氢在多壁碳纳米管上的等量吸附热

为说明氢在多壁碳纳米管(MWCNTs)上所受吸附作用的强弱,文中基于氢和77 K氮的吸附数据,比较了由非局域密度泛函理论(NDFT)、等量吸附线和归一化等温线线性化确定的等量吸附热。结果表明,氢在MWCNTs上等量吸附热随温度变化且表现出在弱的能量不均匀表面吸附的特点,平均值约为3—4 kJ/mol;由77 K氮吸附等温线确定的孔大小分布(PSD)和比表面积影响NDFT计算初始吸附时的精度;在吸附量和吸附温度都较低时,等量吸附线标绘结果与归一化等温线线性化方程确定的等量吸附热之间的偏差较小,在200—290 K温度区间则与ND-FT计算结果更为接近。然而,当温度大于310 K时,3种方法计算结果间的偏差较大。     

用高压气体吸脱附-微量热联用法对CO2和CH4在煤上吸附热力学的研究

为了从热力学角度揭示CO_2和CH_4在煤上的竞争吸附实质,为驱替理论提供热力学参考,利用高压气体吸脱附-微量热联用仪,通过容量法测定30℃,40℃,50℃温度下CO_2和CH_4在煤上的吸附等温线,同时计算等量吸附热和极限吸附热,并测定了CO_2和CH_4在煤上的吸附热。结果表明:CO_2和CH_4吸附等温线均属于Ⅰ型吸附等温线,且均符合Langmuir吸附模型;温度升高,CO_2和CH_4在煤上的吸附量均减小;相同条件下,CO_2在煤上的吸附量明显高于CH_4的吸附量,表明CO_2更容易吸附于煤上。CO_2和CH_4在煤上的等量吸附反映出煤吸附CO_2和CH_4的过程均为物理吸附。同时CO_2在煤上的等量吸附热高于CH_4的等量吸附热,说明CO_2分子与煤分子之间作用力强于CH_4与煤分子之间的作用力,极限吸附热和实验测得的吸附热也显示了同样结果,从热力学角度阐释了CO_2和CH_4在煤体表面竞争吸附的实质。     

氢在YK-1活性炭上的吸附平衡

运用容积法在温度区间113—293 K、压力范围0—12.5 MPa测定氢在椰壳活性炭YK-1上的吸附等温线,由等量吸附线标绘和低覆盖率区域等温线的亨利定律标绘确定等量吸附热和极限吸附热,引入格子理论Ono-Kondo方程对吸附等温线进行模型分析。结果表明,氢在YK-1活性炭上等量吸附热随吸附量的变化平缓,等量吸附热的平均值和极限值分别为4.64 kJ/mol和5.37 kJ/mol;基于Ono-Kondo模型的方程能较好地预测吸附等温线,氢在吸附空间的最大吸附容量随温度变化,其值比液氢在相同吸附空间的吸附容量小。须改善材料结构和降低储存系统温度才能提高活性炭的储氢性能。     

吸附法回收柠檬酸

用静态吸附法测定了四种大孔吸附树脂对柠檬酸的最大吸附量，用间歇法测定了不同温度下的吸附等温线，并筛选出一种最适宜的树脂进行了动态测试。用固定床吸附模型模拟了吸附动态过程。计算值与实验值基本符合。     

多孔蜂窝活性炭材料对CO2吸附热力学研究

以4种不同品牌多孔蜂窝活性炭为研究对象,采用N₂物理吸附、扫描电镜SEM、透射电镜TEM和XRF等方法对活性炭物化结构进行了表征,探讨了不同品牌蜂窝活性炭材料的成分和孔隙结构对CO₂吸附的影响。并通过对比0,20,40℃下CO₂的吸附等温线,分析CO₂在不同蜂窝活性炭材料表面的吸附热力学特性,拟合出CO₂气体吸附量与等量吸附热之间的关系。结果表明：N₂和CO₂吸附等温线属于Ⅰ型吸附等温线,样品主要以微孔结构为主,存在少量介孔结构;不同温度下活性炭吸附CO₂曲线均符合Langmuir吸附模型;温度升高,CO₂在活性炭材料上的吸附量均减小,说明升温不利于CO₂在活性炭上的吸附;相同条件下,泰州800活性炭(TZ800)的吸附量高于淄博800活性炭(ZB800)的吸附量,表明CO₂更易吸附于TZ800活性炭上,这可能得益于其丰富和发达的微孔结构。同时TZ...     

