T H E M A G A Z I N E - Mines Magazine - Colorado School of Mines

Controlled Particle
Movement In
Counter-Current
'low Heating
By
HARRY K. SAVAGE
Research indicates that the current method of heating
broken solids with hot gas by counter-current flow
can be made more effective. To design and operate
equipment for the purpose will require a knowledge of
how broken solids flow. Closely associated with flow
is the feature of agitating in some degree a mass of
broken solids, using no other force than gravity.
Little work has been done on the flow of broken
sohds until in recent years (1), (2). The following data
are offered as a contribution to a better understanding
of the subject.
Experimental work with crashed gravel was done
with a container, so built, that a desired cross-section
of rock flow could be obsei'ved. The floor design of the
container was an adaptation from mining by block
caving which is illustrated by Figure 1. With such a
floor design, two types of flow were developed when
flow into the container was substantially equal to out
flow:
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1—Mass flow in which the individual particles fell
with but httle deviation from the vertical.
Figure I.
THE
HARRY K. SAVAGE
AUTHOR
Although he is a native of Colorado, Harry K. Savage
received his technical education ai Stanford University
where in 1907 he received his A.B. degree in civil engineering.
From 1920 to 1931 he was a field engineer for
D. D. Potter & Associates of Denver.
Early in his career he became interested, in the technical
problems of oil shale. He discovered that the utilization of
oil shale will he a close combination of mining and chemical
engineering. Later he concluded that efficient retorting by
counter-current flow requires a knowledge of how broken
solids behave in motion. Since there was little literature on
the subject, it became necessary for him to undertake research
on the flow of broken solids.
2—Cone flow in which the particles fell at different
rates and directions, mostly non-vertical.
Mass flow was obtained by opening simultaneously
all the outlets shown by Figure 1. This flow is illustrated
by Figure 2 which is a cross section, A-A of Figure 1.
Above B-B of Figure 2, flow did not indicate any stress
except in the vicinity of the sidewalls where friction
caused a variation in velocity. This created shear and
potential tension in that vicinity. Below B-B, mass
flow was gradually superseded by cone flow with increased
shear and tension as the outlets were approached.
Mass flow, because of its stabiMty and lack of independent
particle movement, is not conducive to rapid
heating by counter-current methods. Technical literature
on the subject, (3), states that only a portion of
the voids in broken solids are available as gas carriers
because many of them are blocked by overlying pieces
of solid material and by interlocking. It has been estimated
that only about one-fourth, (4), of the surface
area of the individual particles comes in contact with
the gas stream. Also, volumes of gas tend to set up
convection currents, (5), which result in channeling
and slow up further the heating process. Given enough
time, heating wiU finally be accompfished by indirect
means.
Caking, which sometimes occurs, can be minimized
by agitating the particles at frequent intei-vals or by
removing caking mediums, which are products of
THE MINES MAGAZiNE • MARCH, 1960
distillation, by the use of a sufficient amount of scavenging
gases. Mass flow in the main is inimical to either
of these methods.
On the other hand, cone flow produces varying conditions
of velocity and direction. These induce individual
particle movement and produce stress. This
tends to break up static features in a bed of broken
solids. This type of flow can be induced in a container
of broken sohds by outlet flow through a single orifice.
Its location in regard to the sidewalls will determine
the pattern of flow. An orifice in the center of the
bottom will produce a balanced inverted cone as shown
by Figure 3. li in any other position, flow may be influenced
by the sidewalls. If so, the flow pattern will
be that of an imbalanced cone as illustrated by Figures
4 and 5.
Height of the container in relation to its width is
another important factor. If the height is about two
and one-half times the width, flow at that point and
above, if the container is higher, will be similar to mass
flow shown above B-B by Figure 2. Below the critical
height there will be a combination of mass and cone
flow with the latter increasing in proportion until mass
flow ceases and cone flow takes over completely. As
mass flow diminishes shear and tension increase. The
cone-flow pattern will be that of an inverted cone
flowing thi-ough an upright cone-like figure of static
material. The line of contact between the two cones of
broken rock will be at an angle of 70° from the horizontal,
(Figure 3). Different materials have different
flow angles; catalyst pellets 71", (6): a good grade of
oil shale, 72^ (7).
\
\
A \
7\-
/ - \
^ L \
"I
F/ovf/fty JbfBX^ rock
/ \
/ \
/ \
/ A \
L A L
* 3
Figure 2.
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THE MINES MAGAZINE • MARCH, 1960
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When flow was through outlet #1, (Figure 1), the
pattern was that of an imbalanced cone, (Figure 4).
The pattern was quite different from that of a balanced
cone, {Figure 3), one side of the imperfect cone
was heavier than the other with the lighter side being
between the center of the cone and the side wall. When
flow began through outlet #1, the initial flow in the
upper part of the cone was toward the side wall rather
than toward the outlet. Soon most of the material
changed direction toward the outlet, but part of it
as shown by markers 2, 3 and 4, continued toward
thc side wall until near the bottom of the container.
It then changed direction rather abruptly to flow to
the outlet. The peculiarities of this type of flow indicates
greater shear and tension than that of Figure 3.
_ When outlet #1 was closed and outlet #2 opened a
different type of imbalanced cone flowed as illustrated
by Figure 6. It was more nearly in balance than the
cone illustrated by Figure 4-
Draw thi-ough outlets 1, 4, 13 and 16 will produce
imbalanced cones with similar patterns. Draw through
outlets 2, 3, 5, 8, 9, 12, 14 and 15 wifl produce imbalanced
cones with patterns similar to Figure 6. Draw
through outlets 6, 7, 10, 11 may produce balanced
cones similar to Figure 3. If all the cones were drawn
• Figure 3.
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