Construção de um sistema em fluxo contínuo para a extração de cromo de resíduos de couro curtido assistida por ultrassom

Abstract

In this work, a continuous flow extraction system was developed, characterized and evaluated for the removal of Cr from tanned leather residues, using EDTA as extractor. The system was developed mostly using the 3D printing technique, and several prototypes and configurations were evaluated throughout the work. After the construction of the final prototype, experiments were carried out for the thermal, chemical and dynamic characterization of the system, in addition to the optimization of operational parameters and the evaluation of US energy use. The optimized extraction solution concentration was 14.3 g L-1, corresponding to a molar ratio between EDTA/Cr of three. An optimum value of the recirculation flow of the liquid phase was found (350 mL min-1), in which the entrainment of solid particles was minimized and the internal temperature profile was kept relatively homogeneous. The residence time distribution curves for the solid phase indicated the presence of a significant level of axial dispersion. The relative time to obtain a steady state was similar (θ ≥ 4) for all conditions of temperature (70, 75 e 80 °C), residence time (30, 60, 90 e 120 min) and solid/liquid ratio evaluated (3, 4, 5 e 6%). As the solid/liquid ratio was increased, the extraction efficiency was reduced, where the optimal ratio found was 3%. The effect of the US energy was investigated using a tubular US applicator, with 20 kHz of frequency and electrical input powers of 75, 150, 300 and 600 W. It was observed that the internal temperature profile along the system was affected in the presence of US, and the output temperature of the liquid phase was higher than that of inlet one, which corresponds to an inverse behavior in comparison to the one observed without US. In this way, the simulation of the temperature profile generated by the US, through electrical resistances, was performed to compare the results obtained. The use of the US did not alter the dynamic behavior of the system but increased the extraction efficiency when compared to the condition with simulated temperature profile. US powers above 75 W did not lead to increased extraction efficiency, when the residence time was 30 min. However, when 60 min of residence time were employed, the optimized US power was 150 W, resulting in an extraction efficiency of 71.7 ± 0.7%. In addition, it was observed that with the use of US, the axial dispersion was relatively lower and the average residence time was closer to the theoretical one. The concentration of Cr in the solid phase decreased linearly over the length of the system, with a more pronounced drop in the presence of US. With the use of US, it was observed the presence of reactive power consumed by the system generator, leading to a higher global consumption than that observed without US. Finally, the mass balance allowed to check the consistency of the results, and no significant losses were observed during the operation of the system.