When Higher RPM No Longer Improves Performance: The Onset of Efficiency Plateau and Decline in Rotating Packed Bed (RPB) Reactors

Abstract Rotating packed beds (RPBs) intensify CO2 absorption via high centrifugal fields, yet the assumption that increasing rotational speed continuously improves performance has been challenged by recent experimental observations. This study investigates the geometry-dependent emergence of performance plateaus and efficiency drop behavior in an RPB using a 30 wt % monoethanolamine (MEA) solution. Two fundamentally different packings were compared: a random Raschig ring packing and a structured rod-type bed. Under high rpm operation, the Raschig ring packing exhibited a monotonic efficiency increase from 44.5% at 600 rpm to 73.9% at 1200 rpm. In marked contrast, the rod bed reached a maximum of 65.1% at 1000 rpm, followed by a decline to 64.7% at 1200 rpm. Flow visualization revealed that this anomalous behavior originates from liquid channeling, localized jet-like ejection, and nonuniform wetting patterns that develop in the structured rod bed at elevated rotational speeds. A critical evaluation of widely used semiempirical correlations for effective interfacial area, liquid holdup, and mass transfer coefficients demonstrated that all predict monotonic improvement with rotational speed, exposing a fundamental blind spot: their inherent inability to capture interfacial hydrodynamic collapse and regime transitions. These findings establish that packing geometry imposes critical operational limits on RPB intensification and underscore the need for next-generation models that explicitly incorporate discontinuous flow physics.