Fluidized Bed Of FCC Particles Research Articles

Effect of particle properties on forces on an immersed horizontal slat during start-up of a fluidized bed

To study the effect of particle properties on forces acting on immersed internals during start-up of fluidized bed reactors, forces on a horizontal slat were measured in fluidized beds of four types of particulate materials (i.e. FCC catalyst particles and three silica sands of different sizes). The forces were measured by strain gauges adhered on the slat surface. Transient pressure signals were also measured synchronously by pressure transducers. The experimental results show that a high upward stress impulse appears in the measured stress signals during start-up of the fluidized beds of FCC particles and the two smaller-size silica sands. The peak stress impulse was several times larger than the magnitude of stress signals measured during steady state operation. However, in the fluidized bed of the coarsest silica sand, the peak stress impulse during start-up was much smaller, which is close to the magnitudes measured during steady state fluidization. For the three sizes of silica sands, the effective peak load densities on the slat decreased significantly with increasing particle size and were approximately proportional to the calculated fixed bed pressure drops of these beds based on Ergun equation. On the other hand, during start-up of the fluidized bed of FCC particles, particles near the bottom of the bed were difficult to mobilize and collapse, resulting in the bed expanding considerably and requiring more time to transform into a fluidized state. Potentially, internals immersed in the fluidized beds of small particles are easier to damage during the start-up stage. Effective measures (e.g. starting up the bed gradually with small increments of superficial gas velocity) must be implemented to reduce the possible risks for internals immersed in these fluidized beds.

Effects of operating temperature on the bubble phase properties in fluidized beds of FCC particles

Some important characteristics of the bubbling regime, relevant to a sample of FCC catalyst with an average size of 110μm belonging to group A of Geldart's classification, were investigated at 100, 400 and 700°C. Non-invasive optical techniques were employed for acquiring data of bed collapse tests as well as images of bubble eruption at the free surface of the bed. The experiments, performed at varying fluidization velocity and bed aspect ratio, show that a smoother regime of bubbling is observed at superambient temperature, where thermal enhancement of interparticle forces leads to higher porosity and lower interstitial flow in the emulsion phase while smaller bubbles are observed to flow across the bed at higher frequencies.

Computational Investigation of Hydrodynamics in a Turbulent Fluidized Bed of FCC Particles

The hydrodynamics in a high and narrow turbulent fluidized bed of FCC particles is investigated by using computational fluid dynamics (CFD) method. The axial bed density, particle volume fraction and particle velocity in the fluidized bed are predicted and compared with the experiments. The results indicate that there exist two different coexisting regions in the bed: a bottom dense and an upper dilute region. As increasing superficial gas velocity, bed density decreases in the dense phase whilst increases in the dilute phase. In this high and narrow turbulent fluidized bed, however, the bed density decreases sharply even in the dense phase because of the wall restriction. The hydrodynamics resembles slug flow at the initial fluidization stage; as a steady fluidization is achieved, the fluidized bed produces larger bubbles than the conventional one.

Flow structures inside a large-scale turbulent fluidized bed of FCC particles: Eulerian simulation with an EMMS-based sub-grid scale model

Turbulent fluidized bed reactors are widely used in industry. However, CFD simulations of the hydrodynamic characteristics of these reactors are relatively sparse, despite the urgent demand from industry. To address this problem, Eulerian simulations with an EMMS-based sub-grid scale model, accounting for the effect of sub-grid scale structures on the inter-phase friction, are performed to study the hydrodynamics inside a large-scale turbulent fluidized bed of FCC particles. It is shown that the simulated axial and radial solid concentration profiles, entrained solid fluxes and standard deviation of the solid concentration fluctuation agreed well with experimental data available in the literature. In-depth analysis of time-averaged particle velocity and solid concentration shows that a dense-suspension upflow regime coexists with fast fluidization regime in this bed, which is reminiscent of the hydrodynamic characteristics in high-density circulating fluidized bed (CFB) risers, even though they are operated in different fluidization regimes. The Reynolds stresses in turbulent fluidized beds are anisotropic, but the degree of anisotropy is far less pronounced than the reported values in CFB risers. It was also found that the solid concentration fluctuation and axial particle velocity fluctuation are strongly correlated.

