U.S. patent number 3,583,681 [Application Number 04/825,891] was granted by the patent office on 1971-06-08 for gravity-flow solids blending.
This patent grant is currently assigned to E. I. du Pont de Nemours and Company. Invention is credited to George N. Brown.
United States Patent |
3,583,681 |
Brown |
June 8, 1971 |
GRAVITY-FLOW SOLIDS BLENDING
Abstract
A gravity-flow particulate solids blender having downcomers
provided with traps at their bottom ends retaining solids in static
repose during blender standby provided with gas sweeps effecting
controlled delivery of solids into a common collector during
operation.
Inventors: |
Brown; George N. (Madelyn's
Garden, Wilmington, DE) |
Assignee: |
E. I. du Pont de Nemours and
Company (Wilmington, DE)
|
Family
ID: |
25245160 |
Appl.
No.: |
04/825,891 |
Filed: |
May 19, 1969 |
Current U.S.
Class: |
366/107; 366/191;
366/134; 366/136; 366/192 |
Current CPC
Class: |
B01F
5/245 (20130101) |
Current International
Class: |
B01F
5/00 (20060101); B01F 5/24 (20060101); B01f
005/24 () |
Field of
Search: |
;259/4,36,95,180,18 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Jenkins; Robert W.
Claims
I claim:
1. In a gravity-flow particulate solids blender having an elevated
main storage vessel, a multiplicity of solids withdrawal downcomers
connected at their top ends in open communication at preselected
points with said storage vessel, and a common collector receiving
solids discharged from the bottom ends of said downcomers,
individual traps interposed between the bottom ends of said
downcomers and said common collector retaining solids emerging from
said downcomers in static repose during blender standby, and means
directing a regulated solids-conveying gas stream through solids
held up in said traps and thence into said collector effecting
controlled drawoff of solids from said downcomers during blender
operation.
2. Apparatus according to claim 1 wherein said means directing a
regulated solids-conveying gas stream through solids held up in
said traps is a vacuum source.
3. Apparatus according to claim 2 wherein the gas inlet sides of
said means directing a regulated solids-conveying gas stream
through solids held up in said traps are provided with flow
restrictors such as valves or orifices.
4. Apparatus according to claim 2 provided with a regulable gas
bypass valve connected to maintain a preselected level of vacuum
effectively applied by said vacuum source to said means directing a
regulated solids-conveying gas stream through solids held up in
said traps.
5. Apparatus according to claim 1 wherein said means directing a
regulated solids-conveying gas stream through solids held up in
said traps is a pressure jet disposed with discharge opening in the
range of about 1 inch-- 2 inches from the face of the slope of said
solids adjacent said collector measured during repose.
6. Apparatus according to claim 5 wherein said pressure jet is
mounted generally perpendicularly adjustable of said face of said
solids adjacent said collector.
7. Apparatus according to claim 1 wherein said means directing a
regulated solids-conveying gas stream through solids held up in
said traps comprises a multiplicity of pressure jets, each
individually serving a single trap, a common manifold supplied with
solids-conveying gas from a pressure source, individual gas flow
regulation valves interposed between said manifold and said
pressure jets, and a common gas pressure control valve interposed
between said pressure source and said manifold.
8. In a process for the gravity-flow blending of solids comprising,
in sequence, confining a mass of the heterogeneous solids in an
elevating column, withdrawing as separate fractions from said mass
in a generally vertical direction substantially equal amounts of
said solids per unit time simultaneously from a multiplicity of
different regions of said mass, and combining said separate
fractions in a common collector to produce a solids blend having
improved homogeneity of composition, the improvement consisting of
entrapping said solids in said separate fractions at the discharge
ends of their courses before combining in said common collector to
thereby retain the solids of said fractions in static repose during
blending standby and directing a regulated solids-conveying gas
stream through said solids at said discharge ends of said courses
effecting controlled drawoff of solids from said separate fractions
into said common collector during blending operation.
