OPTIMIZATION OF THERMAL ENERGY CONSUMPTION IN SUGAR CANE FACTORIES

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1 Proceeding of ECOS 2006 Aghia Pelagia, Crete, Greece July 12-14, 2006 OPTIMIZATION OF THERMAL ENERGY CONSUMPTION IN SUGAR CANE FACTORIES Adriano V. Enina; Silvia A. Nebra Energy Department State Univerity of Campina Miguel A. Lozano; Lui Serra * Mechanical Engineering Department Univerity of Zaragoza ABSTRACT Sugar and ethanol production from ugar cane in Brazil i one of the mot competitive ector of the national economy. The ugar production i done baically by everal tep: extraction, juice clarification, evaporation, yrup treatment, ugar boiling, crytallization and centrifugation where the crytal ugar and the molae are obtained. Thi paper preent a thermoeconomic optimization of thermal energy conumption in a ugar production proce looking for the minimum invetment and operation cot. Data from Brazilian ugar factorie are ued to define the proce parameter. The methodology propoed i ued to evaluate the cot of the team conumed by the factory and the optimal deign of the evaporation ytem a well a the juice and yrup heater network. Key-word: ugar production, thermoeconomic optimization, heat recovery. NOMENCLATURE A Surface of heat exchange (m 2 ) b Specific exergy (kj/kg) C Monetary Cot (R$/) d Operation day (operation day/year) E Equipment capital cot (R$) h Specific entalpy (kj/kg) i Annual interet rate (%) j Ueful life (year) k Unit exergetic cot (R$/kJ) m Ma flow rate (kg/) p Preure (bar) Pu Purity (%) Q Heat tranfered (kw) Specific entropy (kj/kg K) t Temperature U Heat tranfer coefficient (W/m 2.K) x Solid ma content (%) Z Cot of equipment (R$/) Greek Letter Scaling exponent Velocity (m/) Amortization factor ( -1 ) Operation factor (operation hour/total hour) Subcript bpe boiling point elevation c condenate tream e equipment evap evaporator he heater in inlet flux j juice let flux r reference equipment team tream at aturation w water 1. INTRODUCTION Sugar and ethanol production from ugar cane in Brazil i one of the mot competitive ector of the national economy. The bagae generated by the productive proce i ued a fuel in cogeneration ytem that offer thermal and electrical energy for the proce. In the lat few year many ugar cane factorie are producing urplu of electricity that can be old for the grid, becoming a new product. * Correponding author: CPS de Ingeniero, C/ Maria de Luna, Zaragoza Spain. erra@unizar.e 569

2 Currently there are more than 300 cane factorie operating all around the country [1]. Thee unit cruhed more than Mt of cane in the harvet eaon 2005/2006, with a total production of more than 26.7 Mt of ugar and 17.0 Mm 3 of ethanol [2]. Sugar production i done baically by everal tep: extraction, clarification, evaporation of the ugar juice, followed by the yrup treatment, crytallization and centrifugation where the crytal ugar and the molae are obtained. demand of the proce occur mainly into the evaporation tation that conume the larget amount of team produced by the cogeneration ytem. Extraction, juice clarification, yrup treatment and crytallization alo conume team that in general came from vapour generated by the evaporation. The reduction of team demand by production proce can increae the urplu of electricity generated by the cogeneration ytem, but it feaibility mut be evaluated together with the proce invetment cot. Since an economic point of view, the price paid for the urplu of electricity generated and the invetment neceary for new heater network and evaporation ytem of the ugar factory determine the feaibility of proce team demand reduction. A thermoeconomic optimization of the proce equipment lay- and operation can indicate the mot adequate invetment. The purpoe of thi paper i to perform a thermoeconomic optimization of the team demand of a crytal ugar factory, analyzing for that the bet deign of the evaporation ytem and the heater network. Thi tudy intend to analyze the poible thermal integration of the factory, conidering the cot involved. Data 1 of equipment cot and ugar proce parameter were obtained from ugar cane factorie in Brazil. 