EMMS-based Eulerian simulation on the hydrodynamics of a bubbling fluidized bed with FCC particles

Although great progress has been made in modeling the bubbling fluidization of Geldart B and D particles using standard Eulerian approach, recent studies have shown that suitable sub-grid scale models should be introduced to improve the simulation on the hydrodynamics of Geldart A particles. In this study, the flow structures inside a bubbling fluidized bed of FCC particles are simulated in an Eulerian approach employing the energy minimization multi-scale (EMMS) model (Chemical Engineering Science, 2008, 63: 1553–1571) as the sub-grid scale model for effective inter-phase drag force, using an implicit cluster diameter expression. It was shown that the experimentally found axial and radial solid concentration profiles and radial particle velocity profiles can be well reproduced.

CFD modeling and validation of the turbulent fluidized bed of FCC particles

AbstractAn experimental and computational study is presented on the hydrodynamic characteristics of FCC particles in a turbulent fluidized bed. Based on the Eulerian/Eulerian model, a computational fluid dynamics (CFD) model incorporating a modified gas‐solid drag model has been presented, and the model parameters are examined by using a commercial CFD software package (FLUENT 6.2.16). Relative to other drag models, the modified one gives a reasonable hydrodynamic prediction in comparison with experimental data. The hydrodynamics show more sensitive to the coefficient of restitution than to the flow models and kinetics theories. Experimental and numerical results indicate that there exist two different coexisting regions in the turbulent fluidized bed: a bottom dense, bubbling region and a dilute, dispersed flow region. At low‐gas velocity, solid‐volume fractions show high near the wall region, and low in the center of the bed. Increasing gas velocity aggravates the turbulent disorder in the turbulent fluidized bed, resulting in an irregularity of the radial particle concentration profile. © 2009 American Institute of Chemical Engineers AIChE J, 2009

Evaluating solids dispersion in fluidized beds of fine particles by gas backmixing experiments

In this study, solids dispersion coefficients in fluidized beds of fine Group A particles were derived by modeling the data obtained in steady-state gas tracer experiments with a two-phase gas mixing model proposed by May [May, W.G., 1959, Fluidized-bed reactor studies. Chem Eng Prog 55 (12): 49–56]. Both baffle-free and baffled fluidized beds of FCC particles were tested in a two-dimensional cold model column with broad operating conditions covering both bubbling and turbulent flow regimes. Modeled results showed the axial solids dispersion coefficient of baffle-free fluidized bed ranges from 0.01 to 0.13 m 2/s, increasing monotonously with increasing superficial gas velocity in both bubbling and turbulent flow regimes, agreeing well with previous solids-tracer studies with similar particles and operating conditions. The modeled results in baffled fluidized beds showed that the axial solids dispersion coefficient is significantly reduced when multilayer louver baffles are inserted in the bed, demonstrating louver baffle's strong suppression on solids backmixing. The suppression on solids backmixing by louver baffles augments with increasing superficial gas velocity. However, for the multilayer mesh-grid baffles, effective suppression of solids backmixing only functions at u 0 ≥ 0.6 m/s. As superficial gas velocity increases, the axial solids dispersion coefficients change in a similar trend as the axial gas dispersion coefficients in both baffle-free and baffled fluidized beds. The effects of different baffles on solids mixing are also similar to those on gas mixing. This demonstrates the close relationship between gas mixing and solids mixing in fluidized bed of fine particles.

Effect of louver baffles on hydrodynamics and gas mixing in a fluidized bed of FCC particles

The effect of louver baffles on the particle concentration profiles, pressure fluctuations, bed expansion, and gas mixing of a fluidized bed was investigated in a transparent 2-D column of cross-section 500×30 mm and height 6 m over a broad range of operating conditions covering both the bubbling and turbulent flow regimes. Visual observations, pressure fluctuations and steady gas tracer experiments showed that louver baffles can break bubbles, as indicted by the lower amplitudes and higher mean frequencies of differential pressure fluctuations, but they were only effective for superficial gas velocities <∼0.7 m/s for the FCC particles considered in this study. The ability of louver baffles to break bubbles reached a maximum near the onset of the turbulent flow regime. A gas cushion of low particle concentration appeared below the louver baffle, and its height increased with increasing superficial gas velocity, indicating increasing suppression of solids backmixing. Internal emulsion circulation was promoted above the louver baffle, causing an uneven distribution of gas flow. The addition of louver baffles reduced the upstream tracer gas concentrations by 80–90%, indicating a significant decrease in the backmixing fluxes of both gas and solids across the baffle layer. The tracer gas concentrations above the louver baffles increased resulting from the promoted emulsion circulation by louver baffles.