Description
BRIEF SUMMARY OF THE INVENTION
Generally, this invention comprises, in a gravity-flow particulate
solids blender having an elevated main storage vessel provided with
a multiplicity of solids withdrawal downcomers connected at their
top ends in open communication at preselected points with the
storage vessel and a common collector receiving solids discharged
from the bottom ends of the downcomers, individual traps interposed
between the bottom ends of the downcomers and the common collector
retaining solids emerging from the downcomers in static repose
during blender standby, and means directing a regulated
solids-conveying gas stream through solids held up in the
individual traps and thence into the collector effecting controlled
drawoff of solids from the downcomers during blender operation,
together with the method of solids feeding from the downcomers.
DRAWINGS
FIG. 1 is a partially broken schematic side elevation view of a
preferred embodiment of gravity-flow solids blender according to
this invention wherein solids drawoff from the downcomers is
effected by vacuum,
FIG. 2 is a side elevation section of a downcomer-collector
juncture taken on line 2-2, FIG. 1, utilizing, however, an orifice
for sweep gas flow control instead of the valve of FIG. 1,
FIG. 3 is a side elevation section of a second embodiment of
downcomer-collector juncture which can be substituted for the FIG.
2 construction where conditions prevent lateral offsetting of the
downcomer discharge ends from the collector,
FIG. 4 is an end view, partly in section, taken on line 4-4, FIG.
3,
FIG. 5 is a partially broken schematic side elevation view of a
preferred embodiment of gravity-flow solids blender according to
this invention wherein solids drawoff from the downcomers is
effected by superatmospheric pressure,
FIG. 6 is a schematic plan view of a downcomer trap-collector
subassembly adapted to use in the apparatus of FIG. 5,
FIG. 7 is a side elevation taken on line 7-7, FIG. 6, only a single
associated downcomer being detailed,
FIG. 8 is a schematic plan view detailing a different design of
downcomer-collector subassembly which can be substituted for that
of FIGS. 6 and 7 where the downcomers are spaced apart sufficiently
at their bottom ends to accommodate the collector therebetween,
FIG. 9 is a sectional view taken on line 9-9, FIG. 8, and
FIG. 10 is a plot of individual downcomer solids discharge rate in
lbs./hr. versus both gas jet throughput in ft..sup.3 /min. and jet
velocity in ft./min. using pressurized air as the sweeping gas.
Gravity-flow solids blending such as disclosed in Reissue Pat. No.
25,687, as to which applicant was a joint inventor, has proved to
be highly effective in the quick and thorough blending of most
particulate solids, quite independent of individual solids particle
mobilities.
Referring to FIG. 1, such blending involves confining a mass of
particulate heterogeneous solids which it is desired to blend in an
elevated column, such as by retention within the upright vessel
indicated generally at 10, provided with loading line 9,
withdrawing from the mass in a generally vertical direction and
within about one-fourth of the distance from the center of the mass
to the confining wall of the column, taken at the level of
withdrawal inwardly from the periphery of the solids mass,
substantially equal amounts of the solids per unit time
simultaneously by gravity flow from a multiplicity of regions
disposed lengthwise of the mass and substantially equiangularly
around the periphery of the mass, and combining these equal amounts
of solids to produce a solids blend having improved homogeneity of
composition.
Solids withdrawal from different regions of the solids mass is
conveniently effected by a multiplicity of open (i.e., unvalved)
solids delivery conduits 11 (in FIG. 1 nine in number, two serving
the cone bottom lying back of the front two, so as not to be
visible in the showing) hereinafter, and in the claims, referred to
as "downcomers," which discharge the gravity flow of solids
therethrough via their open bottom ends into a common collector 12.
