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This study considers the role of the distributor effect during the bubble generation, growth and interaction within a bubbling fluidized bed. To characterize this effect, short-term and long-term dynamics are explored in time and frequency domains. Controversy occurs when inspecting what is actually known of the gas distribution mechanism in bubbling fluidized beds. To sustain discussion, classical and innovative techniques, such as Digital Image Analysis, DIA, and Wavelet Analysis WA, are used. As a result, the conclusion is that the distributor not only has no effect in the averaged long-term dynamics but also has no effect on the general bubble generation characteristic frequencies. The analysis performed on the bubble generation region concludes that the distributor design has a great influence at the short-term scale such as during the bubble generation process controlling the spatial distribution of the bubble nucleation sites over the distributor plate.

This work proposes a new particle receiver for Concentrating Solar Technology (CST) that employs air and particles as heat transfer fluid (HTF). The novel Linear Particle Solar Receiver (LPSR) is located at the ground level receiving the concentrated solar energy linearly from the top. This receiver is formed by several fluidized beds connected consecutively allowing the horizontal movement of solids and the linear absorption of solar energy. A new solar field is proposed and analyzed to redirect the concentrated solar energy towards the receiver linearly. The optic analysis of a linear beam-down system is carried out using a ray-tracing software. Then, the performance of a linear particle solar receiver is analyzed considering target temperatures of 200 °C, 400 °C, 600 °C and 800 °C to reproduce CST integration with medium and high temperature process heat, Rankine power generation and supercritical CO2 (sCO2) cycles, respectively. The temperature of the hot streams of air and sand are calculated considering the heat losses from the receiver and the compressor parasitic consumption. The procedure to determine the optimum design is shown studying one line as a function of the target temperature. For instance, a sand mass flow of 1.75 kg/s in a receiver of 0.5 m width and a secondary reflector eccentricity of 0.8, can be heated up to 600 °C in a length of 280 m, showing a solar field efficiency of 40.25% and a receiver thermal efficiency of 80%. This work has been supported by the Spanish Government under the project ENE2015-69486-R (MINECO/FEDER, UE), by "Comunidad de Madrid" and European Structural Funds under the project ACES2030-CM project (S2018/EMT-4319) and by the fellowship "Ayuda a la investigación en energía y medio ambiente" of Fundación Iberdrola España.

Beam-down concentrating solar power for thermochemical and energy absorption applications stands as an attractive approach that can enhance the renewable energies deployment. This work explores the integration of beam-down optics with fluidized bed technology proposing a model to calculate both gas and bed temperatures. The beam-down system concentrates the energy from the solar field into a fluidized bed receiver. A novel phenomenological model is proposed to adapt the well-known two-phase theory to the heat transfer process of a bed operating in the bubbling regime while it is directly irradiated from the top. In this way, this simple model can be used as a design tool for beam-down fluidized bed receivers. The top bed surface is considered as an opaque diffuse layer formed by gray particles. A single layer model is applied to estimate the effective emissivity between the heterogeneous bed surface and the ambient conditions in the freeboard. The vertical temperature profile is obtained considering particle phase heat conduction, particle to gas heat convection, solid convection, bubble convection and radiation heat transfer mechanisms. The model is validated using silicon carbide and zirconia fluidized bed experiments reported in the literature. The model shows that the solid convection is the dominant heat transfer mechanism for a beam-down fluidized bed receiver. Further results explore the influence of the operating conditions on the fluidized bed receiver for a bed of silicon carbide particles, showing that energy concentration fluxes of 35 . 10(4) W/m(2) can reach bed temperatures of 1000 degrees C when operating at a gas velocities of 3.U-mf.

A severe maldistribution problem was induced with a half-covered gas distributor plate. Videos of the bed surface were recorded and analyzed to study how the boundary between maldistribution and stable fluidization is affected by the gas velocity and the bed aspect ratio. It was observed that the visual inspection of the bed surface reports no information about the maldistribution at the bottom bed. Several cases of moderate maldistribution (i.e. different distributions of open orifices) were investigated by means of pressure fluctuation signal analysis to provide a criterion for maldistribution detection. The effect of the measurement position on maldistribution detection was also investigated. The attractor comparison test, the S-test, as well as the statistical process control of the standard deviation and wide band energy regions, were applied to test their capability of online detecting and monitoring gas maldistribution. The statistical process control methodology, based on standard deviation of pressure fluctuation signals, can be successfully applied to online detection of maldistribution when the pressure probe is located between 50% and 75% the total bed height. Finally, the rotation of the distributor was studied as a counteracting mechanism, showing good results overcoming maldistribution problems in fluidized beds.

