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Cooling System Design for Water and Wastewater Minimization Jin-Kuk Kim* and Robin Smith Department of Process Integration, UMIST, P.O. Box 88, Manchester M60 1QD, U.K.
In this paper, new design methods will be developed aimed at reduction of cooling water makeup. Reduction in makeup will be achieved by introducing water recovery between wastewatergenerating processes and cooling systems. Recognizing that wastewater before or after treatment can replace cooling water makeup, a new design option for the cooling systems will be suggested. As wastewater added to the recirculating cooling water can make the tower bottlenecked, cooling water reuse design can be combined with wastewater recovery. As reuse design of cooling systems allows the cooling tower to take wastewater as makeup, substituting makeup with wastewater can yield water savings, as well as aqueous emissions reduction without overloading the tower. A case study is presented to illustrate the new design methodology. 1. Introduction Increasing environmental concerns have resulted in a focus on wastewater minimization. Reduction of freshwater consumption has become increasingly important in water systems as the supply of the usable water continues to become an increasing problem and the demand of water has increased with industrial development. For water and wastewater minimization, during the past decade, various systematic methods based on pinch analysis have played an important role in saving water resources and developing environmentally friendly designs for water systems. The basic idea is that wastewater can be reused directly in other operations when water-using operations can accept the contamination level of previous operations. Wang and Smith1-3 introduced a design method for water systems and effluent treatment systems. Kuo and Smith4-6 extended the methodology for total water system design by considering system interactions between water minimization, regeneration systems, and effluent treatment systems. In parallel with studies for conceptual design, automated design tools based on optimization techniques were developed to deal with complex problems involving different tradeoffs. Doyle and Smith7 developed a nonlinear programming (NLP) model for water system design by using the solution of linear progamming (LP) model as an initial point to NLP. Later, Alva-Arga´ze8 provided a mixed-integer nonlinear programming (MINLP) framework by combining water-pinch concepts with mathematical programming. While these approaches provide physical insights and design features for water systems, process integration implications between water-using systems and other systems have not been addressed. Water is consumed in many operations for different purposes: (1) extraction, absorption and scrubbing operations, (2) condensation and quenching operations, (3) stripping operations, (4) steam generation, (5) washing operations, and (6) cooling water systems. * To whom correspondence should be addressed. Tel.: +44 (0)161 200 8755. Fax: +44 (0)161 236 7439. E-mail:
[email protected].
Figure 1. New insights for water reuse.
Excluding steam generation and cooling water systems, water contacts process materials in various processes and then contaminated water is sent to wastewater treatment. In practice, not all water is fully reused or recycled in processes, even though its quality is good enough for reuse. For example, steam condensate loss occurs. Not all the condensate is usually recovered. The steam condensate not recovered is a good water source to reuse. While valuable water is not reused, the quality requirements for cooling water makeup are not generally as high as for other industrial processes. Therefore, cooling tower makeup can be changed from freshwater to reused water or even wastewater, if the quality of the water is relatively good. As shown in Figure 1, water consumption and wastewater generation can be reduced simultaneously when wastewater before discharge or treatment is used for cooling tower makeup. Wijesinghe et al.9 executed a feasibility study for cooling tower makeup. They showed that treated sewage effluent as a makeup is technically feasible. Any water treatment problems, such as scaling and biological fouling, in recirculating cooling systems were not anticipated up to 5 cycles of concentration. As cooling tower makeup is one of the largest demands for water, much attention has been paid to measures to reduce makeup. Some guidelines have been suggested so far. But the previous guidelines for makeup reduction have focused on changes of individual processes, rather than on a systematic approach. Kim and Smith10 established a new design methodology for cooling water systems by introducing series
