Note: Descriptions are shown in the official language in which they were submitted.
~ 3
The present invention is directed to an improvement
over the fluidized bed blender disclosed in a Canadian appli-
calion of Zenz et al, Serial No. ~9 ~3~, filed ~5~ 9
entitled Method and Apparatus for slending Powders in a Fluid-
ized Bed, and assigned to the same assignee.
The invention relates to the blending of particulate
solids and in particular to a method and apparatus for con-
verting a heterogenous mixture of fine and cohesive UO2 powders
into a homogenous mixture.
The blending of particulate solids has been accom- ~ '
plished in the past in a variety of ways. Mechanical mixers
of several types, such as tumble mixers~ ribbon blenders and
high shear mixers, ha~e be.en used, as well as so-called bubbling-
bed fluidized bed blenders and spouting bed blenders. In the
prior art, UO2 powders have primarily been blended with
mechanical tumble-type blenders, such as disclosed in U.S.
Patent no. 3,825,230 to Frye et al issued ~uly 23, 1974.
However, problems attendant upon the use o~ mechanical tumble
blenders in general and in particular with the use o~ mechanical
~0 tumble blenders for th,e blending of UO2 powders brought the
development of the-bubblin~bed fluidized be:d blenders. ,
The aforementioned Zenz et al applicati.on discloses
an impro~ed bubbIing-be.d fluidized bed blender. The Zenz et al
blender eliminated th,e la~ge deadzones encountered in prior ~.
art bubbIing-bed fluidized bed blenders by providing a fluid-
izing grid comprIsing a li:near array of pyramidal-shaped hoppers,
each hopper converging into a con.i,cally-sh:aped opening and a
gas orifice for directing a flo~ of fluidizing gas downwaxdly
into the bottom of th.e hopper~ Th.e elim~nat~on of the dead-zones -.
enabled the Zenz et al blender to meet product homogeneity ~,
specifications for powder ha~ing an instantaneous ~low function ;.
of about ~.0 or greater as measured by a Jenike~type flow
~*~
-
factor tester. A compaction and build-up problem developed
with fine, cohesive U02 powders having an instantaneous flow
function less than about 4O0 during blending in the Zenz et al
blender. As used herein, the instantaneous flow function is the
relationship between the unconfined yield strength and the
consolidating pressures for the particles of powder being blended.
The instantaneous flow function and the flow factor tester are
more fully described in Bulletin No. 123, Utah Engineering
Experimental Station, Storage and Flow of Solids by Andrew J.
Jenike. Certain types of these fine and cohesive U02 powders
were found to deposit, compact and bridge in the hoppers of the
Zenz et al blender. Since U02 product homogeneity specifications
require nearly ideal blending, this build-up in the hoppers of
the Zenz et al blender prevented the blender from meeting product
homogeneity specifications with U02 powders having a flow factor
less than about 4.0
A general discussion of the design considerations
involved in designing a prior art bubbling-bed fluidized bed
blender including a consideration of particle properties,
particle size, particle distribution, vessel geometry, superficial
gas velocity, and circulation patterns is found in Fluidization
and Particle Fluid Systems by Frederick A. Zenz and Donald F.
Othmer, Reinhold Chemical Engineering Series, Reinhold Publishing
Corporation, New York, 1960. Design considerations for possible
grid designs are found in Fluidization by J.F. Davidson and
D. Harrison, Academic Press, London, 1971.
The prior art also discloses spouting bed blenders, having
a convergent~ hopper at the bottom of the blender with a plurality
of mixing gas orifices disposed in a circular array at the bottom
of the hopper, for example, see British Patent 900,242 ~ Grun
published July 4, 1962. Although the British patent shows orifices
disposed at angles such that jets of mixing air are directed
-- 2 --
.. I
in a swirl pattern along the walls of the converging hopper,
and in one embodiment the orifices are included in the
closure member of a gate valve, there are differences between
the device disclosed in the British patent and that of the
present invention. These differences stem from the fact that
the British device may be characterized as a spouting bed
blender rather than a bubbling-bed fluidized bed blender.
