(47d) Simulation of Aerosol Synthesis by Simultaneous Coagulation, Sintering and Growth: Structure and Product Property Formation

In general, aerosol synthesis comprises different fundamental mechanisms, as there are nucleation, coagulation, sintering, surface reaction and condensation. Depending on the operational parameters (species, temperature, concentration, gas composition, residence time etc.) several of these formation mechanisms may be relevant simultaneously. In many cases, highly structured aggregates evolve. Relevant particle properties and consequently product value are strongly depending on this aggregate structure. However, this structure is not sufficiently characterized by aggregate volume and surface or primary particle diameter, respectively as typically applied in modern simulation techniques using population balance models. Moreover, further structure parameters, as for example fractal dimension are necessary for a correct description of the relevant properties. Aim of the present work is to simulate the complete aggregate morphology depending on simultaneous coagulation, sintering and surface growth. Furthermore, relevant aggregate properties shall be derived. The approach is based on the stochastic simulation of diffusion controlled cluster-cluster aggregation (Schmid et al. 2005) allowing for simultaneous sintering and growth due to condensation and / or surface reaction. The full geometries of evolving aggregate structures are obtained which are then evaluated for their respective properties: ? Fractal dimension including the pre-factors. ? Free surface area of aggregate as compared to classical sintering theories based on pair-wise particle sintering (important e.g. for reactivity or heat and mass transfer). ? Collision diameter (important to get estimate of the coagulation kernel used in population balances). ? Pair correlation function (detailed measure for the aggregate structure). ? Scattering function expected by static light scattering (very important for comparison of simulation results with experiments since this is the best way to experimentally obtain structure information). Simulations are carried out for a variety of operating conditions and the resulting aggregate structures are depicted and analyzed as described above. Ranges of operating conditions are identified where structure formation is dominated by one, two or all three mechanisms, respectively. Furthermore, a relationship between operation conditions and aggregate structure as well as a relationships between aggregate structure and product properties are obtained. Finally it is discussed how these results can be used for tailoring aggregates with desired properties.

Schmid, H.-J., Al-Zaitone, B.., Artelt, C., Peukert, W. (2005). Evolution of the Fractal Dimension for Simultaneous Coagulation and Sintering. Chem. Eng. Sci., 61, 1, 293-305.