(788f) Two-Phase Flow (solids: interstitial air) Modeling of a Press Feed System

Production rate limitations can develop when feeding fine powders to a press, granulator or mill. These powder feed rate limitations arise when the powder interacts with entrained air, resulting in adverse â??two-phaseâ? (powder-air) flow effects. Two-phase flow problems can result in out-of-specification tablets, requiring operating at below target production rates and can occur in a continuous or batch process. A critical step in developing a robust process, including conducting Quality by Design (QbD) studies, is modeling two-phase flow behavior in the press feed system.

Two-phase flow risks can be accounted for during process design by:

• Measuring key flow properties to use as inputs in the two-phase flow model;
• Computational modeling the two-phase flow effects based upon first principle models;
• Evaluating potential corrective actions through two-phase modeling.

The two-phase modeling software uses key powder flow properties, including:

• Permeability
• Compressibility
• Specific gravity of a particle
• Wall friction angles (phi-prime, fâ??)
• Effective angle of internal friction (delta, d) measured via a cohesive strength test.

The two-phase model as includes the press feed system geometry and process parameters as inputs. The model outputs include parameters that dictate two-phase flow behavior, such as vertical solids stress, interstitial gas pressure, bulk density and normalized gas pressure gradient. These modeling outputs are used to predict whether flow problems will occur and analyze potential corrective actions, including modifying the geometry of the press feed system, increasing the permeability of the powder and adding venting.