Influence of Water Vapor on the Adsorption of VOCs on Lignin‐Based Activated Carbons

Abstract

Activated carbons with a wide range of burn‐off degrees obtained from Eucalyptus kraft lignin have been used to study the influence of the presence of water vapor on VOCs adsorption. The amount adsorbed and the rate of adsorption of both benzene and water vapor increase with activated carbon burn‐off as a consequence of an increase of micropore volume, broadening of micropore size distribution and increasing development of meso‐ and macroporosity. Similar results were found for MEK and methanol. Benzene is only partially desorbed at the adsorption temperature and an appreciable amount of it remains in the carbon, most likely in the narrow micropores. On the contrary, water vapor is completely desorbed at the adsorption temperature and its adsorption profile clearly exhibits two steps with different adsorption rates, associated to water molecules adsorbed on the active sites given rise to cluster formation and further migration and filling of the micropores. Adsorption with mixtures of VOC and water vapor has been carried out. The total amount adsorbed by the carbon, near the equilibrium point, is higher than in the case of the stream containing only the VOC. The adsorption rates for the mixtures streams are similar to that for the corresponding streams containing only the VOC in the case of carbons with a well developed porous structure. However, the presence of water vapor increases the rate of adsorption on the activated carbons with narrower microporosity. Saturation of the activated carbon with water vapor prior to the adsorption of a mixture containing benzene and water vapor has shown little effect on the amount of benzene adsorbed, suggesting that water and benzene molecules are adsorbed in different sites on the carbon surface.     

气固界面吸附团簇分布及相变机制研究

为了深入探讨气固界面吸附过程中团簇分布及其演化规律,结合实验测量和理论分析研究了水蒸气在二氧化硅和石墨表面的吸附特征,获取了不同压比下的团簇分布,确定了吸附相变及润湿转变的临界条件。结果表明,实验测量结果和Zeta吸附理论预测吸附曲线吻合很好,明确了气体分子以团簇形式吸附在固体表面,在低压比区,小分子团簇和零吸附单元占主导地位,随着压比的增加,吸附团簇类型增多,当压比达到某一临界值时,吸附熵达到极大值,界面发生吸附相变。确定了零吸附情况下石墨和二氧化硅表面张力及界面润湿临界条件,润湿压比下,性质均一的大分子团簇聚集,形成类液膜润湿界面。     

硅胶嵌入多孔纸基对苯蒸气吸附性能

苯蒸气作为一种典型的有害VOC污染物，研究对其的高效净化处理具有重要意义。纸基材料是一种密度小、孔隙率高、机械强度高的多孔材料，分析以木浆纤维纸、陶瓷纤维纸作为多孔基材，通过多次硅溶胶浸泡的方法制得硅胶嵌入多孔纸基吸附材料样片。通过扫描电镜SEM研究两种纸基材料表面的形貌及结构，采用重量法蒸气吸附仪对不同上胶次数的两种基材样片进行吸附性能测试。测试结果表明：木浆纤维纸和陶瓷纤维纸6次上胶率高达3.38 g/g和8.66 g/g; 0.1分压苯蒸气下30 min样片吸附量即可达到最大吸附量的80%；两种硅胶嵌入多孔纸基样片吸附容量在0.8分压苯蒸气下均超过100 mg/g；在4次上胶时样片表现出最佳的苯蒸气吸附性能。此外，通过对实验结果进行分段拟合得到吸附特征曲线。     

市政污泥吸附等温线模型和热力学性质

采用静态重量法测定了市政污泥在30℃、40℃、50℃下的吸附等温线,选用11个常见的数学模型对实验数据进行了拟合并对最佳模型进行了解析,通过净等量吸附热q_st、微分熵ΔS、扩散压力π、净积分焓q_in和净积分熵ΔS_in等指标评价污泥的热力学性质。试验结果表明,在温度恒定时,等温曲线属于Ⅱ型,GAB模型拟合效果最佳,能较好地反映平衡含水量随水分活度的变化。应用Clausius-Clapeyron方程,利用等温线模型计算净等量吸附热和微分熵,随着平衡含水率的增加,净等量吸附热和微分熵明显降低,调和平均温度T_hm与等速温度T_l不等,焓-熵补偿理论成立。在一定的水活度下,扩散压力随温度的升高而减小,在温度恒定的情况下,扩张压力随水分活度增大而升高。净积分焓随平衡含水率的增加而减小,而净积分熵在低平衡含水率时随平衡含水率的增加而减小,在30℃、40℃和50℃时分别达到最小值-75.698J/(K?mol)、-78.987J/(K?mol)和-82.687J/(K?mol),然后呈上升趋势。     

