Distinct element analysis of inter-particle coating variability in a batch seed coater

Coating of particulate solids by a thin film layer is of interest in many industrial applications such as seed and tablet coating. In seed processing, seeds are commonly coated with a protective coating layer consisting of fertilisers and crop protection products. Rotary drum batch coaters are typically used for this purpose. The coater consists of a cylindrical vessel with a rotating spray disk in the centre, onto which the coating liquid is fed. The seeds are driven around the vessel by its rotating base, and are mixed by two baffles; one on either side of the vessel. In the present study, DEM simulations are used to analyse the seed coating process. Corn seed is used as a model material and its shape is captured using X-Ray micro-tomography (XRT). The shape is incorporated into the simulations by the clumping multiple spheres to form a particle assembly. The coating uniformity of the seeds is predicted by implementing a coating model, whereby the coating mechanism is represented in the DEM by considering that once a droplet contacts a corn seed, it is removed from the simulation and its mass is attributed to the coating mass of the corn seed. The distribution of mass of sprayed spheres on the corn seeds and the coefficient of variation are evaluated for a range of process conditions, such as spinning disk rotational speed, droplets size and baffle arrangement and designs. In addition to evaluation of coating uniformity of particles, the scale-up rules for rotary drum batch coaters are investigated and reported. The outcome provides guidelines on scale-up rules and improvement of coating uniformity for rotary batch seed coaters

Effect of particle shape on flow in discrete element method simulation of a rotary batch seed coater

In the seed processing industry, rotary batch seed coaters are widely used for providing a protective coating layer (consisting of various ingredients including fertilisers and crop protection chemicals) on the seeds. Seed motion and mixing are important in ensuring uniform coating. In the rotary batch seed coater, the base of a cylindrical vessel rotates, whilst the cylindrical wall is stationary and two baffles turn the bed over for mixing. In the present study, the Discrete Element Method (DEM) is used to simulate the effect of particle shape on motion and mixing in this device. Corn seed is used as a model material and the effect of its shape on motion is analysed by considering two approaches: (1) manipulation of rolling friction to account for shape as it is commonly used in the field; (2) approximation of the actual shape by a number of overlapping spheres of various sizes. The geometry of corn seeds is captured using X-ray microtomography and then the ASG2013 software (Cogency, South Africa) is used to generate and optimise the arrangement of the overlapping spheres. A comparison is made of the predicted tangential and radial velocity distributions of the particles from DEM and those measured experimentally. It is concluded that for rapid shearing systems with short collisional contacts a small number of clumped spheres suffice to provide a reasonable agreement with experimental results. Equally well, manipulating the rolling friction coefficient can provide results that match experiments but its most suitable value is unknown a priori, hence the approach is empirical rather than predictive

Inter-Particle Coating Variability in a Rotary Batch Seed Coater

Coating of particulate solids by a thin film layer is of interest in many industrial applications such as seed and tablet coating. In seed processing, seeds are commonly coated with a protective coating layer consisting of fertilisers and disease control agents, such as pesticides and fungicides. Batch coaters are commonly used for this purpose. A typical coater consists of a vertical axis cylindrical vessel with a rotating base and a spray disc in the centre, onto which the coating liquid is fed to atomise and spray-coat the seeds. The seeds are driven around the vessel by its rotating base, and are mixed by two baffles; one on either side of the vessel. In the present study, Distinct Element Method (DEM) simulations are used to model the seed coating process. Corn seed are used as a model material and their shape is captured using X-Ray Tomography (XRT), which is approximated in the DEM by clumped spheres. The coating uniformity of the seeds is predicted by implementing a coating model in the DEM, whereby the coating droplets are simulated as very fine spheres projecting tangentially from a ring at the edge of the spinning disk. The size and velocity of droplets leaving the spray disk are measured using high speed video imaging and implemented into DEM simulations. The coating mechanism is represented in the DEM by considering that once a droplet contacts a corn seed, it is removed from the simulation and its mass is attributed to the coating of the corn seed. The distribution of mass of sprayed spheres on the corn seeds and their coefficient of variation are evaluated for a range of process conditions, such as the base rotational speed, atomiser disc position relative to the base and baffle arrangement and designs. It is found that the atomiser disc vertical position, baffle angle and clearance to the wall are most influential, whilst the base rotational speed and baffle width and curvature have only minimal effect