Novel ideas in the theory of the physical adsorption of vapors on micropore adsorbents

A general equation has been derived for the physical adsorption of vapors on adsorbents with nonhomogeneous microporous structure, in which the energy of adsorption is increased as a result of the effect of superposition of dispersion force fields of the opposite micropore walls. The equation is based on experimentally determined reciprocal dependence of the generalized dimensions of the micropores expressed as their radii of inertia and the characteristic adsorption energy. The expression of the inertia radii of micropores takes into account their geometrical sizes. The nonhomogeneity of the microporous structure is described in terms of normal distribution of the micropore volumes with respect to reciprocal values of the characteristic adsorption energies. For a homogeneous elementary micropore volume, the adsorption equation derived from the theory of volume filling of the micropores is used. The adsorption equation obtained contains three parameters: the total micropore volume, the characteristic adsorption energy at the maximum of the distribution curve and the dispersion. The equation can be used for micropores with various geometric shapes. It leads to the following consequences: If the dispersion equals zero, then the Dubinin-Radushkewich equation is obtained for homogeneous microporous structures; the Dubinin-Stoeckli equation is obtained for the model of slitlike micropores in carbonaceous adsorbents. The parameters of the equation were found to be effective quantities for increased mesopore surface areas in microporous adsorbents. We considered methods for determining the specific surface areas of mesopores; these methods are necessary for obtaining realistic parameters for the microporous structure of adsorbent.     

Interpretation of chromatographic peaks by fourier analysis

Extraction of model parameters, such as kinetic rate coefficients, equilibrium constants and transport rate coefficients, may be obtained by determining the coefficients of the Fourier series which describe chromatographic peaks. Error buildup due to noise occurs slowly with successive harmonics in contrast to the rapid buildup in successive moments which make higher moments unreliable for extracting model parameters. Fourier analysis is applied to the simplest case of axial dispersion in a non-porous, non-adsorbing packing and to the important case of adsorption on a porous packing. Expressions for the Fourier coefficients, amplitude ratios and phase lags are derived for the axial dispersion case, the Kubin-Kucera model for adsorption on a porous packing and, as well, for the case of axial dispersion and inter-particle diffusion without adsorption.     

Gas–particle partitioning of organic compounds in the atmosphere

Gas–particle partitioning of condensable organic compounds in the atmosphere is described using two methods. The first method is based on the use of a comprehensive mechanistic model of adsorption/absorption processes. The second method is based on aerosol yields estimates. The model parameters in the adsorption/absorption model are evaluated from experimental data. The concepts of concentration of adsorbed molecules on the surface of aerosol particles and diffusion of adsorbed molecules in the liquid phase are used for determining the importance of the adsorption/absorption mechanisms. The model calculations showed a qualitative agreement with available experimental data for alkanes. Furthermore, a modified version of the Carbon Bond IV chemical mechanism including an aerosol yields method to model the formation of organic aerosols in reactive plumes is used in combination with a plume dispersion model. The formation of secondary organic matter in plumes contributes significantly to the total secondary aerosol mass produced.     

Kneading and extrusion of dense polymer-cement pastes

The kneading and extrusion behavior of dense polymer-containing cement pastes has been investigated. Both the polymer to cement ratio (p/c) and the water to cement ratio (w/c) were varied. Three types of polymers - one adsorbing and two non-adsorbing on cement - were used. The kneading and extrusion parameters (dispersion energy, torque plateau value, extrusion pressure, paste ageing) were determined using instrumented kneader and ram extruder. As will be shown, the most important parameters controlling the efficiency of the kneading and extrusion processes are the cement volume fraction on the one hand and the polymer concentration in the interstitial solution on the other hand. Polymer adsorption is not a key phenomenon, whereas hydrodynamic lubrication is important. A simple free volume model for paste extrusion is proposed.     

Sorption dynamics in fixed‐beds of inert core spherical adsorbents including axial dispersion and Langmuir isotherm

The effects of axial dispersion and Langmuir isotherm on transient behavior of sorption and intraparticle diffusion in fixed‐beds packed with monodisperse shell‐type/inert core spherical sorbents are studied. The system of partial differential equations of the mathematical model is solved numerically using finite difference methods. Results are presented in the form of breakthrough curves for adsorption and desorption processes. Results reveal that the shape of the breakthrough curves is influenced by both hydrodynamic and kinetic factors. Hydrodynamic factor is governed by axial dispersion and is controlled by changes of Peclet number. Simulation results reveal that when linear adsorption isotherm is used, the effect of axial dispersion on breakthrough curves of the system is important for Peclet numbers smaller than 50, whereas, for Langmuir isotherm axial dispersion is considerable for Peclet numbers less than 80. In addition, effects of type of adsorption isotherms and size of adsorbents on breakthrough curves are investigated, and results are compared with existing reports in the pertinent literature. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Biofilter modeling for waste air treatment: Comparisons of inherent characteristics of biofilter models

