Basic Study On Desulfurization Performance Of Iron-Based Ionic Liquids

Hydrogen sulfide gas is produced in natural gas development,oil refining,papermaking and sewage treatment.It will not only harm the human body,but also form acid rain and destroy the ecological environment.In the industrial production process,the presence of H₂S will corrode pipelines and production equipment,poisoning the catalyst and losing its activity,reducing production and increasing production costs.Therefore,the removal of hydrogen sulfide becomes very important.The wet oxidation method is the most commonly used desulfurization method,which is widely used in the desulfurization of biogas,syngas and natural gas.However,the existing wet oxidation desulfurization process has many shortcomings:The pH value of desulfurization solution shall be strictly controlled.If the pH is too low,the complex iron will become the precipitation of iron hydroxide and iron sulfide;under alkaline conditions,H₂S will be peroxidized to form thiosulfate,sulfate and other by-products.The by-product salt causes problems such as blockage of pipelines and equipment,affecting production efficiency.Ionic liquids（ILs）as a new type of green solvent have shown excellent performance in the field of desulfurization.Iron-based ionic liquids not only have the advantages of low saturated vapor pressure and good thermal stability of traditional ionic liquids.Their own acidic conditions avoid the problem of the generation of by-product salt in conventional wet oxidative desulfurization production.Considering the source and economy of raw materials,this paper selects N-butylmorpholine as cation and anhydrous ferric chloride as anion to synthesize two kinds of N-butylmorpholine iron-based ionic liquids（rFeCl₃/[Nbmm]Br,r=0.8、1.0）.Then the two N-butyl morpholine iron-based ionic liquids were characterized and analyzed.It can be seen from the infrared spectrum that the cations of the synthesized ionic liquids are identical to those in[Nbmm]Br.It can be seen from the Raman spectrum that the anion of the synthesized ionic liquids is[FeBrCl₃]^-,[FeBr₃Cl]^-,[FeBr₂Cl₂]^-,and[FeCl₄]^-.Four anions exist simultaneously.The acidity of iron-based ionic liquids was analyzed by infrared with pyridine as probe.It was found that the ionic liquids synthesized had Bronsted acidity.The thermogravimetric analysis shows that the ionic liquid has good thermal stability.The density and viscosity of N-butylmorpholine bromide iron-based ionic liquids（rFeCl₃/[Nbmm]Br）with two iron-morpholine ratios（r=0.8 and 1.0）mol·mol^-1 were measured at 303.15～348.15 K and 101.3 k Pa.Density and viscosity data were fitted by empirical correlations for temperature.The average relative errors were less than 0.02%and 2%,respectively.The density and viscosity decrease with increasing temperature.The density increases with the increase of r,while the viscosity shows the opposite trend.The gas-liquid equilibrium data of H₂S in rFeCl₃/[Nbmm]Brwere measured by the isochoric pressure drop method at temperatures of 303.15～348.15 K,pressures of 0～200 k Pa,and r of 0.8 and 1.0.The total solubility of H₂S increases with decreasing temperature,increasing pressure,and decreasing r.The measured solubility data were fitted by the reaction equilibrium thermodynamic model.Chemical reaction equilibrium constant and the Henry’s constant at different temperatures and different r were obtained.The average relative error of model fitting is less than 3%.The results show that the chemical reaction equilibrium constant and the Henry’s constant both decrease with the increase of r,and decrease of temperature.Effect of molecular structure on reaction equilibrium constants was explained by quantum chemistry calculation.The absorption of H₂S in rFeCl₃/[Nbmm]Brat low pressure is dominated by chemical absorption.With the increase of pressure,the proportion of physical absorption continues to increase,and gradually occupies the main part.Iron-based ionic liquids have the disadvantages of high viscosity,low utilization of Fe（Ⅲ）and strong acidity.Adding low viscosity organic solvent and ionic liquid to form compound solvent can reduce the viscosity and acidity of ionic liquid and improve the utilization rate of iron.Density functional theory（DFT）based on M06-2X functional and 6-311+G（d,p）basis set to calculate the interaction energies of H₂S with 11 organic solvents.The order of interaction energy is MDEA>SOO>DMEE>DEAE>NMP>DMAC>DMI>DMF>CH₄O>C₅H₉N>H₂O.AIM topological analysis showed that hydrogen bonds were formed between 11 organic solvents and hydrogen sulfide.The IRI function analysis shows that there is only electrostatic interaction between CH₄O,C₅H₉N and hydrogen sulfide.The other nine organic solvents have both electrostatic and van der Waals interactions with hydrogen sulfide.ESP analysis showed that the hydrogen bond between solvent and hydrogen sulfide was derived from the electrostatic interaction between molecules.According to the viscosity data of the solvent obtained by consulting the data,combined with the viscosity data and interaction energy data,DEAE,SOO,NMP,DMAC,DMI,DMF and CH₄O were selected as organic solvents to form a complex solvent with iron-based ionic liquids.Using RETM model,the solubility data of four ionic liquids under pressure 0～100k Pa were calculated.By comparing the solubility,FeCl₃/[Nbmm]Brwith the largest solubility was selected for subsequent experiments.Seven organic solvents were mixed with FeCl₃/[Nbmm]Brionic liquids to form mixed solvents.Observing the stability of the complex system,combined with the properties of the solvent itself,SOO and FeCl₃/[Nbmm]Brionic liquid were finally selected to form a complex solvent.Then,the response surface analysis method is adopted,with the solubility of H₂S as the objective function,and the iron-salt ratio,compound ratio and temperature as the independent variables,to carry out the optimization design analysis of three factors and three levels.The results show that the iron salt ratio has the greatest influence on the solubility of H₂S,followed by the mixture ratio and temperature,and the interaction between the factors has little influence.The optimal operating conditions are iron salt ratio 0.8mol/mol,mixing proportion 0.2 m L/m L,temperature 26℃and solubility of H₂S 0.0173mol H₂S·kg^-1IL.