2. SUGAR PROCESS DESCRIPTION The crytal ugar production from ugar cane i done baically by the following tep depicted in Figure 1: Extraction Sytem (I): ugar cane bagae and juice are eparated. Traditional ugar factorie ue mill where juice i extracted by compreion. Another ytem that operate with diffuer, extract raw juice by a proce of lixiviation, uing imbibition hot water and recirculation of the juice extracted. Both ytem require previou cane preparation that i done uing knive and hredder that operate with direct drive team turbine. The plant tudied in thi paper operate with a diffuer ytem that demand thermal energy for the heating of recirculating juice. The ugar cane bagae produced at the extraction i delivered to the cogeneration ytem (VII) a indicated in Figure 1, where it i ued a fuel, producing the electricity and team conumed by the proce. Juice Clarification Sytem (II): ome nonugar impuritie are eparated by the addition of ome chemical reactant a ulphur, lime, among other. Juice heating i neceary for the purification reaction. After heating, the juice pa through a flah tank, before entering the clarifier. Evaporating Sytem (III): juice i concentrated in a multiple-effect evaporator. from the cogeneration ytem i ued a thermal energy ource in the firt effect that evaporate an amount of the water preented in the juice, producing like that, the heating team for the next evaporation effect. The ame occur with the other effect. The ytem work with decreaing preure due to a vacuum impoed in the lat effect, to produce the difference of temperature between each effect. The team generated in each effect normally i ued to attend other team demand of the proce. Syrup Treatment (IV): yrup (concentrated juice) from evaporation tation i purified for improving the quality of the final product. Firtly the yrup i heated and then a flotation of the impuritie i done with addition of ome chemical reactant. Sugar Boiling, Crytallization and Centrifugal Separation (V): yrup i boiled in vacuum pan for crytal formation and then directed to crytallizer to complete crytal enlargement. After that, the ugar crytal formed are eparated from molae into centrifugal. Sugar Drying (VI): ugar i dried to reduce it moiture content to be tored 3. OPTIMIZATION PROCEDURE The optimization of the thermal energy conumption in the ugar cane proce wa performed with the EES oftware [3], looking for the minimum total cot including invetment in equipment (juice heater and evaporator) and proce team conumption. The final cot reult are baed in a ugar factory that cruhe 22,000 t 570

3 cane/day. The reference ambient wa conidered at bar of preure and 25ºC of temperature. capital cot of heater; cot of conumed team; SUGAR CANE Vapour Bleeding I II III IV V VI Electricity for the proce Bagae MOLASSE VII SURPLUS ELECTRICITY Exhauted And for the evaporator ytem the following parameter were defined: operation preure of evaporator; intermediate juice concentration; juice boiling point elevation; temperature of boiling juice; urface of heat exchange; capital cot of evaporator; cot of heating team conumed; cot of vapour bleed and condenate generated; The objective function, defined by Equation 1, minimize the total cot of the different ubytem analyzed a a whole, conidering for that, invetment in urface of heat exchange in evaporator and heater, cot of the exhauted team conumed by the evaporator and dicounting the cot of the condenate produced that are aumed a product, being conumed in other part of the ugar proce: C Z C C (1) total e e c c CRYSTAL SUGAR Economic Model Figure 1: Scheme of a ugar factory with the cogeneration ytem. The optimization would define the bet lay of the equipment of heat exchange. The optimization variable choen for the heater and evaporator were repectively, the difference of temperature between the heating team and the juice/yrup leaving each tage of heating and the aturation temperature of the team generated at the evaporation effect. Thu, other parameter could be calculated, defining the bet lay- of the equipment and finally, the minimum total cot of the ytem a a whole. For the heater the following parameter were defined: number of heating tage; intermediate temperature between the tage of heating; urface of heat exchange; type and amount of heating team neceary for each tage of heating; Determination of the team cot The cot of each tream of proce team demanded by the proce wa etimated uing the Theory of Exergetic Cot [4]. Firtly, exergy of the bagae and ugar cane were calculated. For the determination of bagae exergy, a methodology preented by Soa-Arnao and Nebra [5] wa adopted. The referred methodology i a variation of that one propoed by Szargut et al. [6] for wood, with the neceary change in the compoition and Low Heat Value of the fuel. For the bagae at the reference environment condition, it total exergy i equal to it chemical exergy. The following compoition of the bagae in ma and dry bae wa aumed: C(47%), H(6,5%), O(44%) and Ah(2,5%) [7]. The exergy of the ugar cane wa obtained a the um of the bagae exergy and the juice exergy calculated following procedure for ucroe-water olution preented by Nebra and Fernández-Parra [8]. Thu, for a determined production cot of the ugar cane ready to be proceed, the unit 571

