Evaluating the Probabilities of Fluidization Regimes

On the nature of turbulent and fast fluidized beds

The first phase of experimentation for the comprehensive similitude study on the two laboratory scale cold-model circulating fluidized beds has been completed. This first phase required the acquisition and analysis of pressure fluctuation data from a 2.0 inch diameter pressurized circulating fluidized bed. The second phase required the matching of each of the 20 experiments in a cold-model twice the size of the smaller 2.0 inch circulating fluidized bed model using similitude relations. Problems of excessive electrostatic buildup were encountered in this second phase. To alleviate these problems, the large model had to be redesigned and reconstructed. The focus of the research this past quarter has been devoted to these modifications. Currently, the new sections have been constructed and the modified circulating fluidized bed is in the process of reassembly. Two related projects were undertaken while the large circulating fluidized bed model was being modified. A bubbling bed was constructed such that pressure fluctuation data could be measured in both bubbling and turbulent fluidization regimes. The purpose of such tests was to relate pressure fluctuation structure in the lower sections of the circulating fluidized bed with phenomena observed in bubbling/turbulent regimes. Two probes designed to measure heat transfer coefficients in the large and small circulating fluidized bed models were completed and initially tested under bubbling bed conditions. These tests insured the validity and accuracy of the bed to surface heat transfer coefficient measurement. The two probes were constructed as an additional means of validating similitude relations in circulating fluidized beds in addition to pressure fluctuations.

Fluidization regimes of dense suspensions of Geldart group A fluidized particles in a high aspect ratio column

The goal of this research is to determine how spectral analysis of pressure fluctuations can be used as a diagnostic and design tool for both bubbling fluidized beds (BFB) and circulating fluidized beds (CFB). Static pressure fluctuations are measured from two BFB models, two CFB models, and two Pyropower CFB boilers located at the ISU power plant/cogeneration facility. Analyzing the fluctuations using spectral analysis shows that the structure of pressure fluctuations is governed by multiple phenomena. Similarities and differences between the nature of fluctuations in bubbling, turbulent, and fast fluidization regimes are discussed. The Bode plots of pressure fluctuations under bubbling/turbulent fluidization exhibit the characteristics of an oscillatory second order system. This system is governed by a combination of voidage oscillations and surface eruptions. A theoretical model is developed to predict the frequency of oscillation in fluidized systems, and to explain the second order behavior. A comparison of pressure fluctuation behavior in two geometrically similar BFBs is conducted, which validates previously derived fluidized bed similitude parameters under most operating conditions. Multiple second order phenomena acting concurrently also govern CFB pressure fluctuations. Phenomena similar to that proposed for BFBs governs pressure fluctuations in the lower dense regions of the CFB. A siuface wave phenomena is hypothesized to govem CFB fluctuations above the lower dense bed. Similar pressure dynamics and axial voidage profiles are not observed when currently proposed similitude parameters are matched m two CFBs. By replacing the dimensionless soUds flux with a dimensionless riser loading parameter, this modified set of similitude parameters can be used to establish similar hydrodynamics in CFBs . Primarily due to the periodic operation of the coal feed system in the industrial scale CFB Boiler, pressure fluctuation analysis did not result in any definitive hydrodynamic information.

Local solid particle behavior inside the upper zone of a circulating fluidized bed riser

Flow regimes and axial pressure profiles in a circulating fluidized bed

A circulating turbulent fluidized bed connected with a riser and an annular carbon stripper (CS) is proposed to be used as a fuel reactor (FR) in chemical looping combustion. The bottom section of the FR is operated under a turbulent fluidization regime, which can achieve enough solid residence time and enhance the mixing of the oxygen carrier with solid fuel. A 1.5 MWth cold model of the FR was designed, constructed, and tested to investigate the hydrodynamics of solid particles with different sizes. Three kinds of quartz sands with different particle sizes (d50 = 122, 249, and 392 μm) were used as bed materials to simulate the oxygen carrier. Continuous operation with a reasonable pressure balance was achieved in the cold model. The effects of important variables, including gas velocity, static bed height, and particle size, on the gas–solid hydrodynamics of the FR were measured and discussed. It was found that the transition velocities from bubbling to turbulent fluidization for different particles of d50 = 122, 249, and 392 μm were measured to be 0.78, 0.95, and 1.06 m/s, respectively, indicating that the transition velocity increased with increasing the particle size. The solid fraction profile along the reactor height and solid circulation rate were affected by gas velocity and static bed height. A modified correlation was proposed to predict the solid fraction of the annular CS dilute phase, and the predicted results agree well with the experimental data under a wide range of operational conditions.

