Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to the conversion of liquid chemicals such
, as pharmaceuticals, fertilizers such as urea, ammonium nitrate etc., or
other liquid chemicals into discrete substantially spherical solid chemical
particles, generally known as prills.
The formation of liquid chemicals into solid particles of substant-
ially uniform spherical dimension by prilling has been extensively investi-
gated. One of the salient prior art problems entails the necessity of pro-
viding very high and costly prilling towers for the prilling of bulk chemi-
cals such as fertilizers. Another problem encountered, especially with
pharmaceuticals, is the non-uniform size distribution and dimensions of the
prilled product, especially when relatively small prills are desired. The
prilling of chemicals is described in U.S. Patent Nos. 1,951,790; 2,774,660;
2,931,067; 3,334,159; 3,450,804 and 3,538,200, while the production of metal
powders is described in U.S. Patent Nos. 3,009,205 and 3,655,837. Manufact-
ure of metal particles such as shot is described in U.S. Patent Nos. 1,635,
653 and 2,811,748.
In normal prilling operations, height is of prime consideration in
the production of prills since the free fall height is required for actual
formation of the liquid into the sphere shape, followed by an additional sub-
- 20 sequent time function height for the liquid sphere to go from its melted
temperature to crystallization temperature. The size oi the prill formed
in a production head is a function of the hole size in the plates.
Common spray heads are sometimes manufactured with Eacilities for
introducing gas to the liquid stream. However, in these prior art designs,
the gas is always internally premixed with the liquid, and the result is an
atomized spray. These manufactured spray nozzles cannot be used to produce
prills of any re~sonable size. Most attempts result in very minute-sized
prills and fog, which are considered as losses. The prill towers now in
operation utilize flat or dished plates with precision drilled holes of a
,; 30 size required to yield a given sized prill product. The holes are very
. small, requiring a constant maintenance for cleaning, and the tower must be ~-
very high to allow for the liquid sphere formation and the temperature drop
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to the freeze point.
One prior art method of preparing controlled dosage pharmaceutic-
als is to produce small particles or prills of an inert material such as
sugar, and then to coat the sugar particles with a coating of pharmaceutic-
al powder. This procedure is objectionable because of uneven coating and
also because some pharmaceuticals are intended for slow release usages or
for release in the digestive tract rather than in the stomach, and the med-
icinal coating tends to dissolve and be immediately assimilated in the
stomach. It has also been found in practice that the provision of an outer
inert coating outside of a coating of pharmaceutical is not effective due
to uneven coating etc.
In the present invention, a prilling method and apparatus is pro-
vided which features the purposeful dispersion of the liquid chemical into a
plurality of discrete liquid droplets at the upper end of the prill tower,
by forming a plurality of liquid streams and breaking up or dispersing each
stream immediately after formation by directing a plurality of gas streams
into each liquid stream. In most instances the gas stream will consist of
air at ambient temperature, however in some instances, as when prilling an
oxidation-sensitive pharmaceutical or the like, an inert gas such as nitro-
gen, argon or carbon dioxide may be employed as the dispersion gas. In
. many instances optimum results are obtained when the dispersion gas is pre-
cooled to a reduced temperature, typically in the range of -10 C to 20 C
At least three separate gas streams are preferably provided to disperse
each liquid stream, and the gas streams converge on a point wlthin the liq-
uid stream flow path. The gas streams are preferably oriented at an acute
: angle relative to the liquid stream flow, so that the gas stream flow paths
define an inverted conical flow pattern of gas flow having its apex at the
liquid stream flow path. The resul~ant dispersion oE the liquid droplets
is of relatively uniform size distribution, so that when the liquid droplets
fall downwards through the prill tower they congeal into substantially
; spherical particles of quite narrow size distribution range. In other words,
~ a uniform prill is obtained, and in the case of pharmaceuticals or drugs,
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this uniform prill may be of small dimension within a relatively narrow size
range, which is a highly desirable characteristic for pharmaceutical or drug
usages in which controlled and/or uniform release of the medication is highly
desirable. The many pharmaceuticals such as medicinals or drugs which may be
prilled in accordance wlth the present invention are antibiotics, vitamins,
sedatives, tranquilizers, hormones, birth control medications, vaccines, laxa-
tives, etc. In other applications such as the prilling of fertilizers, it
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has been found that the initial purposeful dispersion of the liquid melt or
solution in accordance with the present invention enables the prilling step
to be carried out in a much shorter and smaller prill tower, which results
in a lower capital cost for the chemical facility. Finally, the invention is
advantageous because it enables the prilling of high viscosity materials, or
chemicals which are characterized by slow and lengthy crystallization times.