4 exergetic cot of the ugar cane could be calculated a follow: ccane kcane (2) bcane The unit exergetic cot of the bagae ued a fuel at the cogeneration ytem wa aumed to be the ame of the ugar cane that enter the factory at the extraction ytem. So, the live team produced by the boiler at the cogeneration ytem had it unit exergetic cot obtained uing Equation 3. k ( k cane * m bagae * b m *( b bagae b w ) Z ) boiler (3) The exhauted team from the back preure team turbine of the cogeneration and vapour generated at the evaporator are conidered alternative of heating ource to the proce. So, to perform the optimization, it wa aumed a a hypothei, that they have the ame unit exergetic cot of the live team. The unit exergetic cot of condenate generated after the team condenation at the heat exchanger are alo the ame. So, the cot of each team tream could be calculated multiplying the unit exergetic cot by it exergy (Equation 4). Table 1 how the parameter adopted for the determination of the team cot. C k * m * b (4) Table 1. Data for determination of team cot. Parameter Value Boiler capital cot (10 6 R$) 1 25 Sugarcane production cot (R$/kg) Available bagae (kg/t cane) Live team preure (bar) 63 Live team temperature 480 Boiler feed water temperature 122 Boiler efficiency (%) 85 1 cot of boiler with following characteritic: 63 bar, 480ºC, 200t team/h including cot of intallation and intrumentation [9]. 2 cot of ugar cane ready to be proceed at the State of Sao Paulo, Brazil in 2006[10]. 3 wet bae (50% of moiture) Determination of equipment cot The capital cot of equipment could be calculated uing Equation 5. Data ued are hown in Table 2. Z e E e * (5) Where: Ae E e Er (6) Ar j i *(1 i) j (1 i) 1 (7) 86400* d * Table 2. Data for determination of equipment cot. Parameter Value Evaporator capital cot (10 3 R$) Juice heater capital cot (10 3 R$) 2 90 Evaporator caling factor Heater caling factor Annual interet rate (%) 15 Equipment ueful life (year) 15 Factory operation day 185 Operation factor (%) cot of intalled evaporator type Robert with 4000m 2 of area [10]. 2 cot of intalled carbon teel hell and tube juice heater with 300m 2 of area [10]. 3 Source [11]. 4 Source [12]. 5 Effective operation hour of the total hour of factory operation day 3.2. Phyical Model Evaporation ytem The evaporation ytem model wa compoed by a five effect evaporator type Robert that operate with a vacuum at the lat effect, producing the difference of temperature between each effect. Some retriction are impoed for the optimization: juice enter at Brix 15º and leave at 63º 572

5 maximum temperature of 115ºC for juice boiling at the firt effect to avoid juice ucroe lo and coloration [13] minimal preure of 0.16 bar at the lat effect [14]. A heat demand of ugar boiling ytem wa etimated a 141,6 kwh/t cane [15] and mut be upplied by evaporation ytem vapour bleed with a temperature higher than 100ºC, due to retriction of the proce. The enthalpy of the juice wa calculated uing Equation 8 [16]: h j = ( *x j * x j *Pu j ) * t j *10-5 * x j *t j 2 (8) The temperature of evaporation (Equation 9) in each effect i defined a the um of the temperature of aturation of pure water at the vapour pace for a given operation preure and the boiling point elevation due the concentration of the juice (Equation 10) [17]. The effect of boiling point elevation due the hydrotatic effect of liquid column wa neglected. t BPE t evap = t w,at + t BPE (9) 5,84 *10 6,064*10 7 x t w, at 374,3 t Heater Network 2 0,00072 x 2 0,38 w, at * (10) The heater network include the following ub-ytem: A. Diffuer; B. Juice Clarification; C. Syrup Treatment. Data of each heater ub-ytem are preented in Table 3. The purity of the heated flow i conidered contant at 85%. Table 3. Data of heater ub-ytem. Sub-Sytem m j (kg/) Xj (%) t in t A Diffuer B Juice Clarification 2 C Syrup Treatment Evaporator let raw juice re-circulated in thee different tage during the diffuion [15] 2 10% of the clarified juice i re-circulated[10] Area of heat exchange Heat tranfer area that define the cot of the evaporator and the heater can be calculated by Equation 11. Q A (11) Ut The energy tranferred by the heating team (Q) i equal to it enthalpy of vaporization. Equation 12 [18] and 13 [14] were ued to calculate the heat tranfer coefficient of evaporator and heater repectively. The juice velocity circulation at the heater wa aumed a 1.5 m/. 