Three-phase Inverse Fluidized Bed Research Articles

A modified wake model in bubble-induced three-phase inverse fluidized bed (BIFB)

A modified wake model was proposed for the newly developed bubble-induced three-phase inverse fluidized bed (BIFB), by combining the generalized wake model and the gas-perturbed liquid model. On the basis of the modified wake model, the solids and liquid holdups and the complete fluidization gas velocity in BIFB system have been successfully predicted with two established correlations. The predictions achieved very good agreements with the experimental data.

The axial and radial phase holdup distribution of bubble-induced three-phase inverse fluidized bed

The detailed phase holdup distributions in a 0.33 m ID & 3 m height cylindrical bubble-induced three-phase inverse fluidized bed (BIFB) with air, water and polyethylene particles were investigated using a relatively new telecentric photographic probe. In the complete fluidization regime, the axial and radial distributions of the phase holdups were studied in detail. The axial gas holdup distribution is largely uniform at a low gas velocity and increasing the gas velocity leads to more nonuniformity with a higher gas holdup at the top due to the difference of the bubble size. The solids holdup distribution is less uniform but more uniform axial distribution was observed with the gas velocity as the upper particles being gradually entrained downwards. The liquid holdup is always high in the top region. The effect of the gas velocity and the axial height on the radial distributions of the phase holdups also were discussed.

Hydrodynamic characteristics of bubble-induced three-phase inverse fluidized bed (BIFB)

A new bubble-induced three-phase inverse fluidized bed (BIFB) is proposed, where particles lighter than water are fluidized downwards by upward flowing gas without net liquid flow due to the reduced net gas-liquid mixture density and the turbulence caused by bubble flow. Key advantages include uniform fluidization of particles, zero or low liquid flowrate, low energy consumption and easy re-fluidization, showing great potential in biochemical, bioprocess, food, environmental and petrochemical industry. In a 0.33 m diameter pilot-scale BIFB column, four flow regimes; initial fluidization gas velocity and complete fluidization gas velocity were identified, using particles of different diameters and densities. A new concept, the fluidization ratio, is devised to best describe the fluidization quality in BIFB.

Role of zinc oxide nanoparticles for effluent treatment using Pseudomonas putida and Pseudomonas aureofaciens.

The biological treatment of textile effluent is enhanced by the use of zinc oxide (ZnO) nanoparticles for the reduction in chemical oxygen demand (COD) from its initial value of 1700ppm. The present research investigated the effect of ZnO nanoparticles when microbial cultures of Pseudomonas putida and Pseudomonas aureofaciens were used to treat textile effluent in three-phase inverse fluidized bed bioreactor. The parameters like-size of ZnO nanoparticles, static bed height, superficial gas velocities and solid media particle size-together affected the COD reduction and all of these were investigated in this paper. ZnO nanoparticles of 280nm reduced the maximum COD to 47ppm (97.24%) at low gas velocity of 0.0027m/s at 10% inoculum size and at a static bed height of 2.43cm.

COD reduction of textile effluent in three-phase fluidized bed bioreactor using Pseudomonas aureofaciens and Escherichia coli

This paper investigated the effect of mixed cultures of Pseudomonas aureofaciens and Escherichia coli and air as gas on degradation of chemical oxygen demand (COD) in three-phase inverse fluidized beds (TPIFBs) for textile effluents with initial COD of 1200 ppm (effluent 1) and 640 ppm (effluent 2). Investigators focused on the importance of different design of gas spargers of the TPIFB for checking its effect on COD reduction performance. For this purpose, four different gas spargers were fabricated for gas flow entry into bioreactor with different % open area: Gas sparger 1 (18% open area), sparger 2 (10.42% open area), sparger 3 (8.077% open area) and sparger 4 (1.53% open area). These percentage open area decided the amount of gas flow in a unit time in batch studies, which in turn rely mainly on superficial gas velocity. The reduction in COD was measured at two different superficial gas velocity (0.00343 and 0.004068 m/s) and at two ratio of static bed height to diameter (H/D) of 0.5 and 0.25 in an aerobic mode. Textile effluent 1 resulted in 98.07% COD reduction with sparger 3 and discharged at 23.14 ppm in 28 h. Textile effluent 2 resulted in 96.5% COD reduction with sparger 2 and discharged at 22.4 ppm in 22 h. The resulting COD values of Effluent 1 and effluent 2 were in range of discharge limit and resulted at a low gas velocity of 0.00343 m/s and low H/D ratio of 0.25. The gas holdup correlation was found to be εg = 0.0064 Ug0.98.

