Tomographic analysis of dry and semi-wet bed fluidization: the effect of small liquid loading and particle size on the bubbling behaviour

The hydrodynamic changes resulting from the addition of very small quantities of a non-volatile liquid into a cold conventional fluidized bed has been investigated, and compared with the effects of increasing the particle size in a dry bed. Three different particle mixtures belonging to Group A/B, Group B and Group B/D were assessed. The changes in regime transition velocities, pressure drop, bubble rise velocity, bubble frequency and bubble flow rate have been quantified by employing Electrical Capacitance Tomography measurements. A new analysis method for measuring the effective interparticle forces (F_ip) and the effective drag force (F_d) in a dry fluidized bed is described, and the results are presented in terms of different force ratios including the single particle weight (W). It is shown that the addition of a few drops of liquid to a dry bed of Group B or B/D introduces similar hydrodynamic changes (except, in terms of bubble frequency) as that of increasing the bed particle size, and these particular changes shift the powders away from Group A/B behaviour. It is also illustrated that for beds of different particle sizes, a typical bubbling behaviour can be achieved at a specific gas velocity, this velocity coincides with the point of equality in hydrodynamic force ratios and E_ip/F_d.     

Determination of particle residence time at the walls of gas fluidized beds by discrete element method simulation

This paper presents a study of particle residence time near the walls of gas fluidized beds by DEM simulation. The simulation experiments were performed in two beds, one of cross-section and the other of cross-section. The particles used were 0.5 and in diameter and in density. The number of particles used was 600,000 and 500,000 for simulations of the 0.5 and diameter particles, respectively. The advantage of this approach is that it enables an unprecedented amount of information pertaining to the motion of individual particles near the walls of a fluidized bed to be obtained. These include the distribution of particle residence time, mean residence time, contact frequency, and contact distance at the walls. Variations of particle residence time with particle size, gas velocity, and wall position are examined. Significance of the results to modelling of particle-to-wall heat transfer in fluidized beds is highlighted.     

Particle growth kinetics of calcium fluoride in a fluidized bed reactor

Crystallization process in a fluidized bed reactor to remove fluoride from industrial wastewaters has been studied as a suitable alternative to the chemical precipitation in order to decrease the sludge formation as well as to recover fluoride as synthetic calcium fluoride.In the modeling, design and control of a fluidized bed reactor for water treatment it is necessary to study the particle growth kinetics. Removal of fluoride by crystallization process in a fluidized bed reactor using granular calcite as seed material has been carried out in a laboratory-scale fluidized bed reactor in order to study the particle growth kinetics for modeling, design, control and operation purposes.The main variables have been studied, including superficial velocity (SV, ), particle size of the seed material (L₀, m) and supersaturation (S). It has been developed a growth model based on the aggregation and molecular growth mechanisms. The kinetic model and parameters given by the equation fits well the experimental data for the studied range of variables.     

Study of transition velocities from bubbling to turbulent fluidization by statistic and wavelet multi-resolution analysis on absolute pressure fluctuations

Experiments were conducted in three circulating fluidized beds of high, high and high (with an expanded top section of 1.5 m high) at ambient condition. The spent fluid cracking catalyst (FCC), glass beads and sand particles were taken as bed materials. The transition velocities U_c and U_k were analyzed by means of traditional statistic analysis and multi-resolution analysis (MRA) of wavelet analysis on the absolute pressure fluctuation (APF) signals acquired at different axial positions in the beds. According to the standard deviation of APF, the effects of axial positions and static bed heights on U_c and U_k were systematically investigated. An appropriate measuring interval of relative axial position was recommended to identify U_k and two correlations calculated by regression of data in literatures and this work were proposed to predict U_c and U_k for absolute pressure measurement. By means of MRA of wavelet analysis, a redefined variable, homogeneous index HI, deduced from the energy of SF and LF subsignals, was successfully applied to determine the U_c and U_k and demarcated the dynamic behaviors of Geldart group A (spent FCC) and group B (sand particles and glass beads) particles in the circulating fluidized beds.     

