Impinging Stream Drying
5.1 Basic Features
which moisture evaporation from wet particles or liquid droplets occurs in the impingement zone that develops as a result of the collision of two oppositely directed high-velocity gas streams, at least one of which con-tains the dispersed material to be dried. At the outset, a distinction should be made between the impingement dryers, in which gas jets are directed onto web, sheet-form, or slablike materials (Mujumdar, 2007), and the jet-zone high-velocity airstreams exiting a number of perpendicularly oriented air nozzles (Kudra and Mujumdar, 2007).
streams at the feed plane, then it accelerates from zero velocity to a certain 5.1). After crossing the impingement plane, the particle penetrates into the opposite stream due to its inertia and decelerates to a full stop at some dis-nation point). Thereafter, the particle accelerates in the opposite direction and, after crossing the impingement plane again, penetrates the original gas stream up to the opposite stagnation point. Thus, the deceleration and acceleration processes take place repeatedly. After several damped oscil-lations, the particle velocity in the impingement zone drops to the termi-nal velocity so that it is carried away with the outgoing gas stream. (For
Because of such an oscillatory motion with progressively decreasing amplitude, the residence time of a single particle (of large inertia) in the impingement zone is longer than that of the gas stream. This residence time may be reduced for a multiplicity of particles because of interpar-ticle collisions that lead to enhanced energy dissipation. Although it is possible to feed solids to both gas streams, the rate of particle collision and the resulting loss of momentum are much higher than that for a single stream feed, because particles that do experience nonelastic col-lisions greatly lose their momentum and are driven out of the system.
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depending on the configuration of the impingement chamber.)
The term impinging stream dryer (ISD) refers to a class of flash dryers in
dryers, in which a layer of particulates is pseudofluidized by a multiplicity of
If a solid or liquid particle starts to flow with one of the impinging gas velocity resulting from the hydrodynamics of the gas–solid flow (Figure
tance of penetration within the domain of the opposing jet flow (the
stag-simplicity, Figure 5.1 depicts only the particle trajectory flowing with the upward gas stream, whereas downward or both flows are also possible,
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56 Advanced Drying Technologies
single feed, as well, especially for high solid concentration. In our opin-ion, the problem could be alleviated by intermittent material feed with frequency proportional to the material residence time.
The constraint of reduced residence time is practically eliminated in impinging streams with a mobile impingement zone (Elperin and Meltser, 1978). In this arrangement, the impingement plane is made to move between the two locations (I and II in Figure 5.2) by alternate switching
erate with the original gas stream into one of the impingement chambers, the stagnation point, and then begin to accelerate in the opposite direc-closed, whereas the outlet of the second impingement chamber is open.
into the original gas stream, and the following acceleration toward the the particles is self-controlled by decreasing the inertia force due to the reduction of particle size in the course of processing and reducing the par-ticle mass due to moisture evaporation. Alternatively, a suitable limiting
Impingement plane
Impingement zone
Gas stream I Gas stream II
Particle trajectory
Gas duct I Gas duct II
Gas streamlines Particle or droplet
Feed plane
The principle of ISDs.
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This might result in such a significant decrease of the residence time dramatically reduced. The same effect of particle collisions exists in a and penetration depth that the beneficial effects of impingement are
tion. At this moment, the gas outlet of the first impingement chamber is
first impingement zone are repeated. The period of oscillatory motion of of the gas flows from left to right and then from right to left. The particles duct that links both impingement chambers. In either case, particles accel-can be fed into the accelerating flow duct or into the central (reverse flow)
where they collide with the counter-flowing gas stream, penetrate it up to
ment chamber, where the processes of jet collision, penetration of particles This procedure results in the flow of a secondary gas stream with accel-erating particles along the reverse flow duct toward the second
impinge-© 2009 by Taylor & Francis Group, LLC
grid can be placed at the outlet ducts to restrict the entrainment of the oversized particles.
tains inert spherical particles that oscillate continuously between the two impingement zones throughout the operation of the dryer. This allows drying of slurries or suspensions that are sprayed on the surface of inert particles. Because the inert particles are heated by the hot air, drying of the liquid coat occurs by combined convective and conductive heat trans-fer. Also, drying with simultaneous grinding can be performed if metallic beads are used along with the material being dried.
streams (Figures 5.3 and 5.4). For example, in a system consisting of four the two primary gas streams mixed in an impingement zone are split into two secondary streams carrying the particles off the impingement zone.