Humidity and temperature measurements, together with pressure fluctuation signals, were employed to analyze a drying process in a lab-scale bubbling fluidized bed. The experimental facility was equipped with a rotating distributor. The operational conditions reported in sludge, paste, and granular drying processes were reproduced using silica sand as wet material and as inert medium. Experiments were performed by changing the bed aspect ratio and the water-sand content in the bed operated at ambient conditions. Four drying periods were found when analyzing humidity and temperature signals. The multi-resolution approach of the pressure fluctuation signals showed the effect of the sample drop over the bed surface on the fluidization conditions, by relating the drying periods with the bed dynamics. The drying process can affect the low- and high-frequency details of the pressure signal when the bed state changes towards defluidization, or just the high-frequency detail if the bed was at the maldistributed regime. The drying conditions needed to use the pressure signals to control the drying process are defined by means of a statistical monitoring approach. A comparison between the static and the rotating distributor tests showed clear benefits for the operation with the rotating distributor for shallow and deep beds. When the distributor was rotating, the average improvement of the drying rate was 11%, whereas the drying time was reduced by 40% for the tests of the maldistributed and defluidized regimes.

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. Publicado

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 signals Projects DPI2009-10518 (MICINN) and CARDENER-CM (S2009ENE-1660) Publicado

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.

The Hilbert–Huang transform method, HHTM, is applied to perform a multi-resolution analysis by means of the empirical mode decomposition, EMD, of noisy pressure signals collected from a gas–solid fluidized bed operating in slugging regime. Several important dynamic components, such as local bubble, “bulk” and particle dynamics, are identified. The bulk dynamic component is used to estimate the bed expansion ratio and bed height. The Kolmogorov entropy, K , and the correlation dimension, D 2 , are used for monitoring slugging dynamics from the resulting bubble, particle and bulk time scales. To study the attractor structure, a method for elaborating return-maps, based on the join probability matrix, is used. It has been found that the bulk dynamics provides information of the global behavior of the dense phase and allows characterizing the slugging dynamics with correlation dimension 2.2 D 2 2.6 . In contrast, the local bubble dynamics is characterized by a correlation dimension that ranges from 1.8 to 2.

We present a new method of scaling hydrodynamic data obtained from a 2D gas–solid fluidized bed, establishing links between 2D and 3D geometries. The methodology proposed may also be useful for verifying 3D–3D dynamic scaling. According to the chaos scale-up methodology proposed by Van den Bleek and Schouten [1993. Chemical Engineering Science 48, 2367–2373], the information balance is taken into account. The complexity shown by these systems is measured as the Kolmogorov entropy. Fluidized beds of different geometry (2D, 3D) performing under the bubbling regime were operated at different bed height and bed aspect ratios by fluidizing several particle groups belonging to Geldart groups B and D. Finally, an empirical correlation is proposed to compare fluidized bed hydrodynamics. This correlation relates the global information flow of the system K G = KFr r to the relative Reynolds particle number.

The use of free surface fluctuations in three-dimensional (3-D) fluidized bed dynamic characterization is addressed. This technique provides a noninvasive global measure of the quality of fluidization by measuring at one position in the bed. The well-known effect of particle size on fluidized bed hydrodynamics was explored using a 3D fluidized bed operated with four different particle groups. The analytical methods used were mutual information functions, frequency, and state space analysis of time series collected from bed surface fluctuations. Turbulent, multiple, and noninteracting bubble flow patterns, occurring during bubble regimes, have been characterized. Kolmogorov entropy for the reconstructed attractor is very sensitive to the transition between fluidization regimes. Moreover, correlation dimension estimates ranging between 3 < D2 < 5 indicate apparent low-dimensional dynamics. Additionally for larger particle groups, D2 reaches a value close to 3, indicating slow bubbling controlled by large bubbles before the transition between different bubble flow patterns takes place. Finally, particle size has been found to have strong influence on signal persistence. © 2004 American Institute of Chemical Engineers AIChE J, 50: 3060–3067, 2004

We present a novel method of extracting hydrodynamic information from a gas–solid fluidized bed. The proposed method involves measuring the fluctuations produced in a laser sheet that perpendicularly crosses the free bed surface. This non-local, non-intrusive technique can be applied to 2D and 3D gas–solid fluidized bed systems. Frequency domain and state space analysis were used in a complementary manner to characterize the fluidization regimes of these systems by monitoring EWB, μ, D2, λmax, and K. Finally, we evaluated the influence of H/L on attractor invariants in 3D fluidized beds.

Van Ommen et al. (2000) recently developed a monitoring method that was successfully used for the early detection of agglomeration in fluidized beds. This article shows that this monitoring method is more generic and can also be applied for early detection of foam formation in bubble columns. Knowing that dilute alcohol solutions simulate reasonably well the liquid-phase behavior in bioreactors (Bukur and Patel, 1989), and can also leads to foaming (Pugh, 1996), the system water/ethanol was chosen as a model system. Experiments were carried out in the homogeneous and the heterogeneous regime.

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) Publicado