Spouting bed blenders employ gas mixing jets of sufficient
power and duration to suspend and drive particles through the
bed in a continuous stream that spouts from the top of the
bed. In the sritish patent mixing is accomplished by pressure
waves so dimensioned that the coarsest particles of the highest
specific gravity in the mixture of particles in the mixing
vessel are suspended upwardly. Fluidization in a bubbling-bed
fluidized bed blender occurs at much lower mîxing gas velocities.
In fact, it is the lower mixing gas velocity of the bubbling~bed
fluidized bed blender that makes it particularly suitable for
blending UO2 powders. Spouting bed blenders have not been
adopted for the blending of UO2 powders in particular because '
the violent action of gas jets sufficient to suspend the
coarsest particles causes excessive loss of UO2 powder through
entrainment with the mixing gas.
In bubbling-bed fluidized bed blenders the fluidizing
grid (or gas distributorl at the bottom of the bed exerts a
strong influence on the mixiny process carried out in the
bed. A coarse distributor produces high iniection rates
of gas at local points whi:ch leads to the channeling of gas
within the bed. Channeling of gas causes deadzones of stag-
nant or unfluidized mater~al~ Since the gas mixing orifices
disclosed ~n the British patent were intended to be used at
much higher gas veIocities than those necessary for bubbling~
bed fluidized bed blending/ channeling of gas and deadzones
would occur in the spouting bed blender of the British patent
3~
at lower fluidizing gas velocities.
It is a principal obje:ct of the present in~ention to
- provide a bubbling-bed fluidized bed blender that is capable
of provi.ding a blend of powders ~pproaching ideal homogeneity.
It is another object of the present in~ention to
provide a bubbling~bed fluidized bed blender paxticularly suited
for blendi.ng fine and cohesiYe UO2 powders~
More specifi.c~lly! it is an object of the present
invention to provide a bubbling bed fluidized bed blender that
solves the problem of co,~paction and build-up of fine and
cohesi:Ye UO2 powders encountered with preYious bubbling~bed
fluidized bed hlenaers5
These and other objects of the invention are carxied
out by proYiding a method and appara.tus containing the h.etero~
geneous mixture of powders to be blended, preferabl~ UO2 pQwders!
in a ~rextically-oriente:d! $1ab~shaped, nuclear-safe mixing ~`
vessel. The fluidi:zing gri.d constructed according to this ~`
inventiQn comprises a li.near ~rray~ of generally downwardly -
directed, p~ramidal~shaped hoppers ea:ch haYing ~all$ converging `
into a conically-shaped opening. ~ first set of oEifices :,
is proYided for directing a pluralit~ of fluidizing gas jets in
a circular a~ray abou:t the opening o~ each of`the. ~oppers~
The first set of`orifices~ di,rects. gas upwardly in a diyergent ` `~
swirl-shaped p,a,ttern .along the:~alls of each.of the hoppers.
A plurali.ty of ball valyes incoxporating the first set of
orifices are employed~ one such.`~alYe being di~sposed at the -'~
bottom of each`of the hoppe,rs and each capabIe`of clo$ing
the`opening in the hopper.~ A se:c.ond set of orifices for
directing a flo~ of fluidizing~ gas down~ardly into. the`bottom
of each of the hoppers is also pro~ided in one e~bodiment. ,;
Fluidizing gas is supplied to the first set of ori~icesr or
to the comb,ination of the first and second set of orificesr in
:.
~4~
,
an amount sufficient to cause bubbles of fluidizing gas to
rise through the mixture of powders throughout the bed and
emerge from the top of the powder bed until a homogenous
blend of powders is achieved. If the first set of orifices
is used alone, fluidizing gas is supplied to the orifices on
a continuous basis and the orifices are sized to provide a flow
of fluidizing gas at sonic velocity. When the first and
second sets of gas orifices are used together the flow of
fluidizing gas to the first set of gas orifices is pulsed to
sonic velocity for short periods while the flow of fluidizing
gas to the second set of gas orifices is continuous. In
either case the action of the first set of gas orifices directing
a flow of fluidizing gas along the walls of the hoppers prevents
the compaction of fine and cohesive U02 powders in the bottom
of the hopper encountered in prior fluidized bed blenders of
this type.
