Fluidizing Gas Flow Rate Research Articles

Analysis on chaotic nature of a pressurized spout‐fluid bed by information theory based Shannon entropy

AbstractPressure fluctuation measurements were conducted in a pressurized spout‐fluid bed of 100 mm in diameter with bed pressure up to 0.4 MPa. Information theory based Shannon entropy analysis of pressure fluctuation was developed to recognize chaotic nature of gas–solid flows. It was found that both Shannon entropy and Shannon entropy increment rate were pronounced with the flow patterns and transitions. Moreover, Shannon entropy increment rate could describe the change of chaotic nature with operating conditions. Increasing chaotic nature was observed with increasing fluidizing gas flow rate. When elevating bed pressure, stable flow regions became large and the chaotic nature was weakened.

Cold Flow Model Study on a Dual Circulating Fluidized Bed (DCFB) System for Chemical Looping Processes

AbstractA cold flow model was built and operated in order to study the fluid dynamics of a 120‐kW chemical looping pilot rig for gaseous fuels. The system consists of two interconnected circulating fluidized bed reactors with globally circulating bed material (dual circulating fluidized bed – DCFB). Gas mixing between the two reactors is prevented by moderately fluidized loop seals. In the cold model, both reactors are fluidized with air at two stages to control the solids circulation. The data presented focus on the solids circulation rates and on pressure profiles of both reactors, depending on selected operating parameters like fluidization gas flow rate, level of gas introduction, loop seal fluidization, and solids inventory. The results show high solids circulations in the global loop between the two reactors for the scaled base case operation conditions already at low bed inventories.

Numerical investigation of gas-solid heat transfer in rotating fluidized beds in a static geometry

Gas–solid heat transfer in rotating fluidized beds in a static geometry is theoretically and numerically investigated. Computational fluid dynamics (CFD) simulations of the particle bed temperature response to a step change in the fluidization gas temperature are presented to illustrate the gas–solid heat transfer characteristics. A comparison with conventional fluidized beds is made. Rotating fluidized beds in a static geometry can operate at centrifugal forces multiple times gravity, allowing increased gas–solid slip velocities and resulting gas–solid heat transfer coefficients. The high ratio of the cylindrically shaped particle bed “width” to “height” allows a further increase of the specific fluidization gas flow rates. The higher specific fluidization gas flow rates and increased gas–solid slip velocities drastically increase the rate of gas–solid heat transfer in rotating fluidized beds in a static geometry. Furthermore, both the centrifugal force and the counteracting radial gas–solid drag force being influenced by the fluidization gas flow rate in a similar way, rotating fluidized beds in a static geometry offer extreme flexibility with respect to the fluidization gas flow rate and the related cooling or heating. Finally, the uniformity of the particle bed temperature is improved by the tangential fluidization and resulting rotational motion of the particle bed.

Pyrolysis characteristics of Oriental white oak: Kinetic study and fast pyrolysis in a fluidized bed with an improved reaction system

The kinetic parameters for the pyrolysis of Oriental white oak were evaluated by thermogravimetric analysis (TGA). The white oak was pyrolyzed in a fluidized bed reactor with a two-staged char separation system under a variety of operating conditions. The influence of the pyrolysis conditions on the chemical and physical characteristics of the bio-oil was also examined. TGA showed that the Oriental white oak decomposed at temperatures ranging from 250 to 400 °C. The apparent activation energy ranged from 160 to 777 kJ mol − 1 . The optimal pyrolysis temperature for the production of bio-oil in the fluidized bed unit was between 400 and 450 °C. A much smaller and larger feed size adversely affected the production of bio-oil. A higher fluidizing gas flow and higher biomass feeding rate were more effective in the production of bio-oil but the above flow rates did not affect the bio-oil yields significantly. Recycling a part of the product gas as a fluidizing medium resulted the highest bio-oil yield of 60 wt.%. In addition, high-quality bio-oil with a low solid content was produced using a hot filter as well as a cyclone. With exception of the pyrolysis temperature, the other pyrolysis conditions did not significantly affect the chemical and physical characteristics of the resulting bio-oil.

