Multistage drying processes

The term multistage drying refers to any process that uses high-temperature drying in combination with aeration or natural-air drying. An outline of two such processes, dryeration and combination drying, follows. 

The multistage liquefaction process (Figure 19.17) involves the liquefaction of crushed, dried coal which is slurried with an aromatic recycle solvent and the whole is then charged to a reactor at 415°C-440°C (VSOT-STST) at approximately 2000 psi. An expanded bed reactor system is employed to circumvent the problems that arise through reactor plugging, catalyst deactivation, and inequitable liquid distribution. 

There is some beneficiation of talc by froth flotation (qv), practiced especially on ultramafic-type deposits. In this process (Fig. 2), talc is milled to its hberation size (—100 mesh (ca 0.15 mm)) using ball mills or ring-type roUer mills and then slurried at 10—30% in water. Flotation is done in conventional multistage float cells using methyl amyl alcohol as a frother. Typically two to four stages are required to upgrade the ore from 50—70% talc to 90—98%. The product is filtered and then flash-dried and milled to a final product. 

Both wet and dry granulation methods of tablet manufacture are complex multistage processes, but are necessary to convert the components of the formulation into a state that can be readily compressed into acceptable tablets. If, however, a major component of the formulation already possesses the necessary degree of fluidity and compressibility, granulation would be unnecessary. This is the basis of the direct compression method of tablet manufacture. ... 

The drying of the chlorine is carried out almost exclusively with sulfuric acid of 96-98 wt% in a multistage process (for instance packed towers, counter-current flow). After the drying, another demisting step, now from sulfuric acid, is necessary. 

Spray and fluid bed drying technologies can be combined into one multistage process to accomplish the tasks discussed above. Figure 355 shows as an example the two-stage arrangement of a spray dryer and a vibrating fluid bed. Further... 

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