Effect of powder spray drying on catalyst formulation in tablet form

Abstract: Heterogenous catalysts consist of a large surface area support material applied with the catalytic active phase, and the activity of a catalyst is often correlated to the surface area per volume. Support materials often need to be processed further to be optimal for specific processes. For example, powders can be suitable for minireactors or fluidized-bed reactors. However, in a fixed-bed reactor, powder increases the pressure drop, and in gas-phase reactions, it risks being blown out. Therefore, the support material needs to be processed further in order to maintain the structure or better suit the reactor. The final form is called the carrier and can for example be tablets, extrudates or encapsulated powder. The behaviour of powder is, however, more difficult to predict than fluids or gases. This is because powder behaviour is dependent on the degree of heterogeneity of the particles, the packing history of the powder, and the influence of the environment. To tablet a powder, the flowability of the powder needs to be sufficiently good for the powder to fall down the small holes in the tablet machine. Good flowability of a powder is correlated to spherical particles with sizes above 100μm. This report examines the effect of spray drying on a Y-zeolite powder to increase its flowability and thereby improve the tabletting process of the powder. The best result was also applied to a CeZr powder. The Y-zeolite and CeZr powder have an approximate particle size of 4μm and the objective of utilizing the spray dryer was to generate droplets containing several powder particles, which, upon drying, form larger particles that enhances the flowability of the powder. The flowability was evaluated by calculating the ratio of large particles produced from the spray dryer, along with measuring the angle of repose of the collected powders. The experiments were designed to examine the impact of various settings on the spray dryer and the preparatory steps preceding it. First, the influence of nozzle size was tested, followed by assessing the significance of adding binder to the slurry. Lastly a trial matrix was created, testing the impact of dry content and the feed rate of the slurry. The results were, in most cases, that the flowability of the powder decreased after spray drying. However, there were two exceptions where the flowability of the Y-zeolite powder actually improved, namely for the samples with highest dry-matter content in the slurry (54wt%). As a result, the combination of a dry weight of 5wt% and a feed rate of 1200 ml/h was determined to yield the best results, and these conditions were used for testing the CeZr powder. However, the flowability of the CeZr powder decreased when subjected to spray drying with these settings. Therefore, it was concluded that every powder requires its own investigation to determine the most suitable settings to increase flowability. The influence of the spray dryer settings was assessed using graphs in Excel and statistical analysis software. It was observed that a larger nozzle, and exclusion of binder in the slurry, led to a greater extent of large particles. The statistical software revealed significant impact of dry weight in the slurry for the mass ratio of large particles and flowability of the powder. Specifically, a higher dry-matter content yielded more favourable outcomes. On the other hand, the feed rate showed, in almost all cases, no statistical impact on the results. The spray dried powder was then tableted, and the tabletting performance was compared to that of the original powder. The large particles obtained from the best results of the spray-dried Y-zeolite, which showed enhanced flowability in comparison to the original powder, also improved the tabletting process and more tablets were obtained than from the original Y-zeolite powder. The large particles from the spray-dried CeZr powder, however, showed decreased performance in flowability, and the tabletting process also worked better for the original CeZr powder than the spray dried. It is evident that improved flowability plays a crucial role in enhancing the tabletting process. For future experiments, the impact of the inlet air stream and inlet air temperature would be of interest to examine. Furthermore, better methods to measure flowability should be considered. The method of measuring the angle of repose has many drawbacks where one of them is that it is very sensitive to the operator and another one is low precision.