Conventional fluidization of cohesive powders is challenging due to their strong interparticle forces, often requiring assistance methods. In this study, the hydrodynamics of pulsed and vibrated beds of cohesive Geldart C silica powder (Sauter mean diameter d₃₂=7.9μm) in a 19.2cm diameter column were investigated using X-ray imaging. The results show that low-frequency, moderate-amplitude gas pulsation improves fluidization by disrupting long, persistent gas channels. Higher-frequency pulsation is dampened throughout the bed, resulting in negligible improvement over unassisted systems. When coupled with mechanical vibration, gas pulsation slightly mitigates solid compaction at the bottom section, but the overall flow pattern remains largely unchanged compared to vibration alone. The findings highlight the potential of integrating gas pulsation with other assistance methods to enhance fluidization in practical applications.

Evaluation of the hydrodynamics of opaque multi-phase flows remains a challenging task, with implications for various industrial processes such as chemical processing, pharmaceutical, and mineral processing. Understanding how design and operational variables affect the complex behavior of multi-phase flow systems is essential for optimizing processing conditions and improving efficiency. Radioactive particle tracking (RPT) has been a proven measurement technique to evaluate hydrodynamics in multi-phase flow systems. However, a limitation of the classical RPT technique exists in the assumptions made in the simulation of the count rate received by the detectors in correcting for varying flow-induced fluctuations in the volume fraction of the dispersed phase, often encountered in industrial multi-phase flow systems. In this paper, we introduce a fundamentally novel experimental RPT method that directly uses detected incident photon hit locations for the reconstruction of the three-dimensional radioactive tracer particle position. We argue that this approach is inherently more robust as varying attenuation does not affect the reconstruction. The RPT setup consists of three identical γ-radiation slit collimator detectors that are placed equidistantly at 120° intervals. A subsequent calibration-experimentation procedure is established that allows reconstruction of the tracer particle position with spatial accuracy and precision in the order of 1 mm. We demonstrate the applications of this technique in evaluating hydrodynamics in multi-phase systems by characterizing the flow field of industrial-grade polypropylene reactor powder in a laboratory-scale horizontal stirred bed reactor.

A new X-ray computed tomography technique for the purpose of imaging fluidized beds is presented. It consists of an experimental set-up with three stationary X-ray source and flat panel detector pairs, a geometric calibration and data processing workflow, and an image reconstruction algorithm. The technique enables sparse-angular tomographic reconstruction in large 3D regions of fluidized beds at framerates up to 200 Hz, and therefore images bubbles along their whole trajectories through the volume. It allows for a unique analysis of bubble dynamics in fluidized beds, including bubble velocities, bubble transformations, i.e., time evolution of the bubble distributions in space, and bubble–bubble interactions. In this article, we first analyze the main limitation of the technique, the sparse angular resolution, through numerical simulations. We then test the experimental set-up through imaging a series of phantoms. Lastly, we demonstrate results from a Geldart B bubbling fluidized bed.

Vibro-assisted fluidization of cohesive micro-silica has been studied by means of X-ray imaging, pressure drop measurements, and off-line determination of the agglomerate size. Pressure drop and bed height development could be explained by observable phenomena taking place in the bed; slugging, channeling, fluidization or densification. It was observed that channeling is the main cause of poor fluidization of the micro-silica, resulting in poor gas-solid contact and little internal mixing. Improvement in fluidization upon starting the mechanical vibration was achieved by disrupting the channels. Agglomerate sizes were found to not significantly change during experiments.

Horizontal stirred bed reactors are widely used in the commercial manufacturing of polypropylene. However, a comprehensive understanding of the particle dynamics in horizontal stirred bed reactors remains elusive, primarily due to the lack of detailed experimental data. In this work, we studied the influence of operating parameters on the particle flow dynamics in a laboratory-scale horizontal stirred bed reactor using single-photon emission radioactive particle tracking. The results show that the general solids flow behavior is strongly affected by both the agitator rotation speed and reactor fill level. Operation at low rotation speed and low fill level results in solids flow with poor radial and circumferential distribution due to internal bed circulation. On the contrary, at increased rotation speeds and fill levels, solids motion throughout the bed is continuous resulting in excellent solids distribution. The solids circulation was found to increase for both an increase in rotation speed and reactor fill level. The axial dispersion coefficient, on the other hand, shows a linear relation with the rotation speed, but no conclusive relation between the axial dispersion coefficient and the reactor fill level was found.

