A review of pulsed flow fluidisation; the effects of intermittent gas flow on fluidised gas–solid bed behaviour

Effect of Inlet Flow Strategies on the Dynamics of Pulsed Fluidized Bed of Nanopowder

The use of fluidization assistance can greatly enhance the fluidization hydrodynamics of powders that exhibit poor fluidization behavior. Compared to other assistance techniques, pulsed flow assistance is a promising technique for improving conventional fluidization because of its energy efficiency and ease of process implementation. However, the inlet flow configuration of pulsed flow can significantly affect the bed hydrodynamics. In this study, the conventional single drainage (SD) flow strategy was modified to purge the primary flow during the non-flow period of the pulse to eliminate pressure buildup in the inlet flow line while providing a second drainage path to the residual gas. The bed dynamics for both cases, namely, single drainage (SD) and modified double drainage (MDD), were carefully monitored by recording the overall and local pressure drop transients in different bed regions at two widely different pulsation frequencies of 0.05 and 0.25 Hz. The MDD strategy led to substantially faster bed dynamics and greater frictional pressure drop in lower bed regions with significantly mitigated segregation behavior. The spectral analysis of the local and global pressure transient data in the frequency domain revealed a pronounced difference between the two flow strategies. The application of the MDD inlet flow strategy eliminated the disturbances from the pulsed fluidized bed irrespective of the pulsation frequency.

Pulsed Fluidization of Nanosilica: Rigorous Evaluation of the Efficacy of Pulsation Frequency

Assisted fluidization techniques can significantly improve the hydrodynamics of difficult- to-fluidize solids. Among these techniques, the pulsed flow strategy is highly promising owing to its cost-effectiveness and amenability to implementation for largescale processing. Using commercial-grade, highly porous nanosilica that shows strong agglomeration behavior, we implemented the pulsed flow with square-wave pulsation schemes of 0.05, 0.10, and 0.25 Hz frequencies, and compared their effectiveness in each case. Besides the conventional approach of assessing their efficacy using the pressure drop data, we have proposed a new approach in this work that consists of computing the power of the overall pressure drop transient signals. Using the theoretical value, i.e., the effective bed weight per unit area as a reference, the percentage increase in the power was 27 ± 4, 71 ± 5, and 128 ± 4, respectively, for 0.05, 0.10, and 0.25 Hz pulsation frequencies. In fact, the average pressure drop values were substantially higher when the partial bed collapse occurred between successive pulsations when compared with the case of low-frequency pulsations. The pulsation frequency also affected the evolution of local bed dynamics in various bed regions during the expansion and collapse of the bed. Moreover, the local and global pressure transients have shown interesting mutual correlations which were otherwise not evident from their individual transient profiles.

Hydrodynamics of Pulsed Fluidized Bed of Ultrafine Powder: Fully Collapsing Fluidized Bed

The processing of fine and ultrafine particles using a fluidized bed is challenging in view of their unpredictable hydrodynamic behavior due to interparticle forces. The use of assisted fluidization techniques in such cases can be effective in improving the bed hydrodynamics. This work investigates the dynamics of pulsed fluidized bed of ultrafine nanosilica subjected to square-wave flow pulsations. The pulse duration used in this study is sufficient to allow the complete collapse of the pulsed fluidized bed between two consecutive flow pulsations. The proposed pulsation strategy is carefully implemented using electronic mass flow controllers with the help of analog output signals from data acquisition system. Given that the different regions of the fluidized bed exhibit varying dynamics, which together contribute to overall bed dynamics, the bed transients in the upper, central, and lower regions of the fluidized bed are monitored using several sensitive pressure transducers located along the height of the bed. The effect of the flow pulsation on the hydrodynamics of the fluidized bed is rigorously characterized. A significant reduction in the minimum fluidization velocity was obtained and an increase in the bed homogeneity was observed due to flow pulsations. The frequency domain analysis of the signals clearly delineated the frequency of the various events occurring during the fluidization.

Deagglomeration of Ultrafine Hydrophilic Nanopowder Using Low-Frequency Pulsed Fluidization

Low-frequency flow pulsations were utilized to improve the hydrodynamics of the fluidized bed of hydrophilic ultrafine nanosilica powder with strong agglomeration behavior. A gradual fluidization of unassisted fluidized bed through stepwise velocity change was carried out over a wide range of velocities followed by a gradual defluidization process. Bed dynamics in different regions of the fluidized bed were carefully monitored using fast and sensitive pressure transducers. Next, 0.05-Hz square-wave flow pulsation was introduced, and the fluidization behavior of the pulsed fluidized bed was rigorously characterized to delineate its effect on the bed hydrodynamics by comparing it with one of the unassisted fluidized bed. Flow pulsations caused a substantial decrease in minimum fluidization velocity and effective agglomerate diameter. The frequencies and amplitudes of various events in different fluidized bed regions were determined by performing frequency domain analysis on real-time bed transient data. The pulsations and their effects promoted deagglomeration and improved homogeneity of the pulsed fluidized bed.

