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ENCIT 2020
18th Brazilian Congress of Thermal Sciences and Engineering
EXERGY ANALYSIS OF MECHANICAL VAPOR RECOMPRESSION SYSTEMS TO IMPROVE UTILITY CONSUMPTION AT A CAUSTIC SODA EVAPORATION PLANT
Submission Author:
Italo Breviglieri Sichi de Mello , SP , Brazil
Co-Authors:
Italo Breviglieri Sichi de Mello, antonio esio bresciani, Fernanda Cristina Nascimento Silva, Roberto Guardani, Roberto Nicolas De Jardin Jr, Silvio de Oliveira Junior, Carlos Eduardo Keutenedjian Mady
Presenter: Italo Breviglieri Sichi de Mello
doi://10.26678/ABCM.ENCIT2020.CIT20-0263
Abstract
Operating parameters for the evaporation plant at a caustic soda production unit that consumes, in nominal conditions, 60 t/h of 617 kPa saturated steam from a natural gas-fired boiler, are varied and their impact on the utility demands for the system are evaluated using a detailed physical model. Three alternative configurations using mechanical vapor recompression (MVR) to recover the enthalpy from the low-pressure steam rejected from the evaporator are proposed for the utility plant and evaluated using exergy analysis for different operating conditions. The first alternative, cooling the steam between compression stages with cooling water heat exchangers, reaches up to 94% reduction in steam consumption at the cost of up to 0.83 kWh of electrical power per kg of soda production. The second alternative, which uses de-superheaters to cool the steam and up to 1.15 kWh/kgNaOH electrical power, requires no additional steam from the boiler and produces a surplus of high-pressure steam that can be used in other processes. As a combination of the two previously presented configurations, the third one considers intercooling through the injection of liquid water instead of exchanging heat. A second production unit is considered as a sink for a fixed 10 t/h of steam in the same conditions, and the entire complex is considered in the exergy analysis, including both evaporation plants, the boiler, the MVR system, and the electricity input. Renewable and non-renewable exergy costs and exergy destruction rates were used to compare the configurations overall performances, and exergy destruction rates breakdown by equipment were used to evaluate opportunities for further improvement, showing that intercooling with water injection is the most efficient option, with a specific exergy destruction of 1.13 kWh/kgNaOH in nominal conditions, about 65% lower than the original configuration without MVR, and 25% lower than the other MVR scenarios.
Keywords
exergy, mechanical vapor recompression, caustic soda, Evaporation, MVR
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