Superficial oscillation as an identifier of phenomena governing dynamics of solid-gas fluidized systems

The evolution of the volume of a fluidized bed solid–gas over time is measured by the oscillation of its surface. It is assumed that the system is governed by the pressure gradient between the plenum and the surface; the latter is considered as the reference. The surface oscillation is measured with a laterally located CCD video camera such that it monitors the flattest portion of the surface and records its coordinates. Simultaneously obtain all the phenomena that regulate the hydrodynamics of the bed, the entering bubbles, coalesce, split, increase their volume adiabatically, and explode on the surface. The gas flow and the weight of fluidized material are studied. The time series of the surface oscillation is analysed in the frequency domain and the phase space. The frequencies obtained are approximately between 0.6 Hz and 1.8 Hz. The entropy values behaves linearly proportional to the total number of bubbles

Wide band energy analysis of fluidized bed pressure fluctuation signals using a frequency division method

A statistical method based on approximation of the cumulative energy distribution by Student's t-distribution is proposed for the unbiased frequency domain division. The proposed method fixes the number of samples needed to estimate the power spectrum and its corresponding cumulative energy distribution using the Kolmogorov&-Smirnov test. The reliability of the method to divide the frequency domain was shown for different fluidization velocities by changing the bed aspect ratio and using different pressure probes. Water-induced defluidization tests were conducted to illustrate the use of wide band energy as a monitoring tool. The Student's t-distribution results are compared with an analysis performed using the traditional visual inspection method. The energy of the power spectrum contained within the frequency regions obtained by the visual method is not able to detect changes in the bed aspect ratio or the start of the rotating distributor. No meaningful differences could be observed in the frequency regions using different quality pressure sensors because the approach using Student's t-distribution focuses on the sharp energy increase produced by the primary frequencies of the power spectrum. The sensitivity exhibited by the proposed frequency division approach for the range of fluidization conditions tested improves the use of the energy contained in these regions as a diagnostic tool in fluidized bed processes.The author would like to express appreciation for the financial support from Projects DPI2009-10518 (MICINN) and CARDENER-CM (S2009ENE-1660).Publicad

The effect of temperature on the distributor design in bubbling fluidized beds

In this paper the effect of the temperature on the distributor pressure drop ratio is studied. There is a lack of information in the literature concerning the effect of the temperature on the performance of the distributor plate, and its possible role on non-uniform gas distribution. The effect of temperature on the distributor pressure drop has been experimentally established for the first time for two different air distributor plates, multiorifice and tuyere. The distributor pressure drop curves were obtained at different temperatures by means of pressure measurements. The well-known orifice equation was used to predict the distributor pressure drop at different temperatures and a good agreement with the experimental data was found. A methodology to obtain the distributor open area as a function of the bed temperature for different bed aspect ratios was developed. It was found that when operating at higher temperatures the distributor pressure drop decreases for the same gas velocity due to the decrease in the gas density. The resulting decrease of the distributor to bed pressure drop ratio shows that gas distributor plates have to be designed at the operating temperature instead at the ambient temperature to avoid non-uniform gas distributions and to save cost in manufacturing and operation.The financial support from projects DPI2009-10518 (MICINN) and CARDENER-CM (S2009ENE-1660) is very much acknowledged

Stagnant regions estimation in fluidized beds from bed surface observations

A novel approach to estimate the size of stagnant regions in large-scale fluidized beds by means of experimental data obtained from images recorded on the bed surface was presented. For this purpose, the internal structure of an induced maldistributed pseudo-2D fluidized bed was first studied. Half of the total distributor area was covered to generate an induced stagnant region. The size and shape of this area was studied for several relative gas velocities and bed aspect ratios. The defluidized area was found to be almost independent of the bed aspect ratio, however, it was found to decrease for higher relative gas velocities. A solids recirculation zone was also found above the defluidized zone. The size of this zone increases with relative gas velocity, suggesting that it is strongly related to bubble motion. A correlation was developed to relate the visible bubble flow to the size of the defluidized zone. The results obtained in the 2D bed were extrapolated to a 3D cylindrical fluidized bed with a half-covered distributor plate to estimate the volume of the defluidized zone. The visible bubble flow in the 3D facility was estimated. Using the proposed correlation for the defluidized volume in the 2D bed was used to estimate the defluidized volume in the 3D bed. Finally, the calculated values of the defluidized volume were compared with the experimental values in the 3D facility, obtaining a maximum relative error of 20% in the estimations

Detecting regime transitions in gas-solid fluidized beds from low frequency accelerometry signals

