Experimental investigation and modelling of coalescence and agglomeration for spray drying of suspensions

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The main objective of the research project is related to the modelling of droplet collisions under conditions relevant for spray drying processes, thus for suspension and solution droplets with different drying stage. Consequently, the collision of viscosity dominated droplets and the collision outcome as well as the collision of high viscous droplets with the resulting agglomeration is considered. As a result, the properties of the produced powders, e.g. particle size distribution and agglomerate structure shall be described. The developed models will be implemented in an inhouse Euler/Lagrange code in order to provide a tool for allowing the design and optimisation of stray drying processes.For the realisation of the project goals further measurements of high viscous droplet collisions will be carried out in the last project period. Here a viscosity range up to about 6 Pa s can be achieved with the new droplet generators developed during the last project period. At such high droplet viscosity a partial penetration is expected, yielding binary agglomerates. The penetration depths will be measured by high-speed cameras and summarised using the relevant dimensionless parameters. On the basis of extensive parameter studies (i.e. impact parameter, collision velocity and viscosity ratio) the dependencies will be elaborated and combined to appropriate modelling concepts. On this basis the penetration and structure model of Sommerfeld and Stübing (2012) will be reviewed and adapted according the new findings.The developed droplet collision models as well as a drying model will be implemented in the inhouse Euler/Lagrange code (FASTEST/Lag 3D). For validation, measurements at the modified laboratory spray dryer will be conducted, which is at the inlet equipped with two fan-type atomisers. The separation of the atomisers may be varied in order to determine the spray interaction region. In the near field of the spray nozzles a change of the droplet size due to droplet collisions yielding coalescence or separation, are recorded via optical measurements. This is possible since the fan sprays are rather dilute, so that optical methods, such as high-speed cameras, can be utilised to determine all relevant parameters. Using large nozzle separations spray interaction takes place in the far field where the droplets have dried to a large extent so that agglomeration will be observed. The agglomeration process will be again recorded by high-speed imaging. Moreover, powder probes will be sampled at the end of the test section for allowing a size and shape analysis. After the validation, a simulation tool will be available for predicting the powder properties of spray drying processes.