(77a) Coating on Single Particle Level

Stina Karlsson^a, Anders Rasmuson^b, Ingela Niklasson Björn^c andStaffan Folestad^d

^a stina. karlsson@chalmers.se, Department of Chemical Engineering Design, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden

^b rasmuson@chemeng.chalmers.se, Department of Chemical Engineering Design, Chalmers
University of
Technology, SE-412 96 Gothenburg, Sweden

^c ingela.niklasson-bjorn@astrazeneca.com AstraZeneca Centre of Excellence for Process Analytical Technology, R&DMölndal, SE-431 83 Mölndal, Sweden

^d staffan.folestad@astrazeneca.com AstraZeneca Centre of Excellence for Process Analytical Technology, R&DMölndal, SE-431 83 Mölndal, Sweden

Abstract

The process of coating in a fluidized bed involves a complex thermo- and fluid-mechanical system. Three different phases, gas, particles and coating liquid, mass and heat transferred between phases and particles circulating in the equipment give many coupled parameters influencing the coating. The process involves zones with both high and low particle concentrations as well as high and low velocities. Earlier studies show that the process parameters heavily influence the quality and the quantity of the coating deposited. This makes it difficult to use a fluidized bed as an experimental set-up with the intention of determining the underlying mechanisms that govern the coating process. The mechanisms influencing the coating on single particle level are adhesion of the droplets to the particle surface, film formation and drying.

To study coating on single particle level a new device for coating a levitated particle in a controllable environment is designed and tested. The particle is levitated to get comparable drying condition to fluidized bed.

The device consists of a coating chamber, which contains a capillary tube for levitating the particle, a micro-dispenser for producing discrete droplets of controlled size and velocity and a device for supplying gas with specified temperature and humidity. The coating chamber consists of two parts, a confined space where the particle is levitated and a droplet insertion cone where the coating solution is injected into the particle suspending gas flow, Figure 1. A capillary with a well-defined diameter connects the droplet insertion cone and the area where the particle is levitated. The device is equipped with a piezo-actuated flow-through micro-dispenser that has the ability to produce discrete droplets with high reproducibility in terms of droplet size and velocity. The gas required for the coating process is taken from a gas container where the water content is analysed and kept at a minimum. A liquid flow is then introduced into the gas flow at a well-defined flow rate, mixed and evaporated in a three-way mixing vault.

Figure 1 Schematic drawing of the experimental setup m-flow: liquid mass flow meter, MFC: Mass Flow Controller for the gas, CEM: Controlled Evaporator Mixer

The device is equipped with a high-speed video camera to study the particle surface structure and the particle motion under the coating process. The droplet motion is also studied with the camera. Temperatures and flow rates throughout the device are measured and logged. To evaluate the coating results scanning electron microscope, SEM was used. Glass beads with a diameter of 1200 µm were used as collector particles because of their inert and non-porous surface and their spherical shape. Figure 2 and 3 show examples of particles coated in the device.

Results show the influence of solvent, gas quality and coating procedure on the quality of the coating. Both increased moisture content and increased droplet frequency give a smoother coating. The solvent in the coating liquid and temperature also influenced the coating structure. The influence of coating viscosity, surface tension is investigated to explain how the coating changes with the solvent and temperature.

A model for the coating and wetting condition on a single particle level is developed. The wetting and drying of the single droplets are modeled. The position for the droplets on the particle is coupled to the particle rotation in the experimental set-up. The influence of temperature, moisture content, and droplet volume and frequency is investigated. The model gives the structure of the surface and the moisture content under the coating process. Modeling results is compared to the experiments in the single particle coating device and is used to understand how the mechanisms influence the coating.