High-density Circulating Fluidized Bed Research Articles

Comparison of flow dynamics and reactor performance in gas-solids circulating turbulent fluidized bed with other fluidized beds across regimes

The hydrodynamics and the reactor performance in the recently proposed gas-solids circulating turbulent fluidized bed (CTFB) were compared with that in the low-velocity conventional fluidized beds and the high-density circulating fluidized bed (HDCFB) riser. The average solids holdup distributions, fluctuations, and probability density distributions of the instantaneous solids holdup signals indicate that the CTFB either more resembles a TFB if the Gs/Ug ratio is large or more resembles an HDCFB if the Gs/Ug ratio is small. The distributions of the bubble occurrence, equivalent bubble size, and bubble phase fraction were extracted from the optical fiber signals of the local solids holdup using the wavelet analysis method. The CTFB was found to have a greater number of smaller bubbles than the bubbling and turbulent fluidized beds and relatively fewer and larger bubbles than that in an HDCFB. The effect of flow dynamics and bubble behaviors on the reaction performance in different types of fluidized beds are compared by analytical calculation based on the K-L reactor model. A superior reactor performance can be achieved using the CTFB reactor for a higher effective reaction rate, higher mass transfer rates, and a lower degree of solids backmixing.

A dynamic cluster structure-dependent drag coefficient model applied to the riser in high density circulating fluidized bed

A dynamic cluster structure dependent (DCSD) drag model based on energy dissipation minimization using the two-fluid model (TFM) and the kinetic theory of granular flow (KTGF) is applied to the riser in high density circulating fluidized bed (HDCFB). The characteristics of gas and solid are predicted by means of the DCSD drag model with the convective and local accelerations. The law of granular collision energy dissipation and drag energy dissipation are compared and the influence of granular collision energy dissipation to agglomerate structure is researched. The study shows that granular collision energy dissipation has great influence on heterogeneous structure of riser. Simulated solid volume fraction, axial pressure drop and solid mass flux are compared to experimental measurements, numerical analysis suggests that the temporal-spatial heterogeneous structure characteristics and particle collision affect the flow of the riser in the literature.

Numerical investigations on gas–solid flow in circulating fluidized bed risers using a new cluster-based drag model

A cluster-based drag model is proposed for the gas–solid circulating fluidized bed (CFB) riser by including the cluster information collected from image processing and wavelet analysis into the calculation of system drag. The performance of the proposed drag model is compared with some commonly used drag models. A good agreement with the experimental data is achieved by the proposed cluster-based drag model. Error analysis of the proposed cluster-based drag model based on the local distributions of solids holdup and particle velocity is conducted. The clustering phenomenon in the low-density and high-density CFB risers and the effect of the cluster size on the simulation accuracy are also numerically studied by the proposed drag model.

Numerical Simulation of the Pilot-Scale High-Density Circulating Fluidized Bed Riser

A high-density circulating fluidized bed (HDCFB) has broad prospects in catalytic processes. It can handle higher solid circulation rates than a low-density CFB (LDCFB), leading to more complicated...

Numerical study on influence of exit geometry in gas–solid flow hydrodynamics of HDCFB riser by CPFD

The understanding of gas–solid flow is of great importance for various high-density circulating fluidized bed (HDCFB) practices. In this work, the effects of exit geometry on the gas–solid flow behavior in an HDCFB riser were numerically studied by using the computational particle fluid dynamics (CPFD) method. The simulated results were compared against experimental data on bed pressure and solid volume fraction distributions along the riser and a satisfactory agreement was found between them, which indicated the capability of the CPFD for the HDCFB riser simulation. Thereafter, simulations were conducted for the HDCFB risers with six different exit geometries and comparisons of hydrodynamics characteristics between each other were made. It was found that exit geometry significantly affected the axial solid volume fraction distribution through the whole riser. The abrupt exits gave rise to higher SRT than the smooth ones due to the solids back-mixing and packing effects. The angle between the exit duct and the horizontal plane also has a great influence on the gas–solid hydrodynamics and the abrupt exit with 15° was found to be appropriate for HDCFB riser due to its promotion on solid dispersion and reduction in solids back-mixing and packing.

Three-dimensional full-loop simulation of a high-density CFB with standpipe aeration experiments

Three-dimensional full-loop simulation of a high-density circulating fluidized bed (HDCFB) is conducted using the multiphase particle-in-cell (MP-PIC) method. The computational results are in good agreement with the experimental data of pressure distribution and solids circulation rate under different aeration conditions of the standpipe. The main feature of gas-solid flow in the riser is qualitatively predicted, with coexistence of the developed flow in the upper region and the acceleration flow in the lower region. The flow regime in the standpipe is mainly concurrent downward gas-solid flow in the close packing region. Due to the influence of the aeration gas, the pressure gradient presents a wave variation along the height of standpipe. The predicted solids residence time distribution in the standpipe exhibits a feature of early peak with an extended tail, indicating that the solids flow is not uniform in the standpipe.

