(125c) Fully-Resolved CFD-DEM Simulation for Prediction of Energy Consumption and Erosion in a Ball Mill Used in Food Industry

In the chocolate industry conventional processes like the rolling process are still used to break and open the cocoa beans, despite being relatively time- and energy-consuming.

To overcome these deficiencies research and development departments in leading process unit-producing companies like Bühler AG concentrate on the introduction of alternative processes. A big potential lies in the use of ball mills. Clear advantages in the energy consumption and the resulting product quality can be seen with ball mills. Unlike in the rolling process, raw material with a higher cocoa proportion can be processed due to the reduced smearing within the ball mills.

Despite the advantages, the possible metal debris of the unit’s components and metal beads could have a negative influence to the product quality.

Aim

The aim of this contribution is, firstly, to get an estimation of  the energy consumption of an existing ball mill unit, which was investigated by Buehler AG via momentum measurements. In addition the correct prediction of potential metal debris using simulation within the unit and beads is very important to evaluate the process unit regarding the product quality. Here the results of the simulation are visually compared to images of damaged areas,  which are shown the erosion in a working unit.

Methods

Using numerical methods the whole process will be simulated using the commercial Computational Fluid Dynamics (CFD) Code STAR-CCM+ using the Discrete Element Method (DEM). The fluid flow of the chocolate, the movement of the beads and the rotation of mixing component are included into simulation.

To predict the stress between beads and potential erosion on the unit the particle-particle and particle-wall interaction needs to be analyzed. The particle movement is coupled to the fluid flow which is calculated by Finite-Volume-Method (FVM). The transient movement of the rotating mixer, which affects the particle movement and vice versa the fluid flow, is modelled via the Rigid Body Motion model, which represents a sliding mesh approach.

Results

The momentum acting on the mixer was measured experimentally and was evaluated in numerical investigation. It could be shown, that the CFD/DEM-calculation results fit well with experimental measurements.

The analyses of particle-particle and particle-wall interaction during the transient simulations were used to predict potential particle damage and erosion inside the unit. In comparison to the results taken of the experimental investigation, the simulation provides a good agreement regarding the ratio between the damaged particles to the total particle amount and the localization of the erosion zones.