多孔介质中三组分气体扩散的网络分析

采用有效介质理论和平滑域近似的方法 ,对乙烯、氩气和二氧化碳在γ -Al2 O3 内的三组分扩散进行了网络分析 ,同时又用定态扩散实验加以验证 .研究结果表明 ,在确定了多孔介质孔结构参数 (平均配位数 )和孔尺寸参数 (孔径分布 )的前提下 ,通过网络分析的方法就可以获得有关多孔介质中多组分气体扩散的信息 .从而 ,建立起了用多孔介质微观性质描述宏观扩散过程的基本方法     

曲折因子与多孔介质微观结构的定性关系

采用三维网络模型对多孔介质中的两组分气体扩散过程进行了计算机模拟 .在考虑了多孔介质微观结构的前提下 ,分析了外界因素和多孔介质结构对曲折因子的影响 .结果表明 ,曲折因子仅与多孔介质的孔道连通性———网络配位数密切相关 ,而与孔道的几何尺寸、扩散温度、扩散物种及浓度无关 .经过Wicke -Kallenbach定态扩散实验验证了这一定性结论的可靠性 .     

间断等温边界下多孔介质方腔内非热平衡自然对流传热数值模拟

采用局部非热平衡模型,在方腔两侧壁面布置间断式等温边界,采用SIMPLER算法数值研究了固体骨架发热多孔介质方腔内的稳态非达西自然对流,探讨了6种等温边界布置方案、孔隙率ε及Da数对方腔内自然对流与传热的影响规律。计算结果表明:在左右对称的等温边界条件下,多孔介质方腔内的流场、温度场分布出现了左右对称分布特性。孔隙率ε及Da数的增加有利于提高多孔介质方腔的整体传热量,当Da数小于10-5时,传热量Q值随Da数变化不大,且不同等温边界布置方案的Q值差别不大。随着Da数的增大,多孔介质方腔内的热对流逐渐得到发展,此时不同传热方案的Q值出现了显著的差异,并随着Da数的增大而增加。     

气体有效扩散系数的分形模型

多孔介质中杂乱无序的孔分布结构特性显示了多孔介质具有孔分形结构。气体扩散在孔道中的分形模型是建立在分形理论和气体扩散特点的基础上。在考虑孔的大小分布、连通性分布和2种气体扩散机理的影响下,推导出了气体有效扩散系数和结构参数的关系。从参数分析中,可以得出有效扩散系数与孔的面积分形维数、孔隙率、联通数、气体自由扩散路径、孔最大最小直径比值成正比,与孔道迂曲度、迂曲分形维数成反比。通过实验数据与模型预测值对比验证了分形模型的正确性。     

多孔介质干燥的孔道网络试验及模拟

结合孔道网络及侵入渗流理论。试验并模拟研究了刚性多孔介质的干燥过程。观察了孔道网络内部湿组份的分布状态，孔道网络干燥前沿的形成及发展过程。验证了多孔介质在干燥过程出现的蒸发前沿及其具有的不规则特征。模拟研究了多孔介质在等温条件下不考虑重力时的干燥过程。通过模拟给出了多孔介质在干燥过程内部相的动态分布。模拟结果显示，干燥前沿具有非常不规则的特征。通过相图分析。干燥中内部涅分的迁移不仅是一个从里到外的过程。也同时存在逆向扩散。     

多孔介质中超临界流体可调特性的模拟与分析——温度梯度的作用

多孔介质中超临界流体的可调性与流体流动与传热耦合过程密切相关.今针对各向同性二维正方形区域建立了多孔介质中超临界流体的流动与传热耦合传递过程数学模型.以超临界CO2为模型流体,采用COMSOL Multiphysics软件对模型方程进行数值求解.分析和讨论了多孔介质中温度梯度对超临界流体的流动特性和传递性质的影响特性.研究表明:温度梯度显著地影响计算区域中流体的速度分布,近加热壁区的流体出现了速度的加速层.流体的动力黏度和导热系数等性质的分布特性也受计算区域中温度梯度的作用而存在极小值,导致局部流动性能的提高和传热性能的下降.对于多孔介质中超临界流体的可调性,可通过边界上的温度梯度加以调节和控制.     

