Gilles Hébrard

A comparative study of gas hold-up, bubble size, interfacial area and mass transfer coefficients in stirred gas-liquid reactors and bubble columns

Abstract The overall gas hold-up e g , the volumetric mass transfer coefficient k 1 a , the liquid side mass transfer coefficient k 1 , the volumetric interfacial area a , the bubble size d bs and bubble distribution have been characterised in two bubble columns and a non-standard vessel equipped with various dual-impeller combinations. In the bubble columns, the effects of gas flow rate, sparger type and column diameter were investigated. In the dual-impeller stirred reactor, the effects of rotational speed, gas flow rate, impeller type and diameter were studied. The performances of the two reactors are compared here, and some relationships are proposed and compared with existing correlations.

A unified correlation for predicting liquid axial dispersion coefficient in bubble columns

Abstract Experimental measurements of gas holdup and liquid axial dispersion coefficients have been carried out in two bubble columns (column I; D c =15 cm and column II; D c =20 cm ), operating with co-current upflow of gas and liquid. In addition, measurement of bubble size was investigated. Peclet number and liquid axial dispersion coefficient were calculated from the means and variances of the residence time distribution (RTD) curve. The experimental data obtained show that the hydrodynamic of bubble column depends on superficial gas and liquid velocities, column diameter and the gas flow regime. Present results and the available literature data were used to develop a model involving Bo,Re,Fr and Ga for predicting liquid axial dispersion coefficient.

Hydrodynamics of a three-phase fluidized bed—the inverse turbulent bed

Abstract This paper examines the fundamental characteristics of an inverse gas-liquid-solid turbulent bed. In this reactor, solids lighter than water, of large diameter (3–4 mm) and of densities ranging from 914 to 934 kg/m 3 , are fluidized by only an upward gas flow. The hydrodynamic behaviour is described and characterized by some specific gas velocities. Particles are easily fluidized and can be uniformly distributed over the whole height of the column. The effect of the operating conditions (column diameter, gas sparger, intial weight of particles introduced into the reactor) on the hydrodynamics is presented. A simplified model is proposed to predict the values of one of these gas velocities.

Bubble formation from a flexible hole submerged in an inviscid liquid

Abstract In the waste water treatment industry, a novel gas sparger based on flexible membranes has been used for the last 10 years. The objective of the present work is to study the bubble formation generated from a flexible orifice (membrane). Firstly, the membranes are characterised with regard to their properties: wetting critical surface tension, expanding hole diameter, orifice coefficients, flexibility, critical and elastic pressures. The bubble formation phenomenon in an inviscid liquid at rest is studied experimentally for different membranes and gas flow rates. The variation in the bubble diameter, the bubble centre of gravity and the bubble spread on the membrane are determined as a function of time. An analytic model is proposed to describe the bubble growth and its detachment at a flexible orifice. This theoretical approach, developed by Teresaka and Tsuge (J. Chem. Eng. Jpn. 23 (1990) 160) for rigid orifices, is adapted to take into account the membrane features (elastic behaviour and wettability). The predicted bubble diameters at detachment agree with the experimental measurements; however, the model underestimates slightly the bubble formation times. The calculation of the various forces acting on the bubble in the vertical direction indicates that the real forces governing the bubble growth are the buoyancy force, the surface tension force, and near detachment the inertial force.

Bubble formation at a flexible orifice with liquid cross-flow

In wastewater treatment, biological processes for denitrification and nitrification are performed using oxidation ditches. In these reactors, the mixing and the aeration functions are dissociated: a bubble cloud is generated from flexible membrane spargers and is subjected to a horizontal liquid flow. The objective of this paper is to study the effects of the liquid cross-flow on the bubble formation at a single flexible orifice in water. The several forces acting on the forming bubble have been modelled in order to understand the dynamics of the bubble growth and detachment. The bubble detachment is controlled by the drag force due to the liquid motion and not by the buoyancy force. The experimental analysis of the bubble growth has shown that, under liquid cross-flow conditions, the bubbles move downstream and are flattened during their growth (position of the bubble centre of gravity, bubble inclination angle). The bubbles spread over the orifice surface, and the advancing and the receding bubble angles were measured. The detached bubbles have significantly smaller sizes and higher frequencies when compared to bubble formation under quiescent liquid conditions.

Oxygen mass transfer and hydrodynamic behaviour in wastewater: determination of local impact of surfactants by visualization techniques.

