Spraying method and nozzle for atomization of a liquid
A method for atomizing a liquid (L) in a spraying nozzle ( 1 ), wherein a gaseous phase (G) and a liquid (L) are fed to a mixing chamber ( 30 ) inside a nozzle ( 1 ), obtaining an emulsion of the gas in the liquid, the emulsion being under pressure inside the chamber and formed by gas bubbles enveloped by the liquid in a film state. The speed of the gaseous phase at the inlet of the mixing chamber is around the speed of sound or greater. The atomized liquid is obtained by an expansion of the emulsion at the outlet of the chamber. A suitable nozzle ( 1 ) is also disclosed, comprising a mixing chamber ( 30 ) and a distribution device (D) adapted to provide appropriate gas and liquid feed to form an emulsion.
1. A spraying nozzle for atomizing a liquid, the nozzle comprising:
a gas inlet passage;
a liquid feeding passage;
a mixing chamber; and
a gas and liquid distribution device arranged to provide a gas inlet in an inlet zone of said chamber and a liquid inlet at the same inlet zone of the chamber, so as to put into fluid communication said gas passage and liquid passage with said mixing chamber and to form an emulsion of the gas in the liquid in said mixing chamber;
wherein said gas and liquid distribution device comprises a sleeve and a pin, the pin being coaxially inserted into said sleeve, and wherein:
the sleeve has an axial gas passage with a rear opening in communication with the gas inlet, and a front opening with an internal rounded edge and in communication with the mixing chamber; the pin being shaped so as to define passages at said front opening suitable to generate high-speed gas jets into said mixing chamber; and
the sleeve has a ring portion around said front opening and having a plurality of passages for the liquid, said passages being in fluid communication with the mixing chamber and with the liquid inlet, so that the liquid flow from the passages of said ring portion, in use, is distributed around the high-speed gas jets, at the inlet zone of said mixing chamber.
2. The nozzle according to claim 1 , wherein said gas and liquid distribution device is arranged to provide that the liquid inlet is distributed in a symmetrical way around said gas inlet in the inlet zone of the chamber.
3. The nozzle according to claim 2 , wherein said gas and liquid distribution device is arranged to provide a plurality of gas jets surrounded by a liquid flow distributed over a circumference, at said inlet zone of the mixing chamber.
4. The nozzle according to claim 1 , wherein the pin has
a core portion having a diameter equal to said front opening of the sleeve, and
a head portion having a diameter equal to said rear opening of the sleeve,
wherein longitudinal cuts are formed along the pin, from the head portion to an end of said core portion, a bottom surface of said cuts being at a distance from a longitudinal axis of the pin, which is less than a radius of said core portion, and
wherein the pin is inserted into said sleeve forming the gas-liquid distribution device, so that at an inlet side of said distribution device, the gas flow is allowed by large slits obtained by said longitudinal cuts at the end portion of the pin, while at an outlet side of the distribution device, said passages suitable to generate high speed gas jets are defined by said cuts, between the core portion of the pin and the front opening of the sleeve.
5. The nozzle according to claim 1 , the nozzle having a tip containing said mixing chamber and ending with a convergent portion and with a nozzle orifice, said nozzle orifice being shaped to form a cone or fan-shaped atomized flow.
6. A fluid bed granulation apparatus, comprising at least one nozzle according to claim 1 .