11. Driers

Drying is one of the most frequently encounted chemical engineering operations. Drying generally signifies the removal of liquid from a solid by evaporation, although gases are also dried. Mechanical methods for separating a liquid from a solid are not generally considered drying, although they often precede a drying operation.

Driers may be classified in several ways. The two most useful classifications are based on:

1. the method of transferring heat to the wet solids;

2. The handling characteristics and physical properties of the wet materials.

The first method of classification reveals difference in drier design and operations, while the second one is if most useful in the selection of a drier for a given application.

According to the first method of classification driers are: direct, indirect and mechanical.

In a direct drier the heat is supplied to the wet solid by means of direct contact with hot gases, the gases also carrying away the vaporized liquids.

In an indirect drier the heat is supplied to the wet material from, heated surfaces in the drier, such as tubes or jackets.

In mechanical driers the heat required for drying is generated by mechanical friction.

Further, a drier may be designed for batch or continuous operation. Direct and indirect driers may be batch or continuous.

The second method of classification of driers considers the characteristics of the materials being processed: liquids.

slurries, pastes, free-flowing powders, granular or crystalline solids, continuous sheets (paper, plastic sheets), discontinuous sheets (wallboard, leather) etc. Direct Driers. Direct driers may use air, inert gas, superheated vapooar, or products of combustion as the heating medium» Combustion gases, however, are seldom used la polymer plant because of possible product contamination.

There are many types of direct driers. One of them is a rotary drier. Rotary driers have been the most popular in polymer production. They consist of a revolving drum through which both the -material and gas pass. Gas, concurrent or countercurrent with the polymer, can be introduced at either end of the cylinder. Solids move on through the drier by the effect of the ■ slope (gravity), the rotation of the cylinder, and gas flow (in the case of cocurrent units). Internal scoops, blades, or lif-ters - are provided for better contact between the hot gas and

the solids. Baffles and dam rings are also available to retard the forward motion of the solids, and to increase residence time (-5 to 20 min is common) . The length of the cylinder may vary from four to ten times its diameter; driers having a diameter up to 12 ft. are not uncommon for large polymer plants. This type of drier is common in polystyrene processes, among others.

Commentary Gas, cocurrent or countercurrent with the polymer... -газ, движущийся в а?ом же направлении, что и полимер, или в противоположном