Oxygen transfer in three phase inverse fluidized bed bioreactor during biosurfactant production by Bacillus subtilis

Production of microbial biosurfactants requires development of bioreactors enabling sufficient oxygen supply and foaming control. Gas–liquid mass transfer in a three-phase inverse fluidized bed reactor designed for biosurfactant production by Bacillus subtilis was studied. The influence of main parameters on kLa was investigated in model and real fermentation conditions. Oxygen transfer increased up to 175% and up to 24% with the increase of superficial gas and liquid velocities, respectively. The oxygen transfer obtained in the TPIFB bioreactor (kLa up to 0.015s−1) was higher than in the other bioreactors used for biosurfactant production. A significant decrease of kLa (up to 27%) was measured during fermentation process. It was shown that this decrease of kLa was a result of surface tension decrease due to the production of biosurfactants. However the oxygen transfer remained important and allowed a correct oxygen supply for the aerobic B. subtilis strains. A correlation based on dimensional analysis was proposed to estimate kLa in function of the influencing parameters, integrating surface tension effects. This correlation allowed estimating correctly kLa in the presence or not of solid particles. The correlation being based on dimensionless numbers, it could help for further process control and scale-up considerations.

Heat transfer and bubble dynamics in a three-phase inverse fluidized bed

In this study, surface-to-bed heat transfer experiments were performed to gain insight on heat transfer and hydrodynamics in a three-phase inverse fluidized bed. Air, tap water or 0.5 wt.% aqueous ethanol, and polypropylene were, respectively, the gas, liquid and solid phases. The solid loading was varied from 0 to 30 vol.%, and the gas and liquid superficial velocities from 2 to 50 mm/s and 0 to 21 mm/s, respectively. Visual observations were associated with measured phase holdups and instantaneous heat transfer coefficients. Larger gas velocities lead to an increase in bubble size due to the transition to the coalesced bubble flow regime. The greater turbulence induced by the larger bubbles increases the average heat transfer coefficient. On the other hand, adding ethanol reduces the heat transfer coefficient. Solid concentrations up to ∼13 vol.% increase the average heat transfer coefficient whereas higher solid concentrations tend to lower it. The distribution of instantaneous heat transfer coefficient peak height is wider at higher gas and liquid velocities while the addition of a surfactant narrows it. Gas holdups and average heat transfer coefficients are both compared with existing correlations, which are then adjusted for a better fit.

Modeling of residence time distribution: Application to a three-phase inverse fluidized bed based on a Mellin transform

The study is focused on modeling of gas and liquid residence time distribution in an aerated liquid system of an inverse fluidized bed bioreactor. Two opposite strategies are currently available: the use of powerful complex computational fluid dynamics (CFD) simulation and the phenomenological semi-empirical models. In this work, a specific methodology is proposed, as follows: the reactor is modeled as a reactor network containing a combination of zones with basic ideal flow patterns such as perfect mixed flow (PMF) and plug flow (PF). The approach is based on a Mellin-modification of the Laplace transformation over the relevant equations. The method allows zero-time solutions for identification analysis. The study shows that the increase of the gas flowrate leads to higher mixing intensity of the gas phase. Decreasing the gas velocity, the inverse fluidized bed tends to perform as a plug flow reactor. The liquid phase performs closer to disperse plug flow.

Oxygen transfer and hydrodynamics in three-phase inverse fluidized beds

Experiments were performed at ambient temperature and pressure in a 152 mm inner diameter column with air, tap water or 0.5% wt. aqueous ethanol solution, and polypropylene particles. An increase in liquid velocity and solids loading, and the presence of a surfactant reduces the gas velocity required to reach full bed expansion, which is delimited by the gas sparger. With an increase in gas velocity, solids holdups remain constant after full bed expansion, liquid holdups increase to a maximum and then decrease and gas holdups continuously increase. The addition of ethanol greatly increases the gas holdups leading to significant reductions in liquid holdups. The volumetric gas–liquid mass transfer coefficient, k L a , increases with increasing gas velocity but does not change significantly with liquid velocity. There are complex interaction effects between solids loading and surfactants as the values of k L a in the aqueous ethanol solution were greater than those in water when particles were present and smaller without particles. k L a data in water were found to be proportional to gas holdup whereas for the ethanol solution this proportionality constant first decreased with increasing gas velocity to eventually stabilize at a value smaller than for water.

Particle dispersion in viscous three-phase inverse fluidized beds

Dispersion characteristics of low density fluidized particles such as polyethylene and polypropylene were investigated by using the stochastic method in three-phase inverse fluidized beds with viscous liquid medium ( 0.152 m ID × 2.5 m in height). To establish the relationship between the pressure drop variation and the particle dispersion in test section, the histogram of pressure drop fluctuations were also measured and analyzed. Effects of operating variables such as gas and liquid velocities, liquid viscosity and media particle kind (density) on the fluctuating frequency, dispersion coefficient and exiting rate of media particles from the test section were determined. The fluctuating frequency and dispersion coefficient of particles increased with increasing gas or liquid velocity, but decreased considerably with increasing liquid viscosity in three-phase inverse fluidized beds. The dispersion coefficient of media particles of relatively higher density exhibited a value higher than that of lower density particles. The dispersion coefficients of particles were well correlated with operating variables in terms of dimensionless groups.

