj,k,out
j ¼ 1, . . . , P; k [ S
(13)
j
Ck,Rl ¼ Mk,Rl =FRl,in
k[S
(14)
where subscript Rl means the regeneration water main, so FRl,in is the overall water flow rate in the regeneration water main; subscript j ! Rl means from process j into the regeneration water main; process P is the critical process; Mk,Rl is the overall mass flow rate of contaminant k in the regeneration water main; Ck,Rl is the actual concentration of contaminant k in the regeneration water main. Step 5: Determine the Final Values of the Regeneration Concentrations If the wastewater in the regeneration water main cannot be used in any process of the system, that is, there exists at least one contaminant concentration in the regeneration water main that is greater than the permissible maximum
Trans IChemE, Part A, Chemical Engineering Research and Design, 2004, 82(A10): 1331–1336
DESIGN OF WATER NETWORK WITH INTERNAL MAINS inlet concentration of that process, the calculated regeneration concentrations are the final regeneration concentrations, because the case is the same as the supposition when determining the preliminary position of regeneration water main. On the other hand, if water from the regeneration water main can be used in some processes which are serially higher than the critical process, the water usage and wastewater discharge of that process should be recalculated. Then calculation of the cumulative wastewater discharge in step 3 should be repeated. If the new value is less than or equal to the inherent minimum wastewater discharge, the calculated regeneration concentrations are the final regeneration concentrations. However, if the new value is still greater than the inherent minimum wastewater discharge, the critical process should be further serially elevated, and the corresponding concentrations of the regeneration water main recalculated, until a set of concentrations is found so that the wastewater discharge of some processes between P þ 1 and U is less than or equal to the inherent minimum wastewater discharge. These regeneration concentrations represent the final values.
Step 6: Calculate the Regeneration Water Flow Rate The corresponding flow rate of the regeneration water is the actual water flow rate needed from the regenerated water main.
DESIGN METHODOLOGY The design methodology of water network with internal water mains for wastewater regeneration is quite different from that for simple reuse. With water regeneration, the locations of all processes have been determined when regeneration concentrations are calculated. The design methodology is illustrated in the following: (1) Create a contaminant concentration diagram, as shown in Figure 1, with concentration increasing from the left to the right, including vertical lines to represent external or internal water mains. The freshwater main with 0 ppm contaminants is located on the extreme left, then comes the regenerated water main, followed by the regeneration water main; the last vertical line is the wastewater main. Wastewater to be regenerated flows from the regeneration water main leftward to the regeneration process and then goes leftward again into the regenerated water main. (2) Position a numbered box for each unit process between any two adjacent water mains. For each process, its maximum inlet concentration of any contaminant has to be greater than or equal to that of the water main to its left, but lower than that of the main to its right. (3) Position arrows to represent the streams going into and out of each unit process, all arrows pointing to the right, except for the regeneration process. Water is supplied from the left main for any water-using process, which then discharges into the main at its right. (4) Perform mass balance for each unit process to determine the water flow rate into and out of that process.
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(5) Perform water balance for the vertical water mains. The flow rate of the regenerated water main is determined according to the water needs of the processes with water supplied from regenerated water main. The regeneration concentrations need to be determined to assure that there is always sufficient water to regenerate and pass into the regeneration main and that no water shortage can occur. If there is any excess water in the regeneration water main, it will be discharged into the wastewater main.