FIGURE 1 is a perspective illustration of a bubbling-
bed fluidized bed blender constructed according to the present
invention. `
FIGURE 2 is a vertical section of one hopper of a
prior art bubbling-bed fluidized bed blender utilizing down-
wardly-directed fluidizing gas orifices and showing the gas
flow therein.
FIGURE 3 is a vertical section of one hopper of the
bubbling-bed fluidized bed blender of this inyention illustrat-
ing the gas flow therein and including a schematic of the
associated gas supply system.
FIGURE 4 is a chart illustrating the order of puls-
ing of orifices in a first set of orifices employed in this
invention.
FIGURE 5 is a view of a fluidization grid of a
bubbling-bed fluidized bed blender constructed according to
--5--
this invention.
FIGVRE 6 iS a top view of a rotary closure member
for a ball valve incorporating a first set of fluidizing gas
orifices of the present invention.
FIGURE 7 iS a vertical section of the xotary closure
member illustrated in FIGURE 6 taken along line 7-7 in
FIGURE 6.
FIGURE 8 is a partial section of the rotary closure
member illustrated in FIGURE 6 taken along line 8-8 in FIGURE
7.
FIGURE 1 illustrates one embodiment of the bubblLng-
bed fluidized bed blender constructed according to the present
invention. The blender comprises a vertically-oriented, rec-
tangular, slab-shaped, nuclear-safe mixing vessel 1 haYing a
fluidizing grid 2 disposed at the bottom of the vessel 1.
The fluidizing grid 2 comprises a linear array of ~enerally
downwardly-directed pyramidal-shaped hoppers 3, each having
walls 4 converging into conically-shaped openings 5. A plur-
ality of valves 6 are employed for discharging blended powders,
such as UO2 powders, from the hoppers, one such Yalve being
dlsposed at the bottom of each hopper. A first set of orifices
(not shown in FIGURE 11 is incorporated in Yalve 6, and the
set of orifices iB connected to a source of fluidizing gas 7,
via a common manifold 8 and a plurality of connecting lines 9.
The embodiment of FIGURE 1 also includes a second set of
orifices 12 which direct a flow of fluidizing gas downwardly
into the bottom of the hoppers 3, one such orifice being
provided for each hopper. A source of fluidizing gas at 13
is connected to each of the orifices 12 by a common manifold 14
and a plurality of blowpipes 15.
The vessel 1 of the bubbling-bed fluidized bed
blender is filled through an inlet 16 having a valve 17, such
as a butterfly valve, associated therewith for preventing the
-6-
escape of powders during the blending process. The valve 17
is not shown in detail since it is not part of the present
inventiOn and any suitable type of butterfly valve may be em-
ployed. The vessel 1 is initially filled to about one half of
its height with a mixture of heterogenous or unblended powders.
Thus, the bottom half 18 of ~essel 1 ser~es as a mixing chamber
for the vessel while the top half 19 serves as a gas planum
where powders entrained with the fluidizing gas may settle.
The blender of the present invention includes a fluid-
izing gas off-gas system comprising a fluidizing gas outlet 21
disposed at the top of the gas plenum 19, a cyclone separator
22 connected to recei~e fluidizing gas from the fluidizing ~
outlet 21 and a high efficiency filter 23 connected to receive
fluidizing gas from the cyclone separator 22. Gas discharged
from the high efficiency filter at 23 is e~entually directed
to the factory exhaust system. Solids separated out of the
cyclone separator 22 fall through a pipe 25 into a container 26
disposed below the cyclone separator 22. The off-gas system is
not part of this invention and is indicated only generally in
~0 the drawing. Any suitable con~entional off-gas system may be
employed.