These materials and chemicals are very difficult to prill effectively and
normally require high prill towers. In summary, the prilling method and de-
vice of the present invention can substantially reduce the height required
for the prill solidification and can also produce prills of different sizes
or size distribution using the same holes, by varying the pressure of the
dispersion gas and/or pressure of the gas sent through the gas distributor for
liquid dispersion affects the size of the liquid sphere formed, so that for
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~ the same hole size, more than one sized prill can be produced. An adclitional
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advantage of the present type of prill head is that small diameter holes are
not required to produce small prills, and therefore the possibility of plugging
is minimized.
`.' Thus, in accordance with one aspect of the invention there is
provided a method of converting a liquid chemical compound into discrete sub-
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stantially spherical solid chemical particles which comprises passing a liquid
chemical compound downwards through an externally heated feed zone, discharging
said liquid chemical compound vertically downwards from the lower end of said
feed zone in the ~orm of a continuous stream of liquid chemical compound,
dispersing said liquid chemical compound stream into a plurality of discrete
droplets in a dispersion zone by directing a plurality of gas streams against
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said liquid chemical compound stream, and cooling sald discrete droplets
during free fall of said droplets below said dispersion zone by direct
.- contact with gas in a prill formation ~one, whereby said droplets are
solidified to form a plurality of substantially spherical solid chemical
particles.
In accordance with another aspect of the invention ~here is
provided an apparatus for producing substantially spherical solid chemical
particles from a liquid chemical compound which comprises a vertically
oriented vessel, a liquid distribution container disposed at the upper
end of said vessel, said container being defined by side walls and a
lower substantially horizontal foraminous plate, means to pass a liquid
: chemical compound into said container, means to heat said liquid chemical
compound within said container, whereby said liquid chemical compound
" flows downwards through the openings in said plate as a plurality of
discrete liquid chemical streams, means contiguous with said plate to
direct a plurality of gas streams against each of said liquid chemical
streams whereby each liquid chemical stream is dispersed into a plurality
: of discrete droplets below said plate, means to circulate gas within
said vessel below said container, whereby said discrete droplets flow
. 20 downwards within said vessel in direct contact with said gas and said
discrete droplets thereby solidify in the form of discrete substantial-
ly spherical solid chemical particless and means at the lower end of
said vessel to remove said solid chemical particles from said vessel.
The present invention is illustrated, by way of example, in
the accompanying drawings, in which Figure 1 is a sectional elevation
view of a preferred embodiment of the present invention.. Figure 2 is a sectional plan view of the device of Figure
1, kaken Oll section 2-2
Figure 3 is a sectional plan view of the device of Figure
. 30 1, taken on section 3-3, and
Figure 4 is an enlarged view of the cen~ral portion of Figure
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` Referring now to Figure 1, vessel 1 is a vertically oriented
cylindrical tower in which the prilling procedure takes place. A cylindric-
al container 2 is coaxially disposed at the upper end of vessel 1. Liquid
chemical compound stream 3, which is typically a pharmaceutical compound or
formulation dissolved in a carrier, passes via inlet nozzle 4 into the
upper end of container 2 and joins a body or reservoir of liquid chemical
~; compound 5 within container 2. A lower substantially horizontal and pre-
ferably hollow foraminous disc-shaped plate 6 is disposed at the lower end
of container 2, and plate 6 is usually hollow so that heating fluid stream
7 which usually consists of steam may be passed via pipe 8 into the hollow
interior of plate 6 for heating purposes. In other instances, a heating
; jacket or the like may be disposed about at least a portion or all of plate
6 and/or the balance of container 2. Condensate water produced by the con-
densation of steam stream 7 within the plenum or interior of plate 6 is re-
moved via pipe 9 for discharge via stream 10
-' The heating of plate 6 permits and facilitates the flow of liquid
chemical downwards through the plurality of openings in the foraminous
plate 6, which openings in this embodiment of the invention are characterized
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by an upper-t~re:le~ conical inlet passage 11 from which a lower vertical
cylindrical feed passage 12 extends downwards, so that individual continuous
streams of liquid chemical are formed within and clischarged downwards from
the passages 12.