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design between coolers into networks. This design methodology enables an increase in the operating range of the cooling tower and provides systematic procedures to deal with cooling tower overloading. From the insights of cooling system design, new design features can be provided for saving cooling water makeup. This paper will present a systematic method for the design of cooling water systems that accounts for the interactions between water-using systems and cooling systems to reduce makeup. The conventional methods to reduce cooling water makeup will be first discussed. To overcome the drawbacks of previous methods, new insights for cooling water makeup reduction will be presented following Kim and Smith’s10 procedures for cooling water system design. Finally, a case study will be presented to illustrate a new design option for water and wastewater minimization. 2. Design Options for Cooling Water Makeup Reduction In recirculating cooling tower makeup water is required to cover flowrate loss and blowdown during tower operation. As the evaporative cooling systems remove heat from hot cooling water by a combination of heat and mass transfer between water and air, the water loss comes mainly from evaporation. This is approximately 1% of inlet flowrate for each 10 °F range. Water is also lost from drift, but the amount in conventional cooling towers is no more than 0.2% of the inlet cooling water (Kemmer11). Also, blowdown is extracted from circulation in order to avoid the buildup of undesirable components in cooling systems. Five possibilities related to tower design and operating conditions are summarized below to reduce cooling water makeup. (1) Better cooling water treatment: In recirculating cooling water systems, the cycles of concentration (CC), the ratio of the concentration of a soluble component in the blowdown stream to that in the makeup stream, is used as a measure to control blowdown, makeup, and water treatment. When the plant heat load is fixed, the flowrate loss does not change very much and makeup water demand is dictated by blowdown flowrate or concentration levels. Better cooling water treatment enables high CC to be maintained in the recirculating cooling water, blowdown can be reduced, and therefore makeup water also reduced. It is common practice to divert part of the cooling water stream for filtration. Additional softening and/or clarification of the side stream can improve performance and can reduce makeup water. Reverse osmosis or electrodialysis of side stream is another way to keep CC high, but these methods are extremely expensive. (2) Reduction of cooling load: As the evaporation loss is proportional to the cooling load of tower, makeup can be reduced by decreasing cooling load. This can be achieved by improving the energy efficiency of processes. If the cooling load of the plant can be reduced, it seems to be an easy option. But, if processes are not fundamentally changed to reduce their inherent demand for cooling water, a significant reduction cannot be achieved. Another design option suggested by Lefevre12 is a combination of wet towers and dry coolers as a practical way to reduce evaporation flowrate loss. Usually air coolers are used when overloading of the tower is a problem, and air coolers can be used for reduction of makeup by dissipating some of the heat load to the air coolers. But the capital cost for air coolers is very high
Figure 2. Strategy for wastewater reuse for cooling systems.
and has a negative effect from the viewpoint of thermal performance. As air coolers are usually located in the return cooling water stream, the inlet temperature to the tower is decreased, but flowrate is unchanged. With decreased inlet temperature of the cooling water, the sensible heat transfer is decreased and the latent heat transfer becomes more dominant through the tower. So, the advantages of air cooling are diminished. (3) Monitoring and control: Monitoring for operations using cooling water is vital to reduce makeup. If cooling water leaks somewhere before returning to the tower, it results in an increase of makeup and affects the thermal performance of the cooling-water-using operation. Through monitoring flows, temperature, pressure, or concentrations of process streams and/or cooling water streams, the loss of cooling water on the process side should be examined and appropriate actions be taken, if necessary. (4) Reuse of blowdown after treatment: Blowdown after reclamation can be reused for makeup. As indicated by You et al.,13 higher initial cost of treatment makes it difficult to apply reclamation schemes. (5) Cooling tower upgrade: The cooling tower itself leaves room to reduce the flowrate loss. An effective drift eliminator can reduce the loss of drift water to atmosphere. Also, leaking water around the tower can be prevented by upgrading the tower components. Lefevre12 explained that evaporation depends on other factors such as air flowrate, barometric pressure, ambient temperature, and air humidity. But there are no rules to select optimal conditions of air flowrate, for which the evaporation loss is minimal. Others are uncontrollable variables to the operators. 3. Integrated Design of Cooling Systems As mentioned above, water consumption and wastewater generation can both be reduced when wastewater
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Figure 3. Reuse design of cooling water networks.
