Activated carbon type, influence

Each application for carbon treatment must be cognizant of the characteristics of the contaminant to be removed and designed with the proper carbon type in order to attain optimum results. Basically, there are two forms of activated carbon powdered and granular. The former are particles that are less than U.S. Sieve Series No. 50, while the latter are larger. The adsorption rate is influenced by carbon particle size, but not the adsorptive capacity which is related to the total surface area. —... 

The basic problem with activated carbon is that, intrinsically, it is a poor electrical conductor. Moreover, the use of small particles instead of a bulk crystal adds a contribution to the contact resistance. A binder must be mixed with the powder to stick the carbon particles together. The choice of binder material type and amount is influenced by the carbon surface properties. 

A pressure swing adsorption process (PSA) has been described with high efficiency for separation and capture of C02 in N2 at content from 16 to 25% (22). High purity C02 (> 99%) was recovered with efficiency ranging from 53% to 70% depending on C02 concentration. The selectivity and sorption capacity of zeolite 13X (FAU type) was much better than those of activated carbon. However, the influence of H20 on process efficiency was not reported. It is clear that H20, always present in flue gases from combustion, should first be separated to prevent inhibition of the zeolite. 

The carbon support used was a Norit activated carbon (RX3 extra) having a surface area of 1190 m2.g 1 and a pore volume of 1.0 cm3.g . The Co/C catalyst (4.1 wt% Co) was prepared by pore volume impregnation with an aqueous solution of cobalt nitrate (Merck p.a.) followed by drying in air at 383 K (16 h). The promoted catalyst (1.5 wt% Co, 7.7 wt% Mo) was prepared in a special way to ensure a maximum amount of the Co-Mo-S phase (11). Mossbauer spectroscopy of this promoted catalyst clearly showed that only the Co-Mo-S phase was present after sulfiding (11) and furthermore that this Co-Mo-S is probably a Co-Mo-S type II phase, meaning a minor influence of active phase-support interaction (11,12). The catalytic activity of the sulfided catalysts was determined by a thiophene HDS measurement at 673 K and atmospheric pressure, as described elsewhere (10). The thiophene HDS reaction rate constant kHDg per mol Co present (approximated as a first order reaction) was found to be 17 10 s 1 for Co/C and 61 10 3s 1 for Co-Mo/C. 

This study demonstrated that statistical models may be successfully applied to set up QSPRs available for prediction of adsorption enthalpy of an organic specie on one type of activated carbon. But for generalization, the specific influence of the properties of the GAC have also to be taken into account. 

The selectivity of activated carbons for adsorption and catalysis is dependent upon their surface chemistry, as well as upon their pore size distribution. Normally, the adsorptive surface of activated carbons is approximately neutral, such that polar and ionic species are less readily adsorbed than organic molecules. For many applications it would be advantageous to be able to tailor the surface chemistry of activated carbons in order to improve their effectiveness. The approaches that have been taken to modify the type and distribution of surface functional groups have mostly involved the posttreatment of activated carbons or modification of the precursor composition, although the synthesis route and conditions can also be employed to control the properties of the end product. Posttreatment methods include heating in a controlled atmosphere and chemical reaction in the liquid or vapor phase. It has been shown that through appropriate chemical reaction, the surface can be rendered more acidic, basic, polar, or completely neutral [11]. However, chemical treatment can add considerably to the product cost. The chemical composition of the precursor also influences the surface chemistry and offers a potentially lower cost method for adjusting the properties of activated... 

A wide range of organic products is suitable as feed.stocks for the manufacture of activated carbon. Wood, sawdust, peat, coconut shells and even olive stones are the preferred uncarbonized feedstocks. Of the (already) carbonized feedstocks coal, low temperature lignite coke, charcoal and coke from acid sludges (e.g. from the manufacture of lubricants) are utilized. The properties of activated carbon are very much influenced by the type of feedstock utilized. 