Although not too clearly shown in FIG. 1, the lower end of the
frustoconical bottom of vessel 10 is serviced by an individual
downcomer 11, so that the entire solids mass within vessel 10 is
moved generally downwards during operation. From collector 12 the
recombined solids can be optionally recycled back to the top of
vessel 10 for additional blending through lines 14 and 14' by
either vacuum (FIG. 1) or pressure pneumatic transport (FIG. 5), or
withdrawn as blended product through delivery line 15 by
diversional flow obtained by switching flapper valve 16
approximately 30.degree. clockwise. This closes passage to line 14'
and simultaneously opens passage through line 15, which latter
must, of course, be provided with its own pneumatic transport
facility (not shown) for continued transfer of product to
succeeding process equipment.
The vacuum system for the apparatus of FIG. 1 comprises a blower 18
pulling a vacuum via line 19 through a conventional solids
separator 20 opening into the top of a small volume solids receiver
21 into which line 14' discharges tangentially. Vessel 10 is
maintained at atmospheric pressure by a top open vent connection
23, recycled solids being returned continuously thereto via
conventional motor-driven rotary solids feed valve 22, which
constitutes an air lock between the vacuum system and vessel 10.
Conveying air for solids transport is drawn in through filter 24
disposed adjacent the discharge opening of collector 12, a
regulating valve 25 being interposed therebetween.
Gravity-flow blenders of the prior art have hitherto employed
direct gravity-flow solids discharge out of the downcomers 11 into
the collector 12, and this has been accompanied by difficulties in
flow equalization through the multiple downcomers, particularly
with specific solid materials which, it will be understood, can
range in gravity-flow characteristics from exceedingly mobile to
actually "sticky." As a cure for this problem, flooded operation in
which solids are temporarily held up within collector 12 so that
they bank across the discharge ends of the downcomers to thereby
throttle flow therethrough is disclosed in U.S. Pat. No. 3,158,362.
Applicant, however, preferred the use of a centrally disposed
frustoconical baffle internal of collector 12, as taught in his
U.S. Pat. No. 3,208,737, a modification of which is also the
subject of U.S. Pat. No. 3,347,534.
None of these patented approaches has proved completely
satisfactory with all particulate solids which have to be blended.
Specifically, discharge from downcomers 11 is affected unequally
and unpredictably by the variable amount of material momentarily
disposed across the downcomer outlets. Also, solids hold-up within
collector 12 effectively isolates substantial process material from
the blending process. In addition, variations in flow through
certain downcomers sometimes drastically affects flow through
neighboring downcomers, so that, in very extreme cases, as, for
example, with some fluidizable materials, it has been found that
some flow through a given downcomer reverse to the normal flow can
actually occur during periods of unstable operation. Finally, extra
valves are usually required to insure regulated withdrawal of the
commingled solids from the flooded collector.
This invention solves all of the foregoing problems as well as
provides positive selectability of solids flow rates through
individual downcomers 11 by utilizing indirect transfer of the
discharged solids from the several downcomers via traps into
collector 12 through the agency of air sweeps traversing the bottom
ends of the downcomers and exhausting into the collector.
Referring to FIGS. 1 and 2, a preferred construction utilizes
vacuum to induce the individual downcomer solids-conveying gas
sweeps.
In this embodiment the lower terminal ends of all of the downcomers
11 are offset a short distance laterally from the periphery of
collector 12 at a level slightly below the top thereof. The traps
in this design are inexpensive pipe tees 29, horizontally mounted
so that one straight-run connection of the tee is coupled through
short nipple 30 to the interior of collector 12, whereas the
opposite straight-run connection is provided with an airflow
restrictor such as valve 31 (FIG. 1) or orifice 31' (typically
1-inch dia.), FIG. 2, communicating with the outside, where
atmospheric air is a suitable sweep gas from the process
standpoint. The remaining connection of the tee is oriented
upwardly to receive the bottom end of downcomer 11 in a tight slip
fit, so that solids discharge occurs at a distance approximately 1
inch above the bottom wall of the tee.