465 tevap U evap (12) x U.341* * (5 ) (13) he 1 t, in j For the determination of the difference of temperature (t) in the evaporator and heater, Equation 14 [14] and 15 were ued repectively. t t, t (14) t evap he in evap t t t t 3.3. Bae Cae in t t ln t t in (15) A bae cae i aumed to compare and validate the reult of the optimization procedure. Data of the evaporation ytem deign wa obtained from a real cane ugar factory [15] and i compoed by a five effect evaporator type Robert working at the following preure in each effect: 1.66; 1.32; 0.98; 0.50; 0.17 bar of abolutely preure. A occur in many ugar factorie in Brazil, for thi bae cae, there i not an optimal thermal integration of the proce and all the juice and yrup heater of the factory conume vapour generated at the firt effect of evaporation that attend the heat demand of ugar boiling ytem too. Thu, the heater network i formed by one 573

6 heater in each ub-ytem (Difuer, Juice Clarification and Syrup Treatment). 4. RESULTS Uing the optimization procedure decribed above, the optimum evaporation ytem and heater network of the ugar factory can be determined. Table 4 how the unit exergetic cot calculated. Table 4. Unit exergetic cot determined. Unit Exergetic Cot(10-6 R$/kJ) Sugar Cane The Figure 2 how the lay- of the equipment optimized at the factory. In thi figure the propoed ub-ytem can be een, including alo the ditribution of the vapour bleeding from the evaporator. The detailed parameter of each heater ub-ection are hown in Table 5. Table 5. Optimized heater network. Heating T in Conumed Area (m 2 ) Heating Flow Hate (kg/) T A V B V B V B V B4 673 V C1 402 V C2 387 V A it can be noted in Table 5, after the thermoeconomic optimization a determined ditribution of the vapour generated by the evaporation ytem are obtained. The new lay- promote a better ue of vapour from each evaporation tage, contributing for a higher thermal integration of the factory, with a minimal cot. A1 Juice/Syrup Condenate B4 B3 B2 B1 C2 C1 V1 V2 V3 V4 Condeer Exhauted 1 t Effect 2 nd Effect 3 rd Effect 4 th Effect 5 th Effect Juice Syrup Figure 2: Lay- of the optimized ytem. The higher cot of the exhauted team from the cogeneration ytem had limited the ue of it a heating ource for the heater in an optimum deign. The available vapour bleed had adequate temperature for the heating demand, reducing the total cot of the ub-ytem. The conumption of exhauted team from the cogeneration ytem decreaed when 574

7 compared with the bae cae (Table 6), once it wa ubtituted by the vapour bleed. Table 6. Exhauted Conumption. Exhauted Flow Rate (kg/) Bae Cae Optimized Cae The evaporation ytem wa deigned to have the minimum cot at the optimized cae, being it parameter thoe preented in Table 7. Table 7. Optimized evaporation ytem. Effect Area (m 2 ) p (bar) t w;at t bpe t evap The optimum deign of the evaporation ytem and the heater are related with a minimum cot of invetment in equipment and team conumed. The final reult for bae and optimized cae in each ub-ytem can be een in Table 8 and 9 repectively. Table 8. Cot for the bae cae. Sub-ytem Equipment Heating 1 Total Sub- Sytem Diffuer Juice Clarification Syrup Treatment Evaporator ,276 TOTAL 378 2,761 3,139 1 It i conidered the reduction of cot due the production of vapor and/or condenate that are ued in other part of the factory. A it can be een at Table 8 and 9, after the thermoeconomic optimization performed, a reduction of the total cot wa obtained in each ub-ytem when compared with the bae cae. The invetment in equipment increaed, a heating team of lower quality i ued at the heater, requiring higher urface of heat exchange. On the other hand the reduction of conumed team cot compenate it invetment in new heater and the total cot decreae for each ub-ytem. Table 9. Cot for the optimized cae. Sub-ytem Equipment Heating 1 Total Sub- Sytem Diffuer Juice Clarification Syrup Treatment Evaporator ,053 TOTAL 465 2,189 2,654 1 It i conidered