U.S. patent application number 11/071488 was filed with the patent office on 2005-07-14 for method and apparatus for atomising liquid media.
This patent application is currently assigned to Novel Technical Solutions Limited. Invention is credited to Zhou, Chuanjie.
Application Number | 20050150971 11/071488 |
Document ID | / |
Family ID | 26246046 |
Filed Date | 2005-07-14 |
United States Patent
Application |
20050150971 |
Kind Code |
A1 |
Zhou, Chuanjie |
July 14, 2005 |
Method and apparatus for atomising liquid media
Abstract
There is disclosed apparatus for atomising liquid media
comprising an ultrasonic gas atomisation nozzle (11) having a gas
flow path (12) from a plenum chamber (17) which flow path is
straight and is provided with a plurality of resonance cavities
(31).
Inventors: |
Zhou, Chuanjie; (Altrincham,
GB) |
Correspondence
Address: |
DEMONT & BREYER, LLC
SUITE 250
100 COMMONS WAY
HOLMDEL
NJ
07733
US
|
Assignee: |
Novel Technical Solutions
Limited
Manchester
GB
|
Family ID: |
26246046 |
Appl. No.: |
11/071488 |
Filed: |
March 3, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11071488 |
Mar 3, 2005 |
|
|
|
10475039 |
Oct 16, 2003 |
|
|
|
10475039 |
Oct 16, 2003 |
|
|
|
PCT/GB02/02143 |
May 9, 2002 |
|
|
|
Current U.S.
Class: |
239/1 ; 239/10;
239/290 |
Current CPC
Class: |
B01J 2/04 20130101; B22F
2999/00 20130101; B05B 17/0692 20130101; B22F 2999/00 20130101;
B22F 9/082 20130101; B22F 2009/088 20130101; B22F 2202/01
20130101 |
Class at
Publication: |
239/001 ;
239/290; 239/010 |
International
Class: |
B05D 001/00; B05B
017/00; A62C 005/02; B05B 001/28 |
Foreign Application Data
Date |
Code |
Application Number |
May 9, 2001 |
GB |
0111259.8 |
May 9, 2001 |
GB |
0111257.2 |
Claims
What is claimed is:
1. A method for making a polymer powder comprising: melt-extruding
a polymer material; and impinging a high velocity gas stream on to
the molten extrudate.
2. A method according to claim 1, in which the high velocity gas
stream is supersonic.
3. A method according to claim 2, in which the high velocity gas
stream is at Mach 2.
4. A method according to claim 1, in which the gas does not
adversely react on or with the atomised polymer.
5. A method according to claim 1, in which the gas is nitrogen.
6. A method according to claim 1, in which the extrudate comprises
a film.
7. A method according to claim 1, in which the extrudate comprises
filaments.
8. A method according to claim 7, in which the filaments emerge as
a sheet or ribbon from a line or spinnerettes.
9. A method according to claim 1, in which a film or sheet or
ribbon-like extrudate is impinged on both faces by a gas
stream.
10. Apparatus for making polymer powder comprising a die from which
polymer is extruded and nozzle means impinging a high velocity gas
stream on the extrudate from the die.
11. Apparatus according to claim 10, in which the die comprises a
slit for extruding a film.
12. Apparatus according to claim 10, in which the die comprises a
line of spinnerettes for extruding a sheet or ribbon of
filaments.
13. Apparatus according to claim 10, in which the nozzle means
comprise a slit-form nozzle either side of the die directed towards
the issuing extrudate.
14. Apparatus according to claim 10, in which the nozzle means
impinge the gas stream at an angle to the issuing extrudate so as
to have a component of velocity in the direction of flow of the
extrudate.
15. Apparatus according to claim 14, in which the nozzle means form
a V-shaped gas stream with an included angle between 30.degree. and
90.degree..
16. Apparatus according to claim 10, in which the die comprises a
heater arrangement.
Description
[0001] This invention relates to methods and apparatus for
atomising liquid media, and also to making polymer powder.
[0002] Conventionally, polymer powder is made by grinding extruded
polymer pellets, often under cryogenic conditions. Powder size
distribution and powder morphology are difficult to control, while
the process is expensive and energy-intensive. Moreover, the
grinding equipment can contaminate the product, which is also
susceptible to environmental pollution.
