Study On Absorptve Separation Of HCl By Choline Chloride-based Deep Eutectic Solvents

Hydrogen chloride(HCl)is a bulk by-product of many industrial chemicals,e.g.,vinyl chloride,diphenylmethane diisocyanate(MDI),toluene diisocyanate(TDI),chloromethanes,chlorobenzenes,chlorinated paraffin/polymers,and so on,amounting to millions of tons annually in China.At present,HCl-bearing tail gas is mainly treated by water absorption for the production of industrial hydrochloric acid with limited use and economic value due to its low quality and surplus supply.To upgrade its value and utilization,it is necessary to reclaim anhydride HCl from its tail gas.For this,ionic liquids(ILs)-based adsorption is attractive considering their unique features,e.g.negligible volatility and high solubility for polar gases.In comparison with other gases,HCl has much higher acidity,polarity and H-bonding ability,and thus has higher solubility in ILs.Deep eutectic solvents(DESs)as an ionic mixture are also known as ILs analogues,have wide liquid range and low vapor pressure along with cost effectiveness,environmental friendliness and biodegradability,and thus deserve further exploration.For the adsorption of polar gases like SO2,NH3,H2S,N2O,CO2,and NO2,etc,choline chloride(ChCl)-based DESs have shown promising performance.Therefore,we studied the absorption of HCl by ChCl-based DESs,revealed its absorption mechanism,toward to find a high-efficiency DESs absorbent for HCl gas.Four DESs were synthesized using ChCl as hydrogen bond acceptor(HBA),Urea,glycerol(Gly),glycollic acid(GA)and malonic acid(MA)as hydrogen bond donors(HBD)and HBA:HBD=1:2 and 1:1(mole ratio).The as-synthesized DESs were characterized by FT-IR and NMR,and the formation mechanism of the DESs was determined.The absorption kinetics of the DESs were determined.The results show that the absorption curve follows the first order kinetics equation,being a diffusion controlled process.The solubility of HCl in DESs at 30℃ and 101.3 kPa is consistent with the basicity of HBDs,and the higher the alkalinity,the higher the solubility.The absorption amount of ChCl-Urea,ChCl-Gly,ChCl-GA and ChCl-MA is 0.457,0.256,0.214 and 0.129 g-HCl/g-DESs,respectively.To investigate the absorption of DESs at different temperatures and pressures,the P-T-X data of four systems were measured.Low temperature and high pressure is beneficial to the absorption process,being consistent with the general behavior of gas absorption.The absorption of HCl in ChCl-Urea is mainly a chemical process,and cannot completely desorped,while the HCl in other DESs is mainly physical absorption,and can be desorped completely.FT-IR,1HNMR and 13C NMR were used to analyze the absorption mechanism of HCl in four DESs.The chemical absorption of HCl in ChCl-Urea is mainly due to the strong H-bonding between basic group-NH2 and HCl.H-bonding are also formed between HCl and ChCl-Gly,ChCl-GA and ChCl-MA.Four DESs all have good recycled ability,and their absorption performance is basically unchanged after five absorption/desorption cycles.The throughput of ChCl-Urea and ChCl-Gly are higher,being 0.265 and 0.256 g-HCl/g-DESs,respectively.Further,ChCl-Gly and ChCl-Urea show the best thermal stability,and thus they are chosen as the absorbers,and their viscosity and density are analyzed.The viscosity of ChCl-Gly is decreased greatly after absorbing HCl,and the density is increased slightly,and thus it is suitable for HCl absorbtion.Finally,a conceptual process design is made for the absorption/desorption process of HCl with ChCl-Gly solvent.