MODELING OF SAND GRAIN DISSOLUTION IN INDUSTRIAL GLASS MELTING TANKS

A combination of two models, describing dissolution of sand grains in batch blankets or in the molten glass, is presented: a microscale and a macroscale model. The macroscale model is based on a 3-dimensional calculation procedure to determine the temperature distributions and the flows in industrial glass melting tanks. By means of microscale models, using mass transfer relations for diffusional transport, the dissolution rate of single sand grains can be calculated. The dissolution of the sand is determined by following a large number of single grains during their trajectories through the batch blanket and the molten glass in the glass melting tanks. The dissolution rate of a sand grain is calculated for the temperatures and flow conditions in every volume element in the tank through which the grain proceeds. The dissolution rate in the batch blanket depends strongly on temperature and the stage of the dissolution process. Initially the very fast shrinkage rate of the grains as temperatures exceed 1200-degrees-C results within 10 min in the dissolution of more than 50 % of the sand in the blanket. Forced and free convection in the glass melt leads to increases in the dissolution rate, up to a factor 5 compared to motion-free conditions. Forced bubbling for instance results locally in extremely high mass transfer rates and often improves the melting performance of industrial glass furnaces.