Study On Phase Equilibrium Of 1,4-Bis-(2-hydroxypropyl)-Piperazine-Sodium Sulfate-Water Ternary System

Na2SO4 is produced in the process of desulfurization by renewable organic amines,which results in abnormal operation of the system.Phase equilibrium of HPP-Na2SO4-H2O ternary system was studied in this paper.Aiming at the formation of Na2SO4 in the SO2 removal system of iron pellet sintering flue gas by using self-developed desulfurizer 1,4-Bis-(2-hydroxypropyl)-Piperazine(HPP),and the phase equilibrium data were applied to the freezing crystallization separation of Na2SO4 in ternary system.The phase equilibrium data of HPP-Na2SO4-H2O ternary system at 273.15 K,298.15 K,323.15 K and 343.15 K were determined by isothermal dissolution equilibrium method.The equilibrium solid phase data were determined by wet slag method and the isothermal phase diagram was drawn.The results show that only Na2SO4 crystalline region exists in 273.15 K phase diagram.At 298.15 K,Na2SO4 and Na2SO4·1OH2O crystalline regions exist,and the existence of HPP decreases the transition temperature of Na2SO4 and Na2SO4·1OH2O crystals.At 323.15 K and 343.15 K,there were aqueous two-phase in the phase diagrams.No crystallization region of pure HPP was found in the phase diagrams at four temperatures.The polynomial equation was used to fit the phase equilibrium data at different temperatures,and the calculated results were in good agreement with the experimental data.The Pitzer model of single component electrolyte was used to correlate the phase equilibrium data,and the Pitzer parameters and the solid phase dissolution equilibrium constants were obtained.The calculated results of the Pitzer model were in good agreement with the experimental data.The solid-liquid phase equilibrium(SLE)data of HPP·0.5H2SO4-Na2SO4-H2O ternary system at 273.15 K,298.15 K,323.15 K and 343.15 K were determined by isothermal dissolution equilibrium method.The equilibrium solid phase data were determined by wet slag method and the isothermal phase diagram was drawn.The results show that at 273.15 K and 298.15 K,there were crystallization region of pure Na2SO4 and Na2SO4·10H2O.Because of the existence of HPP.0.5H2SO4,the transition temperature between Na2SO4 and Na2SO4·10H2O decreases more obviously.No crystallization region of pure HPP was found in the phase diagrams at four temperatures.The polynomial equation was used to fit the phase equilibrium data at different temperatures,and the calculated results were in good agreement with the experimental data.Multicomponent electrolyte Pitzer model was used to correlate the phase equilibrium data,and the Pitzer parameters and solid phase dissolution equilibrium constants were obtained by multiple linear regression.The calculated results of Pitzer model were in good agreement with the experimental data.The freezing crystallization of sodium sulfate was studied using ternary phase equilibrium data of HPP-Na2SO4-H2O and HPP·0.5H2SO4-Na2SO4-H2O.The optimum operating conditions for freezing crystallization of HPP-Na2SO4-H2O ternary system are as follows:crystallization temperature 0℃,crystallization time 3 h,stirring rate 200r℉min-1,cooling rate 15℃h-1;After crystallization,the concentration of sodium sulfate in liquid phase is 1.30%,the crystalline solid phase is Na2SO4℉10H2O,and there are no HPP solid.The optimum operating conditions for freezing crystallization of HPP·0.5H2SO4-Na2SO4-H2O ternary system are as follows:crystallization temperature0C,crystallization time 3h,stirring rate 200r·min-1,cooling rate 10℃·h-1;the concentration of sodium sulfate in liquid is 2.80%.The crystalline solid phase is Na2SO4℉1OH2O,and there were no HPP℉0.5H2SO4 solid.