Many solid adsorbents liberate gas as a result of desorption of volatile liquids under the influence of heat. Typical adsorbents with microporous structures such as activated carbons, or precipitated silicas and renewable resources have been used as a coblowing agent in producing low-density extruded polystyrene foam boards. Incorporation of corn cobs or other renewable vegetable matter containing about 10% water together with a primary PBA into polystyrene in the extrusion process produced a low-density polystyrene foam board with bimodal cellular structures. This type of foam with bimodal cell structures has about 10-15% lower K-factor than similar foams without bimodal cellular structures. Similar results were obtained with a precipitated silica for producing a low-density extruded polystyrene foam with bimodal cellular structures. ... 

Volumetric measurements of the adsorption isotherms of hydrogen on SWNTs show that they are type 1 reversible isotherms similar to activated carbons (Fig. 10.7). Although a substantial dispersion in the experimental values of the adsorbed density was observed, improved measurements have been shown to be reliable. The small amount of SWNTs typically available for adsorption experiments requires that adsorbed density measurements be performed with sensitive instruments. The purity of the hydrogen used in the sorption experiments can also be an issue. In addition, the synthesis and preparation of the samples can influence substantially adsorption on SWNTs. Notably, thermal treatments can enhance the adsorption properties of S WNTs by removing guest species blocking access to adsorption sites. 

Adsorption of NOM onto activated carbon has been found to be influenced by a number of physicochemical properties such as e.g. pH value, NOM initial concentration, type and molecular size distribution of NOM, ionic strength, and water temperature (Lee et al., 1981 Cornel et al., 1986 Fettig and Sontheimer, 1987 Summers and Roberts, 1988 Johannsen et al., 1991 Kilduff et al., 1996 Bjelopavlic et al., 1999). Besides physicochemical characteristics of the process water adsorption is also dependent on properties of the activated carbon as e.g. pore volume, pore size distribution (Lee et al., 1981 Bjelopavlic et al., 1999 Ebie et al., 2001) surface area accessible for adsorption and surface functional groups as e.g. carboxyl, hydroxyl and carbonyl groups (Cookson, 1980). Adsorption of organic micro-pollutants is also affected by water and activated carbon properties. 

Mention should be made of a quite different type of influence on shelf life that is found when the source materials contain adsorbable ingredients that give stability to the product. An illustration is supplied by vegetable oils containing natural anti-oxidants that retard the development of rancidity. These natural anti-oxidants are adsorbed by activated carbon therefore to compensate for their loss, suitable synthetic anti-oxidants are added to an oil after treatment with carbon. 

The foregoing approach can have great utility to activation on an industrial scale, but it would be unfortunate to give the impression that the interpretation of the data is easily mastered. Problems arise from varied factors. First of all, in order to examine all diverse properties of the carbon being processed, the study may include so many test solutions that the total data, on initial inspection, may appear more confusing than informative. Some situations are further complicated by the fact that a particular type of adsorptive power may be created by more than one set of activating conditions. Therefore when some properties of the finished carbon are off normal, this may be not a consequence of a deviation in any single factor but rather a resultant of several factors. Still further, activated carbon has a memory and the prior history, e.g., the manner of carbonization, can influence the way in which a char responds to subsequent activation. 

The promoting action is not the same for inactive as for activated carbon, and dissimilar effects are found with different types of activated carbons. Sabalitschka and Moses40 studied the influence of the carrier on a palladium catalyst employed for the hydrogenation of maleic and fumaric acids. They found that the activity of the catalyst depends on the extent to which the palladium compound is adsorbed prior to reduction. With carriers of low adsorbing power, a more efficient catalyst can be prepared by using the readily adsorbable palladium hydroxide rather than the less adsorbable chloride. With good adsorbents, equally efficient catalysts can be prepared with cither of the palladium compounds. 

Figure 15.2 Influence of particle size distribution on activity and filtration behavior for three different types of wood-based activated carbons.

Chlorinations Catalyzed by Active Carbon. When carbon of different densities is subjected to superheated steam, its surface is vaporized, forming capillaries which have the property of absorbing gases and compressing them into much smaller volumes. This compression, possibly combined with the catalytic effect of the metal impurities present in the carbon, promotes reactions such as halogenation, hydrohalogenation, or dehydro-halogenation. The character of the metallic impurities, the absorption power of the carbon, the density of the carbon, and the method of capillary formation all materially influence the type of reaction and the life of the catalyst. Often materials are added to these carbons to modify their properties. 

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