Numerical and experimental study of hydrodynamics in a compartmented fluidized bed oil palm shell biomass gasifier

Numerical and experimental studies of hydrodynamic parameters of fluidized beds formed by either a single component system or a binary mixture in a pilot plant scale model of a Compartmented Fluidized Bed Gasifier (CFBG) have been performed. The numerical study is carried out with an Eulerian-Eulerian description of both gas and particle phases and a standard drag law for multiphase interaction. The numerically simulated results are then compared with the experimental results.The 2D and 3D flow patterns of the combustor and the gasifier are first generated from the numerical study to observe the bubble formation, possible channeling behavior and the binary mixing patterns in the bed.For a single component system, detailed 3D numerical analyses and experimental studies are done to investigate the bed expansion ratio, bubble diameter, bed pressure drop, and fluidization quality in CFBG. Two types of Geldart B inert particles namely river sand and alumina are used in the study.All trends of the aforementioned studies are well-predicted with the numerical values not greater than 15% of the recorded experimental values. Good fluidization is attainable in the combustor side, while the pressure drop behaviour seen for the gasifier with river sand shows that channelling occurs in the bed. The channelling behaviour becomes more severe with alumina bed.The solid circulation rate (SCR) is numerically simulated in this study as well. Solid circulation rate (SCR) increases with the increase in bed height while the main bed aeration does not affect the SCR which is consistent with the experimental data.For a binary mixture system with palm shell and river sand as the second fluidizing material, detailed 3D numerical analysis of the bed expansion ratio is done in parallel with the experimental study. The results of numerical predictions of overall mixing quality and local mixing index are verified by comparing with the experimental results. The actual trends of the studies are modestly captured by the numerical model with under-predicted values of less than 20%. The overall binary mixing quality is enhanced with the smaller palm shell size and larger palm shell weight percent. In addition, increasing the superficial gas velocity increases the local binary mixing index in the experiment.From the studies on bed expansion, bubble formation, steady equilibrium state and overall binary mixing quality, the 2D model provides well over-predicted values compared to the 3D flow model. Also, the local mixing index of the binary system is not captured by the 2D model. The numerical values predicted by 3D model are closer to the actual values.The key findings from the aforementioned studies are used as a guide to develop and operate the pilot plant scale CFBG with 0.5 ton/day of palm shell feed for fuel gas production

Image Processing and Measurement of the Bubble Properties in a Bubbling Fluidized Bed Reactor

The efficiency of a fluidized bed reactor depends on the bed fluid dynamic behavior, which is significantly influenced by the bubble properties. This work investigates the bubble properties of a bubbling fluidized bed reactor using computational particle fluid dynamic (CPFD) simulations and electrical capacitance tomography (ECT) measurements. The two-dimensional images (along the reactor horizontal and vertical planes) of the fluidized bed are obtained from the CPFD simulations at different operating conditions. The CPFD model was developed in a commercial CPFD software Barracuda Virtual Reactor 20.0.1. The bubble behavior and bed fluidization behavior are characterized form the bubble properties: average bubble diameter, bubble rise velocity, and bubble frequency. The bubble properties were determined by processing the extracted images with script developed in MATLAB. The CPFD simulation results are compared with experimental data (obtained from the ECT sensors) and correlations in the literature. The results from the CPFD model and experimental measurement depicted that the average bubble diameter increased with an increase in superficial gas velocities up to 4.2 Umf and decreased with a further increase in gas velocities due to the onset of large bubbles (potential slugging regime). The bubble rise velocity increased as it moved from the lower region to the bed surface. The Fourier transform of the transient solid volume fraction illustrated that multiple bubbles pass the plane with varying amplitude and frequency in the range of 1–6 Hz. Further, the bubble frequency increased with an increase in superficial gas velocity up to 2.5Umf and decreased with a further increase in gas velocity. The CPFD model and method employed in this work can be useful for studying the influence of bubble properties on conversion efficiency of a gasification reactor operating at high temperatures.publishedVersio

