Atomization—a new type for better gas scrubbing

Abstract

Atomization of water by air in a Venturi scrubber was studied by stop-action still photography (0.5 μs) to determine how and where atomization takes place and what happens to the droplets formed. It was shown that two distinct types of atomization can be made to occur: “cloud-type” and “drop-type”. Cloud-type, a unique type of atomization, occurs when water is injected as a stream from nozzles greater than approximately 1 mm i.d.; drop-type occurs when smaller nozzles, spray nozzles, or dropping tubes are used which causes the water to enter the atomizer in drop (or sheet) manner. Atomization of streams produces smaller than predicted (< 30 μ dia.) droplets which group in cloud like form. Atomization of drops gives the predicted “larger” size droplets for which size range and distribution equations have been developed and verified in many previous investigations. Both of these types of atomization were produced in the same apparatus of this study simply by modifying the water inlet nozzle diameters. The difference between the two types of atomization is explained on the basis of surface tension considerations: There is less surface area in a stream of water than in the same volume of water as drops, so less work is required to break up a stream. Cloud-type atomization results in both higher atomization efficiency and in lower acceleration rates for the atomized droplets. These factors are important in obtaining efficient dust collection using inertial impaction devices. Atomization was shown to occur within 1.5 cm from the water inlets, so the lower the water droplet acceleration rate the better the collection results. Once atomized, clouds of water do not appear to agglomerate in the Venturi diffuser. Critical or minimum air velocities required to produce atomization are dependent upon the liquid injection nozzle i.d. Critical air velocities for cloud-type atomization of water can be obtained from the following equation (noting that minimum nozzle i.d. for cloud-type atomization is about 1.0 mm): v o, crit. = ( 8550 d' ) 1 2 + 15.3 where v o , crit. = ft s −1 d' = nozzle i. d., mm. This equation may vary slightly for impure water and other liquids. Cloud-type atomization, in addition to giving superior overall impaction type collection, should result in better absorption or adsorption in an extractor type collector because of the large number of liquid droplets formed (greater cross section and surface areas) and because of the lower acceleration rates. Operating and construction costs should both be lower in scrubbers using cloud-type atomization because larger nozzles are used. Water can be supplied at lower pressures and the numerous smaller inlets (or sprays) are not required. It should be noted that adequate liquid inlet arrangements are still necessary to assure good contact of the liquid streams by the gas stream. A water rate of 5 gal H 2O/1000 ft 3 air appears to provide adequate liquid for scrubbing with cloud-type atomization with scrubber diameter to nozzle diameter ratios in the range of from 9:1 to 16:1 and when the liquid is injected towards the center of the scrubber.