Adsorption of Radon and Water Vapor on Commercial Activated Carbons

Abstract

Equilibrium adsorption isotherms are reported for radon and water vapor on two commercial activated carbons: coconut shell Type PCB and hardwood Type BD. The isotherms of the water vapor were measured gravimetrically at 298 K. The isotherms of radon from dry nitrogen were obtained at 293, 298, and 308 K while the data for the mixture of radon and water vapor were measured at 298 K. The concentrations of radon in the gas and solid phases were measured simultaneously, once the adsorption equilibrium and the radioactive equilibrium between the radon and its daughter products were established. The shape of the isotherms was of Type III for the radon and Type V for the water vapor, according to Brunauer's classification. The adsorption mechanism was similar for both the radon and the water vapor, being physical adsorption on the macropore surface area in the low pressure region and micropore filling near saturation pressure. The uptake capacity of radon decreased both with increasing temperature and relative humidity. The heat of adsorption data indicated that the PCB- and the BD-activated carbons provided a heterogeneous surface for radon adsorption. The equilibrium data for radon were correlated with a modified Freundlich equation.     

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。     

Moisture adsorption isotherms and thermodynamics properties of sucuk (Turkish dry-fermented sausage)

Moisture adsorption isotherms of sucuk were determined using the isopiestic method at 10 °C, 20 °C and 30 °C and within the range of 0.2-0.9 water activity. The isotherms of sucuk exhibited Type II behavior according to BET classification. The adsorption data were analyzed using mathematical equations of Halsey, Caurie, Peleg, Smith, Oswin, Henderson, Modified-BET, GAB, Ferro-Fontan and Harkins-Jura. The best fit of experimental data was obtained with Peleg equation in the range of temperatures and water activities investigated. Thermodynamic properties such as differential enthalpy and entropy, enthalpy-entropy compensation, spreading pressure, net integral enthalpy and entropy were determined from moisture adsorption isotherm data of sucuk. The net isosteric heat of sorption and differential entropy decreased with increasing moisture contents in an exponential function. The spreading pressure increased with increasing water activity, and decreased with increasing temperature. The net integral enthalpy decreased with increased moisture content. The integral entropy increased with moisture content, but it was negative in value. The enthalpy-entropy compensation theory was applied to water adsorption of sucuk.     

Maxwell-Stefan理论模拟NaA沸石膜分离一氯甲烷中微量水的渗透特性

通过热浸渍晶种法制备了高质量的NaA沸石膜,并将其应用于蒸汽渗透脱除一氯甲烷中的微量水. 实验结果表明,NaA膜对该体系显示了优异的分离性能,水对一氯甲烷的分离系数高达74831,产品中的水含量从0.2582%(w)降低到0.005%(w). 将基于Maxwell-Stenfan理论和Langmuir理想吸附理论推导的吸附-扩散模型用于模拟水渗透流速与渗透侧真空度和进料温度的关系,预测趋势与实验值吻合很好,且拟合得到的参数与文献报道较接近,表明水蒸汽在NaA沸石膜中的传递为表面扩散机制,水蒸汽的吸附对渗透速率的贡献很大. 水蒸汽的吸附热为-34.15 kJ/mol.     

Studies on water and nitrogen adsorption on hardened cement pastes. II-Thermodynamics of adsorption

Heats as well as entropies of water and nitrogen adsorption on hardened cement pastes of various pore structures were computed. Water vapor adsorption is accompanied by a decrease in integral entropy relative to the liquid state, while nitrogen adsorption is accompanied by an increase in integral entropy. The results could be related to the pore structures of the various pastes. For water adsorption, the molecules seem to be oriented in a definite array with more order than the liquid structure, and depending on the pore size, the normal liquid structure is resumed at some distance far from the surface. Nitrogen results could be described in terms of the inaccessibility and exclusion of the molecules from parts of the pore system.     

Enhanced sulfate formation on ozone-exposed soot

Gas phase experiments were conducted to examine the heterogeneous oxidation of sulfur dioxide on methane soot in the presence of ozone and water vapor. The enhanced formation of sulfate at atmospheric pressure was confirmed by the use of a particle-into-liquid sampler (PILS) coupled with ion chromatography (IC). Due to the ozone oxidation, multilayer adsorption of sulfur dioxide on soot, as well as sulfate formation and physisorption on secondary surface layer sites was observed. The ozone exposure also caused the soot to become hydrophilic, due to the formation of sulfuric acid and also likely the formation of carboxyl groups on the surface. For the soot and sulfur dioxide concentrations used, the sulfate yield increased with ozone levels, but no increase was observed at ozone concentrations above 2600 ppm.     