CFD study of exit effect of high-density CFB risers with EMMS-based two-fluid model

It has been widely recognized that the exit geometry of circulating fluidized bed (CFB) risers may have significant effects on the hydrodynamics of gas–solid flow. However, a systematical study of exit effect is still lack after extensive experimental studies, possibly due to the fact that systematical modification of exit geometry in experimental study is an expensive and time-consuming task. In this study, after further validation of the recently developed EMMS-based two-fluid model (Wang et al., 2012. Chem. Eng. Sci. 75, 349–358; Zhou et al., 2014. Chem. Eng. Sci. 107, 206–217), the model is used to systematically investigate the effect of exit geometry of high-density CFB risers, fully taking the advantage that bed geometry can be easily modified in computational fluid dynamics study. It is shown that (i) the type of exit has a significant effect on hydrodynamics, the use of abrupt exit results in an increased solids concentration, not only in the immediate vicinity of the exit but also for a considerable distance down to the riser; (ii) the cavity height of abrupt exit, the curvature diameter of smooth exit and the length of the horizontal tube connecting the exit and the primary cyclone only have a minor effect or have no influence on the bed hydrodynamics of the studied risers; (iii) more importantly, the decrease of the diameter of abrupt exit tube results in a remarkable increase of solids holdup, the symmetry of radial solids concentration near the exit can also be enhanced significantly. However, in case of smooth exit, the diameter of the exit tube has no effect on the bed hydrodynamics at all. All of those results are in agreement with conclusions obtained from previous experimental studies, thus offering further validation of the EMMS-based two-fluid model for modelling heterogeneous gas–solid flow.

Three-dimensional simulation of dense suspension upflow regime in high-density CFB risers with EMMS-based two-fluid model

Dense suspension upflow (DSU) regime in high-density circulating fluidized bed (CFB) risers, featuring no downward particle flux and high mean solid holdup, offers significant advantages over conventional fast fluidization, such as much less solid backmixing and higher solid holdup. While most of the previous CFD simulations are limited to the regime of fast fluidization in low-density CFB risers, it is difficult to reproduce the hydrodynamics of DSU regime using CFD simulation. In this study, a recently proposed EMMS-based two-fluid model (Wang et al., 2012. Chem. Eng. Sci. 75, 349–358), where the particle-rich dense phase and gas-rich dilute phase are treated as the two interpenetrating continua instead of treating gas and solid phases as interpenetrating continua as in standard two-fluid models, is first modified and then used to study the hydrodynamics of high-density CFB risers. Extensive simulations have been performed, and the comparison with experimental data indicated that the EMMS-based two-fluid model had the ability to correctly capture main features of the DSU regime. Therefore, present study not only provided further validation of the idea and feasibility of EMMS-based two-fluid model, but also offered new possibility to explore the hydrodynamic characteristics of DSU regime in high-density CFB risers.

Gas–solid flow behavior and contact efficiency in a circulating-turbulent fluidized bed

Abstract Circulating-turbulent fluidized bed (C-TFB) was characterized by high solid holdup, homogenous axial and radial flow structure, no net downflow of solids and high contact efficiency in this study. These flow dynamic properties were mainly represented by solid holdup profiles, the identification of flow regime and cluster dynamics in a riser, 100–150 mm in diameter and 10.06 m in height. To quantify the effect of novel diameter-expanding structure on flow dynamics, a parameter (contact efficiency) was introduced firstly. The effects of gas–solid interaction on flow performance were investigated by a fiber-optic probe to detect solid holdup. CO2 tracer injection and sampling system were used to characterize the flow structure and define the contact efficiency. The experimental results showed that flow regime in C-TFB is belonging to dense riser upflow (DRU) or dense-suspension upflow (DSU) regimes and transient region disappears in axial position. Flow structure is different from previous studies about traditional circulation fluidized bed (CFB) due to the effect of expanding structure and ring-feeder internal. A flow model based on the profiles of solid holdup and CO2 tracer concentration was proposed to account for the gas–solid contact efficiency in the reactor. The contact efficiency in C-TFB is much higher than that of high-density circulating fluidized bed (HDCFB), which means C-TFB reactor would exhibit better performance to optimize the product distribution under the same operating conditions.

