Chemical Mass Transfer Mechanism and Characteristics of Flue Gas Desulfurization of Basic Aluminum Sulfate by Bubbles

Abstract

Flue gas desulfurization (FGD) technology with SO2 recyclable regeneration has been increasingly used for basic aluminum sulfate (aluminum base); in this process, chemical reaction and physical mass transfer synergistically affect desulfurization. In this study, experiments and ANSYS numerical simulation were performed to determine the multicomponent continuity equation, gas-phase volume overall mass transfer coefficient (KGae), and chemical mass transfer complex factor (F) by analyzing the reaction kinetics and using two-film theory. The chemical reaction rate (r) was calculated using the consumption rates of SO2 and active Al2O3, and KGae was obtained using flue gas apparent velocity and SO2 mole fraction. The effect of different factors on the chemical mass transfer of aluminum base FGD was then determined. Results showed that both r and dynamic field of gas–liquid two-phase flow affected the mass transfer capacity and desulfurization effect of the aluminum base. The inlet SO2 concentration exhibited a greater influence on KGae and F than the concentration of the absorption solution. Chemical mass transfer originated at the phase interface, and the reaction mainly occurred in the liquid-phase boundary film. Mass transfer resistance was mainly concentrated in the gas-phase boundary film. The aeration rate affected bubble formation, diffusion morphology, and gas–liquid contact time. Increasing the number of small bubbles and preventing coalescence improved the desulfurization efficiency. Moreover, KGae increased linearly with the increasing aeration rate (0.05–0.15 m/s), and F remained stable at a high level (about 0.05). The mass transfer capacity was mainly controlled by aeration rate, and the comprehensive effect of aluminum base FGD was mainly controlled by liquid-phase mass transfer conditions caused by the chemical reactions. Under the condition with 12.5% absorption solution and 0.05 m/s aeration rate, KGae was only 2.346 × 10–6 kmol/(s·m3·kPa). Furthermore, KGae increased rapidly with increasing concentration of absorption solution, whereas F decreased rapidly to the minimum value of 0.03443 with increasing aeration rate (0.15–0.2 m/s) under the conditions of low absorption solution concentration. The mass transfer capacity and comprehensive effect of aluminum base desulfurization were controlled by both physical mass transfer conditions due to the aeration rate and the liquid-phase mass transfer conditions caused by chemical reactions.