(506e) Simulation of Continuous Boric Acid Slurry Reactors in Series by Microfluid and Macrofluid Models

One of the most important boron minerals, colemanite is reacted with sulfuric acid to produce boric acid. During this reaction, gypsum (calcium sulfate dihydrate) is formed as a byproduct. Gypsum as byproduct needs to be filtered out for high production efficiency and purity of boric acid. In this study, the boric acid production was achieved experimentally both in a batch and four continuous flow stirred slurry reactors (CFSSR) in series system. In this reaction system, there are at least three phases, one liquid and two solid phases (colemanite and gypsum). In a batch reactor all the phases have the same operating time (residence time), whereas in a continuous reactor all the phases may have different residence time distributions. The residence time of both the reactant and the product solids are very important because they affect the dissolution conversion of colemanite and the growth of gypsum crystals. The dynamic behaviors of the continuous flow stirred slurry reactors were investigated by obtaining the residence time distribution of the solid and liquid components. The non-idealities in the reactors have been eliminated by increasing the stirring rate and by modifying the propeller design. Complete mixing was obtained in the range of 400-750 rpm stirring rate. Experiments in both batch and continuous reactors were performed with colemanite particle size less than 150 µm in aqueous sulfuric acid at 85 °C with a stirring rate of 400 rpm and initial/feed molar CaO/SO₄^2- ratio of 1.0. After the fast dissolution reaction of colemanite in aqueous sulfuric acid, the nucleation of gypsum crystals first occurs from the supersaturated solution and then the crystals grow on these nuclei. The gypsum crystal growth rate was found to be second order with respect to calcium ion concentration from the batch experiments. Using kinetic data obtained from batch reactor, the effluent calcium ion concentrations in n-CFSSR's were predicted by macrofluid and microfluid models. To see the effect of increase in number of CFSSR's and the increase in volumetric flow rate on the effluent calcium ion concentration, predictions were extended to total residence times of 20-240 min for one to eight CFSSR's. The results were compared with plug flow reactor assumption. In all predictions, the macrofluid model gave lower effluent calcium ion concentration than that of the microfluid model. Plug flow reactor model gave the lowest effluent concentrations. Model predictions were tested by experiments in four CFSSR in series each having mean residence times of 20 or 60 min. Calcium ion concentration predicted by macrofluid model in the first reactor exit was found to be closer to the experimental value indicating the significance of segregation. However, microfluid model provides the effluent calcium ion concentrations from the second, third and fourth reactors closer to experimental values. Verification of the model values by experimental data reveals that the methodology developed in here is applicable to crystallization in n CFSSR's in series.