Metallic salt deposition on porous particles by dry impregnation in fluidized bed: Effect of drying conditions on metallic nanoparticles distribution

In this paper, the fluidized bed dry impregnation of coarse alumina porous particles by a metallic salt, manganese nitrate, is investigated. In this technique the penetration of each drop of metallic solution in the porous solid particle and solvent evaporation takes place at the same time, then liquid diffusion phenomenon is negligible. So, the metal loading is directly related to the operating time and liquid flow rate and concentration. It is found that the competition between two phenomena, drying and capillary flow, controls the deposit location. In order to determine the importance of the solvent evaporation process compared to the solution penetration by capillarity, an impregnation module, IM, was defined as the ratio between the drying characteristic time and a capillary penetration time. The adequate choice of the operating conditions (bed temperature, liquid and fluidization gas flow rate) allows a uniform deposition of the metallic precursor inside the porous matrix or on the support surface. The impregnation under slow drying conditions (IM ≥ 10 and solvent content in the bed atmosphere τ s ≥ 0.2) leads to a homogeneous deposition inside the pores. Under fast drying conditions (IM < 5 and τ s < 0.2), the deposit is located at the particle external surface. In the case of slow drying, the impregnation kinetics can be represented by a “shrinking core” model. The critical impregnation rate is controlled by the competition between dissolution and recrystallization at the elementary grain scale. The size of the metal crystallites depends on the pore mean size and size distribution and on the drying rate.

Experimental study of fluidization of 1G‐Geldart D‐type particles in a rotating fluidized bed with a rotating chimney

The new, concept of rotating chimney for rotating fluidized beds is experimentally investigated using a step-response technique. The chimney rotates in the same sense as the rotating particle bed, but not necessarily at the same rotational speed. In particular, chimney rotational speeds such that the particles at contact with the chimney obtain a rotational speed higher than the average particle bed rotational speed are possible. The increase of the centrifugal force in the vicinity of the chimney allows to reduce solids losses via the chimney and to build up a higher, solids loading in the fluidization chamber at given fluidization gas flow and solids feeding rates. Alternatively, at given solids loading and fluidization gas flow, rate, the rotating chimney allows to control the particle residence time in the fluidization chamber. The influence of the chimney rotational speed and the fluidization gas flow rate is studied using 1G-Geldart D-type particles. (c) 2008 American Institute of Chemical Engineers.

Experimental investigation of a rotating fluidized bed in a static geometry

Rotating fluidized beds in a static geometry are based on the new concept of injecting the fluidization gas tangentially in the fluidization chamber, via multiple gas inlet slots in its cylindrical outer wall. The tangential injection of the fluidization gas fluidizes the particles tangentially and induces a rotating motion, generating a centrifugal field. Radial fluidization of the particle bed is created by introducing a radially inwards motion of the fluidization gas, towards a centrally positioned chimney. Correctly balancing the centrifugal force and the radial gas–solid drag force requires an optimization of the fluidization chamber design for each given type of particles. Solids feeding and removal can be continuous, via one of the end plates of the fluidization chamber. The fluidization behavior of both large diameter, low density polymer particles and small diameter, higher density salt particles is investigated at different solids loadings in a 24 cm diameter, 13.5 cm long non-optimized fluidization chamber. Scale-up to a 36 cm diameter fluidization chamber is illustrated. Provided that the solids loading is sufficiently high, a stable rotating fluidized bed in a static geometry is obtained. This requires to minimize the solids losses via the chimney. With the polymer particles, a dense and uniform bed is observed, whereas with the salt particles a less dense and less uniform bubbling bed is observed. Solids losses via the chimney are much more pronounced with the salt than with the polymer particles. Slugging and channeling occur at too low solids loadings. The hydrostatic gas phase pressure profiles along the outer cylindrical wall of the fluidization chamber are a good indicator of the particle bed uniformity and of channeling and slugging. The fluidization gas flow rate has only a minor effect on the occurrence of channeling and slugging, the solids loading in the fluidization chamber being the determining factor for obtaining a stable and uniform rotating fluidized bed in a static geometry.