侧壁不连续边界下多孔介质自然对流数值模拟

采用局部非热平衡模型,在方腔左侧壁面布置不连续等温边界条件,数值模拟了固体骨架发热多孔介质方腔内的稳态非达西自然对流传热,探讨了不同等温边界布置方案及方腔的高宽比M/L对方腔内自然对流传热的影响规律。计算结果表明:由于单向重力影响,多孔介质方腔内流函数结构呈现上下不对称特性,而固体相温度场分布呈现出上下对称分布规律;流体相局部Nu数的大小及分布规律与等温边界的位置相关,局部Nu数随着Y=0.5值的增大而增加,固体相局部Nu数则以Y=0.5处为中心呈现上下对称分布规律。存在一个最佳ξ值及高宽比值,使得多孔介质方腔内的整体向外传热量达到最大值,等温边界布置于方腔上侧更有利于强化方腔整体的自然对流传热。     

一种新的计算有效扩散系数的孔结构模型——双参数模型

建立的孔结构模型包含两个孔结构参数.确定模型参数时不涉及由常规方法测得的孔分布数据.该模型用于文献中以及作者的测试结果,表明能良好地反映多孔介质的孔结构特征.     

孔隙分布对多孔介质内流动和传热的影响

采用Sierpinski地毯分形技术建立多孔介质内流动和传热模型,通过改变固体基质位置研究了孔隙分布结构对多孔介质内流动特性和热效率的影响,3种孔隙分布为每分形一次固体基质分布在中心位置(A)、分布在中上方(B)和分布在右上方(C),当流体稳定流过多孔介质时,不同的孔隙分布表现出不同流动和传热特性. 结果表明,孔隙分布是影响多孔介质传输特性和传热效率的重要因素,无量纲渗透率k*C>k*B>k*A,多孔介质孔隙率大于0.8时更明显;流体流过不同孔隙分布的多孔介质时,相同孔隙率时与流体接触的固体基质面积A>B>C,传热效果A最佳、C最差. 孔隙分布影响了无量纲局部熵产率,在3种孔隙分布下用Be表示的热传导引起的熵产率占主导.     

两种毛管压力模型对比的储层孔隙结构分形研究

基于多孔介质分形理论进行Li-Horne毛管压力模型和Brooks-Corey毛管压力模型的推导,证明Brooks-Corey毛管压力模型为Li-Horne毛管压力模型的一种特殊形式。并利用两种毛管压力模型对北部湾盆地不同渗透率岩样的压汞曲线进行拟合,获得孔隙大小分布指数来评价储层的非均质程度,由于Brooks-Corey模型适用于均质储层,而Li-Horne模型对于均质、非均质储层均适用,所以Li-Horne模型拟合结果好于Brooks-Corey模型。同时,根据Li-Horne模型推导出多孔介质孔隙分布的分形概率模型,其指数与分形维数有关,利用该模型计算的孔喉尺寸分布与压汞曲线获得的结果一致。     

确定多孔介质孔结构参数的渗流网络分析法

According to the simulation of nitrogen sorption process in porous media with three-dimensional network model, and the analysis for such a process with percolation theory, a new method is proposed to determine a pore structure parameter--mean coordination number of pore network, which represents the connectivity among a great number of pores. Here the “chamber-throat“ model and the Weibull distribution are used to describe the pore geometry and the pore size distribution respectively. This method is based on the scaling law of percolation theory after both effects of sorption thermodynamics and pore size on the sorption hysteresis loops are considered. The results show that it is an effective procedure to calculate the mean coordination number for micro- and meso-porous media.     