Low frequency accelerometry signals have been applied for detecting regime transitions in a gas solid fluidized bed. Three solids have been fluidized to promote bubbling, churn and slugging regime. The Kolmogorov entropy and the power spectral density have been used to determine the regime transitions as well as to analyze the dynamical features characterizing the different regimes. Pressure and external acceleration measurements have been taken simultaneously. The accelerometry signal was sampled at 10 kHz; then, the envelope was extracted and resampled at 400 Hz. Pressure signal was sampled at 10 kHz and resampled at 400 Hz. Two problems were found during the work: the colored noise present in the envelope and the lack of low frequency information for one of the tested solids. FIR, wavelet and EMD filter strategies have been applied to remove the noise present in the envelope. It is concluded that the envelope of the accelerometry signal might be used to detect regime transition in the same way as the pressure fluctuation signals. Both Kolmogorov and spectral analysis exhibit common features to those obtained from pressure signal analysis, supporting the hypothesis of using low frequency accelerometry instead of conventional pressure measurements for monitoring fluidized bedsThe authors would like to especially thank Prof. María C. Palancar for her contribution to this work and the useful guiding during those years. Moreover, the financial support from the Spanish Ministry of Research, project CTQ2006 15525 C02 01 is kindly acknowledgedPublicad

A novel approach for modeling bubbling gas–solid fluidized beds

A phenomenological discrete bubble model is proposed to help in the design and dynamic diagnosis of bubbling fluidized beds. An activation region mechanism is presented for bubble formation, making it possible to model large beds in a timely manner. The bubbles are modeled as spherical-cap discrete elements that rise through the emulsion phase that is considered as a continuum. The model accounts for the simultaneous interaction of neighboring bubbles by including the trailing effects due to the wake acceleration force. The coalescence process is not irreversible and therefore, the coalescing bubble pair is free to interact with other rising bubbles originating the splitting phenomena. To validate the model, the simulated dynamics are compared with both experimental and literature data. Time, frequency, and state space analysis are complementarily used with a multiresolution approach based on the empirical method of decomposition to explore the different dynamic scales appearing in both the simulated time series and those obtained from experimental runs. It is concluded that the bubble dynamics interactions play the main role as the driver of the resulting bed dynamics, matching the main features of measured bubble dynamics. Exploding bubble phenomena have been identified by establishing a direct relation between the bubble generation, interaction and eruption, and the measured signalsProjects DPI2009-10518 (MICINN) and CARDENER-CM (S2009ENE-1660)Publicad

A novel methodology for simulating vibrated fluidized beds using two-fluid models

The present work considers the use of the two-fluid (Euler-Euler) CFD approach for the continuum description of vibrated fluidized beds as a less computationally demanding alternative to the discrete description given by Lagrangian-Eulerian methods such as DEM. In particular, a novel simulation strategy consisting on solving the two-fluid model equations in a coordinate reference system that moves with the vibrating walls of a gas-solid fluidized bed is proposed. By this way, vibration is transformed into simple alternating acceleration terms that are introduced through body forces in both the gas and the particle phase equations. The results of a series of two-fluid model simulations compare well with discrete particle simulations as well as with experimental data reported for beds containing Geldart group B particles. In general, the results of a series of two-fluid model simulations show similar trends to those seen in discrete particle simulations as well as in experimental data reported for beds containing Geldart group B particles. Exception of that is the velocity of bubbles, for which the two-fluid simulations compare less satisfactorily with the available experimental data. The two-fluid model simulations are also able to reproduce expected phenomena like the bubble growth with the vibration amplitude and the dependence of the pressure drop fluctuation on the vibration strength. In view of these promising results, the proposed two-fluid model formulation opens the possibility of increasing the scale of the vibrated fluidized beds currently simulated.The present work has been funded by the Spanish Ministerio de Ciencia e Innovación through the Project DPI2009-10518. The Authors gratefully appreciate this support.Publicad

Fluidized bed with a rotating distributor operated under defluidization conditions

The fluidization conditions of a rotating distributor applied to a 3-D bubbling fluidized bed was studied to assess its potential use as a counteracting measure of defluidization phenomena. The performance of the fluidized bed operating under nominal conditions was characterized for the rotating and the static distributor configuration. Different methods of analysis in the time and frequency domain were applied to establish the performance of the fluidized bed. The frequency domain analysis suggests some kind of local structuring of fluidized bed dynamics imposed by the distributor motion. The punctual injection of water over the surface of the bed lead to a high cohesive wet region that tend to settle down on top of the distributor giving rise to defluidization. The water-induced defluidization tests reflect an improvement of the fluidization quality with the distributor rotation.The author would like to thank the financial support from Projects DPI2009-10518 (MICINN) and CARDENER-CM (S2009ENE- 1660)Publicad

Characterization of flow-induced vibrations in gas-solid fluidized beds: elements of the theory

This paper revisits the basic hypothesis underlying the measurement of flow-induced vibration in fluidized beds. A novel theoretical approach based on the standing pressure field characterizing the bed dynamics is proposed to link the pressure fluctuations to the measured accelerometer signals. The model provides a reliable prediction of the carrying frequency band and helps in designing the accelerometer measurement process. The model was tested with previous results reported in the literature as well as with piezoelectric accelerometer measurements collected from a lab-scale experimental facility. A study on accelerometer measurements was conducted to identify the main limitations expected for measuring flow-induced vibrations in a gas-solid fluidized bed. The structural response of the vessel to flow-induced vibration was mostly determined by the "bed acoustics" that can be dominated by either elastic or compression waves. Finally, the survival of an envelope process on the measured accelerometer signal guaranteed the quality of the flow dynamical information collected during the measurement process.Financial support from projects DPI2009-10518 (MICINN) and CARDENER-CM (S2009ENE-1660).Publicad