[0003] Methods and apparatus for atomising liquid media are known
for example from U.S. Pat. No. 5,228,620 and earlier publications,
and are used e.g. to produce metal powder by atomising a molten
metal stream into droplets which solidify into spherical or nearly
spherical particles. The most important characteristics of atomised
powders are their morphology shape, size and size distribution. The
powder size and morphology subsequently influences the engineering
properties, i.e. flowability, packability, compressibility, etc.,
and the size distribution indicates the yield of useful material
available for a specific application. It is therefore desirable to
control the average particle size, the morphology, and the powder
size distributions produced during atomisation.
[0004] Prior to the invention, efforts in this area have resulted
in the development of several techniques. One method used a
standing ultrasonic wave generated between two ultrasonic
transmitters to disintegrate a molten material into fine droplets
(European Patent No. 0 308 600). The other design is an ultrasonic
gas atomisation device, in which the gas channel incorporates a
resonance cavity (Hartman shock tube) in order to create a high
frequency pulse in the gas. The atomiser makes use of the
combination of high frequency pulsed gas pressure and supersonic
gas streams will promote efficient atomisation of the molten
material, resulting in a narrow spread of fine droplet size (U.S.
Pat. No. 2,997,245). However, the amount of gas delivered by an
atomisation nozzle is clearly one of the most important design
parameters. The initial design has suffered from a major
disadvantage, in that it requires high operating gas pressure (from
6.5 MPa to 12 MPa) (U.S. Pat. No. 5,228,620). Abrupt fictional
losses at areas in the channel are found to be 36% in total
pressure between the plenum chamber and nozzle exit in the Unal
technical article (#1) "Frictional Losses in Ultrasonic Gas
Atornisation Nozzles", Powder Metallurgy, Vol. 33, No 3, pp.
327-333 (1990).
[0005] The present invention provides methods and apparatus for
atomising liquid media that overcome at least some of the problems
of the prior art.
[0006] The invention comprises, in one aspect, apparatus for
atomising liquid media comprising an ultrasonic gas atomisation
nozzle having a gas flow path from a plenum chamber which flow path
is straight and is provided with a plurality of resonance
cavities.
[0007] The resonance cavities may be spaced apart along the gas
flow path, and may be inclined to the gas flow path in the sense of
being converged therewith in the direction of gas flow.
[0008] The resonance cavities may be such as will impose an
ultrasonic frequency on the gas flow, which may be in the range
20-60 KHz.
[0009] The gas flow path may comprise an annular convergent nozzle
and the resonance cavities then comprise cylindrical cavities
formed in both interior walls of the annular nozzle. The diameters
of the cavities may be between {fraction (1/12)} and 1/8 of the
mean nozzle diameter. There may be between ten and sixty cavities
in such an arrangement spaced along and around the annular
nozzle.
[0010] The gas flow path may, however, comprise a multi-jet
arrangement, and the diameters of the cavities may be between
{fraction (1/10)} and 1/3 of the diameter of the jet passages into
which they open. There may be between two and eight cavities in
each jet. There may be between four and twenty jets spread on one
circle of radius around a liquid stream. The jets may be arranged
in more than one angle toward liquid stream to perform multi-stage
atomisation.
[0011] The cavities may be oriented at between 10.degree. and
60.degree. to the flow direction through the nozzles.
[0012] The invention also comprises a method for atomising a liquid
medium comprising impinging a flow of the liquid medium with a
high-velocity gas stream with a superimposed ultrasonic frequency
generated by resonance in the gas stream.
[0013] The ultrasonic frequency may be in the range 20-60 KHz, and
the high-velocity gas flow may be at supersonic velocity.
[0014] The nozzle used may be of any type including free-fall and
confined types, annular and multi-jet nozzles, and may be of any
miniature type, including inhalers and spray can nozzles.
[0015] Such nozzles may also be used, with the invention, to
atomise various liquids including molten metals, polymer melts,
solvent based solutions, and other forms of liquids. A liquid may
be formed by melting in a crucible or an extruder or dissolving in
a solution, and may be delivered to a die to form liquid
streams.