Porosity and Pressure Waves in a Fluidized Bed of FCC Particles

The speeds of motion of compression waves through a fluidized bed of 75-μm FCC particles between the minimum bubbling and the minimum fluidization velocities were determined by measuring the times of arrival of compression zones using a γ-ray densitometer. The theory of characteristics shows that this speed, which is on the order of 1.4 m/s, represents the maximum velocity of discharge of nonfluidized FCC particles. The velocity of these compression waves was used to calculate the solids stress modulus of the fluidized bed via the one-dimensional mass and momentum balances for granular flow. In addition, measurements of the pressure wave propagation produced a mixture sonic velocity that is an order of magnitude higher than the speeds of the solids compression wave. The sonic velocity of the homogeneous mixture, which is on the order of 20 m/s, gives the maximum achievable velocity for the circulation of FCC particles in a standpipe of a refinery. This study shows that there are two distinct waves that tr...

Numerical simulation of horizontal jet penetration in a three-dimensional fluidized bed

The introduction of reactant gas as a jet into a fluidized bed chemical reactor is often encountered in various industrial applications. Understanding the hydrodynamics of the gas and solid flow resulting from the gas jet can have considerable significance in improving the reactor design and process optimization. In this work, a three-dimensional numerical simulation of a single horizontal gas jet into a cylindrical gas–solid fluidized bed of laboratory scale is conducted. A scaled drag model is proposed and implemented into the simulation of a fluidized bed of FCC particles. The gas and particles flow in the fluidized bed is investigated by analyzing the transient simulation results. The jet penetration lengths of different jet velocities have been obtained and compared with published experimental data as well as with predictions of empirical correlations. The predictions by several empirical correlations are discussed. A good agreement between the numerical simulation and experimental results has been achieved.

Effects of temperature on local two-phase flow structure in bubbling and turbulent fluidized beds of FCC particles

A novel high temperature optical fiber probe has been developed to study the effects of bed temperature on the local two-phase flow structure in a pilot scale fluidized bed of the FCC particles with bed temperatures ranging from 25°C to 420°C, covering both the bubbling and turbulent fluidization regimes. The results show that fluidization is enhanced and fluctuations of the local two-phase flow structure become more intense with increasing bed temperature. At constant superficial gas velocities, the averaged local particle concentration, the dense phase fraction and particle concentration in the dense phase decrease with increasing bed temperature, whereas both the frequency of the dilute/dense phase cycle and the ratio of the dilute phase duration to the dense phase duration increase. In addition, the effects of temperature on the dilute phase depend on superficial gas velocity. The conventional two-phase models fail to predict these changes of the local flow structure with temperature, which may be explained by the fact that the role of interparticle forces is neglected at different bed temperatures. Indeed, fluidization behaviors of the FCC particles tested increasingly shift from typical Geldart A towards B with increasing temperature due to a decrease of the interparticle attractive forces and a simultaneous increase of interparticle repulsive forces.

Characterization of dynamic behaviour in gas–solid turbulent fluidized bed using chaos and wavelet analyses

The hydrodynamics of fluidized beds of FCC particles in a column of diameter 0.29 m were investigated based on gauge and differential pressure signals, as well as optical voidage probe data for conditions approaching and beyond the onset of the turbulent fluidization flow regime. It is shown that any treatment of the system as a dense phase/dilute phase binary is oversimplified given the broad spectrum of properties and the lack of clear delineation between two separate phases. On the other hand, chaos analysis indicates complex bifractal behaviour, with two separate values of the Hurst exponent corresponding to different scales of motion, while wavelet analysis is successful in again decomposing signals into scales of motion associated with voids and separate particles.

Characterization of dynamic behaviour in gas–solid turbulent fluidized bed using chaos and wavelet analyses

The hydrodynamics of fluidized beds of FCC particles in a column of diameter 0.29 m were investigated based on gauge and differential pressure signals, as well as optical voidage probe data for conditions approaching and beyond the onset of the turbulent fluidization flow regime. It is shown that any treatment of the system as a dense phase/dilute phase binary is oversimplified given the broad spectrum of properties and the lack of clear delineation between two separate phases. On the other hand, chaos analysis indicates complex bifractal behaviour, with two separate values of the Hurst exponent corresponding to different scales of motion, while wavelet analysis is successful in again decomposing signals into scales of motion associated with voids and separate particles.