Modiﬁcation of Conventional Sugar Juice Evaporation Process for Increasing Energy E ﬃ ciency and Decreasing Sucrose Inversion Loss

: The evaporation process, boiler, and turbine are the main components of the cogeneration system of the sugar factory. In the conventional process, the evaporator requires extracted steam from the turbine, and bled vapor from the evaporator is supplied to the juice heater and the pan stage. The evaporation process may be modiﬁed by using extracted steam for the heating duty in the pan stage. This paper is aimed at the investigation of the e ﬀ ects of this process modiﬁcation. Mathematical models of the conventional and modiﬁed processes were developed for this purpose. It was found that, under the conditions that the total evaporator area is 13,000 m 2 , and the inlet juice ﬂow rate is 125 kg / s, the optimum modiﬁed evaporation process requires extracted steam at a pressure of 157.0 kPa. Under the condition that the fuel consumption rate is 21 kg / s, the cogeneration system that uses the optimum modiﬁed evaporation process yields 2.3% more power output than the cogeneration system that uses a non-optimum conventional cogeneration process. Furthermore, sugar inversion loss of the optimum modiﬁed process is found to be 63% lower than that of the non-optimum conventional process.


Introduction
The evaporation process, boiler, and steam turbine are the main components of the cogeneration system in the sugar industry. Diluted sugar juice becomes raw sugar and molasses in the evaporation process after a specified amount of water is removed by evaporation. Thermal energy required for water evaporation is provided by steam condensation. The boiler generates high-pressure steam that is supplied to the steam turbine for power generation. Older cogeneration systems use the back-pressure turbine, in which steam is exhausted at a lower pressure, whereas modern cogeneration systems use the extraction-condensing steam turbine, in which some steam is extracted at lower pressure, and the remaining steam is sent to the condenser. Kamate and Gangavati [1] have shown that a cogeneration system using the extraction-condensing steam turbine is more energy efficient than a cogeneration system using the back-pressure steam turbine.
The multiple-effect evaporator is used in the evaporation process. The evaporator is designed to increase the juice concentration from approximately 15% to 70%. The removal of the remaining water content in sugar occurs in the pan stage. The multiple-effect evaporator requires a supply of saturated steam extracted from an extraction-condensing steam turbine at a specified pressure. An adverse consequence of the exposure of sugar juice to high-temperature steam and vapor in the multiple-effect evaporator is sucrose inversion loss, which converts sucrose to glucose and fructose. In order to increase the profitability of raw sugar manufacturing, the amount of required steam at pressure pi+1 is caused by vapor condensation at pressure pi. Concentrated sugar juice from E4 is sent to P. In order to improve the energy efficiency of the process, condensates from E1, E2, and E3 are sent, respectively, to F1, F2, and F3. F2 also receives condensates from P, F1, and H1, and F3 also receives condensates from F2 and H2. Flash tanks (F1, F2, and F3) produce vapor and condensate at a lower pressure from condensate at a higher pressure.
The model of the conventional evaporation process in Figure 1 is similar to the model presented by Chantasiriwan [13]. The difference between the two models is the treatment of condensate from E1. In the model presented by Chantasiriwan [13], the condensate is sent to the boiler. In the model shown in Figure 1, the condensate is sent to F1. It can be shown that this treatment increases the overall energy efficiency of the process.
Due to the similarity between this model and the model presented by Chantasiriwan [13], only different equations are shown for the sake of concise presentation. The different treatment of condensate in this paper gives rise to the following energy equations: Expressions for the other parameters are the same as those in the model presented by Chantasiriwan [13], and an interested reader is asked to consult that reference. The heat transfer equations in this model are also slightly different from those in the model presented by Chantasiriwan [13]. They are shown as follows. In order to improve the energy efficiency of the process, condensates from E1, E2, and E3 are sent, respectively, to F1, F2, and F3. F2 also receives condensates from P, F1, and H1, and F3 also receives condensates from F2 and H2. Flash tanks (F1, F2, and F3) produce vapor and condensate at a lower pressure from condensate at a higher pressure.
The model of the conventional evaporation process in Figure 1 is similar to the model presented by Chantasiriwan [13]. The difference between the two models is the treatment of condensate from E1. In the model presented by Chantasiriwan [13], the condensate is sent to the boiler. In the model shown in Figure 1, the condensate is sent to F1. It can be shown that this treatment increases the overall energy efficiency of the process.
Due to the similarity between this model and the model presented by Chantasiriwan [13], only different equations are shown for the sake of concise presentation. The different treatment of condensate in this paper gives rise to the following energy equations: Expressions for the other parameters are the same as those in the model presented by Chantasiriwan [13], and an interested reader is asked to consult that reference. The heat transfer equations in this model are also slightly different from those in the model presented by Chantasiriwan [13]. They are shown as follows.