Perspectives for Fluidized Bed Nuclear Reactor Technology using Rotating Fluidized Beds in a Static Geometry

The new concept of a rotating fluidized bed in a static geometry opens perspectives for fluidized bed nuclear reactor technology and is experimentally and numerically investigated. With conventional fluidized bed technology, the maximum attainable power is rather limited and maximum at a certain fluidization gas flow rate. Using a rotating fluidized bed in a static geometry, the fluidization gas drives both the centrifugal force and the counteracting radial gas-solid drag force in a similar way. This allows operating the reactor at any chosen sufficiently high solids loading over a much wider fluidization gas flow rate range and in particular at much higher fluidization gas flow rates than with conventional fluidized bed reactor technology, offering increased flexibility with respect to cooling via the fluidization gas. Furthermore, the centrifugal force can be a multiple of earth gravity, allowing radial gas-solid slip velocities much higher than in conventional fluidized beds. The latter result in gas-solid heat transfer coefficients one or multiple orders of magnitude higher than in conventional fluidized beds. The combination of dense operation and high fluidization gas flow rates allows process intensification and a more compact reactor design.

Experimental and Model Investigations on Gas Mixing Behaviors of Spout-fluid Beds

Steady-state tracer gas measurements were carried out to study the gas mixing behaviors in a spout-fluid bed with a cross section of 0.3 m x 0.03 m and height of 2 m. Two different tracer gases were simultaneously injected, one was injected into the spouting gas flow and the other was injected into the fluidizing gas flow. Radial tracer gas concentrations at various bed elevations under different flow patterns were measured. The mechanism of gas mixing was discussed based on the racer gas concentrations and the flow patterns recorded by a high-resolution digital CCD camera. It was found that gas mixing in spout-fluid beds was due to both convection and dispersion. A three-region mixing model was developed to describe the gas mixing in the spout-fluid bed. The spout jet region and the boundary region were modeled with a mass transfer model; the annular region was modeled with a dispersion model. Effects of spouting gas and fluidizing gas flow rate on the gas exchange between the spout jet and the annular dense region, and the gas dispersion in the annular dense region were examined with flow patterns. The results showed that increase in spouting gas velocity and fluidizing gas flow rate could both promote the gas mixing in spout-fluid beds. The gas-solid flow pattern transited from internal jet to spouting to spout-fluidizing, and the gases were better mixed. But the gases became poorly mixed when the flow pattern transited from stable flow to instable flow.

Experimental investigation of particle mixing behavior in a large spout–fluid bed

Experimental investigations on particle mixing in a larger spout–fluid bed with cross section of 1 m × 0.05 m and height of 4 m were carried out. The axial and radial distributions of tracer particle under different spouting gas velocities and fluidizing gas flow rates were measured. The particle mixing behaviors were studied according to the concentrations of tracer particles and flow patterns. Besides, the mechanism of particle mixing was preliminarily discussed. It is found that increase in spouting gas velocity and fluidizing gas flow rate can both promote the particle mixing in spout–fluid beds. Particle mixing in spout–fluid beds is due to three mechanisms: diffusion, transportation and circulation, and bubbles action. However, it changes significantly with increasing fluidizing gas flow rate and spouting gas velocity. Diffusion, transportation and circulation dominate the particle mixing at low fluidizing gas flow rates, while bubble actions dominant the particle mixing at large fluidizing gas flow rates. When increasing the spouting gas velocity, diffusion, transportation and circulation dominant the particle mixing.

CFB air-blown flash pyrolysis. Part II: Operation and experimental results

The main operational characteristics of a novel gasifier operating in the CFB mode are outlined in this paper, based on the experimental results from a total of 11 runs in the pyrolysis mode. The operation runs constituted the main experiments in the CFB reactor, carried out to derive meaningful mass balance and additional operational data for the CFB pyrolyzer. The experiments were conducted in varying operating conditions determined by the most important parameters, i.e., biomass flowrate, fluidizing gas flowrate, air factor, initial bed inventory), temperature in the CFB riser, vapor residence time and nominal air factor – or equivalence ratio, Sb. The results obtained showed that the reactor configuration successfully operated as a biomass fast pyrolysis system to maximize liquid yields reaching 61.50 wt% on a maf biomass basis, with the novel feature of providing for autothermal operation at 500 °C and with 0.46 s gas–vapor residence time, by utilizing the by-product char energy content in a single reactor. The reactor provides a very high specific throughput of 1.12–1.48 kg/h m2 and the lowest gas-to-feed ratio of 1.3–1.9 kg gas/kg feed compared to other fast pyrolysis processes based on pneumatic reactors and has a good scale-up potential, providing significant capital cost reduction. Results to date suggest that the process is limited by the extent of char combustion. Future work should address resizing of the char combustor to increase overall system capacity, improve the solid separation and substantially increase liquid recovery.