is used for cooling tower makeup. It is assumed that water-using systems have abundant wastewater, enough to supply the recirculating cooling water. Two cases can be considered for substituting makeup with wastewater, as shown in Figure 2. If the temperature of the wastewater is lower than that of the inlet cooling water temperature to the network, the wastewater should be added after the cooling tower (Figure 2a). On the other hand, if the temperature of the wastewater is higher than that of the cooling water temperature to the cooling water network, the wastewater should be added before the tower (Figure 2b). Otherwise, the cooling water after the tower gains heat before going to the cooling water network. A problem arises for the latter case, which is the more usual case. When the wastewater is added, the overall flowrate of cooling water is increased for tower operation. If the temperature of the wastewater is much higher than that of the inlet cooling water, the tower may not remove the desired cooling load. For example, steam condensate can be a major water loss in chemical processes and has a high temperature when compared with the cooling water return temperature. When wastewater, especially with a high temperature, is added to the main cooling water header, cooling water systems become bottlenecked. A solution for debottlenecking can be obtained by modifying the cooling water network. Heat removal from the cooling tower can be increased when reuse opportunities between coolers are increased (Figure 3). The current practice for cooling water network design most often follows parallel network configurations, where cooling water with the same temperature is supplied to all coolers. However, coolers do not always require cooling water at the same cooling water supply temperature when inlet cooling water temperature conditions are not too sensitive to the thermal performance of coolers. Appropriate manipulation of cooling water conditions to the coolers might allow the cooling water network to be changed from a parallel to a series design and/or combined parallel-and-series design. Cooling water configurations with cooling water reuse will return cooling water with a higher temperature and lower flowrate, where the cooling tower removes more heat from the water and allows a higher heat load for the tower. Based on this idea, retrofit design methods for cooling systems have been proposed by Kim and Smith,10 and below are brief descriptions of design procedures. (1) Define a limiting cooling water profile: A “limiting cooling water profile” is defined as the inlet and outlet temperatures for a cooling water stream that features the maximum allowable temperatures. These are chosen to comply with the thermal performance of an existing cooler in retrofit cases. These limiting conditions are limited by the “minimum temperature difference” (∆Tmin) or other process constraints (Figure 4).
Figure 4. Defining a limiting cooling water profile.
Figure 5. Composite curve and feasible design region.
(2) Construct a cooling water composite curve: A “cooling water composite curve” is constructed by combining individual limiting profiles. This single composite curve is created by adding all enthalpy changes within the same temperature interval and representing it as a single line (Figure 5). (3) Identifying feasible design region for cooling water supply: The cooling water supply line can be drawn against the composite curve. An increase in slope of a supply line means a decrease in flowrate and increase in temperature for cooling water return to the tower. There are two boundaries that limit the return cooling water conditions to the tower. A lower bound corresponds with a parallel network design of coolers, while an upper bound is limited by the composite curve creating a pinch point (Figure 5). (4) Targeting of cooling water supply conditions to cooling tower: As different settings for return cooling water conditions affect the performance of the cooling tower, the target conditions, where the desired heat removal from the tower is achieved, are searched by using a cooling tower model, as well as the cooling water supply line. (5) Design of a cooling water network: The adjustment of limiting profiles allows a new configuration of coolers to achieve the target conditions obtained in a conceptual way (Kim and Smith10). Also mathematical optimization methods based on the superstructure of a cooler network can be used (Kim and Smith14). 4. Case Study A simplified flow diagram of a high-density polyethylene (HDPE) plant is shown in Figure 6. HDPE is
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Figure 6. Simplified flow diagram of the HDPE plant.
manufactured in a reactor with a catalyst and dispersing agent. The HDPE is separated in a steam distillation kettle, and the dispersing agent is recovered in a distillation unit. The HDPE product is dried by hot air, and then the final product gives its heat to water in the granulator stage. The main freshwater consumption for the HDPE unit is in the supply of water for the HDPE product cooling in the granulator stage, domestic purposes, and cooling tower makeup. Another source of process water in the HDPE plant originates from the medium pressure (MP) and low pressure (LP) steam used as a heating medium. The LP steam is used in the steam distillation kettle for separating dispersing agent from the reactor outlet stream. The LP steam condensate is recovered in the separator and fully recycled to the steam distillation kettle as the temperature of the LP steam condensate is high enough to reuse. The MP steam is consumed in an air heater for generating hot dry air, which is then used for the drying of the product. The quality of the MP steam condensate after air heating is low in terms of heating capacity. Therefore, the MP steam condensate is partially reused for washing of mother liquor and then passed to the effluent header as wastewater. In the dispersing agent distillation unit, MP steam is also required as a hot utility. After MP steam loses its heat to the process stream and condensated, LP steam is generated from it by heat recovery within the distillation process. But all of the MP steam is not used for LP steam generation. Extra MP steam condensate is not fully recycled and currently flows into the effluent header. The other main source of wastewater comes from the granulator stage. Currently, the HDPE product is cooled in the granulator, by direct water cooling, which is circulated between the water tank/pit and the granulator as shown in Figure 7. The circulating water is cooled by cooling water. 20 t/h of water makeup is required to compensate for the 18 t/h of overflow and 2 t/h of loss from the system.
