Experiments and modeling on the deacidification of agglomerates of nanoparticles in a fluidized bed

Abstract

The deacidification of the fumed silica AEROSIL® 200 was studied experimentally in a batch fluidized bed in the temperature range from 250 °C to 400 °C. For a well fluidized bed, the temperature and the steam concentration in the fluidizing gas are the determining parameters for the overall rate of deacidification. If the bed is not well fluidized, e.g. because it is too shallow, or it is fluidized near the point of minimum fluidization velocity, the rate of deacidification drops because channeling and bypassing occur. The adsorption equilibrium of steam and HCl on AEROSIL® 200 was measured for a wide temperature range and the temperature dependency of the Henry coefficient for steam is given. A mathematical reactor model was developed for the adsorption and for the surface reaction on highly agglomerated nanoparticles in a fluidized bed. In applying this model to the experimental data for the deacidification, a simple kinetic rate expression could be derived for the deacidification reaction, which is otherwise not obtainable. The temperature dependency of the rate constant was also determined. All other parameters for the model can either be found through independent measurements (e.g. adsorption equilibrium or fluidizing characteristics) or in literature. The model can be used for sizing and optimizing of fluidized bed reactors in the production of fumed oxides.

Introduction

Fumed oxides (SiO₂, Al₂O₃, TiO₂) are industrially produced in flame reactors. In the case of fumed silica, a chlorosilane vapor (e.g. SiCl₄) is mixed with air and hydrogen and hydrolysis takes place in a flame at temperatures well above 1000 °C. A simplified process schematic is given in Fig. 1. Nano-sized primary particles are formed, which sinter together to aggregates of sub micron size. The morphology of these aggregates is determined by the reaction conditions in the flame reactor. Subsequently, these aggregates adhere to each other to form three dimensional agglomerates, which can be in the several hundred micron range in size. Fig. 2 shows SEM pictures of AEROSIL® 200 at two magnifications showing the aggregate and agglomerate structure of fumed silica. The overall porosity of these agglomerates can exceed 98% and a fixed bed of such material has bulk densities of approximately 20 g/l only. Due to the small primary particles, industrially produced fumed oxides have large specific surface areas of up to 400 m²/g. On their large specific surface there are functional chemical groups, in case of fumed silica these are silanol (Si–OH) and siloxane (Si–O–Si) groups. Some of these functional groups (e.g. the silanol groups) can readily interact with other chemicals or they can be modified in the production process itself.

This unique morphology paired with a wide array of possible surface chemistry make fumed oxides a necessary compound in many products. Fumed silica for example is used in the silicone industry to provide the desired rheology and mechanical strength in silicone adhesives and silicone rubbers. Fumed alumina is used for example to treat ink jet paper for improved ink absorbance and fumed titania is used in cosmetic applications such as sun protection. These same properties also require special considerations in the processing of these solids which often go beyond the conventional text book knowledge for solids processing.

In the industrial processes for fumed oxide manufacturing, fluidized beds are used to remove the byproduct HCl from the fumed oxides (so called deacidification, see Fig. 1) or for chemical modification of the functions surface groups (e.g. to make a hydrophilic fumed silica hydrophobic). Whereas research on gas/solids fluidized bed is extensive, the fluidization of fumed oxides has only been studied recently [1], [2], [3]. Some fumed oxides can be fluidized well (e.g. fumed silica with large specific surface area) and the fluidization is characterized by large bed expansion and no channeling. Whether bubbles are formed, depends on certain fluidization parameters, such as gas distributor type or fluidization velocity [4]. This type of fluidization of nanoparticles has been classified as agglomerate particulate fluidization (APF) [1]. On the other hand, some fumed oxides (e.g. fumed titania or fumed silica with low surface area) are difficult to fluidize and behave like Geldart B type solids with little bed expansion, channeling, and if fluidization can be initiated, it shows strong bubbling. This type of fluidization behavior has been named agglomerate bubbling fluidization or ABF. The research group of Prof. Pfeffer at NJIT (e.g. Yu et al. [5]) has investigated methods to assist fluidization of such particles.

Although fluidized beds have been used for decades in the production of fumed oxides, their design and operation are more based on experience, rather than on a fundamental understanding of the physico-chemical principles governing this process step. No reactor model has been reported so far, 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 for the deacidification of the fumed silica AEROSIL® 200 is the scope of this work.