CASE STUDY In a case study used to illustrate this design methodology, the limiting data for a seven-process system are specified in Table 2. Loss of water in any process is ignored. When only considering water reuse, the minimum freshwater consumption is 148.7 t h21, which can be obtained by solving the mathematical programming developed in Huang et al. (1999) from the data in Table 2. Now we shall see how this 148.7 t h21 could be reduced by using the present methodology following the flowsheet given in Figure 1. In the system, the maximum inlet concentrations of processes 1 and 2 are 0 ppm, so processes 1 and 2 must be fed with freshwater. All the other processes can use regenerated water. Thus the inherent minimum freshwater usage of the system is 100 t h21. As losses are ignored, the discharged wastewater of the system is equal to its freshwater consumption. So the corresponding inherent minimum wastewater discharge is also 100 t h21. Except for processes 1 and 2, the lowest maximum inlet concentrations of the three contaminants of the other five processes are set as the concentrations of the regenerated water main, which are (90, 80, 100). In Table 2, the seven processes are arranged downward in ascending order of their relative composite maximum
Table 2. Limiting data for the case study. Process 1 2 3 4 5 6 7
Contaminant
CMax in (ppm)
CMax out (ppm)
F Lim (t h21)
1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
0 0 0 0 0 0 90 130 100 110 80 150 260 200 180 340 350 400 950 850 900
80 70 80 110 120 100 150 180 210 210 150 220 350 320 310 800 1100 1000 1500 2100 1800
65
1
35
8
40
36
30
48
30
125
64
216
50
343
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outlet concentrations. When process 7 is fed with water from the regenerated water main, its wastewater discharge is 30.9 t h21. Similarly, the discharges of process 6, 5 and 4 are 47.1, 18.6, and 30.0 t h21, respectively. It can be seen that the sum of the wastewater discharge from processes 7, 6 and 5 is 96.6 t h21, which is less than the inherent minimum wastewater discharge of 100 t h21. If process 4 is added, the sum will be greater than the inherent minimum wastewater discharge. So process 4 is set as the critical process. The actual contaminant concentrations of the regeneration water main is calculated to be [122.1, 108.5, 130.6]. Wastewater in such concentrations can be reused by process 7, 6 and 5. According to mass balances, when water is supplied from the regeneration water main, the water usages of the three processes are 31.4, 48.4 and 21.7 t h21, respectively. Thus the sum of the wastewater discharge is 101.5 t h21, greater than 100 t h21. Therefore, process 5 is set as the critical process. In the same way, processes 7 and 6 can be fed with water from the regeneration main, and the corresponding discharges are 31.6 and 49.2 t h21, respectively, the sum of which is 80.8 t h21. This value is less than the inherent minimum wastewater discharge. So the final regeneration contaminant concentrations are [133.2, 124.8, 148.3]. The water flow rate needed for regeneration is 88.6 t h21. The total freshwater consumption of the system is 100 t h21, as shown in Figure 1.
CONCLUSIONS (1) The present paper generalizes a recent methodology (Feng and Seider, 2001) for designing water networks for multi-contaminant systems, and simplifies that methodology by introducing one or two internal water mains. (2) Water networks incorporating regeneration recycle can reduce freshwater consumption and wastewater discharge to the maximum extent, that is, down to the inherent minimum value. (3) This paper proposes a design methodology of water network with internal water mains employing such a regeneration recycle. In such a structure, water is supplied to any of the water-using processes from the freshwater main, the regenerated water main or the regeneration water main in accordance to contaminant concentrations. (4) In the design of water networks with internal mains for wastewater regeneration, the contaminant concentrations of the regeneration water main determine both the freshwater consumption and the regeneration cost. The regeneration concentrations should be the lowest concentrations compatible with the freshwater consumption of the system equal to its inherent minimum value. To ensure that, only wastewater equal to the inherent minimum value can be discharged to the wastewater main, and all other used water with lower contaminant concentrations will discharge to the regeneration water main. Part of water from the regeneration water main is supplied to such waterusing processes as can tolerate its contaminant concentrations, while the remainder reports to the regeneration process.
(5) When water networks are designed by using the proposed methodology in this paper, although the freshwater consumption and wastewater discharge are equal to that in the traditional water networks, and the regeneration water flow rate is more than that in the traditional network, the network has a simpler structure and better operability because of the internal water mains. NOMENCLATURE F C v M N P R U S
water flowrate, t h21 concentration of contaminant, ppm loss flowrate of water in a process, t h21 contaminant flowrate, kg h21 serial number critical process relative composite concentration set of all processes set of all contaminants
Subscripts in out i k Rl
at inlet at outlet process contaminant regeneration water main
Superscripts Max maximum f freshwater w wastewater L loss Lim limiting
REFERENCES Alva-Argaez, A., Vallianatos, A. and Kokossis, A., 1999, Multi-contaminant transhipment model for mass exchange networks and wastewater minimization problems, Comput Chem Eng, 23(10): 1439–1453. Bagajewicz, M.J., Rivas, M. and Savelski, M.J., 2001, A robust method to obtain optimal and sub-optimal design and retrofit solutions of water utilization systems with multiple contaminants in process plants, Comput Chem Eng, 25: 495. Feng, X. and Seider, W.D., 2001, A new structure and design methodology for water networks, Ind Eng Chem Res, 40(26): 6140–6146. Huang, C.-H., Chang, C.-T. and Ling, H.-C., 1999, A mathematical programming model for water usage and treatment network design, Ind Eng Chem Res, 38(7): 2666–2679. Kuo, W.-C.J. and Smith, R., 1998a, Designing for the interactions between water-use and effluent treatment, Trans IChemE, Part A, Chem Eng Res Des, 76(A3): 287– 301. Kuo, W.J. and Smith, R., 1998b, Design of water-using systems involving regeneration, Trans IChemE, Part B, Process Safe Env Prot, 76(B2): 94–114. Mann, J.G. and Liu, Y.A., 1999, Industrial Water Reuse and Wastewater Minimization (McGraw-Hill, New York, USA). Wang, B., Feng, X. and Zhang, Z., 2003, A design methodology for multiple-contaminant water networks with single internal water main, Comput Chem Eng, 27(7): 903–911. Wang, Y.P. and Smith, R., 1994, Wastewater minimization, Chem Eng Sci, 49: 981 –1006.