Blending of the particles 11 in this type of blender is
effected by bubbles 10 of fluidizing gas emitted from the first
set of orifices or from a combination of the first and second
sets of orifices. Bubbles of g~s rise from the orifices through-
out the bed to the top of the bed in wide sweeping zig-zag
motions. Once a bubble is formed/ adjacent particles flow ;
around its upper portion and down to its lower cavity so that
the bubble risesA Particles lying directly above the bubble are
forced upwardly as others are pushed aside with some flowing
down into the lower portion of the bubble filling its path. ,
Thus, a rising bubble spreads particles radially in all directions.
~ 3
As a given bubble rises particles filling its bottom cavity
are packed slightly more tightly than particles immediately
outside the bubble's path. The next bubble rising in that
general region will follow a path through the less tightly
packed particles just to the side of the first bubble's path.
Thus, each successive bubble will tend to rise in a different
location, blending other particles with the particles previously
blended. As more and more bubbles rise through the particle
~ bed, small adjacent bubbles ~ together forming larger ones.
This action, along with the bubbles flowing toward low pressure
regions, causes a wide sweeping zigzag bubble motion, creating
horizontal as well as vertical convective blending. Bubbles
escaping from the top of the particle bed scatter some UO2
powders into the gas plenum 19 at the top of the mixing vessel
1. However, the compressed gas escapes from the particle bed
in intermittent puffs. These intermittent puffs of gas allow
particles that would normally be entrained in the gas flow
an opportunity to fall back into the particle bed rather than
being entrained and swept out with the fluidizing gas. It is
to be emphasized that in the bubbling-bed fluidized bed blender
herein described, although there is the aforementioned circu-
latory blending, there is actually no mass movement of the
particle bed such as that existing in a spouting bed blender.
~C~
~~ne a homogenous blend of powders is achie~ed, the bed
is discharged from the hoppers in such a manner as to maintain -
a homogenous mixture of the powder. This is achieved by main- ;
taining blending conditions throughout the entire discharge
sequence, e.g., keeping the bed fluidized during discharge. The
homogenous mixture of powder falls from the open valves 6 so
rapidly that bubbles from the residual fluidizing gas injected
into the bed are still present substantially throughout the
bed during the rapid discharge. In effect this maintains a
-8-
.. :, . ... .
s ; : :. . :
y~
homogenous mixture of the powder. One possible method to
maintain a homogenous mixture is to open rapidly one half of
the valves (first set) for about one second while maintaining
fluidizing gas through the other half of the valves (second se-t).
Then after a suitable delay during which fluidizing gas is
injected throu~h all the valves, the second set of valves is
opened for about one second while fluidizing gas is injected
through the first set of valves. Each set of valves alternatively
discharges powder in this manner with the suitable delay period
between each discharge until the vessel is empty.
Referring now to FIGURE 2, a vertical section of one
hopper of the fluidized bed blender of the aforementioned Zenz
et al application is illustrated during one moment of operation.
This blender employs a fluidizing grid including a plurality of
hoppers such as the one depicted at 40 arranged in a linear
array, with a set of gas orifices, such as the one at 41, dir-
ecting fluidizing gas downwardly into the bottom of the hopper,
one such orifice being provided for each hopper. The volume 42
illustrates a typical fluidizing gas bubble that is repeatedly
blown by the jet of fluidizing gas emitted from the orifice 41.
The volume 45 adjacent to the volume 42 represents compacted
powder that forms when fine and cohesive types of UO2 powder
having an instantaneous flow function of less than about 4.0
are blended in the Zenz et al device. Volume 43 above the
volume 42 of fluidizing gas shows the particles of powder
suspended in the fluidizing gas.
Referring now to FIGURE 1, which has been previously
described, and to FIGURE 3, there is shown an apparatus of the
present invention for carrying out a method of blending a
heterogenous mixture of fine and cohesive powders, preferably
U2 powders. This method comprises the steps of pro~iding the
nuclear-safe mixing vessel l including the linear array of
. . .:
s
' generally downwardly-directed pyramidal-shaped hoppers 3,
- each having walls 4 converging into a conically-shaped opening.
