Performance Of Adsorption Refrigeration System Based On Metal Organic Framework MIL-101(Cr)/water Working Pair

MIL-101（Cr）is a novel micro-mesoporous material in MOFs.Due to its high specific surface area and high porosity,the researchers in the field of adsorption have paid much attention to it.In this paper,MIL-101（Cr）and water were paired as working fluids to apply to adsorption refrigeration technology.In addition,MIL-101（Cr）was modified by the hydrophilic amino group to improve the adsorption properties of water vapor at low and medium humidity.MIL-101（Cr）with high specific surface area was prepared by hydrothermal synthesis method,and characterized by X-ray diffraction patterns（XRD）,Fourier transform infrared spectroscopy（FTIR）,N₂ adsorption-desorption measurement,thermal gravimetric analysis（TGA）and scanning electron microscope（SEM）.In addition,adsorption isotherms and kinetics of water vapor over MIL-101（Cr）by gravimetric water adsorption analyzer were also experimentally investigated.The results of XRD（2θ=3.29°,5.92°,8.48°,9.12°,10.38°,16.54°）and FTIR(the peak at585cm^-1 is attributed to Cr-O vibrations)showed that the MIL-101（Cr）material was successfully synthesized.The N₂ adsorption-desorption isotherm showed that the N₂equilibrium adsorption of MIL-101（Cr）was 1039cm³/g,and the calculated BET specific surface area and pore volume were 2861m²/g and 1.608m³/g,respectively.The results of thermogravimetric curves showed that the synthesized MIL-101（Cr）can be stable to 270°C.The result of SEM showed that the MIL-101（Cr）presented octahedron configuration with smooth surface.The results of water vapor adsorption test showed that the maximum water vapor adsorption capacity of MIL-101（Cr）was1.1g/g at 308K,and the average adsorption heat was 28kJ/mol.The adsorption rate of water vapor of MIL-101（Cr）at 28.1mbar was much larger than that of other pressures.This paper established the basic adsorption cycle mathematical model based on MIL-101（Cr）/water as the working pair,and used Matlab/Simulink tools to model and simulate.The control variable method was mainly used to simulate the effect on the cooling capacity and the cycle COP when the desorption temperature,evaporation temperature,condensing temperature,and adsorption temperature change,the results indicated that when the desorption temperature reached 367K,the COP of the system reached a maximum value of 0.161,and then decreased with the increase of the desorption temperature.It was helpful to increase the COP of the basic adsorption cycle system by reducing the condensing temperature and the adsorption temperature,increasing the evaporation temperature.The adsorption performance test rig was built based on the liquid level method.By comparing the data of water vapor adsorption measured by water adsorption analyzer and the test bench,the error between them was 10.9%.When the adsorption temperature was 35°C,the evaporation temperature were 15°C and 20°C,respectively.The adsorption capacity of MIL-101（Cr）were 0.45g/g and 0.55g/g respectively,the SCP values were 42.7W/kg and 43.4W/kg respectively,the COP values of the system were 0.07 and 0.09 respectively.In this paper,A series of 3-[2-（2-aminoethylamino）ethylamino]propyl-trimet-hoxysilane（PAPTS）were grafted on coordinatively unsaturated metal centers ofMIL-101（Cr）by postsynthetic modification,the obtained product was named as PAPTS-MIL-101（Cr）,and characterized by XRD,FTIR,N₂ adsorption-desorption measurement,TGA and SEM.In addition,adsorption performance of water vapor of PAPTS-MIL-101（Cr）adsorbents were investigated by gravimetric water adsorption analyzer.The results showed that although the PAPTS-MIL-101（Cr）material has a lower specific surface area and pore volume compared with MIL-101（Cr）material,the PAPTS-MIL-101（Cr）material had higher water uptake capacities in a region of0.30<P/P₀<0.40:at a relative humidity of 0.3,the water vapor adsorption of PAPTS-MIL-101（Cr）-0.5,PAPTS-MIL-101（Cr）-0.75 and PAPTS-MIL-101（Cr）-1.0were comparable to MIL-101（Cr）increased by 6.8%,8.2%,and 16.4%,respectively.At a relative humidity of 0.4,the water vapor adsorption of PAPTS-MIL-101（Cr）-0.5and PAPTS-MIL-101（Cr）-0.75 were comparable to MIL-101（Cr）increased by 23.4%and 20.2%,respectively.