Dryer ♦ Robin Smith

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.

Tunnel dryers are shown in Figure 8.13a. Wet material on trays or a conveyor belt is passed through a tunnel, and drying takes place by hot air. The airflow can be countercurrent, cocurrent or a mixture of both. This method is usually used when the product is not free flowing.

Rotary dryers are shown in Figure 8.13b. Here, a cylindrical shell mounted at a small angle to the horizontal is rotated at low speed. Wet material is fed at the higher end and flows under gravity. Drying takes place from a flow of air, which can be countercurrent or cocurrent. The heating may be direct to the dryer gas or indirect through the dryer shell. This method is usually used when the material is free flowing. Rotary dryers are not well suited to materials that are particularly heat sensitive because of the long residence time in the dryer.

Drum dryers are shown in Figure 8.13c. This consists of a heated metal roll. As the roll rotates, a layer of liquid or slurry is dried. The final dry solid is scraped off the roll. The product comes off in flaked form. Drum dryers are suitable for handling slurries or pastes of solids in fine suspension and are limited to low and moderate throughput.

Spray dryers are shown in Figure 8.13d. Here, a liquid or slurry solution is sprayed as fine droplets into a hot gas stream. The feed to the dryer must be pumpable to obtain the high pressures required by the atomizer.

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.