The types of biofilter modeling may be primarily classified in accordance with whether a biofilm is differentiated from other phases in each model. It may be a secondary classification with regard to biofilter-modeling whether sorption volume and/or adsorption are adopted as reservoirs or not. Thirdly, biofilter models are classified as to whether adsorption is assumed to exist through gas phase and/or a biofilm. Among all the biofilter-models of previous investigators all model-components including gas phase, a biofilm, sorption volume and adsorption surface are considered only in the model of Lim. Since his model does not require a numerical solution but an algebraic solution to describe the concentration of organic pollutants in waste-air-streams along the height of a biofilter even under unsteady-state conditions, it satisfies the condition of simplicity that is one of the important model requirements. In spite of its simplicity, Lim's model predictions are fairly good to fit Hodge and Devinny’s experimental data.     

Role of Steric Hindrance in the Performance of Superplasticizers for Concrete

Interparticle potential energy calculations were performed to investigate the mechanisms by which a new class of concrete admixtures, generally referenced as poly(carboxylic acid)-type (PC) superplasticizers, which aid in dispersing cement particles, are formed. These calculations consisted of long-range Van der Waals, electrostatic, and steric interactions. The repulsive potential that resulted from electrostatic interactions was negligible, which would allow cement particles to flocculate in the absence of steric contributions. A model was developed to describe the adsorption behavior of these superplasticizers, which consisted of grafted polyethylene oxide (PEO) chains on a PC backbone on cement surfaces. Using this adsorption model, the influence of the length of the PEO molecular chain and the density per unit area on the steric contribution was quantified. Steric hindrance effects were the dominant stabilizing mechanism in this system. As expected, enhanced stability was observed with increasing adlayer thickness (and/or density). The results of this study may be useful in designing the molecular structure of this new and important class of dispersion aids for cement-based systems.     

A 3-zone model for protein adsorption kinetics in expanded beds

The adsorption behavior of expanded beds is more complex than that of fixed beds, since the adsorbent particle size, local bed voidage and liquid axial dispersion will vary axially with expanded height. Models applicable to fixed beds maybe not adequately describe the hydrodynamic and adsorption behavior in expanded beds. In this paper, a 3-zone model is developed, in which the model equations are written for the bottom zone, middle zone, and top zone of the column, respectively, and the model parameters, such as the adsorbent particle diameter, bed voidage and liquid axial dispersion coefficient, are zonal values. In-bed breakthrough curves are predicted by the 3-zone model, and tested against literature data for lysozyme adsorption on Streamline SP in an expanded bed.Model parametric sensitivity is analyzed. The effects of film mass transfer resistance, liquid axial dispersion and adsorbent axial dispersion on the breakthrough curves are weaker than that of protein intraparticle diffusion resistance for stable expanded beds. Adsorbent particle size axial distribution and bed voidage axial variation significantly affect in-bed breakthrough curves, therefore, model parameters should not be assigned uniform values over the whole column; instead the model should account for the adsorbent particle size axial distribution and bed voidage axial variation.     

ADSORPTION OF HEXACHLOROPLATINIC ACID ON γ-ALUMINA COATINGS FOR PREPARATION OF MONOLITHIC STRUCTURE CATALYSTS

The adsorption of hexachloroplatinic acid on γ-Alumina coatings is investigated in order to prepare monolithic structure catalysts for carbon monoxide oxidation. For acid concentrations in the range 0.25-1.5 mM the adsorption is examined in a batch process and a kinetic model is proposed and evaluated. The pH of the solution has a major impact on the adsorption rate and dispersion of Pt on Al₂O₃ and when acidity becomes lower than 1.5 pH value the deposition of Pt on the catalyst by pore volume impregnation is favourable. The catalytic properties of Pt-alumina as oxidation catalyst for CO depend strongly on the adsorption rate of H₂PtCI₆. A difference of 80 °C at the light off temperature is observed for catalysts prepared by adsorption from solutions with different acidity.     

