Minimum Liquid Fluidization Velocity Research Articles

Hydrodynamic Properties Investigation of Ebullated Bed Reactor Using Non-Newtonian Liquid

The importance of using EBR has been renewed recently due to the sharp increase in heavy feedstocks sent to refineries and the hydrocracking process. Most of these feedstocks have a non-Newtonian behavior. The performance of this type of reactor using non-Newtonian liquid is complicated and has not been covered well yet. Hence, the present work is devoted to elucidating the effect of the non-Newtonian behavior of fluid on the hydrodynamic properties of a three-phase (gas-liquid-solid) reactor under operating conditions of different values ​​of gas velocity (2, 4, 6) cm/sec, liquid velocity (0.9, 1.39, 1.8, 2.3) cm/sec, and recycle ratio (1.5, 2, 2.5). The study observed the effect of non-Newtonian behavior using polymethyl Cellulose (PMC) at different concentrations (0.1, 0.2, 0.3, and 0.4) wt%. The pressure gradient method was used to elucidate the minimum liquid fluidization velocity and to estimate hold up, while the imaging method was used to measure the bubble's size. The results showed that the higher the gas velocity, the lower the minimum liquid fluidization velocity. As the intensity of the non-Newtonian behavior increased, gas velocity showed the opposite effect. The results also showed that increasing the velocity of liquid and gas and the intensity of the non-Newtonian increase the gas hold-up. The bubbles characteristics, represented by bubble size results, show that small bubbles appear at low gas velocities, and these bubbles collapse as gas and liquid velocities increase as well as liquid viscosity.

Characterization of gas–liquid–solid fluidized beds by S statistics

The hydrodynamics of a gas–liquid–solid fluidized bed was investigated by applying the S statistics method to pressure fluctuations measured under various operating conditions in a laboratory-scale bed. S statistics tests reveal the existence of three transition velocities, especially at low gas velocities. Four distinct fluidization regimes, namely, the compacted bed, agitated bed and coalesced and discrete bubble regimes were detected. A comparison of reconstructed attractors of pressure fluctuations measured at different axial positions along the riser and with various solid loadings showed significant differences in the signals compared before fluidization, especially at minimum liquid agitation velocity. Close to the minimum liquid fluidization velocity and high liquid velocities, the variation in particle size has an insignificant effect on the bed hydrodynamics. Therefore, S statistics is a reliable method to demarcate different fluidization regimes and to characterize the influence of various operating conditions on the hydrodynamics of gas–liquid–solid fluidized beds. The method is applicable in large-scale industrial installations to detect dynamic changes within a bed, such as regime transitions or agglomeration.

Dominant flow structures in gas–liquid–solid fluidized beds

Effective time series analysis techniques in time and frequency domains were applied to characterize three‐phase fluidization under a wide range of gas and liquid velocities. The experiments were carried out in a laboratory scale fluidized bed, operated under ambient conditions. Standard deviation of pressure fluctuations successfully detected four distinct regimes, namely compacted bed, agitated bed, coalesced and discrete bubble regimes (or discrete and dispersed bubble regimes at extremely low gas velocities). A minimum in skewness and average cycle frequency and a maximum in flatness indicated a minimum deviation from larger structures of the bed. A transition between macro and finer structures occurred during agitated bed regime and accordingly, minimum liquid fluidization velocity was perceived by skewness, flatness, and average cycle frequency analyses. The results showed that at very low liquid velocities in the compacted bed regime, the power spectrum at lower frequencies slightly increased with increasing liquid velocity. This results in the consecutive appearance of single bubbles of low frequencies. With further increase in the liquid velocity at agitated bed regime the power spectrum at lower frequencies gradually decreased. At higher liquid velocities near ULmf, the peak dominant frequency is transmitted from low frequency ranges of 2 Hz to the higher frequency of about 10 Hz.

Hydrodynamics of a gas–liquid–solid fluidized bed with hollow cylindrical particles

The hydrodynamic characteristics, viz. the pressure drop, bed expansion and phase holdup profile of a co-current three-phase fluidized bed with an antenna-type air sparger have been determined. Correlations for minimum liquid fluidization velocity, bed voidage and gas holdup have been developed. The bed voidage is found to increase with the increase of both liquid and gas velocities. The gas holdup increases with gas Froude number, but decreases with liquid Reynolds number. The gas holdup is a strong function of the Froude number. The experimental values have been found to agree with the correlations.

