The Design Of Functional Deep Eutectic Solvents And Their Applications In The Absorption Of SO₂ From Flue Gas

Coal-fired power generation plays an important role in the electricity source of our country.The emission of sulfur dioxide(SO2),which mainly originates from the burning of coal,is harmful to human body and causes serious environmental pollution.The removal and recovery of SO2 are significant to our ecological environment and society.Deep eutectic solvents(DESs)are typically mixed by a hydrogen bond acceptor(HBA)and a hydrogen bond donor(HBD),and their melting points are lower than those of HBA and HBD because of the formation of hydrogen bond.DESs possess benign physicochemical properties which show great advantages in SO2 absorption.However,the reported DESs for SO2 absorption mostly exhibited a physical absorption process,which were difficult to apply to remove low-concentration SO2 in flue gas.In this work,several functional DESs that could absorb low-concentration SO2 were designed,and the absorption of SO2 in these functional DESs was studied in details.Meanwhile,a method for energy-saving in the desorption process was proposed via adding lactic acid(HLa)into absorbents.Furthermore,the specific heat capacities(Cp)of DESs were measured.The main results and conclusions are shown as follows.(1)According to the mechanism of SO2 absorption in functional ionic liquids,i.e.replacing a weak acid by a strong acid,four kinds of biodegradable functional DESs based on betaine(Bet)and L-carnitine(L-car)as HBA and ethylene glycol(EG)or H2O as HBD were designed and prepared for SO2 absorption.The results showed that the four kinds of DESs could efficiently and reversibly absorb low-concentration SO2.The SO2 absorption capacity in L-car-EG DES(nL-car:nEG-1:3)reached 0.820 mol SO2/mol L-car(0.151 g SO2/g DES)at 40? with a SO2 concentration of 2 vol%.The mole ratio of HBA to HBD had almost no effect on the absorption of SO2.The SO2 absorption capacity increased with decreasing temperature and increasing SO2 concentration.In addition,the regeneration experiments demonstrated that the SO2 absorption capacities in DESs did not change after five absorption and desorption cycles.Furthermore,the absorption mechanism of SO2 in the DESs was analyzed by FTIR,1H NMR and 13C NMR.It was found that there were strong acid-base interactions between SO2 and-COO-on HBA.(2)Based on the acid-base reaction of SO2 and amino group on functional ionic liquids,four functional DESs based on imidazole(Im),2-methylimidazole(2-MeIm),2-ethylimidazole(2-EtIm),and 2-propylimidazole(2-PrIm)as HBA and glycerol(Gly)as HBD were prepared.All the prepared DESs(nHBA:nHBD=1:2)could efficiently and reversibly absorb 2 vol%SO2 with an absorption capacity of 1 mol SO2/mol HBA at 40?.The alkyl on imidazole had almost no effect on the absorption of SO2.Im-Gly DES had the highest gravimetric absorption capacity compared with those of the other three DESs.N(1)atom on Im was the active site for SO2 absorption according to the 1H NMR spectra of Im-Gly DES and Im-Gly DES+SO2.In addition,CO2 and NO had almost no effect on the absorption of SO2 in Im-Gly DES.The available absorption of SO2 in Im-Gly DES(nIm:nGly=1:2)was high and reached 0.161 g SO2/g DES with 0.2 vol%SO2 at an absorption temperature of 40? and a desorption temperature of 100?.The regeneration experiments showed Im-Gly DES could be reused to absorb SO2 with high capacities and stability for many times.(3)An idea of using cheap,stable,and water-soluble sodium hydroxide to replace the expensive organic bases to prepare a functional DES was proposed,for reducing the cost and improving the stability of absorbent.A novel low-viscosity absorbent,sodium lactate(NaLa)-H2O DES,was prepared for SO2 absorption.NaLa-H2O DES(wNaLa=50%)could absorb 0.130 g SO2/g DES at a SO2 concentration of 2.5 vol%and 40 0C.In addition,NaLa-H2O DES(wNaLa=50%)could remove SO2 from the concentration of 0.096 vol%to 0 vol%in the simulated flue gas.The gravimetric absorption capacity of SO2 in NaLa-H2O DES increased with increasing NaLa content and SO2 concentration,and decreasing temperature.Importantly,NaLa-H2O DES exhibited high reversibility and long-term stability,and its chemical structure did not change after using for 60 days(absorption temperature of 40? and desorption temperature of 100?),indicating a promise for the desulfurization of flue gas.(4)High energy consumption of regeneration is the major cost of renewable desulfurization technology.According to the analysis of SO2 absorption and desorption process in NaLa-H2O DES,we proposed a method to adjust SO2 absorption and desorption process by adding HLa into absorbents to reduce the regeneration energy consumption and the residual content of SO2 in regenerated absorbents.The adjustment of HLa on various regenerable absorbents was investigated.It was found that HLa could greatly reduce the residual content of SO2 in the regenerated absorbents,thereby further reducing the outlet concentration of SO2.HLa played an important role in the absorbents containing water.At conditions of an HLa:NaLa mole ratio of 0.2:1,an inlet SO2 concentration of 5500 mg/m3(0.2 vol%),and an outlet SO2 concentration of 50 mg/m3(0.0018 vol%),30%steam consumption could be saved theoretically for absorption at 40? and desorption at 120?after using HLa.Also,the addition of HLa could accelerate SO2 desorption rate without affecting the regeneration performance of L-car-EG DES and NaLa-H20 DES.(5)Cp of absorbents is an important property,which allows calculating various temperature-dependent thermodynamic properties.Moreover,Cp is significant to many theoretical and engineering calculations.In this work,Cp of two kinds of inner-salt based functional DESs(Bet-EG DES and L-car-EG DES)and four kinds of imidazole based functional DESs(Im-Gly DES,2-MeIm-Gly,2-EtIm-Gly DES,and 2-PrIm-Gly DES)were measured using a Calvet BT 2.15 calorimeter in a temperature range of 45?-90?.The measured Cq as a function of temperature were correlated with a second-order empirical polynomial equation.The results indicated that the equation could accurately fit the measured Cp.All the measured Cp increased with increasing temperature.The Cp of Im-Gly DES was much more sensitive to temperature than those of other imidazole-based DESs.The measured Cp of functional DESs were lower than that of water,which was beneficial to the desulfurization process.