ACKNOWLEDGEMENTS Financial support from the National Natural Science Foundation of China (no. 20376066) and the National Key Basic Research Development Program of China (no. G2000026307) is gratefully acknowledged. The manuscript was received 24 January 2003 and accepted for publication after revision 16 June 2004.
Trans IChemE, Part A, Chemical Engineering Research and Design, 2004, 82(A10): 1331–1336
DESIGN OF WATER NETWORK WITH INTERNAL MAINS FOR MULTI-CONTAMINANT WASTEWATER REGENERATION RECYCLE D. CAO, X. FENG and X. DUAN Department of Chemical Engineering, State Key Laboratory of Multi-Phase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China
W
ater networks incorporating a regeneration recycle can minimize freshwater consumption and wastewater discharge, to the extent of approaching the inherent minimum value. Based on the water network structure provided with internal water mains, this paper addresses the design methodology for water recycling with regeneration, and extends prior work to cases involving multi-contaminant systems. In such a structure water is supplied to each water-using process from freshwater main, regenerated water main, or regeneration water main according to water quality. How to determine the contaminant concentrations in the regeneration water main is the key in such a water network design. The regeneration concentrations should be the lowest compatible with the freshwater consumption of the system equal to its inherent minimum value. The corresponding design methodology focuses on how to determine which main to use as the water source and how much the water flow rate is to be for each water-using process, on the basis of mass balance and the inlet contaminant concentration condition of that process. Finally, an example is given to illustrate the design method. Keywords: water network; regeneration recycle; water mains; water saving; multiple contaminants.
INTRODUCTION Water scarcity and stricter environmental regulations on industrial effluents underlie the growing emphasis on freshwater minimization in industry, which corresponds to wastewater minimization. Reducing freshwater usage and wastewater discharge has become one of the main targets for design and optimization of process systems. At present, research to save freshwater and reduce wastewater mainly focuses on WAP (water allocation planning) technology. WAP technology treats the water utilization processes in an enterprise as an organic whole, and considers how to allocate the water quantity and quality to each water-using unit, so that water reuse of the system is maximized and at the same time the wastewater discharge is minimized (Feng and Seider, 2001; Wang et al., 2003; Wang and Smith, 1994; Bagajewicz et al., 2001; Huang et al., 1999; Mann and Liu, 1999; Kuo and Smith, 1998a; Alva-Argaez et al., 1999; Kuo and Smith, 1998b) When WAP technology is used to design water networks, traditionally new pipes will be installed to connect directly water-using units in the water network. If the Correspondence to: Professor X. Feng, Department of Chemical Engineering, State Key Laboratory of Multi-Phase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China. E-mail: [email protected]
entire plant involves only a few processes, the network is fairly simple and water savings at or near the maximum are achieved. However, for a large petrochemical or chemical complex, with many unit processes, the piping network becomes very complicated and difficult to control and operate. When the quantity or quality of unit processes changes, the operation of other processes will be affected. To make the design, operation and control of water utilization systems easier, Feng and Seider (2001) presented a new water network structure and proposed the design methodology for single contaminant systems. This new structure simplified the water network design and operation by introducing one or more internal water mains. Then this design methodology was extended for multiple contaminant systems (Wang et al., 2003). However, until now, the design methodology for such a proposed water network has only considered water reuse. Using water reuse and water regeneration recycle at the same time can minimize freshwater consumption and wastewater discharge to the maximum extent. In particular, when the freshwater target concerning only water reuse is much higher than the inherent minimum freshwater usage (which will be defined later), in order to further reduce freshwater consumption and wastewater discharge, a water regeneration recycle should be considered. Therefore, in this paper, the design methodology of the water network
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with water mains concerning a water regeneration recycle is developed. Emphasis is placed on positioning of the regeneration water main.