the reduction of cot due the production of vapour and/or condenate that are ued in other part of the factory The higher reduction of cot wa obtained at the evaporation ytem with a difference of 222 R$/h between the bae and optimized cae. Thi repreent 17% of economy in thi ub-ytem and 46% of the total cot reduction obtained. The economy wa reached mainly due the reduction of exhauted team conumed at the firt effect that decreaed 14,3 kg/ or 10%. The clarification that contribute with 29% of the total reduction cot, decreaed it cot in 14% or 140 R$/h. The ue of vapour from the 1 t, 2 nd, 3 rd and 4 th effect reduced it total cot coniderably, even with the increae of invetment in heater that were more than 4 time that one of the bae cae. The optimization at the diffuer reduced 20% of the total cot ubtituting the ue of V1 in the bae cae for V2 in the optimized one and the yrup treatment contributed with 5% of the total reduction. 5. CONCLUSIONS The thermoeconomic optimization preented in thi paper howed to be very ueful to analyze the cot generation on deigning the heater network and evaporation ytem of a ugar factory, looking for the minimum invetment and operation cot, chooing like that, the bet alternative for thermal integration of the factory. The evaporation ytem repreent the larget part of the total cot of the factory in thermal energy conumption. It invetment cot i ubtantially higher than the heater howing the importance of it optimum deign. Moreover it 575

8 produce the heating ource for the other ubytem, influencing their deign either. The optimization of the thermal energy conumption in ugar factorie can alo be important to evaluate the cot of proce team demand reduction. The deciion of bet the technology to be implemented in the cogeneration ytem depend of the quantity of team conumed by the proce and the analyi of both ytem mut be made together looking for the better alternative for the factory a a whole. ACKNOWLEDGEMENTS The author would like to acknowledge Uina Cruz Alta, Guarani and Santa Iabel engineer, technician and manager for available proce data and FAPESP, CAPES and CNPq for the financial upport to do thi tudy. 6. REFERENCES [1] UNICA União da Agroindútria Canavieira de São Paulo. Available at: [2] CONAB (2006) - Companhia Nacional de Abatecimento. Minitério da Agricultura Pecuária e Abatecimento. Cana de Açúcar Safra 2005/2006. Terceiro Levantamento - Dezembro Available at: [3] EES Engineering Equation Solver. F-Chart [4] Lozano. M. A. and Valero. A. Theory of the exergetic Cot. Energy. v.18. nº 9. pp [5] Soa-Arnao, J.H. and Nebra, S. A. Exergy of Sugar Cane Bagae. Proceeding of 14 th European Bioma Conference & Exhibition. Bioma for Energy, Indutry and Climate Protection October 2005, Pari France. [6] Szargut. J.. Morri. D.R.. Steward. F.R. Exergy Analyi if thermal. Chemical and Metallurgical Procee. Hemiphere Publihing Corporation. New York. USA p. [7] Tone Baloh. E. W. Energy Manual for Sugar Factorie. 2º Ed. Verlag Dr. Albert Barten. Berlin p. [8] Nebra. S.A. e Fernandez Parra. M.I The exergy of ucroe-water olution: propoal of a calculation method. Proceeding of ECOS th International Conference on Efficiency. Cot. Optimization. Simulation and Environmental. Trondheim. Norway June. [9] DEDINI S/A Indutria de Bae - Peronal Communication. July, [10] Uina Santa Iabel - Novo Horizonte, SP - Brazil. Peronal Communication. January, [11] Chauvel, A.; Fournier, G.; Raimbaut, C. Manuel d evaluation économique de procédé - Nouvelle Édition Revee et Augmentée. Edition Technip Pari, France. 488p. [12] Ulrich, G.D. Dieño y economía de lo proceo de ingeniería química [13] Zuckertechniker-Tachembuch, 7.Aufl., Berlin 1966, p. 250 apud Baloh, T. & Wittwer, E. Energy Manual for Sugar Factorie. 2 nd edition, Verlag Dr. Albert Barten, Berlin [14] Hugot, E. Handbook of Cane Sugar Engineering. 2 nd Ed. Elevier Publihing Company, Amterdam, [15] Uina Cruz Alta Olímpia, SP -Brazil. Peronal Communication. September, [16] Kadlec, P.; Bretchneider, R. ; Dandar, A.; The meaurement and the calculation of the phyical chemical propertie of waterugar olution, La Sucrerie Belge, Vol. 100, Feb. 1981; pp (In French). [17] Peacock, Stephen; Predicting phyical propertie of factory juice and yrup; International Sugar Journal, 1995, Vol. 97, No. 1162, pp [18] Van der Poel, P.W. Schiweck, H. and Schwartz, T. Sugar Technology. Beet and Cane Sugar Manufacture. Verlag Dr. Albert Barten, Berlin