Assessment of CO2 capture by calcium looping (CaL) process in a flexible power plant operation scenario

Multi-scale numerical simulation of fluidized beds: Model applicability assessment

In many process engineering fields, gas-particle fluidized beds are widely used. In fluidized bed research, the discrete element method, or DEM, has been a powerful tool for design and operation purposes. However, with the use of Type-A powders, fluid catalytic cracking or FCC particles being classical cases, they have hardly been reported in DEM simulations of fast fluidization. This study paid close attention to the suitable selection of a stiffness constant and a DEM time step. To reflect their respective effects and complicated interactions, a so-called relative DEM time step was defined. The drag coefficient was correlated using the energy-minimization multi-scale (EMMS) approach to reasonably calculate the gas–solid interaction. Six representative cases with different relative time step values were chosen to simulate a micro-fluidized bed of Type-A FCC powders. The results showed that DEM employing EMMS-based drag force was able to greatly enlarge the suitable range of relative time steps in a fast fluidization simulation of Type-A powders. In addition, the typical macro flow structures of fast fluidization was successfully captured: axially dilute in the top and dense in the bottom, and radially dilute in the core and dense near the wall. Moreover, the distinct gas–solid backmixing, which is considered one of the most important pieces of evidence for the determination of fast fluidization regimes, was modeled. It was indicated that the EMMS-based drag model attenuated the overestimated drag force so that the soft-sphere contact model would be able to more appropriately deal with particle collision, and thus improve the suitable relative DEM time step range.

Statistical and frequency analysis of the pressure fluctuation in a fluidized bed of non-spherical particles

Reliable computational methods can provide valuable insight into gas–solid flow processes and can be used as a design tool. Of particular interest in this study is the hydrodynamics of a binary mixture of sand–biomass in a fluidized bed. Biomass particulates vary in size, shape, and density, which inevitably alter how well the particles fluidize. Our study will use computational fluid dynamics (CFD) to interpret the hydrodynamic states of a fluidized bed by analyzing the local pressure fluctuations of beds of sand and a binary mixture of cotton stalks and sand over long time periods. Standard deviation of pressure fluctuations will be compared with experimental data to determine different fluidization regimes at inlet gas velocities ranging from two to nine times the minimum fluidization velocity. We will use Bode plots to present the pressure spectra and reveal characteristic frequencies that describe the bed hydrodynamics for different fluidization regimes. This work will present CFD as a useful tool to perform that analysis. Other important contributions include the study of pressure fluctuations of a fluidized bed in bubbling, slugging, and turbulent regimes, and the analysis of a binary mixture using CFD.

Modeling the hydrodynamics of downflow gas-solids reactors

From bubbling to turbulent fluidization: Advanced onset of regime transition in micro-fluidized beds

Changes in the coherent standard deviation and in the average frequency of measured pressure time series with gas velocity, are proposed, as unique and unambiguous criteria to mark flow regime transitions in slurry bubble columns. In a 2‐dimensional (2‐D) slurry bubble column, pressure time series are measured at different gas velocities simultaneously with high‐speed video recording of the gas‐liquid flow. The frequency of occurrence and the average diameter of the large bubbles are determined from video image analysis. The gas velocity where the first large bubbles are detected, with an average diameter of 1.5 cm, and with a frequency of occurrence of one bubble per s, is designated as the first regime transition point (transition from the homogeneous regime to the transition regime). At this point, the coherent standard deviation of the measured pressure fluctuations clearly increases from zero. The gas velocity where the average diameter and the frequency of occurrence of the large bubbles become constant, is designated as the second regime transition point (transition from the transition regime to the heterogeneous regime). From this point onward, the slope of the coherent standard deviation of the measured pressure fluctuations clearly decreases with gas velocity, while the average frequency becomes constant. These clear changes with gas velocity in the coherent standard deviation, and in the average frequency are also demonstrated in a 3‐D slurry bubble column. © 2005 American Institute of Chemical Engineers AIChE J, 2005