Controllable Preparation Of Activated Carbon Fiber And Their Adsorption Properties For H₂O/N₂/O₂

In the human production process and living environment,air humidity has become an important parameter that affects industrial and agricultural production,environmental quality,and indoor comfort.Therefore,it is of great significance to control indoor humidity by solid dehumidification technique.Among the adsorbents for solid dehumidification,activated carbon fiber has a good prospect in dehumidification applications due to its large specific surface area,uniform micropore distribution and perfect stability.This paper aimed to prepare an adsorbent with efficient water vapor adsorption capacity at high humidity.The effects of pore structure characteristics and surface functional groups of two kinds of activated carbon fibers on dehumidification performance were investigated.Firstly,commercial activated carbon fibers were used as raw materials to improve the physicochemical property and adsorption performance by the vacuum activation method.Secondly,biomass-based activated carbon fibers were prepared by pre-oxidation and zinc chloride activation using natural fiber-palm fiber as raw material to use biomass resources and reduce material cost entirely.In the process of commercial activated carbon fiber activation modification or biomass-based activated carbon fiber preparation,a variable control method was employed to prepare the series of activated carbon fiber samples under different activation conditions（temperature,time and impregnation ratio）.The physicochemical properties of the activated carbon fibers were characterized by thermogravimetric analyzer,automatic specific surface and pore size analyzer,fourier transform infrared spectrometer and scanning electron microscope.A constant temperature and humidity test chamber was used to test the two activated carbon fibers’water vapor non-equilibrium adsorption properties.The relationships between pore structure,surface chemical properties of activated carbon fibers,and adsorption properties were investigated.Water vapor adsorption isotherms of activated carbon fibers were measured experimentally and fitted by the finite simplified model of the Do-Do adsorption model.The adsorption thermodynamics of water vapor on activated carbon fibers was analyzed by the Clausius-Clapeyron equation.Adsorption isotherms of nitrogen and oxygen were measured experimentally and fitted by the Henry adsorption model.Multi-component adsorption isotherms were fitted according to the ideal adsorbed solution theory,and the adsorption selectivity of water vapor to nitrogen and oxygen was calculated and analyzed.The adsorption mechanism of water vapor on activated carbon fiber was preliminarily elucidated.The main research conclusions are as follows:（1）Commercial activated carbon fiber（CACF）:The pore structure characteristics of CACFs were improved by vacuum activation,including specific surface area,pore volume,and pore size.However,the content of oxygen-containing functional groups（OFGs）was affected little.The water vapor adsorption capacity of modified CACF was significantly improved under high relative humidity conditions.Under the water vapor non-equilibrium adsorption conditions of 25°C&80%RH,the final adsorption capacity of CACF10-5 was 0.9817 g/g,which was 73.7%higher than that of A0（the final adsorption capacity of 0.5698 g/g）.（2）Palm-activated carbon fiber（PACF）:The pre-oxidation process removed part of the groups of palm fiber by pyrolysis,and a small amount of pore structure appeared.During the Zn Cl₂ activation process,many substances were removed by a reaction between Zn Cl₂ and pre-oxidized samples,and many cylindrical pores were formed inside PACF.The PACF with rich functional groups and good pore structure characteristics was constructed by pre-oxidation and Zn Cl₂ activation.Low-temperature activation was beneficial to retaining hydrophilic functional groups on the material’s surface,which promoted water vapor adsorption capacity and adsorption rate at low relative humidity.High-temperature activation and increasing impregnation ratio were beneficial to forming pore structure,thereby improving PACF water vapor adsorption capacity under high relative humidity.（3）At room temperature and atmospheric pressure,the two activated carbon fibers’nitrogen and oxygen adsorption capacities were extremely low.And the nitrogen and oxygen adsorption capacities in the low-pressure zone were fitted well by Henry’s law.The selectivity coefficients of H₂O/N₂ and H₂O/O₂ exceeded 1000,which indicated that the ideal adsorption solution theory could reasonably predict the adsorption isotherm of the ternary mixture（H₂O/N₂/O₂）.The water vapor adsorption on the two types of activated carbon fibers was dominant,and there was almost no competitive adsorption.（4）The adsorption processes of water vapor on two activated carbon fibers were similar.The adsorption steps were as follows:（a）initial adsorption of hydrophilic functional groups;（b）The formation,growth and aggregation of water clusters at the initial adsorption nucleation sites and subsequent micropore filling;（c）capillary condensation in mesopores.And water vapor adsorption process was exothermic and was dominated by physical adsorption,which was beneficial to achieve low energy consumption desorption and cyclic dehumidification.