Study On Physical Properties And Mass Transfer Coefficient Of PDS Desulfurization System

Among the many desulfurization methods,the PDS method has been widely used in the gas industry,nitrogen fertilizer industry and coking desulfurization and other related industries,large sulfur capacity,large sulfur foam particles,easy separation and no leakage,and achieved a significant desulfurization effect.For the entire desulfurization process,the pH of the system is the most important factor affecting the overall desulfurization process.PDS desulfurization processes with Na₂CO₃-NaHCO₃ buffer as absorbent,in order to achieve the absorption of H₂S and desulfurization liquid pH stability,but due to the existence of side effects and the phenomenon of absorption of CO₂,it need to regularly add to the desulfurization solution Na₂CO₃ to maintain the system PH stability.In the actual production,people often rely on experience to adjust,lacking of theoretical depth,so the nature of Na₂CO₃-NaHCO₃ buffer in this thesis and its effect on the gas-liquid mass transfer process were studied in order to guide the production.PDS desulfurization is the buffer solution composed of Na₂CO₃ and NaHCO₃,which can stabilize the pH value of the solution to a certain extent.In order to study the effect of buffer composition on the pH value,Na₂CO₃-NaHCO₃ solution with different concentration was prepared,The changes of pH value were examined and the properties of the buffer were studied.Then the pH value?8.5?of the solution was fixed,and the ratio of Na₂CO₃ and NaHCO₃ and the effects on the physical properties?density,viscosity and surface tension?were studied,which laid a foundation for the study of gas-liquid mass transfer.In the process of desulfurization,there are side reactions,which produce NaSCN,Na₂S₂O₃,Na₂SO₃ and Na₂SO₄,and break the equilibrium relationship between Na₂CO₃ and NaHCO₃,and the salt in the desulfurization solution has a certain influence on the physical properties and solubility of NaHCO₃.The pH value of the absorption solution decreases with the increase of the salt content,With the increase of secondary salt content,the density,viscosity and surface tension of the absorption liquid increased,but the degree of increase was not the same,and the viscosity changed the most.With the increase of the secondary salt content,the solubility of NaHCO₃ decreases,and the temperature also has some influence on the solubility,which basically shows the law of decreasing temperature and decreasing the solubility.In order to prevent the occurrence of NaHCO₃ precipitation,this paper gives a formula for calculating the upper bound of the density and the density operating area in the actual operation,which decreases with the increase of the secondary salt content.Meanwhile,the density of liquid?or the content of secondary salt?and the content of NaHCO₃ should be monitored simultaneously.Through the simulation of desulfurization process in self-made packed column,and combined with the response surface design method showed that in the range of experiment,the Na₂CO₃ concentration has the greatest influence on the total mass transfer coefficient K_Ga,this is because of the change of Na₂CO₃ concentration changes the p H value of the system,thus changing the chemical absorption rate.The secondary salt in the system also affects the pH of the solution.By studying the effect of Na₂CO₃ and secondary salt on K_Ga,it is found that there is a sensitive area in the mass transfer coefficient K_Ga.By comparing the three groups of experimental data,it was found that the influence of Na₂CO₃ and secondary salt on K_Ga was mainly achieved by the pH value of the system.The absorption process of wet desulfurization was described by double membrane theory.The mechanism of material transfer in gas-liquid mass transfer was studied,and the mathematical model of the process was put forward.In this paper,the gas phase mass transfer coefficient kG and the liquid mass transfer coefficient kL were calculated by using the correlation formula of Onda et al.,And the results showed that Na₂CO₃ and secondary salt changed kL by changing the physical properties of the system and did not affect kG,And from the value of kL,kL changes in the amount is not large.At the same time,the corresponding second-order reaction rate constant k2 was calculated.The results show that with the increase of the pH value of the system,the variation of k2 is approximately the same as that of K_Ga,which indicates that the change of the total mass transfer coefficient K_Ga is changed by the k2 of the system.