Pore types straight cylindrical

There are two main types of single-pore models. In the first, the approach of Giner and Hunter is taken in which there are straight, cylindrical gas pores of a defined radius. These pores extend the... 

The most simple pore morphologies (Figs. 2.1A and B) are those of more or less straight cylindrical or conical-shaped pores. This type of pore is formed in so-called track etch and in "anopore" membranes. The latter is obtained by anodic oxidation of A1 metal foils and results in porous (amorphous) alumina (mesoporous) membranes. A detailed discussion is given by Burggraaf and Keizer in Ref. [1]. These types of membrane are useful for fundamental trans-... 

Fig. 9.2. Schematic representation of main types of pore structures and membrane architectures (A) straight cylindrical pores (B) straight as)onmetric pores (C) tortuous pore system.

When the pore diameter is small compared with the mean free path, the mode of gaseous diffusion takes on quite another aspect. Instead of colliding with its own type, the molecule will collide much more frequently with the wall. The molecule s progress down its pore will thus depend on pore geometry as well as on the physical characteristics of the diffusing substance. For a straight cylindrical pore of radius d, Knudsen showed that Fick s law could be used with a diffusion coefficient... 

Our discussion of the various types of diffusion has presumed that the diffusion takes place in a well-characterized pore stmcture, that is, straight cylindrical pores. However, the catalysts used in industry have extremely complex stmctures with interconnecting pores, tortuous pores, and wide variations in pore diameter as one moves along the length of the pore. Consequently, we need to convert the combined diffusivities that are appropriate for Knudsen and/or bulk diffusion into effective... 

For gas-solid systems with a low gas density in which gas molecules diffuse through long narrow pores, the mean free path of the molecules is much larger than the pore diameter. In this type of diffusion, so-called Knudsen diffusion, the transport properties are essentially determined by eollisions of the gas moleeules with the pore walls rather than by collisions with other gas molecules. Based on the kinetic theory of gases, in the Knudsen approach all rebounds are assumed to be governed by the cosine law of reflection. In a straight cylindrical pore of diameter d ore, the Knudsen diffusion coefficient Dk,i for component i with molecular mass Mi is given by... 

Figure 2.2 Schematic representation of the main types of membrane pores (a) isolated (b) dead-end (c) straight cylindrical (d) constricted (e) conical.

The other type of porous glass that has cylindrical pores is mesoporous silicate (MPS) (14,15). The advantage of MPS is in its feasibility to make a small pore diameter, typically below 10 nm. A columnar-phase liquid crystal, formed from surfactant molecules with a long alkyl chain tail and silicate molecules, is calcined to remove hydrocarbons. At the end, a hexagonal array of straight and uniform cylindrical holes is created in a crystalline order. MPS is not available commercially either. 

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