The present invention relates in general to spray dryers, and more particularly, to a spray nozzle for atomizing and spraying a slurry of finely divided material into a spray drying chamber to be contacted with a hot drying gas therein.
Spray drying is well-known in the prior art and has been used for drying solutions or slurries of finely divided particulate material dissolved or suspended in a volatile carrier liquid, most commonly water. The slurry or solution to be processed is atomized and sprayed into a hot gas stream passing through the spray dryer. The volatile carrier liquid is evaporated, and the dissolved or suspended particulate material dried to a fine powder. Spray drying has for many years been used for drying and processing food products, pharmaceuticals, and many other powder products.
More recently, spray drying has been found to be an effective method for treating flue gases from fossil fuel-fired furnaces to remove gaseous pollutants, most commonly sulfur dioxide, therefrom. In such an application, a solution or slurry of sulfur oxide absorbent, such as lime, limestone, soda ash, or caustic soda, in water is atomized and sprayed into the spray drying chamber to contact hot flue gas from the furnace. As the water in the slurry or solution is evaporated by heat from the hot flue gas, the sulfur oxide absorbent reactant contained therein reacts with sulfur dioxides contained in the flue gas. The resultant product is a dry powder of sulfur-containing salt.
A typical spray dryer generally comprises a housing defining a spray drying chamber designed to provide the proper environment and residence time for efficient drying of the solution or slurry. The drying gas is typically introduced to the vessel through an inlet at the top thereof and an outlet near the bottom thereof. The solution or slurry of particulate material to be dried is sprayed into the vessel in a finely divided form through atomization means. The atomized solution or slurry is sprayed into the hot drying gas as it enters the spray drying chamber so as to intermix with the hot gas so that the volatile carrier liquid is evaporated and the dissolved or suspended particulate material reduced to a fine, dry powder.
One type of atomization means being used in spray drying applications is a sonic atomizing spray nozzle. In such a device, sound waves are generated by impinging a high-velocity stream of a gas, most commonly air traveling at substantially sonic speed, against a tuned cavity, termed a resonator cup, disposed at the outlet of the spray nozzle. Simultaneously, the liquid solution or slurry to be atomized is injected through a plurality of small orifices into the zones of sound waves generated by discharging the high-velocity gas into the resonator cup. The vibrations from the resonating sound waves possess considerable energy and as a result, atomize the liquid into very fine droplets.
One problem encountered in using such sonic spray nozzles to spray slurries into spray dryers, such as slurries of sulfur oxide absorbent particles in a carrier liquid such as water, is the tendency of the orifices through which the slurry is sprayed to become plugged by particles in the slurry. Another problem encountered is the uneven distribution of slurry spray in the spray dryer resulting from the fact that injecting the slurry through a plurality of orifices yields a series of discrete spray streams, one emanating from each orifice, rather than a continuous spray sheet.