Porous solids simulation results

The results of simulations demonstrating the effect of various factors on nitrogen desorption from porous solids are presented in Figs. 16-18. To describe the desorption process, we use Eqs. (24) and (30). The adsorption branch of the isotherm is described by Eq. (18). The size distributions of necks and voids are assumed to be lognormal ... 

The results of simulations demonstrating the effect of various factors on mercury intrusion into porous solids are shown in Figs. 24-26. All the intrusion curves presented have been calculated for the same void radius distribution [Eq. (41) with v = 3000 A and = 0.5]. The mercury intrusion process is seen to start at higher pressures with decreasing Zo (Fig. 24), r (Fig. 25), and o- (Fig. 26). 

A method of using GCMC simulation in conjunction with percolation theory [74,75] has been suggested for simultaneous determination of the PSD and network connectivity of a porous solid [76]. In this method, isotherms are measured for a battery of adsorbate probe molecules of different sizes, e.g., CH4, CF4, and SFg. As illustrated in Fig. 9a, the smaller probe molecules are able to access regions of the pore volume that exclude the larger adsorbates. Consequently, each adsorbate samples a different portion of the adsorbent PSD, as shown in Fig. 9b. By combining the PSD results for the individual probe gases with a percolation model, an estimate of the mean connectivity number of the network can be obtained [76]. 

The heat transfer between the solid particles and the fluid follows the local thermal non-equilibrium model. The volumetric heat transfer coefficient hv represents the heat exchanger capacity of the porous media. In the current paper, we will investigate the volumetric convection heat transfer coefficient by numerical simulation results of the... 

Figure 6.4 uses the same simulation result as that shown in Fig. 6.2, with the fluid flow velocity profiles, where the flow in the porous package is defined by the Brinkman equation, but at different cross-section lines along the package. The solid line is the same as that in Fig. 6.2, i.e. the middle of the package (Plate I, y=0), the dotted line is near the top of the package (y=0.06) and the dashed line is near the bottom of the package (y=-0.06). 

We test three theories for adsorption and capillary condensation in pores against computer simulation results. They are the Kelvin equation, and two forms of density functional theory, the local density approximation (LDA) and the (nonlocal) smoothed density approximation (SDA) all three theories are of potential use in determining pore size distributions for raesoporous solids, while the LDA and SDA can also be applied to mlcroporous materials and to surface area determination. The SDA is found to be the most accurate theory, and has a much wider range of validity than the other two. The SDA is used to study the adsorption of methane and methane-ethane mixtures on models of porous carbon in which the pores are slit-shaped. We find that an optimum pore size and gas pressure exists that maximizes the excess adsorption for methane. For methane-ethane mixtures we show the variation of selectivity with pore size and temperature. 

It is well known that the energy of interaction of an atom with the continuous solid is 2-3 times less than with the discrete (atomic) model (cf., e.g., Ref. [38], Figs. 2.2-2.4). Thus, to obtain the same Henry s Law constants with the two models, one has to increase e for the continuous model. This, however, does not discredit the continuous model which is frequently used in adsorption calculations. In particular, we can use the above mentioned results of Ref. [37] to predict the value of e for Ar which would have been obtained if one had carried out Henry s Law constant calculations for Ar in the AO model of Ref. [17] and compared them with experiment. One can multiply the value of e for CH4 obtained from AO model by the ratio of e values for Ar and CH4 in the CM model [36] to obtain tjk = 165A for Ar in the AO model. This is very close to the value of 160 K obtained in Ref. [21, 28] by an independent method in which the value of the LJ parameter e for the Ar - oxide ion interaction was chosen to match the results of computer simulation of the adsorption isotherm on the nonporous heterogeneons surface of Ti02. Considering the independence of the calculations and the different character of the adsorbents (porous and nonporous), the closeness of the values of is remarkable (if it is not accidental). The result seems even more remarkable in the light of discussion presented in Ref. [28]. Another line of research has dealt with the influence of porous structure of the silica gel upon the temperature dependence of the Henry constants [36]. 

Fig. 18. PSDsfor model porous silica glasses [25]. A, B, C, and D are sample glasses prepared by quench MD the samples differ in mean pore size and porosity. The solid curves are exact geometric PSD results for the model adsorbents the dashed lines are the PSDs predicted from BJH pore size analysis of simulated nitrogen isotherms for the model porous glasses.

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