U.S. patent number 3,807,705 [Application Number 05/267,200] was granted by the patent office on 1974-04-30 for process and apparatus for solids blending.
This patent grant is currently assigned to E. I. du Pont de Nemours and Company. Invention is credited to Robert Garnett Humkey, Donald Raymond Marshall, Donald James Ryan.
United States Patent |
3,807,705 |
Humkey , et al. |
April 30, 1974 |
PROCESS AND APPARATUS FOR SOLIDS BLENDING
Abstract
A process and apparatus for the blending of a bed of particulate
solids operating cyclically by selectively regulated downward
gravity flow of a fraction of the solids, fluidized mixing outside
of the bed, followed by recycle to the top of the bed.
Inventors: |
Humkey; Robert Garnett (Orange,
TX), Marshall; Donald Raymond (Orange, TX), Ryan; Donald
James (Orange, TX) |
Assignee: |
E. I. du Pont de Nemours and
Company (Wilmington, DE)
|
Family
ID: |
23017747 |
Appl.
No.: |
05/267,200 |
Filed: |
June 28, 1972 |
Current U.S.
Class: |
366/101 |
Current CPC
Class: |
B01F
13/0244 (20130101) |
Current International
Class: |
B01F
13/02 (20060101); B01F 13/00 (20060101); B01f
005/12 () |
Field of
Search: |
;259/4,18,36,95,96,97,98,180,DIG.17 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Jenkins; Robert W.
Assistant Examiner: Coe; Philip R.
Claims
What is claimed is:
1. The method of blending a bed of particulate solids comprising,
in sequence, maintaining different residence times of separate
weight fractions of said solids within said bed while withdrawing
said solids in downward gravity flow from said bed, feeding said
weight fractions of solids to a common gas fluidization mixing
zone, intimately mixing said solids by gas fluidization, and
recycling said solids from said mixing zone to said bed.
2. The method of blending a bed of particulate solids according to
claim 1 wherein the fraction of said solids fluidized at any one
time is in the range of about 0.01 to about 0.20 of the total
quantity of said solids.
3. The method of blending a bed of particulate solids according to
claim 1 wherein said separate weight fractions of solids withdrawn
from said bed exceed five in number, all substantially equal in
magnitude.
4. The method of blending a bed of particulate solids according to
claim 1 wherein said separate weight fractions are substantially
equal in magnitude and the residence time distribution of solids
constituting said separate weight fractions of said solids
withdrawn from said bed is preselected to be approximately linear
with respect to the number of said separate weight fractions.
5. The method of blending a bed of particulate solids according to
claim 1 wherein said separate weight fractions are substantially
equal in magnitude and the ratio of the maximum residence time to
the minimum residence time of solids constituting said separate
weight fractions of said solids withdrawn from said bed is in the
range of about 1.5 to about 5.0.
6. Apparatus for blending a bed of particulate solids comprising,
in combination, a vessel containing said bed of particulate solids,
gas fluidization solids mixing means, means maintaining different
residence times of separate weight fractions of said solids within
said bed while withdrawing said solids in downward gravity flow
from said bed and delivering said solids to said gas fluidization
mixing means, and solids recycling means delivering said solids in
intimately mixed condition from said gas fluidization solids mixing
means back to said bed.
Description
BRIEF SUMMARY OF THE INVENTION
Generally, this invention comprises a process for blending a bed of
particulate solids comprising, in sequence, maintaining different
residence times of separate weight fractions of the solids within
the bed while withdrawing the solids in downward gravity flow from
the bed, feeding the weight fractions of solids to a common gas
fluidization mixing zone, intimately mixing the solids by gas
fluidization and recycling the solids from the mixing zone to the
bed, together with apparatus for conducting the process.
