A dimensionless analysis of residence time distributions for continuous powder mixing

For continuous manufacturing of pharmaceuticals, understanding the dynamics of how a material flows through the process is critical with respect to the development of a control strategy for product quality assurance. Such understanding of the process dynamics can be obtained by characterization of the residence time distribution (RTD). The RTD for a process is not fixed and can vary due to changes in operating conditions or physiochemical properties of the blend. As such the RTD needs to be evaluated over the range of operating condition that can impact process dynamics (e.g. throughput, impeller rotation rate etc.). In this paper, we demonstrate that the dimensionless RTD (normalized with respect to the mean residence time) is invariant with throughput and impeller rotation rates under certain conditions for the two continuous direct compression processes. We present a case study to illustrate the utility of this relationship for predicting the process dynamics at different operating conditions (i.e., throughputs) and evaluating the impact of variations in the process dynamics on the control strategy for a continuous direct compression process.