Particle
suspension in liquids is a unit operation commonly encountered
in the process industry. Although it is usually carried out in baffled
stirred tanks, there are some specific applications where the presence
of baffles may be undesirable. In the present work solid–liquid
suspensions are investigated in a radially stirred unbaffled tank
provided with a top cover. The minimum impeller speed at which all
solid particles get suspended (N
js) and
the relevant power requirements (P
js)
are assessed. The dependence of these two parameters on physical properties
(liquid viscosity, particle concentration, and size) and system geometrical
configurations (impeller diameter and clearance) is investigated,
and a correlation for the N
js estimation
is proposed. Results mark some notable differences with respect to
baffled tanks: N
js appears to be almost
independent of particle diameter, a feature that may suggest the adoption
of unbaffled tanks when large heavy particles are to be dealt with.
Also, N
js was found to decrease as liquid
viscosity increases, once again marking a favorable discrepancy with
respect to baffled vessels. As concerns the system geometrical configuration,
a tank stirred by a D = T/3 six-bladed
Rushton turbine offset by T/3 from the tank bottom
is found to be the most convenient configuration among those investigated
in this work. Present results are finally compared with literature
information on solids suspension in baffled or unsteadily agitated
unbaffled systems. In both cases, top-covered unbaffled stirred vessels
are found to be a convenient choice, provided that mixing times are
not a controlling factor.