Solids Holdup of High Flux Circulating Fluidized Bed at Elevated Pressure

AbstractExperimental studies on the solids holdup of a high‐flux circulating fluidized bed (HFCFB) at an operating pressure up to 0.5 MPa were carried out. The effects of operating pressure, solids mass flux and superficial gas velocity on the solids holdup distribution were systematically tested. It was found that the solids holdup at elevated pressure increases with increasing solids mass flux but decreases with increasing superficial gas velocity, which is similar to the trends at atmospheric condition. As a result, the condition of a high‐density circulating fluidized bed (HDCFB), i.e., solids holdup in everywhere of the riser is larger than 10 %) is easier to be achieved at elevated pressure than in a HDCFB operated at atmospheric pressure. In the current work, the condition of a full HDCFB with Geldart group B particles has been achieved successfully at 0.5 MPa.

A revised drag force model and the application for the gas–solid flow in the high-density circulating fluidized bed

Abstract The behavior of the gas–solid two-phase flow plays a vital role in the heat and mass transfer in the high-density circulating fluidized bed. In this paper, based on the energy-minimization multi-scale (EMMS) model, a drag force coefficient model, which can be applied to different kinds of particle, is developed for investigating the hydrodynamic characteristics of the high-density circulating fluidized bed (HDCFB). Coupled with the two-fluid model and the kinetic theory of granular, a 3D full-loop numerical simulation is implemented to test the validity of the revised drag force coefficient model, and excellent numerical performances and encouraging results are obtained. Under the operated conditions, the simulation results are in excellent agreement with the experimental data, which reveals that the revised drag force coefficient model is competent for the HDCFB’s numerical simulation.

Circulating turbulent fluidization—A new fluidization regime or just a transitional phenomenon

While circulating fluidized bed (CFB) reactor has many advantages over the more conventional turbulent fluidized bed (TFB) reactor, it does at least have one significant shortcoming—the rather dilute solids volume concentration in CFB reactor gives rise to less ideal reaction intensity. On the other hand, while having higher reaction intensity, TFB reactor has one fatal drawback of particle back-mixing, making it not suitable for certain reactions such as catalytic reaction where the catalyst requires frequent regeneration. This paper describes some key issues in the development of a circulating turbulent fluidized bed (CTFB) reactor that combines the advantages of both TFB and CFB, that is, to have the high reaction intensity as in TFB but and also to have a suppressed solids back-mixing as in CFB due to a continuous net upflow of solids flux through the bed. Experimental results show enough evidence to suggest that a new fluidization regime is formed, the characteristics of which appears to be distinct from those observed in a regular TFB and from those in either the bottom or the upper sections of regular CFB and/or high-density CFB (HDCFB). Fundamentally, the difference is that particle–particle interaction (collision) dominates the motion of particles in CTFB and TFB, while gas–particle interaction (drag force) is the key element that determines the two phase flow in CFB including HDCFB.

Demarcation of a new circulating turbulent fluidization regime

AbstractTransient flow behaviors in a novel circulating‐turbulent fluidized bed (C‐TFB) were investigated by a multifunctional optical fiber probe, that is capable of simultaneously measuring instantaneous local solids‐volume concentration, velocity and flux in gas‐solid two‐phase suspensions. Microflow behavior distinctions between the gas‐solid suspensions in a turbulent fluidized bed (TFB), conventional circulating fluidized bed (CFB), the bottom region of high‐density circulating fluidized bed (HDCFB), and the newly designed C‐TFB were also intensively studied. The experimental results show that particle‐particle interactions (collisions) dominate the motion of particles in the C‐TFB and TFB, totally different from the interaction mechanism between the gas and solid phases in the conventional CFB and the HDCFB, where the movements of particles are mainly controlled by the gas‐particle interactions (drag forces). In addition, turbulence intensity and frequency in the C‐TFB are significantly greater than those in the TFB at the same superficial gas velocity. As a result, the circulating‐turbulent fluidization is identified as a new flow regime, independent of turbulent fluidization, fast fluidization and dense suspension upflow. The gas‐solid flow in the C‐TFB has its inherent hydrodynamic characteristics, different from those in TFB, CFB and HDCFB reactors. © 2009 American Institute of Chemical Engineers AIChE J, 2009

Comparative study of flow structures in a circulating-turbulent fluidized bed

This paper presented a comparison of flow structures in a newly designed circulating-turbulent fluidized bed (C-TFB), which has a high solids holdup and solids flux, with two commonly used fluidized beds reactors (circulating fluidized bed (CFB) and turbulent fluidized bed (TFB)). Experimental results included instantaneous solids concentration and particle velocity, and local solids flux. Results indicated that the flow behavior in the C-TFB possessed both similarities and differences with the TFB and high-density CFB. Many distinct advantages of the C-TFB flows were found.

