Modelling spray coating using a combined CFD–DEM and spherical harmonic formulation

• Modelling of fluidised bed spray coating system using a coupled CFD–DEM approach.
• Modelling of coating distribution at an individual particle level using spherical harmonic technique.
• Examination of operating factors governing coating uniformity at micro- and macro-scales.

The ability to coat particles with a uniform film of a desired thickness is a key requirement in many industries such as pharmaceuticals, food processing and chemical manufacture. Controlling the distribution of the coating material during coating operations is essential for ensuring factors such as homogeneity in the coating thickness and prevention of agglomeration, which are common requirements in most types of coating applications. Despite the significant industrial and commercial importance of spray coating, few numerical models have been developed to model this process. Here, we detail the first computational model to incorporate particles, coating spray droplets and gas flow, dynamically interacting over an entire coating system. Particles and gas flow are modelled using a coupled DEM-CFD method, and spray droplets are modelled as individual Stokesian particles within the gas flow field. The coating model uses a newly developed method for mapping the coating coverage over each particle, based on a spherical harmonic formulation. This allows the coating to be evaluated at an intra-particle level on each individual particle. Distribution functions, such as coating quality and volume deposition, can be easily determined for the entire system at the inter-particle level. The method is applied to a test case of an fluidised bed spray coater, and the effects of varying geometry and system operating conditions are evaluated in terms of the coating coverage and quality. The method enables factors affecting coating distribution in such systems to be investigated and understood, and potential design optimisations to be evaluated.