Competitive Adsorption Of Freon-134a And Water Vapor On Chemical Protection Carbon Materials

HFC-134a is chemically stable,environmentally friendly,easy to be desorbed and detected in the low concentration range.It is often used as a tracer gas for non-destructive mechanical leakage detection of filter which is the core equipment of collective protection systems.The adsorption behavior of the tracer gas on the chemical protective carbon materials,especially the competitive adsorption process with water vapor（H₂O）on the chemical protective carbon materials,is the theoretical basis for establishing the non-destructive mechanical leakage detection method.In this paper,based on the systematic characterization of physical and chemical properties for the chemical protective carbon materials,the competitive adsorption process of HFC-134a and H₂O on the chemic a l protective carbon materials was researched from two aspects including adsorption equilibrium and adsorption kinetics by experimental research,model analysis and process simulation.The physical structure and surface chemical property of three kinds of chemic a l protective carbon materials,including AC,ASZM-T and ASC-T,were investigated by automatic specific surface area and porosity analyzer,X-ray photoelectron spectroscopy and so on.The results show that the order of specific surface area and pore volume is AC>ASC-T>ASZM-T.The oxygen-containing functional groups on the surface are mainly hydroxyl and carboxyl,and the order of relative percentage is ASC-T>ASZM-T>AC.The adsorption isotherms of single component H₂O and HFC-134a on three kinds of chemical protective carbon materials were measured by gravimetric vacuum vapor adsorption apparatus and a self-built micro-freon adsorption equilibrium test system based on volumetric method,respectively.The classical adsorption equilibrium mode ls were used to fit the single component adsorption data.The ideal solution theory（IAST）was used to calculate the two-component adsorption equilibrium.The competitive adsorption behavior of HFC-134a and H₂O under the equilibrium state was further discussed.The results show that:（1）All the adsorption isotherms of H₂O are type-V,the adsorption capacity is positively correlated with the concentration of surface oxyge n-containing functional groups in the range of P/P₀<0.7,and the pore volume in the range of P/P₀>0.7.All the adsorption isotherms of HFC-134a are type-I which belongs to physical adsorption.The adsorption capacity was positively correlated with pore volume and negatively correlated with temperature.（2）Micro-porous DD model and Mahle model could accurately fit the adsorption data in the range of P/P₀<0.4 and P/P₀>0.4,respectively.The adsorption isotherms of HFC-134a in the low concentration range can be accurately fitted by DR equation.The adsorption characteristic curve acquired by DR equation plotting can predict the adsorption equilibrium data of HFC-134a at the room temperature.（3）The results calculated by IAST preliminarily revealed the competitive adsorption rule of HFC-134a and H₂O on ASZM-T and ASC-T:With the increase of H₂O content,the adsorbed amount of HFC-134a decreases.The reasons are mainly due to the effect of concentration,pore structure and surface oxygen-containing functional groups.The breakthrough curves of HFC-134a and H₂O on the fixed adsorption beds loade d by chemical protection carbon materials under different experimental conditions were measured by using a self-built adsorption kinetics evaluation system and a fiber bragg grating temperature evaluation system.The adsorption behavior of HFC-134a unde r different conditions,including the types of chemical protection carbon materials,airflow velocity,gas concentration,bed thickness,bed temperature,relative humidity and moisture content,were investigated.The variation of temperature field in the fixed bed during the adsorption process was analyzed.The competitive adsorption of HFC-134a and H₂O under dynamic conditions was discussed.The results show that:（1）The breakthrough time of HFC-134a on fixed bed is positively correlated with pore volume and bed thickness,and negatively correlated with airflow velocity,gas concentration and bed temperature.（2）The thermal effect of H₂O in the adsorption/desorption process was obvious,and the adsorption process of HFC-134a and H₂O was in a non-isothermal state during a certain period of time.（3）With the increase of relative humidity,the breakthrough time and the saturated adsorption capacity of HFC-134a both decreased,and the breakthrough curves appeared"Rolling up".The competitive adsorption process can be divided into four stages:co-adsorption,breakthrough,HFC-134a desorption and dynamic adsorption equilibrium.The reason can be analyzed:in the adsorption process,the heat released by H₂O adsorption causes the bed temperature rising up.And the adsorbed HFC-134a was desorbed resulting in"Rolling up"phenomenon.The larger relative humidity of bed is,the more significant thermal effect is,and the more obvious the phenomenon is.（4）With the increase of moisture content,the breakthrough time of HFC-134a decreases and the dynamic saturated adsorption capacity increases.The breakthrough curves appear"pseudo adsorption platform".The competitive adsorption process can be divided into four stages:co-adsorption,breakthrough,HFC-134a pseudo adsorption equilibrium and dynamic adsorption equilibrium.The reason can be analyze d:pre-adsorbed H₂O could be desorbed resulting in the bed temperature reducing and the adsorption capacity of HFC-134a increasing.At the moment,a large number of adsorption sites were released after H₂O desorption.So"pseudo-adsorption platform"phenomenon happened.The larger moisture content of bed is,the more signific a nt thermal effect is,and the more obvious the phenomenon is.（5）The strong interaction of H₂O and oxygen-containing function on the surface of impregnated carbon causes the incomplete desorption of H₂O.And a large number of residual H₂O attached to the pore wall,resulting in the pore volume increasing and the pore size distribution changing,further resulting in dynamic saturated adsorption capacity of HFC-134a increasing.The breakthrough curves and adsorption load curves of HFC-134a and H₂O in ASZM-T fixed bed under different experimental conditions were calculated by Aspen Plus simulation software.The results show that the software can simulate the breakthrough curves of HFC-134a and H₂O well under the condition that H₂O exists in the gas stream.The simulated adsorption load curve can describe the concentration distribution of the two adsorbents at different positions of the bed and any time in the dynamic adsorption process.Under the condition that H₂O exists in the fixed bed,there is a larger error between the simulation results and the experimental data.