Lift force in bubbly flow systems

From the significance of three-dimensional simulation of dispersed flow systems in many engineering fields, extensive study was conducted for lift force in a single particle system as well as a multiparticle system. In this study, the lift force in a single particle system was modeled by considering the effect of bubble deformation on the lift force. The model was finalized based on existing data obtained in the range of particle Reynolds number from 3.68 to 78.8, viscous number from 0.0435 to 0.203 and Eötvös number from 1.40 to 5.83. The viscous number is defined by where μ_f, ρ_f, σ, g and Δρ are, respectively, fluid viscosity, fluid density, surface tension, gravitational acceleration and density difference between phases. The applicability of the model to higher particle Reynolds number system such as an air-water system was qualitatively examined. The lift force model developed in a single particle system was extended to a multiparticle system. The applicability of the extended lift force model was qualitatively examined. The similarity between drag and lift forces were also discussed.     

Wall-to-liquid mass transfer in fixed beds at low flow rates

The wall-to-fluid mass transfer coefficient was obtained from coated wall dissolution experiments, for water flow through fixed beds of spheres with tube-to-particle diameter ratios of 2.9-11.6, and for particle Reynolds number (Re) in the range 3-200. The coefficients, in the form of dimensionless Sherwood numbers (Sh_wf), were shown to approach a nonzero limit as Re→0. The data were well-represented by the equation

     

Enzyme kinetics of kinetic resolution of racemic ibuprofen ester using enzymatic membrane reactor

An ultrafiltration hollow fiber enzymatic membrane reactor was employed to study the kinetics of lipase-catalyzed kinetic resolution of racemic ibuprofen ester. Lipase from Candida rugosa was employed in the hydrolysis reaction both in free form in a batch system and in immobilized form in an enzymatic membrane reactor (EMR). The half life (t_1/2) of immobilized lipase on spongy layer was 105 h at reaction temperature of and 62 h at . This value was 94 h for lipase immobilized on the inner lumen and 45 h for free lipase in batch system at . Excessive substrate was found to inhibit the reaction as an uncompetitive inhibitor. The by-product 2-ethoxyethanol was found to be non-competitive inhibitor to the reaction when it was present in an excess. Michaelis constant (K_m) and maximum reaction rate (V_max) for immobilized lipase were and , respectively; and that for free lipase were and h^-1, respectively.     

Phase space structure and multi-resolution analysis of gas-solid fluidized bed hydrodynamics: Part II: Dynamic analysis

The dimension of interaction dynamics, , is used to study the nonlinear interactions in a fluidized bed between the local bubble, “bulk” bed and particle dynamic components. The different timescales are obtained by a multi-resolution analysis based on the Hilbert-Huang Transform Method (HHTM). The state space analysis is used to study the time-space structure of the fluidized bed dynamics. A method for constructing Poincaré projections based on the join probability matrix computation is provided. The Kolmogorov entropy, K, and the correlation dimension, D₂, are used to compute the invariant properties of the corresponding attractor. It is concluded that the local bubble dynamics has chaotic features and is responsible for the determinism found in pressure fluctuation signals. It has been found that, as expected, the bubble component is the driver of the gas-solid fluidized bed hydrodynamics.     

CFD simulation of gas solid flow in FCC strippers

In this paper, the hydrodynamic characteristics in bubbling fluidized beds (FCC Strippers) were simulated by using computational fluid dynamics (CFD) code (Fluent 6.2.16). The modified Gidaspow drag model based on the effective mean diameter of the particle clusters predicted the expected bubbling fluidization behavior and bed expansion. Compared with the bed densities of in the empty-cylinder stripper, bed densities in the V-baffled stripper were at the superficial gas velocity of 0.10-0.20 m/s. The overall trend of the time-averaged bed density at various superficial gas velocities were in agreement with the experimental data. The results illustrated that internal baffles had an important effect on the fluidization hydrodynamics. Internal baffles improved break-up and redistribution of bubbles and intensified the gas-solid contact. The simulation results also indicated that appropriate modification of the internal configuration eliminated the dead flow region in the strippers, and enhanced the gas-solid mixing remarkably, showing benefit for the mass and heat transfer in the fluidized bed.     

