Pores pore diameter

Pores are classified into two types open pores, which connect to the outside of the material, and the closed pores, which are totally within the material. Penetrating pores are kind of open pores these have at least two openings located on two sides of a porous material. Penetrating pores are permeable for fluid, and therefore are important in applications such as filters. Many porous materials have been used in many applications. They are classified by many different criteria such as pore size, pore shape, materials and production methods. Classification by pore size and by pore shape is useful while considering the applications of porous materials. The classification of porous materials by pore size (according to Schaefer30) differentiates between microporous pores (pore diameter < 2 nm), mesoporous pores (2 nm < pore diameter <50 nm) and macroporous pores (pore diameter > 50 nm). 

Trade name or code Chemical composition Specific surface area (m /g) Mean pore Pore diameter volume Particle (A) (ml/g) shape ... 

Framework Type Channel System Pore Pore Diameter [A]... 

Commercial activated carbons are generally produced in granular, bead, pellet, or extrudate forms. The particles contain a complex network of meso-macro pores (pore diameters ranging between 30 A to several microns) and micropores (pore diameter <30 A) of different shapes and sizes. The larger pores act as arteries for the gas molecules to be transported from the external gas phase to the mouth of the micropores. Most of the adsorption capacity of a gas on the carbon is created by adsorption within the micropores. Figure 22.2 shows the cumulative pore size distribution of the carbons of Table 22.2 [18]. They were also obtained from the manufacturers data sheet. 

Fig. XVII-27. Nitrogen adsorption at 77 K for a series of M41S materials. Average pore diameters squares, 25 A triangles, 40 A circles, 45 A. Adsorption solid symbols desorption open symbols. The isotherms are normalized to the volume adsorbed at Pj = 0.9. (From Ref. 187. Reprinted with kind permission from Elsevier Science-NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.)...

Anotlier important modification metliod is tire passivation of tire external crystallite surface, which may improve perfonnance in shape selective catalysis (see C2.12.7). Treatment of zeolites witli alkoxysilanes, SiCl or silane, and subsequent hydrolysis or poisoning witli bulky bases, organophosphoms compounds and arylsilanes have been used for tliis purjDose [39]. In some cases, tire improved perfonnance was, however, not related to tire masking of unselective active sites on tire outer surface but ratlier to a narrowing of tire pore diameters due to silica deposits. 

The limiting cases of greatest interest correspond to conditions in which the mean free path lengths are large and small, respectively, compared with the pore diameters. Recall from the discussion in Chapter 3 that the effective Knudsen diffusion coefficients are proportional to pore diameter and independent of pressure, while the effective bulk diffusion coefficients are independent of pore diameter and inversely proportional to pressure. 

Thus, when the pore diameters are sufficiently small or the pressure suffi-e B... 

The Knudsen diffusion coefficients are given by equations (2.11) in which K. is independent of pressure and proportional to pore diameter a, so that we can write... 

The limiting form of the flux equations for large pore diameters or high pressure is best approached starting from equations (5.7) and (5.8). 

This determines the total flux at the li/nic of viscous flow. Equations (5.18 and (5.19) therefore describe the limiting form of the dusty gas model for high pressure or large pore diameters -- the limit of bulk diffusion control and viscous flow,... 

Consideration of the behavior of equations (5.26) and (5,27) when the pressure or the pore diameter becomes Large illustrates the care which must... 

It ls not surprising chat such a relation should hold at the Limit of Knudsen diffusion, since Che Knudsen diffusion coefficients are themselves inversely proportional to the square roots of molecular weights, but the pore diameters in Graham s stucco plugs were certainly many times larger chan the gaseous mean free path lengths at the experimental conditions. 

When the mean free paths are long compared with all pore diameters, condition (i) above determines the form of the flux relations completely in isothermal systems, and no further modelling is required if is regarded... 

Let us now assemble the complete set of dimensionless parameters for the problem. These are set out in Table 11.1, where the last column indicates the nature of their dependence on the external pressure p, the mean pore diameter and the pellet radius a. Symbols ft and 0... 

At Che opposite limit, where Che density Is high enough for mean free paths to be short con ared with pore diameters, the problem can be treated by continuum mechanics. In the simplest ease of a straight tube of circular cross-section, the fluid velocity field can easily be obtained by Integrating Che Nsvler-Stokes equations If an appropriate boundary condition at Che... 

These calculations lend theoretical support to the view arrived at earlier on phenomenological grounds, that adsorption in pores of molecular dimensions is sufficiently different from that in coarser pores to justify their assignment to a separate category as micropores. The calculations further indicate that the upper limit of size at which a pore begins to function as a micropore depends on the diameter a of the adsorbate molecule for slit-like pores this limit will lie at a width around I-So, but for pores which approximate to the cylindrical model it lies at a pore diameter around 2 5(t. The exact value of the limit will of course depend on the actual shape of the pore, and may well be raised by cooperative effects. 

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