Bed Pressure Drop Research Articles

CFD analysis of fluidized beds using wastes from post-consumer carton packaging

Fluidized bed reactors can be used for fast pyrolysis of polyethylene-aluminum composite (LDPE/Al) to achieve good gas-solid mixing together with high rates of heat and mass transfer in bed columns. Pyrolysis of waste from post-consumer carton package (i.e., polyethylene-aluminum composite) reduces the manufacturing costs of new packages as it permits to produce them using the recovered aluminum and polyethylene in the process. Therefore, this research aims at modelling and analyzing the gas-solid flow in fluidized beds composed of sand and LDPE/Al mixtures. For all mixtures used, results show that gas-solid flow can be well described by the Eulerian-Eulerian Granular model coupled with the usage of parameterized Syamlal-O’Brien’s model for momentum transfer between phases. For bed mixtures with up to 20% LDPE/Al composite, the bed pressure drop can be predicted by the model with an error of less than 18%. In addition, for bed mixtures operating at air velocities higher than 50% minimum fluidization, significant segregation of LDPE/Al was not observed. This result indicates that bed mixtures can achieve a good gas-solid contact in the bubble fluidization regime and can be used as a fluidized bed reactor for pyrolysis of wastes from post-consumer carton packages.

Experimental validation of CFD hydrodynamic models for catalytic fast pyrolysis

Hydrodynamics for catalyst fluidization and circulation for Catalytic Fast Pyrolysis (CFP) were investigated using cold flow experiments and used to validate predictions from computational fluid dynamics (CFD) simulations. Proprietary fresh and equilibrium CFP catalyst were tested in lab-scale fixed fluidized bed (FFB) and circulating fluidized bed (CFB) setup. Bed expansion and pressure drop were recorded for fresh catalyst in the fixed fluidized bed setup for a range of gas velocities (0–0.04m/s) which encompassed minimum fluidization and bubbling regimes. Catalyst circulation in the circulating fluidized bed setup on the other hand, was quantified using mass holdup in the mock reactor, particle size distribution for this holdup and pressure profile for a range of transport gas velocities (0.094–0.561m/s, 10–60 SLPM) and catalyst feedrates (3, 6, 12kg/h; G=0.46, 0.92, 1.84kg/m2-s) for a single-inlet and single-outlet mock reactor configuration. Flow patterns and gas jet penetration through the mock reactor provided qualitative observations that were used for evaluating CFD models. CFD code based on CPFD method (Barracuda Virtual Reactor®) was used to simulate these gas-solid fluidized beds based on three-dimensional grids. Grid independence was established for both fixed and circulating fluidized bed models such that an average grid volume of 23–29mm3 was sufficient to effectively predict quantitative and qualitative observables. Several CFD parameter sets were tested to evaluate combinations of drag models, blended acceleration model, particle-wall interactions and collision and stress models. Parker-drag model with blended acceleration and advanced settings for particle-wall interactions, collision and stress parameters is determined to provide the best agreement with experimental results.

Experimental investigation on gas-solid hydrodynamics of coarse particles in a two-dimensional spouted bed

The gas-solid hydrodynamics of coarse particles in a two-dimensional spouted bed (2DSB) are measured using 6mm glass beads to obtain quantitative experimental results. A one-dimensional mathematical model based on gas-solid mass and momentum balance is used to characterize the spout-annulus interaction and to further interpret the experimental results. The superficial velocity and static bed height are in the range of 2.58 to 4.39m/s and 8.0 to 13.3cm, respectively. Pressure fluctuation periodicity of the bed pressure drop is characterized by power spectral density (PSD) analysis. The whole-field particle velocity profiles in 2DSB are obtained by high speed CCD camera and PIV technique. The results show that particle motion in the spout in the present 2DSB experiments is dominated by the balance of drag force, solid stress and gravitation. The existence of obvious solid stress from particle impaction and friction leads to energy dissipation, which limits the increase of vertical particle velocity along the spout axis (vyc) in the spout and results in a flat stage of vyc with slight fluctuation near the bottom of the fountain. The solid stress also contributes to the nearly flat peak of lateral profile of downward particle velocity in the annulus. These detailed results are helpful for understanding the gas hydrodynamics and particle motion behavior of coarse particles in 2DSB, and for verification of the extended CFD-DEM coupling method for particle size approaching fluid cell size.

