Role Of Polysulfide In Binary Catalystic Oxidation Desulfurization Reaction System

Tannin extract desulfurization technology?TEDT?and modified Stretford process all belong to vanadium-base binary catalystic oxidition desulfurization process and are the most common wet desulfurization process.The two desulfurization method would produce lots of by-products in the reaction process.In order to repress or avoid side reaction,it is essential to study further mechanism of the desulfurization process.In this paper,the results of reaction between HS^-and V?V?indicated that there produces polysulfide ions Sx2-.Baesd on the former research findings,a new method for the determination of Sx2-was established,and absorption and transformation of H₂S was investigated.Then,the reaction between sulfur-containing substances from absorption and transformation of H₂S and oxidation catalyst was researched using the new mothed.And reaction mechanism of binary catalystic oxidition desulfurization process was discussed.The main conclusions are as follows:Reaction between disulfide HS^-and pentavalent vanadium V?V?and influencing factors of polysulfide ions Sx2-formation were investigated,the results shown:The reaction process can be divided to initial fast reaction stage and stable reaction stage,and there is Sx2-formed in the reaction with right reactants concentration.The influence of reaction temperature on equilibrium conversion of HS^-is not appreciable,however higher temperature is helpful for the generation of Sx2-.With the increasing of pH value,equilibrium conversion of HS' decreases,while higher pH value may suppress the transformation from HS^-to Sx2-.A new method for the determination of Sx2-from binary catalystic oxidition desulfurization process was established.The mothed is cyclic voltammetry?CV?method with hanging mercury electrode?HME?as working electrode.Experiment results shown sulfur-containing ions Sx2-and HS^-were separated and determined effectively in the cyclic voltammetry curves.Meanwhile,other substances in the desulfurization solution were determined using CV mothed with HME as working electrode,the results indicated these substances did not interfere the determination of Sx2-.It demonstrated that the new mothed was feasible and reliable in the determination of Sx2-.While,the optimum experimental conditions of CV method were investigated:the best scan rate is 400mV/s and the best concentration of electrolyte KCl 0.5mol/L.Absorption and transformation of H₂S in binary catalystic oxidition desulfurization process was studied using CV method with HMD as working electrode.The results show that there were four oxidation peaks in CV curves.It indicated that there produce three kinds of polysulfide ions Sx2-except for HS^-under oxidation of O₂?aq?after absorption of H₂S.The analysis indicated that the three kinds of Sx2-are S₂^2-×S₄^2-and S₆^2-respectively.Absorption and transformation mechanism of H₂S is as follows:H₂S?g????H₂S?aq?H₂S?aq?+ OH?H₂O+HS^-4HS^-+O₂?aq??2S₂^2-+2H₂O4S₂^2-+O₂?aq?+2H+?2S₄^2-+2OH-S₂^2-+S₄^2-+O₂?aq?+2H+?S₆^2-+2OH-In the reaction between imitation H₂S absorption solution and pentavalent vanadium V?V?,concentration of sulfur-containing substance decreases with increase of reaction time,and there is a rapid reaction process in initial 5s.While sulfur-containing substance HS^-,S₂^2-,S₆^2-and S₄^2-respectively disappear.Throughout the reaction process,no new polysulfide ion is formed,and there appears milk-white precipitate element sulfur S8.The analysis of the exist state of vanadium species in V-H₂O system indicated that the exist state of catalyst vanadium will transform from pentavalent vanadium HV₂O₅^-to tetravalent vanadium HV₂O₅^-.Based on these conclusions,reaction mechanism between sulfur-containing substance and V?V?was given:2HS^-+HV₂O₇^3-?S₂^2-+HV₂O₅^-+2OH-2S₂^2-+HV₂O₇^3-+2H₂O?S₄^2-+HV₂O₅^-+4OH-S₂^2-+S₄^2-+2HV₂O₇^3-+2H₂O?S₆^2-+2HV₂O₅^-+4OH-S₂^2-+S₆^2-+4HV₂O₇^3-+2H₂O?S₆^2-+4HV₂O₅^-+4OH-2S₆^2-+HV₂O₇^3-+2H₂O?S8?+S₄^2-+HV₂O₅^-+4OH-2S₄^2-+4HV₂O₇^3-+4H₂O?S8?+4HV₂O₅^-+8OH'Compare with reaction between sulfur-containing substance and V?V?,the reaction rate between sulfur-containing substances and oxidation-state tannin extract?TEos?was slower,while there was not elemental sulfur precipitate formed.UV-vis spectrum and infrared spectrum of reaction solutions shown that sulfur-containing substances were oxidized and to generate an organic compound with sulfur-containing functional groups based on tannin extract basic molecule structure.The reaction between imitation H₂S absorption solution and pentavalent vanadium V?V?indicated that ADA can directly react with sulfur-containing subtances in the solutions.Concentration of sulfur-containing substance decreased with the increase of reaction time,and the reaction process can be divided to initial fast reaction stage and stable reaction stage.Kinetics of stable reaction stage between sulfur-containing substance and ADA was investigated using trial and error method,the results shown that the reaction was a one-order reaction,and that reaction rate constant was 0.00175s-1.Relationship between by-products and residence time of desulfurization reaction solution was researched,the results shown conversion of HS^-increases rapidly and reaches more than 95%.The amount of by-products S2O32-and SO42-increases rapidly in a very short time,it shown unconverted HS^-and Sx2-are sources of by-products.With the increasing of time and conversion of HS^-reaches more than 95%,there are lots of by-products formed.It goes to show Sx2-is the primary source of by-products at the moment.