Computational fluid dynamics of high density circulating fluidized bed riser: Study of modeling parameters

CFD modeling of air and fluid catalytic cracking (FCC) particles in the riser of a high density circulating fluidized bed (HDCFB) has been performed. The implementation of correct inlet conditions was found to be critical for the successful simulation of the hydrodynamics. The simulated profiles of gas and solid velocity and volume fraction were overall in good agreement with experimental data reported in the literature. However, due to the difficulties in accurate modeling of the solid segregation toward the wall, the solid volume fraction was under predicted near the walls. The effect of modeling parameters including different drag models, wall restitution coefficient values, and solid slip conditions have been evaluated. While the wall restitution coefficient did not exhibit a significant effect on the riser hydrodynamics, the appropriate slip condition aided in predicting the solid segregation toward the wall.

New closure models for CFD modeling of high-density circulating fluidized beds

Experimental results from a high-density circulating fluidized bed (CFB) riser have been used to develop new closure models for the drag coefficient and the gas–solid mixture viscosity. The models predict a rapid increase in both viscosity and drag associated with the high solids concentrations near the riser wall. These new models have been incorporated into the commercial Computational Fluid Dynamics software FLUENT and the predictions of FLUENT have been compared with experimental data from the literature. With the inclusion of the new closure models, FLUENT was able to predict the radial distribution of solids concentration and solids mass flux found experimentally in three different cold-flow CFB risers operated at solids mass fluxes between 148 kg/m 2·s and 455 kg/m 2·s and superficial gas velocities between 4.7 and 10 m/s. These conditions lead to average solid concentrations in excess of 10 vol%, which corresponds to high-density CFB operation.

Novel multifunctional optical‐fiber probe: II. High‐density CFB measurements

AbstractThe novel parallel three‐fiber optical probe described in Part I of this article is applied to determine simultaneously local instantaneous solids volume concentration, velocity, and flux in multiphase suspensions. Radial distributions of local particle concentration, velocity, and flux and their fluctuations in a high‐density circulating fluidized bed (HDCFB) are presented. A strong correlation exists between fluctuations of local particle velocity and particle concentration. The optical probe is shown to provide a means of evaluating alternative measurement techniques that have commonly been employed in earlier studies.

Profiles of particle velocity and solids fraction in a high-density riser

Radial profiles of particle velocity and solids fraction in a high density circulating fluidized bed (HDCFB) at average cross-sectional solids fraction up to 0.21 were measured by an improved optical fiber laser doppler velocimeter and an optical fiber density sensor. The axial development of these radial profiles and the influence of operating conditions on the profiles were examined. The results showed that similar radial profiles of solids fraction exist in the HDCFB. The following Boltzman function can correlate well the solids fraction profile: (1 − ɛ)/(1 − ɛ) = 2.2 − (2)/(1 + exp(10· r/ R −7.665)). The radial profiles of particle velocity in HDCFB can also be described by the Boltzman function, that is, ( V p)/ ( U g) = (2.7)/(1 + exp(10· r/ R −10· X o) − 0.2). The Boltzman profiles of particle velocity in the high density operating regime was different with the parabolic shape operating in dilute phase regime. The local particle fluctuation velocity in the center of the riser increases with average solids concentration, while the fluctuation velocity decreases sharply as the radial position near the wall.

Radial solids fraction profiles in inlet region of high density circulating fluidized bed

AbstractThe radial solids fraction profiles in the inlet region of a high density circulating fluidized bed (HDCFB) are studied by a dual‐beam optic density sensor. Three kinds of inlet structures are employed to examine the axial developing process of the profiles. The acceleration effect of solids is analyzed by comparing the cross‐sectional average solids fraction obtained by the density sensor and differential pressure method. The results show that the radial solids fraction profile is largely influenced by the inlet structure. The length for the inlet effect region is no more than 1.5 m above the air injection.

The characteristics of cluster in a high density circulating fluidized bed

AbstractThe cluster images in a high density circulating fluidized bed (HDCFB) measured by a one‐dimensional optical fiber image analysis system are studied. The experimental results show that the characteristics of cluster in the riser vary greatly at different operating conditions. The radial cluster size, the cluster interception time and the probability of cluster appearance increase with bed density and decrease with gas velocity. Based on the analysis of the cluster image at various operating conditions, correlations of the radial cluster size, the cluster interception time and the probability of cluster appearance in a HDCFB are proposed. It is found that they are related to local solids concentration and gas velocity.