Classification carbonate pore types

FIGURE 1.7 Petrophysical classification of carbonate pore types. Based on Lucia (1983, 2007). 

As first shown by Boehm in 1975 [41] on CDC synthesized from TaC and SiC, and later for most carbide precursors [33], the resulting carbons have type I isotherms in the Brunauer classification, which are indicative of microporous carbon having pore sizes less than 2nm and relatively high surface areas up to 2000m2/g [37,39,42], The pore size of CDC can be tailored by the selection of carbide precursors with different spatial distributions of carbon atoms in the initial carbide lattice, changing the nanotextural ordering in the CDC by varying the synthesis temperature, and posttreatment in a... 

Although some of the isotherms in Figures 9.18-9.22 are more complex than others, they are all essentially Type I in the IUPAC classification. The five carbons are evidently predominantly microporous, but with different ranges of pore size. Before any attempt is made to assess the pore size distribution of each carbon, it is worth examining the significance of the various characteristic features of the isotherms. 

ACs are the most commonly used form of porous carbons for a long time. Typically, they refer to coal and petroleum pitch as well as coconut sheUs-based AC. In most cases, ACs are processed to be filled with rich micropores that increase the surface area available for gas sorption and separation. For this category, to get a definite classification on the basis of pore structure is difficult due to their countless products as well as their complex pore features. Based on the physical characteristics, they can be widely classified into the following types powdered, granular, extruded, bead ACs, etc. For the pore structure of ACs, actually, all the three types of pores (micropore, mesopore, and macropore) are included in one product (Fig. 2.1), with a wide pore size distribution [1, 2]. Up to now, many kinds of ACs have been well commercialized in gas sorption/separation including CO2 capture. For example, the BPL type with specific area of 1,141 m g is able to adsorb 7 mmol g CO2 under the conditions of 25 °C and 35 bar, while under the same conditions MAXSORB-activated carbon with specific area of 3,250 g can capture up to 25 mmol g [3]. 

For water-rinsed and acid-leached rice husks. Fig. 13.11a, b shows lower nitrogen capacity and no apparent desorption hysteresis loop, indicating that the porosity of the two raw materials is relatively lower than that of heat-treated rice husks samples. For carbonized in nitrogen and burned in air atmosphere husk samples. Fig. 13.11c, d shows that the isotherms are of type 1 according lUPAC classification. The hysteresis loops (associated with capillary condensation) found in both samples are of various shapes. According to these observations, BRHA is mainly microporous with narrow pore size distribution while WRHA contains both micro- and mesopores. 

The definition of the different types of pores is based on their width, which represents the distance between the walls of a slit-shaped pore or the radius of a cylindrical pore. This classification, which is not entirely arbitrary, is now widely accepted and used. It takes into account differences in the behaviour of molecules adsorbed in micropores and in mesopores. It appears that for pore widths exceeding 1.5-2.0 nm, the gaseous adsorbate condenses in a liquid-like state and a meniscus is formed. As a consequence, a hysteresis loop appears on desorption and its interpretation can lead to the distribution of the mesopores in the adsorbent [23]. The limit between mesopores and macropores at 50 nm is more artificial, and corresponds to the practical limit of the method for pore-size determination based on the analysis of the hysteresis loop. As a rule, the porous structure of the usual types of activated carbons is tridisperse, i.e. they contain micropores, mesopores and macropores. Micropores are of the greatest significance for adsorption owing to their very large specific surface area, and their large specific volume. At least 90-95% of the total surface area of an activated carbon can correspond to micropores. 

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