Stirring has been recognized in the literature as a promising technique for facilitating fluidization of cohesive powders, via inputting additional energy to counteract interparticle forces. However, the influence of operating conditions and stirrer configurations on flow behavior remains largely unknown, which impedes the practical implementation of stirred fluidization. Utilizing X-ray imaging, this research demonstrates that stirring enhances fluidization in cohesive micron-silica powder (Sauter mean diameter [Formula presented]) by collapsing the powder packing structure, and transitioning channeling beds into bubbling states. Comb-like configurations featuring fewer stirrers and blades, placed in the bottom region, have shown to be highly effective. Excessive stirring can lead to air pockets and a compacted phase of particles on the column walls, undermining the interaction between particles and stirrers. Additionally, the experiments show that maximizing the sweeping coverage, employing complex asymmetrical configurations, and avoiding tortuous gas pathways are preferable.

Mechanical vibration has been broadly used to assist fluidization of cohesive powders, because of its capability to disrupt gas channels and agglomerates. However, the improvement reported in literature is mostly deduced from bulk response and ex-situ measurements, whereas the induced fluidization behavior and underlying physics remain largely unexplored. In this work, the fluidization behavior of micron-sized cohesive silica (Sauter mean diameter D₃₂ = 7.9 μm) has been investigated experimentally under vibration of varying conditions. X-ray imaging was carried out to directly capture the temporal evolution of system hydrodynamics, and identify in-situ powder stratification, bubbling and channel formation. The study demonstrates that vibration effectively collapses gas channels, yet facilitates powder stratification and compaction, therefore developing three distinctive flow regions inside the bed with different fluidization states. Consequently, common measurements, such as pressure drop and bed expansion, tend to overestimate the improvement. In addition, increasing frequency, from 10 Hz to 30 Hz, is observed to increase the number of bubbles by 60 %, whereas a large amplitude (e.g., 2 mm) leads to a 10 % compaction in the top flow region.

In many industrial applications, particles used in fluidized bed clearly deviate from ideal spheres. This leads to an increasing need for better understanding and developing better simulation models for fluidization of non-spherical particles. So far, the literature is quite scarce when it comes to experimental results which can be used for validation of numerical models. Also, the exact difference in fluidization behavior between spherical and elongated particles in dense fluidizing conditions is not well understood. In this work, we apply X-ray tomography to compare the fluidization behavior of a bed of a Geldart D-type spherical particles of aspect ratio 4 to that of volume equivalent spherocylindrical particles for different gas velocities. Even though the beds of both spherical and elongated particles are operating in the slugging regime, due their size and high bed height to width ratio, we see clear differences in their fluidization behavior. Our results indicate that the bed of elongated particles is slugging less than the one with spherical particles. This is indicated by a lower average bubble size in the case of elongated particles, together with a higher bubble rise velocity. The bed of elongated particles has a considerably higher distribution of small and medium bubbles. The slug waiting time distribution and slug frequency distribution indicate that a bed of elongated particles periodically switches between slugging and turbulent fluidization, unlike the bed of spherical particles which remains in the constant slugging regime.

Due to the presence of inter-particle cohesive force, cohesive particles reveal totally different fluidization behaviors as compared to the non-cohesive system. This paper studies the fluidization dynamics of Geldart B particles with varying thermal-induced cohesive forces. Multi-source X-ray tomography was applied to reconstruct 3D temporal images of bubbles, based on which, various bubble properties were extracted. The results show that increasing cohesive force will decrease bubble number while increase bubble size, implying that the presence of cohesive force facilitates bubble coalescence. By examining the bubble size distribution, cohesive force is found to have no effect on the number of median bubbles but greatly influence small and large bubbles. When the cohesive force is strong, the bubbles grow to a considerable size similar with bed dimension, giving rise to slugging near bed surface. With the action of inter-particle cohesive force, particle slug gradually grows by capturing other freely fluidizing particles, finally inducing “whole-bed” slugging. The particle slug may rupture in the rising process, and the bed turns back to normal fluidization. In comparison to normal bubbles, the gas slug has much larger size but far smaller frequency. The rise velocity of gas slug is also very low due to the particle-wall friction and gas-solid momentum dissipation. Therefore, the averaged values of bubble properties dramatically changed as bed temperature exceeds 35 °C. When the temperature attains 45 °C, the cohesive force is so strong that the fluidization completely fails in terms of stable whole-bed slugging.