It is essential that the downcomer ends be disposed well within the
tees 29 in order to provide open space for the sweep gas to clear
past the pile of solids gathered at the discharge openings of the
downcomers 11 in order to entrain particles on the outside surfaces
of the piles and deliver them to collector 12. Typically, a
downcomer disposition in which the discharge end lies approximately
at the horizontal axis of the tee 29 gives good performance.
It is frequently practicable to employ polymeric tubing for the
downcomers 11, in which case it is relatively easy to adjust the
clearance between the downcomer discharge ends and tees 29 by
simply manually slipping the tubing into or out of the upwardly
oriented tee connections an appropriate amount, this adjustment
being facilitated by the inherent flexibility of the polymer.
It will be understood that the offset A from the centerline of
downcomer 11 to the inside periphery of collector 12 must slightly
exceed the radius of the base of the solids pile (typically, by
about 1 inch for granular polyethylene) as it subsists in static
repose, which is the condition depicted in FIG. 2. Then, in standby
condition with no sweep gas drawn through the flow restrictors 31,
31', the solids simply accumulate at the exit ends of the
downcomers, thereby blocking all downcomer solids throughput and
all delivery to collector 12.
However, when sufficient vacuum (typically, 3 inch H.sub.2 0) is
applied to collector 12 by blower 18 during blender operation,
sweep air is drawn through all of the flow restrictors 31, 31'
simultaneously at a conveying velocity through the traps of
approximately 4000 ft./min. or higher, and solids are thereupon
entrained from the sides of the solids piles and conveyed out of
traps 29 through nipples 30 into collector 12 and immediately
discharged therefrom into line 14, without any solids accumulation
within the collector. Close equality of solids discharge rates from
downcomers 11 is automatically attained with orifices 31', whereas
regulation to equality or, if desired, preselected difference in
discharge rates, is obtainable by suitable adjustment of valves 31.
An overall control of vacuum applied is provided by manually
operated bypass valve 32 equipped with its own filter 33, which is
connected from atmosphere into line 14 downstream from collector
12. The size of valve 32 is chosen so that, when opened wide, it is
effective to admit a large enough flow of bypass air to cut off all
solids delivery out of traps 29.
Where space limitations prevent lateral accommodation of downcomers
11, the design of FIGS. 3 and 4 is preferred, wherein the traps 37
are boxlike structures mounted within collector 12. Here the
inboard end of the trap is provided with a weir lip 38 defining a
solids delivery opening 39 with the top closure of the collector.
Sweep gas introduction is effected through radially disposed
nipples 40, again provided with valves 31 or orifices 31' (not
shown) as hereinbefore described for FIGS. 1 and 2, aligned with
openings 39.
Gravity-flow solids blending with superatmospheric pressure
pneumatic solids conveying is entirely practicable, a typical
apparatus being shown schematically in FIG. 5. Here the number of
downcomers 11 has, for simplicity of representation, been limited
to five, four of which draw from the upper cylindrical section of
solids storage vessel 10 whereas the fifth is connected to the
vertex of the vessel's cone bottom. As in FIG. 1, vessel 10 is
provided with a heterogeneous solids supply line 9 for introducing
solids to be blended and a top vent 23 opening to atmosphere, which
is fitted with a dust filter 44 to remove entrained dust from the
exhaust.
Downcomers 11 discharge into collector 12 via traps hereinafter
described, the collector outlet being connected to line 14 through
conventional continuously operating motor-driven rotary solids feed
valve 45, which constitutes an air lock therebetween. Rotary valve
45 is operated at a rotational speed high enough to continuously
remove all solids from collector 12, so that there is no solids
accumulation therein. The product removal piping system is
identical with that of FIG. 1, constituting branched recycle line
14' and product delivery line 15, optionally selected by flapper
valve 16. Pressurized pneumatic solids conveying air is supplied by
blower 46 connected upstream from rotary valve 45.
