Adsorbent pellets, properties

The macropores are not straight cylinders but bounded with changing widths and sometimes narrow slits or dead-end pores. Therefore, a tortuosity factor is introduced in the balance equations. Principally speaking, the tortuosity factor should be dependent only on the inner geometry of the adsorbent pellet but not on operation parameter of fluid properties such as the pressure, the temperature, the density, the viscosity, or the molar fluxes of the components. However, this is not true for some publications because of the difficulty to clearly separate the contributions from the particle porosity Sp and the Knudsen diffusion. Therefore, the tortuosity factors published in the literature are often greater than 3 up to 6 which can be expected due to the inner structure of the porous material. 

Tab. 3.3.1 Physical properties of the packed porous particles. The relaxation times were determined at a Larmor frequency of 300 MHz for protons of water adsorbed into saturated catalyst pellets (average error 2%). The equivalent diameter is defined by 6 Vp/Ap where Vp and Ap are volume and external surface of the particles, respectively.

The discussion so far has concentrated on mass transfer. The transfer of the heat liberated on adsorption or consumed on desorption may also limit the rate process or the adsorbent capacity. Again the possible effects of the boundary-film and the intra-pellet thermal properties have to be considered. A Biot number for heat transfer is hri/ke. In general, this is less than that for mass transfer because the boundary layer offers a greater resistance to heat transfer than it does to mass transfer, whilst the converse is true in the interior of the pellet. 

The designer now needs to make some estimates of mass transfer. These properties are generally well known for commercially available adsorbents, so the job is not difficult. We need to re-introduce the adsorber cross-section area and the gas velocity in order to make the required estimates of the external film contribution to the overall mass transfer. For spherical beads or pellets we can generally employ Eq. (7.12) or (7.15) of Ruthven s text to obtain the Sherwood number. That correlation is the mass transfer analog to the Nusselt number formulation in heat transfer ... 

In most spectroscopic studies, the solids to be studied are usually compressed to form pellets under pressures around 1.5-2 kbar. From an academic point of view, the stability of MTS towards pressure is very important, since most spectroscopic studies of lattice groups or adsorbed probes might be affected by a degradation of MTS during compression. For industrial applications compaction is crucial to handle the powder. Thus the mechanical properties of MTS are a very sensitive topic if we think about the future of these materials. Solids with such high porosity and small wall thickness are very likely to be crushed. Previous studies point out a very weak mechanical strength of MTS [3,4J which can jeopardize further industrial development. It has been demonstrated that these materials have the lowest mechanical stability among the... 

A differential permeation method was used to determine diffusion kinetics of strongly adsorbing vapors through an Ajax activated carbon (type 976) (whose physical properties [7] particle density of 733 kg/m micropore porosity 0.40, macropore porosity 0.31 and mean macropore radius 0.8 pm). An activated carbon pellet was carefully mounted in a copper block, separating two reservoirs. One reservoir is much larger in volume than the... 

We believe that the difference 1n N2 capacity for a packed-bed vs. a shallow bed 1s directly attributable to the poor hydrothermal stability of this adsorbent. Nevertheless, the self-bound CaLSX after dehydration 1n a packed-bed displays the best air separation properties of any pelletized N2-select1ve adsorbent reported. 

Air separation properties (both Na/Oj selectivity and Na capacity) for CaLSX are superior to any other pelletized Na selective adsorbent reported. 

The combined application of PFG NMR self-diffusion and tracer desorption experiments has thus proved to be an effective tool for studying the hydrothermal stability of A-type zeolites with respect to their transport properties [186]. It turns out that with commercial adsorbent samples there are considerable variations in hydrothermal stability between different batches of product and even between different pellets from the same batch. As an example. Fig. 24 shows the distribution curves [A(Tin,ra) versus Ti ,r.j] measured with ethane as a probe molecule at 293 K for two different samples of commercial 5A zeolites. Evidently batch 1 is more resistant to hydrothermal deterioration, because the lengthening of Tjn,ra is less dramatic than with batch 2. Since the intracrystalline diffusivity was the same for all samples, the deterioration can be attributed to the formation of a surface barrier. 

The most common sample forms used for the measurement of electrical properties are pressed pellets, thin films, and single crystals. Electrical measurements on pellets are often difficult to interpret because of the presence of polycry staUinity, grain boundaries, large surface area, and unknown amounts of adsorbed gases. Films are useful in some instances, but in many cases also cause uncertainties because of poly cry staUinity. Electrical properties are most easUy and reUably interpreted with single crystals free from unintentional impurities and solvent occlusions. 

Due to its predominantly hydrophobic surface properties, activated carbon preferentially adsorbs organic substances and other non-polar compounds from gas and liquid phases. Activated alumina, silica gel and molecular sieves will adsorb water preferentially from a gas-phase mixture of water vapor and an organic contaminant. In Europe cylindri-cally-shaped activated carbon pellets with a diameter of 3 or 4 mm are used for solvent recovery, because they assure a low pressure drop across the adsorber system. Physical and... 

The most common adsorbant used is granular or powdered activated carbon. This material, which is available from almost all forms of organic carbon-containing matter, is a microcrystalline nongraphite form of carbon. The production of activated carbon can be achieved by use of rotary kilns, hearth furnaces, or furnaces of the vertical shaft or fluidised bed type, and each is suitable for the generation of different pore size and the source of carbon. The pore volume and size are influenced by both the carbon source and method of production. The adsorption properties are directly related to the pore volume, pore size distribution and the nature of the functional groups on the surface of the carbon. Activation is achieved chemically, by treatment by dehydration with zinc chloride or phosphoric acid, or by treatment with steam, hot carbon dioxide or a mixture of both. The activated carbon is available in three basic forms, powder, granules or as cylindrical or spherical pellets. For solvent recovery systems the carbon is usually obtained from either wood charcoal, petroleum residues or coconut shells and is often used in the form of pellets. 

Most practical zeolitic adsorbents are used in a pellet form (with or without binders) where a network of meso-macro pores provide the access of the gases to the adsorption sites (inside the micropores of crystalline zeolites). The zeolite crystal and the pellet radii are typically in the range of 0.5-2.0 pm and 0.5-2.0 mm, respectively. Consequently, the kinetics of ad(de)sorption of Nz and Oz are often controlled by the transport of these gases through the mesoporous network, and the ad(de)sorption kinetic (Knudsen, molecular and Poiseuille flow) time constants are large (>0.5 seconds 1). Thus, the kinetics of ad(de)sorption processes may not be critical. The thermodynamic adsorptive properties (a,b) and the desorption characteristics (c) under local equilibrium conditions often determine the separation performance of a zeolite. 

Sepiolite is a fibrous silicate, Sii2MggOjo(OH)4(H20)4, made up of microporous channels parallel to the fiber axis. The chemical composition and stmcture of sepiolite are responsible for good adsorbent behavior towards polar molecules such as water, ammonia, amines and aldehydes in both gas and liquid phases because of its hydrophilic surfaces. Activated carbon is essentially microporous and hydrophobic, making it suitable for nonpolar molecules such as hydrocarbons. As these properties are complementary, a mixture of both could be useful in specific applications such as adsorption of mixtures of molecules. The preparation of discs or pellets is straightforward because in mixtures of carbon and sepiolite, the latter acts as a binder when adding small quantities of water. 

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