Process intensification of gas–liquid downflow and upflow packed beds by a new low‐shear rotating reactor concept

In the present work, a new low‐shear rotating reactor concept was introduced for process intensification of heterogeneous catalytic reactions in cocurrent gas–liquid downflow and upflow packed‐bed reactors. To properly assess potential advantages of this new reactor concept, exhaustive hydrodynamic experiments were carried out using embedded low‐intrusive wire mesh sensors. The effect of the rotational velocity on liquid flow patterns in the bed cross‐section, liquid saturation, pressure drop, and regime transition was investigated. Furthermore, liquid residence time and Péclet number estimated by a stimulus‐response technique and a macro‐mixing model were presented and discussed with respect to the prevailing flow patterns. The results revealed that the column rotation induces different flow patterns in the cross‐section of the packed bed operating in a concurrent downflow or upflow mode. Moreover, the new reactor concept exhibits a more flexible adjustment of pressure drop, liquid saturation, liquid residence time, and back‐mixing at constant flow rates. © 2016 American Institute of Chemical Engineers AIChE J, 63: 283–294, 2017

Hydrodynamic modeling strategy for dense to dilute gas–solid fluidized beds

When investigating the hydrodynamic behavior of gas–solid flow systems, there are several options for the drag function, viscosity model, and other parameters. The low accuracy obtained with a random trial and error modeling strategy has led researchers to develop new drag models that are fine-tuned for their specific studies. However, besides the drag functions, an appropriate viscosity model together with radial distribution function have a great impact on the hydrodynamic modeling of fluidized beds. In this study, a detailed validation and verification task is conducted using three different experimental datasets to derive a modeling strategy for predicting hydrodynamic behavior in dense to dilute flow regimes of various fluidized beds. For this purpose, the steady-state Reynolds-averaged Navier–Stokes equations are solved in a finite volume scheme using the twoPhaseEulerFoam solver in the OpenFOAM 2.1.1 software. A comparative study of different drag and viscosity models enables an optimal modeling strategy to be determined for the accurate prediction of the bed pressure drop, bed expansion ratio, time-averaged solid hold-up, and bed height in various dense and dilute flow regimes. Our results show that the modeling strategy prescribed in this study is widely applicable for identifying the hydrodynamic characteristics of various gas–solid fluidized beds with different operating conditions.

Performance of drag models for simulation of fluidized beds with Geldart D particles

CFD-DEM algorithm which couples the macroscopic governing equations for gas phase and the second law of motions for individual particles in the system, is more accurate than the continuum based description of the particle phase. However, certain key challenges still remain which are the massive computational time and the selection of appropriate drag correlation. In the present work, coupled CFD-DEM simulations of two and three dimensional fluidized beds are carried out using the open source code MFIX, first, to understand the performance of different drag laws and second to investigate the effect of scaling down the actual geometry on fluidization behavior. The average gas and solid phase properties such as bed pressure drop, and the mean particle velocity, are determined and compared with the experimental results to benchmark the numerical simulations.

Experimentally validated numerical study of gas-solid vortex unit hydrodynamics

A three-dimensional numerical analysis of the flow in a Gas-Solid Vortex Unit (GSVU) is carried out. The numerical model is first validated by comparing the bed pressure drop and solids velocity with experimental data. Next, the influence of gas flow rate, solids density, and particle diameter on the pressure drop, solids velocity, bed void fraction and slip velocity between the two phases is studied. A stable, solids bed is achieved for the entire range of gas flow rates tested (0.1–0.6Nm3/s). No particle entrainment is observed when varying the solid density (1800–450kg/m3) or the particle diameter (2–0.5mm). A shift to bubbling fluidization regime is observed for small particle diameters (0.5mm). The observed changes in the GSVU flow patterns are discussed by analyzing the changes in the cumulative centrifugal to drag force ratio over the bed.