Modified Evaporation Process
The conventional evaporation process uses bled vapor from the first effect of the evaporator for the pan stage. A consequence of this requirement is that the extracted steam pressure (p 0 ) must not be lower than the minimum value that corresponds to a specified juice mass flow rate. It is possible to remove this constraint by using extracted steam instead of bled vapor for the pan stage in the modified evaporation process.
The modified evaporation process is depicted in Figure 2. It can be seen that extracted steam at pressure p a is supplied to the pan stage. The model of this process is the same as that of the conventional process with m a deleted from Equation (1). The mass flow rate of extracted steam required by the pan stage is

Modified Evaporation Process
The conventional evaporation process uses bled vapor from the first effect of the evaporator for the pan stage. A consequence of this requirement is that the extracted steam pressure (p0) must not be lower than the minimum value that corresponds to a specified juice mass flow rate. It is possible to remove this constraint by using extracted steam instead of bled vapor for the pan stage in the modified evaporation process.
The modified evaporation process is depicted in Figure 2. It can be seen that extracted steam at pressure pa is supplied to the pan stage. The model of this process is the same as that of the conventional process with ma deleted from Equation (1). The mass flow rate of extracted steam required by the pan stage is It is interesting to note that, under the same operating conditions, mf,4, x4, and p4 of the modified and conventional evaporation processes are identical. Therefore, the values of ma of both processes are the same if pa = 150 kPa.

Performance Parameters
This paper is intended to demonstrate that the modified evaporation process requires extracted steam at a lower pressure than the conventional evaporation process, which leads to the enhanced performance of the modified evaporation process compared with the conventional process. Comparison between both processes is based on two performance parameters, which are turbine power output of the cogeneration system and sucrose inversion loss. It is interesting to note that, under the same operating conditions, m f,4 , x 4 , and p 4 of the modified and conventional evaporation processes are identical. Therefore, the values of m a of both processes are the same if p a = 150 kPa.

Performance Parameters
This paper is intended to demonstrate that the modified evaporation process requires extracted steam at a lower pressure than the conventional evaporation process, which leads to the enhanced performance of the modified evaporation process compared with the conventional process. Comparison between both processes is based on two performance parameters, which are turbine power output of the cogeneration system and sucrose inversion loss.

Turbine Power Output
Steam economy is the performance parameter that may be used to evaluate the energy efficiency of an evaporation process. It is equal to the ratio of the mass flow rate of evaporated water to the mass flow rate of extracted steam. Therefore, for the conventional evaporation process, and for the modified evaporation process. Steam economy is an appropriate parameter for comparing different conventional evaporation processes because the extracted steam pressure in the first effect of the multiple-effect evaporator is fixed. The process having larger steam economy is considered to be more energy efficient. However, steam economy should not be used to compare the conventional and modified evaporation processes because extracted steam pressures in both processes may be different. To identify a more suitable performance parameter, it is necessary to consider the cogeneration system.
The cogeneration systems for the conventional and modified evaporation processes are depicted in Figure 3. In each system, the mass flow rate, pressure, and temperature of steam generated by the boiler (B) are, respectively, m s , p s , T s . Steam is extracted at the pressure of p 0 in the conventional evaporation process. The mass flow rate of extracted steam is m v,0 . The extracted steam is used for evaporation in the first effect of the evaporator. The remaining steam is condensed at the pressure of p c . The mass flow rate of condensed steam (m c ) is, therefore, m s − m v,0 . The modified evaporation process requires not only extracted steam at the pressure of p 0 for evaporation in the first effect of the evaporator but also extracted steam at the pressure of p a for evaporation in the pan stage. The corresponding mass flow rates of extracted steam are m v,0 and m a . The remaining steam is condensed at the pressure of p c . The mass flow rate of condensed steam (m c ) is, therefore, m s − m v,0 − m a . Inspection of Figure 3 reveals that the inputs of both systems are sugar juice and bagasse, and the outputs are turbine power, sugar, and molasses. Both systems are assumed to have the same juice processing capacity. This means that m f,in , x in , and x 4 are the same in both the conventional evaporation process and the modified evaporation process. Moreover, both systems are assumed to consume the same amount of fuel (m fuel ) in their boilers. Based on these assumptions, the only difference between both systems is turbine power output, which is expressed as where η t is isentropic efficiency of the steam turbine, h s is specific enthalpy at pressure p s , and temperature T s , h 0s , h as , and h cs are specific enthalpies at, respectively, pressures p 0 , p a , and p c , and the same entropy as the inlet steam. It should be noted that m a is zero in the cogeneration system for the conventional evaporation process.
performance parameter, it is necessary to consider the cogeneration system. The cogeneration systems for the conventional and modified evaporation processes are depicted in Figure 3. In each system, the mass flow rate, pressure, and temperature of steam generated by the boiler (B) are, respectively, ms, ps, Ts. Steam is extracted at the pressure of p0 in the conventional evaporation process. The mass flow rate of extracted steam is mv,0. The extracted steam is used for evaporation in the first effect of the evaporator. The remaining steam is condensed at the pressure of pc. The mass flow rate of condensed steam (mc) is, therefore, ms  mv,0. The modified evaporation process requires not only extracted steam at the pressure of p0 for evaporation in the first effect of the evaporator but also extracted steam at the pressure of pa for evaporation in the pan stage. The corresponding mass flow rates of extracted steam are mv,0 and ma. The remaining steam is condensed at the pressure of pc. The mass flow rate of condensed steam (mc) is, therefore, ms  mv,0  ma.  Figure 3 reveals that the inputs of both systems are sugar juice and bagasse, and the outputs are turbine power, sugar, and molasses. Both systems are assumed to have the same juice processing capacity. This means that mf,in, xin, and x4 are the same in both the conventional

