Figure 8.13. Four of the more common types of thermal dryer.
Drying refers to the removal of water from a substance through a whole range of processes, including distillation, evaporation and even physical separations such as centrifuges.
Here, consideration is restricted to the removal of moisture from solids into a gas stream (usually air) by heat, namely, thermal drying. Some of the types of equipment for removal of water also can be used for removal of organic liquids from solids.
Four of the more common types of thermal dryers used in the process industries are illustrated in Figure 8.13.
The product tends to be light, porous particles. An important advantage of the spray dryer is that the product is exposed to the hot gas for a short period. Also, the evaporation of the liquid from the spray keeps the product temperature low, even in the presence of hot gases. Spray dryers are thus particularly suited to products that are sensitive to thermal decomposition, such as food products.
Another important class of dryers is the fluidized-bed dryer. Some designs combine spray and fluidized-bed dryers. Choice between dryers is usually based on practicalities such as the materials’ handling characteristics, product decomposition, product physical form (e.g. if a porous granular material is required), and so on. Also, dryer efficiency can be used to compare the performance of different dryer designs. This is usually defined as follows:
If the total heat consumed is from an external utility (e.g. mains steam), then a high efficiency is desirable, even perhaps at the expense of a high capital cost. However, if the heat consumed is by recovery from elsewhere in the process, as is discussed in Chapter 22, then comparison based on dryer efficiency becomes less meaningful.
Fuente: Chemical Process Design and Integration, Dryer, page 153.
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