Mass Transfer around gas bubbles in reacting liquids
The project deals with the quantification of mass transfer around rising single bubbles and bubble swarms in reacting systems as in bubble columns. Experimental work is focussed on the clarification of the interactions between fluid dynamics, mass transfer and chemical reactions. The following parameters will be varied: chemical system, reaction rate (concentrations, temperature and pressure), bubble diameter and number, contact time, degree of turbulence in the liquid phase.To characterise mass transfer by measuring mass transfer coefficients two novel image analysis based methods are further developed and applied. Using high speed cameras the decrease of bubble sizes due to chemical reaction is followed over time. As a result the time resolved mass transfer averaged over the bubble surface can be calculated. These measurements will be automated based on an adapted image analysis. Due to refraction effects and irregularly shaped bubbles this already existing method has to be improved. Beside the integral mass flux locally resolved information about mass transfer will also be gathered using a novel colourimetric method. It is based on an oxidation reaction where a coloured product is generated. A highspeed camera follows the expansion of the dyestuff into the continuous phase. By grey scale analysis the local concentration of the dyestuff can be determined allowing an analysis of the local mass fluxes. This method has to be further improved within the project.Both measurement techniques will be applied for single bubbles and bubble swarms. In bubble swarms the impact of bubble induced turbulence on mass transfer can be studied. Only diluted swarms will be investigated initially.The main focus of the project is the impact of homogeneous chemical reactions on mass transfer. As reacting systems sodiumhydroxide solutions and CO2 bubbles are used representing systems with only one irreversible reaction. Additionally, oxidation reactions leading to coloured products are used where also parallel reactions occur. Here, the time scales of the reaction will be changed over a broad range by varying temperature and concentrations.