Cristalização de ácido cítrico em leito vibrado

Abstract

Crystallization data of citric acid are extremely important for applications in chemical, food processing and pharmaceutical industries. The equipment design usually requires the use of a reliable experimental data. The aim of this work was to study the crystallization process of the commercial citric acid and proceeding from Tahiti lime juice, in vibrated bed, at 55.0ºC. The effects of vibration applied to the crystallization of citric acid using a conictrunk crystallizer followed by a cylindrical part and a vibration system constituted of four perforated disks, containing 0.005 m of perforations, were studied for the experimental condition of supersaturation from literature, which did not occurred nucleation of the system. The growth kinetics of the crystal was also studied in the best operating condition of vibration and supersaturation intensities. The mass and the dimension of the citric acid seeds and crystals were quantified and presented results very close to the literature. In a second part of the work, the process of crystallization of citric acid was studied in a crystallizer with a truncated conical shape, built in stainless steel and jacketed, with a vibration system consisting of two perforated discs. The independent variables: dimensionless variables number of vibration, supersaturation and population of seeds were studied using a central composite design (CCD) and commercial citric acid. In preliminary tests with the commercial citric acid, it was determined the point of maximum production of acid through an optimization of the experimental results. The growth kinetics of the crystal was studied using the optimized condition of the vibration intensity, supersaturation and number of seeds. It were verified this work, the behavior of the response variable (crystals mass per seeds mass), the characteristic dimension and mass of the crystals as a function of crystallization time. For a crystallization time of about 2 h the relation mass of the crystals per seeds mass increased 157.1% and the characteristic dimension of the crystal increased approximately 42.3%. The citric acid in the lime juice was extracted, centrifuged and filtered, then it was realized the stage of precipitation of calcium citrate at 60.0ºC in a borosilicate glass reactor. Citrate was dried at 75.0ºC for 48 h and then it was realized the reconstitution of the citric acid with diluted sulfuric acid. The citric acid solution obtained was purified with activated charcoal and concentrated until saturation. The stage of crystallization occurred in the optimized condition of the dimensionless number of vibration, supersaturation and population of seeds. The data of growth kinetics using citric acid from the lime juice showed that in 2 h of crystallization, the characteristic dimension of the crystals grew 49.5%, while the mass increased 242.9% compared to the seeds. Deviations related to the growth rates were calculated by comparing the data of crystallization of the commercial citric acid with the lemon citric acid. Chromatographic analysis were performed to quantify the purity of citric acid at different stages of the process, that is in citric acid solution prepared with lemon juice after centrifugation and filtration, in the solution obtained after the recovery of citric acid and without treatment with activated charcoal in a purified solution and in a solution using the crystals obtained in the crystallization process in the vibrated bed.