Note: Descriptions are shown in the official language in which they were submitted.
217059~
, WO 95/06615 PCT/EP94/01270
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Specification
This invention relates to a process for the production of
granulated sodium percarbonate by fluidised bed spray
granulation.
Various processing principles are known for the production
of sodium percarbonate of the formula 2 Na2CO3 3 H2O2: (i)
reaction of hydrogen peroxide with sodium carbonate in the
aqueous phase, crystallisation of the sodium percarbonate
and separation thereof from the mother liquor; (ii)
reaction of solid soda with aqueous hydrogen peroxide;
(iii) fluidised bed spray granulation, wherein a hydrogen
peroxide solution and a soda solution are sprayed in a
fluidised bed apparatus onto sodium percarbonate nuclei and
water is simultaneously evaporated. While the processing
principle according to (i) is used on an industrial scale,
auxiliary substances such as sodium chloride for salting
out and metaphosphates to control crystallisation are,
however, necessary and purification and/or partial
discharge of the mother liquor is required in order to
achieve good product quality. Due to irregularities and
unsatisfactory storage stability, the quality of the sodium
percarbonate produced using processing principle (ii) does
not generally approach that of sodium percarbonate produced
according to (i) or (iii).
Processes according to principle (iii) are attracting
increasing interest as they give rise to no waste water and
also result in an abrasion-resistant sodium percarbonate at
very high yield. German patent 20 60 971 teaches such a
` 21705~3
Wo 95/06615 PCT/EP94/01270
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process: in this process, a sodium percarbonate solution or
sodium percarbonate suspension or separately and
simultaneously an aqueous hydrogen peroxide solution and an
aqueous sodium carbonate solution are continuously
introduced into a fluidised bed which contains sodium
percarbonate nuclei, the ~;m~n~ions of which are smaller
than those of the granulate particles to be produced, and
water is continuously evaporated from the aqueous medium
containing sodium percarbonate and granulate particles of a
certain size are removed from the fluidised bed. When a
sodium percarbonate solution or an H2O2 solution and an
Na2CO3 solution are used, nuclei are simultaneously fed into
the fluidised bed.
The above-stated process exhibits a series of
disadvantages: in one embodiment a sodium percarbonate
solution or suspension must first be produced, which
entails an additional process stage. Introducing a sodium
percarbonate suspension or a supersaturated sodium
percarbonate solution into a fluidised bed is liable to be
troublesome because the spray nozzle used rapidly becomes
plugged. On the other hand, if a dilute sodium percarbonate
solution is used much water must be vaporised, so raising
costs.
German patent 27 33 935 points out problems associated with
another embodiment disclosed in DE 20 60 971 C3: when an
aqueous hydrogen peroxide solution and an aqueous sodium
carbonate solution are sprayed through two separate spray
nozzles, such as conventional two-fluid nozzles for
spraying a solution, using air as the propellant gas, it is
difficult to achieve sufficiently intimate mixing of the
two solutions in the fluidised bed, a feature which is
however necessary in order to obtain homogeneous sodium
percarbonate particles. However, if the two solutions are
together introduced into the fluidised bed through a single
spray nozzle, crystallisation occurs in the spray nozzle,
WO 95/06615 217 0 5 9 3 PCT/EP94/01270
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generally after a short period of operation, which gives
rise to plugging and results in downtime.
In order to eliminate the above-stated problems, it is
5 proposed in DE 27 33 935 C2 to use a common spray nozzle
for both solutions and, in order to avoid plugging of the
spray nozzle, to dissolve a metaphosphate in at least one
of the two solutions. The two solutions are mixed within or
at the inlet of the spray nozzle. The quantity of
10 metaphosphate used is conveniently between 0.1 and 20 g per
kg of sodium percarbonate.
On the one hand, the additional use of a metaphosphate in
the process of DE 27 33 935 C2 increases raw material costs
15 and, on the other hand, the phosphate introduced into the
sodium percarbonate and thus into the detergents, bleaches
and cleaning products containing the sodium percarbonate,
is a component in which there is increasing interest in
eliminating for environmental reasons.
The object of the present invention is thus to provide a
process for the production of granulated sodium
percarbonate of the formula 2 Na2CO3 3 H2O2 by fluidised
bed spray granulation, wherein an aqueous hydrogen peroxide
25 solution and an aqueous sodium carbonate solution are
sprayed using a single spray nozzle into a fluidised bed
containing nuclei, the dimensions of which are smaller than
those of the granulate particles to be produced, and water
is simultaneously evaporated at a fluidised bed temperature
30 in the range from 40 to 95C, which process does not
exhibit the disadvantages of the process known from DE
27 33 935 C2.
This object is achieved by using a three-fluid atomising
35 nozzle with external mixing of the solutions for spraying
the two solutions, to which no crystallisation inhibitor
containing phosphorus is added.