正戊烷和正己烷在5A分子筛上的液相吸附性能研究

测定了373~453K温度条件下正戊烷和正己烷在5A分子筛上的液相吸附平衡数据,用Langmuir方程和Freundlich方程对实验吸附等温线数据进行了拟合计算。结果表明,在453K温度下,正戊烷和正己烷在5A分子筛上仍可以达到7%左右的吸附容量,其吸附等温线符合Langmuir吸附等温类型,Langmuir吸附方程能够很好的拟合实验结果。Clapeyron-Clausius方程计算得到的正戊烷和正己烷在5A分子筛上吸附热、吸附熵等热力学数据表明,正戊烷、正己烷均易吸附于5A分子筛上,其吸附热为-24kJ/mol左右。     

乙醇在MIL-101上的吸附相平衡及其吸附机理

主要研究了MIL-101材料对乙醇的吸附性能和吸附机理。采用水热合成法制备了MIL-101(Cr),并分别应用N₂静态吸附、X射线粉末衍射(PXRD)、傅里叶红外光谱(FTIR)等分析手段对MIL-101晶形结构、孔隙结构参数进行分析表征。应用静态吸附法测定乙醇和水蒸气在不同温度下的吸附等温线,并讨论乙醇吸附在MIL-101(Cr) 4种吸附位的机理,根据吸附等温线估算出乙醇和水蒸气在MIL-101上的等量吸附热,并测试了乙醇在MIL-101上的吸附循环性能。研究表明,在298 K下,MIL-101的乙醇吸附容量为20.3 mmol·g^-1,远高于传统吸附材料。在低压下MIL-101对乙醇的吸附量高于水蒸气的吸附量,这是由于乙醇的偶极矩和分子动力学直径均比水大,使得乙醇分子在孔道中受到更大吸附力场作用;在低吸附量范围,乙醇在MIL-101上的等量吸附热要高于水蒸气的等量吸附热。在较高吸附压力条件下,主要发生多层吸附或孔填充,受吸附剂的孔容限制效应,尺寸越大的分子被吸附的物质的量会越少,由于乙醇的动力学直径(0.45 nm)大于水分子的动力学直径(0.268 nm),所以在较高吸附压力下乙醇在MIL-101上吸附量要小于水蒸气的吸附量。多次吸附脱附等温线测试显示MIL-101具有良好的乙醇吸附循环性能。     

柠檬酸改性柚子皮纤维素对废水中铜离子的吸附

采用静态平衡吸附法用柠檬酸纤维素(Cell-CA)吸附废水中Cu2+,考察了溶液的初始浓度、吸附剂颗粒大小、pH值、吸附时间和温度对吸附的影响,分析了吸附过程的热力学、动力学和等温吸附规律.结果表明,在pH值为5.5时Cell-CA对废水中Cu2+的吸附率为92.7%,吸附容量高达18.54mg/g;对不同温度下的吸附等温线采用Freundlich和Langmuir方程进行拟合,结果显示Langmuir方程的拟合效果更好.模拟吸附动力学实验数据符合准一级和准二级动力学模型,Cell-CA对Cu2+的吸附是放热反应.     

活性炭对甲苯的吸附及其等温线预测

以微波椰壳活性炭和微波再生后的活性炭吸附甲苯.测定了在20,30和40℃条件下甲苯在活性炭上的吸附等温线,采用Langmuir方程对实验数据进行拟合.结果表明,在30℃下椰壳活性炭和再生活性炭吸附甲苯的理论饱和吸附量分别为0.323 和0.273 g/g;采用Polanyi吸附势理论预测了苯在活性炭上的吸附等温线,其中...     

