(357d) Flow Properties Characterization for the Analysis of Catalyst Support Impregnation Process Performance
AIChE Annual Meeting
Tuesday, October 18, 2011 - 4:15pm to 4:35pm
Powders and granular materials can be found at any stage of processing in catalyst manufacturing; they exhibit a variety of flow patterns, and their state and behavior differs from application to application. Since there is a lack of fundamental understanding of powder behavior, multiple problems can be encountered during production, such as jamming of the hoppers, sub-standard blending performance, and weight variability of final products due to segregation and/or agglomeration. Scale-up could also be a challenge, since the lack of constitutive equations for granular materials forces most scale-up efforts to follow the trial-and-error route. The field of powder characterization is employed as both, a distinguishing method for choosing the best-fit material and a predictive tool to analyze the process performance, and therefore, plays a very important role in process and product development. There are numerous methods to characterize the flow properties of granular materials, such as avalanching testers, fluidizers, shear cells, indicizers, density methods, angle of repose, etc.; however, most of them are application-specific, and it is not clear, how they correlate with each other or with process performance. For this reason, the use of most of these testers is restricted to a specific application, for which they were designed, and any attempts to apply the results of such experiments to a different application frequently result in process failure. In this respect, relating the variables and parameters outputted by various testers to the fundamental properties of materials is of great interest for successful implementation of powder characterization as a predictive tool for process optimal performance.
The research efforts of this study concentrated on applying powder characterization methods to optimization of manufacturing processes; in particular, studying the effects of impregnation processing on the flow and shear properties of catalyst support materials. During a dry impregnation process a solution of catalyst metal is typically sprayed onto a dry catalyst support powder. In this case the DI water was used instead of metal suspension to emulate the impregnation process onto a set of blends containing coarse alumina boehmite (~60μm) and Benecel as a binder (HPMC, ~70μm). Shear cell, compressibility tester, and moisture content measurements were used to characterize the axial uniformity of the impregnated blends within the double-cone impregnation setup; SEM imaging and particle size distribution measurements were used to characterize the changes in the microstructure and connect these changes with the bulk properties measurements. Bulk flow properties were evaluated with the GDR (an avalanching tester) and dilation measurements. Methods for improving impregnation uniformity were evaluated for efficiency.
The study has found a procedure regularly used in industry for impregnation produced mixtures that were very non-uniform; the use of characterization methods have correctly identified the flaw in the blender design that prevented a better water distribution in the blend (a “dead zone” within the blender). With respect to blend uniformity within the impregnator it was also observed that the addition of an intermediate amount of water (50% of the particle pore volume) resulted in greater flow properties variability within the blender when compared to the blends with significant amount of water (95% of particle pore volume). This variability was also more pronounced at higher concentrations of HPMC. This finding suggests that the transfer of water in the impregnator bulk does not occur uniformly, but rather is dependent upon the pore saturation of the particles exposed to the water stream. A particle is more likely to transfer water to the neighboring particles when its pores are mostly saturated with water and no significant absorption can occur at this point.
Two different approaches to enhance mixing performance were tested for efficiency: mixing for an extended period of time after the solution has been added – a method currently in use in industry, and applying shear with an intensifier bar at a very low level (~300rpm), while the solution is being added. The blend uniformity for each case was tested using the methods of the base case study. The testing showed that applying low levels of shear during impregnation results in a much more uniform distribution of solution within the blender, while extensive mixing fails to perform to the same extent for the mixtures containing binders (such as HPMC). These findings may also be helpful in designing wet granulation systems for pharmaceutical applications, as the process for water-binder solution addition is similar.