Solids withdrawal into collector 12 is accomplished with the
construction of FIGS. 6 and 7, which utilizes internally mounted
traps 49 into the tops of which downcomers 11 open, as particularly
detailed for the single downcomer shown at the left-hand side of
FIG. 7. Vertex downcomer 11 is served by centrally disposed trap
49, thus permitting a straight-run connection for this vital
downcomer. The upper closure of collector 12 is provided with a
central opening 35 aligned with the opening in square frame plate
36 to which the lower ends of downcomers 11 are attached,
permitting exhaust of sweep gas from the collector. Traps 49 are
generally similar to traps 37 of FIGS. 3 and 4, except that they
taper downwardly into V-troughs, the open ends of which are fitted
with weir lips 50. The downwardly depending wall sections 49' and
upwardly extending weir lips 50 define, between them, solids
discharge openings 51.
Solids-entraining sweep gas is supplied through air jets 55 aligned
with openings 51, the jets preferably being adjustably mounted
longitudinally of traps 49 by slip-fitting through threaded nipple
retainers 56 fixedly secured through openings in the collector wall
into the traps. Jet pipes 55 are held fixedly in preselected
longitudinal position within nipple retainers 56 by locknuts 57. In
the FIG. 5 apparatus, air delivery to jets 55 is regulated by
valves 52 interposed between the jets and an air supply manifold
53, hereinafter described in greater detail, which obtains its air
in turn from either a high pressure plant air supply line 54 or, as
is sometimes more convenient, directly from blower 46 by the
cross-connection 58, indicated in broken line representation,
provided with regulating valve 59.
If desired, pressurized sweep gas can, of course, be supplied to
jets 55 through individual lines connected to the outboard ends;
however, it is most convenient to utilize a manifold system such as
that detailed for the alternate arrangement of FIGS. 8 and 9.
The latter apparatus services eight downcomers 11 which discharge
into open-topped collector 12 through laterally oriented elbows 61
of sufficient horizontal run to constitute traps retaining the
solids in static repose during blender standby when no sweep gas is
supplied to gas jets 62, the condition depicted in FIG. 9.
Collector 12 is, in this instance, a very small volume receptacle
connecting to the input side of conventional motor-driven rotary
solids valve 45, which is continuously operated at a high enough
rate of rotation to immediately remove substantially all solids out
of the discharge end of the collector, so that there is never any
accumulation of solids therein. Rotary valve 45 discharges into
line 14, solids delivered thereto being immediately conveyed away
by pressurized gas supplied by blower 46, FIG. 5.
Gas jets 62 are preferably adjustably mounted longitudinally of the
horizontal runs of elbows 61 by assemblies such as those already
described for jets 55 with reference to FIG. 7.
A common pressurized gas supply (e.g., 1.5 to 2.5 lbs. operating
pressure for a 1-inch dia. pipe manifold) is provided for the eight
jets 62 (typically, 1/4-inch dia.) by use of the square manifold 64
which symmetrically encloses collector 12, connections to
individual gas jets 62 being made via manual control valves 63
interposed between the manifold and the jets. Preferably, a main
electrical solenoid gas supply control valve 65 is interposed
between the primary air supply line 67 and manifold 64 for control
purposes hereinafter described.
When solenoid valve 65 is open, and individual valves 63 are all
adjusted to desired settings, gas jets 62 discharge adjacent the
solids slope disposed towards collector 12 and the solids are
conveyed into the collector at a regulated rate. Jet air exhausts
freely through the open top of the collector, covered with a
protective screen 60, which can be further protected from ingress
of foreign material, rain water or other adulterants by provision
of a surmounting protective roof cap, not detailed. Simultaneous
shutoff of all solids flow is obtained by closure of solenoid valve
65.
Where entraining jets 55 or 62 are employed, with their relatively
high exit velocities, there is a critical position of the jet
outlet with respect to the slope of the solids pile adjacent
collector 12. Thus, if the jet outlet is less than about 1 inch
from the face of the pile, there is a tendency to blast the
particles from in front of the nozzle mouth, which reduces
effectiveness as a solids feeder. Similarly, if the jet is spaced
farther back than about 2 inches from the solids slope line, the
jet does not positively entrain particles.
