Figure 3. Sifting segregation after pile formation; light-colored fines remain
in center while darker, coarser particles concentrate at perimeter.
driving force. The driving force is the difference between the
bulk gas and the temperature of the liquid on the particle surface. The specific surface area of the bulk material is inversely
proportional to the particle diameter, while the heat-transfer
coefficient generally becomes smaller with increasing particle diameter. The equilibrium moisture content is frequently
greater for smaller particles, due to the higher capillary forces
that hold water associated with smaller pores.
Hence, the residence time of the solids inside the dryer and
the particle size of the material can have a dramatic effect on
dryer performance. Because material is typically introduced
from a surge or feed hopper, its design is critical. The discharge
rate of the hopper must be steady to ensure a controlled dryer
residence time. Care must be taken to ensure that segregation
by particle size, which can occur when the hopper is filled,
does not lead to a dryer feed that is variable in particle size
Under some conditions, fine particles will percolate or sift
through coarse particles. For example, if the feed is relatively
free flowing, large particles will tumble towards the walls of
the surge hopper and fine particles will become concentrated
beneath the fill point (see Figure 3). As a result, side-to-side
variation of particle size will take place.
SIFTING SEGREGATION CONSEQUENCES
The consequences of sifting segregation depend on the flow
pattern that occurs when material is discharged from the surge
hopper. In general, there are two primary flow patterns that
can occur: funnel flow and mass flow. In funnel flow, an active
flow channel forms above the outlet, with stagnant material remaining (i.e., ratholes) at the periphery. Funnel flow can cause
erratic flow due to collapsing ratholes and flooding in the case
of fine powders, exacerbate segregation, and allow particle
degradation (e.g., caking, spoilage) in stagnant regions. Flow
patterns are illustrated in Figure 4.
In mass flow, the entire bed of solids is in motion when material is discharged from the outlet. This behavior eliminates
stagnant regions in the vessel and affords a first-in, first-out
flow sequence, which provides a more uniform discharge rate.
Mass flow also provides a more uniform velocity profile, which
reduces the effects of sifting
segregation. Hence, a mass
flow hopper should be used to
handle material fed into a con-
Design charts originally developed by Jenike [ 2] provide
allowable hopper angles for
mass flow given values of wall
friction. The angle of wall friction φ’ is obtained by following
the method described in ASTM
D-6128  where a sample of
powder is placed inside a retaining ring on a flat coupon
of wall material, and a normal load is applied. The powder is
forced to slide along the stationary wall material, and the resulting shear stress is measured as a function of the applied
normal stress. The wall yield locus is then constructed by
plotting shear stress against normal stress. The angle of wall
friction φ’ is the angle that is formed when a line is drawn from
the origin to a point on the wall yield locus. This angle of wall
friction is the inverse tangent of the wall friction coefficient.
The outlet of the hopper section must be large enough to
prevent stable obstructions to flow from developing. The required outlet size depends on the cohesive strength and the
bulk density of the powder. The relationship between the cohesive strength of a powder and its consolidation pressure (i.e.,
its flow function) is measured by shear cell testing as described
in ASTM D-1628  or D-6773 [ 3].
From cohesive strength and bulk density test measurements,
the minimum outlet size required to overcome arching Bmin can
be calculated from:
H (θ’)σcrit Bmin = ρbg
where H(θ’) is a function given by Jenike [ 2], σ crit is the
critical cohesive strength taken from the flow function follow-
ing a method described by Jenike [ 2], ρb is the bulk density of
the powder, and g is equal to the gravitational constant.
In summary, dryers run optimally when operated with uniform feeds. Mass flow hoppers mitigate sifting segregation,
which can cause feed non-uniformities, and provide steady
feeds to continuous dryers. Wall friction, bulk density, and cohesive strength tests must be performed to be able to specify
wall materials, geometry, hopper angle, and outlet dimensions
that ensure mass flow and prevent obstructions to flow. ■
1. ASTM D-6128, “Standard Test Method for Shear Testing of
Bulk Solids Using the Jenike Shear Cell”, ASTM Int. (2006).
2. Jenike, A. W., Storage and Flow of Solids, Bulletin 123,
University of Utah Engineering Station, 1964 (revised, 1976).
3. ASTM D-6773, “Standard Shear Test Method for Bulk Solids
Using the Schulze Ring Shear Tester”, ASTM Int. (2008).
Figure 4. Funnel flow and mass flow behavior in a
surge or feed hopper.