DRAWINGS
In the drawings,
FIG. 1 is a partially schematic cross-sectional side elevation view
of one embodiment of blending apparatus according to this invention
provided with planar baffles,
FIG. 1A is a partially schematic cross-sectional side elevation
view of the main entrainment gas supply nozzle of the apparatus of
FIG. 1,
FIG. 2 is a full plan view taken on line 2--2, FIG. 1,
FIG. 3 is a partially schematic cross-sectional side elevation view
of a second embodiment of blender according to this invention
provided with helical baffles,
FIG. 4 is a partially schematic cross-sectional side elevation view
of a third embodiment of blender according to this invention
provided with both planar baffles and an enlarged antechamber at
the inlet end of the draft tube,
FIG. 5 is a partially schematic cross-sectional side elevation view
of a solids flow throttling means for a fourth particularly
preferred embodiment of this invention,
FIG. 5A is an enlarged (2X) view looking upwards from line 5A--5A,
FIG. 5,
FIG. 6 is a log-log plot of Mixer Weight Fraction versus Number of
Displacements to Blend for the two conditions (a) layer fill and
(b) 0.5 fill for apparatus according to this invention,
FIG. 7 is a plot of Residence Time Ratio (Maximum/Minimum) versus
Number of Displacements to Blend as to which the Mixer Weight
Fraction (f) was preselected to be 0.05, 0.10 and 0.20,
respectively,
FIG. 8 is a plot of Residence Time Ratio (Maximum/Minimum) versus
Number of Displacements to Blend wherein the Column Residence Time
was preselected to increase linearly with column number for Mixer
Weight Fractions (f) of 0.02, 0.05 and 0.10, respectively,
FIG. 9 is a plot of Residence Time Ratio (Maximum/Minimum) versus
Number of Displacements to Blend, with Mixer Weight Fraction (f) =
0.10 in all cases for the different total number of columns of
solids exiting the apparatus preselected to be 3, 4, 5, 6 and 12,
respectively,
FIG. 10 is a plot of Residence Time Ratio (Maximum/Minimum) versus
Number of Displacements to Blend for two different two
column-blenders operating with 0.5 fill and having preselected
Mixer Weight Fractions (f) of 0.1 and 0.2, respectively,
FIGS. 11A and 11B are plots of Number of Displacements versus
Marker Concentration in Exit for two column apparatus
configurations operating with 0.2 Mixer Weight Fractions for an
in-phase Residence Time Ratio (Maximum/Minimum) of 2.1 and an
out-of-phase Residence Time Ratio (Maximum/Minimum) of 1.6,
respectively,
FIG. 12 is a schematic side elevation crosssectional view of a
4-column physical model apparatus which was constructed to verify
the mathematically predicted operation of blending apparatus
constructed according to this invention,
FIG. 13 is a comparative plot of Residence Time Ratio
(Maximum/Minimum) versus Number of Displacements to Blend (a) as
computed and (b) as measured using the apparatus of FIG. 12,
FIG. 14 is a plot of Column Number versus Column Displacement
Time/Blender Displacement Time for the conditions (a) column
residence time linear with column number (full line plot) and (b)
column velocity linear with column number (broken line plot),
respectively,
FIG. 15 is a broken section plan view looking upwards of a
preferred design of solids flow-constricting valves employed in a
preferred embodiment of this invention, and
FIG. 16 is a sectional view taken on line 16--16, FIG. 15.
DETAILED DESCRIPTION
Solids blending according to this invention is intended to have
application to quite uniformly sized particulate solids, such as
pelletized polyethylene and the like, as distinguished from solids
mixtures having a relatively wide distribution of particle sizes
such as, for example, dry Portland cement-aggregate mixes.
This invention utilizes sequential
1. gravity-flow downward progression of solids through a relatively
dense bed, during which a substantial proportion of the solids are
delayed in passage by baffles or other flow regulators whereas the
remainder of the solids gravitates downwardly relatively
unhindered,
2. fluidized solids mixing and
3. recycling of solids to the bed.
Referring to FIG. 1, one embodiment of our apparatus comprises a
vessel having a cylindrical top section 11 closed at the bottom by
a frusto-conical bottom section 12. A draft tube 15 is, in this
design, mounted co-axially of top section 11 with lower end in open
communication with bottom section 12 and upper end in open
communication with top section 11.
Optimum solids loading level for the apparatus is along line a--a,
disposed slightly below the top end of tube 15.
The roof closure of the apparatus, which is not critical to
operation, in this instance consists of a squat frusto-conical
member 16 which is provided with a centrally located gas discharge
pipe 17 and a raw unblended solids introduction pipe 18 in circuit
with which is valve 19.
To prevent entrainment discharge from draft tube 15, a solids
deflector 20 is disposed coaxially with respect to tube 15 about
one tube radius thereabove and provided with a circularly dished,
downwardly oriented deflection hood 20a. Solids striking hood 20a
are deflected downwardly into the main bed of solids, denoted at
23, whereas gas exhausted from tube 15 is led out through annular
passage 20b and thence through discharge pipe 17 to a dust
collector (not shown) or to the atmosphere, as desired.
Conical bottom section 12 is provided with a plenum chamber 24
extending around the entire circumference of the cone at about half
height, the bottom edge of which is sloped downwardly to provide,
with the inner wall overhang 12a of the cone, a continuous
peripheral slot 25 through which solids fluidizing gas is supplied
via nozzles 26 spaced at equal angular intervals around the cone
section outside periphery.