Nanopowder Fluidization Using the Combined Assisted Fluidization Techniques of Particle Mixing and Flow Pulsation

In the present study, we report the fluidization behavior of ultrafine nanopowder using the assisted fluidization technique of particle mixing, which was further superimposed with the pulsation of the inlet gas flow to the fluidized bed. The powder selected in the present study was hydrophilic nanosilica, which shows strong agglomeration behavior leading to poor fluidization hydrodynamics. For particle mixing, small proportions of inert particles of Geldart group A classification were used. The inlet gas flow to the fluidized bed was pulsed with a square wave of frequency 0.1 Hz with the help of a solenoid valve controlled using the data acquisition system (DAQ). In addition to the gas flow rate to the fluidized bed, pressure transients were carefully monitored using sensitive pressure transducers connected to the DAQ. Our results indicate a substantial reduction in the effective agglomerate size as a result of the simultaneous implementation of the assisted fluidization techniques of particle mixing and flow pulsation.

Enhancement of CO2 Removal Efficacy of Fluidized Bed Using Particle Mixing

The present study proposes a cost-effective assisted fluidization technique of particle mixing to improve the carbon capture effectiveness of a fluidized bed containing fine adsorbent powder. Using activated carbon as the adsorbent, we mixed external particle of Geldart group B classification in different fractions to examine the effectiveness of the proposed strategy of particle mixing. Four different particle-mixing cases were considered by varying the amount of added particle—0, 5, 10, and 30 wt %—on external particle-free basis. The inlet flow of the nitrogen was fixed, while two different flows of carbon dioxide were used. The adsorption experiment consisted of a three step procedure comprising purging using pure nitrogen, followed by adsorption with fixed inlet CO2 concentration, and finally the desorption step. Inlet flows of both nitrogen and CO2 were separately controlled using electronic mass flow controllers with the help of data acquisition system (DAQ). The CO2 breakthrough was carefully monitored using the CO2/O2 analyzer, whose analog output was recorded using the DAQ. Best results were obtained with 10% external particles. This is in conformity with the results of our previous study of bed hydrodynamics, which pointed to clear improvement in the fluidization behavior with particle mixing.

Improving Fluidization Hydrodynamics of Group C Particles by Mixing with Group B Particles

We have developed a new particle-mixing strategy for improving the fluidization hydrodynamics of Geldart group C powders by mixing with small proportions of group B particles. Two different group C particles with widely different physical properties, i.e., 1 μm calcium hydroxide powder and 27 μm porous activated carbon, were selected for investigation in the present work. A carefully sieved sample of inert sand was used as external group B particles for mixing. Fluidization experiments were carried out, and the quality of the fluidization was assessed using the fluidization index. For the monocomponent fluidization of fine calcium hydroxide powder, pressure drop was sometimes as much as 250% higher than the effective weight of the bed. The proposed strategy of particle mixing substantially improved its fluidization hydrodynamics. On the other hand, the development of channels and cracks during the monocomponent fluidization of the activated carbon led to gas bypassing, resulting in low pressure drop and poor contact of phases. Particle mixing was found to improve fluidization behavior, and the chi-squared test showed that the best results were obtained with 13 wt% particle mixing.

Hydrodynamics, numerical study and application of spouted bed

This article reviews the major research and development on spouted beds (SBs). Due to its unique structural and flow characteristics, the SB is a very successful system in most applications. Two-phase and sometimes three-phase interactions generate a large number of variables to be noted in each process. Up-to-date information on the fundamentals and applications of SBs has been briefly presented, based on the published works. Thousands of interesting studies on hydrodynamic characteristics, numerical simulations, and new applications of SBs are reported. In the first step, the present work presents a review of hydrodynamic characteristics (circulation of solids in SB, measurement techniques for particle tracking and empirical hydrodynamics, pressure drop, maximum spoutable height, minimum spouting velocity, and diameter of the spout). In the second step, main mathematical models and computational fluid dynamics (CFD) simulation of the SB to predict and analyze different processes are described. Some main mathematical modeling and the recent advances of two fluid methods and discrete element method approaches in CFD simulation of SBs are summarized. In the last step, some new applications of the SB are presented. As the result of this review, we can observe the importance of further development of hydrodynamics structure, working on modeling and related correlations and improve the applications of SBs.