Water vapor adsorption and the microporous structures of carbonaceous adsorbents

The principal role in adsorption of almost all vapors organic and inorganic substances on nonporous and microporous carbonaceous adsorbents is played by dispersion interactions. They are characterized by a considerable increase in adsorption potentials as a result of superposition of the fields of the opposite pore walls. This effect determines the entire specifics of adsorption in micropores and, in particular, the substantial increase in adsorbability. A theoretical estimate of the adsorption potentials of benzene and water in adsorption on graphite, and a comparison of the differential heats of adsorption of water vapors on a non-porous carbon black previously heated in a vacuum at 950°C and on an active carbon show that water adsorption is due to the formation of hydrogen bonds both between the oxygen complexes on the surface of carbonaceous adsorbents and between the adsorbed molecules themselves. Dispersion interactions are weak and can be neglected to a first approximation. It has been shown for microporous structures and the slitlike model that one can calculate, from the parameters W₀ and E₀ of the adsorption equation of the theory of volume filling of micropores (determined from the adsorption isotherm of a standard vapor, benzene) the volume of the micropores, their halfwidth, and the specific surface area of the micropore walls. The latter are in good agreement with the specific surface areas of the micropores, as estimated by the independent method of similarity of the adsorption isotherms of water in micropores and on the surface of a nonporous carbonaceous adsorbent. The application of the BET and t-methods to microporous carbonaceous adsorbents is physically unsubstantiated.     

EXERGETIC CRITERIA FOR THE DEVELOPMENT OF ABSORPTION HEAT PUMPS

Lumped parameter mass transfer models based upon a tanks-in-series representation of fixed bed adsorption columns are derived. The effects of external and internal diffusion and axial dispersion are incorporated in the model for both single and multisolute (competitive) adsorption. The advantages of this type of model as compared with distributed parameter models are discussed. The model is validated against experimental data for both the single solute and bi-solute adsorption.     

LUMPED PARAMETER MODELS OF ADSORPTION KINETICS IN FIXED BEDS

Lumped parameter mass transfer models based upon a tanks-in-series representation of fixed bed adsorption columns are derived. The effects of external and internal diffusion and axial dispersion are incorporated in the model for both single and multisolute (competitive) adsorption. The advantages of this type of model as compared with distributed parameter models are discussed. The model is validated against experimental data for both the single solute and bi-solute adsorption.     

EPA和DHA在超临界流体色谱柱中的传质动力学

用超临界流体色谱 (SFC)法研究了EPA和DHA在C18上的吸附和传质性能 .建立了色谱动力学模型并获得了模型参数 ,较好地拟合了实验流出曲线 .结果表明吸附平衡、轴向弥散、流体相传质和颗粒内有效扩散都是影响SFC分离EPA和DHA的重要因素     

An overview of the basic aspects of polymer adhesion. Part I: Fundamentals

Adhesion between two substrates is a complex phenomenon which at present is still not well understood. The important existing adhesion models (electrical, diffusion, thermodynamic adsorption, chemical, etc.) are reviewed in order to try to explain their mechanisms. Thermodynamic adsorption is now believed to be one of the most importnat mechanisms by which adhesion is achieved. Difusion and wetting are kinetic means in attaining good adsorption of a polymer at the interface. In the case of this model (thermodynamic adsorption), the notion of surface energy is developed and the importance of this property in the understanding of adhesion phenomena is emphasized. The methods of determining the surface characteristics of low and high energy solids are presented. The role played by acid-base interactions in adhesion is also mentioned.     

Using a Brush Copolymer as Efficient Dispersant for the Preparation of Highly Stabilized Ag Nanoparticles in Aqueous Suspensions

Silver (Ag) nanoparticle has extremely high surface energy and it is difficult to find an efficient dispersant to prevent its agglomeration in suspensions. A new brush copolymer, succinic anhydride modified epoxy-amine poly[(propylene oxide)-co-(ethylene oxide)]-grafted polymer (EPOA), which can efficiently disperse concentrated aqueous suspensions of Ag nanoparticles is revealed. The dispersion efficiency of EPOA for the dispersion of a 60 nm-Ag nanoparticles in aqueous suspension is studied by measuring its sedimentation and rheological behavior, and the results are compared with those of a commercially available dispersant, ammonium poly(acrylic acid) (PAA-NH₄). Interactions between the dispersants and the Ag nanoparticles are characterized by zeta potential and adsorption analyses. Theoretical calculations are conducted to clarify the adsorption and the dominant dispersion stabilization mechanisms of the dispersants. Compared with PAA-NH₄, EPOA obtains a higher stable suspension of Ag nanoparticles with less significant sedimentation over 1 month. The dispersion homogeneity of the suspension remains excellent even at an extremely high solid loading of 30–40 wt%. According to adsorption analysis, it is suggested that both EPOA and PAA-NH₄ adsorb via single-point attachment through the carboxyl group on the Ag surface. Based on theoretical calculations, the Ag nanoparticles are better stabilized by EPOA via an electrosteric dispersion mechanism.     