Sucrose Inversion Loss
Sucrose inversion is the chemical reaction that transforms sucrose into glucose and fructose, which do not crystallize and cannot be recovered as sugar. Main factors that influence sucrose inversion in the multiple-effect evaporator are temperature, time, juice acidity, and juice concentration. Sucrose inversion loss may be estimated by using the Vukov model [18]. According to this model, the mass fraction of lost sucrose is expressed as where t is retention time (in minutes) of sugar juice in an evaporator vessel. The reaction rate (k) is determined from Equation (20) is applicable when the juice temperature is 25 • C. At a different temperature, the corrected pH value is For simulation purposes, pH 25 is assumed to be 6.0. The retention time (t) is proportional to the evaporator surface area (A), and inversely proportional to sugar juice mass flow rate (m f ). It may be approximated by assuming that sugar juice flows through N tubes, of which diameter and length are D and L, in an evaporator vessel at the speed of V. The expression of V is Consequently, Typical tube diameter varies from 38 to 51 mm. It is assumed that D is 45 mm in this paper.

Results and Discussion
The parameters of both evaporation processes are x in = 15%, x out = 70%, p 4 = 16 kPa, and T h,2 = 30 • C. In each process, the total surface areas of the multiple-effect evaporator and the juice heater are, respectively, 13,000 and 2500 m 2 . Multiple-effect evaporators in both systems are designed to process 125 kg/s (or 450 t/h) of juice. The optimum distribution of the total evaporator surface area that maximizes the steam economy at a specified extracted steam pressure (p 0 ) may be determined for each system.
The procedure for determining the optimum distribution of the evaporator surface area in the conventional evaporation process that maximizes the steam economy (SE) is shown in Figure 4. Figure 4a shows that, for the first-effect area (A 1 ) of 6000 m 2 and the second-effect area (A 2 ) of 1200 m 2 , the optimum value of the third-effect area (A 3 ) that yields the required juice mass flow rate of 125 kg/s and the maximum steam economy (SE) is 1233 m 2 . Figure 4b shows that, for the same value of A 1 , the optimum value of A 2 that results in maximum SE is 1251 m 2 . Figure 4c shows that, as A 1 increases, SE decreases, and first-effect pressure (p 1 ) increases. By requiring that p 1 is 150 kPa, the optimum value of A 1 is found to be 4518 m 2 . The corresponding value of SE is 2.508. Therefore, the mass flow rate of extracted steam for the evaporator (m v,0 ) is 41.63 kg/s. The parameters of both evaporation processes are xin = 15%, xout = 70%, p4 = 16 kPa, and Th,2 = 30 C. In each process, the total surface areas of the multiple-effect evaporator and the juice heater are, respectively, 13,000 and 2500 m 2 . Multiple-effect evaporators in both systems are designed to process 125 kg/s (or 450 t/h) of juice. The optimum distribution of the total evaporator surface area that maximizes the steam economy at a specified extracted steam pressure (p0) may be determined for each system.
The procedure for determining the optimum distribution of the evaporator surface area in the conventional evaporation process that maximizes the steam economy (SE) is shown in Figure 4. Figure 4(a) shows that, for the first-effect area (A1) of 6000 m 2 and the second-effect area (A2) of 1200 m 2 , the optimum value of the third-effect area (A3) that yields the required juice mass flow rate of 125 kg/s and the maximum steam economy (SE) is 1233 m 2 . Figure 4(b) shows that, for the same value of A1, the optimum value of A2 that results in maximum SE is 1251 m 2 . Figure 4(c) shows that, as A1 increases, SE decreases, and first-effect pressure (p1) increases. By requiring that p1 is 150 kPa, the optimum value of A1 is found to be 4518 m 2 . The corresponding value of SE is 2.508. Therefore, the mass flow rate of extracted steam for the evaporator (mv,0) is 41.63 kg/s. The procedure for determining the optimum distribution of evaporator surface area in the modified evaporation process that maximizes SE is shown in Figure 5. Figure 5(a) shows that, for the first-effect area (A1) of 4000 m 2 and the second-effect area (A2) of 1100 m 2 , the optimum value of the third-effect area (A3) that yields the required juice mass flow rate of 125 kg/s and the maximum SE is 1723 m 2 . Figure 5(b) shows that, for the same value of A1, the optimum value of A2 that results in the The procedure for determining the optimum distribution of evaporator surface area in the modified evaporation process that maximizes SE is shown in Figure 5. Figure 5a shows that, for the first-effect area (A 1 ) of 4000 m 2 and the second-effect area (A 2 ) of 1100 m 2 , the optimum value of the third-effect area (A 3 ) that yields the required juice mass flow rate of 125 kg/s and the maximum SE is 1723 m 2 . Figure 5b shows that, for the same value of A 1 , the optimum value of A 2 that results in the maximum SE is 1342 m 2 . Figure 5c shows the optimum value of A 1 that results in the maximum SE is 2074 m 2 . The corresponding value of SE is 2.345. Since the mass flow rate of juice leaving E4 (m f,4 ) is 26.79 kg/s, and the mass flow rate of extracted steam for the pan stage (m a ) is 13.16 kg/s, the value of m v,0 is found to be 31.53 kg/s. boiler of each system is 21 kg/s, the higher heating value of