A T-fitting 28 is connected to the apex of conical section 12 with
the axial branch fitted with a solids gravity-flow drain valve 28a.
The side branch, 28b, is a nozzle for the supply of entrainment gas
during solids blending operation of the apparatus. In addition,
main entrainment gas supply nozzle 29 is inserted through the side
wall of cone 12 with discharge end aligned axially upward of draft
tube 15 at a level approximately one draft tube diameter below the
lower end of tube 15, giving particularly good solids
entrainment.
As detailed in FIG. 1A, nozzle 29 comprises an upwardly bent
pressurized air supply conduit 29a provided at its discharge end
with an upwardly directed conically-formed hollow check valve
element 42, which is raised axially upward from its seat at the
discharge end 29b of conduit 29a to permit air discharge upwardly
from nozzle 29. To effect this operation, and at the same time
retain the check valve in correct vertical alignment for accurate
gravity reseating within discharge end 29b, the valve is provided
with an axially disposed threaded guide shaft 43 which is secured
against rotation at its upper end within the hollow base member 42a
of valve element 42 by pin 44 inserted through drilled hole 45
aligned with a companion blind bore 46 disposed diametrically
opposite thereto. Pin 44 lies loosely across a transverse flat 47
formed by cutting away approximately one half of the thickness of
the upper end of shaft 43. The lower end of shaft 43 is slidably
guided within a vertical tube piece 48 coaxially supported by an
open spider horizontal framework 49 weld-attached to the inside of
conduit 29a. A set screw 50 attached to the base end is slidable
within longitudinal slot 50a cut in piece 48, thereby locking shaft
43 against rotation during screw adjustment of valve element 42
longitudinally of shaft 43. In operation, when air is supplied
under pressure through nozzle 29, element 42 rises from seat 29b,
thus providing solids entrainment air supply into the bottom of
draft tube 15. When air supply is discontinued, check valve element
42 immediately drops back on seat 29b, thereby preventing back flow
of solids into nozzle 29.
Referring also to FIG. 2, the blender of FIG. 1 is provided with
four planar baffles 30 at the same horizontal level supported by
struts 30a connecting with the inside wall of section 11 and the
exterior of draft tube 15. Baffles 30 are disposed equiangularly
around the annular blender inner space at approximately equal end
spacings of, typically, 6.5 percent of the diameter of section 11
from confronting apparatus elements and at inward inclinations of,
typically, 34.degree. with respect to the vertical. The upper ends
of baffles 30 lie at, typically, 10 percent bed depth level below
a--a, with the lower ends at about 45 percent total bed depth
level. As seen in FIG. 2 particularly, the collective projected
areas of baffles 30 aggregate about 50 percent of the full annular
cross-sectional area bounded by the inside periphery of top section
11 and the outside periphery of draft tube 15.
A second set of four planar baffles 31, not shown in full in FIG.
1, identical in all respects with baffles 30, is mounted coparallel
with and in vertical alignment with respect to baffles 30 at a
typical spacing therefrom of approximately 10 percent of bed depth.
The upper ends of baffles 31 thus lie at about 55 percent bed
depth, whereas the lower ends lie at about 90 percent bed
depth.
In operation, start-up is preferably effected by first introducing
the solids to be blended and then supplying fluidizing gas, usually
air, unless a different gas is required for chemical activity
reasons, via nozzles 26. The fluidizing gas pressure is preselected
in relationship to bed 23 depth so that the bed seals against
excessive gas leakage, a pressure drop of, typically, 2 inches
H.sub.2 O/ft. of bed depth being taken in the blending of high
density polyethylene having a roughly spherical particle size of
2.5 to 3.5 mm. in diameter. At this stage, valve 29c and the gas
supply valve (not shown) connected with side branch 28b are opened
to introduce enough entrainment gas to elevate solids from the
bottom of section 12, whereupon a fluidized solids zone extending
over substantially the full depth of cone 12, i.e., below line
b--b, for typically, 80 to 90 percent of the diameter of the
cylindrical section, is maintained below the lower end of draft
tube 15, with some solids entrainment through tube 15 occurring.
However, for best control of entrainment recycle, it is preferred
to supply a portion of entrainment gas via nozzle 29, and this is
accomplished by suitable adjustment of control valve 29c. Also,
nozzle 29 can be adjustable in elevation for further control of
solids entrainment. Solids flow in lbs./min. can be adjusted over a
broad range, as detailed in the report of comparative operation set
out in Table I infra.
