Gas-adsorbent carbon

Coconut shells are a very suitable raw material for gas-adsorbing carbon. On carbonization, they shrink, yielding a hard porous structure structure that can withstand a considerable amount of steam activation without becoming soft. In times of peace, coconut hells provide much of the carbon used in America for gas-adsorption applications. However, the available supply does not fill all requirements in time of war, particularly because coconut shells must be imported and cargo space is then at a premium. Consequently, efforts have been directed to utilize other raw materials. Few other materials, in their natural state, can be converted into a satisfactory hard activated carbon, but some can be activated successfully when given various preparatory treatments.1,4, 63,64 In a typical process, a base material, e.g., coal, is pulverized and mixed with sufficient binder to make a plastic mass... 

Gas-adsorbent carbon. This is made by carbonization of coconut shells, fruit pits, coal, lignite, and wood. It must be activated, essentially a partial oxidation process, by treatment with hot air or steam. It is available in granular or pellet form and is used for recovery of solvent vapors from gas mixtures, gas masks, collection of gasoline hydrocarbons from natural gas, and the fractionation of hydrocarbon gases. It is reviviHed for reuse by evaporation of the adsorbed gas. 

Of the many carbonaceous materials that form active charcoal, only relatively few— coconut shells, fruit pits, and cohune and babassu nutshells—readily yield chars with all of the properties desired for gas-adsorbent use. Becau.se of the limited supply of these materials, special preparatory treatments have been developed to enable other base materials to be used for gas-adsorbent carbon. In its most common form, the pretreatment consists of pulverizing carbonaceous material, incorporating a suitable binder, and pelleting or extruding to form a dense, compressed material. ITie pellets or spaghetti-like extrusions are then carbonized at temperatures from 700° to 900°C. Various types of wood and coal have been found to be suitable base materials, and materials such as sugar, tar, and lignin can be used as binders. 

Tully J C 1980 Dynamics of gas-surface interactions reactions of atomic oxygen with adsorbed carbon on platinum J. Chem. Phys. 73 6333... 

In these processes, a carbon monoxide containing gas is fed to an adsorber bed containing copper, typically dispersed on a high surface area support such as alumina or carbon. The copper is present predominately as Cu", which selectively adsorbs carbon monoxide. The remainder of the gas stream passes through the adsorbent bed. The carbon monoxide is then removed from the adsorbent by lowering the pressure. Figure 6 shows a typical process for a CO-PSA process. Process conditions are typically adsorption pressures of 0.68—204 MPa (6.8—20.4 atm) and temperatures of 313—373 K. Regeneration occurs at reduced pressure or by vacuum. 

Remick, R.J., Tiller, A.J., Advanced Methods for Low Pressure Storage of CNG, Non-petroleum Vehicular Fuels Conference, Washington, April 1985 Otto, K., Alternative Energy Sources IV, Vol 6 p241, Ann Arbor Science, MI Barton, S.S., Holland, J.A. Quinn, D.F., "The Development of Adsorbent Carbon for Storage of Compressed Natural Gas, Report AF-85-01, Ontario Ministry of Transportation, 1985 1201 Wilson Ave, Downsview, Ontario, Canada M3M 1J8... 

Adsorption A physical process in which a molecule of a vapor or gas (adsorbate) is condensed on and taken up by the surface of a porous material (adsorbent) such as silica gel or activated carbon. 

IR spectroscopy of adsorbed carbon monoxide has been used extensively to characterize the diluted, reduced Cr/silica system [48-54,60,76,77]. CO is an excellent probe molecule for Cr(ll) sites because its interaction is normally rather strong. The interaction of CO with a transition metal ion can be separated into electrostatic, covalent a-dative, and 7r-back donation contributions. The first two cause a blue shift of the vco (with respect to that of the molecule in the gas phase, 2143 cm ), while the last causes a red shift [83-89]. From a measurement of the vco of a given Cr(II) carbonyl complex, information is thus obtained on the nature of the Cr(II)- CO bond. 

Adsorbed carbon monoxide on platinum formed at 455 mV in H2S04 presents a thermal desorption spectrum as shown in Fig. 2.4b. As in the case of CO adsorption from the gas phase, the desorption curve for m/e = 28 exhibits two peaks, one near 450 K for the weakly adsorbed CO and the other at 530 K for the strongly adsorbed CO species. The H2 signal remains at the ground level. A slight increase in C02 concentration compared to the blank is observed, which could be due to a surface reaction with ions of the electrolyte. Small amounts of S02 (m/e = 64) are also observed. 

