Vertical tubes in bubbling fluidized beds: A magnetic resonance imaging study of particle and bubble behavior

Vertical tubes are commonly introduced into fluidized bed reactors, for example, to promote heat exchange. It has been shown previously that such vertical tubes affect the bed hydrodynamics, however, it is still unclear (a) whether vertical tubes cause gas channeling along the tubes and (b) how these internals affect the motion of the particulate phase within the bed. In this work, we used real-time magnetic resonance imaging (MRI) to study the influence of vertical tubes of varying lengths and diameters on the bed hydrodynamics and gas bubble behavior. Spatially and temporally resolved MRI measurements of the local particle concentration, particle velocity, as well as fluidization sensitive measurements were carried out. Local particle concentration measurements show that vertical tubes do reduce the average size of gas bubbles in the bed and lead to a more homogeneous radial distribution of the gas bubbles. In addition, we find that tubes cause axial gas channeling, which might decrease the heat transfer between the bed and the tubes. Fluidization sensitive MRI measurements show that long tubes starting from the distributor plate homogenize the fluidization. Vertical tubes reduce the mean particle velocity compared to a fluidized bed without internals. The findings presented in this work might help to design more efficient internal geometries and provide a dataset that can be used to validate numerical simulations.