Study On Physical Properties Of New Absorption-compression Refrigerant Working Pair For CO₂-ionic Liquid Complex Solvent

Nowadays,with the rapid development of the global economy,human beings are facing pressure from the rapid population increase and environmental degradation.In particular,the two major environmental problems,global warming and the destruction of the ozone layer,are threatening the survival of mankind.The use of CFCs,HCFCs and HFCs refrigerants is one of the main reasons for these two major problems.As a refrigerant,CO₂ is the most competitive green refrigerant with environmental friendliness and excellent thermophysical properties.However,the problem of high pressure in the CO₂ transcritical refrigeration cycle cannot be ignored,which seriously hinders the development of CO₂ as a refrigerant.In order to solve this problem in CO₂ transcritical cycle,researchers have proposed a new type of CO₂ refrigeration system,namely absorption-compression refrigeration system.It achieves to reduce the system pressure by introducing an absorbent which has a high boiling temperature and excellent CO₂ absorption performance.In the current research,the ionic liquids（ILs）as a CO₂ absorbent have attracted much attention due to their strong CO₂ solubility,excellent thermal stability and other advantages.Though some relevant physical properties of different types of CO₂-ILs binary systems have been studied,there are still some application difficulties such as large viscosity,slight corrosion,and difficulty in desorption.Therefore,this thesis developed an environmental friendliness CO₂ absorbent with good absorption capacity,low regeneration energy consumption,weak corrosion and low viscosity.This new CO₂absorbent is suitable for CO₂ absorption-compression refrigeration system,making the system a better operation performance and lower energy consumption.The physical properties of this new CO₂absorbent have been studied.The specific research contents and conclusions are as follows:1.[emim][FAP]and[hmim][FAP]were studied as absorbents;polyethylene glycol（PEG200）was selected as the co-solvent considering the comprehensive factors such as cost,physical characteristics,the ability to absorb CO₂ and energy consumption.Mass ratio of 90%[emim][FAP]+10%PEG200 and 90%[hmim][FAP]+10%PEG200 were studied.2.The experimental study on the CO₂ absorption performance of 90%[emim][FAP]+10%PEG200 and 90%[hmim][FAP]+10%PEG200（including tests on CO₂ solubility and system viscosity under phase equilibrium）were carried out at the temperature of293.15～343.15 K（one experimental point for every 10 K）and the inlet pressure of1.0～10.0 MPa（one experimental point for every 1.0 MPa）,.The effects of temperature,pressure and the addition of PEG200 on the solubility and viscosity of CO₂ were analyzed.The experimental results showed that:1)In the complex solvent,reducing the temperature and increasing the pressure both contribute to the absorption of CO₂ by the solvent;The addition of PEG200 to the ionic liquids[emim][FAP]and[hmim][FAP]is able to promote the absorption of CO₂;During the desorption process,the addition of 10%PEG200 can shorten the desorption time of the system,reduce the energy consumption of the system and improve the cycle efficiency.2)At the same temperature,the viscosity of CO₂ and the complex solvent decreased with the increase of the equilibrium pressure.When the equilibrium pressure was low（less than 5.0 MPa）,the viscosity changed significantly with the increase of pressure;when the equilibrium pressure was high（more than 5.0MPa）,the viscosity changed weakly with the pressure.At the same pressure,the viscosity of the system decreased with the increase of temperature.The addition of 10%PEG200can reduce the viscosity of the mixture,facilitating the flow of the working medium pair in the absorption-compression refrigeration system.3.The corrosion performance of pure ILs[emim][Tf₂N]and 90%[emim][Tf₂N]+10%PEG200 complex solvent on 316L stainless steel was studied by electrochemical test method and scanning electron microscope.The effect of corrosion inhibitor PEG200 on the corrosion behavior of ILs was analyzed.The experimental results showed that the impedance modulus of the electrode in the ILs added with inhibitor PEG200 was larger than that in the pure ILs under the same conditions,that is,the former was less prone to corrosion than the latter.In addition,the addition of PEG200 caused a positive shift in the self-corrosion potential(E_corr)of the sample in the solution,an increase in the pitting potential（E_b）and a decrease in the self-corrosion current(I_corr),all of which indicated that the addition of PEG200 in[emim][Tf₂N]could effectively slow down the corrosion of 316L stainless steel by solvent.4.Fitting regression of physical parameters of ionic liquid[emim][FAP]was carried out by Aspen Plus software.NRTL-RK model was used to simulate the vapor-liquid equilibrium of CO₂-[emim][FAP]binary system,and the simulation results were compared with the experimental results.Based on the theoretical simulation results of binary system,the calculation equation and related parameters applicable to the ternary system were derived,and the vapor-liquid equilibrium data of CO₂ and 90%[emim][FAP]+10%PEG200 complex solvent were obtained by theoretical calculations,and the results were compared with the experimental data.The experimental results showed that the results obtained by the NRTL-RK model were in good agreement with the experimental results when the vapor-liquid equilibrium of CO₂-ionic liquid system was simulated by Aspen software.The average absolute relative deviation（AARD%）between the molar fraction of CO₂（that is,the solubility of CO₂）in 90%[emim][FAP]+10%PEG200 and the experimental value in this paper is 6.954%at the maximum and 3.315%at the minimum,which is within 7%and the overall deviation is small.It shows that the equation derived from the obtained data of CO₂-[emim][FAP]vapor-liquid equilibrium theory can be used to calculate the phase equilibrium of the complex solvent under the experimental conditions in this paper after it is improved,and the calculation results are satisfactory.This provides a reference for the theoretical calculation of vapor-liquid equilibrium of ternary substances not contained in Aspen software.5.Based on the experimental data,the numerical calculation of the kinetic parameters of CO₂ absorption by the complex solvent（including the reaction equilibrium constant,absorption enthalpy and diffusion coefficient）.The CO₂ absorption performance of complex solvent was analyzed and evaluated from the perspective of absorption kinetics.The results showed that the enthalpy of absorption of CO₂ by 90%[emim][FAP]+10%PEG200 complex solvent is 2.52 k J/mol lower than pure[emim][FAP],90%[hmim][FAP]+10%PEG200 complex solvent is 2.72 k J/mol lower than pure[hmim][FAP].It showed that the addition of PEG200 can reduce the absorption enthalpy and the system has good reversibility.In addition,the diffusion coefficient of the complex solvent was greater than that of the pure ionic liquid at each temperature,which indicated that the addition of PEG200 makes CO₂ have a stronger diffusion ability in the solvent and promotes the mass transfer effect of vapor-liquid.The calculation results of the absorption kinetic parameters of the CO₂-ILs system can provide reference for the design and optimization of the system process conditions in the future.