吸附制冷用复合吸附剂的吸附等温线推算

用真空重力法测定了水和乙醇在自制复合吸附剂上的吸附等温线。采用由微孔填充理论导出的吸附平衡方程对所测得的等温线进行了拟合,计算相应的吸附热;对用Clausius Clapeyron方程推算吸附等温线的方法进行了讨论,并推算有关吸附等温线。结果表明:水在自制复合吸附剂M1 0001和M1 9906上平衡吸附量的推算值与实测值之间的相对误差<10%;乙醇在自制复合吸附剂上的推算误差<15%。给出了推算等温线和获得等压线的简便方法。     

Experimental study and artificial neural network simulation of methane adsorption on activated carbon

The adsorption of methane on two activated carbons with different physical properties was measured. Adsorption isotherms were obtained by static volumetric method at different temperatures and pressures. The experimental results sow the best gas storage capacity was 113.5 V/V at temperature 280 K and pressure 8.5MPa on an activated carbon with surface area 1,060 m²/gr. An artificial neural network (ANN) based on genetic algorithm (GA) was used to predict amount of adsorption. The experimental data including input pressure, temperature and surface area of adsorbents as input parameters were used to create a GA-ANN simulation. The simulation results were compared with the experimental data and a good agreement was observed. The simulation was applied to calculate isosteric heat of adsorption by using the Clausius-Clapeyron equation. Comparison of the calculated adsorption heat showed different surface heterogeneity of the adsorbents.     

Experimental and theoretical study of methane adsorption on granular activated carbons

The experimental and theoretical study of methane adsorption on granular activated carbons is presented. The adsorption data are modeled by various isotherm equations. Toth equation is found to have the best fit. The isosteric heat decreases with loading and increases weakly with temperature, which is an indication of heterogeneity of the methane and granular activated carbon system. Using optimized parameters from Toth equation, a novel procedure is developed to calculate the integral heat of adsorption, which is the total amount of isosteric heat of adsorption at a given temperature and pressure during the adsorption process. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Effect of osmosis and ultrasound pretreatment on the moisture adsorption isotherms of quince

The effect of pretreatment osmotic–ultrasonic dehydration on the sorption isotherms of quince was determined by static gravimetric method at temperatures of 30 °C, 45 °C, and 60 °C. The curves obtained can be considered as type II according to the Brunauer–Emmett–Teller (BET) classification. Adsorption data were fitted into seven isotherm models. The best fit of the experimental data was obtained with Peleg for both fresh- and pretreated dried quince slices.Thermodynamic properties such as net isosteric heat, differential entropy, enthalpy–entropy compensation, and spreading pressure were determined from moisture adsorption isotherm data of quince. The net isosteric heat of sorption and differential entropy decreased with increasing moisture contents in an exponential function. A plot of differential heat versus entropy satisfied the enthalpy–entropy compensation theory. The spreading pressure increased with increasing water activity, and decreased with increasing temperature. The value of net isosteric heat, differential entropy and spreading pressure of untreated samples is higher than that of pretreated samples of quince.     

Analytic expressions for the isosteric heat of adsorption from adsorption isotherm models and two-dimensional SAFT-VR equation of state

The isosteric heat of adsorption is an important thermodynamic property used to characterize and optimize adsorption processes. In this work, analytic expressions for isosteric heats of adsorption are derived for a collection of commonly used isotherm models and a two-dimensional molecular equation of state based on the SAFT-VR approach. The use of these expressions is presented with an example of adsorption of nitrous oxide, N₂O, on biochar, which is a waste biomass charcoal that exhibits high adsorption potential. The results show that accurate fitting of the adsorption isotherms leads to consistent results obtained with different approaches; however, the predicted isosteric heat of adsorption exhibits strong variations in the regions where experimental data is insufficient such in the region of low pressure/low coverage. Convergence on the prediction of the isosteric heat of adsorption by the different models is only observed in the region where no extrapolation of experimental data is needed.     