PROCESS MODEL AND MASS BALANCE Water-using units can be classified into three types: (1) water is a reactant—in such processes, water may generate or lose, e.g. water and coal are reacted to generate synthesis gas; (2) water is a mass transfer medium—in such processes, water is used to remove contaminants from process streams in the given units, for example, steam stripping, liquid – liquid extraction and washing operations. In this paper, only the water used in such processes is studied; (3) water is an energy medium in cooling or heating, for instance, steam or circulating cooling water in process industries—in such processes, the contaminant concentration in water normally remains unchanged. Here we use U and S to represent respectively the number of water-using processes and the number of contaminants: U ¼ {iji is the number of water-using processes, S ¼ {kjk is the number of contaminants; k ¼ 1, 2, 3, . . . , S}
(1) (2)
Mass Balance of Process For each water-using process, the mass balance of water and the mass balance of contaminant are the most fundamental formulae. The water balance is: Fi,in ¼ Fi,out þ vi
i[U
(3)
where F denotes the water flow rate, subscript i means process i, U is the set of all processes, subscripts in and out represent inlet and outlet of the process, respectively, and v represents the loss flow rate of water. The contaminant balance is as follows: Fi,in Ci,k,in þ Mi,k ¼ Fi,out Ci,k,out þ vi Ci,k,L (4) i [ U, k [ S where C denotes contaminant concentration, subscript k means contaminant k, S is the set of all contaminants and subscript L means loss. Normally, vi and Ci,k,L are given before design. The constraint conditions are the maximum inlet and outlet concentrations. Max Ci,k,in Ci,k,in
i [ U, k [ S
(5)
and Max Ci,k,out Ci,k,out
i [ U, k [ S
where superscript Max denotes maximum.
Based on the mass conservation of water, the total freshwater consumption flow rate of the whole system can be expressed as: X f Fi,in i[U (7) Ff ¼ i
Process Model
i ¼ 1, 2, 3, . . . , U}
Mass Balance of System
(6)
where the superscript f denotes freshwater. The total water loss of the whole system is: X vi i [ U FL ¼
(8)
i
where superscript L denotes loss. The total flow rate of the wastewater discharge of the whole system is: X w Fw ¼ Fi,out i[U (9) i
where the superscript w denotes wastewater. To the whole water utilization system, the water balance should be Ff ¼ Fw þ FL
(10)
WATER NETWORK STRUCTURE In traditional water networks, water reuse is realized by connecting units directly by pipes. To make water networks easier to design, operate and control, a new water network structure with installing internal water mains was proposed (Feng and Seider, 2001). All plants always contain a freshwater main and a wastewater main, which are external water mains. The ‘internal water mains’ are similar mains possessing uniform contaminant concentrations between freshwater and wastewater. They receive water from some water-using processes at outlet contaminant concentrations less than or equal to their contaminant concentrations, and supply water to unit processes at inlet concentrations greater than or equal to their contaminant concentrations. In this way, all waterusing units are only connected to water mains, including external and internal water mains. Figure 1 gives an example of such a water network, in which the left vertical line is the freshwater main, the right vertical line the wastewater main, and the two vertical lines in the middle the internal water mains. The numbers above each main and in parentheses are the contaminant concentrations (the water system in Figure 1 involves three contaminants), and below each main is marked its flow rate. The numbered boxes represent water-using units which are connected by water mains, that is, they receive water from the main on the left, and discharge water to the main on the right. Obviously, as a water source, an internal water main should be positioned at a contaminant concentration less than or equal to that needed by the water sinks. Furthermore, as a water sink, an internal water main must receive water from at least one unit process. When a water regeneration recycle is considered, except for the processes that must use freshwater, all other waterusing processes can use regenerated water or wastewater
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Figure 2. Qualitative relationship between freshwater consumption F f and regeneration concentration Ci.
