Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Wall effects porous solid

Diffusion within the largest cavities of a porous medium is assumed to be similar to ordinary or bulk diffusion except that it is hindered by the pore walls (see Eq. 5-236). The tortuosity T that expresses this hindrance has been estimated from geometric arguments. Unfortunately, measured values are often an order of magnitude greater than those estimates. Thus, the effective diffusivity D f (and hence t) is normally determined by comparing a diffusion model to experimental measurements. The normal range of tortuosities for sihca gel, alumina, and other porous solids is 2 < T < 6, but for activated carbon, 5 < T < 65. [Pg.600]

Streaming Potential When the solution is forced through the porous solid under the effect of an external pressure P, the character of liquid motion in the cylindrical pores will be different from that in electroosmotic transport. Since the external pressure acts uniformly on the full pore cross section, the velocity of the liquid will be highest in the center of the pore, and it will gradually decrease with decreasing distance from the pore walls (Fig. 31.5). The velocity distribution across the pore is quantitatively described by the Poiseuille equation. [Pg.603]

The study of how fluids interact with porous solids is itself an important area of research [6], The introduction of wall forces and the competition between fluid-fluid and fluid-wall forces, leads to interesting surface-driven phase changes, and the departure of the physical behavior of a fluid from the normal equation of state is often profound [6-9]. Studies of gas-liquid phase equilibria in restricted geometries provide information on finite-size effects and surface forces, as well as the thermodynamic behavior of constrained fluids (i.e., shifts in phase coexistence curves). Furthermore, improved understanding of changes in phase transitions and associated critical points in confined systems allow for material science studies of pore structure variables, such as pore size, surface area/chemistry and connectivity [6, 23-25],... [Pg.305]

The electronic structures of porous solids have been examined by X-ray photoelectron spectroscopy (XPS). However, the penetration depth of electrons is 1 nm at best and XPS cannot examine electronic structures of inner pore-walls. XPS has been often used for the determination of surface chemical structures such as surface functional groups in activated carbon. Ar etching leads to the depth profile of electronic structures. This depth profile is often effective to evidence the presence of nanoporosity. [Pg.13]

From the theory of irreversible thermodynamic processes, one can conclude that mass transfer in porous capillary walls occurs much more effectively with the pressure difference along the channel length rather than diffusion. In other words, the pressure difference in the capillary channel entrance can ensure abnormally rapid filling of the capillary with a liquid. Seemingly, this mechanism provides the filling of cracks of nonsoluble solid particles of plankton under action of ultrasonic cavitation. There is no necessity for long exposures for the activation of plankton particles because two or three periods of cavity pulsation (about 100-150 ps) are sufficient for the cavity collapse and filling of the capillary with a liquid metal under action of the impulse of 102 MPa. [Pg.141]

In this section, we consider the problems relevant to equiUbriiim of the two multicomponent phases separated by a curved interface. This is the classical and the most well-studied case of the thermodynamic equilibrium involving surface effects. Such equilibrium is present in macro-porous and mesoporous media, like the porous rocks of petroleum reservoirs, where it accompanies adsorption. In the pores of smaller sizes, the forces produced by the solid surfaces may modify the properties of the bulk (Uquid and gas) phases. However, the present study is also important to the pores of smaller sizes, as it makes it possible to separate the effects connected with the gas-liquid surface tension (and, of course, the contact angle) from additional contributions of the solid walls. The corrections related to the last type of interactions have been considered in, for example. Refs. [13-15]. For brevity, we will apply the term capillary equilibrium to the narrow case being described, but it must be remembered, however, that a wider understanding of the capillary equilibrium is available. [Pg.381]

There are two basic disadvantages to the coated capillary column. First, the limited solute retention that results from the small quantity of stationary phase in the column. Second, if a thick film is coated on the column to compensate for this low retention, the film becomes unstable resulting in rapid column deterioration. Initially, attempts were made to increase the stationary-phase loading by increasing the internal surface area of the column. Attempts were first made to etch the internal column surface, which produced very little increase in surface area and very scant improvement. Attempts were then made to coat the internal surface with di-atomaceous earth, to form a hybrid between a packed column and coated capillary. None of the techniques were particularly successful and the work was suddenly eclipsed by the production of immobilize films firmly attached to the tube walls. This solved both the problem of loading, because thick films could be immobilized on the tube surface, and that of phase stability. As a consequence, porous-layer open-tubular (PLOT) columns are not extensively used. The PLOT column, however, has been found to be an attractive alternative to the packed column for gas-solid chromatography (GSC) and effective methods for depositing adsorbents on the tube surface have been developed. [Pg.1067]

For example, the determining factors for the filtration are the initial rate of liquid movement (6 to 8 cm s l) and the rate of the liquid rises to several centimeters during the first seconds of the ultrasonic action. On the other hand, due to adhesion of dispersed particles of solid nonmetallic inclusions onto the walls of a capillary, the effective cross-section of the latter continuously changes in the process of filtration. Finally, with actual filtration we deal not with the regularities of the flow through a capillary channel but with the statistics of melt movement through a porous medium with a host of channels. [Pg.133]


See other pages where Wall effects porous solid is mentioned: [Pg.502]    [Pg.216]    [Pg.413]    [Pg.240]    [Pg.147]    [Pg.172]    [Pg.147]    [Pg.151]    [Pg.138]    [Pg.236]    [Pg.270]    [Pg.121]    [Pg.430]    [Pg.296]    [Pg.606]    [Pg.36]    [Pg.801]    [Pg.371]    [Pg.802]    [Pg.240]    [Pg.482]    [Pg.931]    [Pg.25]    [Pg.254]    [Pg.175]    [Pg.139]    [Pg.218]    [Pg.238]    [Pg.77]    [Pg.482]    [Pg.584]    [Pg.144]    [Pg.520]    [Pg.494]    [Pg.203]    [Pg.9]    [Pg.519]    [Pg.585]    [Pg.23]    [Pg.636]   
See also in sourсe #XX -- [ Pg.264 , Pg.265 , Pg.266 ]




SEARCH



Effect solids

Porous solids

Solid walls

Wall effects

© 2024 chempedia.info