Experimental study and X-ray microtomography based CFD simulation for the characterization of pressure drop in sinter bed

In this study, pilot-scale sinter pot tests under three granulation moisture conditions were conducted. Porous structures of the sampled sinter cakes were reconstructed by X-ray microtomography and CFD simulations were applied based on the real geometry. The pressure drop of serial subzones along the sinter bed including sintered zone, high-temperature zone and humidified zone were discussed by analyzing the experimental measured airflow rate and CFD simulation results together. The results show that sinter cake porosity increases from 0.48 at 6.3wt% H2O to 0.59 at 7.3wt% H2O. The reconstructed sinter cakes show various complex porous structures, resulting in remarkable flow heterogeneity and complicated velocity field. A high porosity bed has many more gas channels and the flow distribution is developed better. The pressure drop in sintered zone is ∼1.5kPa/m, which is much lower compared to the high-temperature zone and humidified zone. The high-temperature zone where main reactions and bed structure changes happen has a controlling effect on the pressure drop across the sinter bed. The sintering bed permeability greatly determines the final productivity, which is affected by the bed temperature profile and bed porous structure.

Thermal Non-equilibrium Heat Transfer and Entropy Generation due to Natural Convection in a Cylindrical Enclosure with a Truncated Conical, Heat-Generating Porous Bed

Natural convection in enclosures driven by heat-generating porous media has diverse applications in fields like geothermal, chemical, thermal and nuclear energy. The present article focuses on heat transfer and entropy generation characteristics of a heat-generating porous bed, placed centrally within a fluid-filled cylindrical enclosure. Pressure drop and heat transfer in the porous bed are modelled using the Darcy–Brinkmann–Forchheimer approximation and the local thermal non-equilibrium model, respectively. Energy flux vectors have been utilised for visualising convective energy transfer within the enclosure. The study of a wide range of Rayleigh number (\(10^{7}\)–\(10^{11}\)) and Darcy number (\(10^{-6}\)–\(10^{-10}\)) reveals that heat transfer in the porous region can be classified into conduction-dominated and convection-dominated regimes. This is supplemented with an entropy generation analysis in order to identify and characterise the irreversibilities associated with the phenomenon. It is observed that entropy generation characteristics of the enclosure closely follow the above-mentioned regime demarcation. Numerical computations for the present study have been conducted using ANSYS FLUENT 14.5. The solid energy equation is solved as a user-defined scalar equation, while data related to energy flux vectors and entropy generation are obtained using user-defined functions.

CPFD 시뮬레이션을 통한 Shroud 노즐 및 수직 구조물이 설치된 기포 유동층 반응기 내에서의 기포 흐름 해석

본 연구에서는 내경 0.3 m, 높이 2.4 m인 기-고 유동층 반응기 내에서 수직 방향의 내부 구조물과 shroud 노즐 분산판이 기포 흐름에 미치는 영향을 CPFD (Computational Particle-Fluid Dynamics)를 이용하여 모델링을 수행하였다. 층 물질로는 Metal-grade 실리콘 입자(MG-Si)가 사용되었으며 <TEX>$d_p=149{\mu}m$</TEX>, <TEX>${\rho}_p=2,325kg/m^3$</TEX>, <TEX>$U_{mf}=0.02m/s$</TEX>이다. 전체 층물질의 양은 75 kg이며 정적층(static bed) 높이는 0.8 m이다. 수직 내부 구조물이 기포 상승속도에 미치는 영향을 파악하였다. 내부 구조물이 분산판으로부터 0.45 m 높이에 설치되었을 때 기포의 분쇄가 일어났다. 유동층의 압력강하 및 수직 고체체류량 분포는 내부 구조물의 영향을 크게 받지 않는 것으로 나타났다. 하지만 내부 구조물이 제트에 너무 가까운 경우 기포가 분쇄되지 않고 내부 구조물을 우회하여 상승하였으며 내부 구조물이 없는 경우나 0.45 m 높이에 설치된 경우에 비해 더 빠른 속도로 상승하였다. The effect of internal and shroud nozzle distributor to bubbling fluidized beds which has the size of <TEX>$0.3m-ID{\times}2.4m-high$</TEX> column was modeled by CPFD (Computational Particle-Fluid Dynamics). Metal-grade silicon particles (MG-Si) were used as bed materials which have <TEX>$d_p=149{\mu}m$</TEX>, <TEX>${\rho}_p=2,325kg/m^3$</TEX> and <TEX>$U_{mf}=0.02m/s$</TEX>. Total bed inventory and static bed height were 75 kg and 0.8 m, respectively. Effect of vertical internal on the bubble rising velocity was investigated. Bubbles were split by internal when the axial position of the internal from the distributor, z = 0.45 m. Bed pressure drop and axial solid holdup were not affected by internal. However, in the case that axial distance of internal from distributor was too close to jet penetration length, bubbles were not separated and bypassed internal, and faster than without internal or z = 0.45 m.

