Fluid sprinkling – spraying – and diffusing – Processes – Including centrifugal force or spattering
Reexamination Certificate
2000-06-22
2002-01-15
Scherbel, David A. (Department: 3752)
Fluid sprinkling, spraying, and diffusing
Processes
Including centrifugal force or spattering
C239S398000, C239S406000
Reexamination Certificate
active
06338438
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing droplets with a narrow size distribution from liquids. The term liquid used in connection with the invention covers both clear liquids, such as solutions, and for instance metal melts and flowable dispersions, like for instance suspensions.
2. Description of the Prior Art
Producing droplets from liquids is often termed atomization. Common atomizing processes used commercially at a large scale are spraying under pressure in single-fluid pressure nozzles, for instance hollow cone nozzles, spraying with a gas in two-fluid nozzles or pneumatic atomization and the atomization with rotary atomizers. The invention relates to processes according to the last-mentioned principle.
In many technical processes a narrow droplet size distribution is desired. This is, inter alia, because spray drying plants must be dimensioned according to the biggest droplets of the spray, as these droplets require the longest residence time in the drier. Thus a broad droplet spectrum means, in spite of a lower average droplet size, large and consequently unfavorable dimensions. The smallest droplets in the spray necessitate high costs for cleaning the discharge air in form of filters and cyclones or like devices. A broad droplet size spectrum moreover leads to a broad particle size distribution of the produced spray-dried powder and, consequently, in some cases to undesirable technical properties.
Up until now all known atomization processes which are used technically at a large scale, i.e., a production capacity of more than 100 kg/h, produce droplets with comparatively broad size spectrums. See for instance Chem.-Ing. Techn. 62 (1990) 12, pp. 983-994.
Admittedly, droplets with a fairly narrow size distribution may be obtained with rotary atomizers of commonly used design. Thereby, the effect of the laminar jet disintegration is utilized. If the liquid is delivered to the center of a plane around a rotating disc, it flows—when a certain limited liquid production is kept—as a laminar film radially outwards and forms at the trailing edges of the disc threads of liquid. The liquid threads are created at the periphery of the trailing edge in a natural way with regular spacing. The subsequent disintegration of the liquid threads results in droplets with a very narrow size spectrum. If the size distribution of the droplets thus attained is described, for instance with the RRSB function according to DIN 66 141, then an evenness parameter of 6<m<8 is attained. As average droplet size d
v.50
is in the present text the droplet diameter defined at which the 50%-value of the volume distribution is obtained; i.e. that 50% of the sprayed liquid volume includes smaller droplet diameters, and 50% of the sprayed liquid includes bigger droplet diameters than d
v50
.
A considerable drawback of the atomizing method with plane rotating discs is that the amount of liquid passing this flow area is very small. As an estimate, the passing amount {dot over (V)} of low-viscous liquids will lie in the range of 0.21<{dot over (V)}(&rgr;
3
n
2
/D
3
&sgr;
3
)
0.25
<0.32. D is the disc diameter, &rgr; the liquid density, &sgr; the surface tension of the liquid, and n the speed of rotation. Both the narrow limits of the throughput range and the low value of the throughput of liquid hinder widespread use of this process.
In order to obtain a higher throughput it has been suggested to arrange several discs over each other, Chem.-Ing.-Techn. 36 (1964) 1, pages. 52-59. A uniform distribution of the liquid on the discs is however difficult to obtain with a device that does not easily clog. The narrow throughput range is also in this connection a drawback.
Recently, discs or cuts have been used, which at their periphery are provided with evenly spaced notches or grooves, for spraying of lacquers. In this way the throughput range can be broadened while maintaining laminar jet formation. However, also with this embodiment the throughput range is insufficient for many technical purposes.
FR A-2 662 374 discloses an atomizer rotor which is capable of working with varying volume, a homogenous atomization being obtained even with high-viscous liquids. This atomizer rotor is on the outside provided with grooves, to which the liquid to be atomized is supplied through perforations in the cylindrical wall of the rotor. It is stated that the length of these perforations must never exceed double their diameter. The liquid to be atomized is distributed on the inner side of the rotor by a stationary tube. It seems in particular to be grooves arranged on the exterior of the rotor that are to ensure an even droplet size at the atomization, and the advantage obtained is limited.
The atomizer normally used for spray drying consists of a low, cylindrical body, most frequently called atomizer wheel, said body having apertures or ducts. The diameter of the apertures are normally in the range of 5-30 mm. The liquid is often supplied centrally and flows radially outwards and leaves the atomizer through the apertures. The design has admittedly the advantage that the comparatively big apertures normally do not clog, but the throughput for large scale technical uses is chosen so high that the liquid leaves the apertures in thick turbulent jets. Due to the high relative speed between the liquid and the surrounding gas, the liquid jets which already leave the openings turbulently are dispersed. Thereby droplets are created at high rotational speed which is necessary in respect of small droplet dimensions, the droplets having a very broad size spectrum. At the same time a considerable wear of the walls of the apertures occurs when suspensions are atomized on account of the high flow speed.
SUMMARY OF INVENTION
The turbulence in the liquid jets is on account of the high relative speed between liquid and the gas surrounding the atomizer still further enhanced. It is known that a high jet turbulence always leads to droplets with a broad size spectrum. Usual evenness parameters of the RRSB distribution lie approximately in the range 2<m<4. Typical liquid throughputs lie with the conventional process, for instance for an average droplet size of 250 &mgr;m at approximately 20-200 l/h per aperture. Typically a number n of revolutions per minute of 10,000-30,000 is used, which depending on the diameter results in centrifugal accelerations a in the range from 5.10 to 1.10
6
m/s
2
. Here the limit is set by the strength of the material of which the wheel is made.
According to the invention, these drawbacks are eliminated by adjusting the throughput of the liquid in the apertures in the wall of a rotating hollow cylindrical body (cylinder) to a comparatively small and equal value. At the same time on account of the throughput limit per aperture a plurality of apertures is necessitated, in order to obtain technically desired throughputs. The liquid flows laminarily at suitable low throughputs through the apertures, so that when leaving the aperture a laminar jet disintegration takes place. Provided that the throughput per aperture remains the same and sufficient aperture lengths are provided for, the diameter of the apertures may surprisingly be changed within broad limits without any perceptible effect on the droplet size. In this way, fine droplets with narrow size distribution may be obtained at comparatively low speed and with comparatively big apertures, with very little tendency to clog. Thereby the droplet size is determined to a high degree by the throughput and the number of apertures, to a small degree by the rotary speed of the atomizer, and to a very little degree by the liquid density and the surface tension. The small flow rate in the apertures further provides the advantage that no substantial wear occurs.
The minimum throughput per aperture is determined by the lower limit, which is necessary for the formation of a jet. For low-viscous liquids, the throughput per aperture amounts according to measurements to:
{dot over (V)}
A
=1.0(&
Bach Poul
Flyger Soren Birk
Funder Christian Reedtz
Walzel Peter
Hwu Davis
Niro Holdings A/S
Scherbel David A.
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