A mixture of powders such as UO2 powders, to be blended, is
supplied to the vessel 1 in amount sufficient to partially fill
the vessel, usually to about one half the height of the vessel
1. A first set of fluidizing gas orifices illustrated at 50
in FIGURE 3 is disposed in a circular array about the opening 5
of each of the hoppers 3. The first set of fluidizing gas
orifices directs a plurality of fluidizing gas jets upwardly
in a divergent swirl-shaped pattern along the walls 4 of each
hopper 3. The volume 51 in FIGURE 3 illustrates where fluidizing
gas is directed in each hopper by the first set of orifices.
It may be seen from the shape of the volume 51 that compaction
and build-up of fine and cohesive powders within and about the
hopper 3 (as indicated by the volume 45 in FIGURE 2) is sub-
stantially eliminated due to fluidizing gas from the first set of ~.
orifices. Volume 51' shows a bubble rising from the hopper
toward vessel 1. Compaction and build up of UO2 powders within
the entire hopper is prevented by impingement of the first set
of orifices upon the possible compaction volume (volume 45
shown in FIGURE 2), and by the flow of fluidization gas from the
first set of orifices along the walls 4 of the hopper 3 which
reduces friction between the UO2 powders and the walls 4 of the .
hoppers. Volume 53 above gas bubble 51' shows the region where
the particles of powder are suspended in the fluidizing gas.
Volume 51" shows fluidizing gas from orifice 12 discussed more
fully below.
Refe.rring also to FIGURE 3, valves 6 are welded to
the bottom of the hoppers 3 at 59. The valves 6 are ball valves
having in one embodiment a 1-1/ 3 inch diameter full throat 61.
Each valve has an operating lever 64 associated with a valve
! stem 63 to allow the valves to be individually opened for
-10-
7'~
discharging portions of the fluidized bed blender into transport
containers (not shown) disposed below each valve.
Fluidizing gas is supplied to the first set of orifices
50 in an amount sufficient to cause bubbles of fluidizing gas
to rise through the mixture of powders and emerge from the top
of the powders. Flow of the fluidizing gas through this set
of orifices is contained until a homogenous blend of powders is
achieved. More specifically, fluidizing gas is supplied to the
first set of orifices in an amount sufficient to provide a
superficial gas velocity in the mixing vessel in a range of 1.25
to 2.0 feet per second. A typical blen~ing period for achieving
a blend of fine and cohesive powders to nearly ideal quality is
about 50 seconds to about 5.5 minutes. To prevent plugging of
the first set of orifices 50, the orifices 50 are sized such
that the fluidizing gas is emitted therefrom at sonic velocity.
Normally, the fluidizing gas employed in all of the steps of the
present blending method is dry nitrogen or dry air at ambient
temperature.
The method of the present invention may further include
the step of directing a second set of fluidizing gas orifices
12 downwardly into the opening 5 of the hopper 3. The second
set of orifices in the preferred embodiment comprises a single
orifice the center of which is positioned along the center line
of the pyramidal-shaped portion of the hopper. Fluidizing gas
is then supplied to the first and second sets of orifices in a
combined amount sufficient to cause bubbles of fluidizing gas
to rise throughout the mixture of powders and emerge therefrom
until a homogenous blend of powders is achieved.
In this step of the method, the flow of fluidizing
gas tothe first set of orifices is pulsed for a short portion
of a cycle with the balance of the cycle being a purge flow,
and the flow of fluidizing gas to the second set of orifices
.
'7 ~ ~ ~
is continuous. Most of the fluidizing gas being introduced to
-~ the mix~ng vessel during this step of the method is introduced
ro~
the second set of orifices. The first set of orifices
is periodically pulsed to eliminate compaction and bridging
of fine and cohesive powders within the entire volume of the
hoppers. It has been determined that it is desirable to pulse
the first set of orifices at sonic velocity for a period of one
second out of every period of six seconds that the process is
run.
Further, to prevent surging or periodic lifting of
the bed of powder, all the orifices contained within one hopper
are pulsed for one second out of every period of six seconds
at the same time and a sequence of randomly pulsing the different
hoppers is established. By the term "randomly", as employed
in the preceding sentence, it is intended to indicate that the
pulsing arrangement is such as to avoid pulsing simultaneously
adjacent hoppers in the linear array. Such pulsing would tend
to have the effect of causing surging or periodic lifitng of the
~ p~~
bed of powders, preventing e~ operation of the apparatus.