fuel is 9000 kJ/kg, the boiler efficiency is 70%, the pressure and temperature of superheated steam generated by the boiler are 4.5 MPa and 440 C, and the turbine efficiency is 85%. Figure 6 shows variations of mv,0 and P with p0 in cogeneration systems for the conventional and modified evaporation processes that have the optimum distributions of evaporator surface areas. It can be seen that, in each system, there exists the optimum value of p0 (p0,opt) that results in the maximum turbine power output (Pmax). In the cogeneration system for the optimum conventional evaporation process, p0,opt is 186.8 kPa, and Pmax is 29,286 kW. In the cogeneration system for the optimum modified evaporation process, p0,opt is 157.0 kPa, and Pmax is 29,442 kW. It is interesting to compare the cogeneration systems for the optimum modified evaporation process and a non-optimum conventional evaporation process, in which p0 is 200 kPa. The non-optimum conventional process has the same juice processing capacity as the optimum conventional process, but it is less energy efficient. The value of SE in this process is 2.411, and the value of mv,0 is 43.31 kg/s. The turbine power output of the cogeneration system that uses this process is 28,789 kW, which is 2.3% lower than the turbine power output of the cogeneration system that uses the optimum modified evaporation process. Table 1 shows simulation results of cogeneration systems for the non-optimum conventional evaporation process, the optimum conventional evaporation process, and the optimum modified evaporation process.  The calculation of the turbine power output (P) of a cogeneration system requires information about the fuel, the boiler, and the steam turbine. It is assumed that the fuel consumption rate in the boiler of each system is 21 kg/s, the higher heating value of fuel is 9000 kJ/kg, the boiler efficiency is 70%, the pressure and temperature of superheated steam generated by the boiler are 4.5 MPa and 440 • C, and the turbine efficiency is 85%. Figure 6 shows variations of m v,0 and P with p 0 in cogeneration systems for the conventional and modified evaporation processes that have the optimum distributions of evaporator surface areas. It can be seen that, in each system, there exists the optimum value of p 0 (p 0,opt ) that results in the maximum turbine power output (P max ). In the cogeneration system for the optimum conventional evaporation process, p 0,opt is 186.8 kPa, and P max is 29,286 kW. In the cogeneration system for the optimum modified evaporation process, p 0,opt is 157.0 kPa, and P max is 29,442 kW. It is interesting to compare the cogeneration systems for the optimum modified evaporation process and a non-optimum conventional evaporation process, in which p 0 is 200 kPa. The non-optimum conventional process has the same juice processing capacity as the optimum conventional process, but it is less energy efficient. The value of SE in this process is 2.411, and the value of m v,0 is 43.31 kg/s. The turbine power output of the cogeneration system that uses this process is 28,789 kW, which is 2.3% lower than the turbine power output of the cogeneration system that uses the optimum modified evaporation process. Table 1 shows simulation results of cogeneration systems for the non-optimum conventional evaporation process, the optimum conventional evaporation process, and the optimum modified evaporation process.  Table 1 shows that steam and vapor pressures in the optimum modified evaporation process are lower than those in the non-optimum and optimum conventional evaporation processes. Sucrose inversion losses in all effects of evaporators in the three processes are compared in Table 2. It can be seen that sugar inversion loss is largest in the first effect of each process. Sucrose inversion loss in the first effect of the optimum modified evaporation process has the lowest value due to the smallest extracted steam pressure and temperature. As a consequence, the total sucrose inversion loss of the optimum modified evaporation process is 63% lower than that of the non-optimum conventional evaporation process. Table 1. Simulation results of cogeneration systems for the non-optimum conventional evaporation process, the optimum conventional evaporation process, and the optimum modified evaporation process.   Table 1. Simulation results of cogeneration systems for the non-optimum conventional evaporation process, the optimum conventional evaporation process, and the optimum modified evaporation process.  Table 1 shows that steam and vapor pressures in the optimum modified evaporation process are lower than those in the non-optimum and optimum conventional evaporation processes. Sucrose inversion losses in all effects of evaporators in the three processes are compared in Table 2. It can be seen that sugar inversion loss is largest in the first effect of each process. Sucrose inversion loss in the first effect of the optimum modified evaporation process has the lowest value due to the smallest extracted steam pressure and temperature. As a consequence, the total sucrose inversion loss of the optimum modified evaporation process is 63% lower than that of the non-optimum conventional evaporation process. Table 2. Comparison of sucrose inversion losses in the non-optimum conventional evaporation process, the optimum conventional evaporation process, and the optimum modified evaporation process.

