Carbon, activated adsorbent nature

The use of activated carbons as a natural gas storage medium for vehicles is attractive because the gas may be stored at significantly lower pressures in the adsorbed state (3.5 - 4.0 MPa) compared to pressurized natural gas (20 MPa), but with comparable storage densities. The development of an adsorbed natural gas storage system, and suitable adsorbent carbons, including novel adsorbent carbon... 

In some of the earliest recorded examples of adsorption, activated carbon was used as the adsorbent. Naturally occurring carbonaceous materials such as coal, wood, coconut shells or bones are decomposed in an inert atmosphere at a temperature of about 800 K. Because the product will not be porous, it needs additional treatment or activation to generate a system of fine pores. The carbon may be produced in the activated state by treating the raw material with chemicals, such as zinc chloride or phosphoric acid, before carbonising. Alternatively, the carbon from the carbonising stage may be selectively... 

S. Biloe, V. Goetz, A. Guillot, Optimal design of an activated carbon for an adsorbed natural gas storage system, Carbon 40 (2002) 1295— 1308. 

It is generally accepted that the oxidative power of charcoal is due to the chemisorbed layer of oxygen on its surface.1 This oxygen layer appears to be important also in other ways T. W. J. Taylor s studies on the interconversion of stereoisomeric oximes and sulphoxides show that oxygen is necessary for this reaction. The mechanism appears to be an exchange of the active, adsorbed oxygens on the surface of the carbon, with the molecules of the substances undergoing isomerization in solution. The nature of the layers adsorbed on charcoal influences, naturally, its behaviour as an electrode, and the kind of ions which it sends into solution ... 

Single-component adsorption equilibria on activated carbon of the n-alkanes Q-C4 and of the odorant tert-butyl mercaptan were measured at the operating conditions expected in a large-scale facility for adsorbed natural gas (ANG) storage. The experimental data were correlated successfully with the Adsorption Potential theory and collapsed into a single temperature-independent characteristic curve. The obtained isotherm model should prove to be very useful for predicting the adsorption capacity of an ANG storage tank and to size and optimize the operation of a carbon-based filter for ANG applications. 

In a recent review of technological alternatives for NOM removal, Jacangelo et al. [558] presented field data that illustrate wide variability in adsorption capacities of activated carbon in a single location (e.g., exhaustion periods between 41 and 182 days). They concluded that these results are evidence of the site-specific nature of [dissolved organic carbon] removal by [activated carbon] and that the concerns regarding reliability of treatment practices to meet the new [regulations] have a sound basis. Clearly, much fundamental work remains to be done to understand fully the complex nature of these adsorbent/adsorbate interactions and thus be able to optimize both the physical and the chemical accessibility of the carbon surface to natural organic matter. 

TABLE 32 Effect of pH on the Surface Charge of an Activated Carbon in the Presence and Absence of Adsorbed Natural Organic Matter... 

The capacity and intensity with which either GAC or PAC adsorbs a given solute has been modeled in several ways. The most commonly applied method is referred to as a Freundlich isotherm. The capacity of the activated carbon is defined as a function of the intrinsic characteristics of the carbon itself, the nature of the solute, and the final equilibrium concentration of the solute. The eapacity is defined as ... 

The presented data show possibilities of activation at high temperatures of carbon-silica adsorbents, i.e. microporous structure creation and change of chemical nature of the surface adsorption sites. As an illustration, adsorption from the aqueous solutions of chloro-phenols and methane trihalotanes was discussed [37-39]. Activation and deactivation of carbosils (in physical and chemical adsorption) through their HTT under the water vapour high pressure conditions and at moderate temperatures (250-500°C) were discussed [59]. Some results are presented in Figs. 7 - 10. 

Much less clear is the nature of basic sites on the surface of carbons. It is well known for a long time that carbons can adsorb acids. Hydrochloric acid is usually taken for the determination of acid-binding sites, and a preferred concentration is 0.05N [37, 38], analogous to the titration of acidic functions with NaOH. In particular, such carbons show basic behavior, which have been outgassed at high temperatures, e.g., 800-1100°C, and cooled to ambient temperature in a high vacuum or under an inert gas. Thus, many freshly produced carbon materials, such as activated carbons or carbon blacks, show basic reaction. 

Activated Carbon Sulfur removal from natural gas by adsorption at ambient temperature on carbon, activated with cupric oxide, is widely used. Carbon physically adsorbs sulfur compounds to its surface and the cupric oxide reacts with hydrogen sulfide. The activated carbon is typically regenerated every 30 days by passing steam through the bed at a temperature of 230°C (450°F) for 8—10 hr while air is injected. Oxygen in the air reacts with the metal sulfide to form the metal oxide and sulfur dioxide. These reactions are ... 

Ebie, K., Li, F., Azuma, Y., Yuasa, A. and Hagishita, T., 2001 Pore distribution effect of activated carbon in adsorbing organic micropollutants from natural water, Wat. Res. 35, 167-179. 

As mentioned above the most important features of activated carbons for adsorption me the textural parameters. For years natural surface chemistry of carbons, however studied in details [38, 39, 40, 45], was not really strongly linked to the performance of carbons as adsorbents. Its role was rediscovered when US EPA introduced strict envirorunental regulations and activated carbons were proven to be the best adsorbents working at ppm or even ppb levels. Moreover, very often the matrix (gaseous or liquid) from which the species have to be separated can interfere with the adsorption process causing that in many cases the specific adsorption forces have to be applied. Those forces can be provided by surface chemistry of carbons. 

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