Hydrophilic Activated Carbon Preparation And Its Adsorption Performance For Water Vapor Analyzed By Experiments And Theoretical Simulation

Abstract:Some adsorbents,such as silica gel,molecular sieve and activated carbon, were usually used to remove water vapor in environment. In the recent years,there are many researches on water vapor adsorption in silica gel and molecular sieve,and corresponding adsorption theories are comparatively perfect.However, to water vapor adsorption in activated carbon,there still have many challenges to face and many problems to solve.Based on this situation, it is of great significance to studying the variation rule of water vapor adsorption in activated carbon, investigating the action mechanism of water molecules adsorbed on the surface of activated carbon, perfecting and optimizing relative adsorption theories for water vapor, and establishing simulation system to describe the change law of heat and mass transfer for water vapor in activated carbon adsorption column.To investigate the effect of different modification conditions to the properties of activated carbon,the activated carbon was pre-modified with nitric acid.Then,modified sample was divided into several portions, and was further modified at different oxygen concentration, temperature and time,respectively. The quantitative relations between modification conditions and BET surface area, total pore volume surface oxygen content were established by using the software of Design Expert7.0.The results indicate that the quadratic equations with the variable of oxygen concentration, temperature and time can well predict the values of property parameters of activated carbon.It is beneficial to form oxygen-containing functional group on the surface of activated carbon with the higher oxygen concentration,lower temperature and longer modification time.The activated carbon with the good properties are prepared by the two modes as follow:(1)In the extremely low oxygen concentration environment, activated carbon was modified in a higher temperature and a longer time;(2) In a higher oxygen concentration environment, activated carbon was modified in a lower temperature and a shorter time.The thermal stability of oxygen-containing functional groups on activated carbon surfaces in a thermal oxidative environment was studied. The raw activated carbon was first treated with nitric acid,and the resulting nitric acid-treated activated carbon (ACn) was further oxidized under2.5%O2(in N2) atmosphere at different temperatures.The types and the amount of oxygen-containing functional groups were analyzed by TGA, FTIR, Boehm titration, and XPS. Both oxygen-and nitrogen-containing functional groups were introduced onto the ACn surface.Under thermal oxidative conditions,hydroxyl was oxidized to the corresponding carboxyl group in the temperature range of378-473K, and epoxy groups and lactones were generated between573to773K via oxidation reactions between graphitized carbon and oxygen.In contrast, carboxyl decomposition occurred at around573K. Lactones, ketones, and quinones exhibited better thermal stability, undergoing decomposition between773to973K.Ether and epoxy groups exhibited the best thermal stability, decomposing only at temperatures above973K.Five different kinds of hydrophilic organic salts were used to modify commercial activated carbon in order to prepare hydrophilic carbon materials.Properties of the samples were analyzed by surface area analyzer and SEM-EDX.The hydrophilic organic salts,with the different properties, were introduced into activated carbon and significantly affected the properties of the samples.During adsorption experiments,the adsorption amount of modified samples for water vapor increased0.57to17.12times in temperature range from303to323K as well as relative pressure below0.50.Water molecules combined with surface hydrophilic groups through H-bonding exhibited good thermo stability. Effect of temperature, oxygen content and properties of the hydrophilic organic salts to water vapor adsorption were studied as well.It indicates that water vapor adsorption in modified samples are mainly effected by the surface oxygen content. The carboxylate radical in the hydrophilic organic salts greatly affect micropore structure of the modified samples, while the metal ion in them exhibit limited affection.Difference adsorption capacity of modified samples can be explained with the electronegativity of elements presented by Pauling.Hydrophilic-activated carbon was prepared by ester hydrolysis reactions,and was characterized by surface area analysis,FTIR, SEM-EDX, XRD and XPS.Hydrophilic groups that were introduced on activated carbon surface through ethyl acetate hydrolysis in sodium hydroxide were more efficient than those introduced with sodium acetate. During adsorption, the hydrophilic groups on modified activated carbon surface bound with water molecules through H-bonding and increased the adsorption capacity of water vapor. The adsorption isotherms of water vapor were well fitted by the Do model.Water molecules generated larger water clusters around the functional groups at303and313K.In addition, water desorption from the samples was analyzed by TGA.Water molecules that were hydrogen-bonded to functional groups exhibited higher thermal stability than those adsorbed in the micropore of activated carbon.Hydrophilic groups that were introduced on activated carbon surface through ethyl acetate hydrolysis in calcium hydroxide was used to verify the universal of method.The modified activated carbon were regenerated at different temperatures,and it exhibited best regeneration capabilities as the regenerating temperature was at398K, the regeneration rate is up to93.32%.Besides,the process of sodium acetate formation on the surface of modified activated carbon was discussed.A simplified model was developed to describe the water adsorption on activated carbon.The development of the simplified model was started from the original model proposed by DO and his co-workers.Two different kinds of carbon materials were prepared for water vapor adsorption,and the adsorption experiments were conducted at different temperatures (293-323K) and relative humidities (5-99%) to test the model.It is shown that the amount of adsorbed water vapor in micropore decreases with the temperature increasing, and the water molecules form larger water clusters around the functional group as the temperature is up to a higher value.The simplified model describes reasonably well for all the experimental data. According to the fitted values,the parameters of simplified model were represented by the temperature and then the model was used to calculate the water adsorption amount at298K and308K. The results show that the model can get relatively accurate values to calculate the water vapor adsorption on activated carbon.A new model for calculating the adsorption equilibrium constant of water vapor in the micropores of activated carbon was established,the mechanism of which is based on the penetration of water clusters into micropores. Two kinds of carbon materials with significantly different pore and surface structures were prepared for water vapor adsorption, and the adsorption experiments were conducted in different conditions to test the accuracy of the new model.The new model was also applicable to water adsorption on bituminous-based activated carbon.Furthermore,being able to calculate the values for enthalpy of activation and entropy of activation, the new model can provide universal thermodynamic criteria for the water adsorption on activated carbon.A mathematical model of heat and mass transfer for water vapor adsorption on activated carbon was established.Effects of related parameters on the adsorption were studied.With temperature increasing, the diffusion speed of water molecules in adsorption column increased, the breakthrough time, as well as the change rate of column temperature decreased.The influence of model parameters on the rule of heat and mass transfer of the adsorption column were discussed.The mass transfer regularities of water/toluene and water/acetone mixed vapor during adsorption process were simulated as well,it indicates that the mass transfer model including Langmuir equation can well describe the mass transfer of mixed vapor in initial stage of adsorption. The adsorption performance of activated carbon for the three adsorbents follow the order of toluene>water>acetone, and its adsorption performance for all of three adsorbents decreased with temperature increasing.