Granular Coalescence

This chapter deals with the study of transformation of a solid that does not lead to the creation of a new phase - the coalescence of the grains of a powder, that is, the enlargement of the grains imder the influence of a given temperature and in the presence of a gas atmosphere, without reversal being possible. 

These transformations generally involve only one process with several elementary steps. We will assume pure or mixed kinetic modes with only one or two steps as rate determining and of separable rate. 

It is noted that the change in Gibbs energy is negative if the final radius R2 is larger than the initial one, Ri. 

If a powder consists of two populations of grains, the larger grains and the smaller grains, the small ones tend to disappear with deposition on the large ones this phenomenon is known as Ostwald ripening. 

It should be noticed that taking into account the magnitude of the surface tensions (see Table 3.8), the changes in Gibbs energy associated with these phenomena are much smaller than those that accompany chemical reactions. Thus, 

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Either on a continuous oily film, if it is established in contact with a granular medium (sand, anthracite) and this is the case for granular coalescers or even filters. 

Tlie slop tanlcs can be heated to 40-45 C or a little higher (60°C), except when regulated for safety reasons, and the intermediate emulsion layer can be recycled upflow through a granular coalescer if the layer is not rich in SS. An attempt should also be made to keep naphthenic... 

Catalyst-supporting materials are used to immobilize catalysts and to eliminate separation processes. The reasons to use a catalyst support include (1) to increase the surface area of the catalyst so the reactant can contact the active species easily due to a higher per unit mass of active ingredients (2) to stabilize the catalyst against agglomeration and coalescence (fuse or unite), usually referred to as a thermal stabilization (3) to decrease the density of the catalyst and (4) to eliminate the separation of catalysts from products. Catalyst-supporting materials are frequently porous, which means that most of the active catalysts are located inside the physical boundary of the catalyst particles. These materials include granular, powder, colloidal, coprecipitated, extruded, pelleted, and spherical materials. Three solids widely used as catalyst supports are activated carbon, silica gel, and alumina ... 

The ability to separate a mixture of two liquid phases is critical to the successful operatiou of mauy chemical aud petrochemical processes. Besides its obvious importauce to liquid-liquid extractiou aud washing operations, liquid-liquid phase separation can be a critical factor in other operations including two-liquid-phase reaction, azeotropic distillation, and industrial wastewater treatment. Sometimes the required phase separation can be accomplished within the main process equipment, such as in using an extraction column or a batch-wise, stirred-tank reactor but in many cases a stand-alone separator is used. These include many types of gravity decanters, filter-type coalescers, coalescers filled with granular media, centrifuges, and hydrocyclones. 

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