| Absorption chillers and heat pumps are energy-saving and environmentally friendly devices which can be driven by low-temperature waste heat.LiBr/H2O and NH3/H2O are two widely used working fluids in the absorption cycle.However,due to the corrosion and crystallization problems of LiBr/H2O,toxicity and volatility of NH3,which limits their application.Therefore,many researchers are committed to the exploration of new working fluids in absorption cycle.Ionic liquids(ILs)are expected to be the new working fluid for absorption cycles due to their low melting point,low vapor pressure,high thermal stability and chemical stability.Among them,the binary solution of 1-Ethyl-3-methylimidazolium Dimethylphosphate([Emim][Dmp])-water has the potential to be new working fluids because of low vapor pressure,high heat capacity and negative excess enthalpy.But there is not much thermo-physical property data about the above binary solution,and the research on its microstructure is lacking.Therefore,the thermmo-physical properties and liquid structure,vapor/liquid interface of[Emim][Dmp]-H2O system were investigated by Molecular Dynamics(MD)simulations,and the relationship between structure and properties for the above binary solution is revealed,which can provide a theoretical basis for the search of new working fluids.Mass transfer between vapor/liquid two-phases are widely exists in chemical industry,such as gas dehumidification,liquid rectification.The vapor/liquid mass transfer performance of the absorber and generator in the absorption cycle also has an important influence on the performance of the equipment.Therefore,to study the mass transfer between vapor/liquid phases will help to find the control step during this mass transfer process,which provides theoretical guidance for macro-mass vapor/liquid transfer.However,up to now,there are few researches about the micro-vapor/liquid mass transfer process,so in this paper,the absorption and desorption processes of[Emim][Dmp]-H2O binary solution were studied at molecular level through MD simulation,and the time-dependent behavior of vapor/liquid phases in micro-absorption and desorption process was reproduced.The main contents and achievements of this paper are as follows:(1)The stable configuration of[Emim][Dmp]was optimized by Density Function Theory(DFT)and the binding energy between the anion-cation,[Emim][Dmp]and H2O was calculated.The electrostatic potential surface and the frontier molecular orbitals were analyzed.The results of infrared vibration frequency and attenuation total reflection spectrum were compared,and the hydrogen bond strength in the system was analyzed.It is found that the binding energy of[Emim][Dmp]to H2O is much larger than the hydrogen bond energy between water molecules,making[Emim][Dmp]and H2O soluble with each other at any mole fracrtion.In[Emim][Dmp](1)-H2O(2)binary solution,when the molar fraction of water in is less than 0.4(x2≤0.4),the structure of the binary system is similar to that of the pure ionic liquid.When the molar fraction of water is greater than 0.6(x2≥0.6),the structure of the binary system is similar to that of pure water.(2)The force field parameters of[Emim][Dmp]were constructed based on OPLS_AA force field and DFT calculation results,and the properties and structure of[Emim][Dmp](1)-H2O(2)binary solution for liquid phase were studied by MD simulations.It is found that the relative deviation between the simulated value and the experimental value of each system density is 3%;the self-diffusion coefficient increases significantly with the increase of temperature,and the viscosity decreases with the increase of temperature.In binary solution,when 0.6,its properties are similar to those of pure ionic liquid;when x2≥0.8,its properties are similar to water.The reason for the above properties is that one anion can form hydrogen bonds with two water molecules,and the hydrogen bond formed between H atom in water molecule and the double bond oxygen atoms(021/022)in anion is the strongest;this hydrogen bond also makes[Emim][Dmp]miscible with H2O,so[Emim][Dmp]-H2O can be used as working fluids for absorption cycle.(3)MD simulation results of vapor/liquid interface showed that the relative deviation between simulated surface tension and experiment data for each system is less than 20%.When[Emim][Dmp]is contained in the system,a thin layer with increased density is formed at the interface,and the density distribution of the liquid bulk has a large fluctuation.In the interface layer,the density distribution continuously transitions from the vapor phase to the liquid phase,and anions are arranged close to the vapor side with two methyl groups being toward the gas phase,and two double bond oxygen atoms being toward the liquid phase.Cations are arranged near the liquid bulk,with ethyl group being toward the vapor phase and methyl group being toward liquid bulk.Because the double bonded oxygen atoms in anions play the most important role in the absorption of vapor water molecules in vapor phase,the position and orientation distribution of the anions in the interface layer are more beneficial to the absorption of water molecules in the vapor phase.(4)MD simulations for absorption and desorption processes showed that absorption rate decreases with the increase of H2O mole fraction in binary solution,while desorption rate increases with the increase of H2O mole fraction.Water molecules in the vapor phase are absorbed into the interface within 2 ns after the absorption process started and a mass density gradient of H2O forms from the interface to the center of liquid bulk.The mass density gradient for[Emim][Dmp]is on the opposite trend to that of H2O in liquid bulk.The formation of mass density gradient is conducive to mass transfer in liquid phase.In the process of micro-absorption and desorption,the equilibrium time for liquid phase(about 8-14 ns)is longer than that of vapor phase(about 2 ns),which indicates that the resistance of micro-mass transfer process is mainly on the liquid side.Therefore,in order to strengthen the mass transfer process,the mass transfer in liquid phase should be strengthened first.In the process of two-component absorption,the two components interact with each other,and the insoluble component will hinder the diffusion of the soluble component in the vapor phase,thus increasing the time for the system to reach equilibrium and reducing the absorption rate. |
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