[0016] In particular the invention also comprises a method for
producing polymer powder comprising melt extruding a polymer
material and impinging a high velocity gas stream on to the molten
extrudate.
[0017] A single liquid stream may be impinged while in free fall
from a die. A liquid stream may comprise a film or filaments, in
which latter case the filaments may emerge as sheet or ribbon from
a line of spinnerettes. The film or sheet--or ribbon-like liquid
stream, may be impinged on both faces by gas streams.
[0018] The gas stream velocity may be up to Mach 2.
[0019] The die may comprise heater arrangements to ensure the
liquid is evenly heated, and still molten in the region of
impingement.
[0020] Air, nitrogen and argon may be used as atomising gas.
Atomising gas may be heated by a gas heater to atomise certain
types of liquids. Using the invention, the cost of special
gases--such as nitrogen and argon--used in gas atomisation, can be
substantially reduced. Not only is less gas used, but the maximum
working pressure, of about 17 bar, generated from conventional
cryogenic supply of such gases is suitable for use with methods and
apparatus of the invention, avoiding the need for high pressure
cryogenic pumping and high pressure storage vessels used in
conventional gas ultrasonic atomisation. The gas used should, of
course, not adversely react on or with the atomised polymer or
other material.
[0021] The invention also comprises apparatus for making polymer
powder comprising a die from which polymer is extruded and nozzle
means impinging a high velocity gas stream on the extrudate from
the die.
[0022] The die may comprise a slit for extruding a film or a line
of spinnerettes for extruding a sheet or ribbon of filaments.
[0023] The nozzle means may comprise a slit-form nozzle either side
of the die directed towards the issuing extrudate. The nozzle mans
may impinge the gas stream at an angle to the issuing extrudate so
as to have a component of velocity in the direction of flow of the
extrudate. The nozzle means may form a V-shaped gas stream with an
included angle between 30.degree. and 90.degree..
[0024] The die may comprise heater arrangement to ensure the
extrudate is evenly heated and still molten in the region of
impingement.
[0025] The invention also includes powder, inter alia polymer
powder, made by methods or apparatus as herein disclosed. Such
powders may be characterised by comprising spherical or nearly
spherical particles.
[0026] Methods and apparatus for atomising liquid media according
to the invention will now be described with reference to the
accompanying drawings, in which:
[0027] FIG. 1 is a diagrammatic cross-section of a conventional
flow channel;
[0028] FIG. 2 is a diagrammatic cross-section of a flow channel
modified in accordance with the present invention;
[0029] FIG. 3 is a detail not shown on the cross-section of FIG.
2;
[0030] FIG. 4 is a cross-section like FIG. 12 of another type of
gas flow arrangement;
[0031] FIG. 5 is a comparative graphical depiction of particle size
distribution of a typical product of a prior art process and a
process according to the invention;
[0032] FIG. 6 is a cross-section of a melt die with gas stream
nozzle means;
[0033] FIG. 7 is a view on arrow A of FIG. 6 of a first
embodiment;
[0034] FIG. 8 is a view like FIG. 7 of a second embodiment; and
[0035] FIG. 9 is a graphical depiction of particle size
distribution and a typical polymer product of a process according
to the invention.
[0036] FIG. 1 illustrates a conventional gas atomisation nozzle 11,
following U.S. Pat. No. 2,997,245. The flow channel 12 comprises
first and second legs 14, 15, joined at right angles, with a
resonance cavity 16. The abrupt change in the direction of flow
between the two legs 14, 15 gives rise to considerable energy loss
and limits nozzle efficiency.
[0037] FIG. 2 shows an improved design according to the invention
in which the flow channel 12 has a single straight line leg from
the plenum chamber 17 to the nozzle exit 18. Elimination of the
right-angled leg arrangement of FIG. 1 improves the efficiency of
the arrangement by eliminating energy losses involved in
redirecting the direction of gas flow.
[0038] Not shown in FIG. 2 are alternative arrangements for
generating ultrasonic frequency sound in the gas flow. These are
indicated, however, in FIG. 3, where more resonance cavities 31 are
shown opening into the flow channel 12.
[0039] FIG. 3 shows opposed cavities 31 in a circular section jet
flow channel 12, the cavities 31 comprising cylindrical bores
having a diameter `d` of {fraction (1/10)} to 1/3 of the diameter
`D` of the channel 12. The cavities 31 could be of other shapes,
but it is easier to machine circular-section cavities usually.
