Preparation And Study Of The Hygrothermal Performance Of Zeolite-based Humidity Control Materials

As functional materials which are used to regulateindoor air relative humidity,thehumidity control materials are of great importance in a wide range of areas,such as building energy conservation,national relic protection,medical surveillance,etc.Due to the complex pore structures,each kind of humidity control material havephysical adsorption and chemical adsorption and nonlinear variation of humidity controlling properties,the heat and mass transfer characteristics of humidity control materialshas not yet fully understand on the macroscopic scale.As a kind of ideal humidity control material,natural zeolite hasexcellent humidity control characteristics of high porosity and bigger surface area.For these reasons,we preparethe zeolite based humidity control materials by modification of nature zeolite and apply the fractal theory,capillary condensation theory,random generation reconstruction and B value methodto investigate the hygrothermal performance of ZBHCM by experinment and numerical simulationand obtained the following conclusions:(1)Experimental study on humidity control performance optimizationof zeolite.A new humidity controlling materialwith great absorption and desorption is developed by the modification of microwave assisted ammonium chloride on natural zeolite.The microscope pore appearance of natural zeolite before and after modifications are described by scanning electron microscope(SEM),FTIR spectroscopy and BET surface area measurement,respectively.The analysis results that the modification of zeolite can effectively increase pore volume and form microscopic pores,thereby the moisture absorption and desorption characteristic of zeolite can be improved.Zeolites samples,which are modified by 3%saturated ammonium chloride and 100W microwave heating 3 minutes can significant enhance the humidity conditioning function.(2)Experimental study on preparation and structural characteristicsof zeolite based humidity control materials.Based on the prepared ZBHCM blocks with great absorption and desorption,the thermal and physical parameters of ZBHCM blocks are analyzed theoretically and experimentally,including vapor water uptake,water vapor transmission,capillary condensation and pore size distribution et.al.The fractal spectral dimension can effectively analyze and reflect pores connectivity of porous media from the microscopic perspective.The results show that the parameters of the modified zeolite based humidity controlling material,such as porosity,fractal spectral dimension and tortuosity fractal dimension are greater than that of the natural zeolite based humidity controlling material.The capillary condensation phenomenon of natural zeolite based humidity control materialsand modifiedzeolite based humidity control materials first appear in relative humidity of 0.74,aperture of 40nm and relative humidity of 0.62,aperture of 35nm.(3)Experimental study on humidity adjusting properties of zeolite based humidity control materials.To studies the humidity adjusting characteristics of ZBHCM,two-set experimental devices are designed including the static and dynamic systems.In the static systems,curves of isothermal adsorption are used as the evaluation index and analyzed the influence of ambient temperature and relative humidity on its humidification performance.In the dynamic systems,the capacityand rate ofmoisture absorption and desorption are used as the evaluation index and analyzed the effect on indoor air temperature,relative humidity and moisture content with or without the zeolite based humidity control materials under natural ventilationand air conditions.The results show that the increasing of ambient temperature has few effects on the moisture adsorption isotherms and moisture absorption/desorption velocity of ZBHCM in the static systems with constant temperature.Meanwhile,the relative humidity in the air contribute to the humidity adjusting characteristics of ZBHCM,with greater difference of moisture content between environment and pores in ZBHCM does more effective.Comparing the relative humidity in natural ventilation space with common walling materials,the relative humidity and humidity ratio change slowly and uniformly in natural ventilation space with ZBHCM.Also,the larger the area of humidity controlling materials is,the slower the change of relative humidity and humidity ratio in the experiment room.The analysis result indicates that ZBHCM assist with the interim operation of the air conditioner can help blunt the indoor air humidity parameters gyrations,such as ambient temperature,relative humidity and moisture content.(4)Numerical study on hygrothermal performanceof zeolite based humidity control materialsin macro scale.Considering the experimental results of physical parameter and humidity adjusting performance tests of ZBHCM,a one-dimensional heat and moisture migration mathematical model is established.The relationship between heat and moisture migration process within the pores and the indoor air humidity parameters are simulated.The results indicates that the humidity-controlling properties of ZBHCM is sensitive to pore structure,the adsorption and desorption curves are nonlinear variation along with the difference of pore structure.Materials with higher porosity have higher moisture absorbing and releasing property.Environmental temperature has hardly effect on the heat and moisture migration process of ZBHCM.Better humidity control performance demands greater concentration gradient differences in environment and pores of ZBHCM.(5)Numerical study on reconstruct and humidity adjusting performanceof zeolite based humidity control materials inmicro scale.Considering the experimental results of physical parameter of ZBHCM,different pore size and distributions are reconstructed by random growth method.Pore structure and pore size distribution were characterized to analyse the influence of different pore configuration on moisture diffusion process in humidity-controlling material.Simulated results show that materials with higher porosity have better pores connectivity and bigger connected area.With the increase of growth kernel probability,the skeleton and pore structure,pores connectivity become fine.The parameters of porous medium,such as the coefficient of moisture equilibrium diffusion,moisture concentration at node3 of z cross section in porous medium when equilibrium and the time when moisture diffusion process achieve balance can reflect the connectivity and moisture diffusion properties in pores of humidity controlling material.With the increase of moisture concentration variation at 3 nodes of z cross section in porous medium and the decrease of the time when moisture diffusion process achieve balance,the pores connectivity and moisture diffusion property in porous media get better.(6)Application study on zeolite based humidity control materials in buildings.Indoor air temperature,RH and moisture content in a enclosed space with/without ZBHCM and enclosed natural ventilation space with ZBHCM are simulated using the effective moisture penetration depth model of Energyplus under typical climateconditions of Nanjing in winter.The results show that humidity material can mitigate the fluctuation of indoor air parameters.The thickness of humidity material should be set to the optimal value.When the thickness of humidity material isdeviated from optimum thickness,the humidity control properties get weaker.The RH in natural ventilation space with 66m~2ZBHCM is lower than that of in the area with 39m~2 ZBHCM about 5.1%.In summary,the above researches systematically acquired the modification and preparation of ZBHCM,pore structure and characteristics of ZBHCM,establishment of heat and moisture migration mathematical model from aspects ofmacro scale and micro scale of ZBHCM,simulation when ZBHCM was applied to the inner side of buildings to get the humidity control mechanism.The current work in theory and experiments provide a experimental method and data support for further studies on other kinds of humidity control materials.