Page 48 - Chemical and process design handbook

Speight_Part 1_N&O 11/7/01 3:02 PM Page 1.34

1.34 REACTION TYPES
decanted from the spent acid and are washed with dilute alkali. The spent
acid is sent to some type of recovery system and yields of 98 percent can be
anticipated.
Considerable heat evolution accompanies the nitration reaction, oxida-
tion increases it, and the heat of dilution of the sulfuric acid increases it still
further. Increased temperature favors dinitration arid oxidation, so the
reaction must be cooled to keep it under control. Good heat transfer can be
assured by the use of jackets, coils, and good agitation in the nitration ves-
sel. Nitration vessels are usually made of stainless steel, although cast iron
stands up well against mixed acid.
When temperature regulation is dependent solely on external jackets, a
disproportional increase in nitration vessel capacity as compared with
jacket surface occurs when the size of the machine is enlarged. Thus, if the
volume is increased from 400 to 800 gallons, the heat-exchange area
increases as the square and the volume as the cube of the expanded unit.
To overcome this fault, internal cooling coils or tubes are introduced,
which have proved satisfactory when installed on the basis of sound cal-
culations that include the several thermal factors entering into this unit
process.
A way of providing an efficient agitation inside the nitration vessel is
essential if local overheating is to be mitigated. Furthermore, the smooth-
ness of the reaction depends on the dispersion of the reacting material as it
comes in contact with the change in the nitration vessel so that a fairly uni-
form temperature is maintained throughout the vessel.
Nitration vessels are usually equipped with one of three general types of
agitating mechanism: (1) single or double impeller, (2) propeller or turbine,
with cooling sleeve, and (3) outside tunnel circulation.
The single-impeller agitator consists of one vertical shaft containing
horizontal arms. The shaft may be placed off center in order to create rapid
circulation past, or local turbulence at, the point of contact between the
nitrating acid and the organic compound.
The double-impeller agitator consists of two vertical shafts rotating in
opposite directions, and each shaft has a series of horizontal arms attached.
The lower blades have an upward thrust, whereas the upper ones repel the
liquid downward. This conformation provides a reaction mix that is essen-
tially homogeneous.
The term sleeve-and-propeller agitation is usually applied when the
nitration vessel is equipped with a vertical sleeve through which the charge
is circulated by the action of a marine propeller or turbine. The sleeve is