Theoretical And Experimental Study On The Vapor-liquid Equilibria Of The Binary Mixtures Containing HFOs

Refrigerant is the blood of refrigeration,heat pump and air conditioning system,which directly affects the performance of the system.HFOs with pretty low GWP and short atmospheric lifetime is one kind of eco-friendly refrigerants,and attracts more and more attention gradually.However,compared with HFCs(such as R134 a,R410A,etc.),the latent heat of evaporation of HFOs is relatively lower.Moreover,most HFOs have certain flammability.In order to improve the safety and system performance of HFOs,many researchers mixed HFOs with other green and efficient refrigerants.In addition,HFOs also have the potential to be used as refrigerants in absorption systems,but there is little related literature.In the main heat exchanger devices of refrigeration,heat pump and air conditioning system,such as evaporator,condenser,generator,and absorber etc.,the vapor-liquid transition process of working fluid is involved.Therefore,the investigation of vapor-liquid equilibrium(VLE)of mixed working fluid is an important basis for the analysis on system performance.The excess free energy mixing rule can effectively couple the equation of state(Eo S)and the activity coefficient model(ACM),which is widely applied to the VLE calculation of mixed working fluid.However,there is still not a unified reference to choose sutiable excess free energy mixing rule,and the reproducibility of excess Gibbs free energy(gE)using excess free energy mixing rule has an important effect on the accuracy of the calculation for enthalpy,entropy and other properties.In view of the above situation,many experiments and theoretical studies have been carried out in this paper:(1)Aiming at the problem that the previous excess free energy mixing rule(MR1)can not be used in the high reduced temperature area,a modified mixing rule,MMR1,was proposed in this work.In the MMR1 mixing rule,the dimensionless volume parameter ui of pure substance was set as the limit value ui,lim at high reduced temperature.the MMR1 mixing rule could be successfully extended to the high reduced temperature area,which improved its application potential in phase equilibrium calculation;(2)The reproducibility of gE for different excess free energy mixing rules was analyzed.For the mixing rules referred at infinite reference pressure(like HV,WS,etc.),due to the difference between the excess properties at infinite pressure and those at low pressure,their reproducibilities were usually poor.For the approximate zero reference pressure mixing rules(like MHV1,MHV2,etc.)which were obtained by using some hypothesis on the basis of the rigorous zero reference pressure mixing rule(EXACT),they only showed the relatively close reproducibilities to EXACT mixing rule within the fitting interval of the constants in the rule.Outside the fitting interval,the reproducibilities would become worse.Generally,the MMR1 mixing rule,and the MR3 mixing rules proposed by our team in previous work,had better reproducibilities of gE than the approximate zero reference pressure mixing rules.However,in the high reduced temperature area,or for the asymmetric system in the molecular structure,the abilities of MMR1 and MR3 to reproduce gE would reduce;(3)Four VLE models with different excess free energy mixing rule,MHV1,WS,MMR1 and MR3,couplied with PR equation and NRTL model,were applied into the the VLE correlation calculation of various mixed working fluids(50 kinds of refrigerant + refrigerant and 15 kinds of refrigerant + organic solvent).The calculated results indicated that PR+MHV1+NRTL and PR+WS+NRTL had better accurancy than PR+MMR1 +NRTL and PR+MR3+NRTL,but showed much worse reproducibilities of gE than the latter two;(4)On the basis of the approximate equality between the excess Helmholtz free energy(aE)at infinite pressure and gE at zero pressure,and with the linear simplification between q(?)and ?,the new mixing rules,NMR-1 and NMR-2,were proposed.By adjusting the value of the term ln(7)(8)i m ii?x b b in the equation of state to replace the excess contribution term originally obtained by the activity coefficient model,the NMR-1 and NMR-2 mixing tule did not need to use any activity coefficient model to obtain gE or aE in the actual calculation.By applying the NMR-2 mixing rule into the VLE calculations of various mixture working fluids and solution systems,it was proofed that the NMR-2 mixing rule had a wider application range than the vd W mixing rule,and an excellent accuracy which was equal to or even better than the MHV1 mixing rule;(5)The VLE data of four mixtures(refrigerant + refrigerant)containing HFOs,R1234yf+R1216,R1234ze(E)+R1216,R1234yf+R1234ze(E),and R1336mzz(E)+ R32,were measured respectively.Three models,PR+NMR-1,PR+NMR-2,and PR+MHV1+ NRTL,were used to correlate the experimental data respectively.The results showed that the calculated results using the three models were all in good agreement with the experimental results,and the results using PR+NMR-2 were slightly better than those using PR+NMR-1;(6)The VLE data of two mixtures(refrigerant + organic solvent)containing HFOs,R1234ze(E)+NMP and R1234ze(E)+DMETr EG,were measured respectively.Three models,PR+NMR-1,PR+NMR-2,and PR+MHV1+NRTL were used to correlate the experimental data respectively.The results showed that the calculated results were in good agreement with the experimental results using PR+NMR-2,and its accuracy was equivalent to that using PR+MHV1+NRTL.In addition,the calculated results using PR+NMR-1 were not satisfactory.