Analysis and utilization of mass and photon transport phenomena in aerosol media

The world is transitioning towards green chemistries and sustainable processes. Playing a key role in many fields, aerosols can facilitate this transition due to their intrinsic properties. In chemical industry, mass transfer limitations are a main concern in reactor design and operation for multiphase reactions. Mass transfer limitations occur when diffusion rate of molecules is slower than reaction rate. These limitations decrease productivity of reactors and lead to higher processing costs. Utilization sustainable processes in industry is often challenging due to high costs associated to these new technologies. Alleviating mass transfer limitations by better reactor designs with a high surface to volume ratio can foster the transition. Aerosols often have a high surface area to volume ratio, which makes them an ideal platform for gas-liquid reactions. The aim of this thesis is to investigate mass and photon transport phenomena in aerosols to intensify several processes. Aerosols are characterized and utilized in many fields such photochemistry, carbon capture and dentistry. Mass transfer in aerosols is the central aspect of the thesis.

Photochemical reactions are often considered as green chemistries. Photoreactions are fast and selective. As a result, they can reduce equipment volume. However, photochemistry has not been exploited to its full potential in industry due to difficulties of  photoreactor scale-up. Photon transport adds a great complexity to photoreactor design. Light can decay substantially in a few millimeters in a photoreactor. Due to their small characteristic lengths, microreactors are shown to boost apparent reaction rates. However, their application to an industrial level is challenging because light and flow need to be distributed to too many channels. In this thesis, aerosols were utilized to circumvent this issue. In an aerosol photoreactor, each droplet works as a microreactor. When light hits a droplet, it is absorbed and scattered to all other droplets. They can be scaled up easily by increasing amount and volume of droplets. It was shown that both mass and photon transfer limitations can be overcome easily in an aerosol photoreactor. A photo-sulfoxidation reaction was shown to take place in an hour in a small vial in literature. The same reaction went to almost complete conversion under 25 s in the aerosol photoreactor. This huge enhancement in residence time shows the potential of aerosols to intensify photoreactors. Still, for an efficient photoreactor operation, radiant fields inside aerosol photoreactors needs to be solved. In this thesis, a new and simpler way of calculation of radiative fields in an aerosol photoreactors was suggested. The new model is called the shadow area model and was shown to predict the light transmission and absorption with a similar accuracy to the current simplified models of radiative transfer equation such as two flux model and six flux model. 

One of the main challenges that world is currently facing is global warming. The most mature technology is CO2 capture into monoethanolamine (MEA). However, the process is still not economically viable due to high energy requirement of desorption. An aqueous 30 wt.% MEA is used as a benchmark in this field. Higher MEA concentration are believed to hinder mass transfer rates due to the high viscosity of MEA. However, this belief lacks experimental evidence. In this thesis, CO2 capture into pure MEA was studied in a spray column. Highest overall mass transfer coefficients (KGɑ) was obtained as 11.7 kmol.m-³∙kPa-1∙h-1. This value was around 10 times higher than most other literature values. Although the solubility of gases in liquids decreases with increasing viscosity, having more MEA molecules at the surface of the droplets seem to be the main reason of the very high KGɑ values. Since less liquid needs to be pumped and heated up in desorption column, high MEA concentrations have the potential to decrease the CO2 capture costs significantly.

Another challenge that the world faced during the course of this thesis was COVID-19 pandemic. To assess the potential risks of disease transmission in dentistry, the atomization mechanisms and dynamics of aerosols generated by most commonly used dental instruments were studied The findings of this work can pave the way for classification of dental instruments based on aerosol generation. This work gives insights for better designs of dental instruments and safer dental practices. 

In summary, understanding and solving mass and photon transfer phenomena in aerosols are shown to improve several processes. Valorization of these applications is discussed further in the thesis.