During blending, the particles in bed 23 are supported in air to an
extent where they possess a low angle of repose of about
18.degree.. As downward gravitation occurs in replacement of
entrained solids recycled by draft tube 15, the particles overlying
baffles 30 and 31 are delayed somewhat as compared to the downward
progress of the remaining particles. Thus, different residence
times of separate weight fractions of the solids within the bed 23
are maintained while withdrawing the solids in downward gravity
flow from the bed, which effects blending during progress of the
solids through the bed. Vigorous blending next ensues within the
fluidization zone of conical section 12, followed by yet more
blending during solids entrainment recycle through tube 15.
Referring to FIG. 3, there is shown a second embodiment of blending
apparatus employing a pair of identical twin flight, half turn
helical plate baffles 33 and 34 mounted coaxially with respect to
central draft tube 15'. These baffles have a radius, referred to
the longitudinal axis of the blender apparatus such that the outer
limit line of the baffles lies at approximately 60 percent of the
bed 23' radial extent. Upper baffle 33 occupies approximately the
first quarter of bed 23' depth, whereas lower baffle 34 occupies
approximately the last quarter. Helical baffling has proved
especially effective for high ratio length-to-diameter blender
configurations. All remaining construction is as hereinbefore
described for the blender of FIG. 1.
Referring to FIG. 4, there is shown a design of blender wherein the
draft tube 15" is expanded at the lower end to provide an
ante-chamber 37 having its lower end completely within
frusto-conical bottom section 12". The antechamber can conveniently
comprise two frusto-conical elements 37a and 37b joined by welding
at their large diameter ends to give a maximum diameter at the
junction line approximately twice the diameter of draft tube 15".
The inlet opening 38 to antechamber 37 is approximately the same
diameter as the inside diameter of tube 15", and the converging
walls of frusto-conical element 37b diverge slightly away from the
confronting walls of frusto-conical bottom section 12" after
approaching to the closest clearance corresponding to about 90
percent of the radius of the vessel cone at the junction line of
element 37a with element 37b. Angle .alpha. of antechamber 37
measured, typically, 113.degree., so that the wall 37a is sloped
away from the horizontal at an angle of about 57.degree., much
greater than the angle of repose of about 27.degree. for the
polyethylene pellets being blended. The foregoing angles apply for
vessels having conical sections 12" of 60.degree. included
angle.
The annular area measured at the level of the juncture line of
elements 37a and 37b should be in the range of about 5-20 percent
of the full horizontal section area of cylindrical shell 11",
dependent on the diameter and height of draft tube 15". This is
done to decrease the bed air flow necessary to obtain the .DELTA.P
for conveying in the draft tube. The .DELTA.P/ft. in the annular
area must not exceed about 6 inches H.sub.2 O/ft., otherwise one
does not get downward pellet flow.
The apparatus of FIG. 4 is otherwise similar in all respects to the
design hereinbefore described with reference to FIG. 1, except that
only four planar baffles 30" instead of eight are required due to
its lower height.
Operation of the apparatus of FIG. 4 is the same as described for
the other embodiments, except that, due to the placement nozzle 29"
within opening 38, fluidization of the solids processed occurs
largely within antechamber 37 and, as to it, within the side angle
regions almost exclusively. The central path is taken by entrained
particles, as indicated by the smaller number particle density
represented within this space.
An important advantage of the antechamber construction described is
that much more stable fluidization is obtained, because of removal
of the weight of the higher pellets in the bed. The added
resistance presented to gas leakage out through annular passage 40,
of course, reduces markedly the operating gas requirement and thus
realizes economies.
The following five examples present a comparison of operation of
the several embodiments of blending apparatus hereinbefore
described wherein the material blended was particulate polyethylene
in the general size range of 2.5 to 3.5 mm. diameter. ##SPC1##
In Example I, the blending apparatus of FIG. 3 was operated without
baffles 33 and 34 installed; however, blending was conducted with
sequential fluidization in the conical bottom section followed by
draft tube entrainment, pellet and air separation and gravity flow
through the packed bed, and uniform blended product was obtained
after 35 minutes of operation with the gas (in this instance air)
and pellet flow reported. This Example shows that relatively long
blending times tend to be required for vessels having a large
overall height diameter ratio.
Example II shows that the blending time for the same material
processed in the same height/diameter ratio blender using the same
air flow rate, but with the apparatus provided with helical baffles
33 and 34 as shown in FIG. 3, can be dramatically reduced to 5
minutes.