Evaluation of mercury porosimeter experiments using a network pore structure model

A square network model has been developed to interpret mercury penetration and retraction behaviour in the widely employed mercury porosimetry technique for investigating pore structure and pore size distribution. A network of arbitrary size is constructed by assembling cylindrical pore segments of equal length and pseudo-random number generation is used to assign pore diameters according to any stipulated size distribution function. Application of the simple Washburn equation then predicts movement of mercury into the network under increasing pressure (penetration) and the corresponding withdrawal under reducing pressure (retraction) The network model is superior to the classical parallel bundle model, since it implicitly produces hysteresis between penetration and retraction, predicts that mercury entrapment on retraction is a result of interconnectedness of pore segments and provides a better estimate of the intrinsic distribution of segment sizes. Comparison with porosimeter experiments on a commercial hydrodesulphurisation catalyst show that the approach can be applied to practical measurements and the model may provide an improved basis for the study of diffusion, reaction and deactivation in catalyst pellets.     

Characterisation of porous solids using integrated nitrogen sorption and mercury porosimetry

The two different techniques of nitrogen sorption and mercury porosimetry, which are generally utilised completely separately, have been integrated into the same experiment to improve upon the information obtained from both methods. Nitrogen sorption isotherms have been run both before and after a mercury porosimetry experiment on the same sample. This experiment has revealed that for a particular type of sol-gel silica catalyst support the entrapped mercury is confined to only the very largest pores in the material. Light micrograph studies have shown that the spatial distribution of entrapped mercury is highly heterogeneous. These results suggest that mercury entrapment within the material is caused by a mechanism involving macroscopic () heterogeneities in the pore structure. These findings conflict with the usual assumptions generally made in simulations of porosimetry based on random pore bond network models. The new work has shown that, in conjunction with computer simulations involving the correct mercury retraction mechanism, mercury porosimetry and nitrogen sorption can be used to study the spatial distribution of all pore sizes within a mesoporous material. A percolation analysis of the nitrogen sorption data, obtained both before and after mercury entrapment, allowed broad features of the spatial disposition of variously sized pores to be determined. The results reported here also support the use of new, semi-empirical alternatives to the Washburn Equation to analyse raw mercury porosimetry data, rather than the traditional approach.     

Mercury porosimetry and the interpretation of pore geometry in sedimentary rocks and artificial models

The major objectives of this paper are to investigate how the form and hysteresis of mercury injection, withdrawal and reinjection capillary pressure curves are affected by the geometry of pores and their connections in samples of sedimentary rocks and also in artificial and theoretical pore-network models.In particular, those aspects of pore systems which may influence trapping of mercury during pressure reduction and withdrawal are considered. These are: pore to throat ratio, throat to pore coordination number or connectivity and the types and extent of random and non-random heterogeneities within the system.These aspects of pore systems influence the threshold pressure and the gradient of injection curves as well as the gradients and degree of hysteresis displayed by withdrawal and re-injection curves. Such curves are useful in interpreting pore geometry and give information which is valuable in assessing multiphase fluid behaviour in oil and gas reservoir rocks. In the case of water displacing oil or gas, in a strongly water-wet system, the trapping of oil or gas is controlled mainly by capillary forces and a direct analogy with the air-mercury system is possible.     

Influence of catalyst pore network structure on the hysteresis of multiphase reactions

The effects of the catalyst pore network structure on multiphase reactions in catalyst pellets are investigated by using the experimentally validated pore network model proposed in our recent work (AIChE J, 62 , 451, 2016). The simulations display hysteresis loops of the effectiveness factor. The hysteresis loop area becomes significantly larger, when having small volume‐averaged pore radius, wide pore‐size distribution, and low pore connectivity; however, the loop area is insensitive to pellet size, even though it affects the value of the effectiveness factor. The hysteresis loop area is also strongly affected by the spatial distribution of the pore size, in particular for a bimodal pore‐size distribution. The pore network structure directly influences mass transfer, capillary condensation, and pore blocking, and subsequently passes these influences on to the hysteresis loop of the effectiveness factor. Recognizing these effects is essential when designing porous catalysts for multiphase reaction processes. © 2016 American Institute of Chemical Engineers AIChE J, 63: 78–86, 2017     

