(187c) Heterogeneous Model for the Adsorption and Reaction on the Surface of Highly-Agglomerated Nanoparticles in a Fluidized Bed

The industrial production of fumed Oxides (SiO2, Al2O3, TiO2) from Chlorine containing precursors requires quantitative removal of the byproduct HCl bound to the surface of the particles. Besides this so called deacidification, some fumed Oxides are also post treated with hydrophobizing agents and industrially, both process steps can be carried out in fluidized beds. The equipment is typically sized based on experience rather than on a physico-chemical basis.

Fumed Oxides have in common, that they are nano-sized in their primary particles, which are highly agglomerated to sizes of several hundred micrometers. The overall porosity of these agglomerates can exceed 98%, and a fixed bed of such a material has bulk densities of 20 to 100 g/l only. In order to understand the fluidization process and to design the equipment, the fluidization behaviour of such agglomerates has frequently been studied and reported in literature. However no reactor model exists, which can describe adsorption and chemical reaction of such low-bulk density particulate systems in fluidized beds. The presentation of such a model and the comparison with experimental data is the scope of this work.

In the reactor-model of the batch fluidized bed three phases are considered: The first phase is the fluidized agglomerates, which are assumed to be spheres with constant diameter and porosity. These agglomerates are suspended in the suspension phase. Solid free spherical bubbles of constant size make the third phase. Adsorption and / or reaction of HCl take place on the surface of the primary particles within the agglomerate. Adsorption equilibrium is calculated using the linear Henry law, which was found applicable from experimental measurements. Chemical reaction of surface-bound HCl with steam is described by a formal kinetic term. Linear kinetic terms were used to describe the mass-transfer in boundary layers between the phases. There is no diffusion limitation within the agglomerate. The presented model allows the calculation of the overall mass transfer from the suspended agglomerates to the fluidizing gas as a function of time with and without chemical reaction. It can be applied to both the homogeneous fluidized bed and the bubbling heterogeneous fluidized bed.

The presented model was applied to experimental data of the deacidification of the fumed silica AEROSIL® 200 from Degussa. Values for most model parameters (e.g. agglomerate size and morphology and equilibrium data) were found through own independent experiments or from literature. A simple formal kinetic expression for the chemical reaction in the fluidizing gas at various temperatures could be derived from the comparison with experimental data. This model can be used for the sizing or optimizing of process equipment in industry. With modification of the kinetic expression (through experiments), other post treatment steps could be calculated with this model as well.