Fluid mechanics and mass transfer of reactive sprays

In a considerable number of industrial processes in bio-, food-, chemical-, pharmaceutical- and environmental engineering liquid feed materials are contacted with a process gas in form of a spray. Recently a growing interest in the analysis of spraying systems can be recognized even in non-combustion technologies. Tools for fluid mechanical design and especially mass transfer intensification of such processes and plant components need to be developed and improved. In gas cleaning technologies typically chemical sorbents are added to the liquid phase to increase the absorption efficiency and to eliminate waste gas components specifically from a process gas stream. The proposed research program intends to tackle the challenging problem of spray absorption and desorption in gas scrubbers. In case of injecting a reactive spray into a hot gas atmosphere (exemplarily in a quenching chamber) several momentum-, heat- and mass transfer processes take place at the gas/liquid interphase. Simultaneously to the evaporation of water from the droplet surface, a diffusion of a soluble waste species from the surrounding gas into the droplet may occur. Furthermore the absorption may be accompanied by chemical reactions in the droplet phase itself. Under special circumstances the evaporation induced droplet cooling may produce an enhanced rate of absorption and therefore increase cleaning efficiency. A fundamental description of separation processes involving sprays will be based on a numerical solution of the detailed fluid mechanical model for the spray/gas two-phase flow. It must be coupled to a description of molecular exchange processes like interfacial mass transfer and chemical reactions. Contrary to the demands of simulation in spray combustion engineering the challenge in process engineering spray applications lays in resolving mass transfer and chemical reactions, which are located in the droplet phase itself and moreover may involve additional types of complexity, like aqueous phase chemistry. The numerical simulations will be accompanied by experimental studies of evaporation and chemisorption of falling single droplets and droplet chains. It is intended to validate the computational results with the help of a spray quenching chamber experiment, to simulate real flue gas quenching conditions.

Spray processes are in widespread industrial use in bio-, food-, chemical-, pharmaceutical- and environmental engineering. Recently a growing interest in the analysis of spraying systems can be recognized even in non- combustion technologies. In environmental engineering spray application for flue-gas cleaning focuses on optimal cleaning efficiencies at minimal operating costs. In gas cleaning technologies typically chemical sorbents are added to the liquid phase to increase the absorption efficiency and to eliminate toxic components from a process gas stream. Exemplarily the process of flue-gas desulphurization by a gypsum/limestone-slurry is investigated in detail. The research program deals with the challenging fluid-mechanical and chemical aspects of gas absorption in spray towers of caloric power plants as well as the rapid cooling of raw gases from industrial burners and ovens. During spray injection in hot raw gas streams a fluid-mechanical and thermodynamical non-equilibrium exists between the moving spray and the gas stream. Thereby, coupled transfer processes of heat, mass and momentum occur. Starting from considerations of plant construction and plant operation mathematical models are generated for the transfer processes on a single droplet level of description. These models capture the physico-chemical mechanisms of the waste component absorption from the raw gas into the washing agent, where the absorption mechanism for sulphur dioxide is investigated as a specific case of interest. Embedding of the mass transfer description into a computational fluid-dynamics software enables the process engineer to visualize the spray tower`s flow-field and furthermore, to gain inside into the distribution of waste component concentration and the temperature field in the plant. The numerical simulations are accompanied by experimental studies of chemisorption in lab-scale and in pilot- scale. A model gas-scrubber was constructed in technicum-scale to analyze chemical effects in the aqueous ionic solution of the absorbing liquid on the desulphurization efficiency under various raw gas conditions. Keywords: reactive sprays, chemisorption, droplet evaporation, spray/gas-two-phase flow, rapid quenching of flue gases, flue- gas desulphurization, spray tower.