9.1 INTRODUCTIONThe drum dryer is commonly used to dry viscous,concentrated solutions, slurries or pastes on rotatingsteam-heated drums.1,2 It can also be used to dryconcentrated solutions or slurries that become moreviscous or pasty because of flashing or boiling off ofmoisture or of irreversible thermochemical transformationsof their content that occur on their first contact with the hot drum surface.3–5 The viscous slurry or paste is mechanically spread by the spreading action of two counter-rotating drums into a thin sheet that adheres on the hotter drum in single drum dryers or split sheets on both hot cylinders in double drum dryers. The adhering thin sheet of paste is then rapidly dried conductively by the high heat flux of the condensing steam inside the drum. For very wet slurries that produce wet sheets,the drying of the wet thin sheet can be further enhanced by blowing hot dry air on the sheet surface. The thin sheet containing heat-sensitive materials, such as vitamins, can also be dried at a lower temperature in a vacuum.The irreversible thermochemical transformations during the slurry’s first contact with the hot drum can also be used to simultaneously impart certain required quality of the dried product. 6 Starch slurries can be gelatinized or ‘‘cooked’’ before the sheet is
đang được dịch, vui lòng đợi..