Pore structural models

Newson (1975) was among the first to develop a pore plugging model of demetallation to predict catalyst life. By using the pore structure model of Wheeler (1951), the pellet was assumed to have N pores of identical length but with a specified distribution of pore radii. Metal deposition was assumed to be a first-order reaction over an outer fraction of the pore length and to have a uniform thickness. This model showed that the broadness of the size distribution had little effect on the catalyst life for the same average radii, but that increasing the radii from 45 to 65 A more than doubled the catalyst life. The restricted form of the diffusivity (see Section IV,B,5) was not employed in this model. 

FIGURE 2.17 Scanning electron micrograph of an activated carbon and pore structure model. 

The Corrugated Pore Structure Model (CPSM), [8,9] was used to simulate the sorption data of both types of materials under consideration and the results are presented in Fig. 1 (continuous lines through the points). It is obvious from the plots of Fig. 1 that in all cases an excellent fit was achieved for the set of CPSM parameters reported in Table 1. 

Simulation of Nitrogen Sorption Data Using the Corrugated Pore Structure Model -CPSM... 

Additionally to the above, the so-called Corrugated Pore Structure Model CPSM [13,15,16] was employed to simulate the N2 adsorption - desorption experimental data. The intention is to work out true PSD curves based on a simulation of the entire hysteresis loop and to a single PSD prediction and thus to overcome the difficulty of choosing either one or the other branch of hysteresis loops, which result in di-similar predictions. The PSD predicted by the CPSM model (i.e. solid line PSDs in Fig. 2) indicate Dmax values that approach those... 

It is known that the effect of the surface area in the gasification of charcoal is intimately related to the very broad pore size distribution of this material. Random pore structure models accounting for the effects of pore growth and coalescence have been proposed by various authors and have often shown satisfactory agreement between theory and experiment, but none of the proposed kinetic relations describes the charcoal reactivity in the conversion range beyond X 0.7 satisfactorily. For the latter conversion... 

The relevance of the second approach stems from the possibility to use the same pore-structure model as used in description of the process in question (counter-current (isobaric) diffusion of simple gases, permeation of simple gases under steady-state or dynamic conditions, combined diffusion and permeation of gases under dynamic conditions, etc.). 

Differences in conductivity data of modified perfluorosulfonate membranes can be related to structural differences on the basis of the pore structure models. Smaller equivalent weights (e.g., for Nafion 105, Dow, Membrane C), that is, higher specific ion content, lead to superior performance compared to Nafion 117 due to narrower psds and, thus, more homogeneous water distributions. 

Variable and random parameters in pore structure modeling... 

The severe computational burden associated with assembling and carrying out adsorption calculations on disordered model microstructures for porous solids, such as those discussed in Sections ILA and II.B, has until recently limited the development of pore volume characterization methods in this direction. While the reahsm of these models is highly appealing, their application to experimental isotherm or scattering data for interpretation of adsorbent pore structure remains cumbersome due to the structural complexity of the models and the computational resources that must be brought to bear in their utilization. Consequently, approximate pore structure models, based upon simple pore shapes such as shts or cylinders, have been retained in popular use for pore volume characterization. 

McBratney, A.B., andC.J. Moran. 1993. Soil pore structure modeling using fuzzy random pseudofractal sets. p. 495-506. In A.J. Ringrose-Voase and G. Humphreys (ed.) Studies in management and genesis. Proc. 9th Int. Work. Meet. Soil Micromorphol., Townsville, Australia. July 1992. Elsevier, Amsterdam. 

Although the volume parameter can be measured directly, the characteristic pore size is always calculated from some measured physical quantities in terms of some pore structure model. Hence, the pore size distribution is not a well-defined property because, by and large, it depends very much on the particular method used in its determination. 

One-Dimensional Pore Structure Models and Pore Size. Experimental data invariably have been characterized in terms of an arbitrary model of pore structure. The most common method consists of a bundle of parallel capillary tubes of equal length and a distributed size. The pore size would be unique only if the pores were tubes of uniform size and cross-section or spherical bodies. As neither is the... 

Table III shows some tortuosity values for various sandstones, where rc is the tortuosity value computed from equation 61. We observe that the prediction by equation 61 is poor because of the heterogeneity of the sandstones. Hence, whenever possible, the measured value should be used. The tortuosity value can be estimated rather well from the pore size distribution results with a proper pore structure model (31).

Tortuosity and connectivity are difficult to relate to the nommiformity and anisotropy of a medium. Attempts to predict permeability from a pore structure model require information on tortuosity and connectivity. 

Dynamic methods rely on the study of fluid flow properties of porous membranes, which are extremely sensitive functions of the pore size distribution, f(r), and of additional pore structural characteristics, such as the pore connectivity, z. The dynamic data, if analyzed in combination with other measurements obtained by equilibrium methods, can offer important structural information, relevant to the evaluation of performance of membranes, provided that an appropriate pore structural model is used for the data interpretation. 

The pore structures of various solid are very anfractuous and multifarious, and their lag rings are different from each other. Der Booer considered that it would not exceed the five kinds of a, b, c, d, e as shown in Fig. 7.10. In the study of pore structure of catal3dic materials, the analysis of these five lag rings can help to provide useful information for the understanding of the pore structural characteristics and for the reasonable choice of the pore structural models. 

Figure 1. Fouling by wedge layering parallel bundle pore structure model...

Pore Structure Models (distributed, random,pore tree) Bhatia,Perlmutter (1980) Sotirchos,Yu (1985) Christman,Edgar (1983) Bhatia,Perlmutter (1981,83) Simons et al (1985,86,87) Bhatia (1985)... 

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