Mixing dynamics in bubbling fluidized beds

Abstract

Solids mixing affects thermal and concentration gradients in fluidized bed reactors and is, therefore, critical to their performance. Despite substantial effort over the past decades, understanding of solids mixing continues to be lacking because of technical limitations of diagnostics in large pilot and commercial‐scale reactors. This study is focused on investigating mixing dynamics and their dependence on operating conditions using computational fluid dynamics simulations. Toward this end, fine‐grid 3D simulations are conducted for the bubbling fluidization of three distinct Geldart B particles (1.15 mm LLDPE, 0.50 mm glass, and 0.29 mm alumina) at superficial gas velocities U/Umf = 2–4 in a pilot‐scale 50 cm diameter bed. The Two‐Fluid Model (TFM) is employed to describe the solids motion efficiently while bubbles are detected and tracked using MS3DATA. Detailed statistics of the flow‐field in and around bubbles are computed and used to describe bubble‐induced solids micromixing: solids upflow driven in the nose and wake regions while downflow along the bubble walls. Further, within these regions, the hydrodynamics are dependent only on particle and bubble characteristics, and relatively independent of the global operating conditions. Based on this finding, a predictive mechanistic, analytical model is developed which integrates bubble‐induced micromixing contributions over their size and spatial distributions to describe the gross solids circulation within the fluidized bed. Finally, it is shown that solids mixing is affected adversely in the presence of gas bypass, or throughflow, particularly in the fluidization of heavier particles. This is because of inefficient gas solids contacting as 30–50% of the superficial gas flow escapes with 2–3× shorter residence time through the bed. This is one of the first large‐scale studies where both the gas (bubble) and solids motion, and their interaction, are investigated in detail and the developed framework is useful for predicting solids mixing in large‐scale reactors as well as for analyzing mixing dynamics in complex reactive particulate systems. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4316–4328, 2017

Multistage Shelf Devices with Fluidized Bed for Heat-Mass Transfer Processes: Experimental Studies and Practical Implementation

“…The complex hydrodynamics of the fluidized bed by a gas flow is described by disordered mixing of solid particles and the formation of gas bubbles, that is confirmed in [25][26][27][28][29]. The effect on the fluid dynamics of a fluidized bed of various parameters was studied: gas injection time into the bed, gas flow velocity and bubbling regime [25], bubble dynamics during nonspherical particles fluidization [26], continuous injection of the central air jet into the bed [27], and features of the pseudo-two-dimensional two-zone gas-solid fluidized bed [28], from the features of solid particles and gas bubbles [29]. Researchers also paid attention to the study of the interphase heat transfer intensity between particles and the gas flow [30] and heat transfer to heat exchange surfaces located in the layer [31][32][33][34].…”

On the fast modeling of species transport in fluidized beds using recurrence computational fluid dynamics

“…Following the deep fluctuated pressure, the solid flux ɛ s u s , reveals a coherent relevance to the uniform flow periodicity (note the signal of w s in Figure b). Namely, it highly discloses the fingerprints of rising bubbles passage, with high effective solid mixing through the wake effect particles, emulsion drift particles, and the bubble eruptions particles . The gas flow, however, evolves as dense flow in rich solid areas, visible bubbles, and throughflow which bypasses through the bubbles .…”

“…Namely, it highly discloses the fingerprints of rising bubbles passage, with high effective solid mixing through the wake effect particles, emulsion drift particles, and the bubble eruptions particles. 37 The gas flow, however, evolves as dense flow in rich solid areas, visible bubbles, and throughflow which bypasses through the bubbles. 38 Hence ϵ g u g includes high fluctuated dynamics that absentees the recurrence between states and can be less appropriate for the periodicity disclosure.…”

Manufactured chemistry: Rethinking unit operation design in the age of additive manufacturing

“… A new reactor design methodology is envisioned which couples multiscale and multiphysics modeling (top), topology optimization design methods (lower right) and advanced manufacturing technologies (lower left). …”