Experimental Study Of SO₂ Absorption From Flue Gases By Quaternary Phosphorus-based Deep Eutectic Solvents

Because of the rising demand for energy,SO₂emissions from the burning of fossil fuels will seriously impact both human health and the environment.Nowadays,limestone-gypsum flue gas desulfurization technique is the most popular and efficient flue gas desulfurization method in the industry.However,the incompletely oxidized gypsum created following desulfurization,which readily forms solid debris and leads to secondary environmental damage,is difficult to handle.The needs of businesses have been challenging to satisfy with the current flue gas desulfurization technology as national environmental standards have improved.In addition,SO₂serves a vital function in other industrial applications,including organic solvents,refrigerants,and reducing bleach,etc.Therefore,in order to further respond to the requirements of the national"green chemical industry",a new type of low viscosity quaternary phosphorus-based DESs was prepared for the absorption of low concentration SO₂in flue gas,and the utilization rate of DESs was improved by loading DESs with activated carbon（AC）.With ethylene glycol（EG）and imidazole（Im）as hydrogen bond donors and methyltriphenylphosphonium bromide（MTPB）as hydrogen bond acceptor,binary EG-MTPB（4:1）DESs and ternary EG-Im-MTPB（1:2:1）DESs for absorption of low concentration SO₂were prepared by physical heating and stirring method.The desulfurization performance of DESs was studied by bubbling absorption device.The effects of DESs type,molar ratio,flue gas temperature and flue gas composition on the absorption of low concentration SO₂by DESs were investigated.The mechanism of SO₂absorption by DESs was analyzed by Fourier transform infrared spectroscopy（FT-IR）,nuclear magnetic resonance hydrogen spectrum（¹H-NMR）and DFT theoretical calculation system,and the detailed mechanism between SO₂and DESs in the desulfurization process was clarified.Aspen plus V12 software was used to simulate its industrial application.AC was impregnated into EG-Im-MTPB（1:2:1）DESs by equal volume impregnation method to obtain EG-Im-MTPB DES/AC solid desulfurizer.The desulfurization performance and adsorption kinetics of EG-Im-MTPB DES/AC solid absorbent were investigated by using a fixed bed flue gas purification device.The main research contents and conclusions are as follows:（1）With EG as hydrogen bond donor and MTPB,ethyltriphenyl phosphorus bromide（ETPB）,propyl triphenyl phosphorus bromide（PTPB）and butyl triphenyl phosphorus bromide（BTPB）as hydrogen bond acceptors,four binary quaternary phosphorus DESs with low viscosity were synthesized by a simple heating and stirring method.The results showed that EG-MTPB（4:1）DES with short alkyl chain substituents had the best absorption capacity for low concentration SO₂（1000ppm）,and the absorption capacity of EG-MTPB（4:1）DES to SO₂remained unchanged after five SO₂absorption-desorption cycles.The active sites for chemical interactions between EG-MTPB（4:1）DES group and SO₂molecule were identified to be Br through the analysis using FT-IR,¹H-NMR,and quantum chemical calculation analysis.（2）On the basis of the preparation of EG-MTPB DESs,a new ternary EG-Im-MTPB DESs was synthesized by adding Im as the second hydrogen bond donor.Through the regulation of the molar ratio between EG,Im and MTPB,it was found that when EG:Im:When the molar ratio of MTPB is1:2:1,the saturation absorption of EG-Im-MTPB（1:2:1）DES for low concentration SO₂（3000ppm）is 4.43 mol SO₂/mol DES（0.65g/g）.Compared with the binary EG-MTPB（4:1）DES（0.194 mol SO₂/mol DES）,the absorption performance increased nearly 20 times,and the cyclic stability and thermal stability were excellent.Through FT-IR,¹H-NMR and quantum chemistry analysis,it is found that there are multiple sites of interaction between EG-Im-MTPB（1:2:1）DES and SO₂,and the active absorption sites are two N atoms in Im and Br atoms in MTPB.Further thermodynamic calculation and the Aspen Plus process simulation of the absorption process of SO₂by EG-Im-MTPB（1:2:1）DES were carried out.The results showed that EG-Im-MTPB（1:2:1）DES could regenerate completely at 90℃after absorbing SO₂.In addition,the purity of SO₂and EG-Im-MTPB（1:2:1）DES obtained after separation were 94.16%and 98.82%,respectively,which realized the recovery of sulfur resources and provided a certain theoretical basis for the industrial application of DESs.（3）The Ca O contained in the ash on the surface of AC has an adverse effect on the process of desulfurization and AC regeneration.Ca O interacts with SO₂to form the refractory Ca SO₄·2H₂O whiskers which exist on the surface and pore structure of AC,leading to the blockage of the pore channels of AC and the reduction of the specific surface area of AC.Therefore,EG-Im-MTPB DES/AC sorbent was synthesized by using AC after acid cleaning to load EG-Im-MTPB（1:2:1）DES.The SO₂absorption performance of EG-IM-MTPB DES/AC sorbent was evaluated by a self-made fixed bed flue gas integrated device.The results showed that the absorption performance of SO₂was the best when AC was 25%of EG-Im-MTPB（1:2:1）DES.Combined with the analysis methods of Brunauer-Emmett-Teller（BET）surface area analysis,FT-IR and X-ray diffraction spectroscopy（XRD）,it was found that EG-Im-MTPB（1:2:1）DES mainly existed in the mesopore structure of AC in the form of amorphous.When the content of DES loaded on AC was too much,the pore structure of AC would be blocked.SO₂can not interact with DES in the pore of AC,and the absorption capacity of EG-Im-MTPB DES/AC sorbent is reduced.Further,four kinetic models,including Lagergren,Mc Kay,Boyd and Bangham models were used to model the absorption process of SO₂by EG-Im-MTPB DES/AC desulfurizer.The results showed that,under the conditions of different flow rates and space velocity,the adsorption kinetics of SO₂on EG-Im-MTPB DES/AC sorbent was predicted in the order:Bangham kinetics model≈Ho quasi second-order kinetics model>Lagergren first-order kinetic model>Mc Kay second-order kinetic model,Bangham kinetic model and Ho quasi second-order kinetic model more effectively demonstrate that the EG-Im-MTPB DES/AC sorbent SO₂absorption process is the result of surface reaction and microporous diffusion.In addition,the dynamic absorption process of SO₂on the surface of EG-Im-MTPB DES/AC sorbent was expressed by the Shilov equation.It was found that the effective time of SO₂absorption by EG-Im-MTPB DES/AC sorbent decreased with the increase of flue gas flow rate.The interaction time between SO₂and EG-Im-MTPB DES/AC sorbent is reduced,and the utilization rate of sorbent is decreased.