Suitable gaskets or seals against fluid flow, such as gasket 13,
will be provided in practice. The gasket 13 is disposed between the lower
ili surface of plate fi and the horizontal dispersion gas inlet manifold 14. A
suitable dispersion gas stream 15 which usually consists of air or an inert
gas as mentioned supra passes through heat exchanger 16 and is cooled by
heat exchange with a suitable coolant fluid or refrigerant such as freon,
ammonia, chilled water, brine or solid carbon dioxide, commonly known as
dry ice. The coolant flows into unit 16 via stream 17~ and warmed coolant
is removed via stream 18. The cooled dispersion gas stream 19, now at a
temperature typically in the range of -10C to 20C, flows via pipe 20 into
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manifold 14.
A substantially horizontal disc-shaped dispersion gas distribution
plate 21 is disposed below manifold 14. The plate 21 is characterized by
the provision of a plurality of inclined passages 22 through which the
dispersion gas flows for contact with the down-flowing liquid chemical
streams. Plate 21 is also provided with a plurality of vertical passages 23
disposed directly below the openings or passages 12 in plate 6, so that the
individual liquid streams formed in passages 12 flow downwards through the
passages 23 as individual continuous liquid streams 24.
The dispersion gas streams flowing downwards through the inclined
passages 22 contact and intersect the streams ~ immediately below plate 21,
so that the gas streams disperse the liquid streams into a plurality of
discrete droplets of substantially uniform dimension in a dispersion zone A
immediately below plate 21. In most instances, the angle of contact b~tween
the gas streams and the individual liquid streams will generally be an acute
angle typically in the range of 20 to 60. As is evident from Figure 1 and
as will appear Lnfra, a plurality of gas streams contact and intersect each
liquid stream 24 in zone A, so that the liquid is thoroughly and uniformly
dispersed into a plurality of down-flowing discrete liquid droplets. The
angle of contact in a specific facility will depend on specific characteris-
tics of the liquid chemical such as viscosity and crystallization-rate.
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Similar considerations apply to the pressure, flow rate and/or temperature
o the dispersion gas stream 15, and suitable values for these operating
parameters will be provided in practice for a specific facility and liquid
chemical, dependLng on the characteristics of the l:Lquid chemical as well as
other factors such as the number of passages 22 provided in plate 21 about
each liquid stream 24. In most instances, at least three passages 22 will ` '
;. be provided about each passage 23 and for each liquid stream 24.
The resultant liquid droplets of substantially uniform dimension
now flow downwards within the prilling vessel or tower 1. The free fall of
. the droplets within vessel 1 takes place in direct contact with a cooling
gas such as air in a lower prill formation zone within the middle and lower
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portion of vessel 1, so that each droplet is individually solidified to form
a substantially spherical solid chemical particle, generally known as a
prill. The resultant plurality of substantially spherical solid chemical
particles or prills collect in the lower end of vessel 1 and are removed via
lower outlet nozzle or prill feader 25 as stream 26.
The gas utilized for the cooling of the liquid droplets so as to
produce solidification and solid prill formation is generally air, which
may be precooled, however in suitable instances such as when processing an
` ~ oxidation-sensitive chemical, an inert gas such as nitrogen or argon ~'~K-
~ may be utilized. In this embodiment of the invention, pre-cooled air
is employed in the prill formation zone. Thus, ambient air stream 27 passes
through heat exchanger 28, which may be similar in configuration to unit 16
; described supra. A coolant stream 29 which is usually comparable to stream
17 described supra is passed into unit 28 and cools the air to a reduced
. temperature generally below 30C and typically in the range of -10 C to 20 C.
