Diffusion through solids

Facilitated transport has been briefly described in Chapter 1. In facilitated transport, the selective transport medium is a liquid or molten salt contained or immobilized in a porous support. The liquid membrane is held tightly in the support pores by capillary forces. The liquid or molten salt selectively reacts with a gas or vapor species and the reacting species diffuses across the liquid or salt and desorbed on the other side of the facilitated transport membrane. The major advantage of the facilitated transpoa is that diffusion is generally several orders of magnitude faster than diffusion through solid membranes. The support is, therefore, not a membrane by definition. Comprehensive... 

All of the comments above apply to diffusion through solid coal, not through coal s pore structure. Diffusion through the pore network has been studied thoroughly and seems to be understood (8-12). The considerations above are relevant to a reagent penetrating the solid coal to reach a functional group that is not located at a surface. 

Properties Bp -268.9C (1 atm), fp -272.2C (25 atm) lowest of any substance, bulk d 0.1785 g/L (0C). Very slightly soluble in water insoluble in alcohol rate of diffusion through solids is three times that of air an asphyxiant gas. 

SFC is the application of a supercritical fluid, any substance at a temperature and pressure above its thermodynamic critical point (Figure 9.2) with both gas- and liquid-like abilities to diffuse through solids, and dissolve materials, respectively, as the mobile phase in the chromatographic process. The most widely used mobile phase for SFC is carbon dioxide because of its low critical pressure (73 atm), low critical temperature (31°C), inertness, low toxicity, and high purity at low cost [12,13], Historically there were two approaches in developing modern SFC the use of either the packed and microbore columns designed for HPLC application or the open-tubular capillary GC type columns [13,14], The conventional packed HPLC... 

Diffusion through solid product "ash" layer Hydrodynamic factors are important... 

Figure 4.6.4 Conversion of a non-porous solid with a gas for different cases (i) absence of any diffusional resistances, Eq. (4.6.36), (ii) control by diffusion through solid product layer, Eq. (4.6.37), and (iii) gaseous product and control by external diffusion, Eq. (4.6.38).

In the diffusion through solids, several distinct cases arise that depend on the nature of the diffusing species and of tiie solid medium. The diffusing... 

Supercritical fluids are liquids at a temperature and pressure above their critical point, when distinct gas and liquid phases do not exist. They dissolve materials like a liquid but have very low viscosity and surface tension and diffuse through solids like a gas. 

In any given situation of practical interest, the concentration profile is obtained by solving the appropriate form of Eq. (13.2.9). For diffusion through solids and liquids (this is the situation of interest to us here), the flow terms are always small in comparison to the other terms. Consequently, we solve Eq. 

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