A Multi-Point Feed Increases Macromixing Rate by an Order of Magnitude in Simulated Multi-Impeller Vessels

Tall vessels equipped with multiple impellers, such as many gas-liquid contactors in bioprocessing, often suffer from poor macromixing of feeds, which leads to heterogeneities and complicates the scale-up. Here, a diffusion equation was used as a simple model of macromixing in multi-impeller tanks, and the model fitted previously published experimental tracer curves. The model was then noted to predict that the macromixing time-scale is decreased to a quarter by relocating the feed point from the top to the middle. Furthermore, dividing the vessel axially in symmetric proportions and locating a feed point in the center of each compartment was found to reduce the time-scale even further. These theoretical results were put to test by compartment model simulations of 17 L to 22 m³ tanks with one to four impellers. Order-of-magnitude improvements were seen in the multi-impeller setups: up to 29-fold macromixing rates compared to a single-point top feed were realized with multiple feed points in a 22 m³ bioreactor stirred with four Rushton turbines. Considerable improvements were observed also in the other scales and geometries with fewer than four impellers, which suggests that the herein proposed division of feed points may greatly facilitate the scale-up of tall multi-impeller vessels.