[0040] In a convergent annular type nozzle, the cavities 31 would
be much as illustrated in FIG. 3 but spaced apart circumferentially
around the annular nozzle as well as lengthwise along the flow
path.
[0041] For annular nozzles the bore diameter of the cavities can be
between {fraction (1/12)} and 1/8 of the mean nozzle diameter.
[0042] Between two and eight resonance cavities can usually be
arranged in each jet of a multi-jet arrangement; between ten and
sixty resonance cavities can be used in annular nozzle
arrangements.
[0043] The geometry, distribution and number of resonance cavities
will determine the intensity and frequency of the ultrasonic
superimposition. Typical frequencies are 20-60 KHz, produced in a
nitrogen gas stream generated by a plenum pressure between 1.4 and
1.7 MPa at up to Mach 2.
[0044] FIG. 4 illustrates a confined type nozzle (which may be
either annular or multi-jet) according to U.S. Pat. No. 3,252,783
and U.S. Pat. No. 5,228,620 adapted to the present invention.
[0045] In a typical arrangement a melting furnace was charged with
30 Kg of 316 stainless steel, melted by induction and heated to a
temperature of 1600.degree. C. Eight gas jet discharge orifices of
free fall type were arranged to define an apex angle of 45.degree..
The nozzles were supplied with nitrogen gas at 1.4 MPa. For
comparison, nozzles with and without resonance cavities were used
In nozzles with cavities, there were six, each of 1 mm diameter
uniformly arranged in each gas channel, formed at an angle of
15.degree. to the direction of the channel.
[0046] Atomised droplets were collected after solidifying and size
classified, the results being shown in FIG. 5. About 40% by weight
of the particles produced by the nozzles with resonant cavities
according to the invention were of less than 38 .mu.m diameter,
compared to only about 15% of those produced by nozzles without
resonant cavities, indicating that the ultrasonic superimposition
produced by the resonant cavities has significantly enhanced the
atomisation efficiency of the nozzles.
[0047] The FIGS. 6 to 8 illustrate apparatus for atomising liquid
streams e.g. of polymer material comprising a die 111 from which a
melt 112 is delivered in the form of a film (FIG. 7) or a sheet or
ribbon of filaments (FIG. 8), and gas stream nozzle means 113
impinging a high velocity, e.g. Mach 1 or above, stream of gas on
either side of the melt 112.
[0048] The die 111 has a heater arrangement shown diagrammatically
as an electric resistance element 114 to ensure the melt 112 is
evenly heated and molten where the nozzle arrangement 113 impinges
the melt 112.
[0049] The nozzle arrangement 113 comprises nozzles 113a directed
at the melt 112 from either side thereof and angled so that the gas
stream from each has a component velocity in the direction of flow
of the melt 112, which is itself in free fall from the die 111. The
nozzles 113a are outlets from plenum chamber means 113b and are
directed so as to form a V-shaped flow enclosing an angle B between
30.degree. and 90.degree..
[0050] The extruder is arranged to deliver melt to the die 111 so
that the cross-section of the melt 112 is equal to that of the die
orifice. The gas stream is desirably at least supersonic, possibly
up to Mach 2 for best atomisation. The particle size of the product
powder is inter alia governed by the cross-section of the melt
112.
[0051] In a typical arrangement an extruder was used to melt
PE-based polymer to a temperature of 150.degree. C. Eight gas jet
discharge orifices were arranged to define an apex angle of
45.degree.. The nozzles were supplied with compressed air at 0.4
Mpa. Compressed air was heated to a temperature of 150.degree. C.
by a gas beater. In nozzles with cavities, there were six, each of
1 mm diameter uniformly arranged in each gas channel, formed at an
angle of 15.degree. to the direction of the channel.
[0052] FIG. 9 shows the particle distribution of atomised polymer
powder produced by such an arrangement. The product powder is found
to comprise spherical or nearly spherical particles of defined size
distribution depending on the dimension of the die orifice and the
viscosity of the melt. The process can be carried out under
conditions such as to avoid risk of contamination of the
product.
* * * * *