Guided Design of Heating and Cooling Mains for Lower Water and Energy Consumption and Increased Efficiency

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1 Guided Design of Heting nd Cooling Mins for Loer Wter nd Energy Consumption nd Incresed Efficiency Vincent Gololo 1, Thoko Mjozi 1*, Toshko Zhelev 2, Krum Semkov 2 1 University of Pretori, Deprtment of Chemicl Engineering, Lynnood Rod, Pretori, 0002, South Afric 2 University of Limerick, Deprtment of Chemicl nd Environmentl Science, Irelnd thoko.mjozi@up.c.z Wter cooling nd ter heting is n importnt source of energy consumption, ccounting for more thn 20% of ll energy consumption in mnufcturing industry. It is cler tht the development of het recycling schemes nd better structurl design cn llo reching significnt svings. It is lso knon from poer genertion nd from the bulk chemicls production tht the cooling ter systems re generlly designed ith set of het exchngers rrnged in prllel. This rrngement results in higher cooling ter florte nd lo cooling ter return temperture thus reducing cooling toers efficiency. This indictes the importnce of the system structure, the possibility of mixing of heting or cooling ter; recycling nd reuse of heting nd cooling resources nd finlly the importnce of distribution of the driving forces to het trnsfer. This pper reports technique for grssroot design of centrlised cooling nd heting ter system for ter nd steter minimiztion hich incorportes the performnces of the cooling/heting providers involved. The study focuses on cooling/heting systems consisting of multiple chillers nd/or boilers tht supply common set of het exchngers. The pper tkes conceptul pproch to the stted problems, but trces the y for exct solution using the superstructure bsed mthemticl pproch (formulting the problem). It explores cooling/heting ter reuse opportunity nd predicts therml performnce of therml systems involved. 1. Introduction Cooling ter systems re used to remove ste energy from the process to the tmosphere. The systems consist of cooling toers, re-circulting system nd cooling ter netork. Wter is used s medium to remove energy from the process through the cooling toers into the tmosphere. Previous reserch on cooling ter systems hs focused minly on debottlenecking the cooling toers through synthesis nd optimiztion of the cooling ter netorks. The common technique used in this regrd s bsed on grphicl nlysis pproch (Kim nd Smith, 2001; Kim et l., 2002). This technique s derived from the principles of pinch nlysis developed for het exchnger netorks synthesis by Linnhoff nd coorkers (Linnhoff nd Floer, 1978; Linnhoff et l., 1979; Floer nd Linnhoff,

2 1980). The principles of pinch nlysis ere lso dpted for mss exchnge netork synthesis (EL-Hlgi nd Mnousiouthkis, 1989) nd lter pplied for trgeting nd synthesis of steter minimiztion problems (Wng nd Smith, 1994). Fe uthors used the superstructurl modelling techniques to optimize the cooling ter systems (Mjozi & Moodley, 2008; Kim nd Smith, 2003). This pper presents technique for synthesis nd optimiztion of cooling ter system hich incorportes the performnces of the cooling toers involved. The study focuses minly on cooling systems consisting of multiple cooling toers tht supply common set of het exchngers. The het exchnger netork is synthesized using the mthemticl optimiztion technique. The cooling toer model is used to predict the therml performnce of the cooling toers hilst tking the therml conditions of the ssocited het exchnger netork into ccount. 2. Cooling ter systems model development The cooling ter system consists of cooling toers nd het exchnger netork. Therefore the mthemticl model for designing cooling system entils the het exchnger netork model nd the cooling toer model. The het exchnger model entils superstructure in hich ll possible cooling ter reuse opportunities re explored. The mthemticl optimiztion formultion s developed from the superstructure given in Figure 1 by considering energy nd mss blnce equtions cross ech cooling ter using opertion nd t ech node. Figure 1: Superstructure for cooling ter system The optimum het exchnger netork design is found by minimizing the cooling toer inlet flortes. The interction beteen the het exchnger netork nd the cooling toers is investigted using the cooling toer model derived by Kröger (2004) using the control volume in Figure 2. The governing equtions tht predict the therml performnce of cooling toer re given by Equtions (1), (2) nd (3). Equtions (1)