Characterization of hydrodynamic properties of a gas–liquid–solid three-phase fluidized bed with regular shape spherical glass bead particles

The hydrodynamic characteristics, viz., the pressure drop, bed expansion and phase hold-up of a cocurrent gas–liquid–solid three-phase fluidized bed has been studied using liquid as the continuous phase and gas as the discontinuous phase. These have been done in order to develop a good understanding of each flow regime in gas–liquid and liquid–solid fluidization. Air, water and glass beads (2.18, 3.05 and 4.05 mm, respectively) are used as the gas, liquid and solid phases, respectively. The experiments were carried out in a 100 mm ID, 2 m-height vertical Plexiglas column. The column consists of three sections, viz., the gas–liquid disengagement section, test section and gas–liquid distributor section. Bed pressure measurements have been made to predict the minimum liquid fluidization velocity. By keeping gas velocity at a fixed value, the liquid velocity was varied and the effect on phase hold-up, minimum liquid fluidization velocity, pressure drop and the expansion ratio was studied for different particle size and static bed height. Experimental study based on statistical design has been made to investigate the expansion ratio of fluidized bed and a correlation has been developed for gas hold-up. It is evident from the correlation that gas hold-up is strongly function of modified gas Reynolds number and independent of liquid Reynolds number. The experimental values have been compared with those predicted by the

Some hydrodynamic aspects of 3-phase inverse fluidized bed

Hydrodynamics of 3-phase inverse fluidized bed is studied experimentally using low density particles for different liquid and gas velocities. The hydrodynamic characteristics studied include pressure drop, minimum liquid and gas fluidization velocities and phase holdups. The minimum liquid fluidization velocity determined using the bed pressure gradient, decreases with increase in gas velocity. The axial profiles of phase holdups shows that the liquid holdup increases along the bed height, whereas the solid holdup decreases down the bed. However, the gas holdup is almost uniform in the bed.

Hydrodynamics of three-phase fluidized bed systems examined by statistical, fractal, chaos and wavelet analysis methods

Hydrodynamic studies were conducted in gas–liquid–solid systems (0.1 m ID, 2 m high) of 3.0 mm glass beads and of 2.1 mm polypropylene low-density particles, with particles densities of 2471 and 1290 kg/m 3, respectively. Simultaneous measurement of differential pressure and bubble conductivity probe signals sampled at 500 Hz for 60 s enabled the investigation of the change in flow structure in relation to the flow regime transitions. Superficial gas velocities ranged between 0.010 and 0.052 m/s for polypropylene particles, and extended to 0.12 m/s for glass beads, while the superficial liquid velocities covered the ranges of 0.0007–0.045 m/s for polypropylene particles, and ranged up to 0.056 m/s for glass beads. Spectral analysis of the pressure fluctuations revealed a transition from dispersed to coalesced bubbling flow with decreasing liquid velocity for a given superficial gas velocity. The use of a conductivity probe facilitated characterization of the local flow structure in terms of bubble movement. The measurements were extensively analyzed using fractals and chaos, power spectra frequency analysis and wavelet decomposition in addition to the standard statistical analyses. The coefficient of variation of the bubble probe signals was found to be the most effective in deducing the transition velocity between coalesced and dispersed bubbling flow regimes, while wavelet energy confirmed the similarity in the distribution between two axial positions once operated in the dispersed flow regime. Comparison of the flow structure between glass beads and polypropylene particles showed that both the minimum liquid fluidization velocity and the transition velocity between the bubble flow regimes were much higher for the glass beads than for the lighter polypropylene particles. Furthermore, the standard deviations of the decomposed bubble probe signals through wavelet transformation successfully highlighted the difference between the two systems of particles.

The Variable-Voidage Effect on Modeling the Minimum Liquid Fluidization Velocity of Gas-Liquid Fluidized Beds

By taking account of the experimentally determined variation of voidage with the gas velocity at minimum gas-liquid fluidization of a bed of solid particles, it is shown, using the same experimental database as in the earlier paper by Lee et al. (2001a), that the model which best predicts the minimum liquid fluidization velocity is a variant of the gas-perturbed liquid model of Zhang et al. (1995), but with the buoyancy term based on the gas-liquid mixture rather than the liquid alone. The best value of the constant in the viscous term of the Carman–Ergun drag equation involved is 165, intermediate between Carman’s 180 and Ergun’s 150.

