Porosity development

MODELING POROSITY DEVELOPMENT DURING KOH ACTIVATION OF COAL AND PITCH-DERIVED CARBONS FOR ELECTROCHEMICAL CAPACITORS... 

While keeping in mind all these implications, the primary requirement in an attempt to store a huge charge based on the electrostatic forces seems to be high surface area of an activated carbon used. Among different ways of porosity development in carbons, the treatment with an excess of potassium hydroxide is most efficient in terms of microporous texture generation. Porous materials with BET surface areas in excess of 3000 m2/g could be prepared using various polymeric and carbonaceous type precursors [5,6]. 

The reagents ratio, reaction temperature, reaction time and inert gas flow have been reported as the variables of KOH activation process, which influence effectively the porosity development on the treatment of a given carbonaceous material with potassium hydroxide [6],... 

Varying KOH ratio in the mixture is a very effective way of controlling porosity development in resultant activated carbons. The trend in the pore volume and BET surface area increase seems to be similar for various precursors (Fig. la). It is interesting to note, however, a sharp widening of pores, resulting in clearly mesoporous texture, when a large excess of KOH is used in reaction with coal semi-coke (Fig. lb). Increase in the reaction temperature within 600-900°C results in a strong development... 

The soak time seems to have a less pronounced effect on porosity development during treatment at 800°C. A considerable porosity (VT = 1.04 cm3/g and Sbet = 2570 m2/g) is created within the early period of reaction and a limited change of the parameters occurs with prolonged heating to 3 h (Fig. 3a). Again, the widening of pores, as evidenced by an increase of both the mesopore ratio and the micropore width, is the most noticeable result of extended reaction time (Fig. 3b). 

In order to evaluate the potential to porosity development, which is intrinsically associated with a given precursor nature, all parent materials were treated using drastic conditions reagent ratio 4 1, temperature 800°C, time 5 hours. Porosity parameters of resultant carbons are given in Table 2. 

Activation with KOH was recognized originally as an efficient way of producing microporous carbons with relatively narrow pore size distribution and extremely high surface area. The results of present study demonstrate a considerable flexibility of the process in terms of porosity development and, to some extent, surface properties. 

Porosity can be primary or secondary. Primary porosity develops as the sediment is deposited and includes inter- and intraparticle porosity (Figure 3.1). Secondary porosity develops after deposition or rock formation and is referred to as diagenesis (Figure 3.2). 

No porosity develops, ho vever, and the sample surface area does not change (Sidhu et ah, 1977). 

By cross-plotting the data in Figs. 13 and 14, the relation between the specific surface area and the porosity of the carbon rods after reaction at different temperatures can be presented, as in Fig. 15. It is seen that the surface area developed in the rods is not only a function of the porosity developed but also a function of the reaction temperature. The development... 

Fig. 16. Variation of effective diffusion coefficient of carbon dioxide through carbon monoxide at N.T.P. with porosity of spectroscopic carlion rods. Porosity developed by reaction with carbon dioxide at 950°.

Interpretation of the timing of dolomitization is important in a variety of studies, including hydrocarbon maturation and migration and porosity development. In Chapter 10 we discuss further the dolomitization process as a clue to ocean-atmosphere evolution. 

Figure 8.34. Hypothetical porosity-depth curves for various diagenetic situations. A. "Normal" burial curve of chalk sequences. B. A typical curve stemming from porosity changes brought about by dissolution and cementation processes during meteoric zone diagenesis. C. A scenario for late stage porosity development brought about by a dissolution event related to hydrocarbon maturation and destruction. D. Porosity preservation resulting from conditions of overpressuring. (After Choquette and James, 1987.)...

Lindquist S.J. (1977) Secondary porosity development and subsequent reduction, overpressured Frio Formation Sandstone (Oligocene), South Texas. Trans.-Gulf Coast Assoc. Geol. Socs. 27, 99-107. 

Moore C.H., Jr. (1989) Carbonate Diagenesis and Porosity. Developments in Sedimentology 46, Elsevier, Amsterdam, 338 pp. 

Groen, J. C., Peffer, L. A. A., Moulijn, J. A. and Perez-Ramirez, J. Mechanism of hierarchical porosity development in MFI zeolites by desilication the role of aluminium as a pore-directing agent, Chem. Eur. J., 2005, 11, 4983 1994. 

Urbonaite, S., Juarez-Galan, J.M., Leis, J., Rodriguez-Reinoso, F., and Svensson, G. Porosity development along the synthesis of carbons from metal carbides. Micropor. Mesopor. Mat., 113(1-3), 2008 14-21. DOI 10.1016/j. micromeso.2007.10.046. 

Thus, the objective of this section is to make a comparison between the results obtained by SAXS technique and by gas adsorption (nitrogen at 77 K and C02 at 273 K). These results include pore size and porosity development. For this study, ACFs, which have been prepared using C02 and steam as activating agents up to different burn-off degrees, have been used. 

---