~ WO 95/06615 2170 5 ~ 3 PCT/EP94/01270
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A three-fluid atomising nozzle comprising a nozzle body and
a nozzle mouthpiece is conveniently used, the nozzle
mouthpiece of which has a central tube and two jacket tubes
arranged coaxially around the central tube, wherein one of
the solutions is introduced into the central tube and one
into the annular gap formed between the central tube and
the inner jacket tube and a propellant gas is introduced
into the outer annular gap formed between the jacket tubes.
According to a particularly preferred embodiment, a three-
fluid atomising nozzle of the above-stated type is used
having a central tube which, at the nozzle tip, extends
beyond the end of the jacket tubes by at least one radius
of the central tube.
By using a three-fluid atomising nozzle with external
m;x;ng, the two solutions each conta;n;ng one reactant are
sprayed into the fluidised bed by means of a single nozzle,
wherein mixing of the two solutions and, thereafter,
formation of the sodium percarbonate, occur outside the
nozzle before the water evaporates from the liquid
droplets. Sodium percarbonate particles of a homogeneous
structure are obtained in this manner without the nozzle
becoming plugged. By using a nozzle with a nozzle
mouthpiece-according to the invention and in particular
such a nozzle with an extended central tube, it is possible
to avoid encrustation on the nozzle tip and so possibly to
avoid operational stoppages even after an extended period
of operation. Simultaneously, it is possible to dispense
with the use of a metaphosphate or another crystallisation
inhibitor containing phosphorus, such that the granulated
sodium percarbonate produced is substantially free of
phosphorus compounds. A very low phosphorus content in the
sodium percarbonate is not excluded in the event that
hydrogen peroxide stabilised with phosphates, as is
commercially conventional, is used.
The basic principle of the three-fluid atomising nozzle is
similar to that known for conventional commercial two-fluid
WO 95/06615 217 0 5 ~ 9 pcT/Epg4/ol27o
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nozzles, but it additionally contains devices for
introducing and guiding the second liquid into the nozzle.
The nozzle thus comprises a nozzle body with separate
channels and connections for the media and a nozzle
mouthpiece with the features according to the claims.
Figures 1 and 2 illustrate the structure of a convenient
three-fluid atomising nozzle; the particularly preferred
arrangement of the nozzle mouthpiece is simultaneously
shown: figure 1 shows a longitudinal section of a
particularly preferred three-fluid atomising nozzle; figure
2 shows a cross-section through the plane A-B shown in
figure 1:
A nozzle body (1) is connected to a nozzle mouthpiece (2)
in such a manner that the liquid media to be conveyed do
not come into contact with each other until they are
outside the nozzle. The connection between (1) and (2) may
take the form of a plug, bayonet or screw fixing or sleeves
or the like. In the preferred embodiment according to
figure 1, the jacket tubes ~11) and (12) of the nozzle
mouthpiece are connected to the nozzle body by means of
screw threads (9a and b). The nozzle body contains the
connections (3) and (4) for the two liquids and (5) for the
propellant gas, together with the separate channels (6) and
(7) for the two liquids and (8) for the propellant gas.
The nozzle mouthpiece (1) comprises a central tube (10) and
two jacket tubes (11) and (12) arranged coaxially around
the central tube. The central tube (10) is connected with
channel (7); in figure 1, channel (7) and the central tube
(10) take the form of a continuous tube. The annular gap
(13) formed between the central tube (10) and the inner
jacket tube (11) is connected with channel (6) and the
annular gap (14) formed between the inner (11) and outer
(12) jacket tube is connected with channel (8). An
essential feature of a particularly preferred embodiment is
the central tube extension (15), which extends beyond the
WO 95/06615 21705 9 9 PCT/EP94/01270
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ends of the jacket tubes at the nozzle tip. One or both
jacket tubes and the central tube may taper inwards towards
the nozzle tip ((17a) and (17b) in figure 1) in order to
increase the exit velocity of the media and to promote
break up of the liquid stream and propellant gas stream
exiting from the annular gaps. The central tube (10) or its
extension (15) and/or one or both of the annular gaps may
additionally contain swirl inducers (16a and b). The
propellant for the nozzle may be air or another inert gas,
such as for example nitrogen or also superheated steam.
According to a preferred embodiment of the nozzle, the
central tube of the nozzle mouthpiece extends beyond the
ends of the jacket tubes by at least one radius of the
central tube, preferably by 2 to 10, in particular 3 to 6
radii of the central tube. The jacket tubes preferably end
at the same level. The jacket tubes may, however, also end
at different levels, but the central tube extension
according to the claims must be longer than both jacket
tubes. Provided that the outer jacket tube extends beyond
the inner jacket tube, the liquid in the annular gap and
the propellant gas are premixed within the nozzle, but the
liquids themselves do not come into contact with each other
until they are outside the nozzle. The optimum size of the
central tube extension is dependent upon the radius of the
central tube and the flow area of the inner annular gap at
the outlet. As the central tube radius increases, it is
generally favourable to shorten the central tube extension;
at a central tube radius of, for example, at least-2 mm,
the central tube extension will generally be between 3 and
5 central tube radii.
