Modelling of Composition and Growth of Aerosol Droplets in Absorption Columns
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To meet future energy demand and at the same time reduce our carbon footprint are main challenges of the current era. The global CO2 concentration has been increasing since the beginning of the industrial revolution and is mainly caused by our consumption of fossil fuels to fulfill the steadily rising global energy demand. The high atmospheric CO2 content leads to increased global temperatures, commonly referred to as a global warming. Melting of ice caps, rising sea levels and severe weather conditions are outcomes of global warming and with the passage of time, these scenarios are getting worse and worse. In order to minimize the adverse effects of CO2 every option to capture and to store CO2 should be looked into. The world is now looking for energy efficiency improvements like switching from high carbon fossil fuels, such as coal, to low carbon fuels. In the meanwhile carbon capture and storage, CCS, from large point sources is a way to enable a continued use of fossil fuels until sufficient renewable energy becomes available. It is also a technology for atmospheric CO2 content reduction through so-called CO2 negative solutions. Incredible efforts have been made in research and development in order to improve carbon capture technologies in recent years. Absorption of CO2 in chemical solvents, as currently the most mature technology, comes with some challenges as being energy-demanding and having issues related to amine loss and associated health risks and environmental concerns. Extensive work is going on for successful implementation of carbon capture technologies by developing new low-energy penalty solvents, by reducing aerosols emissions, and through scale-up and environmental standards for the processes. Formation of aerosols can cause serious complications in industrial exhaust gas cleaning processes. Small mist droplets and fog formed can normally not be removed in conventional demisting equipment because their submicron size allows the particles or droplets to follow the gas flow. Undesired aerosol formation may lead to amine emissions many times larger than what would be encountered in a mist-free gas phase in a PCCC process. Recent experimental investigations indicate significant aerosol based amine emission from PCCC plants. It is thus of crucial importance to understand the formation and build-up of these aerosols in order to mitigate the problem. This work is dedicated to develop a rigorous model for prediction of aerosol growth and composition changes. In order to understand the changes taking place with a particle entering an absorber an implementation of the model is created in Matlab. The model predicts the development in droplet size, droplet internal variable profiles and explains how the composition of the droplets are changing with respect to position. In the last part of the work droplet size distributions are introduced and followed through the process. The Matlab model is based on a subclass method of weighted residuals for boundary value problems named, the orthogonal collocation method. The model comprises a set of mass transfer equations for transferring components which are coupled with necessary equilibrium, kinetics, heat and mass transfer models to describe the droplet internal profiles for all relevant constituents. Also included is heat transfer across the interface and inside the droplet. The model is developed and expanded in steps and is described in chapter 2. The model also explains gas phase absorber profiles and the effect aerosol droplets can have on gas component depletion. The model covers both the absorber and water wash sections. This thesis presents the developed simulation tool for the characterization of aerosols formed in CO2 absorption columns and results from a number of case studies.