Charcoal, pore size distribution

It is known that the effect of the surface area in the gasification of charcoal is intimately related to the very broad pore size distribution of this material. Random pore structure models accounting for the effects of pore growth and coalescence have been proposed by various authors and have often shown satisfactory agreement between theory and experiment, but none of the proposed kinetic relations describes the charcoal reactivity in the conversion range beyond X 0.7 satisfactorily. For the latter conversion... 

Table 1 summarizes the information required for a detailed characterization of a supported metal catalyst for supported bimetallics there are additional questions, e.g., the distribution of atoms in bimetallic clusters and the surface composition of larger alloy crystallites. For the support and the prepared catalyst, the total surface area, pore size distribution, and surface acidity are routinely measured, if required, while other characteristics, e.g., thermal and chemical stability, will have been assessed when selecting the support. The surface structure of alumina, silica, charcoal, and other adsorbents used as catalyst supports has been reviewed. Undoubtedly, the most commonly measured property is the metal dispersion, often expressed in terms of the specific metal area and determined by selective chemisorption or titration but, as discussed (Section 2), there is the recurring problem of deciding the correct adsorption stoicheiometry. 

Since carbon molecular sieves are amorphous materials, the dimensions of their pore structures must be measured phenomenologically by the adsorption of small probe molecules with different critical dimensions. There is insufficient long range order to utilize standard x-Ray diffraction methods for characterization. The earliest reports of molecular sieving carbons dealt primarily with coals and charcoals. Sorption of helium, water, methanol, n-hexane, and benzene was measured and related to the porosity of the carbon. Pore-sizes were estimated to be two to six angstroms (3-6). In a classic paper P.H. Emmett described methods for tailoring the adsorptive properties and pore size distributions of carbon Whetlerites. 

The most common adsorbant used is granular or powdered activated carbon. This material, which is available from almost all forms of organic carbon-containing matter, is a microcrystalline nongraphite form of carbon. The production of activated carbon can be achieved by use of rotary kilns, hearth furnaces, or furnaces of the vertical shaft or fluidised bed type, and each is suitable for the generation of different pore size and the source of carbon. The pore volume and size are influenced by both the carbon source and method of production. The adsorption properties are directly related to the pore volume, pore size distribution and the nature of the functional groups on the surface of the carbon. Activation is achieved chemically, by treatment by dehydration with zinc chloride or phosphoric acid, or by treatment with steam, hot carbon dioxide or a mixture of both. The activated carbon is available in three basic forms, powder, granules or as cylindrical or spherical pellets. For solvent recovery systems the carbon is usually obtained from either wood charcoal, petroleum residues or coconut shells and is often used in the form of pellets. 

The influence of sulfur surface compounds on the adsorption of polar and nonpolar vapors of varying molecular dimensions was examined by Puri and Hazra. The adsorption of water vapors increased appreciably at relative pressures lower than 0.4 and decreased at higher relative pressures. The effect increased with increase in the amount of sulfur fixed and was attributed to the variation of the pore-size distribution caused by the fixation of sulfur along the pore walls. The adsorption isotherms of methanol and benzene vapors indicated that these larger molecules found smaller and smaller areas as more and more sulfur was being incorporated into the pores. Bansal et prepared carbon molecular sieves by blocking pores of PVDC charcoals by... 

FIGURE 4.7 Pore-size distribution curves for Saran charcoal before and after pore blocking with sulphur. (After Bansal, R.C., Bala, S., and Sharma, N., Indian J. TechnoL, 27, 206, 1989. With permission.)... 

FIGURE 4.11 Pore size distribution of charcoals prepared from balsa, grant ipil-ipil, and bamboo, (a) Pore size distribution of raw wood and charcoal carbonized at 700°C for 1 h at a heating rate of 50°C/min and (b) dependence of pore volume on heat treatment temperature and rate. 

Production of activated carbon using activation of lignite or charcoal by supercritical water or carbon dioxide has been patented by Salvador-Palacios (2001) who claims that a maximum in porosity development is reached for about 45-50 wt% bum-off. When comparing supercritical activation with steam activation, it was found that the former is about 10 times faster, leading to activated carbons with larger volumes of micropores and a wider pore-size distribution, with lower ash content. 

Figure 1 compares the pore size distribution of micropores of various substrates using the methanol adsorption technique. The activated charcoal shows a large surface area at > 100 m /g. However, many of the pores consist of micropores with radii < 1 nm. The ion-exchange resin (lER) similarly contains micropores. On the other hand, the microfibrilated fibers exhibit a surface area of 200m /g, which is as large as 10-20% of... 

Active carbon or charcoal is an important modification of carbon in catalysis. It consists of carbonized biopolymer material which is activated in a second step. This procedure creates a high specific surface area by oxidative generation of micropores of very variable size and shape distribution. A more controlled activation is achieved by the addition of phosphoric acid or zinc chloride to the raw product. The additive is incorporated during carbonization into the hard carbon and is subsequently removed by leaching creating the empty voids in a more narrow pore size sistribution as achievable by oxidation. Other activation strategies... 

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