DETERMINING CONTAMINANT CONCENTRATIONS OF REGENERATION WATER MAIN Figure 1. Water network of example 1.
from other processes. In this paper, we only consider the water network with two internal water mains, one of which is the regeneration water main, the other, the regenerated water main, as has already been shown in Figure 1. This is a design decision that simplifies the water network. To save on the costs of regeneration, it may be preferable to use two or more regenerated water mains. So in the future, perhaps a synthesis strategy can be developed that positions additional regenerated water mains on the basis of regeneration cost estimates as a function of concentrations. In such a structure, the regeneration water main receives wastewater from some water-using units (in Figure 1, from process units 1, 2, 3, 4 and 5), supplies part of the water to some other units at concentrations higher than or equal to its concentrations (in Figure 1, to process units 6 and 7), and also sends the other part of the water to the wastewater regeneration process. The regenerated water from the water regeneration process goes to the regenerated water main and is used as a water source to supply water to some water-using units (in Figure 1, to process units 3, 4 and 5). Positioning the internal water mains properly is very important, as it determines the freshwater consumption and the regeneration cost. For the water system with regeneration recycle, normally the freshwater consumption changes with the contaminant concentration of the regeneration water, having the relationship shown qualitatively in Figure 2. If the concentrations are high enough, the freshwater consumption will remain unchanged. However, if the contaminant concentrations of the regeneration water main reduce to below certain limits (Ck,R1 in Figure 2), the freshwater consumption will increase with the decrease of the contaminant concentrations of the regeneration water main. The contaminant concentrations of the regeneration water main should therefore not be lower than this limiting value of Ck,R1.
It should be pointed out that, different from Feng and Seider (2001), the contaminant concentrations of the regeneration water main in this paper are the actual concentrations after full blending. A single-contaminant system is a specific case of multiple-contaminant systems and is much easier to deal with than multiple-contaminant systems. Therefore, in this paper, only multiple-contaminant systems are considered. Concentrations of the regenerated water main can be simply fixed at certain values (Wang and Smith, 1994; Mann and Liu, 1999; Kuo and Smith, 1998a). Normally the concentrations are set very low (Wang and Smith, 1994; Mann and Liu, 1999), for example, 10 ppm (Wang and Smith, 1994). However, actually the concentrations can be set slightly higher. Except for the processes that can only use freshwater, all other processes can use regenerated water. Consequently, the post-regeneration concentrations can be set as the lowest concentrations of all the maximum inlet concentrations of the processes that use regenerated water (Mann and Liu, 1999). Higher postregeneration concentrations can reduce the regeneration cost. Therefore, in this paper, the post-regeneration concentrations will be set in this way. The contaminant concentrations of the regeneration water main are the lowest concentrations with minimum freshwater consumption. If the concentrations are higher, the regeneration cost will increase, but the freshwater consumption cannot be reduced anymore. If the concentrations are lower, the freshwater consumption will increase unavoidably. The procedure to determine the regeneration concentrations comprises of the following six steps. Step 1: Determine the Inherent Minimum Freshwater Usage and the Inherent Minimum Wastewater Discharge of the System The inherent minimum freshwater usage of a system is defined as the sum of the minimum freshwater consumed by the processes that can only use freshwater. For example,
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if the maximum inlet concentration of at least one contaminant in a process is 0 ppm, this process has to consume freshwater, and if the wastewater discharges from the process at its maximum outlet contaminant concentrations, the freshwater consumption of the process has reached its minimum. The other processes in the system can use water from the regenerated water main or from the regeneration water main. Because water can be recycled, there is always sufficient water in the regenerated water main. When the system consumes freshwater equal to the inherent minimum freshwater usage, the corresponding wastewater discharge is defined as the inherent minimum wastewater discharge, which can be calculated through the water balance, equation (10).