Powerful techniques, based on the Planar Laser Induced Fluorescence (PLIF) technique, are deployed to locally visualize and quantify the impact of surfactants in wastewaters on hydrodynamics and oxygen mass transfer. Bubble diameter, aspect ratio, rise velocity, contamination angle, as well as flux, flux density, liquid side mass transfer and diffusion coefficients of transferred oxygen are determined based on these techniques applied in the wake of rising bubbles of diameter 1xa0mm and through planar gas/liquid interfaces. The initial experiments were performed in demineralized water containing small amounts of surfactant. Different concentrations of surfactant were added to finally reach the Critical Micelle Concentration (CMC). Bubbles have classically been found to be more spherical with a reduced rise velocity in the presence of surfactants up to the CMC. Above the CMC, these hydrodynamic characteristics were found to be almost constant, although the oxygen mass transfer decreased due to the presence of surfactants. Experimental results were markedly lower than predicted by the well-known Frössling equation with rigid surfaces. This is believed to be caused by a barrier of surfactants hindering the oxygen mass transfer at the interface. Similar hindrance of oxygen mass transfer applies to waters from sewage plants (filtered raw water and treated water), making accurate design of aeration tanks difficult.

Potential of using Hydrocyclone and Hydrocyclone equipped with Grit pot as a pre-treatment in run-off water treatment

The large nominal diameter Hydrocyclone process and the new process, Hydrocyclone equipped with Grit pot, were tested as a pre-treatment for run-off water. Many operational parameters such as inlet pressure, feed concentration and type of suspension solid were varied to study their performances. Both processes showed high efficiency in terms of solid separation and cutting diameter while the Hydrocyclone equipped with Grit pot has a very high concentration and low volume sludge compared to the classical Hydrocyclone process. Moreover, the advantage of the Hydrocyclone equipped with Grit pot was clearly demonstrated by comparing with the classical sedimentation tank.

Effect of a new high porosity packing on hydrodynamics of bubble columns

Abstract The purpose of this work was to study the effect of a new high porosity packing (solid fraction corresponds to 0.5%) on the hydrodynamics of two bubble columns of different diameters (15 and 20 cm) operating with co-current up-flow of gas and liquid. This specific packing is a stainless steel welded grid with a mesh size of 12.5 mm. Two types of gas sparger were used. Gas hold up, gas hold up profiles bubble size, slip velocity and liquid axial dispersion coefficients were determined with and without packing. The results obtained show that this type of packing has a considerable effect: it delays regime transition and can maintain a homogeneous regime over a large range of superficial gas velocities. Liquid axial mixing was also greatly affected by the presence of this packing, tending toward plug flow.

Determination of the water quality correction factor α using a tracer gas method

Abstract The purpose of this work is to evaluate, in a bubble column pilot using different wastewaters, the water quality correction factor α , defined as the ratio between the volumetric mass transfer coefficient kla O2 in the wastewater and kla O2 in clean water. The characteristic of this paper is that the volumetric mass transfer coefficient kla O2 is determined using propane as a tracer gas as it is neither produced nor consumed in biological reactors. Three types of water were tested in a bubble column pilot: raw wastewater, the supernatant of mixed liquor from an aeration tank and treated water after the secondary settler. The first results show that the alpha factor depends on water quality and seems to be linked to organic carbon and surface tension. The separation of the parameters kl and a allowed us to understand the variations of the alpha factor.

Characterization of axial liquid dispersion in gas–liquid and gas–liquid–solid reactors

Abstract Axial liquid mixing was studied in gas–liquid systems (bubble columns) and two types of gas–liquid–solid system (the turbulent and inverse turbulent beds) for different types of gas sparger (membrane and perforated plate). In the turbulent and inverse turbulent beds, solids, larger and lighter than water and of large diameter (3–4xa0mm) are fluidized only by an upward gas flow. In two-phase systems, the type of gas sparger has a strong effect on the gas flow regime and consequently on the axial liquid mixing. A general diagram with co-ordinates Pe versus Fr (1/3) has been established in order to explain all the results. It allows the hydrodynamic characteristics to be predicted by a simple measurement of the axial liquid dispersion coefficient E z l . In the gas–liquid–solid reactors tested, the effect of the gas sparger on axial liquid mixing can be pronounced. A flow regime diagram is proposed, which shows a heterogeneous flow behaviour independent of the gas sparger when the solid content of the column is sufficiently high.