The influence of moisture on the fluidization behaviour of porous pharmaceutical granule

The influence of moisture content on the fluidization behaviour of placebo pharmaceutical granule has been studied in a 14 cm diameter cylindrical fluidized bed column. The dry granule has a mean diameter of and exhibits a bimodal size distribution with modes of 169 and . Bed pressure drop profiles and tapped density measurements were generated for granule moisture contents between 5 and 30 wt%, which corresponds to typical final and initial moisture contents experienced during drying. At high moisture contents, the wet granule exhibits Geldart C type powder behaviour as channelling and defluidization exist. As the moisture content is reduced, the granule fluidity improves and demonstrates behaviour characteristic of Geldart B powders. The changing fluidization behaviour was quantified using parameters such as the full support velocity, full support bed voidage and Hausner ratio. These parameters were found to increase significantly above granule moisture contents of 10 wt%. The increase in the Hausner ratio suggests that the interparticle force load in the bed increases. This change in interparticle force load is responsible for the increase in the full support velocity and bed voidage.     

Fracture energy-fracture stress relationship for weak polymer-polymer interfaces

The bulk thick films of high-molecular-weight atactic polystyrene (PS) were brought into contact at a small contact pressure ≤0.2 MPa at a constant healing temperature T_h below the calorimetric glass transition temperature of the bulk . Fracture energy G and fracture stress σ of the auto-adhesive joints PS-PS were measured at ambient temperature in the T-peel test and the lap-shear joint geometry, respectively. In the framework of the diffusion controlled mechanism of the development of these two mechanical properties suggesting their evolution as and (t_h is the healing time), and as G∝1/T_h and σ∝1/T_h, a linear relationship between G^1/2 and σ, valid over a temperature range of , has been found. The penetration depth of 0.5 nm corresponding to the value of G calculated using the measured value of σ developed at for 24 h was reasonably smaller than the thickness of the surface mobile layer of 1 nm predicted by Wool's rigidity percolation theory for thick bulk PS films. The feasibility of a full healing of polymer-polymer interfaces below has been discussed. The dependence of an apparent activation energy characterising the process of segmental motions at PS surfaces and interfaces on the approach and the physical property chosen for its calculation has been analysed.     

Laser desorption-ionization time of flight mass spectrometry of various carbon materials

The possibilities of commercially available MALDI-TOF mass spectrometric instrumentation equipped with 337 nm nitrogen laser in carbon materials analysis have been examined. Laser desorption-ionization of synthetic diamond, graphite, glassy carbon, carbon nano-tubes, and diamond-like thin layer results in the formation of two positively charged ( and odd-numbered 44-346) and one negatively charged carbon cluster set (). Fullerene C₆₀ was analyzed for comparison. From the clusters mass spectra, several similarities, concerning “magic number” ions with high signal intensities, can be observed. It is concluded that differing carbon materials are subjected by UV-laser pulses to similar physico-chemical changes. Heavy carbon clusters of low stability and characteristic loss of neutral C₃ particle in case of the smaller clusters during post-source decay (PSD) implies that the species observed do not possess stable fullerene structure, which might have been expected for C_n species with n ? 20.     

Comparative study of different crystallographic structure of LiNi0.5Mn1.5O4−δ cathodes with wide operation voltage (2.0-5.0 V)

LiNi_0.5Mn_1.5O_4−δ cathode materials with two different structures ( and P4₃32) were synthesized by ultrasonic spray pyrolysis method. The X-ray diffraction (XRD) data was confirmed that face-centered spinel () transformed into primitive simple cubic (P4₃32) structure by annealing process at 700 °C. In spite of two electrons operated cut-off voltage range between 2.0 and 5.0 V, LiNi_0.5Mn_1.5O₄ with P4₃32 structure has better electrochemical behaviors than the cathode with simple cubic structure. Ex situ XRD study of the electrode revealed that LiNi_0.5Mn_1.5O₄ (P4₃32) has reversible crystal transformation between fully lithiated state (2.0 V) and delithiated state (5.0 V) whereas LiNi_0.5Mn_1.5O_4−δ () showed irreversible phase transformed at two voltage region. The LiNi_0.5Mn_1.5O_4−δ () has voltage drop was occurred after 20th cycled compared without any voltage drop of the P4₃32 structure.     