The influence of surface hydrophobicity in fluidization of ultrafine Al2O3 particles

A new approach is hereby presented to study the influence of surface properties on ultrafine alumina (Al2O3) particles towards a fluidized performance by changing the ultrafine particles (Al2O3) surface hydrophobic degree. A number of variables that affects fluidization performance of ultrafine particles (Al2O3), including hydrophobicity has been studied. The minimum full fluidization velocities (Umff), average bed pressure drop (ΔPa), standard deviation in pressure drop (σp) and so on, has been found to depend on the hydrophobic degree, which is a function of the ultrafine particles (Al2O3) surface properties. After treatment of the ultrafine particles (Al2O3) with a surfactant, the liquid-solid contact angle on the ultrafine particles (Al2O3) raises, thus changing the interaction between the ultrafine particles (Al2O3) and also between the ultrafine particles (Al2O3) with the micro-fluidized bed wall surface. Furthermore, some changes in important parameters such as particle size (simple agglomerate size), bulk density, flow function plots, maximum wall friction angle, BET parameters and so on were also evaluated. The fluidization of ultrafine particles is attractive, if the interaction forces between the ultrafine particles (Al2O3) and also between the ultrafine particles (Al2O3) with the micro-fluidized bed wall surface can be artificially controlled. Moreover, it is possible to achieve a better fluidization performance on the ultrafine particles (Al2O3). However, more investigation is required to improve the fluidization performance of the ultrafine particles.

The Effect of a Porous Medium on Fluidization Characteristics in Air Dense Medium Fluidized Bed

A porous material (sponge) was introduced into a typical air dense medium fluidized bed (ADMFB) as an air distribution layer to improve the uniformity and stability in fluidized beds. An ADMFB, an ADMFB with a secondary air distribution plate (PFB), and an ADMFB with a sponge (SFB) were compared by measuring the bed pressure drop and shooting image. Results shows that the introduction of a sponge reduces critical fluidization gas velocity and facilitates high-grade fluidization in beds. A sponge with smaller pores yields better effect on sustaining bed uniformity. In fluidized beds, the trend is that uniform stability is initially promoted and subsequently diminished as the fluidization number N increases. The optimal fluidization is reached when the fluidization number is 1.36. Fluidization quality is improved as the sponge thickness increases. In addition, the findings elucidate the behavior of air bubbles in a bed. The maximum diameter of an air bubble is 4.297 cm under the condition that the fluidization number is 1.36 in an ADMFB with a sponge.

CFD modeling of conical spouted beds for processing LDPE/Al composite

Several routes of reusing carton packaging waste have been proposed along the years. Pyrolysis of carton waste has been highlighted due to obtain high value-added products such as paraphinic oils and aluminum. Recently, the spouted bed has been investigated as a promising reactor for pyrolysis of solids. By suitably setting up pyrolysis, a fully investigative procedure of three phase (air-LDPE/Al-sand) flow dynamics in the conical spouted bed is needed. Thus, this research aims at analyzing the fluid dynamics behavior of conical spouted beds comprised mixtures of sand and LDPE/Al composite. Eulerian-granular multiphase model for describing the air-particle mixture flow in a conical spouted bed is used. By using the computational fluid dynamics and bi-dimensional structured grid, the solution of model equations is obtained. In addition, the following models are tested: laminar, mixture k-ε, per phase k-ε and disperse k-ε models. Based on data analyses, it can be identified that disperse k-ε model describes better air-particles flow in conical spouted beds comprised sand and LDPE/Al composites mixtures. The results show that bed pressure drop and fountain height are in agreement with those predicted by disperse k-ε model.