Heat Transfer and Hydrodynamics in Two- and Three-Phase Inverse Fluidized Beds

Characteristics of heat transfer and hydrodynamics have been investigated in two- and three-phase inverse fluidized beds whose diameters are 0.152 and 2.5 m in height. Effects of gas and liquid velocities and particle density (polyethylene and polypropylene) on the immersed heater-to-bed heat-transfer coefficient, individual phase holdup, and minimum liquid fluidization velocity have been determined. It has been found that the minimum liquid fluidization velocity decreases with increasing gas velocity in the inverse fluidized beds. The gas and liquid holdups increase with an increase in the gas or liquid velocity in the beds. The heat-transfer coefficient in two-and three-phase inverse fluidized beds of relatively high density particles (polyethylene) has been higher than that in the beds of relatively low density particles (polypropylene). The heat-transfer coefficient increases with increasing gas velocity; however, it exhibits a maximum value with increasing liquid velocity in liquid-solid as well as three-phase inverse fluidized beds. The liquid velocity at which the h value attains its maximum value decreases with increasing particle density or gas velocity. The immersed heater-to-bed heat-transfer coefficient has been well correlated in terms of dimensionless groups as Nu 1 =0.001(Pr)(Re 1 ) 1.282 , Nu 2 =0.084(Pr)-(Re 2 ) 0.944 , and Nu 3 =0.050(Pr)(Re 3 ) 0.810 .

Dimensional hydrodynamic similitude in three-phase fluidized beds

This study tests the scaling approach for three-phase fluidized bed hydrodynamics proposed by Safoniuk, Grace, Hackman, & McKnight (Chem. Eng. Sci. 54 (1999) 4961) based on geometric and dynamic similitude with a limited number (5) of dimensionless groups. Experiments were carried out in two systems in which all five dimensionless groups were matched: an aqueous glycerol solution with glass beads (system 1) and silicone oil with porous alumina particles (system 2), with air as the gas in both cases. Although bed expansions were similar for the two systems, trends differed. Gas holdups were always slightly higher for system 1. The dimensionless transition velocities from dispersed to coalesced flow were similar. The minimum liquid fluidization velocity Reynolds number was slightly higher for system 1 without gas, but somewhat lower with gas present. Differences between the systems are statistically significant, but generally less than 12%, so the dimensional similitude approach gives a reasonable basis for estimating global hydrodynamic parameters under the present operating conditions. The differences between the two systems are attributed to the complex coalescence behavior of liquid mixtures, suggesting that additional dimensionless groups are needed to fully characterize the local dynamic bed behavior.

Models for Minimum Liquid Fluidization Velocity of Gas-Liquid Fluidized Beds.

The gas-perturbed liquid model of Zhang et al. (1995) is modified in an effort to improve its prediction of the minimum liquid velocity of fluidization, Ulmf, of a bed of solid particles in the presence of a low or moderate cocurrent flow of gas. Variants of the model are considered where the buoyancy term is based on the gas-liquid mixture, instead of the liquid alone, and with the frictional pressure gradient given by several alternative equations to the Ergun equation employed in the original gas-perturbed liquid model. All versions of the model provide similar dependence on such factors as gas velocity, particle diameter, particle density and liquid viscosity as those seen experimentally. The mixture buoyed equation with the drag based on an equation suggested by Foscolo et al. (1983) gives improved predictions over the original Zhang et al. (1995) model, but the best overall agreement is with buoyancy based on the liquid alone and the first term in the drag equation with the Carman (1937) constant of 180 instead of Ergun's 150. The predictions are sensitive to the minimum fluidization voidage, which is measured, assumed, or estimated. Further work is required to investigate minimum liquid fluidization velocities experimentally for particles of density closer to that of the liquid, and for high-viscosity liquids.