Bubbles emerging from a submerged granular bed

This paper explores the phenomena associated with the emergence of gas bubbles from a submerged granular bed. While there are many natural and industrial applications, we focus on the particular circumstances and consequences associated with the emergence of methane bubbles from the beds of lakes and reservoirs since there are significant implications for the dynamics of lakes and reservoirs and for global warming. This paper describes an experimental study of the processes of bubble emergence from a granular bed. Two distinct emergence modes are identified, mode 1 being simply the percolation of small bubbles through the interstices of the bed, while mode 2 involves the cumulative growth of a larger bubble until its buoyancy overcomes the surface tension effects. We demonstrate the conditions dividing the two modes (primarily the grain size) and show that this accords with simple analytical evaluations. These observations are consistent with previous studies of the dynamics of bubbles within porous beds. The two emergence modes also induce quite different particle fluidization levels. The latter are measured and correlated with a diffusion model similar to that originally employed in river sedimentation models by Vanoni and others. Both the particle diffusivity and the particle flux at the surface of the granular bed are measured and compared with a simple analytical model. These mixing processes can be consider applicable not only to the grains themselves, but also to the nutrients and/or contaminants within the bed. In this respect they are shown to be much more powerful than other mixing processes (such as the turbulence in the benthic boundary layer) and could, therefore, play a dominant role in the dynamics of lakes and reservoirs

Hydrodynamic Characterization of 3D Fluidized Beds Using Noninvasive Techniques

Fluidized beds are useful processing systems that are employed by many industries for their relatively unique operating properties. Low pressure drops, uniform temperature distributions, and high heat/mass transfer rates occur through the action of vertical gas injection into a column of solid particles. Although these properties give fluidized beds great advantages over other processing systems, the hydrodynamic characterization of fluidized beds is important for the efficient processing of many consumer products. However, fluidized bed hydrodynamics are difficult to visualize and quantify because most fluidized beds are opaque. Traditionally, the monitoring of local fluidized bed hydrodynamics has been done with intrusive probes that disturb local structure and the collection of data over large areas is time consuming. X-ray computed tomography (CT), as a noninvasive technique, can quantify local time-average phase fractions in highly dynamic multiphase systems without disturbing local structure. Using X-ray visualization techniques, methods have been developed in this study to: 1) test the repeatability of calculating local time-average gas holdup values using X-ray CTs; 2) find the fluidization uniformity of a non-reactive cold-flow fluidized bed; 3) compare local time-average gas holdup values in various bed materials, diameters, and operating conditions; and 4) compare annular hydrodynamic structures within the beds. Tests for the first two objectives were completed using a 15.2 cm ID reactor, while varying between two bed materials (crushed walnut shell and glass beads) of the same size and two gas flow rates. The third objective used a 10.2 cm and 15.2 cm ID reactor, varied between three bed materials (ground corncob, crushed walnut shell, and glass beads) of the same size, and over four and five relative superficial gas velocities and side-air injection gas flow rates respectively. The fourth objectives mirrored the third, however, did not use side-air injection. Observations show that local time-average gas holdup values can be calculated through the use of multiple X-ray CTs. The method of calculation is shown to be highly repeatable over the various flow rates, bed materials used, and ambient environmental conditions. Axisymmetric fluidization uniformity of the bed is also confirmed using the same method, while some differences are observed with varying materials and flow rates. Uniformity is observed to increase with bed height and increased gas flow rates, due to increased dispersion of gas into the bed and mixing rates respectively. Local time-average gas holdup is observed to differ somewhat between reactors. However, the overall results show that the hydrodynamic structures, i.e. aeration jets, bubble coalescence zones, bubble rise zones, particle shearing zones, and the side-air injection plume, within the fluidized beds for each reactor are very similar. These structures coupled with axisymmetric fluidization uniformity indicate that gas flow and material circulation tend to be annular in shape. Moreover, changes in the shape, size, number, and location occur with changes in superficial gas velocity, bed diameter, and bed material density. It is also suspected that the aeration scheme of the bed and the bed material properties i.e. shape factor, coefficients of restitution, and porosity play a role in the development of these structures. The aeration jets are similar in length in all beds regardless of material density or bed diameter. They also tend to decrease in height and become increasingly wall leaning as superficial gas velocity increases. The coalescence of bubbles tends to occur in regular locations near the reactor wall just above the aeration jets within all beds regardless of material density, bed diameter, and gas flow rates. The rise paths of bubbles through all beds emanate from the coalescence zones with relatively small widths and increasing in width as bed height increases. Particle shear zones occur in differing size, shape, number outside of all other hydrodynamic structures while migrating around the bed with changing material density, bed diameter, and superficial gas velocity. The diffusion of gas into the fluidized bed from the side-air injection plume in each bed is similar, due to advection dominance within the plume. Gas dispersion does not seem to occur by similar means between materials though, because crushed corncob and ground walnut shell are natural systems and have a higher porosity and lower density than glass beads. The natural materials also have non-uniform shape factors causing behavior differences with the fluidization gas. The time-average bed height between bed diameters is different for each material density and gas flow rate, where the height in the 10.2 cm diameter reactor is observed to be greater on average in all tests than in the 15.2 cm reactor, due to wall effects. Lastly, the techniques used for analysis in this study are valuable to computational fluid dynamicists for direct comparison to simulation and models of fluidized beds

