Environmental Engineering FOURTH EDITION

284 ENVIRONMENTAL ENGINEERING

"Q 100 200 300 400

Terminal settling velocity, cmls

Figure 14-9. Terminal settling velocities of various components of MSW. No single
air speed will result in complete separation of organic material (plastics and paper)
from inorganic (steel and aluminum). Idealized curves.

bottom is the reject or underjbw. The recovery of organic materials by air classification
is adversely influenced by two factors:

0 Not all organic materials are aerodynamically less dense, nor are all inorganic

0 Perfect classification of more and less dense materials is difficult because of the

materials denser.

stochastic nature of material movement in the classifier.

The first factor is illustrated in Fig. 14-9, in which the terminal settling velocity (the

air velocity at which particles will just begin to rise with the air stream) is plotted

against the fraction of particles of various materials. Complete separation of organic

from inorganic material can never occur, regardless of the chosen air velocity.

The second factor is illustrated in Fig. 14-10, which shows the efficiency, as

defined by Eq. (14.9), vs the feed rate to the classifier. As solids loading increases,

more particles that would have exited with the less dense fraction may be caught in

the underflow stream, and vice versa. Recovery of organic material in the product

stream may be plotted against the recovery of inorganic material in the reject stream.

The ideal performance curve may be obtained from the terminal settling plots like

Fig. 14-9 and by calculating &) and for a continuous classifier at different

speeds. The closer the two curves, the more effective the air classifier. Moreover,

as may be seen in Fig. 14-10, the performance deteriorates as the loading on the air

classifier is increased. The throat of the classifier can become clogged at high feed

rates, rendering a clean separation impossible.

EXAM~E 14.4. An air classifier operates with an air velocity of 200 cds, and the feed

contains equal amounts (by weight) of paper, plastics, aluminum, and steel, having