基于流固耦合的蒸汽发生器换热管结构应力分析

以核电站蒸汽发生器为研究对象,采用两流体模型描述二次侧汽液两相流动及沸腾换热,利用CFX对一、二次侧流体与换热管及支撑板的耦合流动换热进行三维数值模拟,在此基础上提取一、二次侧流体压力载荷,在Workbench平台中实现载荷向结构模型的传递,计算换热管结构应力。流固耦合计算结果表明：支撑板位置处二次侧流体流速迅速增大又快速减小,并且流体湍流耗散急剧增大造成二次侧流体压力损失增大;换热管压力应力取决于一、二次侧流体压差,整体平均压力应力约为58 MPa,与实际测量参数相符。     

The devolatilisation of millimetre sized coal particles at high heating rate: the influence of pressure on the structure and reactivity of the char

Most studies on the influence of pressure on the combustion of coal particles have shown that for a constant oxygen concentration, an increase of pressure leads to a decrease of combustion rate. Among the different phenomena, which can explain this behaviour, the influence of the devolatilisation pressure on the structure and reactivity of the char formed may be important. The aim of this paper was to obtain a quantitative characterisation of the physical and chemical structure of chars formed during pyrolysis under a large range of pressure. Experiments of single coal particle pyrolysis were conducted in a laser reactor with pressure ranging from 0.014 to 2.1 MPa in a nitrogen atmosphere. As expected, an increase of pressure lead to a decrease of the volatile matter yield, which can be related to the secondary reactions of volatile matter. A characterisation of the char was performed by gas adsorption methods: nitrogen adsorption, carbon dioxide adsorption and active surface area (ASA) measurement. True and apparent densities, porosities and swelling of the particles were also investigated. Although the volatile matter yield decreases, the porosity and the swelling of the char increases with increasing pyrolysis pressure. We observed an increase in surface area and microporosity with increasing pressures up to 0.6 MPa. The ASA surface also increases in this temperature range, but the ratio of ASA to CO₂ surfaces shows that the intrinsic reactivity of the surface decreases with increasing pyrolysis pressure.     

不同种类超临界流体异质结构及相变分析

为了从微观角度揭示不同种类超临界流体性质,对超临界流体氩和水进行分子动力学模拟。分析了温度和压力对不同种类超临界流体局部密度时序曲线波动,物理团簇及不同密度区占比的影响。模拟结果表明,两种超临界流体物理参数的变化具有较强的一致性。首先,密度时序曲线的均方根误差的最大值所对应的温度均偏离拟临界点,随着压力增大,近临界压力出现的“脊”也逐渐减弱或消失。定压工况,径向分布函数随温度的升高,峰值和谷值均减弱,呈现出从类液状态向类气状态过渡的规律,物理团簇的个数随着温度的升高逐渐增大,最大团簇原子数的占比随着温度升高逐渐减小。定温时,随着压力的增大,表现出相反的变化趋势,物理团簇和最大团簇的占比均与系统密度有较强的依赖关系。不同压力下,系统内平均密度区占比随温度的升高,呈现出先减小后增加的变化规律,且随压力的增大,整体均匀性增强。其次通过理论方法确定两种超临界流体两相区的起止温度,发现从类液状态向类气状态转变的相变焓随压力的增加逐渐增大,是压力的线性函数。最后根据系统熵和温度的关系,阐述熵对超临界流体有序性的影响,指出熵是驱动超临界流体相变的重要作用机制。     

Water vapor adsorption on chars and active carbons-oxygen sensors prepared from a tropical tree wood

Carbonizates and active carbons (AC) are useful as sensors of oxygen. Adsorption of water vapor on such materials, prepared from the wood of a tropical tree, Peltogyne from Surinam, was investigated. Reproducible and reversible water vapor isotherms were obtained of preliminarily unheated samples, as well as of heated samples after several adsorption-desorption cycles. Water vapor adsorption revealed very fine (ultramicropores) structure in the investigated materials. Such ultramicropores can not be detected by traditional adsorption of nitrogen at 77 K because of a molecular sieve blocking effect. Several carbonizates at high water vapor pressure exhibited large weight reduction. This weight loss is the result of expulsion of organic impurities from the porous structure under the influence of adsorbed water. These data were confirmed by mass-spectrometric thermodesorption analysis. X-ray data and water vapor adsorption isotherms indicated that the strongly bound water stabilizes the carbonizate structure. When heated at 623 K, the AC exhibited desorption of strongly bound water and modified AC structure.     

CO2和N2吸附的研究札记

Experiments were made for the adsorption of CO2 and N2 on typical adsorbents to investigate the effects of porous structure and surface affinity of adsorbents as well as those of adsorption temperature and pressure that might cause the variation of adsorption mechanism. It is shown that polar surface tends to enlarge the adsorption difference between CO2 and N2, and the difference is more sensitive to temperature than the adsorbents with non-polar surface. The adsorbents with non-polar surface are not much sensitive to the effect of water vapor, though the water vapor interferes the separation remarkably. The separation coefficient linearly increases with the micropore volume per unit surface area of activated carbons, but no rule is shown on mesoporous silicon materials. The function of adsorption mechanism on the separation is not as much as expected.     