Conclusions
The comparison between the cogeneration system that used the conventional evaporation process and the cogeneration system that used the modified evaporation process was investigated in this paper. Bled vapor and steam extracted from the turbine were used, respectively, by the first and the second systems for heating duty in pan stages. Both conventional and modified evaporation processes had the total evaporator surface area of 13,000 m 2 and total juice heater surface area of 2500 m 2 . They were designed to process 125 kg/s of inlet sugar juice. The distribution of evaporator surface area of the optimum modified evaporation process resulted in the maximum steam economy. The pressures of extracted steam supplied to the optimum modified evaporation process were chosen so that the turbine power output of the cogeneration system that used this process was maximized. According to simulation results obtained from the mathematical models developed for this investigation, extracted steam at a mass flow rate of 31.53 kg/s and a pressure of 157.0 kPa was required for the evaporator of the optimum modified evaporation process, and extracted steam at a mass flow rate of 13.16 kg/s and a pressure of 150.0 kPa was required for the pan stage of this process. The turbine power output was 29,442 kW for the cogeneration system that used the optimum modified evaporation process. This power output was 2.3% larger than the power output of the cogeneration system that used a non-optimum conventional evaporation process. Furthermore, since the pressure profile in the evaporator of the optimum modified process was lower than that of the non-optimum conventional process, sucrose inversion loss in the modified process was 63% lower.
Funding: This research received no external funding.

Conflicts of Interest:
The authors declare no conflict of interest.

Nomenclature
A heat transfer surface of evaporator, m 2 A h heat transfer surface of juice heater, m 2 c p specific heat capacity, kJ/kg· • C h specific enthalpy, kJ/kg I mass fraction of lost sugar due to inversion m mass flow rate, kg/s P turbine power output, kW p pressure, kPa SE steam economy T temperature, • C t retention time, min U heat transfer coefficient, kW/m 2 · • C x concentration of sugar juice, % Greek Symbols ε heat loss coefficient in evaporator η τ turbine efficiency ρ density, kg/m 3