Bubble properties in three-phase inverse fluidized beds with viscous liquid medium

Bubble properties such as size, rising velocity and frequency have been measured and investigated in a three-phase inverse fluidized bed with viscous liquid medium, whose diameter and height are 0.152 and 2.5 m, respectively. Effects of gas and liquid velocities, liquid viscosity and particle density (particle kind) on the bubble properties have been determined. It has been found that the size and frequency of bubbles have increased with increasing gas or liquid velocity, while the bubble rising velocity increases with increasing gas velocity but decreases with liquid velocity. The bubble size and rising velocity have increased but the frequency has decreased, with increasing liquid viscosity. The values of bubble properties have been well correlated in terms of operating variables.

Minimum Fluidization Velocity in a Three Phase Inverse Fluidized Bed Reactor with Newtonian and Non-Newtonian Fluids

The influence of operational parameters and the effect of viscosity on minimum fluidization velocity, Ulmf (minimum fluidization liquid velocity) and Ugmf (minimum fluidization gas velocity) were investigated in a three-phase inverse fluidized bed reactor using low-density polyethylene (LDPE) and polypropylene (PP) particles. The Ulmf showed an increasing trend with an increase in particle diameter. The effect of superficial gas velocity, Ug on Ulmf and the effect of superficial liquid velocity, Ul on Ugmf for water, glycerol and carboxy methyl cellulose (CMC) as the liquid phase were studied. With an increase in Ug, the Ulmf decreased and with an increase in Ul, the Ugmf also decreased. With an increase in the viscosity (concentration) of glycerol and CMC, Ulmf decreased. Correlations for the prediction of Ulmf in Newtonian and non-Newtonian systems are proposed.

Phase hold-up and critical fluidization velocity in a three-phase inverse fluidized bed

We studied the hydrodynamic characteristics of a three-phase inverse fluidized bed made of a transpar- ent acrylic column of 0.115 m inner diameter and 2 m heights. Air, water and polyethylene particles were used as the gas, liquid and solid phase, respectively. We used both hydrophobic low density polyethylene (LDPE) and hydrophilic LDPE as solid phase, and distilled water as liquid phase, and filtered air as gas phase. The LDPE was chemically treated by chlorosulfonic acid to change the surface property from hydrophobic to hydrophilic. We tried to solely investigate the effect of the surface hydrophilicity of polymeric particles on the phase holdup and the critical fluidization velocity of three-phase inverse fluidization. Thus, we measured the static pressure and eventually observed critical fluidization velocity. Critical fluidization velocity became smaller in case of using MDPE hydrophobic particles than LDPE hy- drophilic particles. This was thought to be due to the retardation of rising bubbles near hydrophobic particles and, sub- sequently, the increase of gas hold-up.

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 .

Gas-liquid mass transfer in bioreactor with three-phase inverse fluidized bed

In the present study the oxygen mass transfer from the gas to the aqueous phase in a Three-Phase Inverse Fluidized Bed (TPIFB) has been studied. A pilot scale TPIFB has been designed and constructed. For determination of the volumetric oxygen mass transfer coefficient the elegant dynamic method, described by Dang et al. (1977) was used. The influence of hydrodynamic parameters, e.g., superficial velocities of the gas and liquid phases on the mass transfer rate was studied. In the range of variables covered, it was found that the superficial liquid velocity had a weak effect on the mass transfer whereas the gas flowrate affects the mass transfer positively. The results for the volumetric oxygen transfer coefficient in the TPIFB were compared to reported values of that coefficient, measured in a classic three-phase fluidised bed under similar hydrodynamic conditions and solid phase properties. The comparison demonstrated a two-fold increase of the oxygen transfer rate in the inverse bed over that in the classic one.

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.

Effect of coalescence inhibitors on the performance of three-phase inverse fluidized-bed columns

Three-phase inverse fluidized beds can be used for wastewater treatment or biochemical reactors. The coalescence inhibitors in the liquid of such reactors greatly affect the hydrodynamics of three-phase inverse fluidized beds. Their effect can not be predicted from the surface tension or even from their behavior in a bubble column. The effect of the inhibitors on the minimum fluidization velocity can be predicted from their effect on gas holdup with an adaptation of the model from Zhang, Epstein, Grace and Zhu (Transactions of the Institution of Chemical Engineers, 73 A, 1995, 347–353).