Step 2: Arrange all Unit Processes in the Order of their Maximum Outlet Concentrations For single contaminant systems, unit processes can be arranged directly in the order of their maximum outlet concentrations. However, for multiple contaminant systems, the following rule is proposed to arrange processes in a multiple contaminant system. For each contaminant, k, rank-order its maximum outlet concentration from the unit processes in increasing order. Let the serial numbers, Ni,k, contain an integer ranking, where subscript i refers to process i and k refers to contaminant k. Then, the composite maximum outlet concentration, Ri, is defined as the repeated product of Ni,k, k ¼ 1, . . ., S, for each unit process i as given by: Y Ni,k i ¼ 1, . . . , U; k ¼ 1, . . . , S (11) Ri ¼ As a simple demonstration, for the system specified in the first five columns of Table 1, the calculated result is shown in columns 6 and 7. For example, N1,2 means the serial number of contaminant 2 in process 1. Because the maximum outlet concentration of contaminant 2 in process 1 is 400, which is greater than that in process 3, i.e. 45, and less than that in process 1, i.e. 12,500, N1,2 ¼ 2. In a similar manner, contaminant 1 for process 1 is the lowest of all the three processes, and therefore, N1,1 ¼ 1. Likewise, N1,3 ¼ 1. According to equation (11), R1 ¼ N1,1 . N1,2 . N1,3 ¼ 1 . 2 . 1 ¼ 2. Then list unit processes according to the rising order of their values of Ri, that is, in the order of R1 ¼ 2, R3 ¼ 9 and R2 ¼ 12, or process 1, 3 and 2, as shown in Table 1. Note that if the Ri of one process is equal to another, their relative list sequence should be arranged in the order of their maximum inlet concentrations. Table 1. Limiting data for a three-process system and computed results for their R. Process
F Lim (t h21)
1
45
2
34
3
56
Contaminant
CMax in (ppm)
CMax out (ppm)
Ni,k
Ri
1 2 3 1 2 3 1 2 3
0 0 0 20 300 45 120 20 300
15 400 35 120 12,500 180 220 45 9500
1 2 1 2 3 2 3 1 3
2 12 9
Step 3: Determine the Preliminary Position of Regeneration Water Main Start with the process with the highest relative composite maximum outlet concentrations (that is, the highest value of Ri), that is, the last one in the above sort, or process U. Let this process use the water from the regenerated water main (or freshwater main, depending on its maximum inlet concentrations) and discharge wastewater at its maximum outlet concentrations. Then calculate the water usage and discharge of this process by performing mass balances of contaminants and water [equations (3) and (4)]. In the same way, calculate the water usage and discharge of the next lower-ordered process, U 2 1, in the sort, one by one, and sum the discharged wastewater until the sum is just greater than the inherent minimum wastewater discharge that is determined in step 1. At this time, the processes taking part in summing up the discharged wastewater are from process U to a certain process U 2 i. This last process is called the critical process P, serially related to U by the relation P ¼ U 2 i. The preliminary position of the regeneration water main is set between the critical process P and the process P þ 1. In this way, the wastewater discharge of the system will be the inherent minimum wastewater discharge. If the preliminary concentrations of the regeneration water main are the same as those of process P, the wastewater discharge will be greater than the inherent minimum wastewater discharge, while, if they are the same as those of process P þ 1, the wastewater discharge will be less than the inherent minimum wastewater discharge. That is to say that the preliminary concentrations of the regeneration water main should lie between process P and process P þ 1. Step 4: Calculate the Actual Concentrations of the Regeneration Water Main The actual concentrations of the regeneration water main are determined by the following equations by considering full blending: X Fj!Rl j ¼ 1, . . . , P (12) FRl,in ¼ j
Mk,Rl ¼
X
Fj!Rl Cj,k,out
j ¼ 1, . . . , P; k [ S
(13)
j
Ck,Rl ¼ Mk,Rl =FRl,in
k[S
(14)
where subscript Rl means the regeneration water main, so FRl,in is the overall water flow rate in the regeneration water main; subscript j ! Rl means from process j into the regeneration water main; process P is the critical process; Mk,Rl is the overall mass flow rate of contaminant k in the regeneration water main; Ck,Rl is the actual concentration of contaminant k in the regeneration water main. Step 5: Determine the Final Values of the Regeneration Concentrations If the wastewater in the regeneration water main cannot be used in any process of the system, that is, there exists at least one contaminant concentration in the regeneration water main that is greater than the permissible maximum
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DESIGN OF WATER NETWORK WITH INTERNAL MAINS inlet concentration of that process, the calculated regeneration concentrations are the final regeneration concentrations, because the case is the same as the supposition when determining the preliminary position of regeneration water main. On the other hand, if water from the regeneration water main can be used in some processes which are serially higher than the critical process, the water usage and wastewater discharge of that process should be recalculated. Then calculation of the cumulative wastewater discharge in step 3 should be repeated. If the new value is less than or equal to the inherent minimum wastewater discharge, the calculated regeneration concentrations are the final regeneration concentrations. However, if the new value is still greater than the inherent minimum wastewater discharge, the critical process should be further serially elevated, and the corresponding concentrations of the regeneration water main recalculated, until a set of concentrations is found so that the wastewater discharge of some processes between P þ 1 and U is less than or equal to the inherent minimum wastewater discharge. These regeneration concentrations represent the final values.
