10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Mathematical Modeling of Aerosol Formation from Binary Vapor Mixtures
Ali Rostami, Sergey Fisenko, SERGEY N. MAXIMOFF, David Kane, Yezdi Pithawalla, Mohamed El-Shall, Altria Client Services LLC
Abstract Number: 354 Working Group: Aerosol Physics
Abstract Characterizing the aerosol produced by an Electronic Nicotine Delivery System (ENDS) is necessary for regulatory compliance and for designing and optimizing device performance. ENDSs produce condensation aerosol by mixing a high temperature vapor mixture with ambient air, resulting in a vapor mixture at high supersaturation. To characterize the resulting aerosol, it is important to estimate the homogeneous condensation nucleation rate, droplet size distribution, droplet composition as well as vapor-liquid partitioning. In circumstances where the supersaturation is very high and the critical cluster size is smaller than a dimer, the Classical Nucleation Theory (CNT) does not apply.
We addressed this limitation by developing a computational model that uses kinetic theory of gases to calculate nucleation rate and aerosol formation for binary mixtures of propylene glycol (PG) and glycerin (G) vapors. The process involves ternary collision of two vapor molecules of different kinds and any third molecule. The model accounts for non-isothermal growth of clusters with free molecular regime approximation where the cluster size is much smaller than the mean free path of vapor molecules. The latent heat of phase change, that affects the temperature difference between clusters of molecules and gaseous mixture, was accounted for in the model. An ideal liquid mixture solution was assumed for the droplets to account for the vapor-liquid partitioning of each constituent. After 200 ns of free molecular condensation the main mechanism responsible for droplets growth will be coagulation.
Computations were performed for different PG/G mixture ratios, ranging from 0/100 to 100/0 (mole based). Under ideal adiabatic mixing with air, at a vapor temperature of 570 K (close to glycerin boiling temperature), the initial supersaturation of glycerin drops several orders of magnitude. Particle size distribution over time and particle composition were calculated and presented. It was shown that final droplet size distribution is determined by coagulation. The temperatures of droplets and gaseous mixture are the same, due to high heat transfer coefficient at the particle-gas interface. The droplet composition for PG/G mixture strongly depends on the glycerin partial supersaturation in the initial mixture. The mass median diameter of aerosol particles, 1 s after mixing reaches 1-1.5 µm, and shows small dependence on the PG/G ratio and temperature. For all practical purpose, coagulation stops when the number density of micron size droplets decreases to less than about 1014 droplets/m3.