Analyses of (AD)-sorption for the estimation of pore-network dimensions and structure

The interpretation of pore dimensions based on physical ad-desorption analyses is central to the characterization of pore network structure. Several approaches have been proposed and are commonly employed in the analysis of physical adsorption and/or desorption to deduce the dimensions of the porous network. These approaches assume either theoretical (e.g., BET, the Halsey equation as interpreted by Pierce et al., or the more recent analyses of microporosity) or standard isotherms as model(s) for the sequential calculations required in estimating the pore network dimensions. Subsequent representation of the pore dimensions and the relationship between these distributions in dimension and other experimental parameters (such as catalytic activity, adsorptivity or transport); thus, depend explicitly on the model employed in the analyses. Each instrument currently available for the measurement of porous solid structure by sorption employs the same specific models for the relationship between the volume ad-desorbed and the dimensions of the porous network that is being characterized.This paper analyzes the interpretation of porous dimensions based on the sequential calculations required in the analyses. A new approach is proposed which is based on a modification to current practices reflecting Halsey's original theory for the thickness of the adsorbed layer (as a function of P/P ₀). Further, the calculations of the incremental changes in the exposed surface area are discussed as they relate to pore network structure. A method is proposed to infer the differences in pore shape. Sorption data are analyzed by these new approaches, and these analyses will be compared with those approaches currently employed. Analyses based on these modified approaches provide a dramatically more consistent interpretation of the sorption data and the corresponding pore network structures.     

Determination of Both Mesopores and Macropores in Three-Dimensional Ordered Porous Materials by Nitrogen Adsorption

Three-dimensionally ordered silica structures containing both mesopores and macropores are created using polystyrene coacervate spheres with a diameter of ca. 146 nm. The close-packed polystyrene coacervate spheres are intercalated with tetraethyl orthosilicate. The spheres are removed by calcination leaving an inverse silica replica with a spherical macropore cavity diameter of 110 nm. Due to the nature of these porous structures, pores leading into the macropore cavity are in the mesopore regime, 40 nm in diameter. The nitrogen adsorption data described in the following paper gives a pore size for both the macropore cavity and the mesopore openings leading into the cavity. The pore sizes as determined by nitrogen sorption are in good agreement with the pore sizes observed by scanning electron microscopy. Mercury intrusion porosimetry results confirm the size of the mesopore openings leading into the macropore cavity, however due to destruction of the sample upon intrusion, extrusion results can not be obtained to determine main cavity diameters. As a result, nitrogen sorption may be a viable option for determining pore sizes with these three-dimensionally ordered materials containing both mesopores and macropores.     

Pore geometry and permeation characteristic of 3D–Cf/SiC composites fabricated via precursor infiltration and pyrolysis

To investigate the correlation of pore geometry and permeation characteristic, this paper evaluated the three-dimensional braided and/or woven carbon fabrics reinforced silicon carbide (3D–C_f/SiC) composites by mercury intrusion porosimetry, scanning electron microscopy and bubble point measurement. The flowrate–pressure curves of N₂ through C_f/SiC panels were measured by pressure apparatus at room temperature, then the flow modes conversion were analyzed, and permeability K was calculated. The pore geometry of 3D–C_f/SiC is supposed to be a three dimensional network composed of multi-sized interconnecting chambers, channels and cracks with sizes from microns to nanometers. The permeability prediction by porosity proves that the contents and sizes of the full open inter-bundle channels are the determinant factors for the intrinsic through-flow capability of the composite. The capillary bundle model displays feasibility to predict K when the actual full-open pore size distribution is obtained by appropriate means, such as bubble point method.