One particular type of pulsing arrangement which is
~0 embraced by the term "randomly" is best illustrated in FI~URE 4
which indicates the order of pulsing of the groups comprising
a first set of orifices providing fluidizing gas to a linear
array of fourteen hoppers. In this figure, the valves which
are associated with the fourteen hoppers and which include the
groups of orifices of the first set of orifices are numbered
along the abscissa from 1 through 14 and the time in seconds
is indicated on the ordinate. The X's in the chart shown in
FIGURE 4 indicate the groups of orifices associated with par-
ticular valves which are pulsed during each second of the six-
second cycle involved. Thus, referring to FIGURE 4, it can
be seen that during the first second, valves 1, 7 and 14, which
are separate from each other, are pulsed. During the second
-12-
,
.
second, valves 3 and 9, which are also separate from one another,
are pulsed. During the following four seconds, valves 4 and
10 are pulsed, then valves 2, 8 and 13 are pulsed, then valves
5 and 11 are pulsed and finally valves 6 and 12 are pulsed, there-
by completing the cycle which results in pulsing of fluidizing
gas through the groups of orifices in all fourteen valves. It
will be understood that thereafter the sequence is repeated,
beginning again with valves 1, 7 and 14.
In FIGURE 3 the means for pulsing the orifices 50
in a ball 65 of a ball valve 6 is shown. ~ pulse manifold
100 supplies sufficient gas pressure through electrically
operated solenoid valve 101 in line 9 to create a sonic flow
condition (pulsing) from the eight orifices 50. This solenoid
valve 101 is operated by a programmable controller 103 to provide
the timing for pulsing as discussed above with reference to
FIGURE 4. A similar arrangement has other solenoid valves
regulating the supply of fluidizing gas to the ball valves
orifices for the other hoppers in the linear array. During the
time the orifices 50are~not being pulsed, a purge gas flow
prevents back flow of gas and/or solids into the orifices 50.
20 ~ The purge gas flows from manifold ~d~into flow line 105 through
orifice plate 106 to line 9 and orifices 50. This flow is
negligible during the pulse period because the high pressure
through valve 101 and line 9 stops flow from flow line 105
through line 9.
Referring now to FIGURE 5, which shows further details
of the fluidized bed blender, it may be seen that the hoppers
3 are welded to adjacent hoppers at 56 and to a rectangular
transition piece 57 at 58 to form a linear array. The trans-
ition piece 57 is connected to the bottom of the mixing vessel
1 in any suitable fashion, for example by bolts. In a
preferred embodiment, to insure a nuclear-safe vessel 1 for
blending UO2 powder enriched with the U-235 isotope in
in amounts from about 0.7% up to 4.0% by weight, the vessel
1 has a maximum width of about five inches corresponding to the
top dimensions (width and length) of each hopper. The
transition piece 57 is of L-shaped cross-section and forms a
central rectangular opening corresponding in size to the tops of
the array of hoppers 3. The transltion piece thus surrounds the
hoppers and is welded thereto as previously indicated.
Referring now to EIGURES 3, 6/ 7 and 8, the rotary
closure member 65 of each ball valve 6, held between gasket 68,
is provided with a group of orifices 50 disposed in a generally
circular array. Fluidizing gas flows into the body 66 of the
valve 6 and into the throat 61 of the closure member 65 and
through the orifices 50 into the bottom of the hopper 3. Together
the groups of orifices of the several valves utilized with the
linear array of hoppers comprise the first set of orifices,
which feed fluidizing gas to the hopper when the ball valves are
in the closed position shown in FIGURE 3. When a Yalve is
turned by its lever 64 to the open or discharge position, its
throat 61 is brought into alignment with the bottom of the
hopper and the orifices 50 are moved to a position out of
communication with the hopper.