Cavitation is a complicated multiphase phenomenon, where the production of vapor cavities leads to an opaque flow. Exploring the internal structures of the cavitating flows is one of the most significant challenges in this field of study. While it is not possible to visualize the interior of the cavity with visible light, we use X-ray computed tomography to obtain the time-averaged void fraction distribution in an axisymmetric converging-diverging nozzle (’venturi’). This technique is based on the amount of energy absorbed by the material, which in turn depends on its density and thickness. Using this technique, two different partial cavitation mechanisms are examined: the re-entrant jet mechanism and the bubbly shock mechanism. 3D reconstruction of the X-ray images is used (i) to differentiate between vapor and liquid phase, (ii) to obtain radial geometric features of the flow, and (iii) to quantify the local void fraction. The void fraction downstream of the venturi in the bubbly shock mechanism is found to be more than twice compared to the re-entrant jet mechanism. The results show the presence of intense cavitation at the walls of the venturi. Moreover, the vapor phase mixes with the liquid phase downstream of the venturi, resulting in cloud-like cavitation.

In this work, the effect of an electrolyte (up to 2 M of NaCl dissolved in water) on a homogeneous dense bubbly flow, in an airlift bubble column, is studied using nonintrusive techniques. X-ray and high-speed imaging are used to investigate the bubble size distribution, the local and the global gas-fraction profiles. The major effect of the electrolyte is the bubble size distribution at the fine-pore sparger, which is a consequence of the bubble coalescence inhibition promoted by the electrolyte. The bubble plume widening, the increase in overall gas fraction, and the onset of bubble recirculation in the column can all be explained by the bubble size reduction at the fine-pore spargers. As a result of the bubble size reduction, the overall role of the electrolyte is in a reduction of the driving force for the liquid recirculation. Furthermore, an accumulation of the small bubbles causes a layer of foam at the free surface, which is dynamic in nature and induces additional bubble recirculation.

The performance of fluidized bed reactors strongly depends on the bubble behavior, for which reason knowledge concerning the bubble properties is important for modeling and reactor optimization. X-ray measurements can be used to characterize bubbles within the cross-section of a fluidized bed on a laboratory scale, but cannot easily be extended to hot, pressurized large scale plants. For future measurements at hot conditions in a fluidized bed methanation reactor, we have developed an optical probing system that can be employed under these conditions. However, optical sensors are only able to investigate the local fluidization patterns at a defined position in the bed. The objective of this study is to characterize differences in bubble properties between local optical measurements and an X-ray tomography method that is able to detect bubbles over the entire cross-section. Therefore, an artificial optical signal is created out of existing hydrodynamic X-ray measurement data obtained at a cold flow model of a pilot scale methanation reactor. The determined bubble properties of both methods (i.e. evaluation of the derived artificial optical probe signal and image reconstruction based on the evaluation of original X-ray tomographic data) are compared with regard to the bubble rise velocity and the bubble size (for the X-ray method) or pierced chord length (for the optical evaluation method), respectively. The comparison shows that for the evaluation of the optical probe data, statistical effects have to be considered carefully. The detected mean chord length of the optical method does not immediately correspond to the mean bubble size determined by the X-ray method. Moreover, also differences regarding the bubble rise velocity were detected for some fluidization states. The reason for the discrepancies between both methods could be identified and corrected, amongst others by means of a Monte Carlo simulation in which rising bubbles in a fluidized bed were simulated and characterized by a local virtual optical sensor.