Efeito do teor de solvente sobre a separação e o consumo de energia no processo de destilação extrativa.

Abstract

The question of a well-designed extractive distillation system has been the subject of several works, all involving sensitivity analysis using a process simulator, which usually involves repeated simulations, which lead to a large number of results. However, the findings are limited because the main decision variables, reflux ratio, solvent flow rate and size of the extractive region, are not analyzed simultaneously. This paper proposes a new approach to evaluate separation and energy consumption of extractive distillation columns, using the solvent content throughout the column as the main parameter, more specifically, at the solvent feed stage. Unlike the classical approach, this new approach allows to evaluate simultaneously the effect of the major decision variables.Moreover, it was possible to calculate the minimum solvent flow rate and minimum energy consumption, thus clarifying the discrepancies found in the literature regarding the existence or not of proportionality between the minimum energy consumption and minimum solvent flow rate. The study was performed for two columns of different sizes. The effect of EG content on the separation factor (SF) of the mixture was also considered. Given the overall obtained results, it was observed that operation at higher solvent content at extractive section displays considerably better performance, standing this, the energy consumption in the reboiler of the extractive column is heavily dependent on the solvent content throughout the column and that the minimum energy consumption of this reboiler does not occur for the minimum solvent flow rate; but for the lowest reflux ratio.The results also indicate that increasing the content of solvent causes the energy consumption to become independent of the number of stages of the column. The vast majority of papers consider only separation and the energy consumption of the extractive column. From the use of this new approach, it is observed that the convergence problem disappears (as long as the restrictions are met), for this reason, it was decided to extend the use of such a procedure for the complete extractive distillation system (involving both distillation columns - extractive and recovery). According to the results, the global minimum energy consumption does not occur for the minimum solvent flow rate, but for lowest reflux ratio of the extractive column, which implies a higher EG content throughout the column. This behavior is independent of the optimization of the recovery column and it results in a negligible loss of solvent. However, depending on the EG content throughout the extractive column, the energy consumption of the recovery column may represent a significant percentage in the overall energy consumption. Due to its industrial importance, the dehydration of aqueous mixtures of ethanol using ethylene glycol (EG) as solvent was chosen as a case study; however, this proposed approach is successful for any azeotropic mixture separation system using a solvent with high boiling point in relation to the other components of the mixture.