Example III shows the increase in air consumption required as the
diameter of the blending vessel is increased. With eight planar
baffles and the high pellet flow rate of 260 lbs./min. reported,
blending time was reduced to the very low value of 2.5 minutes, but
at the cost of a considerably increased air flow rate.
In an effort to reduce air consumption, the identical apparatus of
Example III was altered for Example IV by disposing the lower end
of draft tube 15 well below, i.e., 12 inches below, the knuckle
line, whereupon air consumption was reduced by at least 35 percent.
However, the blending time increased approximately in the same
proportion to 3.5 minutes.
Example V illustrates the advantage of the antechamber in reducing
air consumption, the apparatus of FIG. 4 realizing a blend time
change over the unbaffled apparatus of Example I about in the same
proportion to the increase in air consumption. As compared with the
apparatus of Example IV, the apparatus of Example V also shows
enhanced efficiency.
While symmetrical designs of apparatus having the draft tube
mounted concentrically within the blending vessel are preferred,
because of the more efficient utilization of space which they
provide for the containment of solids to be blended, asymmetrical
designs of apparatus operate approximately as well. Thus, an
apparatus which was effectively a vertical quarter section of the
apparatus of FIG. 4, utilizing only one planar baffle 30 and having
its draft tube mounted vertically along the junction of the two
radial section planes, operated with good efficiency. This
demonstrates that there is little criticality as regards relative
spatial location of the different elements of the apparatus, which
is advantageous as regards fabrication tolerances as well as ease
of accommodation of apparatus within restricted spaces in
manufacturing plants.
It has been noted that apparatus of this invention performs an
incidental classification action during blending operation, in that
fines are entrained out of discharge pipe 17 and can be readily
recovered in a downstream air filter. This can be a concurrent
benefit accompanying the blending, especially where the solids are
ultimately used as extrusion stock where particle size uniformity
is essential for long term extrusion machine set-up adjustments and
the like.
Another design of apparatus utilizing an expanded antechamber 37'
is that detailed in FIGS. 5 and 5A, in which the upper cylindrical
shell 11'" is omitted for simplicity in the showing. Here, baffles
30 and 31 are omitted and an arcuate solids flow restricter plate
53, sloped outwardly an amount of about 30.degree. at the top edge
to prevent hang-up of solids and attached around the lower end of
element 37b', is substituted. Plate 53 overlies the solids escape
end of frusto-conical section 12'" and thereby throttles flow of
solids therethrough to preserve a preselected differential flow
throughout the annular space surrounding draft tube 15'". For best
results in the prevention of solids cross-flow, it is preferred to
mount vertical baffles 54 around the entire circumference of the
annular volume defined by element 37b' and the inside surface of
cone bottom 12'". A typical number of ten such baffles is shown in
FIG. 5A, in this instance disposed circumferentially at equiangular
spacings of 36.degree. apart, although, if desired, a different
circumferential distribution can be employed in order to obtain yet
other individual solids weight fraction flows as hereinafter
described.
The profile of plate 53 was chosen to preserve a solids residence
time, by which is meant the time a particle dwells in bed 23,
varying linearly with the number of individual solids delivery
channels defined by baffles 54, starting from 9 o'clock position
and going to 3 o'clock position, as seen in FIG. 5A. Since the
plate is symmetrical about the 3 o'clock-9 o'clock axis, apertures
in mirror image relationship on opposite sides of this axis deliver
equal weight fractions of solids, so that the term "different
residence times," as employed in the claims, is not limited to the
situation where the residence time of each separate solids weight
fraction delivered from the bed 23 is, in fact, different from all
others delivered from the bed.
It is convenient to visualize the separate solids weight fractions
as increments extending from the top of bed 23 to the point of
discharge into the gas fluidization zone at the base of draft tube
15, and the term "column" is hereinafter employed in designation of
these individual increments.
Research work in the field of gravity-flow blending leading to this
invention has revealed certain novel principles of operation which
are not known to the prior art, and, more importantly, are usually
violated in practice, with the result that less effective blending
is obtained than might otherwise be the case.
For example, it has been found that the "Mixer Weight Fraction"
(MWF, or f), by which is meant the proportion of solids maintained
in a highly agitated and fluidized state, is a critical parameter,
particularly in the range of fractions from about 0.01 to about
0.20. It has further been found that control of individual solids
residence times is also very critical, especially in terms of the
aspects
a. distribution of solids circulation and
b. Residence Time Ratio, i.e., RTR, which is treated herein as the
ratio of the longest circulation time to the shortest. These
aspects are particularly important in reducing the energy and time
required for blending.
