oa South African Journal of Chemical Engineering - Simulation of RTD in a Closed Comminution Circuit using the Selective Recycle Model



The experimental Residence Time Distribution (RTD) curves were obtained using sodium chloride as a tracer during the comminution of apatite ore in a ball mill. The generalized number of cycles distribution (NCD) and the inverse Laplace transform for a perfectly mixed reactor and a pure time delay unit were used in curve fitting of RTD with internal circulation and RTD with dead space. The iterated parameters of the NCD probability mass functions and the corresponding number of cycles as well as the time constants in the two units were used to simulate the experimental RTD curves. The values of the probability mass functions and the corresponding number of cycles obtained in the simulation were used to calculate the moments of the NCD. The moments of the NCD were then used to calculate the moments of the theoretical RTD and these were then compared with the moments of the experimental RTD curves. Any malfunctioning of the comminuting unit, such as dead space in the mill, can result in an uneconomical utilization of the equipment, loss of optimum output of material per given time and a high recycle ratio. It is therefore essential from time to time to check for any malfunctioning of the equipment using for example a tracer to determine the RTD of the material through the comminuting unit. Any deviation from the normal RTD curve means unsatisfactory performance of the unit.


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