Figure 7. Current situation of granulator cooling.
As shown in Figure 6, wastewater around the steam distillation kettle, air heater, and separator units is generated by leaks and overflow during operation and is referred to as the HDPE channel water. The steam condensate, overflows, and the HDPE channel water are the main source of the 55.4 t/h of wastewater in the central treatment unit. 4.1. Reduction of Freshwater Consumption. Overflow is currently extracted to prevent the buildup of undesirable materials in the circulating water in the granulator stage. The circulating water is contaminated by HDPE product, but there is no chemical or biological contamination and the water contains only dust. Therefore, a filter can be introduced for separation of undesirable solids rather than the use of a purge. When the filter is introduced into the system for regulating the quality of the water, the overflow from the water tank can be prevented, and hence water makeup can be reduced. Also, the high quality overflow from the MP condensate tank can be replaced by the water makeup because steam condensate is almost pure. As shown in Figure 8, the freshwater makeup is reduced from 20 to 0.44 t/h by the introduction of the filter and reuse of the overflow from the MP condensate tank. A reduction of 18 t/h of effluent is achieved, while the water makeup savings is 19.56 t/h. 4.2. Cooling System Design for Reduction of Cooling Water Makeup. A simplified diagram of the cooling system is shown in Figure 9. A cooling tower with a capacity of 2201 t/h supplies cooling water for
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Figure 8. Proposal for water cooling for the HDPE product.
Figure 10. Proposal for the cooling water system.
Figure 9. Cooling water system of the case study. Table 1. Cooling Water Conditions of Base Casea cooler
flowrate (t/h)
Tout (°C)
C1 C2 C3 C4 C5 C6 C7 C8 C9 C 10
578 596 58 95 208.7 16 166.2 26 430 27
35.22 35.11 30 31 29.8 37 38.8 33 33 33
total a
2201
Figure 11. Proposal for the cooling water network.
34.15
Cooling water inlet temperature: 28.8 °C.
the HDPE unit, and 25.5 t/h of makeup water is consumed to compensate for the evaporation loss and blowdown from the tower. To investigate new design options for the cooling system, it was assumed that existing features of heat exchanger network and cooling system are accepted. The cooling water data for individual coolers are shown in Table 1. There are two wastewater streams from the HDPE units. One is from the HDPE channel water, which would not create any treatment problems in cooling systems because it originates from steam condensate. The other is from the ML washer tank, which might increase fouling or scaling problems, because it is contaminated by minerals during washing. As this wastewater flowrate is very small (3 t/h) when compared with the overall flowrate (2201 t/h) and CC is controlled below 8, mixing the ML washer overflow with the cooling water header does not degrade cooling water quality very much. Makeup can be reduced from 25.5 to 11.5 t/h by introducing wastewater into the cooling water header. As an additional 14 t/h of wastewater is mixed into the cooling water header, the effluent flowrate is reduced by 14 t/h.
Figure 12. Final design of the cooling water system.
