Nucleation and Growth of Aerosol in Chemically Reacting Systems: A Theoretical Study of the Near-Collision-Controlled Regime

Abstract

A theoretical treatment of the formation and growth of aerosols in systems where condensable molecules are generated at a constant rate is presented. Previous investigations of this type have focused either on collision-controlled (coagulation-limited) nucleation or on condensation /evaporation-controlled nucleation. In the latter case, classical nucleation theory has typically been used to determine particle formation rates, and coagulation of subcritical clusters is neglected. The present theory accounts for both coagulation and condensation / evaporation processes, and serves to bridge the gap between the two limiting cases. The aerosol population balance equations are cast in a nondimensional form and are solved numerically for the time-dependent size spectrum. A key aspect of this work is the identification of dimensionless parameters that have a significant influence on aerosol formation. The most important of these parameters is the evaporation parameter, E =

, where N _s is the saturation concentration of the nucleating vapor, β₁₁ is the monomer collision frequency function, and R is the rate of monomer production by chemical reaction. A second parameter, A, indicates the extent of enhancement in evaporation of small droplets due to the Kelvin effect. It is shown that the collision-controlled limit applies when E is sufficiently small (< 0.1 when A = 12), and the condensation/ evaporation limit applies when E is sufficiently large (> 1.0 when A = 12). The effects of deposition on and evaporation from the reactor walls were also considered. A wall deposition parameter, W is defined that can be used to estimate the magnitude of these effects. Definitions of A and W are included in the nomenclature.

*This is Particle Technology Laboratory Publication No. 640.

Notes

*This is Particle Technology Laboratory Publication No. 640.