Hydrodynamic transition from fixed to fully fluidized beds for three-phase inverse fluidization

Hydrodynamic transition experiments, involving both visual observations and pressure measurements, were performed using a 127-mm diameter Plexiglas column for threephase inverse fluidized beds of 5.8-mm polyethylene spheres. Observations of interest not hitherto reported include: (1) A marked hysteresis effect (even when starting from a loose-packed condition) between inverse fluidization and defluidization which disappears when a wetting agent is added to the downflowing water. (2) An initially abrupt decrease of the minimum fluidization voidage, єemf, followed by a gradual rise of єemf with increasing superficial gas velocity, Ug. (3) Lower values of emf for three-phase systems than for the corresponding two-phase (liquid-solid) fluidized beds because local agitation by the gas bubbles causes bed compaction near the minimum liquid fluidization velocity, Ulmf. (4) Ulmf vs. Ug curves which, though they always show Ulmf decreasing as Ug increases, sometimes display concave-downward, sometimes concaveupward and sometimes S-shaped behavior.

Minimum fluidization velocities for gas—liquid—solid three-phase systems

Cocurrent upward gas—liquid fluidization of coarse solids is actuated primarily by the motion of the liquid at relatively low gas velocities and by the momentum of the gas at zero or low liquid velocities. Our gas-perturbed liquid model, which has previously been shown to give good predictions of the minimum liquid fluidization velocity, U lmf, at a fixed low gas velocity, is shown here also to give reasonable agreement with U lmf measurements for inverse three-phase fluidization at a given upward gas velocity, using the coefficient in the gas hold-up equation of Yang et al. [X.L. Yang, G. Wild, J.P. Euzen, Int. Chem. Eng. 33 (1993) 72] as an adjustable parameter. It is further shown that a liquid-buoyed solids/liquid-perturbed gas model can predict with moderate success the minimum gas fluidization velocity, U gmf, for three-phase cocurrent upward fluidization of coarse solids at zero or low liquid velocities.

Minimum liquid fluidization velocity in two- and three-phase fluidized beds with non-Newtonian fluids

Abstract Measurements have been taken of the relationship between flow rate and pressure drop for non-Newtonian fluids in two-and three-phase fluidized beds and the minimum fluidization velocities have been determined. Aqueous solutions of carboxymethyl cellulose and xanthan gum have been used as the non-Newtonian liquids. The measurementscovered Reynolds numbers at minimum fluidization from 0.0377 to 540 and flow indices in a power-law model from 0.489 to 1. The minimum fluidization velocity decreased with an increase in the gas velocity and the magnitude of this decrease became smaller as particle size decreased. The increase of the viscous non-Newtonian flow behaviour or pseudoplasticity led to a decrease of the minimum fluidization velocity. The result for the minimum liquid fluidization velocity for two-phase systems have been examined using the correlation based on the free-surface cell model. The predicted minimum fluidization velocities were in reasonable agreement with the experimental data for non-Newtonian liquid-solid two-phase systems over the whole Reynolds number region.

Minimum liquid fluidization velocity in gas-liquid-solid fluidized beds of low-density particles

Phase properties evolve in biological and biochemical three-phase fluidization processes and thus it is essential to continuously monitor the fluidization regime. The density of the solids used in these processes is usually only slightly higher than the liquid density. Many techniques used to determine the minimum liquid fluidization velocity of systems with heavier solids do not work with lower-density solid systems. For gas-liquid-solid systems over a wide range of solid densities, the minimum liquid fluidization velocity can be determined from the coefficient of variation of V-statistic of the integrated-bed pressure gradient and the derivative of local conductivity.

Minimum liquid fluidization velocity in gas‐liquid‐solid fluidized beds

AbstractAccurate detection of minimum liquid fluidization is essential to the successful operation of gas‐liquid‐solid fluidized beds, especially when particle or liquid properties evolve. A gas‐liqid‐solid system of 3‐mm glass beads exhibits three distinct flow regimes as the liquid velocity is increased: compacted, agitated and fluidized‐bed regmes. Measurements showed that the bed is not fluidized in the agitated bed regime. Pressure gradient and bed height measurements do not provide the minimum liquid fluidization velocity; instead, they offer the velocity between the compacted and agitated bed regimes. Time‐averaged signals are not reliable for determining the minimum liquid fluidization velocity. It can be obtained from the standard deviation, the average frequency, the Hurst exponent and the V statistic of the cross‐sectional average conductivity, which can be measured under many industrial conditions.