The simplified diagram for the suggested cooling system is represented in Figure 10. However, modification of the cooling water network is needed as the inlet temperature and flowrate to the tower increase. Unfortunately, it is difficult to modify the cooling system design because 60% of the cooling water (C1-C4 in Table 1) is used in the reactor coolers. A limited possibility for cooling water reuse is possible. Reuse of cooling water is possible between C5, C6, and C7, as shown in Figure 11. The outlet stream from C5 is cool enough for reuse in the other coolers. By introducing reuse between the coolers, the temperature of the overall inlet stream to the tower is increased by 0.52 °C but the flowrate of the overall inlet stream to the tower is decreased by 7.6%. From a cooling tower performance analysis, there is no problem for the current tower to manage with different inlet conditions to the tower. Also, different cooling water conditions affect the amount of evaporation loss, but the difference is marginal with the modified design. The final design of the cooling system is illustrated in Figure 12.
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By considering water minimization combined with cooling water system design, the water savings identified a potential to reduce 45% of the cooling water makeup, 98% of water makeup for HDPE units, and 58% of the wastewater. This would reduce the demand for raw water by 168216 t/y and for cooling water by 120400 t/y. Aqueous emissions would be reduced by 275200 t/y. 5. Conclusion As cooling water makeup is a major part of the industrial water supply, significant effort has been made to reduce this. The previous guidelines for makeup reduction have focused on the process changes rather than on a systematic approach. Because cooling tower overloading can be prevented by changing the cooling water network design to a series arrangement, a new design method for cooling systems has been suggested for makeup reduction. Reduction in makeup can be achieved by introducing water recovery between wastewater-generating processes and cooling systems, while the cooling tower can reliably manage the increased heat load. A case study illustrates the integrated analysis of water and cooling systems that can achieve a significant reduction in aqueous emissions. Acknowledgment The authors would like to express their appreciation to Engineers India Limited for help given during the research project. Nomenclature C ) cooler CT ) cooling tower CW ) cooling water OP ) water-using operation TR ) treatment unit
Tincw ) inlet cooling water temperature to network Tww ) wastewater temperature
Literature Cited (1) Wang, Y. P.; Smith, R. Wastewater Minimisation. Chem. Eng. Sci. 1994, 49, 981-1006. (2) Wang, Y. P.; Smith, R. Design of Distributed Effluent Treatment Systems. Chem. Eng. Sci. 1994, 49, 3127-3145. (3) Wang, Y. P.; Smith, R. Wastewater Minimisation with Flowrate Constraints. Trans. IChemE 1995, 73, Part A, 889-904. (4) Kuo, W. J.; Smith, R. Effluent Treatment System Design. Chem. Eng. Sci. 1997, 52, 4273-4290. (5) Kuo, W. J.; Smith, R. Designing for the Interactions between Water-use and Effluent Treatment. Trans. IChemE 1998, 76, Part A, 287-301. (6) Kuo, W. J.; Smith, R. Design of Water-using Systems Involving Regeneration. Trans. IChemE 1998, 76, Part B, 94114. (7) Doyle, S. J.; Smith, R. Targeting Water Reuse with Multiple Contaminants. Trans. IChemE 1997, 75, Part B, 181-189. (8) Alva-Arga´ze, A. Integrated Design of Water Systems. Ph.D. Thesis, UMIST, Manchester, U.K., 1999. (9) Wijesinghe, B.; Kaye, R. B.; Fell, C. D. Reuse of Treated Sewage Effluent for Cooling Water Makeup: A Feasibility Study and a Pilot Plant Study. Water Sci. Technol. 1996, 33 (10-11), 363-369. (10) Kim, J.; Smith, R. Cooling Water System Design. Chem. Eng. Sci. 2001, 56 (12), 3641-3658. (11) Kemmer, F. N. The NALCO Water Handbook, 2nd ed.; McGraw-Hill: New York, 1988. (12) Lefevre, M. R. Reducing Water Consumption in Cooling Towers. Chem. Eng. Prog. 1984, July, 55-62. (13) You, S.; Tseng, D.; Guo, G.; Yang, J. The Potential for the Recovery and Reuse of Cooling Water in Taiwan. Resour., Conserv. Recycl. 1999, 26, 53-70. (14) Kim, J.; Smith, R. Cooling System Design with Pressure Drop Constraints. PRESS 2002, Praha, Czech Republic, August 2002: Paper 740.
Received for review November 4, 2002 Revised manuscript received June 3, 2003 Accepted October 3, 2003 IE020890M
