Experimental Study Of Vapor-Liquid Equilibrium For The Mixtures Containing CO₂ And High Boiling Point HFOs

With the rapid increase of energy consumption for building heating,the energy-saving and environmentally friendly heat pump has become one of the important alternatives to coal-fired heating in northern China.In order to take into account the environmental protection,safety and efficiency,this paper selects the vapor-liquid equilibrium of the mixed refrigerants（CO₂+R1234ze（Z）and CO₂+R1336mzz（E））as the research contents,which are suitable for the large-temperature-span heat pump.The theoretical and experimental researche results have laid a good foundation for the subsequent accurate simulation and design of the large-temperature-span heat pump.The main contents and conclusions of this paper are as follows:（1）We designed and built a VLE experimental apparatus suitable for CO₂+R1234ze（Z）and CO₂+R1336mzz（E）,and took several innovative measures to solve the problems caused by high pressure and high boiling point components.There are several problems that seriously affect the measurement accuracy.The reliability test results show that the VLE experimental apparatus has good reliability and accuracy.（Ⅰ）The problem of the condensation in the pressure measuring pipeline is solved by adding a constant temperature cover,and the reliability of the pressure is checked up by measuring the vapor pressure of R1234ze（Z）.When the temperature of the constant temperature cover is set to 30℃,the maximum relative deviations of R1234ze（Z）and CO₂ vapor pressure are 1.32%and 3.30%,respectively.（Ⅱ）A sampling method with a cache tube,a throttle valve and a pressure gauge is proposed,which can solve the problem of the excessive pressure and sample losses in sampling processes,and avoid the condensation of vapor phase sample and incomplete gasification of liquid phase sample in sampling processes,respectively.（Ⅲ）A vapor phase thermostatic sleeve is added to the vapor phase sampling pipeline to prevent the condensation of the vapor phase sample.The reliability test was conducted with CO₂+R600,and it was found that the higher the temperature of the vapor phase cache tube,the smaller the effect of vapor phase condensation.The maximum absolute deviation of vapor composition is 1.08%.Therefore,it is more appropriate for the temperature of the vapor phase thermostatic sleeve as 120°C.（Ⅳ）A liquid phase thermostatic sleeve is added on the liquid phase sampling pipeline to prevent the incomplete liquid phase gasification.Through the reliability test with CO₂+R600,it is found that the superheat of liquid phase thermostatic sleeve should not be too high or too low:60℃is more appropriate when the temperature of equilibrium cell is 20～10℃,and 40℃is more appropriate when the temperature of equilibrium cell is 20～60℃.When the appropriate superheat is taken,the maximum absolute deviation of liquid composition is 0.72%.（2）For the first time,the static-analytical method was addopted to measure 148groups of VLE experimental data of CO₂+R1234ze（Z）and CO₂+R1336mzz（E）at nine temperatures ranging from 253.15 K to 333.15 K and CO₂ liquid phase mole fraction in the range of 0～100%.The maximum uncertainty of the pressure measurement is about 6 k Pa,the uncertainty of the temperature measurement is about 0.1 K,and the maximum uncertainty of the composition measurement is about 0.009.（3）The PR,SRK,and PC-SAFT equations of state in Aspen Plus were used to fit the experimental data,which laid a good foundation for the accurate simulation and design of the large-temperature-span heat pump using the mixed working fluid.The isothermal-curve shows that the two mixed working fluids are non-azeotropic mixtures with large temperature glide and low nonideality.The Gibbs free energy and-envelope are also calculated.The correlation results of different equations of state on the experimental data show that the PR and SRK equations of state can fit the experimental data well,and their fitting results are relatively close,while the PC-SAFT equation of state shows a poor fitting effect.