In principle, the aqueous H2O2 solution or the Na2CO3
solution may be conveyed through the central tube of the
nozzle and the other solution may be conveyed through the
adjacent annular gap. Preferably, however, the solution
present in the smaller volume (at preferred solution
WO 95/06615 2170 5 ~ 9 PCT/EP94/01270
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concentrations, this will be the hydrogen peroxide
solution) is passed through the central tube.
The aqueous hydrogen peroxide solution and the aqueous
sodium carbonate solution are introduced into the fluidised
bed maintained in an apparatus for fluidised bed spray
granulation in a ratio such that the molar ratio of Na2CO3
to H2O2 is in the range between 1:1.4 and 1:1.7; a molar
ratio of 1:1.5 and 1:1.65 is preferred.
The concentration of the solutions may vary over a broad
range; the highest possible concentration is preferably
selected in order to keep the quantity of water to be
evaporated low. According to a particularly preferred
embodiment, the Na2CO3 solution and the H2O2 solution will
have a very high concentration, so that the sodium
percarbonate solution initially present in droplet form in
the mixing-zone in front of the nozzle tip is
supersaturated. The aqueous hydrogen peroxide solution
conventionally contains from 30 to 75 wt.~, preferably 40
to 70 wt.~, of H2O2. The Na2CO3 concentration of the sodium
carbonate solution is conveniently above 10 wt.~ of Na2CO3,
preferably between 20 wt.~ and the saturation concentration
at the particular temperature; Na2CO3 concentration is
particularly preferably 30 wt.~. One or both solutions, but
preferably the soda solution, may be used in preheated form
at 30 to 70C, instead of at a conventional storage
temperature.
Reference is made to the cited prior art documents with
regard to performance of fluidised bed spray granulation.
In the case of continuous operation, a sufficient number of
nuclei must always be present in the fluidised bed. In
order to control grain size distribution, between 0 and
30 kg, preferably between 1 and 10 kg of nuclei are
introduced into the fluidised bed per 100 kg of granulated
sodium percarbonate discharged from the fluidised bed. The
weight of nuclei is determined by the desired grain size
` WO 95/06615 217 0 5 9 3 PCT/EPg4/01270
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range and in particular by the size of the nuclei. It is
necessary to optimise operating conditions with regard to
the introduction of nuclei at a given desired fluidised bed
temperature in order to achieve steady state operation.
Recirculating an excessively large quantity of very finely
divided material as nuclei may result in excessively low
particle growth, while an excessively high moisture content
in the fluidised bed may result in unwanted aggregation.
Reference is made to the article by H. Uhlemann in Chem.-
10 Ing. Technik 62 (1990), no. 10, pp. 822-834 with regard to
the general technique of fluidised bed spray granulation,
the mutual interaction of operating parameters and control
of particle size and distribution and to suitable equipment
for continuous fluidised bed spray granulation.
The temperature of the fluidised bed is maintained between
40 and 95C, preferably between 40 and 80C and in
particular between 50 and 70C.
The temperature of the inlet air for drying and maintaining
the fluidised bed is adjusted to above 120C, preferably
between 200 and 400C and in particular between 300 and
400C. The person skilled in the art will adjust the
temperature and mass flux of the inlet air in such a manner
that a well fluidised bed is produced, the required
performance may be achieved and an excessively large
quantity of product need not be returned from a downstream
dust separator. The velocity of the drying air in the empty
tube is conventionally between 1 and 4 m/s. The fluidised
bed apparatus is generally operated in such a manner that
approximately standard pressure (around 1 bar) prevails in
the area of the fluidised bed; it is, however, also
possible to operate at a pressure below or above standard
pressure. One or more three-fluid atomising nozzles may be
arranged in the fluidised bed spray granulation unit,
wherein the direction of spraying may be substantially
cocurrent or countercurrent to the stream of drying air or
may assume an intermediate position.
- Wo 95/06615 21705 9 9 PCT/EP94/01270
g
It has proved convenient in the case of continuous
operation to discharge the sodium percarbonate at a
residual moisture content of up to 10 wt.~, preferably of
between 3 and 9 wt.~ and in particular between 5 and 8 wt.~
and, if desired, to dry it in a downstream apparatus to the
residual moisture content of the conventional commercial
product (below 1 wt.~) or to pass it on for post-
treatment. Post-treatments which may in particular be
considered are processes for providing a shell around the
particles for the purpose of increasing storage stability.