Fluidized and intensely agitated solids blending and reaction
systems, wherein most or all of the solids are fluidized and moving
at relatively high velocities, have long been known. U. S. Pat.
Nos. 3,159,383 and 3,386,182 describe mixing systems which use
these principles. These systems are, however, expensive, especially
for larger and more dense solid particles. High power input is
necessary to suspend and move the particles, and a larger vessel is
required due to the expansion of the solids bed. We have found that
it is not necessary to have the major portion of the solids bed in
such a fluidized state, but that maintaining a relatively small
portion of the solids in well-mixed condition attains a
surprisingly large reduction in blend time.
It is helpful to consider the manner in which solids flow downward
by gravity in vessels generally used for blending. As solids are
removed from an opening at the extremity of the bottom cone, the
higher pellets in contact with the lower pellets tend to move
downward to replace the pellets removed. This movement is
transmitted up through the bed in a manner depending upon the flow
properties of the solid. The amount of lateral movement occurring
is controlled by the geometry of the vessel and the flow properties
of the solid. In any case, a column of downwardly moving pellets
exists above every bottom exit. If the pellets removed from the
exit are returned to the top of the vessel, then the pellet flow
can be described by:
1. an average circulation time, i.e., the average time to move the
top of the bed to exit and return,
2. a ratio between the longest and shortest residence times,
and
3. any function describing the residence time of each weight
fraction in the moving column.
A number of such columns can be considered until the total blender
weight is being recirculated and the Residence Time Ratio (RTR) for
the slowest to fastest moving fractions and the Residence Time
Distribution (RTD), i.e., the residence time relationship
one-to-another of all solids columns in circumferential order
around the blender vessel, are defined for all fractions of the
total blender. In our calculations of RTR, the mixer residence
time, being negligible, was not included and thus the RTR and RTD
are based exclusively on the transport times in the gravity flow
bed.
The effects which the foregoing have on blending are complex, and
physical model studies of all the variables would be extremely
time-consuming, if not impossible. Fortunately the important
elements of the process, i.e., the mixer and its volume, numbers of
and volumes of solids columns, and the residence time in each
column can be represented accurately by mathematical expressions. A
large number of experiments can then be run on a computer within a
reasonable length of time. While our calculations permit accurate
definition of the performance of a particular blending system, we
have not yet developed a model allowing prediction of the optimum
system.
Since the course of blending in any given case depends upon the
relative dispositions of differing quality particles as they exist
at startup in the main solids body, i.e., within cylindrical vessel
11 (FIG. 1), of the blending apparatus, applicants chose two widely
different test conditions for purposes of comparison, these
being:
1. layer fill, consisting of loading a distinctively colored layer
of one resin on top of a differently colored main resin in the
arbitrary proportion of less than about 1 percent for the former to
about 99 percent of the latter, and
2. 0.5 fill, consisting of half the total solids volume being a
layer of one type of resin, whereas the remaining 50 percent is a
layer of another type.
Actually, this latter condition is much more frequently encountered
in industry than the former, a typical case being that wherein one
batch of resin contains 1,000 ppm antioxidant whereas a previous or
succeeding batch contains zero ppm of the additive. The system is
then arbitrarily accepted as blended when all volume elements are
within .+-.0.1 of the final average value.
FIGS. 6-10 and FIGS. 11A and 11B illustrate some of the results of
blending experiments conducted on a computer with the mathematical
model. Referring now to FIG. 6, which is a log-log plot of Mixer
Weight Fraction versus Number of Displacements (turnovers through
blending apparatus) to Blend (NDB), it is seen that blending
efficiency increases rapidly with Mixer Weight Fraction and that,
while it is slightly more difficult to effect blending in the layer
fill situation than for 0.5 fill, parallel conditions exist and
there is no disabling factor barring blending in either case. It
will be noted that a Residence Time Ratio (RTR) (of particle flows
in columns) of 1.0 was employed in all FIG. 6 tests, meaning that
the transport times or residence times of all pellets in all equal
weight fractions withdrawn were equal as the pellets were
circulated from the mixer to the solids bed and flowed by gravity
back to the mixer.