Warmed coolant is removed from unit 28 via stream 30, while the cooled air
~ stream 31 discharged from unit 28 flows via inlet nozzle or air distributor
32 into the lower end of vessel 1. The rising cold air within vessel 1
c0019 and solidifies the freely falling liquid droplets to form prills of
substantially uniform dimension, and the resultant warmed air is discharged
from the upper end of vessel 1 via outlet 33 as stream 34, which may be
further processed by filtration, or in cyclonic means for separation of
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entrained solid particles from a gas stream, or by scrubbing with a suitable
liquid or solution, or the like, to recover minor quantities of dust and/or
fines prior to atmospheric discharge or recycle via stream 27. In instances
when the initial feed liquid chemical stream 3 contains a volatile solvent,
carrier or other component, stream 34 may contain a viable proportion of
this component due to vaporization during prill formation. In this case,
stream 34 may be cooled to condense and recover the volatile component
phase, or stream 34 may be scrubbed with a suitable liquid to dissolve and
recover the vaporous component phase~ or stream 34 may be otherwise suitably
processed by suitable methods and means to recover the vaporized component
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phase.
Referring now to Figure 2, a sectional plan view of Figure 1 taken
on section 2-2 is shown. The liquid chemical stream formation passages hav-
ing the upper inverted conical inlet sections 11 and the lower vertical cylin-
drical sections 12 may be disposed within the horizontal plate 6 in any suit-
: able configuration, thus the passages 11-12 may be arranged and disposed in a
pattern of concentric circles within plate 6, or these passages 11-12 may be
disposed in a radlal pattern extending outwards from the center of plate 6,
etc.
Figure 3 is a plan view of Figure 1 taken on section 3-3, and Fig-
ure 3 shows the plurality of dispersion gas flow passages 22 disposed about
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each passage 23 in plate 21. Each continuous liquid chemical stream 24 is
. also shown in cross-section.
Figure 4 is an enlarged plan view of one of the units for dispersion
of the liquid chemical streams 24 into a plurality of liquid droplets. The
passage 23 in plate 21 for downwards flow of stream 2~ is centrally oriented
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with respect to the plurality of dispersion gas passages 22.
Numerous alternatives within the scope of the present invention will
; occur to those slcilled in the art, besides those alternatives mentioned supra.
An example of testing and application of the present invention will
now be described.
Example
~ testing and development program was carried out in order to devise
a suitable method, procedure and apparatus Eor prillLng a pharmaceutical drug
; dispersed in a carrier, which in the test work was polyethylene glycol. The
concentration of the active pharmaceutical drugs in the carrier was 20% by
weight. The obJective was to produce uniform small prills oE approximately 1
mm in d:Lameter. It was primarily important that,the prill should be of a
specified size; of secondary importance was ~e the product could be prilled
from some reasonably low height.
: The material had a freezing point of about 55 C. The kinematic vis-
cosity of the PEG (polyethylene glycol) varies from about 12 centistokes at
200C to 250 centistokes at the freezing point o~ 55 C. The ~e~t- samples
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had the drug already dispersed in the carrier. The PEG appeared to be self-
insulating in that, after attaining the spherical shape using prior art de-
vices and cooling, the prill would flow after the free fall drop and this
resulted in half-sphere shapes. The crystal surface apparently was not
sufficiently strong to maintain the sphere shape while the interior cooled.
During the test program, three types of prilling heads were used
in attempts to produce quality prills. The first was a flat prill plate
with a number of precision-drilled holes. This type is normally used in
~- production prilling of urea and the like. The second was cone-shaped.
Finally, a head was made in accordance with the present invention, which
. utilized external air in the liquid stream after the liquid stream cleared
the hole.