Step 6: Calculate the Regeneration Water Flow Rate The corresponding flow rate of the regeneration water is the actual water flow rate needed from the regenerated water main.
DESIGN METHODOLOGY The design methodology of water network with internal water mains for wastewater regeneration is quite different from that for simple reuse. With water regeneration, the locations of all processes have been determined when regeneration concentrations are calculated. The design methodology is illustrated in the following: (1) Create a contaminant concentration diagram, as shown in Figure 1, with concentration increasing from the left to the right, including vertical lines to represent external or internal water mains. The freshwater main with 0 ppm contaminants is located on the extreme left, then comes the regenerated water main, followed by the regeneration water main; the last vertical line is the wastewater main. Wastewater to be regenerated flows from the regeneration water main leftward to the regeneration process and then goes leftward again into the regenerated water main. (2) Position a numbered box for each unit process between any two adjacent water mains. For each process, its maximum inlet concentration of any contaminant has to be greater than or equal to that of the water main to its left, but lower than that of the main to its right. (3) Position arrows to represent the streams going into and out of each unit process, all arrows pointing to the right, except for the regeneration process. Water is supplied from the left main for any water-using process, which then discharges into the main at its right. (4) Perform mass balance for each unit process to determine the water flow rate into and out of that process.
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(5) Perform water balance for the vertical water mains. The flow rate of the regenerated water main is determined according to the water needs of the processes with water supplied from regenerated water main. The regeneration concentrations need to be determined to assure that there is always sufficient water to regenerate and pass into the regeneration main and that no water shortage can occur. If there is any excess water in the regeneration water main, it will be discharged into the wastewater main.
CASE STUDY In a case study used to illustrate this design methodology, the limiting data for a seven-process system are specified in Table 2. Loss of water in any process is ignored. When only considering water reuse, the minimum freshwater consumption is 148.7 t h21, which can be obtained by solving the mathematical programming developed in Huang et al. (1999) from the data in Table 2. Now we shall see how this 148.7 t h21 could be reduced by using the present methodology following the flowsheet given in Figure 1. In the system, the maximum inlet concentrations of processes 1 and 2 are 0 ppm, so processes 1 and 2 must be fed with freshwater. All the other processes can use regenerated water. Thus the inherent minimum freshwater usage of the system is 100 t h21. As losses are ignored, the discharged wastewater of the system is equal to its freshwater consumption. So the corresponding inherent minimum wastewater discharge is also 100 t h21. Except for processes 1 and 2, the lowest maximum inlet concentrations of the three contaminants of the other five processes are set as the concentrations of the regenerated water main, which are (90, 80, 100). In Table 2, the seven processes are arranged downward in ascending order of their relative composite maximum
Table 2. Limiting data for the case study. Process 1 2 3 4 5 6 7
Contaminant
CMax in (ppm)
CMax out (ppm)
F Lim (t h21)
1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
0 0 0 0 0 0 90 130 100 110 80 150 260 200 180 340 350 400 950 850 900
80 70 80 110 120 100 150 180 210 210 150 220 350 320 310 800 1100 1000 1500 2100 1800
65
1
35
8
40
36
30
48
30
125
64
216
50
343
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outlet concentrations. When process 7 is fed with water from the regenerated water main, its wastewater discharge is 30.9 t h21. Similarly, the discharges of process 6, 5 and 4 are 47.1, 18.6, and 30.0 t h21, respectively. It can be seen that the sum of the wastewater discharge from processes 7, 6 and 5 is 96.6 t h21, which is less than the inherent minimum wastewater discharge of 100 t h21. If process 4 is added, the sum will be greater than the inherent minimum wastewater discharge. So process 4 is set as the critical process. The actual contaminant concentrations of the regeneration water main is calculated to be [122.1, 108.5, 130.6]. Wastewater in such concentrations can be reused by process 7, 6 and 5. According to mass balances, when water is supplied from the regeneration water main, the water usages of the three processes are 31.4, 48.4 and 21.7 t h21, respectively. Thus the sum of the wastewater discharge is 101.5 t h21, greater than 100 t h21. Therefore, process 5 is set as the critical process. In the same way, processes 7 and 6 can be fed with water from the regeneration main, and the corresponding discharges are 31.6 and 49.2 t h21, respectively, the sum of which is 80.8 t h21. This value is less than the inherent minimum wastewater discharge. So the final regeneration contaminant concentrations are [133.2, 124.8, 148.3]. The water flow rate needed for regeneration is 88.6 t h21. The total freshwater consumption of the system is 100 t h21, as shown in Figure 1.