However, where the jets are disposed within the 1-inch-- 2-inch
critical spacing, near-linearity of solids feeding is achieved, as
is indicated by the operational plot of FIG. 10. The slight
nonlinearity of solids feed which does exist is believed due to the
varying friction of solids material in flow through downcomers
11.
The plot of FIG. 10 graphically depicts the operation of a 1/4
-inch dia. air jet (having its orifice spaced 1.5 inches inwards
from the solids slope) passing pressurized air at essentially room
temperature. The solids utilized were granular polyethylene cubes
measuring approximately one-eighth inch on a side, with somewhat
rounded edges, having an unpacked density of approximately (1) 35
lbs./ft..sup.3 under which circumstances the following
relationships existed: ##SPC1##
In summary, the delivered rate of solids is a function of: (1)
total gas flow from the jet, (2) exit velocity of the jet and (3)
the immersion depth of the jet within the solids pile, as measured
horizontally from nozzle to surface.
Solids feeding by jet as taught in FIGS. 5--10, inclusive, is
independent of the level of solids maintained within vessel 10 and,
thus, there is no requirement for changing the settings of valves
52 (FIG. 5) or 63 (FIG. 9) to accommodate periodic emptying or
refilling of the blender.
With vacuum solids feeding in accordance with FIGS. 1--4,
inclusive, however, each solids-free downcomer 11 effectively
constitutes a bypass of gas which correspondingly reduces the
vacuum applied to the several traps 29 (FIG. 2) or 37 (FIGS. 3 and
4). Surprisingly, solids feeding entrainment by gas continues even
with one or more downcomers completely empty, but the solids feed
rate, of course, decreases proportionately. This can be compensated
for by providing a conventional automatic throttling control for
bypass valve 32, progressively reducing its air bleed flow to a
degree compensating for the progressive uncovering of the
downcomers 11 during emptying of vessel 10 and, conversely,
progressively increasing the air bleed flow as successive
downcomers each fill in turn during loading.
An important advantage of this invention is the fact that the
solids hold-up traps operate completely independent of one another
and thus can discharge into collector 12 in practically any
direction without interference from neighboring traps, and, also,
at different vertical levels. This permits the designer wide
latitude in the accommodation of a large number of downcomers, both
as to relative orientation and points of introduction into the
collector, which is a particular aid where the downcomers are
crowded closely together, as they frequently are in manufacturing
plants.
It is entirely feasible to use jets such vacuum-type those denoted
55 (FIG. 7) and 62 (FIG. 9) embedded in the piles of solids
discharged from downcomers 11 as gas flow restrictors for vacuum
systems instead of the valves 31 or orifices 31' taught with
respect to FIGS. 1 and 2, respectively. This necessitates drawing a
somewhat higher vacuum of, typically, 4 inches of Hg to obtain the
requisite jet air throughput; however, this is not disadvantageous.
In fact, where a large number of downcomers are involved, a smaller
size blower 18 is required for vacuum-type jets than would be the
case for the valve 31 orifice 31' restrictors.
In addition, it is practicable to use pressure jet feeding of
solids from downcomer traps to a system which employs vacuum
conveying away from the collector discharge, except that it is then
necessary to interpose a rotary feeder valve such as 22 or 45
between the outlet of collector 12 and line 14 to serve as an air
lock. With such a design, operation remains entirely unaffected by
any air bypassing through downcomers running empty, such as occurs
during filling or emptying of blender vessel 10, or the blending of
relatively small lots of material which are insufficient to cover
all of the downcomer inlets.
From the foregoing, it will be understood that this invention can
be modified extensively without departure from its essential
spirit, and it is accordingly intended to be limited only by the
scope of the following claims.
* * * * *