(509a) A Piecewise Parametric Model for Open Loop Screw Feeder Flow Rates

Authors: 
Johnson, B., Carnegie Mellon University
Sahinidis, N., Carnegie Mellon University
Sen, M., Eli Lilly and Company
Hanson, J., Eli Lilly and Company
Slade, D., Process Systems Enterprise Limited
Screw feeders, a type of loss-in-weight feeder, serve as the crucial first unit in a pharmaceutical drug product continuous manufacturing (CM) process. They are responsible for feeding the API and excipient powders in a controlled manner. Therefore, there is industrial interest in an accurate and computationally efficient flow model of a screw feeder for use in process development [1]. Some proposed models have used statistical analysis [2], response surface methodologies [3], and semi-empirical differential equations [4]; however, these methods only model the mass flow under steady state or closed loop control, obfuscating the underlying dynamics of powder flow.

This work proposes a novel parametric piecewise-linear model to approximate the mass flow rate of powder discharged from a feeder under open loop operation. The piecewise model represents two flow regimes: a constant flow observed at higher powder masses in the hopper and a linearly declining flow observed at lower powder masses. First, a custom process model was developed in gPROMS™ (Process Systems Enterprise, London, UK). Then, open loop experiments were performed using six different powders, two different Coperion K-Tron screw feeders, and multiple different motor speeds. Using the feeder’s operational data, parameter values for each run configuration were estimated. It was observed that for powders that mirrored our models assumptions, the model was accurate over the entirety of the run. For powders that did not display strictly constant mass flow in the first regime, the model would instead capture the average mass flow rate. Overall, this model is easily fit, computationally inexpensive, and reliably describes the general open loop flow behavior of a powder. It is suitable for use in the early stages of CM process development.

References

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