CONCLUSIONS (1) The present paper generalizes a recent methodology (Feng and Seider, 2001) for designing water networks for multi-contaminant systems, and simplifies that methodology by introducing one or two internal water mains. (2) Water networks incorporating regeneration recycle can reduce freshwater consumption and wastewater discharge to the maximum extent, that is, down to the inherent minimum value. (3) This paper proposes a design methodology of water network with internal water mains employing such a regeneration recycle. In such a structure, water is supplied to any of the water-using processes from the freshwater main, the regenerated water main or the regeneration water main in accordance to contaminant concentrations. (4) In the design of water networks with internal mains for wastewater regeneration, the contaminant concentrations of the regeneration water main determine both the freshwater consumption and the regeneration cost. The regeneration concentrations should be the lowest concentrations compatible with the freshwater consumption of the system equal to its inherent minimum value. To ensure that, only wastewater equal to the inherent minimum value can be discharged to the wastewater main, and all other used water with lower contaminant concentrations will discharge to the regeneration water main. Part of water from the regeneration water main is supplied to such waterusing processes as can tolerate its contaminant concentrations, while the remainder reports to the regeneration process.
(5) When water networks are designed by using the proposed methodology in this paper, although the freshwater consumption and wastewater discharge are equal to that in the traditional water networks, and the regeneration water flow rate is more than that in the traditional network, the network has a simpler structure and better operability because of the internal water mains. NOMENCLATURE F C v M N P R U S
water flowrate, t h21 concentration of contaminant, ppm loss flowrate of water in a process, t h21 contaminant flowrate, kg h21 serial number critical process relative composite concentration set of all processes set of all contaminants
Subscripts in out i k Rl
at inlet at outlet process contaminant regeneration water main
Superscripts Max maximum f freshwater w wastewater L loss Lim limiting
REFERENCES Alva-Argaez, A., Vallianatos, A. and Kokossis, A., 1999, Multi-contaminant transhipment model for mass exchange networks and wastewater minimization problems, Comput Chem Eng, 23(10): 1439–1453. Bagajewicz, M.J., Rivas, M. and Savelski, M.J., 2001, A robust method to obtain optimal and sub-optimal design and retrofit solutions of water utilization systems with multiple contaminants in process plants, Comput Chem Eng, 25: 495. Feng, X. and Seider, W.D., 2001, A new structure and design methodology for water networks, Ind Eng Chem Res, 40(26): 6140–6146. Huang, C.-H., Chang, C.-T. and Ling, H.-C., 1999, A mathematical programming model for water usage and treatment network design, Ind Eng Chem Res, 38(7): 2666–2679. Kuo, W.-C.J. and Smith, R., 1998a, Designing for the interactions between water-use and effluent treatment, Trans IChemE, Part A, Chem Eng Res Des, 76(A3): 287– 301. Kuo, W.J. and Smith, R., 1998b, Design of water-using systems involving regeneration, Trans IChemE, Part B, Process Safe Env Prot, 76(B2): 94–114. Mann, J.G. and Liu, Y.A., 1999, Industrial Water Reuse and Wastewater Minimization (McGraw-Hill, New York, USA). Wang, B., Feng, X. and Zhang, Z., 2003, A design methodology for multiple-contaminant water networks with single internal water main, Comput Chem Eng, 27(7): 903–911. Wang, Y.P. and Smith, R., 1994, Wastewater minimization, Chem Eng Sci, 49: 981 –1006.
ACKNOWLEDGEMENTS Financial support from the National Natural Science Foundation of China (no. 20376066) and the National Key Basic Research Development Program of China (no. G2000026307) is gratefully acknowledged. The manuscript was received 24 January 2003 and accepted for publication after revision 16 June 2004.
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