Analysis Of Characteristics Of Ammonia + Water + Lithium Bromide Mixture In Vapor-liquid Equilibrium And Its Effect On GAX Absorption Cycle

In view of the high energy consumption of rectification of ammonia-water absorption system and the shortage of large equipment,scholars at home and abroad have proposed adding lithium bromide to the ammonia solution to form ammonia-water-lithium bromide ternary mixed working fluid instead,using the hydrophilic characteristic of lithium bromide to improve the concentration of the ammonia component in vapor phase,thereby reducing the energy consumption of rectification and reducing the volume of system equipment.The operating characteristics of the ternary working fluid absorption refrigeration are closely related to the phase equilibrium physical properties of the ternary working fluid.Therefore,this article intends to start from the basic theory of phase equilibrium and initially explore the calculation method of the phase equilibrium physical properties of the ternary working fluid in a wide range,and use it for system performance analysis.In this paper,the physical property parameters of the ammonia-water-lithium bromide ternary mixture in the vapor-liquid equilibrium state are deeply explored.The physical property data obtained from the existing experiments in the literature are numerically analyzed,and the activity coefficient method is used for the vapor-liquid equilibrium of the ternary mixture.The solvation model is used for the electrolyte dissolution process,while the NRTL equation is used to calculate the activity coefficients of the components and ionic complexes in the solution.Combined with the vapor phase model,the thermodynamic balance and the mass balance relationship of substances,a calculation model for the vapor-liquid equilibrium physical properties of the ammonia-water-lithium bromide ternary system is proposed.The comparison results show that the theoretical calculation data are basically consistent with the experimental results,and the calculated relative deviations of pressure and mole fraction of ammonia in vapor phase do not exceed 7%.Using the proposed calculation model for NH₃-H₂O-LiBr ternary mixed working fluid in the vapor-liquid equilibrium,this paper explores and explains the specific mechanism of lithium bromide in the mixed solution.The study found that under the action of a certain temperature,the solvation effect of LiBr and H₂O is significantly stronger than that of LiBr and NH₃;In the temperature range from 333 K to 413 K,with the initial mole fraction of lithium bromide increases,the solvation coefficient of LiBr and H₂O will gradually decrease,and the solvation coefficient of LiBr and NH₃ will increase very slowly,but when the initial mole fraction of lithium bromide（?）<0.4,the solvation coefficient of LiBr and H₂O is always maintained above 1.5,and the solvation coefficient of LiBr and NH₃ is always0<n<0.5;NH₃-H₂O-LiBr mixed working fluid can effectively overcome the shortcomings of high working pressure of the ammonia water binary system in terms of pressure-temperature characteristics.In the range from 293 K to 373 K,as the concentration of lithium bromide increases,the phase equilibrium pressure of the ternary working fluid will decrease;when the initial mass fraction of ammonia is constant in the liquid phase,the increase of temperature and the increase of lithium bromide concentration will both lead to the increase of ammonia concentration in the vapor phase.In the temperature range from 293 K to 433 K,when the initial mass fraction of ammonia in the liquid phase is low,the lithium bromide concentration has a significant effect on the ammonia concentration in the vapor phase,and when the initial mass fraction of ammonia in the liquid phase is more than 45%,under the conditions of high temperature,the mole fraction of ammonia in the vapor phase is close to 1,and the influence of lithium bromide on the concentration of components in the vapor phase gradually weakens.According to the physical property parameters of some state points in the cycle process obtained by the calculation model of NH₃-H₂O-LiBr ternary mixed working fluid in the vapor-liquid equilibrium,the performance analysis of the NH₃-H₂O-LiBr absorption refrigeration system based on the GAX cycle is carried out using Aspen Plus software.The research results show that the coefficient of performance of the GAX absorption cycle increases with the increase of the heat source temperature under certain design parameters.Under the design parameters of evaporating temperature₀=5℃,condensation temperaturet₀=39℃,and lithium bromide mass fractiont_h=10%,when the heat source temperature is higher than 113℃,compared with the general single-stage cycle,the existence of the heat exchanger can significantly improve the coefficient of performance of the ternary absorption refrigeration system.When the temperature of the heat source reaches 140℃or more,the coefficient of performance of the system is about 18%higher than that of the cycle without GAX heat exchange,and when the temperature of the heat source is lower than 113℃,compared with the general single-stage absorption cycle,the GAX cycle has no coefficient improvement advantage;the ternary absorption GAX cycle performance coefficient decreases with the increase of the cooling water temperature under certain design parameters,but when the cooling water rises to a certain temperature point,it will no longer have an effect on the coefficient of performance.Under the working conditions of evaporation temperature t₀=0℃and heat source temperature t_h=140℃,the critical temperature of cooling water is 50℃;as the lithium bromide concentration ω_LiBr increases,the COP of the absorption cycle increases first and then decreases.For each different working condition,there is an optimal lithium bromide concentration.