Solid Mixing Behavior Research Articles

Assessment of heat transfer performance in a swirling fluidized bed with binary mixture

• Heat transfer in a SFB with binary mixture is numerically investigated. • Bed-to-wall and gas–solid heat transfer characteristics are analyzed. • Effect of swirling angle and operating velocity on heat transfer is evaluated. A swirling fluidized bed reactor has its potential in the enhancement of gas–solid mixing behaviors and heat transfer performance via the introduction of lateral momentum. A deep insight into heat transfer mechanism and enhancing effects in a swirling fluidized bed is essential for the reactor optimization and process regulation. In this work, the heat transfer performance of a swirling fluidized bed is investigated by means of a hybrid Eulerian-Lagrangian approach. The solid holdup and temperature fields are predicted. The bed-to-wall heat transfer and gas–solid interphase heat transfer are revealed. The results demonstrate that the increase of operating velocity is beneficial to mixing behaviors of binary mixture and enhancement of heat transfer performance. Meanwhile, the impact of swirling angle is also analyzed. When the inclination blade angle is changed from 45° to 12°, the bed to wall heat transfer coefficient can be improved by about 20%.

Particle Mixing in Bubbling Fluidized Bed Reactors with Immersed Heat Exchangers and Continuous Particle Exchange

Tracer experiments were conducted in a lab-scale cold flow model of 0.4 m by 0.2 m in cross section to investigate the solid mixing behavior of bubbling fluidized beds under continuous particle exc...

Discrete Element Study of Solid Mixing Behavior with Temperature Difference in Three-Dimensional Bubbling Fluidized Bed

This work numerically evaluates the mixing behavior of a binary mixture with the same physical properties but different temperatures in a three-dimensional bubbling fluidized bed based on the coupling of computational fluid dynamics and discrete element method. General mixing procedure and mixing index are adopted to investigate the macroscopic mixing behavior of the solid phase. Moreover, the effects of superficial velocity and the initial temperature configuration of particle groups on the temperature evolution of the system are discussed. The results show that three mixing mechanisms induced by the bubbles can be identified. The time required for a system to reach the temperature dynamic equilibrium is different from that required for the spatial mixing. With an increase in the superficial velocity, the mixing procedure is enhanced, and the temperature of each particle group distributes more uniformly. In addition, the effect of temperature configurations of different particle groups appears at the initial stage of solid mixing. This initial effect weakens slowly and even disappears with time evolution.

Numerical studies of solid–solid mixing behaviors in a downer reactor for coal pyrolysis

A downer reactor is ideal for fast pyrolysis during coal gasification as solids holdup distributions and residence time distribution in the reactor are fairly uniform and residence time is short. Because pyrolysis is a rapid reaction, in the downer solid–solid mixing is critical to promote pyrolysis reaction. Thus, solid–solid mixing in the downer was studied through numerical modeling in this study. An Eulerian–Lagrangian method was applied and implemented by combining Computational Fluid Dynamics and Discrete Element Method (CFD–DEM). The heat carrying media of silica sand was modeled to flow from the top of the downer while coal particles were introduced through two lateral nozzles in normal and tangential arrangements. Numerical results showed that tangential arrangement had lower solids holdups while higher particle velocities than normal arrangement. Mixing among binary solid particles was poor at the entrance of downer. The extent of mixing increased rapidly and approached an almost constant value in the downstream regions of the downer. High air velocity and small solid particle size could lead to better mixing for both normal and tangential arrangements. But tangential arrangement had better mixing of binary solid particles than normal arrangement, and when the particle sizes were reduced from 2 to 1mm, the influence of nozzle arrangement was reduced.