Axial segregation behaviour of a reacting biomass particle in fluidized bed reactors: experimental results and model validation

Axial segregation behaviour of a single biomass particle in a lab-scale bubbling fluidized bed has been investigated from both experimental and modelling perspectives. Experiments were conducted using beech wood particles of different sizes, ranging from 8 to 12 mm under either oxidizing or inert conditions. The fluidized bed reactor was operated at temperatures and fluidization velocity ratios, U/Umf, in the range of 500–650 °C and 1–2, respectively. A one-dimensional model has been developed to predict the axial location of the particle over time, taking into account both dynamic and thermal conversion mechanisms. X-ray imaging techniques allowed to identify endogenous bubbles released during devolatilization and carry out direct measurements of their size. This information was used to propose an expression for the lift force acting on the fuel particle. The model showed very accurate predictions and the segregation behaviour of the fuel particle appeared to be independent of the nature of the fluidizing medium

Non-invasive and non-intrusive diagnostic techniques for gas-solid fluidized beds – A review

Gas-solid fluidized-bed systems offer great advantages in terms of chemical reaction efficiency and temperature control where other chemical reactor designs fall short. For this reason, they have been widely employed in a range of industrial application where these properties are essential. Nonetheless, the knowledge of such systems and the corresponding design choices, in most cases, rely on a heuristic expertise gained over the years rather than on a deep physical understanding of the phenomena taking place in fluidized beds. This is a huge limiting factor when it comes to the design, the scale-up and the optimization of such complex units. Fortunately, a wide array of diagnostic techniques has enabled researchers to strive in this direction, and, among these, non-invasive and non-intrusive diagnostic techniques stand out thanks to their innate feature of not affecting the flow field, while also avoiding direct contact with the medium under study. This work offers an overview of the non-invasive and non-intrusive diagnostic techniques most commonly applied to fluidized-bed systems, highlighting their capabilities in terms of the quantities they can measure, as well as advantages and limitations of each of them. The latest developments and the likely future trends are also presented. Neither of these methodologies represents a best option on all fronts. The goal of this work is rather to highlight what each technique has to offer and what application are they better suited for

Computer simulation of the hydrodynamics of a two-dimensional gas-fluidized bed

A first principles model of a gas-fluidized bed has been applied to calculate the hydrodynamics of a two-dimensional (2-D) bed with an orifice in the middle of a porous plate distributor. The advanced hydrodynamic model is based on a two fluid model approach in which both phases are considered to be continuous and fully interpenetrating. Conservation equations for mass, momentum and thermal energy have been solved numerically by a finite difference technique on a mini-computer. Our computer model calculates the porosity, the pressure, the fluidum phase temperature, the solid phase temperature and the velocity fields of both phases in 2-D Cartesian or axisymmetrical cylindrical coordinates. The new feature of the present model is the incorporation of Newtonian behaviour in the gas and solid phases. Our preliminary calculations indicate that the sensitivity of the computed bubble size with respect to the bed rheology (i.e. the solid phase viscosity) is quite small. However the bubble shape appears to be much more sensitive to the bed rheology. Results of the calculations have been compared with data obtained from an experimental cold-flow model (height: 1000 mm, width: 570 mm, depth: 15 mm)