甲烷在石墨化炭黑上吸附平衡的分子模拟

采用GCMC法对不同温度和压力下纯CH4在石墨化炭黑上的吸附平衡进行了分子模拟研究,选用Lennard-Jones 12—6势能函数和联合原子力场参数（TraPPE）对体系进行势能计算,并将吸附平衡预测结果与实验数据进行了比较分析。结果表明,在所研究的温度范围内,除了在低压的一定区间外,GCMC模拟结果在中压和高压条件下与实验数据基本吻合,表明采用GCMC方法模拟甲烷分子在石墨化炭黑上的吸附平衡数据是可以进行准确预测的。在此基础上,利用模拟的吸附平衡数据计算不同温度下CH4的Henry常数进而到得极限吸附热,计算结果与实验数据接近。     

Molecular modeling and adsorption properties of porous carbons

In this work, we calculate the adsorption isotherms and isosteric heat of argon in molecular models of saccharose coke obtained via the Hybrid Reverse Monte Carlo method. In the first route (method A), the molecular models were built by considering only carbon atoms, and all other heteroatoms present were neglected. In the second route (method B), the molecular models were built by considering carbon and hydrogen atoms. We find that the models obtained via method B have smaller pores as compared to the models obtained via method A. This is reflected in the adsorption properties. The amount adsorbed is less in models obtained via method B as compared to method A. We also find that the isosteric heat calculated in the models obtained via method B match the experimental data more closely as compared to models obtained from method A.     

THERMODYNAMIC ANALYSIS OF DRYING FOODS

Thermodynamic functions computed for food systems showing hysteresis indicate considerable differences. There appears to exist no qualitative agreement between the values obtained. Large differences in values between the adsorption an desorp-tion branches of isotherms make the absolute magnitude of the entropy and enthalpy changes uncertain. The desorption iso-steric heat was found to decrease with temperature while the adsorption branch of the isotherm for a few foods were found to exhibit temperature reversal effect giving higher values at low temperatures. Such trend may indicate either transition in the form of binding energy or the effect of temperature on structural alterations. In designing drying equipment, the isosteric heat of desorption will provide “a worst case” analysis as the true isosteric heat will lie somewhere between the adsorption and desorption isosteric heat. Any overcalculation in designing equipment by using the desorption branch of hysteresis data can only be in favor of increased throughput capability. The use of isosteric heat in freeze drying and in air drying calculations were undertaken and found to rise at a faster rate than the energy expended in moisture removal.     

THERMODYNAMIC ANALYSIS OF DRYING FOODS

Thermodynamic functions computed for food systems showing hysteresis indicate considerable differences. There appears to exist no qualitative agreement between the values obtained. Large differences in values between the adsorption an desorp-tion branches of isotherms make the absolute magnitude of the entropy and enthalpy changes uncertain. The desorption iso-steric heat was found to decrease with temperature while the adsorption branch of the isotherm for a few foods were found to exhibit temperature reversal effect giving higher values at low temperatures. Such trend may indicate either transition in the form of binding energy or the effect of temperature on structural alterations. In designing drying equipment, the isosteric heat of desorption will provide “a worst case” analysis as the true isosteric heat will lie somewhere between the adsorption and desorption isosteric heat. Any overcalculation in designing equipment by using the desorption branch of hysteresis data can only be in favor of increased throughput capability. The use of isosteric heat in freeze drying and in air drying calculations were undertaken and found to rise at a faster rate than the energy expended in moisture removal.     

Isosteric heats of adsorption in the Henry’s law region for carbon single wall cylindrical nanopores and spherical nanocavities

The isosteric heat of adsorption in the Henry’s law region is calculated as a function of the pore width for carbon single wall cylindrical nanopores and spherical nanocavities. The maximum isosteric heat of adsorption is obtained for six gas molecules: argon, methane, carbon dioxide, hydrogen, helium, and nitrogen. In addition, the results for cylindrical carbon nanopores are compared with adsorption data on single-wall carbon nanotubes from the literature. We find the pore width where the isosteric heat of adsorption is a maximum for both geometries. The effect of solid–fluid parameters on the pore diameter for the maximum isosteric heat of adsorption is determined for any system described by a Lennard–Jones potential. Constant relationships between the pore diameters for the maximum isosteric heat of adsorption and the specific solid–fluid parameters are found for cylindrical nanopores, spherical nanocavities, and parallel-wall slit-shaped pores. Surface mean curvature has a significant influence on the isosteric heat of adsorption.