Cristalização de ácido cítrico: otimização operacional

Abstract

In this work the crystallization of citric acid using Tahiti lemon juice was studied. The experiments were carried out in a batch crystallizer, with agitation and temperature control. The influences of three variables according to a composite experimental design on the production of citric acid were analyzed: super saturation, crystal number (seeding) and speed stirred. The effects of these variables and their interactions were analyzed by the response surface methodology. The biggest influences were exerted by supersaturation and by speed stirred. Through a canonic analysis of the adjusted response surface, the optimal variables values that led to the biggest production of citric acid were determined: R = 356 rpm (speed stirred); S = 1.26 super saturation and N = 27 seeding. In these conditions, the relations of the crystal mass and seed mass for citric acid (y) reached 2.2700. A kinetic study into the growth from the crystal of citric acid showed that the average increase of the mass of crystals was 1.375x10-6 kg (102% of the initial mass) and it showed presented an average increase of the characteristic dimensions of 0.48x10-3 m after two hours of crystallization. The calculated growth rate decreases throughout the time, probably because of the reduction of the driving force and for the dependence that the growth rate of crystals has in related to the size of the same ones. Being that lesser crystals possess bigger growth rate that the crystals biggest not following. The smaller crystals show a larger growth rate than the bigger crystals and crystallization didn t follow the McCabe ΔL law. The kinetic study of the growth rate showed that KG changed through the experiments, probably because of hydrodynamic influence in the agitation, that is, bigger particles behave in a different way from smaller particles on the crystallizer in a certain agitation.