MODELING AEROSOL BOLUS DISPERSION IN HUMAN AIRWAYS

Aerosol bolus dispersion has attracted attention due to its value in non-invasive clinical diagnosis of lung function and also due to its influence on particle deposition in the pulmonary region of the lung. The two salient observations from bolus dispersion experiments are that (1) dispersion increases linearly with inhaled volume and (2) dispersion is higher in a diseased lung than a healthy one for similar volume penetration. A number of plausible causes for dispersion have been proposed and discussed in the literature. However, a clear physical mechanism responsible for dispersion is still not available. Using a simple hypothesis for convective mixing, we propose an approximate physically based model to quantify bolus dispersion in the human airways. We then develop a computational model to simulate dispersion in the human lung and compare the model predictions to experimental data. This simple model, employing no adjustable parameters, predicts many observations of bolus dispersion.     

基于有序多孔材料磷酸盐吸附剂的研究进展

过量的磷流入水体易导致水体富营养化等环境问题,吸附法由于操作方便、经济高效等特点被广泛应用于水体磷酸盐的去除。有序多孔材料具有孔道规则,比表面积大,孔容大等特点,其作为载体可有效提高活性物种的分散性,从而提高吸附剂吸附磷酸盐的效率。综述了基于不同有序多孔材料的吸附剂应用于水体磷酸盐去除的进展,主要包括有序多孔碳材料、有序多孔硅材料和金属有机框架。讨论了基于有序多孔材料吸附剂的磷酸盐吸附性能、主要吸附机理、影响因素及回收利用。总结了基于有序多孔材料吸附剂吸附磷酸盐存在的问题,展望了其未来研究方向及应用前景。     

基于有序多孔材料磷酸盐吸附剂的研究进展

过量的磷流入水体易导致水体富营养化等环境问题。吸附法由于操作方便、经济高效等特点被广泛应用于水体磷酸盐的去除。有序多孔材料具有孔道规则，比表面积大，孔容大等特点，其作为载体可有效提高活性物种的分散性，从而提高吸附剂去除磷酸盐的效率。该文综述了基于不同有序多孔材料合成的吸附剂应用于水体磷酸盐去除的进展，主要包括有序多孔碳材料、有序多孔硅材料和金属有机框架。讨论了基于有序多孔材料吸附剂的磷酸盐吸附性能、主要吸附机制、影响因素及回收利用等。总结了基于有序多孔材料吸附剂吸附磷酸盐存在的问题，展望了其未来研究方向及应用前景。     

Thermal modeling of activated carbon based adsorptive natural gas storage system

A homogeneous, isotropic porous matrix of activated carbon inside a portable steel cylinder is considered as the adsorption bed for natural gas (NG) which is idealized as pure methane for the purpose of simulation. The heat and fluid flow inside the porous adsorption bed are modeled using a volume averaging technique and Darcy-Brinkman formulation. The effective thermal conductivity of the activated carbon-methane system is calculated as a function of uptake according to the Luikov model. Heat generation due to the exothermic process of adsorption is considered. The governing equations are solved using an implicit finite volume method for the given boundary conditions. Three different models of adsorption are considered, namely (i) a no-flow model, (ii) flow model with uniform adsorption and (iii) a flow model with local adsorption. For each of these models, transient temperature profiles in the adsorption bed during the charging process are obtained, and the corresponding mass adsorption potentials are calculated. Parametric studies are performed to investigate the effects of gas inlet temperature and rate of charging on the maximum bed temperature and the time required to fill the cylinder.     

Y型沸石吸附噻吩的能量及结构异质性的分子模拟

采用GCMC方法计算了噻吩在全硅Y和NaY沸石上的吸附,通过变化不同的噻吩吸附量得到吸附能量的异质性,并基于主客体径向分布函数剖析了吸附能量的异质性。根据对噻吩分子与沸石之间能量的分析,噻吩分子与沸石之间的作用主要由短程的色散作用控制。NaY沸石上吸附表现出与全硅Y沸石明显不同的吸附位特征,钠离子的引入明显地增强了对噻吩的吸附作用。钠离子与噻吩之间的静电和色散作用是引起这两种不同化学组成沸石吸附热不同的重要因素。进一步对径向分布函数分析确定了NaY沸石中S—Na,CHsp2—Na之间存在独特的作用位。