Influence of tube configuration on the gas–solid hydrodynamics of an internally circulating fluidized bed: A discrete element study

Three-dimensional modeling of the gas–solid flow in an internally circulating fluidized bed is conducted by means of the computational fluid dynamics combined with the discrete element method to explore the effect of the tube bundle on the gas–solid hydrodynamics of the system. Gas motion is resolved at the computational grid level, while solid motion is obtained in the Lagrangian view. The influences of tube bundle on the interaction of two chambers, the circulating and resident behaviors of solid phase are evaluated. Moreover, solid mixing behavior and tube erosion are discussed. The results show that the immersed tubes obviously enlarge the gas/solid velocity in the vicinity of the partition plate and lower the interaction intensity of the two chambers. Solid cycle time of the system with or without tubes processes a log-normal distribution. Furthermore, different resident behaviors of the solid phase can be obtained in the two chambers, and inserting the tube bundle in the reactor chamber enlarges the solid residence times of the two chambers. More tubes inserted into the bed enlarge both the cycle time and residence time of the solid phase. On the other hand, the presence of tube bundle enhances the solid mixing intensity. Finally, different erosion distributions appear around the circumferential positions of each tube, and the latent erosion pattern can be identified from the distribution of the time-averaged solid flux.

Computational Fluid Dynamics-Discrete Element Method Investigation of Solid Mixing Characteristics in an Internally Circulating Fluidized Bed

Solid mixing dynamics is of vital important to the processing rate, achievable homogeneity and product quality in the related industries of granular material. In this paper, solid mixing behaviors within a baffle-type internally circulating fluidized bed (ICFB) are numerically investigated using a three-dimensional computational fluid dynamics-discrete element method (CFD-DEM), in which the gas motion is modeled by means of large eddy simulation (LES) while the solid kinematics is handled by a soft-sphere model. On the basis of the simulation results, typical snapshots of granular mixing dynamics in the bed are extracted to reveal the mixing process of different initial segregation conditions. The mixing quality, which is described by Lacey mixing index, is evaluated. Meanwhile, the solid circulation pattern is illustrated by tracking tracer positions both in the three-dimensional bed and along the horizontal and vertical directions as simulation time advances. Furthermore, the influence of different para...

A study of gas and solid mixing behaviors in a three partitioned fluidized bed

A previously unknown partitioned fluidized bed gasifier (PtFBG) has been developed for improving coal gasification performance. The basic concept of the PtFBG is a fluidized bed divided into two parts, a gasifier and a combustor, by a partitioned wall. Char is burnt in the combustor and the generated heat is supplied to the gasifier along with the bed materials. During that time, highly concentrated CO2 is inevitably generated in the combustor. Therefore, vigorous solid mixing is an essential precondition as well as minimizing horizontal gas mixing. In this study, gas and solid mixing behaviors were verified in a cold model three partitioned fluidized bed (3-PtFB). Glass beads with an average diameter of 150μm and a particle density of 2500kg/m3 were used as bed materials. For the gas mixing experiments, CO2 and N2 were introduced into the beds through each distributor. Then, outlet gas flow rates and concentrations were measured by gas flow meters and an IR gas analyzer respectively. The calculated gas exchange ratios ranged from 3% to 10% with varying gas flow rates. For the solid mixing experiments, 1000μm polypropylene particles with a density of 883kg/m3 were continuously fed into the reactor. Then, the polypropylene particles were distributed to the entire beds evenly. Solid mixing behaviors were very analogous to liquid mixing behaviors in a continuous stirred tank reactor (CSTR).

Investigation of solid mixing mechanisms in a bubbling fluidized bed using a DEM—CFD approach

ABSTRACTSolid mixing has a great influence on heat transfer and reaction processes in fluidized bed reactors; however, a review shows that the quantitative results are scattered. In this work, the mechanisms of solid mixing in a bubbling fluidized bed are investigated by numerical simulation. A three‐dimensional discrete element model (DEM)–computational fluid dynamic (CFD) simulation tool is developed and employed to investigate solid mixing behaviors due to the passage of isolated bubbles, continuous bubbles, and interacting bubbles. The vital role of bubbles on solid mixing has been clearly demonstrated. The results show that: (a) solid mixing in the vertical direction is dominant; (b) when a bubble forms near the air distributor, the continuous jet directly penetrates into the bubble, leading the bubble growing up; (c) lateral solid transport is promoted by bubble interaction, especially when the bubble sizes are different. © 2011 Curtin University of Technology and John Wiley & Sons, Ltd.