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. Initial test runs were made at high pressure (200 psig.) using
": normal flat prill plates with hole sizes of 0.010, 0.013, 0.015, 0.025 and
0.029 inches diameter. Near the freezing point of about 55 C and prilling
from a height of 60 feet, no prills formed or solidified. Some runs resulted
in all of the sample being splattered, while when runs were made with prill
plates having small holes, the plates plugged continually because the holes
were too small. In summary, all attempts to make prills with the flat
; 20 plate were unsuccessful, generally because the prill plates with smaller
dimension holes constantly plugged and in other cases the exiting material
or solution did not ~ool sufficiently in the free fall and this resulted in
splatter. A run was also made in which preconditioning of the prill tower
was accomplished by cooling the induced ambient air with blocks of dry ice.
The ice was stacked in the bottom of the tower and the aLr was drawn past
the ice by connecting the suction end of a blower to an outlet at the top
of the tower. The temperature was about 9C near the tower bottom and 12C
~` near the top of the tower when prilling was started. Most of the material
adhered to the side walls of the tower.
A series of runs were made which utilized commercial or convent-
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ional type spray nozzles, with and without air mixing. Two types of commer-
cial spray nozzles and a venturi gun were used. The material either did
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not prill or was all atomized to fines. In other words, a form of prill
could be made with all three types of nozzle but the size of the resultant
prill was extremely small and attempts to increase the size to the desired
0.5 mm to 1.0 mm range were unsuccessful. The use of these spray nozzles
and the venturi gun resulted in atomization of the material. The so-called
"Bette" nozzle requires a certain pressure which imparts a spin to the so-
lution, as it exits the nozzle, due to configuration. As the pressure is
increased, the liquid velocity is also increased and this results in larger
liquid angles e~iting the nozzle; the larger the angle, the smaller the drop-
let size. All the runs made with this nozzle produced solid prills, but the
largest obtainable prill size was still much smaller than that which was
desired. The free fall required for this nozzle was about 6 feet The ven-
turi gun produced about the same results as the Bette nozzle. A larger prill
was produced and more control of size variation was an advantage of the ven-
turi throat principle, however the practical feasibility of the system was
not established. Another type oE nozzle which was tested during the program
was one in which air was injected into the solution just prior to solution
discharge from the nozzle hole. Here again, the product prills were extreme-
ly small and size-variation control was poor. The injected air had the added
disadvantage of some times cooling the solution to its freezing point in the
nozzle internals, and thereby causing a plugging of the discharge hole. In
; some cases the material extruded from the hole. In summary, in most attempts
to utilize air which was brought into contact with the solution before
exit:Lng the hole, the solution was atomized to fine prills (fog-like) which
were much too small and were unacceptable. Attempts to increase the prill
size by changing the air pressure were partly successful in that some in-
crease was obtained, but the desired size range could not be obtained.
The final prilling test apparatus which proved successful was in
accordance with the present invention. A 0.040 inches diameter hole was
drilled in a flat plate. Air from an external source was then directed at
. the liquid stream after it had cleared the prill plate. The air stream
caused the continuous liquid stream to break into spheres, and it also
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quenched the liquid with cold air. The prill is formed very near the
dispersing gas distribution plate, and is sufficiently quenched by the
cool air to form soli~ prllls of the proper size in a free fall of about
25 feet. The size of the prill is effected by the amount of air directed
at the liquid stream. Ir. a typical run, liquid temperature was 57C,
air temperature was 20C, prilling height was 20 feet, vessel pressure
was 14 psig and internal prill head pressure was 15 psig. A screen analysis
of the product prills produced from this run showed 6.0% of 1.168 mm
or greater size ~+14 mesh), 13.0% of 0.833-1.168 mm, 43.9% of 0.589-0.833
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mm, 30.8% of 0.42-0.589 mm, 6.0% of 0.25-0.42 mm, and 0.3% fines, all
percentages by weight. Using the same technique but varying the amount
of air slightly to increase the prill size, the following size distribution
was obtained: 5.26% greater than 1.168 mmJ 35.78% of 0.833-1.168 mm,
; 43.15% of 0.589-0.833 mm, 13.68% of 0.42-0.589 mm, and 2.10% of 0.25-0.42 mm,
balance fines (0.23%).
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