Detailed calculation of the grinding and classifying processes in a vertical roller mill by coupled hybrid modeling
M. Mersmann, aixergee GmbH, A. Omer, aixprocess GmbH, Aachen, Germany
The vertical roller mill (VRM) combines the basic functions of material comminution and material classification within one machine housing. From the process engineering aspect this involves a grinding and classifying circuit in which the recirculation of material at several points within the housing is determined by the flow properties of the gas and of the material being ground. In particular, segregation processes and shifts in the particle size distributions can occur at different points in the mill and then affect the efficiency of the grinding and classification processes. An understanding of the inherent process conditions is absolutely essential for further optimization of the design and operation of vertical roller mills. However, the essential information about internal mass flows, their particle size distributions and the intake conditions before the rollers is not available because of the inaccessibility for measurements. Investigations on a laboratory scale can also only be scaled up to a limited extent so this information can only be provided by a simulation calculation. The complexity of the 3-dimensional multiphase flow in conjunction with the particle-dominated comminution and classification processes has meant that so far the calculations have either concentrated on pure flow processes or else the coupled processes taking place have been described by means of a one-dimensional function description through integral-descriptive equations. An integral and stationary calculation is often carried out in practice while ignoring the 3-dimensional multiphase flow. Calculations using CFD (Computational Fluid Dynamics) are based on a 3-dimensional resolution of the flow area in which the comminution and classifying processes are not included and, as a rule, it is not possible to consider the particle-driven processes because the particles are present in different loadings at different points within the mill. There are too many particles in the system for application of the DEM (discrete elements method). For the first time this article presents a method that makes it possible to manage the different requirements for a detailed, 3-dimensional, and substantially physically rigid calculation of the processes taking place in a vertical roller mill with their complex interaction. Different methods of calculation are linked with one another to ensure both a replication that is physically as exact as possible and also to be able to calculate the different coupled sub-processes.
Summary CEMENT INTERNATIONAL 04/2016 pp 52-59
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