Adsorbent forms monoliths

From isotherm measurements, usually earried out on small quantities of adsorbent, the methane uptake per unit mass of adsorbent is obtained. Sinee storage in a fixed volnme is dependent on the uptake per unit volume of adsorbent and not on the uptake per unit mass of adsorbent, it is neeessary to eonvert the mass uptake to a volume uptake. In this way an estimate of the possible storage capacity of an adsorbent can be made. To do this, the mass uptake has to be multiplied by the density of the adsorbent. Ihis density, for a powdered or granular material, should be the packing (bulk) density of the adsorbent, or the piece density if the adsorbent is in the form of a monolith. Thus a carbon adsorbent which adsorbs 150 mg methane per gram at 3.5 MPa and has a packed density of 0.50 g/ml, would store 75 g methane per liter plus any methane which is in the gas phase in the void or macropore volume. This can be multiplied by 1.5 to convert to the more popular unit, V/V. 

In addition to the particulate adsorbents listed in Table 16-5, some adsorbents are available in structured form for specific applications. Monoliths, papers, and paint formulations have been developed for zeolites, with these driven by the development of wheels (Fig. 16-60), adsorptive refrigeration, etc. Carbon monoliths are also available as are activated carbon fibers, created from polymeric materials, and sold in the forms of fabrics, mats, felts, and papers for use in various applications including in pleated form in filters. Zeolitic and carbon membranes are also available, with the latter developed for separation by selective surface flow [Rao and Sircar, J. Membrane Sci., 85, 253 (1993)]. 

The heart of an adsorbent wheel system is a rotating cylinder containing the adsorbent. Figure 16-60 illustrates two types horizontal and vertical. In some adsorbent wheels, the adsorbent particles are placed in basket segments (a multitude of fixed beds) to form a horizontal wheel that rotates around a vertical axis. In other instances, the adsorbent is integral to the monolithic wheel or coated onto a metal, paper, or ceramic honeycomb substrate. These monolithic or... 

The catalyst, used in the form of a ceramic honeycomb monolith, is constituted, as in mobile applications, by a noble metal and an absorber element, such as potassium, deposited on a Y-AI2O3 wash-coat layer. In the oxidation and absorption cycle, the SCON Ox catalyst works by simultaneously oxidizing CO and UHCs to CO2 and H2O, while NOj, are captured on the adsorber compound. Catalyst regeneration is accomplished by passing a controlled mixture of regeneration gases across the surface of the catalyst in the absence of oxygen. 

Activated carbon fibers (ACFs) offer a choice of other carbon forms for VOC removal. As discussed earlier, the narrow diameter of the fibers provides ready access of adsorptive species to the adsorbent surface. The incorporation of ACF into permeable forms such as felt, paper, and rigid monoliths helps to surmount the disadvantages of using loose fibers. Rigid ACF composites have been prepared at the University of Kentucky and examined for their potential for the removal of low concentrations of VOCs [31]. 

To return to activated carbons, these can occur in very different physical forms granular (or particulate), powdered, fibrous, or even membrane (the latter can be either unsupported, or, more commonly, supported). These basic physical forms can be combined with binders and extruded to form pellets, monoliths, or even paper. All of these materials, which are very frequently used as adsorbents, differ significantly in shape, but not in their intrinsic nanotextural features. All of them are isotropic and have their BSUs randomly oriented. 

On the other hand, the experimental results show that the pellets resulting from the heat treatment at 500 °C are consistent when kept under water this is not the case for pellets dried at 110 °C or heat treated at 300 °C. Thus, a multi-purpose adsorbent is easily prepared, which can he used either in gas- or liquid-phase applications in the form of pellets or monoliths. 

In order to withstand the process environment, adsorbents are usually manufactured in granular, spherical or extruded forms with sizes most often in the range 0.5-8 mm. Special shapes such as tri-lobe extrudates are available so that pressure drops can be kept low when the adsorbent is packed in a vessel. Other forms are available for special purposes, such as powders and monoliths. Some adsorbent materials, particularly zeolites, require a binder material in order not only to provide mechanical strength but also to provide a suitable macropore structure such that adsorbate molecules can gain ready access to the internal microporous structure. Example adsorbents are shown in Figure 2.7. 

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