Three-dimensional Simulation of Gas/Solid Flow in Spout-fluid Beds with Kinetic Theory of Granular Flow

A three-dimensional Eulerian multiphase model, with closure law according to the kinetic theory of granular flow, was used to study the gas/solid flow behaviors in spout-fluid beds. The influences of the coefficient of restitution due to non-ideal particle collisions on the simulated results were tested. It is demonstrated that the simulated result is strongly affected by the coefficient of restitution. Comparison of simulations with experiments in a small spout-fluid bed showed that an appropriate coefficient of restitution of 0.93 was necessary to simulate the flow characteristics in an underdesigned large size of spout-fluid bed coal gasifier with diameter of 1m and height of 6m. The internal jet and gas/solid flow patterns at different operating conditions were obtained. The simulations show that an optimal gas/solid flow pattern for coal gasification is found when the spouting gas flow rate is equal to the fluidizing gas flow rate and the total of them is two and a half times the minimum fluidizing gas flow rate. Besides, the radial distributions of particle velocity and gas velocity show similar tendencies; the radial distributions of particle phase pressure due to particle collisions and the particle pseudo-temperature corresponding to the macroscopic kinetic energy of the random particle motion also show similar tendencies. These indicate that both gas drag force and particle collisions dominate the movement of particles.

Maximum spoutable bed height of spout-fluid bed

Experimental study on the maximum spoutable bed height of a spout-fluid bed (cross-section of 0.3 m × 0.03 m and height of 2 m) packed with Geldart group D particles has been carried out. The effects of particle size, spout nozzle size and fluidizing gas flow rate on the maximum spoutable bed height were studied. Experimental data were compared to some published experiments and predictions. The results show that the maximum spoutable bed height of spout-fluid bed decreases with increasing particle size and spout nozzle size, which appears the same trend to that of spouted beds. The increasing of fluidizing gas flow rate leads to a sharply decrease in the maximum spoutable bed height. The existent correlations of the maximum spoutable bed height in the literature were observed to involve large discrepancies. Additionally, the flow characteristics when bed materials deeper than the maximum spoutable height were summarized. Under this condition, the spout-fluid bed operated without a stable and coherent spout or fountain assembles the characteristics of jetting fluidized bed. Besides, the mechanisms of spout termination were investigated. It was found that slugging in the spout and growth of instabilities would cause the spout termination in spout-fluid bed.

Experimental study of gas mixing in a spout‐fluid bed

AbstractGas mixing in a spout‐fluid bed with a cross section of 0.3 × 0.03 m and height of 2 m was investigated by simultaneously injecting two different tracer gases. One was injected into the spouting gas flow and the other was injected into the fluidizing gas flow. Steady‐state tracer gas measurements were carried out to obtained radial tracer gas concentrations at various bed elevations. Effects of two important operating parameters—spouting gas velocity and fluidizing gas flow rate—on the gas mixing were discussed with flow patterns recorded by a high‐resolution digital CCD camera. The results show that increasing spouting gas velocity and fluidizing gas flow rate can both promote the gas mixing in spout‐fluid beds. Increasing fluidizing gas flow rate is the more effective way, given that a satisfactory mixing condition at a relatively low bed height can be obtained by increasing the fluidizing gas flow rate. For both cases, it is difficult to obtain a good mixing condition in the wall layers. Besides, the mechanism of gas mixing was preliminarily discussed. Results indicate that gas mixing in the spout‐fluid bed is caused by both convection and diffusion. Diffusion other than molecular diffusion should not be neglected, especially at high spouting gas velocity or fluidizing gas flow rate. © 2005 American Institute of Chemical Engineers AIChE J, 2006

Characterization of dynamic behavior of a spout-fluid bed with Shannon entropy analysis