As best shown in FIGURES 6, 7 and 8, the first set of
orifices is provided by drilling a plurality of gas passages
70 in each rotary closure member 65. The orifices 50 formed
by the passages 70 at the outer surface of the closure member -
65 are disposed in a circular array. The gas passages 70 extend
to the throat 61 and are inclined at an angle ~ with respect
to a line 71 which coincides with the central axis of the hopper
(shown in FIGURE 7~. The passages 70 are also inclined at an
angle 9 (shown in FIGURE 8? with respect to a line 72 normal
to the surface of the rotary closure member 65 at the orifice
50~ For/hopper having the~ dimensions set forth above, the angle
&~ in FIGURE 7 is the angle (about 15) between the line 71
-14-
,: , ;
, ,., : .
and the centerline of passage 70. The angle ~ (about 30)
is formed between the normal line 72 and a line 74 passing
axially through the bore of the gas passage 70, shown in FIGURE
8. This location of the passages 70 prevents plugging of the
passages 70 during loading of the blender as powder charged to
the blender does not enter passages 70.
Referring now to FIGURE 8, a detailed view of one of
the drilled gas passages 70 is illustrated. The passage 70 is
0.0625 + 0.0005 inch in diameter for a hopper having about a
1.5 inch diameter outlet. A total of eight passages are provided
in each rotary closure member 65 as shown in FIGURE 6. The
orifices 50 are sized so that when the full flow of fluidizing
gas is passed therethrough in an amount sufficient to provide
a superficial velocity in the mixing vessel 1 in a range of 1.25
to 2.0 feet/second sonic flow is achieved in the orifices.
Referring again to FIGURE 3, a second set of fluidizing
gas orifices 12 is also provided. Each orifice is disposed at
the end of elbow-shaped blowpipes 15 which direct fluidizing
gas jets downwardly into the bottom of each hopper 3. The
blowpipes 15 are bolted to the transition piece 57 at 82 and
are removably connected to the manifold 8 in a T connection so
that different blowpipes having different orifice sizes may
be substituted, if desired. The size of the orifices 12 is
normally in a range of 5/16 to 3/8 of an inch in diameter. This
is sufficient to blow fluidizing gas bubbles~ in a column of
U2 powders about five inches high, of approximately 2-1/2
inches in diameter, when dry nitrogen or dry air at ambient
temperature and 3 to 4 psi is used as a fluidizing gas. In
the preferred embodiment of this invention the mixing vessel 1
has a maximum width, indicated by W in FIGU~E 3, of about 5
inches and the hoppers 3 are arranged as shown, in a single
linear array. The top of each hopper 3 has a five-inch square
-15-
cross section, indicated at 52 in FIGURE 3. Each hopper tapers
from this cross section at the top to a circular cross section
about 1 1/2 inches in diameter at the bottom adjacent the
first set of orifices 50. In one specific embodiment the height,
indicated at H in FIGURE 3, of each hopper is about 6.53 inches
and the walls 4 of the hopper form an angle, indicated at ~ in
FIGURE 3, of 75 with the horizontal.
When only the first set of orifices is employed,
fluidizing gas is supplied to this set of orifices in a con-
tinuous manner in an amount sufficient to cause bubbles of
fluidizing gas to rise through the mixture of powders and
emerge from the top of the powder until a homogenous blend of
powder is achieved. As indicated earlier the fluidizing gas
so supplied has a velocity in the range of 1.25 to 2.0 feet/
second.
When both the first and second sets of orifices are
employed, the primary flow of fluidizing gas is supplied in a
continuous manner through the second set of orifices and addi-
tional fluidizing gas is supplied in a randomly pulsed manner,
as described above through the first set of orifices. The
combined gas flow through the two sets of orifices is in an
amount sufficient to cause bubbles of fluidizing gas to rise
through the mixture of powders and emerge at the top surface
thereof until a homogenous blend of powder is achieved. The
combined gas flow has a veIocity in the range of 1.25 to 2.0
feet/second.
Other forms, embodiments and applications of the
inyention may occur to those skilled in the art and it is
intended by the appended claims to cover all such modifications
coming within the proper scope of this inyention.
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.; , . ,' ~ . .;, !