Enhanced growth of carbon nanotube bundles in a magnetically assisted fluidized bed chemical vapor deposition

The efficient fabrication of carbon nanotube (CNT) bundles is the prerequisite for their bulk applications. However, the poor fluidization behavior of metal nanoparticle catalysts hinders the effective growth of high quality CNT bundles. Herein magnetically assisted fluidized bed (MFB) chemical vapor deposition was proposed for bulk growth of long CNT arrays. By introducing a uniform magnetic field into the fluidized bed reactor, the fluidization behavior of the ultrafine catalyst particles derived from FeMgAl layered double hydroxides (LDHs) was significantly improved with reduced bed pressure drop and increased bed expansion. Possibly, the magnetic LDH-catalyst flakes were arranged orderly in a magnetic field, which facilitated the steady and durative growth of CNT bundles. Compared with the CNT products grown in a routine fluidized bed reactor, the as-produced CNTs in the MFB reactor were with a much higher yield of 9.1 gCNT/gcatal, improved graphitic degree, increased length up to ∼100 μm, together with smaller diameters and narrower diameter distribution as well. A gradient-magnetically assisted fluidized bed reactor was also explored for facile fabrication of very long CNT bundles over 100 μm.

Modeling and simulation of catalytic coal gasification in a pressurized jetting fluidized bed with embedded high-speed air jets

A pressurized jetting fluidized bed catalytic coal gasification process has been developed in China. In order to avoid the hot spot at the bed bottom occurring in the conventional coal gasifier, and to reassign the oxygen distribution in the bed, a pair of embedded high-speed air jets was employed in addition to a bottom central jet. To examine the effects of the embedded air jets on the gas–solid hydrodynamics and the gasification process, a comprehensive model has been constructed by using the two-fluid method incorporated with the kinetic theory of granular flows. The established model was applied in the simulation of a 0.3t/d process development unit. The bed expansion was validated against the empirical correlation, and the bed pressure drop agreed with the value obtained by the Ergun equation. Some key jetting characteristics, including the jet height, the distributions of particle content, gas and solid velocities were presented. Comprehensive analyses of the simulation results showed that the embedded jets were able to split large bubbles, and improve the gas–solid contacts. A simulation for a conventional gasifier with a single bottom jet was performed, and the results were compared with the proposed embedded jets gasifier. For both schemes, a high temperature zone was predicted along the jet pathway due to a rapid combustion of char particles, the maximum temperature was, however, controlled well below the ash softening temperature for the embedded jets gasifier, mainly due to distributed oxygen supply. The carbon and steam conversions for the conventional gasifier were predicted lower than that of the embedded jets gasifier, indicating the embedded jets have enhanced the reactor performance due to the improvement of gas–solid mixing.

Computational and experimental studies on bed dynamics of a gas–solid fluidized bed using Geldart-A particle: A comparison

ABSTRACTThe bed dynamics of a two-dimensional gas–solid fluidized bed is studied experimentally and computationally using Geldart-A particles. Commercial software ANSYS FLUENT 13 is used for computational studies. Unsteady behavior of gas–solid fluidized bed is simulated by using the Eulerian–Eulerian model coupled with the kinetic theory of granular flow. The two-equation standard k−ϵ model is used to describe the turbulent quantities. The simulation predictions are compared with experimentally observed data on volume fraction, bed pressure drop and bed expansion ratio. The results of simulations are found to be in close agreement with the experimental observations, implying that computational fluid dynamics (CFD) can be used for the design of an efficient bench-scale catalytic fluidized bed reactor.

