Porous Solids—Continuous Operations

The intensive rapid solid mixing in the reactor leads to a wide range of residence times of individual particles in the reactor for continuous operations this gives poorer performance since the removal of fully reacted particles will inevitably be associated with removal of unreacted carbon for catalytic reactions, the movement of porous catalyst contributes to the backmixing of gaseous reactant. 

Section V deals with the drying of porous solids in continuous operations. The study of drying in rotary and tunnel dryers is presented based on the relationships derived from basic theory. The effect of the operating variables on drier performance is discussed. A suitable procedure is developed for sizing rotary and tunnel driers. 

Porous metallic structures have been used for electrocatalysis (Chen and Lasia, 1991 Kallenberg et al., 2007). Porous electrodes are made with conductive materials that can degrade under high temperatures at high anodic potential conditions. This last problem is of less importance for fuel cell anodes, which operate at relatively low potentials, but it can be of importance for electrochemical reactors. Porous column electrodes prepared by packing a conductive material (carbon fiber, metal shot) forming a bar are frequently used. Continuous-flow column electrolytic procedures can provide high efficiencies for electrosynthesis or removal of pollutants in industrial situations. Theoretical analysis for the electrodeposition of metals on porous solids has been provided by Masliy et al. (2008). 

Various designs are used to achieve either a clarification of liquors, or a significant increase in solids concentration resulting in the production of a wet solid filter cake of minimal moisture content. This can be driven by either an applied pressure to the substrate over a porous filter medium (i.e. pressure filtration), or a vacuum applied behind the filter medium (vacuum filtration). The process can be on a batch basis, or via a continuous operation where, by a mechanical rotary means, new filtration areas are offered to the substrate. Chemical conditioning, including flocculation and coagulation, can be used to increase throughput, cake solids or fines capture. 

The discovery of solid catalysts led to a breakthrough of the chemical process industry. Today most commercial gas-phase catalytic processes are carried out in fixed packed bed reactors. A fixed packed bed reactor consists of a compact, immobile stack of catalyst pellets within a generally vertical vessel. On macroscopic scales the catalyst bed behaves as a porous media. The fixed beds are thus employed as continuous tubular reactors in which the reactive species in the mobile fluid (gas) phase are reacting over the catalyst surface (interior or exterior) in the stationary packed bed. Compared to other reactor types or designs utilizing heterogeneous catalysts, the fixed packed bed reactors are preferred because of simpler technology and ease of operation. 

Adsorption is a physical phenomenon in which some components (adsorbates) in a fluid (liquid or gas) move to, and accumulate on, the surface of an appropriate solid (adsorbent) that is in contact with the fluid. With use of suitable adsorbents, desired components or contaminants in fluids can be separated. In bioprocesses, the adsorption of a component in a liquid is widely performed by using a variety of adsorbents, including porous charcoal, silica, polysaccharides, and synthetic resins. Such adsorbents of high adsorption capacities usually have very large surface areas per unit volume. The adsorbates in the fluids are adsorbed at the adsorbent surfaces due to van der Waals, electrostatic, biospecific or other interactions, and thus become separated from the bulk of the fluid. In practice, adsorption can be operated either batchwise in mixing tanks, or continuously in fixed-bed or fluidized-bed adsorbers. In adsorption calculations, both equilibrium relationships and adsorption rates must be considered. 

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