Droplet and Particle Dynamics in Aerosol Reactors and Environmental System

A WashU Dissertation by: Sukrant Dhawan (Dept. of Energy, Environmental, and Chemical Engineering)

Abstract: Aerosol science and engineering is an enabler for continual advances in nanomaterial synthesis. The spray-based techniques have been used extensively for the large-scale production of nanoparticles. Synthesis of particles with a desired the size and morphology is of key importance for exploiting their properties for their use in several emerging technologies. In contrast to useful nanomaterials, the aerosols from industrial flue gas, dust, indoor cooking, pathogens, and wildfire etc. are harmful to human health. It is important to understand how these harmful aerosols travel through the environment and potentially impact the health. It is also very critical to improve the accuracy of indoor aerosols sampling instruments for accurate assessment of the health impacts of these aerosols. Many potentially harmful indoor aerosols such as viruses (including the SARS-COV-2 virus) and protein fragments lie in the nanometer size ranges, and it is therefore important to improve existing technologies or develop low-cost alternatives that efficiently capture harmful, nanometer-sized aerosols. In order to control the harmful aerosol emissions, and tailor the properties of synthesized aerosols, a thorough understanding of nanoparticle formation and their dynamics in different reactor systems and environments is needed, which is the main focus of my graduate work. My dissertation is divided into three sections.

The first section of my dissertation focuses on understanding the particle formation in the aerosol reactors that employ liquid-to-particle conversion route (spray synthesis). The particles with different morphologies, mainly solid and hollow, are produced using spray drying depending on the process conditions. A model for simultaneous droplet heating, evaporation, and dynamics and transport of solute and particles within the droplet was developed, to investigate the effect of different conditions during spray drying on the dried particle morphology. The drying process was modelled in two separate stages in this work, initial drying stage before shell formation, and the transition stage, in which shell formation was modelled till the solid crust formation takes place. Using this model two cases were analyzed, 1) drying of droplet with dissolved solute, and 2) drying of droplet with suspended solids. Next, the developed droplet drying model was advanced further to understand and predict structure and conductivity of PEDOT (poly(3,4- ethylenedioxythiophene)) nanoparticles synthesized using aerosol vapor polymerization. The model was modified to additionally account for gas phase transport of monomers and polymerization reaction inside the droplet. The effect of different reactor conditions was examined on the average chain length of polymers in synthesized PEDOT nanoparticles as it directly affects their conductivity.

The second section of my dissertation focuses on understanding and accurately assessing the impact of harmful aerosols on human health. Semi-Volatile Organic Compounds (SVOCs) are very common indoor pollutant which are present in every household. These compounds can phase-partition and exists in the air in both gas and particle phase. Diffusion denuders are used to separate gas and particulate SVOCs, and measure both phases separately to accurately access their transport in an indoor environment and their subsequent health risks. However, there are artifacts associated with this sampling method. A theoretical model for simultaneous gas diffusion and aerosol evaporation in the parallel plate denuder was developed to investigate the effects of denuder sampling artifacts on gas–particle partitioning measurements of SVOCs. The effect of the denuder design parameters and organic species properties, which may influence the evaporation of the particulate phase, was studied on sampling artifacts. The next part of my thesis focuses on understanding the spread of airborne pathogens like SARS CoV-2. A comprehensive model for respiratory emissions of droplets, droplet evaporation, and transport due to diffusion, gravitational settling, and ambient air flow, was developed. The considerations for viral load in droplets and virus decay were accounted for in the model to determine the spatiotemporal concentration of viable virus exhaled by the infected individual. The exposure to viable virus and risk of infection was determined using respiratory deposition curves and dose-response approach. The effect of the different parameters such as viral load, physical separation, ambient air velocity, mask usage etc. was determined on the risk of infection transmission.

The third section of my dissertation focuses on the fundamental understanding of particle charging in a non-thermal plasma reactor, with a vision to incorporate plasma reactors in conjunction with the conventionally used particle capture devices, thereby increasing their efficiency for particle capture. We tested a new design concept for enhancing aerosol nanoparticle charging in plasmas by introducing a DC field downstream of the plasma volume in the spatial afterglow to potentially prevent neutralization of the particles. Premade, chargeneutral nanoparticles were introduced into the plasma reactor with a downstream DC bias and the charge fraction of the particles was examined at the reactor outlet for different particle diameters as the function of reactor operating conditions. The mechanism of particle charging in plasma reactor was proposed based on experimental observation sand characteristic charging time scale calculations.

Link to Dissertation: https://www.proquest.com/docview/2572571676?pq-origsite=gscholar&fromopenview=true