【作者】 姚小龙；

【导师】 李立清；

【作者基本信息】 中南大学 ， 土木工程， 2014， 博士

【摘要】 摘要：对于利用吸附法处理微环境中水蒸气时,常选用硅胶、分子筛、活性炭等材料作为吸附剂。有关水蒸气在硅胶与分子筛中的吸附机理研究较多,其吸附相关理论也已比较完善。而对于水蒸气在活性炭中吸附过程与机理还有很多问题需要认识与解决。鉴于此,研究活性炭吸附水蒸气的变化规律、深入探索活性炭在吸附水蒸气过程中的作用机理、完善与优化水蒸气在活性炭表面吸附相关理论、建立描述水蒸气吸附过程传热传质数学模拟体系具有重要的意义。本文以研究改性条件变化对活性炭物性影响作为出发点,研究了硝酸改性活性炭孔结构与表面氧含量在不同氧气浓度、温度与时间下改性后的变化规律。通过Design Expert7.0软件建立了改性条件与活性炭物性参数间的量化关系,探讨不同改性方法对活性炭物性的影响规律。结果表明：通过响应曲面法得到的二次方程可以准确对活性炭相关物性进行预测。较高的氧气浓度、较低的改性温度与较长的改性时间有利于增加活性炭表面氧含量。对活性炭物性参数优化可通过两种方式实现：(1)：在氧气浓度极低的环境中,适当提高改性温度与增加改性时间；(2)：增加氧气浓度,降低改性温度与缩短改性时间。在研究活性炭物性在热氧联合改性中变化规律的基础上,利用TGA、FTIR、Boehm滴定、XPS等测试手段,探讨了活性炭表面官能团在热氧环境中的变化规律。研究表明：硝酸改性活性炭表面会生成含氧、含氮官能团。酚羟基/羟基官能团在378-473K内氧化生成羧基官能团；本底碳在573-773K内氧化生成环氧基/内酯基官能团。羧基官能团的热稳定性最差,在573K时受热分解；内酯基／酮基/醌基官能团的热稳定性好于羧基官能团,分解温度范围在773-973K；醚基/环氧基官能团热稳定性最好,分解温度在973K以上利用五种不同的亲水性有机盐改性制备亲水性活性炭。通过孔径分析、SEM-EDX测试样品物性。结果表明：五种不同亲水性有机盐引入活性炭,制得物性各不相同的炭材料。在吸附实验中,相对压力小于0.50时,改性活性炭在303-323K内对水蒸气的吸附量相较原始活性炭增加了0.57-17.12倍。水分子与活性炭表面亲水性官能团以氢键连接,增加了其热稳定性。研究了吸附温度、表面氧含量、孔容、有机盐性质对水蒸气吸附性能的影响。发现改性活性炭对水蒸气的吸附性能主要受其表面氧含量的影响。有机盐中所含羧酸根会对活性炭微孔结构产生显著影响,而其所含金属离子对微孔影响有限。改性活性炭对水蒸气的吸附量差异可通过Pauling提出的电负性理论解释。在活性炭表面,采用乙酸乙酯-氢氧化钠碱性水解制备亲水性活性炭。利用孔径分析、FTIR、SEM-EDX、 XRD、XPS等测试活性炭物性。发现利用碱性水解法在活性炭表面引入亲水性官能团相较利用盐溶液直浸法更加有效。在吸附实验中,亲水性官能团与水分子以氢键连接,增大了活性炭对水蒸气的吸附量。利用DD模型可以准确拟合水蒸气等温吸附线。随着温度的升高,围绕官能团形成的水分子簇逐渐增大。通过TGA对水分子脱附性能进行研究。发现以氢键连接的水分子具有更好的热稳定性。利用乙酸乙酯-氢氧化钙碱性水解验证该方法的普适性。同时,讨论了该方法制备活性炭的再生吸附性能。发现MAC-398的再生吸附性能最好,再生率达到了93.32%。基于DD模型理论,提出含有较少参数,可直接测量活性炭对水蒸气吸附量的简化等温吸附模型。同时,基于假设条件,理论推导得到计算活性炭微孔吸附平衡常数的数学模型。利用不同物性活性炭吸附水蒸气,验证简化模型与微孔吸附平衡常数模型的准确性。结果表明：简化模型与微孔吸附平衡常数计算模型适用性良好。对于简化模型,通过有限次吸附实验,依次将其中参数与温度建立数学关系,得到以宏观量表示的简化模型,可实现其它工况下水蒸气吸附量的计算。对于微孔吸附平衡常数计算模型,可计算活性炭吸附水蒸气过程中的热力学参数,为活性炭吸附水蒸气性能提供普适性热力学判据。建立了活性炭吸附水蒸气传热传质数学模型,研究了实验过程中相关参数变化对吸附过程的影响。发现随着温度的升高,水分子吸附扩散速度加快,吸附穿透时间缩短,吸附柱温度变化率降低。讨论了模型参数变化对吸附柱传热传质规律的影响。尝试研究了水蒸气／甲苯与水蒸气／丙酮混合蒸气在吸附过程中传质规律变化。研究发现：以Langmuir方程为基础建立的传质模型可对吸附初始阶段传质过程较好描述。三种吸附质的吸附性能强弱依次为甲苯>水蒸气>丙酮。随着吸附温度的升高,活性炭对每种组分的吸附能力均下降。更多还原

【Abstract】 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.更多还原