Controllable Construction Of Activated Carbon Fiber From Palm Fiber And Its Water Vapor Adsorption Performance

The increasing shortage of conventional fossil energy and the deterioration of the global ecological environment have led to a low-carbon transition in the global energy market,which makes the development and use of biomass materials particularly important.At present,the commonly used adsorbents in the field of drying and dehumidification are silica gel,activated alumina and activated carbon,etc.There is a lack of research on the development and use of new drying adsorbents.Therefore,in combination with regional characteristics,this paper developed and prepared lignin-based carbon fiber with low-cost and widely sourced palm fiber waste as raw material and explored its value in the field of water vapor adsorption,in order to achieve the dual purpose of environmental protection and energy conservation.In this paper,the palm fiber was selected as a raw material and the chemical composition of palm fiber was determined.The lignin was successfully extracted from palm fiber using an ethanol organic solvent method.The effects of four extraction conditions,namely reaction time,extraction temperature,ethanol concentration and material-to-liquid ratio,on the lignin extraction rate were optimized by single-factor experiments and response surface methodology.The extracted lignin was also subjected to various characterization analysis.Immediately afterwards,the extracted lignin was blended with polyvinyl alcohol for electrostatic spinning.The spinnability of the spinning solution with different lignin contents was studied.Lignin-based carbon fibers（LCF）and lignin-based activated carbon fiber（LACF）were finally prepared by pre-oxidation,carbonization and activation of the electrostatic spun lignin-based raw fiber.At this stage,the effects of different charring temperatures and KOH activation ratios on the water vapor adsorption performance of LCF and LACF were investigated and the physicochemical properties of LCF and LACF were characterized.In order to further improve the adsorption performance of water vapor,the LACF was then loaded with calcium chloride by impregnation to obtain LACF-Ca.The water vapor adsorption isotherms and adsorption kinetics of the three prepared adsorbents LCF,LACF and LACF-Ca were tested at different temperatures using a dynamic adsorption experimental system.The adsorption isotherms were fitted using a finite simplified model of the Do-Do model.The lignin-based activated carbon fiber were analyzed for the adsorption of water vapor by fitting the corresponding kinetic curves to the Lagergren quasi first-order kinetic equation,quasi second-order kinetic equation and Bangham equation.The following conclusions were ultimately reached:（1）Palm fiber contains about 34.6%lignin.Through the optimization of single factor and response surface method,the optimum conditions for extracting lignin were as follows:reaction time 111 min,extraction temperature 174℃,ethanol concentration73%,solid-liquid ratio 1:16 g/m L.The response surface model predicted that the extraction rate was 55.9%.The experiment verifies that the extraction rate of lignin under the optimized conditions could reach 56.2%.The characterization analysis of lignin shows that the relative molecular weight of lignin is relatively small,the micro morphology is mainly spherical particles,and has good thermal stability.At the same time,it also contains rich functional groups.（2）In the preparation of electrostatic spinning raw fiber,it was found that the relative lignin content in the range of 30%-50%could form relatively good shape of raw fiber.However,the fiber continuity was higher and the fiber diameter was more uniform and fine only when the lignin ratio was 40%,the average fiber diameter reached about 336 nm.The BET specific surface area and pore volume of LCF and LACF obtained by charring at 700℃with a 4:1 activation ratio of KOH were the highest,also their saturation adsorption of water vapor was the largest,reaching 259.94mg/g and 401.25 mg/g,respectively.Through the analysis of Fourier transform infrared spectroscopy（FT-IR）and X-ray photoelectron spectroscopy（XPS）of LCF and LACF,it was found that LACF has more abundant hydrophilic oxygen-containing functional groups.（3）The DO-DO finite simplified model was used to fit the adsorption isotherms of the three adsorbents LCF,LACF and LACF-Ca at different temperatures,and the fitting coefficient R~2 was close to 1 indicating that the model could accurately fit the water vapor adsorption isotherms.According to the IUPAC classification,the adsorption isotherm of water vapor on activated carbon fiber belongs to the common V-shaped isotherm.It is clear that the saturation adsorption of water vapor by activated carbon fiber decreases with the increase of adsorption temperature,which indicates that the higher the adsorption temperature is,the more unfavorable the behavior of water vapor adsorption by activated carbon fiber.（4）From the results of water vapor adsorption kinetics,it can be seen that the adsorption rate of the three adsorbents on water vapor was the fastest in the early stage of adsorption,the adsorption amount increased slowly with time and eventually stabilized.Compared with LACF-Ca,both LCF and LACF had shorter equilibrium adsorption times at different temperatures,the adsorption equilibrium time decreased with increasing temperature.Among the fitting results of the three adsorption kinetic models,the correlation coefficient R~2 value of the Bangham kinetic model was the closest to 1,which indicated that the Bangham model can well describe the dynamic adsorption behavior of activated carbon fiber on water vapor.（5）The results of the isosteric heat of adsorption of water vapor indicate that the adsorption process of water vapor is exothermic.The isosteric heat of adsorption showed a sharp decrease followed by a slow increase with the increase of the cumulative adsorption of water vapor,the magnitude of the isosteric heat of adsorption of the three adsorbents was LACF-Ca>LACF>LCF.This proved that the adsorption of water vapor by activated carbon fiber is mainly physical adsorption in the early stage,and a significant chemisorption phenomenon appears in the later stage.