Molecular simulation of temperature-programmed desorption

We use accelerated molecular dynamics to probe the thermal desorption of n-alkanes from the Au(1 1 1) and C(0 0 0 1) surfaces. Studying an alkane series ranging from CH₄ to C₁₆H₃₄, we find that the desorption prefactor increases with increasing chain length for the short chains until a certain chain length is reached when it becomes essentially constant and independent of chain length. We can understand the dependence of the preexponential factor on alkane chain length in terms of conformational changes within the alkane molecules. For the shorter molecules, the desorption temperatures probed in experimental temperature-programmed desorption studies lie below those for which torsional motion is activated. Thus, the short alkanes can be treated as rigid rods, and the loss in translational and rotational entropy upon adsorption leads to a preexponential factor that increases with increasing chain length. As the alkane chain length increases, the binding energy and the experimental desorption temperatures also increase. Thus, torsional motion is activated to an extent that increases with increasing chain length. This torsional activation increases the entropy of adsorption and counteracts the entropy decrease due to a loss of translation and rotation. This leads to a virtually constant prefactor for sufficiently long chains. Our findings are consistent with experimental data for the thermal desorption of alkanes from the Au(1 1 1), Pt(1 1 1), MgO(1 0 0) and C(0 0 0 1) surfaces.     

Kinetics of Anatase TiO2 Surface Area Reduction in a Mixture of HCl, H2O, and O2: l, Experimental Study

Changes in the surface area of anatase TiO₂ were investigated at 690 K as a function of the gas composition in mixtures of hydrogen chloride, water vapor, and oxygen. The chlorine content on the catalyst support was not dependent on the annealing time, which indicates that the adsorption equilibrium is reached very quickly. The kinetics of surface area were obtained as a function of partial pressure of HCI (from 0.1 to 40 kPa), water vapor (from 0.05 to 10 kPa), and oxygen (from 0.4 to 20 kPa). A mathematical expression for the rate of surface area loss was obtained which includes the partial pressure of water and the chlorine content at the surface of anatase.     

蒸发器沸腾两相段熵产分析

以管内侧制冷剂,管外侧水的蒸发换热器为研究对象,基于热力学第一、第二定律建立了蒸发器沸腾两相流段的熵产计算模型,用量纲1熵产数N_s表示管内外流体因温差传热和压降损失引起的不可逆损失。分析水侧、制冷剂侧质量流量,水侧入口温度和管长尺寸对系统熵产数的影响。结果表明：系统熵产主要由制冷剂侧两相压降引起;系统熵产数随水侧质量流率的增大迅速降低并趋于稳定,随制冷剂侧质量流率的增大呈指数增长,随水侧进口温度和管长尺寸的增大而增加。     

复合吸附剂MWCNT/MgCl2的水蒸气吸附性能

通过研磨将多壁碳纳米管分别与质量分数为30%、40%和50%的无水氯化镁复合，制备了3种不同配比的复合吸附剂MWCNT/MgCl₂。采用数字化扫描电子显微镜（SEM）观察复合吸附剂表面材质的结构样貌，通过Hot Disk热常数分析仪测得复合吸附剂的热导率，使用恒温恒湿箱选取具有代表性的温湿度，测试复合吸附剂在不同工况下的水蒸气吸附性能，并采用准二级动力学模型对25℃、50% RH工况下的实验数据进行拟合，应用Autosorb-IQ全自动气体分析仪测试了三种样品在25℃下的等温吸湿曲线。实验结果表明，相同温湿度工况下，随着氯化镁含量增加，复合吸附剂的吸附量提高，25℃、50% RH下氯化镁含量为30%、40%和50%的复合吸附剂M1、M2和M3的吸附量分别为0.62、0.79和0.94 g/g；恒定湿度为50% RH，温度变化为15~35℃时，复合吸附剂吸附量受温度和饱和水蒸气分压力的双重影响，表现为先增加后减小；温度固定为25℃，相对湿度从50% RH增加到80% RH时，复合吸附剂吸附量均大大提升；复合吸附剂在35℃、25% RH中高温、低湿条件下仍表现出较好的吸附能力；在相对压力P/P₀为0.3时，M1、M2和M3的吸附量分别为0.24、0.25和0.30 g/g，随着吸附压力的增加，复合吸附剂的吸附量也不断提升，最大吸附量分别达到3.54、3.75和4.42 g/g。复合吸附剂MWCNT/MgCl₂的制备研究，为吸附剂的性能研究提供了基础，对太阳能吸附式空气取水的研究具有潜在意义。