DEM-LES simulation of coal combustion in a bubbling fluidized bed Part II: coal combustion at the particle level

The discrete element method-large eddy simulation (DEM-LES) is used to model coal combustion at the particle level in a bubbling fluidized bed. The gas phase is modelled as a continuum and the solid phase is modeled by DEM. Chemical reactions consist in the heterogeneous reactions of char with O₂, CO, CO₂, NO, and N₂O, and in the homogeneous reactions involving CO, O₂, NO, and N₂O. The colliding particle-particle heat transfer is based on the analysis of the elastic deformation of the spheres during their contact. The model predicts the effects of the particle heterogeneous flow structure on the thermal characteristics of coal particles when heating and burning, and the gaseous emissions from a fluidized sand-coal binary mixture. The heating rates are 1627 and for, respectively, 0.8 and diameter coal particles fed into the fluidized bed. The instantaneous contribution of the collision heat transfer is weak, less than 5.0% of the total power exchanges (coal combustion, radiation, convection and collision) during the heating and 1.5% during the combustion. The temperature of the coal particles exceeds the bed temperature, which is in qualitative agreement with experimental data from literature. The effects of the diameter of coal particles, of the bed temperature, and of the inlet gas velocity on the thermal characteristics are also studied.     

On the quantification of energy dissipation in the impeller stream of a stirred vessel from fluctuating velocity gradient measurements

The turbulence energy dissipation rate (ε) in the impeller stream of a vessel of diameter stirred by a Rushton turbine of diameter D=T/3 was directly measured with particle image velocimetry (PIV). Both 2-D and 3-D PIV techniques were employed to measure the mean velocities, Reynolds stresses and ε in the vessel for Reynolds numbers of 15 000-40 000. ε was determined directly from measurements of the fluctuating velocity gradients by analysing the PIV images with a resolution of . The values of the normalised ensemble-averaged dissipation rate (ε/N³D²) in the impeller stream were in the range 5-10. The measured fluctuating velocity gradients compared well with similar data obtained using a four-channel laser anemometer. The results are also compared with those of earlier works employing non-direct methods to estimate ε and show that some of these methods yield comparable values, although the spread of the some of the data previously reported is significant. The present results show the feasibility of direct measurement of the ε distribution with PIV and provide useful information for the design of mixing processes as well as for its more accurate estimation in future work.     

Hydrogen generation in a Pd membrane fuel processor: Productivity effects during methanol steam reforming

Performance analyses are carried out for the palladium membrane fuel processor for catalytic generation of high purity hydrogen. The reactor model includes detailed particle-scale multi-component diffusion, multiple reversible reactions, flow, and membrane transport. Using methanol steam reforming on Cu/ZnO/Al₂O₃ catalyst as the test reaction, a systematic examination of the effects of operating and reactor design parameters on key performance metrics is presented. Single particle simulations reveal a complex interplay between nonisobaric transport and the reversible reactions (methanol reforming and decomposition, and water-gas shift), which impact overall reactor performance. An analysis of characteristic times helps to identify four different productivity controlling regimes: (i) permeation control, encountered with thick membranes and/or insufficient membrane area; (ii) catalyst pore diffusion control encountered with diffusion of reacting species in larger particles; (iii) reaction control, encountered when intrinsic catalytic rates are too low because of inadequate activity or catalyst loading; and (iv) feed control, encountered when the limiting reactant feed rate is inadequate. The simulations reveal that a maximum in the hydrogen productivity occurs at an intermediate space velocity, while the hydrogen utilization is a decreasing function of space velocity, implying a trade-off between productivity and hydrogen utilization. The locus of productivity maxima itself exhibits a maximum at an intermediate membrane surface to volume ratio, the specific value of which is dependent on the particle size, membrane thickness and reaction conditions. At moderate temperature and total pressure (, 10 bar), particles smaller than 2 mm diameter, Pd membranes with thickness less than , and membrane surface to volume ratio exceeding are needed to achieve viable productivity . A comparison between the packed-bed membrane reactor and conventional packed-bed reactor indicates a modest improvement in the conversion and productivity due to in situ hydrogen removal.