Análise exergética do sistema de evaporadores de múltiplo efeito da etapa de recuperação da indústria Kraft de obtenção de celulose

Abstract

The Kraft Recovery cycle is important for the economic and environmental viability of the pulping process. In addition to recovering the chemical compounds used in part of the process, the cycle also promotes the generation of electric power and steam from the factory by burning the black liquor, a byproduct generated in the process of cooking the wood, in the recovery boilers. However, the liquor needs to be concentrated before proceeding to the firing process, which is done in the multi-effect evaporator system. This equipment is characterized by involving large amounts of energy. A rational and optimized use of this can be achieved through the exergetic analysis technique, which is capable of identifying and quantifying energy losses and inefficiencies in this industrial process. Thus, the objective of this work is to apply the energy and exergetic analyzes in the multiple evaporator effect, in the Kraft recovery stage. Through the realization of the modeling and validation of the multiple effect evaporator was evidenced that the model is able to describe its steady state behavior, with a maximum prediction error of 17.3% in relation to the operating conditions adopted in the factory and in the literature. In another study, where the exergetic analysis of a single-effect evaporator was carried out, it was possible to prove that the adoption of different correlations of boiling point elevation and heat capacity for pine and eucalyptus did not influence the exergetic efficiency, since the values obtained for this were very close, differing at most 0.05% from pine to eucalyptus. However, the fact that the pine liquor has a higher EPE than eucalyptus (different correlations) makes the former more exergetic than the second. The real system with multiple effects studied is composed of 6 effects (1st to 6th effect), where the first effect has three equipments (1A, 1B and 1D). When performing the energy and exergetic analysis of this system, the exergetic efficiencies of each effect were obtained, ranging from 79.6 to 96.2%. It is possible to identify that the 1D and 6th effects are the ones that had the highest energy and effect 1B and 2º those who had the minors. Finally, a sensitivity analysis was performed in order to observe and study the behavior of the main output variables of the model when changing some input variables