3 nd (2) define the mss nd energy blnce for the control volume, respectively. Eqution (3) defines the ir enthlpy chnge for the control volume. Figure 2: Control volume dm dt dh = m d m = cp m cp 1 K fi A = m fi dh T d ( Le ( H H ) + ( 1 Le )H ( )) f s (3) The het exchnger netork model is bsed on the folloing possible prcticl cse study. 3. Cse study A set of cooling toers ith specified mximum cooling ter return temperture to the cooling toers ithout dedicted source or sink for ny cooling ter using opertion is given. This sitution rises hen pcking mteril inside the cooling toer is sensitive to temperture nd ny cooling toer cn supply ny ter using opertion hilst the ter using opertion cn return to ny cooling toer. The ppliction of the proposed technique is demonstrted by considering one exmple. This exmple s extrcted from the pper by Mjozi nd Moodley (2008). The formultion for the Cse study consists of biliner terms hich re nonconvex thus rendering the model NLP. This model is difficult to initilize becuse the strting point might be infesible or the solution might be loclly optimum (Gololo & Mjozi, 2010). To overcome these difficulties the technique proposed by Quesd nd Grossmnn (1995) s used to linerize the biliner terms. This technique uses the upper nd the loer bounds to crete convex spce for the biliner terms. f v s (1) (2)

4 3.1 Bse cse Cooling ter system in Figure 3 shos set of het exchnger netorks hich re supplied by set of cooling toers. Ech cooling ter using opertion is supplied by freshter from the cooling toer nd return bck to the cooling toer. The impliction of these rrngements results in higher return cooling ter florte nd lo return cooling ter temperture thus reducing cooling toer efficiency (Bernier, 2004). Figure 3: Bse cse (Mjozi nd Moodley, 2008) 3.2 Cse study In this cse ech cooling toer cn supply ny cooling ter using opertion. The return strems from ny cooling ter using opertion cn go to ny cooling toer. The return temperture to ny cooling toer is hoever specified. Figure 4 shos the het exchnger netork fter pplying the methodology described bove. By exploiting the opportunity for cooling ter reuse, the overll circulting ter decresed by 22 % nd one cooling toer s eliminted. The cooling toer inlet tempertures re t their mximum vlues.

5 Figure 4: Finl design of the cooling ter system These results sho the opportunity to increse the het duties, through expnsions, ithout investing on ne cooling toer. The only dditionl investment required is on piping for reuse strems. For this cse study the mkeup nd the blodon s lso decresed by 7 %. Hoever the decrese in mkeup nd blodon cnnot be gurnteed for ll prcticl cse studies nd this is not the intended purpose in this study. The results summry is shon in Tble 1. Tble 1 Results summry Strem Bse cse(kg/s) Results(kg/s) Mkeup Blodon Circulting ter

6 4. Conclusions The mthemticl technique for cooling ter system synthesis ith multiple cooling toers hs been presented. This technique is more holistic becuse it cters for the effect of cooling toer performnce on the het exchnger netorks. The cooling toer therml performnce is predicted using the mthemticl model. The proposed technique debotllenecked the cooling toers by decresing the circulting ter florte. This implies tht given set of cooling toers cn mnge n incresed het lod. From the cse study, 22 % decrese in circulting ter florte s relized. This decrese in the overll circultion ter hs n dded benefit of decresing the overll poer consumption of the circulting pumps. The blodon nd mkeup ere lso decresed by 7 %, hoever this cnnot be gurnteed for ll prcticl cse studies. References El-Hlgi M.M. nd Mnousiouthkis V. Synthesis of mss-exchnger netorks, AIChE. Sci. 1989, 8, Floer J.R. nd Linnhoff B. Thermodynmic-combintoril pproch to the design of optimum het exchnger netorks, AIChE J. 1980, 26(1), 1-9. Gololo K.V. nd Mjozi T. On synthesis nd optimiztion of cooling ter systems ith multiple cooling toers. Submitted to Ind.& Eng.Chem.Res. Journl, Kim J.K., Lee G.C., Zhu X.X. nd Smith R. Cooling system design, Het Trnsf. Eng. 2002, 23(6), Kim J.K. nd Smith R. Automted retrofit design of cooling ter systems, AIChE J. 2003, 49(7), Kim J.K.nd Smith R. Cooling ter system design, Chem.Eng.Sci. 2001, 56, Kröger D.G., 2004, Air-Cooled Het Exchngers nd Cooling Toers: Mss Trnsfer nd Evportive Cooling. Penn Well Corportion, USA. Linnhoff B., Dvid R.M. nd Wrdle, I. Understnding het exchnger netorks., Comput. nd Chem. Eng. 1979, 3, Linnhoff B. nd Floer J.R. Synthesis of het exchnger netorks, AIChE J. 1978, 24(4), Mjozi T. nd Moodley A. Simultneous trgeting nd design for cooling ter systems ith multiple cooling ter supplies, Comput. nd Chem. Eng. 2008, 32, Quesd I. nd Grossmnn I.E. Globl optimiztion of biliner process netorks ith multicomponent flos, Comput. nd Chem.Eng. 1995, 19(12), Tkm N., Kuriym T., Shiroko K. nd Umed T. Optiml ter lloction in petroleum refinery, Comput. nd Chem. Eng. 1980, 4, Wng Y.P. nd Smith R. Wsteter minimiztion, Chem.Eng.Sci.1994, 49(7)