FIG. 7 (for a 100 equal weight fraction, i.e., column, apparatus)
is a linear plot of the Number of Displacements to Blend (NDB)
versus the Residence Time Ratio (RTR) with parameters of MWF (Mixer
Weight Fraction). Thus the combined effects of Residence Time Ratio
and MWF can be seen. For these tests, the residence time in each
column was determined by increasing the pellet velocity directly
with the weight fraction number while employing the slowest and
fastest column velocities to obtain the indicated Residence Time
Ratio (Max./Min.). As can be seen from FIG. 7, a blender with a
0.05 fraction "Mixer" will blend in about 3.2 Displacements while a
0.10 MWF reduces the NDB to about 2.75.
FIG. 8 is again a plot of the NDB (Number of Displacements to
Blend) versus Residence Time Ratio with parameters of Mixer Weight
Fraction. However, in these tests, the Residence Time Distribution
(RTD) has been changed. The residence time (rather than velocity)
for each weight fraction was increased linearly with weight
fraction number while maintaining the ratio of the slowest to the
fastest as shown. As can be seen in comparing FIGS. 7 and 8, a
surprising reduction in turnovers to blend from 3.2 (FIG. 7) to
about 2.1 (FIG. 8) for a 0.05 "Mixer" was obtained with this manner
of adjusting Residence Time. Note also that the optimum RTR was
about 4.0 for a 0.05 fraction "Mixer."
FIG. 9 is a plot of the NDB against the RTR with parameters of the
number of equal weight fractions withdrawn from the blending
system. The well-mixed volume contains 0.1 of the total weight of
pellets and the residence time is distributed linearly with weight
fraction number. Layer fill was used in this test to determine the
number of weight fractions required. The plot shows that increasing
the number of equal weight fractions withdrawn from 3 to 6 reduces
the NDB and decreases the sensitivity to Residence Time Ratio
(RTR). Further increasing the number of equal weight fractions
withdrawn from 12 to 100 was found to make almost no change in the
NDB. Comparing FIGS. 8 and 9 permits another comparison of the
increased time to blend (approximately one additional displacement)
when testing with a thin layer of markers on top of the bed.
The data from blending tests with the physical models and with the
computer model appear to contain some inexplicable results. For
example, as seen in FIG. 8, as the RTR is increased from 3.8 to 4.0
the NDB abruptly drops by about one turnover for the "Mixer Weight
Fractions" of 0.02 and 0.05, while with a MWF of 0.1 the NDB drops
rather abruptly between an RTR of 2.6 and 2.8. Consideration of the
many data displays and tests conducted on the computer and physical
models indicate, surprisingly, that these results are indeed real
and attributable to a phenomenon similar to resonance.
The phenomenon of resonance may be described by referring to FIG.
10, and FIGS. 11A and 11B. FIG. 10 relates the NDB versus the RTR
for a simple two equal weight fraction model with the well-mixed
fractions of 0.1 and 0.2. Note the wide oscillation in NDB as the
RTR is increased from 1 to 3 and note that two minima are obtained.
To further search for an understanding, the concentration of
markers exiting the mixer (by which is meant the concentration of
black, blue or other distinctively colored minority particles
incorporated as trace material) was plotted as a function of number
of displacements (time) and is shown in FIG. 11A for the RTR (2.1)
coinciding with a maximum NDB. FIG. 11B indicates the marker
concentration vs. number of displacements (time) for the RTR (1.6)
coinciding with a minimum NDB.
It must be remembered that both minima and maxima marker
concentrations exiting the mixer are returned to the downward
moving bed and are thus stored until they again flow into the
mixer. In FIG. 11A, with the Residence Time Ratio of 2.1, the two
equal weight fractions and their stored concentrations, minima and
maxima, are fed to the mixer such that the concentration maxima are
reinforced and the concentration minima are also reinforced. As
shown in FIG. 11B, with an RTR of 1.6, these concentration minima
and maxima which are stored in the weight fractions are fed to the
mixer at rates such that the minima from one weight fraction very
nearly enters the mixer with maxima from the other weight fraction.
Thus, they are out of phase, and no concentration reinforcement
occurs. Blending, therefore, is achieved most rapidly with the
out-of-phase condition shown in FIG. 11B. This reinforcement or
resonance phenomena explains the seemingly erratic behavior of many
blending systems comparatively tested.
The mathematical model is basically very straightforward, and the
only assumption required is that the solids can be made to flow in
the manner described. However, referring to FIG. 12, to further
substantiate its validity, a four-column physical model was built
and tested. Separate conveying lines were used such that adjusting
the air flow permitted adjusting the Residence Time in each column.
Separate inlet pipes were used to assure maximum uniformity of
residence time in each column. The conveyors empty into a
separating vessel and the impingement created a region of high
pellet velocity and a great deal of random movement Measurements
indicated that about 2 percent of the total pellets were in this
well-mixed zone at any given time.