Differential pressure fluctuation time series were obtained at different locations in a two-dimensional spout-fluid bed with a cross section of 300 × 30 mm and height 2000 mm. Shannon entropy analysis of differential pressure fluctuations was developed to characterize the dynamic behavior. Effects of two important operating parameters (spouting gas velocity and fluidizing gas flow rate) on the Shannon entropy were examined. It was demonstrated that a spout-fluid bed at a high spouting gas velocity or fluidizing gas flow rate was a deterministic chaos system since the Shannon entropies at all bed locations increased sharply and asymmetric unstable flows occurred. Shannon entropies were found to be significantly different at various bed locations. Shannon entropies of different flow regimes were distinct, so they were used to identify the flow regimes. The results show that the Shannon entropy helps to grasp the complex characteristics of dynamic behavior in spout-fluid beds.

Pressure fluctuation frequency characteristics in a spout-fluid bed by modern ARM power spectrum analysis

Pressure fluctuation frequency characteristics were investigated in a two-dimensional spout-fluid bed with its cross section of 300 mm×30 mm and height of 2000 mm. A multi-channel differential pressure sampling system was established to obtain pressure fluctuations and the modern ARM (Auto-Regressive Model) power spectrum was developed to estimate the pressure fluctuation major frequency. Effects of spouting gas velocity, fluidizing gas flow rate and static bed height on the pressure fluctuation major frequency were systematically examined. Differences of the major frequencies at various bed regions were analyzed. Experimental results show that the pressure fluctuation major frequency increases with increasing spouting gas velocity, while it decreases with increasing static bed height at any bed region; for Q f /Q mf≤1.0, the major frequency at any bed region increases with increasing fluidizing gas flow rate; for Q f /Q mf>1.0, the major frequency above the V-type distributor region decreases with increasing fluidizing gas flow rate, while the major frequency within the V-type distributor region increases all along with increasing fluidizing gas flow rate. In addition, a digital camera and a digital video were used to record the gas–solid two-phase interactions in bed. Preliminary investigation on the flow characteristics with the pressure fluctuation major frequencies indicates a promising way to study the complex gas–solid interactions and flow pattern transitions in spout-fluid beds. Some suggestions were also proposed for the operation of spout-fluid bed coal gasifiers.

Jet penetration depth in a two-dimensional spout–fluid bed

The jet penetration depth was proposed to be an important parameter to describe the jet action during the chemical process of spout–fluid bed coal gasification. A two-dimensional cold model of a spout–fluid bed coal gasifier with its cross section of 300 mm × 30 mm and height of 2000 mm was established to investigate the jet penetration depth. Four types of Geldart group D particles were used as bed materials. A multi-channel pressure sampling system and a high-resolution digital CCD camera were employed for experimental investigations. The effects of spouting gas velocity, spout nozzle diameter, static bed height, particle property and fluidizing gas flow rate on the jet penetration depth have been systematically studied by pressure signal analysis and image processing. Experimental results indicate that the jet penetration depth increases with increasing spouting gas velocity and spout nozzle diameter, while it decreases with increasing particle density, particle diameter, static bed height and fluidizing gas flow rate. Additional, a new correlation considered all of the above effects especially static bed height and fluidizing gas flow rate, was developed for predicting the jet penetration depth in spout–fluid beds. The correlation was compared with published experimental data or correlations, which was in well agreement with the present experimental results and some other references.

Thermal treatment of petroleum contaminated soils by a fluidized bed desorber

A novel type of fluidized bed desorber was developed for the remediation of petroleum-contaminated soils at low temperature with high efficiency. The performance of the fluidized bed desorber was experimentally investigated in two different operation modes. Results of continuous operation of fluidized bed indicated that the effect of the mass ratio of the fluidizing gas flow rate to the feed rate of contaminated soil was not significant at temperatures above 300 ‡C as long as proper fluidization was ensured. Periodic fluidization mode was tested aiming at reduction of off-gas volume. Effects of cycle time and split fraction on desorption efficiency were investigated. Within the range of experimental conditions, desorption efficiency decreased with increase of cycle time and split due to defluidization in the bed. With suitable choice of cycle time and split, periodic fluidization of the desorber enabled considerable reduction of off-gas volume without compromising desorption efficiency.