CFD simulation of an agitated gas-fluidized bed: Effects of particle–particle restitution coefficient on the hydrodynamics

A gas–solid fluidized bed with a frame impeller was simulated using the computational fluid dynamics (CFD) method that integrated the Eulerian–Eulerian two-fluid model, the kinetic theory of granular flow (KTGF) and the multiple reference frame (MRF) model. The pressure fluctuation used to verify the numerical model depends strongly on the particle–particle restitution coefficient, while the bed pressure drop is independent of it. As the particle collisions become ideal, it reaches the homogenous fluidization regime with non-bubbling, low pressure fluctuations and granular temperatures. Meanwhile, the agitation of the frame impeller improves fluidization quality with reduced bubble diameters. The minimum limit of agitation speed for homogenous fluidization decreases with the increase of particle–particle restitution coefficient, i.e. a high agitation speed is needed to strengthen fluidization for inelastic particles, and the fluidization performs well enough for elastic particles that agitation has little influence.

Hydrodynamics of three-phase fluidization of homogeneous ternary mixture in a conical conduit — Experimental and statistical analysis

Hydrodynamics of conical fluidized bed differ from that of columnar beds by the fact that a velocity gradient exists along the axial direction of the bed. The gas–liquid–solid fluidized bed has emerged in recent years as one of the most promising devices for three-phase operations. Such a device is of considerable industrial importance as evident from its wide applications in chemical, refining, petrochemical, biochemical processing, pharmaceutical and food industries. To explore this, a series of experiments have been carried out for homogeneous well-mixed ternary mixtures of dolomite of varying compositions in a three-phase conical fluidized bed. The hydrodynamic characteristics determined included the bed pressure drop, bed fluctuation and bed expansion ratios. The single and combined effects of operating parameters such as superficial gas velocity, superficial liquid velocity, initial static bed height, average particle size and cone angle on the responses have been analyzed using response surface methodology (RSM). A 25 full factorial central composite experimental design has been employed. Analysis of variance (ANOVA) showed a high coefficient of determination value and satisfactory prediction second-order regression models have been derived. Experimental values of bed pressure drop, bed fluctuation and bed expansion ratios have been found to agree well with the developed correlations.

Prediction of bed pressure drop, fluctuation and expansion ratios for three-phase fluidization of ternary mixtures of dolomite in a conical conduit

AbstractHydrodynamics of conical fluidized bed differ from that of columnar beds by the fact that a velocity gradient exists along the axial direction of the bed. The gas–liquid–solid fluidized bed has emerged in recent years as one of the most promising devices for three-phase operations. Such a device is of considerable industrial importance as evident from its wide applications in chemical, refining, petrochemical, biochemical processing, pharmaceutical, and food industries. To explore this, experiments have been carried out to find the bed pressure drop, bed fluctuation, and expansion ratios for ternary mixtures of dolomite in a three-phase conical fluidized bed. The effect of superficial liquid and gas velocity, initial static bed height, average particle size, and cone angle on the above-mentioned three responses have been studied. Mathematical models have been developed for the responses using both dimensional and statistical analyses. The calculated values of the responses from the developed model...

Influence of the specularity coefficient on hydrodynamics and heat transfer in a conical fluidized bed combustor

In this paper, the hydrodynamics and wall-to-bed heat transfer of gas–solid flow in a conical fluidized bed were simulated using the Eulerian–Eulerian model. Sand with a diameter of 560μm was used as bed material where its properties were calculated by applying the kinetic theory of granular flow (KTGF). Different amounts of slip between particle and wall were given by varying the specularity coefficient. The effect of different values of specularity coefficient (0, 0.2, 0.5, and 1.0) on hydrodynamic and wall-to-bed heat transfer at superficial gas velocity of 0.65m/s was investigated. It was found that the wall-to-bed heat transfer coefficient decreases with increasing specularity coefficient. The specularity coefficient had no clear effect on the bed pressure drop. Different values of specularity coefficient resulted in almost the same hydrodynamic behavior near the wall and the center of bed.