The data from tests of this physical model were compared with
calculations from the mathematical model in FIG. 13. Two tests with
a linear Residence Time Ratio (Max./Min.) were made and these
points appear as squares on the diagram, and check the calculated
performance very closely. A single test with the column velocity
distribution adjusted to be linear with column number is indicated
by the triangle and confirms that Residence Time Distribution
linear with column number does indeed give reduced NDB as compared
with the linear velocity distribution.
FIG. 14 is included to better describe the concepts of Residence
Time Ratio (RTR) and Residence Time Distribution (RTD). In FIG. 14
the Column Residence Time divided by the Blender Displacement Time
is plotted against the column number. The column residence time is
that required for a pellet within said column to travel from the
top of the column and return to its starting position, since no
mixer was used. All pellets within a single column have been given
the same residence time and, in the example illustrated, eight
columns are used to make up the blender. Thus each column
represents one-eighth of the total weight to be blended. The
blender displacement time was found by dividing the blender weight
by the sum of the flow from each column. This sum, or the total
recirculation flow, was held constant.
Two different Residence Time Distributions are illustrated. In the
RTD shown with circles, the column residence time was increased as
a linear function of the column number while maintaining an RTR of
3.33. In the second case, the column velocity was increased with
column number. The reason that the linear RTD gives the shortest
blend time is not completely understood but may be a result of more
uniform feed of concentration layers to the mixer.
Turning to the mechanical design of the draft tube blender, it is
most advantageous to obtain columns with different downward
velocities and thus different residence times around the central
bulb and conveyor. Since the free-flowing solids generally tend to
flow directly downward, this can be conveniently achieved by the
valve arrangement shown in FIGS. 15 and 16. These valves permit
adjusting the flow area at the bottom of each column.
The design of FIGS. 15 and 16 is shown as having only four solids
flow control valves, although, of course, a greater number is
preferred and can be readily accommodated without interference one
with another.
The individual valves 56 conveniently constitute cut-out sections
of an inwardly nested false conical bottom section telescoped
within conical bottom section 12", each section being
hinge-supported at the upper ends 56a to permit lateral movement
with respect to element 37b. Push rods 57 are provided for
preselected settings of each individual valve.
In order to close off the interspaces between adjacent edges of
valves 56, pyramidal filler blocks 58, rigidly supported against
movement by secure attachment at their outboard faces to the
stationary skirt section 12", are mounted between valve pairs.
Then, as valves 56 open or close, the edges thereof slide over the
pyramidal sides of blocks 58 close enough thereto to bar escape of
any solids particles being blended but with sufficient clearance to
avoid frictional binding of the moving valve edges with respect to
the filler blocks.
If the valve pieces 56 are edge cut sufficient amounts at their
lower terminal edges, the inboard edges of pyramidal blocks 58 can
extend inwardly far enough to constitute partially effective
separators barring any circumferential displacement of solids from
one column to another. Full length baffling equivalent to that
hereinbefore described with reference to FIGS. 5 and 5A can then be
achieved by welding fins 54' to the inboard edges of the pyramidal
blocks as shown in FIG. 16, in which case the inner edges abut at
all times against the confronting outside surface of element 37b
(omitted from this view to better show the details of the filler
blocks).
Similar results can be obtained by properly sized orifices at the
bottom of each column sector. However, the adjustable valves are
useful, particularly where the flow properties of the solids are
not adequately known or where the blender is operated on more than
one type of solid.
Considering all of the test results obtained and the discovery of
the resonance phenomenon, it is concluded that it is most
advantageous to account for the following in designing and
operating systems to blend free-flowing solids:
1. A substantial well mixed fraction (0.01-0.20) is desirable to
decrease blend time, but it is also obviously more expensive, due
to the power required to keep a larger volume of solids in a highly
agitated state. Thus the size chosen will be the smallest which
will give the desired blend times with optimization of all other
blender variables.
2. Residence Time Ratio should be controlled between about 1.5 and
5.0, depending on the mixer size, number of columns, composition
difference to be blended, and the Residence Time Distribution. An
RTR smaller than 1.5 invariably cause excessive concentration
amplitude peaks as the material exits the mixer, and thus longer
blend times. An RTR larger than 5.0 causes longer blend times due
to the time required to displace the slower moving columns. There
is not a direct relationship between RTR and blend times, but the
optimums lie within the 1.5-5.0 range.
3. Residence Time Distribution and the number of columns employed
should be designed to obtain the most constant feed of the
concentration disturbance into the mixer and therefore the smallest
amplitude of concentration cycles exiting the mixer. Thus, a
residence time distribution that is linear with column number and a
larger number of columns are desirable. The number of columns
should preferably be more than five for most blending tasks.
* * * * *