Ind. Eng. Chem. Res. 2009, 48, 1905–1913
1905
Fluidized-Bed Coating with Sodium Sulfate and PVA-TiO2, 2. Influence of Coating Solution Viscosity, Stickiness, pH, and Droplet Diameter on Agglomeration Peter Dybdahl Hede,†,‡ Poul Bach,‡ and Anker D. Jensen*,† CHEC Research Center, Department of Chemical and Biochemical Engineering, Technical UniVersity of Denmark, Building 229, DK-2800 Kgs. Lyngby, Denmark, and Solid Products DeVelopment, NoVozymes A/S, SmørmoseVej 11, DK-2880 BagsVærd, Denmark
In the first part of this study [Hede, P. D.; Bach, P.; Jensen, A. D. Ind. Eng. Chem. Res. 2009, 49, 1914], agglomeration regime maps were developed for two types of coatings: sodium sulfate and PVA-TiO2. It was observed here how the agglomeration tendency is always lower for the salt coating process than for the polymer coating process, under similar process conditions. This is investigated further in this second part and concluded to be due to differences in coating solution stickiness, rather than differences in bulk viscosities. Furthermore, results show that it is possible to optimize the PVA-TiO2 coating formulation and process to achieve a low tendency of agglomeration, similar to that of the salt coating process. The best results for the PVA-TiO2 solution are obtained by substituting the PVA-TiO2 in equal amounts with Neodol 23-6.5 and further reducing the pH value in the coating solution to pH 4. 1. Introduction Fluid-bed coating is, in principle, a technique that is used to apply a shell layer onto individual core particles during fluidization. In terms of this process, it is possible to encapsulate active ingredients such as pharmaceuticals and enzymes, and the desired product consists thereby of unagglomerated individual carrier particles, each coated homogeneously. Different types of coating materials may be used, depending on their abilities, in terms of, e.g., controlled release properties and mechanical and dust-reducing properties, as well as moist barrier properties.1 Agglomeration of core particles during the coating process is a severe industrial problem, and, unfortunately, different coating materials behave differently, in terms of agglomeration tendency, even when processed under similar conditions. It is still not clear what causes these differences. In the first part of this study,2 a thorough literature review provided as the background for the detailed study of two types of industry-relevant coatings: sodium sulfate and PVA-TiO2. The sodium sulfate coating solution is a fairly simple chemical system, whereas the PVA solution that contains dispersed TiO2 particles is a complex system, where colloidal phenomena and rheology must be taken into account in the coating process. It was observed how the agglomeration tendency is always less for the salt coating process than for the polymer coating process, under similar process conditions, but also that rough agglomeration regime maps may be built based on the coating solution spray rate and the drying force, which is a thermodynamic property that links the bed temperature with the humidity of the outlet air. However, for the PVA-TiO2 coating solution, it was not possible to obtain agglomeration percentages of
