| SO2 pollution, sulfur resource demand in China and the present technical state and application of flue gas desulfurization around the world were analyzed in this article. Therein the wet flue gas desulfurization technology is more suitable for resolving conflicts of sulfur resource shortages and growing demand in China, for the characteristics of high purification of flue gas, PM2.5 source reduction absorbent recycling use and recovery of SO2. However, the technology had many problems, such as long process, high capital cost and regeneration energy consumption. What’s more, sulfite is easily oxidized into thermal stability sulfate and the stability of absorbent was poor, causing large consumption, low desulfurization rate and recovery rate of SO2, fouling and plugging of equipment. By far, the current application proportion of this technology has not reached 5%. Therefore, it will be the fundamental way to promote regenerative flue gas desulfurization technology and a major future development direction of flue gas desulfurization in the future in China by selecting or developing appropriate renewable desulfurizer and innovating technology and equipment. Therefore, we choose sodium phosphate buffer solution as the flue gas desulfurization agent, which is of thermal and chemical stability, cheap and renewable, and the rotating packed bed (RPB) which is remarkable for the mass and heat transfer enhancement as the absorption and desorption equipment. Theoretical and experimental study on flue gas desulfurization using rotating packed bed-sodium phosphate buffer solution method was preceded, which could provide a theoretical basis and scientific data for the industrial application.In terms of desulfurization theory, through analyzing the flue gas desulfurization mechanism of sodium phosphate buffer solution, combined with multi-buffered solution theory, chemical equilibrium and phase equilibrium theory, equations of distribution coefficient of ions, buffer capacity, theoretical sulfur capacity and phase equilibrium in sodium phosphate salt and desulfurization solution were established. By numerical calculation methods using the MATLAB software, the effects of the acid concentration, pH value and SO2 partial pressure on the distribution coefficient, buffering capacity, and the theoretical sulfur capacity is theoretically calculated and analyzed. The buffer capacity change rules of the sodium phosphate salt solution with the pH were experimentally measured, and the experimental results are close to the theoretical calculation except that the useful buffer range of pH is wider and more in line with the requirements of flue gas desulfurization. The pH range of the solution is 4.5 to 7 for the virtuous cycle of SO2 absorption and desorption; NaH2PO4 and Na2HPO4 play the main buffer action. These analyses provide a theoretical guidance for the flue gas desulfurization experiments and optimization using sodium phosphate buffer solution.With respect to mass transfer performance of the absorbent, the desulfurization mechanism and mass transfer process by phosphate buffer solution were analyzed, and results find that the SO2 absorption rate is mainly controlled by mass transfer rate. Thereby, it is necessary to develop a new desulfurization equipment and process to enhance the mass transfer and absorption efficiency. For this purpose, the characteristics of the mass transfer and desulfurization performance were experimented in the absorption process of SO2 from simulated flue gas, using high efficient mass transfer devices of RPB and packed column respectively, and the absorbent of sodium phosphate buffer solution. The effects of process parameters (β, U, u, Cp and initial pH value of absorbent, y1, etc.), liquid-gas contact mode, absorption equipment, packing styles on the gas phase volumetric overall mass transfer coefficient Kya were investigated. Results indicate that, at the same conditions, the Kya of RPB is 1 order of magnitude higher than high efficiency packed column; The Kya of RPB with layered packing is higher by 50% than that filled packing; The Kya of RPB with θ ring packing is higher by around 15% than that with corrugated wire mesh packing; The Kya of counter-current absorption process is a little higher than that of co-current one, except that at a higher sprinkle density, the Kya is a little lower than the latter. The correlation of some operation parameters is proposed after fitting the experimental data for RPB with filled and layered packing under counter-current and cocurrent processes, using the nonlinear regression program in MATLAB software:Kya= mβaubUc, and the average error is (2.82-7.85)%, which can well reflect the effect rules of these operation parameters on the mass transfer coefficient during the desulfurization process in RPB using sodium phosphate absorbent.In terms of desulfurization process, the effecting rules of parameters (β, L/G, u, Cp and initial pH value of absorbent, C1 and t, etc.), packing type and structure on the absorption efficiency η are detailed investigated in both co-current and counter-current processes in RPB, through the orthogonal experiments and single factor experiments. The results show that, the η increases with the increase of initial pH value of absorbent, L/G, β and Cp, and the increasing rate decreases; while the η decreases with the increase of temperature; at a relatively higher U (>7 m3/m2h), the η firstly increases a little and then decreases with the increase of C1 and u but the changes are small. However, at lower U (<4 m3/m2 h), the η decreases with the increase of C1 and u, and the η reduces rapidly. At the same operation conditions, the η in co-current mode is lower than counter-current mode, but the difference becomes smaller with the increase of L/G, β and initial pH value of absorbent; the η in RPB with θ ring packing is higher than that with corrugated wire mesh packing. The optimum process conditions are as follows:initial pH of 5.5-6,t<50℃, in co-current mode:gas-liquid ratio of 2～3 L/m3, high gravity factor of 80～100, Cp of around 1.5 mol/L; in counter-current mode:gas-liquid ratio of 1.5～2 L/m3, high gravity factor of around 80, Cp of 1～1.5 mol/L. When u is 0.3～1.2 m/s and C1 is lower than14 g/m3, the η in both processes maintains above 98%, and C2 is lower than 200mg/m3 or 100 mg/m3 and even 50 mg/m3, which reaches the new national emission standards.On behalf of regenerative process, the regeneration experiments of the enriched absorbent were carried out in two devices of RPB and packed column. The effects of process parameters were mainly investigated and the results were compared at similar conditions:both devices could efficiently desorb the enriched solution, and desorption efficiency may maintain above 88%. The desorption efficiency in RPB is higher than the packed column by 4%, and the preheating temperature of the enriched solution may be reduced 5℃; in both devices, the desorption efficiency increases with the increase of the preheating temperature and SO2 concentration in the rich solution, gas-liquid ratio; but decreases with the increase of enriched solution pH, flow rate and the phosphate concentration. The optimum desorption process parameters are as follows:β of 60～80, enriched solution pH<5, liquid flow rate of around 30 L/h, and gas-flow ratio of around 1000 m3/m3, phosphate concentration CP≤3 mol/L and desorption temperature t of around 90℃.At these conditions, the desorption efficiency could maintain above 91%.The results of 10 times repeat absorption and desorption experiments in RPB indicate that:the decay of desorption efficiency is only 0.05%, while the desorption efficiency increases a little; the oxidation rate of sodium sulfite is 0.003 mol/(L·h), which is only 7.9% andl.7% of sodium phosphate method and sodium sulfite method in packed columns, respectively. At the optimum conditions of co-current absorption and desorption process, the results of consecutive operation for 5 hours indicate that:the absorption efficiency and desorption efficiency maintains around 98.2% and 81.1%, and the outlet SO2 concentration maintains around 100 mg/m3, which is lower than the flue gas emission standards. |
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