exergy sucrose water solutions proposition Nebra

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THE EXERGY OF SUCROSE-WATER SOLUTIONS: PROPOSAL OF A CALCULATION METHOD Silvia Azucena Nebra Energy Department / Mechanical Engineering Faculty Interdisciplinary Centre of Energy Planning - NIPE State University of Campinas - UNICAMP Campinas, São Paulo ,Brazil sanebra@fem.unicamp.br M. I. Fernández-Parra Universidad de Oriente, CEEFE/FIM/UO, Avenida Las Américas s/n esquina L, Santiago de Cuba, Cuba.

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ABSTRACT The present paper discusses a calculation method for the sucrose – water solution exergy. To perform this calculation, correlations are presented to obtain some properties: density, solubility, specific heat, boiling point elevation, enthalpy and entropy. These properties permit to calculate the physical exergy of the solution. Moreover, a correlation for the water activity coefficient in the solution was obtained from the literature and an equation to calculate the sucrose activity coefficient is proposed. Both coefficients are used to determine the mixture exergy. Finally, a discussion of the reference system is performed, to complete the calculation of the chemical exergy.

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reasons I have seen that people are not taking care of the calculation of exergy of solutions. Solutions like LiBr-water or, as in this case, sucrose-water Somebody does not take care about the mixer exergy Other does not take care about the reference system (he / she ignores it) To be careful with these aspects is very important for a reliable exergy system analysis

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Density of sucrose – water solutions Above de solubility:

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solubility Based on d’Grut data:

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Specific heat Sucrose – water solutions Pure sucrose

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Enthalpy Entropy Neglecting the second terms… just everybody does…

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Enthalpy

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Entropy

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Measurements of boiling point elevation permit to obtain activity coefficient values : . Activity Where: And the reference values are, for pure water:

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Using Gibbs – Duhem equation: Where the reference is established by the superior limit, which is the saturation point, where we just have also pure sucrose so, our reference condition will be: Solving the integral, we get: Sucrose activity

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Activity

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Exergy

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The values of the reference exergies, for sucrose and water, were taken from Szargut et al. [20], so, the reference environment proposed by these authors was adopted. Considering the elemental substances present in the system, which are carbon, oxygen and hydrogen, the reference substances adopted are: carbon dioxide, oxygen and water (in vapour state), all them diluted in the standard atmosphere detailed in [20]. What this means is that the sucrose was treated as a fuel, and, if we think that it is used for human feeding, it acts really as a fuel, because the man produces carbon dioxide when breathes. Anyway, it can also be burnt. Reference environment

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Physical exergy Mixture exergy Physycal exergy + mixture exergy Total exergy

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The reference state adopted in this work is different of that of Baloh [5] and Guallar [7], whom adopted the minimum value of mixture exergy at 25 °C (or 20 °C) as zero level, so, the value corresponding to minimum free energy and maximum solubility. Baloh [5] adopted also a null value of exergy for the sucrose at reference temperature. Guallar [7] adopted two different reference systems, when he analysis the sugar production process. Since our final objective is to perform a complete exergetic analysis of the sugar and alcohol production process from sugar cane, where the elemental substances, carbon, oxygen and hydrogen are present also in the bagasse, used as fuel, we think that a unique reference system must be adopted to perform the exergy calculations.

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CONCLUSIONS A research on the properties of water – sucrose solutions was made; being as final objective to present an exergy calculation methodology. In order to reach this objective, values of density, solubility, specific heat, enthalpy and entropy are presented. Correlation and values from different authors were carefully compared. An expression for the sucrose activity was developed from a correlation for the water activity in the solution, using the Gibbs – Duhem equation. In order to calculate the exergy, we need to adopt as reference states pure water and pure sucrose, this was also performed in the calculation of the activity coefficient. As this type of solutions have a solubility limit, careful calculations and considerations needed to be done. The reported correlations permit the complete exergy calculation of sucrose-water mixtures at any concentration.

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ACKNOWLEDGMENTS The authors want to express their gratitude to Research Foundation of Sao Paulo State – FAPESP, to Coordination for the Improving of Superior Education - CAPES and to Research Council of Scientific and Technological Development – CNPq, for the financial resources to do this work. THANKS!