Such post-treatment is preferably baæed on applying
solutions cont~ining one or more shell components, such as
for example boron compounds, soda, sodium sulphate,
magnesium sulphate and water glass onto the previously
produced granulated sodium percarbonate in a fluidised bed
while simultaneously evaporating water and forming a firmly
attached outer shell.
If required, the H2O2 solution and/or Na2CO3 solution to be
sprayed may also contain additives (with the exception of
crystallisation inhibitors containing phosphorus) in order
to influence product properties and in particular to
increase the active oxygen stability of the hydrogen
peroxide used and of the sodium percarbonate to be
produced. Stabilising additives which may be considered are
preferably magnesium salts (conventionally added to the H2O2
solution in sulphate form) and water glass (usually added
to the soda solution); further additives may be, for
example, stannates, complexing agents and dipicolinic acid.
While phosphorus-free crystallisation inhibitors may be
present, they are not conventionally used. Speed of
dissolution may, for example, be increased by adding
surface-active substances.
The apparatus for fluidised bed spray granulation may be
those as are described in DE 27 33 935, EP o 332 929 B1 and
in the already cited article by H. Uhlemann. The fluidised
bed may be equipped with one or preferably with two or more
WO 95/06615 217 05 9 3 PCT/EP94/01270
-- 10 -
three-fluid atomising nozzles according to the invention.
Apparatus having an inlet for nuclei and a classifying
granulate outlet is preferred. The nuclei to be introduced
into the fluidised bed may originate from dust separation,
screening and/or partial comminution.
In addition to the above-stated apparatus having a steady
state fluidised bed, the process may also be performed in a
fluidised bed flow channel, which is equipped with one or
more nozzles arranged in series; the product is classified
at the end of the flow channel and undersized product,
optionally together with comminuted oversized product, is
returned to the flow channel.
Although the process of the invention is preferably
performed continuously on an industrial scale, i.e. with
continuous introduction of the solutions and discharge of
the granulated product of the desired size, it may also be
operated discontinuously (spraying terminated once the
desired grain size range has been achieved and the
granulated product is then discharged).
It is possible by means of the process according to the
invention to obtain granulated sodium percarbonate starting
from a hydrogen peroxide solution and a sodium carbonate
solution by fluidised bed spray granulation on an
industrial scale without stoppages due to plugging or
encrustation of the nozzles in a virtually quantitative
yield with an elevated active oxygen content, elevated
abrasion resistance, elevated bulk density and very good
storage stability without there being any need to use
crystallisation inhibitors. In order to increase stability,
the product obtainable according to the invention may,
immediately after production, be enclosed in a shell in a
manner known per se, wherein due to the dense and
substantially spherical structure of the grains, a smaller
quantity of shell material than is required for enclosing
Wo 95/06615 217 05 9 9 PCT/EP94/01270
sodium percarbonate obtained by crystallisation processes
in a shell is sufficient.
It could not be predicted that it would be possible by
using a three-fluid atomising nozzle according to the
invention not only to achieve adequate external mixing of
the solutions and to obtain a homogeneous product, but also
to dispense with a crystallisation inhibitor containing
phosphorus; operating costs are thus reduced and an
environmental problem is avoided. It was moreover not to be
expected that by using a three-fluid atomising nozzle with
a central tube extension it would be possible virtually
completely to avoid stoppages due to encrustation on the
nozzle.
Examples
An aqueous hydrogen peroxide solution and an aqueous sodium
percarbonate solution, which contained no crystallisation
inhibitors, were introduced into a fluidised bed using air
as the propellant in an apparatus for fluidised bed spray
granulation with devices for dust recirculation and the
introduction of nuclei and a classifying granulate outlet,
once an initially introduced quantity of sodium
percarbonate had been fluidised. The nozzles were located
within the fluidised bed and the direction of spraying was
cocurrent with the stream of drying gas. The central tube
extension was 3 central tube radii in length, the jacket
tubes ended at the same level.
The table shows the essential operating parameters and
material data for the sodium percarbonate produced. Even
after several days' continuous operation, there was no
plugging or encrustation in or around the nozzle.
WO 95/06615 2 17 0 5 9 9 PCT/EP94/01270
.,
-- 12 --
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- 13 -
List of reference numbers
1 Nozzle body
2 Nozzle mouthpiece
3 Connection for liquid (i)
4 Connection for liquid (ii)
Connection for propellant gas
6 Channel for liquid (i)
7 Channel for liquid (ii)
8 Channel for propellant gas
9a Thread
9b Thread
Central tube
11 Jacket tube (inner)
12 Jacket tube (outer)
13 Annular gap for liquid (i)
14 Annular gap for propellant gas
Central tube extension
16a Swirl inducer
16b Swirl inducer
17a Taper
17b Taper