Figure 5.5 shows the principle of semicircular impinging streams in which a primary gas stream is split equally into two secondary streams
thickness of the boundary layer at the solid surface.
Another variant of the impinging stream concept can be formed when one or two gas–solid suspensions are brought into collision inside a
Impingement chamber Material feed
Impingement zone
Acceleration zone Deceleration zone
Stagnation point
Impingement zone I Impingement zone II
Constraining grid
Gas feed duct
H
u0 D u0
L
H H
FIGURE 5.2
Coaxial ISD with a mobile impingement zone.
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Another modification of the ISD with the mobile impingement zone
con-Similar transport phenomena occur in other configurations of impinging mutually perpendicular ducts of the same diameter (the “X” configuration),
Reverse flow duct
flowing inside two ducts bent to form semicircular channels. In this case, carrying gas) is affected by centrifugal forces, which might reduce the the flow of the drying material (and, to a smaller extent, the flow of the
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58 Advanced Drying Technologies
tion), (e) counterrotating countercurrent, and (f) corotating countercurrent.
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tion, (c) curvilinear countercurrent, (d) semicircular impinging streams (chain Basic types of impinging stream flows: (a) Coaxial, (b) mutually perpendicular X
configura-© 2009 by Taylor & Francis Group, LLC
(a) (c)
(b) (d)
FIGURE 5.4
F Fc Fcp
FD x2
x1
FIGURE 5.5
Particle trajectory for double penetration in a semicircular impinging stream (Fc, centrifu-gal force; Fcp, centripetal force; FDDD, drag force; F, lift force).
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Impinging plane/zone geometries: (a) Planar with radial flow, (b) planar with circumferen-tial flow, (c) tubular, and (d) annular.
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60 Advanced Drying Technologies
Grinholtz, 1987). Figure 5.6 presents some of these impinging stream reac-tors that could be used for drying granular or liquid feeds (Tamir, 1994).
Product Feed Air
(a) (b) (c)
(d) (e) (f )
(g) (h)
FIGURE 5.6
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confined volume that forms a cyclone, for example (see Figure 5.3c).
ical reactions, it is commonly termed the impinging stream reactorr (Tamir and Because such a configuration was originally developed to carry out
chem-Some configurations of the impinging stream reactors.
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according to
a. Rectilinear, where streamlines are parallel with either aligned
b. Rotating (swirling), where streamlines do follow a helix
c. Curvilinear, where streamlines are represented by circular arcs
2. Flow direction
in opposite directions the same direction 3. Streamline direction
a. Counterrotating, where gas (or solid suspension) streams rotate in opposite directions
b. Corotating, where gas (or solid suspension) streams rotate in the same direction
4. Type of impingement zone
a. Stationary, in which the position of the impingement plane does not change with time
b. Mobile, in which the position of the impingement plane changes periodically or continuously
5. Geometry of impingement plane/zone toward the impingement plane
concentric circles in the impingement plane
c. Tubular, in which impinging streams meet at the cylindrical surface
d. Annular, in which impinging streams meet in a sector of annulus
Single impinging stream units can be combined in series or parallel to form a system, allowing extension of the residence time and develop-ment of different hydrodynamic or temperature regimes. Also, hybrid
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5.4), the following basic variants of impinging streams can be identified
configurations such as the coaxial impinging streams combined with the Depending on the geometry and flow direction (see Figures 5.3 and
1. Type of flow
axes of the gas jets (coaxial flow) or displaced axes of the gas jets (eccentric flow)
a. Countercurrent, where gas (or solid suspension) streams flow b. Concurrent, where gas (or solid suspension) streams flow in
a. Planar with radial flow, in which streamlines diverge radially b. Planar with circumferential flow, in which streamlines form
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62 Advanced Drying Technologies
semicircular ones have found certain advantages over the simple systems (Dengying et al., 1999; Huai et al., 2000).
Besides the high intensity of turbulence in the impingement zone, a common feature of any impinging stream is the unsteady particle motion, acceleration, deceleration, and movement of the particles against the gas enhance the heat- and mass-transfer processes and hence reduce drying times (see Table 5.1).
The unique characteristics of impinging streams also offer special advantages for thermal processing. An example is the intermittent drying that takes place when the opposing streams are supplied at different tem-peratures. Thus, particles are heated and cooled periodically when oscil-of grains (thermal micronizing), which occurs when wet grain, for exam-ple, is subject to high moisture and temperature gradients.