Effective pore volumes, definition

Internal resistance relates to the diffusion of the molecules from the external surface of the catalyst into the pore volume where the major part of the catalyst s surface is found. To determine the diffusion coefficients inside a porous space is not an easy task since they depend not only on the molecules diffusivity but also on the pore shape. In addition, surface diffusion should be taken into account. Data on protein migration obtained by confocal microscopy [8] definitely demonstrate that surface migration of the molecules is possible, even though the mechanism is not yet well understood. All the above-mentioned effects are combined in a definition of the so-called effective diffusivity [7]. 

In Figure lb, each figure is considered as a unique stracture, which is repeated to form the porous solid. In each model, the variation of the micropore volume with the pore width (w) has been calculated. (It must be noted that w is the effective pore width, according to definition of Kaneko et al [5]). 

If the catalyst particles are not completely wetted by the liquid phase and the pores consequently not completely filled with liquid phase (static holdup gives some indication of whether this is the case or not), the situation is considerably more complex. In addition to being a function of the Thiele modulus, the catalytic effectiveness will now depend on the fraction of external wetting, rjcs, and the fraction of pore volume filled with liquid, rji. Dudokovic [M.P. Dudokovic, Amer. Inst. Chem. Eng. Jl., 23, 940 (1977)] proposed a reasonable approach that accounts for all three factors. If the reaction proceeds only on the catalyst surface effectively wetted by the liquid phase and components of the reaction mixture are nonvolatile, then one can in principle modify the definition of the Thiele modulus to... 

For applications in modem technology fields, not only high surface area and large pore volume, but also a sharp pore-size distribution at a definite size and control of strrface natirre of pore walls are strongly required. In order to control the pore stmcture in carbon materials, studies on the selection of prectrrsors and preparation conditions have been extensively carried out and certain successes have been achieved [ 1-3]. Pore sizes and their distributions in adsorbents have to comply with requirements from different applications. Thus, relatively small pores are needed for gas adsorption and relatively large pores for liquid adsorption, and a very narrow PSD is reqtrired for molecular sieving applications. Macropores in carbon materials were fotmd to be effective for sorption of viscous heavy oils. Recent novel techniques to control pore stmcture in carbon materials can be expected to contribute to overcome this limitation [41-46]. 

In resin-coated, impregnated, or laminated nonwoven composites, a small portion of the pores is not accessible (ie, not connected to the fabric surface), and the definition of porosity in above equation gives the so-called total porosity of the fabric. Thus, the open porosity (or effective porosity) is defined as the ratio of accessible pore volume to total fabric volume, which is a part of the total porosity of the fabric. 

Thus, there are two limitations of the pycnometric technique mentioned possible adsorption of guest molecules and a molecular sieving effect. It is noteworthy that some PSs, e.g., with a core-shell structure, can include some void volume that can be inaccessible to the guest molecules. In this case, the measured excluded volume will be the sum of the true volume of the solid phase and the volume of inaccessible pores. One should not absolutely equalize the true density and the density measured by a pycnometric technique (the pycnometric density) because of the three factors mentioned earlier. Conventionally, presenting the results of measurements one should define the conditions of a pycnometric experiment (at least the type of guest and temperature). For example, the definition p shows that the density was measured at 298 K using helium as a probe gas. Unfortunately, use of He as a pycnometric fluid is not a panacea since adsorption of He cannot be absolutely excluded by some PSs (e.g., carbons) even at 293 K (see van der Plas in Ref. [2]). Nevertheless, in most practically important cases the values of the true and pycnometric densities are very close [2,7],... 

They believed that the effect of dissolved solids in the pore fluid should not change the basic relationship of Se = w G. They also proposed that the definition of water content, in this case, should be the ratio of the mass of freshwater required to fill the volume occupied by the pore fluid (seawater) to the mass of the soil solids. Using this relationship results in the desired form of the equation S e = iv Gs. The primes indicate values corrected for salt content. The water content relationship is thus expressed as follows ... 

A porous medium may be described as a solid coutainii many holes and tortuous passages. The number of holes or pores is sufficiently great that a volume average is needed to estimate pertinent properties. Pores that occupy a definite fiuction of the bulk volume consdmte a complex network of voids. The manner in which holes or pores are embedded, the extent of their interconnection, and dieir location, size and shape characterize the porous medium. The term porosity refers to the fraction of the medium that contains voids. When a fluid is passed over the medium, the fraction of the medium (i.e., the pores) that contributes to the flow is referred to as the effective porosity. 

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