Exepriments in the gas phase have supplied us with considerable knowledge on the state of adsorbed carbon monoxide on platinum. 

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... 

The simplest theories of reactions on surfaces also predict surface rate laws in which the rate is proportional to the amount of each adsorbed reactant raised to the power of its stoichiometric coefficient, just like elementary gas-phase reactions. For example, the rate of reaction of adsorbed carbon monoxide and hydrogen atoms on a metal surface to produce a formyl species and an open site,... 

A fundamental question concerns the state of the adsorbed gas, namely whether it is closer to the gaseous or the liquid state. At 301 K, the solvent shift is mainly observed on the terminal carbon atoms which are more exposed to intermolecular interactions (22). The carbon Cj and C4 of 1-butene experience a small low field shift with respect to the gas, the carbon a small high field shift, while the methinic C2 carbon atom is much more influenced than the other carbon atoms (low field shift) suggesting a specific interaction at this site of the molecule. 

Prior to 1970 our understanding of the bonding of diatomic molecules to surfaces, and in many cases the type of adsorption (i.e., molecular or dissociative) was almost entirely dependent on indirect experimental evidence. By this we mean that deductions were made on the basis of data obtained from monitoring the gas phase whether in the context of kinetic studies based on gas uptake or flash desorption, mass spectrometry, or isotopic exchange. The exception was the important information that had accrued from infrared studies of mainly adsorbed carbon monoxide, a molecule that lent itself very well to this approach owing to its comparatively large extinction coefficient. 

If we consider the surface of quartz to be divisible into two areas on which the lengths of life of the carbon monoxide molecule are 1 and the amount of gas adsorbed on the total area at a pressure p will evidently be... 

Finally, one must consider the possibility of an involatile residue being formed by one or more of the atomic species present in the molecular gas, e.g., adsorbed carbon from CF. Unless this residue is removed by some mechanism (step 5), it will terminate the etching reaction. Electron and ion bombardment could enhance residue removal by many of the same mechanisms mentioned above in connection with steps 2-4. 

S.W. Martin, USP 2812246 (1957) CA 52, 3346 (1958) claims prepn of impact-resistant LOX from an adsorbent carbon obtained by flash-oxidation-calcination of a finely divided swelling bituminous coal (with a volatile content of >12%) in a stream of oxygen-contg gas at 800° or more. An expl prepd from this carbon was insensitive to impacts of >1200ft-lb. It failed to detonate when burned in semi-confinement with a 0.25 inch... 

Additional studies by Menon (32) have indicated the p can occur at lower pressures then those predicted by Equation 1 depending on the pore structure associated with the adsorbent. Empirically, adsorbents possessing microporosity exhibit a p that is 0.6-0.8 of the value predicted by Equation 1. This observation is attributed to the overlapping of potential fields in the adsorbent pores, thereby enhancing sorption of the gas at lower pressures. Experimental studies by Ozawa (33) have verified this trend as shown in Figure 4 for the C02/activated carbon system. Here the adsorption maxima for the gas occurs at a lower pressure than the critical pressure of carbon dioxide. It should also be noted that the amount of gas adsorbed is decreased at higher reduced temperatures and that additional compression is required to reach a defined adsorption maxima (i.e., at very high values of T it is sometimes difficult to discern a well-defined adsorption maximum). The above trend has also been found for other adsorbent/adsorbate systems, such as silica gel/C02. 

Alternative techniques do exist, however, for obtaining information regarding the distribution and number of catalytic components dispersed within or on the support. Selective gas adsorption, referred to as chemisorption, can be used to measure the accessible catalytic component on the surface indirectly by noting the amount of gas adsorbed per unit weight of catalyst. The stoichiometry of the chemisorption process must be known in order to estimate the available catalytic surface area. One assumes that the catalytic surface area is proportional to the number of active sites and thus reaction rate. This technique has found use predominantly for supported metals. A gas that will selectively adsorb only onto the metal and not the support is used under predetermined conditions. Hydrogen and carbon monoxide are most commonly used as selective adsorbates for many supported metals. There are reports in the literature of instances in which gases such as NO and O2 have been used to measure catalytic areas of metal oxides however, due to difficulty in interpretation they are of limited use. 

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