Effect of the liquid layer on the impact behavior of particles

During a spray granulation process the moisture loading in fluidized beds has a great influence on the inter-particle collision properties and hence on the flow behaviour. To study the influence of the liquid layer as well as granule impact velocity on the impact behaviour free-fall experiments were performed. During these experiments the g-Al2O3 granules were dropped from a predefined height onto a liquid layer on the flat steel wall and the velocity-time curves were obtained using highspeed video recording. The height of the liquid layer was varied from 50 mm to 1 mm. Moreover, the tests were performed at different velocities and viscosities of liquid layer in the range of 1-300 mPa.s. Both distilled water and water solutions of hydroxypropyl methylcellulose with different concentrations (3, 6, 10 mass-%) were used. The obtained restitution coefficients were compared with the experiments performed without liquid film on the surface. For a granule impacted on a liquid film on the wall, the increase of liquid viscosity decreases the restitution coefficient and thickness of liquid layer at which the granule sticks. In the examined velocity range, with decreasing impact velocity the restitution coefficient greatly decreases. To explain the obtained effects the force and energy balances for a particle impacted on a liquid layer on the wall were derived. Both contributions to energy absorption (granule-liquid layer and granule-wall contacts) have been taken into consideration.