Liquid phase mass transfer in spray contactors

Yeh, Norman Kirk
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Spray absorbers are used in the majority of limestone slurry scrubbers to remove sulfur dioxide from the flue gas of coal-fired power plants, and in these systems, the mass transfer is primarily liquid film controlled. Improving the fundamental understanding of spray towers should be helpful in the selection, design, and optimization of nozzles, spray scrubbers, and other gas-liquid contactors. Liquid phase mass transfer in sprays has been measured with carbon dioxide desorption by collecting and analyzing samples of the spray. Commercial hollow cone nozzles were used to determine the effects of spray distance, nozzle pressure drop, and nozzle selection on mass transfer performance. Experiments were conducted with laboratory (1/8 to 3/8 inch) and pilot scale (3 inch) nozzles at pressures of 5 to 20 psi. Significant mass transfer occurred during sample collection, and a quench sampling method was developed to minimize this effect. Spray impact in the sample collector without quenching resulted in 0.2 to 0.7 liquid phase transfer units (NL), compared to 0.5 to 1.2 transfer units in the spray. Of the mass transfer in the spray, approximately 60% occurs in the liquid sheet before droplet formation. The droplet region can account for less than half of the total NL. Increasing the nozzle pressure drop resulted in substantially higher mass transfer during spray impact but had negligible effect on the NL of the spray itself. The spray NL decreased with nozzle size, and spray distance appears to scale with the nozzle orifice diameter up to 60 orifice diameters.