33 research outputs found
Bubble formation from a flexible hole submerged in an inviscid liquid
In the waste water treatment industry, a novel gas sparger based on flexible membranes has been used for the last ten 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 & Tsuge (1990) 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
Influence of liquid surface tension (surfactants) on bubble formation at rigid and flexible orifices
The influence of liquid surface tension on the bubble formation from both rigid and flexible
orifice has been investigated. The liquid phases under test are aqueous solutions with butanol
or surfactants (cationic, non-ionic and anionic); static and dynamic measurements of liquid
surface tension have been performed to characterise them. This study shows that the effect of
surface tension on the bubbles generated cannot be analysed only in terms of the static surface
tension, but also depends on whether the bubbles are generated from a rigid orifice or from a
flexible orifice. The kinetics of adsorption and diffusion of the solute molecules towards the
bubble interface have to be taken into account insofar as their time scales are comparable to
those of the bubble formation phenomenon
Dynamics of bubble growth and detachment from rigid and flexible orifices
The objective of this paper is to understand how and why the orifice nature (rigid or flexible) governs
the bubble generation. The differences in orifice nature and properties have strong consequences on the bubbles
generated. Indeed, the dynamics of the formation and the nature of the detached bubbles are fundamentally
different depending on whether the bubbles are generated from the rigid orifice or from the flexible orifice.
Keywords. Gas-Liquid reactors, aeration, rigid and flexible orifices, bubble formation dynamics.
Résumé. L’objectif de cette étude est de comprendre comment et pourquoi la nature de l’orifice (rigide ou
flexible) contrôle la génération de bulles. Les différences de nature et de propriétés entre les deux orifices ont des
conséquences notables sur les bulles générées. En effet, la dynamique de formation et la nature des bulles
détachées sont fondamentalement différentes selon si elles sont générées par un orifice rigide ou par un orifice
flexible
Effect of surfactants on liquid side mass transfer coefficients: a new insight
Specific experiments are proposed to investigate the effect of surfactants on liquid side mass
transfer coefficients. They are based on the determination of the liquid side mass transfer
coefficient kL at a free gas-liquid interface, under controlled temperature and hydrodynamic
conditions. Firstly, the methodology is validated in water at various rotation speeds and
temperatures. In a second time, it is applied in aqueous and pure solutions of anionic
surfactants: a decrease of kL with an increase of surfactant concentrations is then observed
until leveling off when the CMC is reached. Deduced from experimental results, the
equivalent diffusion coefficients describe an identical behavior. These results demonstrate
that the lowest kL are directly linked to the presence of surfactants at the gas-liquid interface
which makes the diffusion coefficients of oxygen be reduced. At last, a comparison is
performed with the data of [1-2] obtained from a chain of bubbles having diameters above to
3.5 mm. A quasi-linear relation between the kL issued from both hydrodynamic
configurations is revealed in the whole range of surfactant concentrations. Such findings
would prove that, in both cases, the impact of surfactants on liquid side mass transfer
coefficient is correlated with the changes in the diffusion coefficients of oxyge
Bubble formation at a flexible orifice with liquid cross-flow
In waste water 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 crossflow
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
Effect of surfactants on liquid side mass transfer coefficients
In the present paper, the effect of liquid properties (surfactants) on bubble generation phenomenon, interfacial area and liquid side mass transfer coefficient was investigated. The measurements of surface tension (static and dynamic methods), of Critical Micelle Concentration (CMC) and of characteristic adsorption parameters such as the surface coverage ratio at equilibrium (se) were performed to understand the effects of surfactants on the mass transfer efficiency. Tap water and aqueous solutions with surfactants (cationic and anionic) were used as liquid phases and an elastic membrane with a single orifice as gas sparger. The bubbles were generated into a small-scale bubble column. The local liquid side mass transfer coefficient (kL) was obtained from the volumetric mass transfer coefficient (kLa) and the interfacial area (a) was deduced from the bubble diameter (DB), the bubble frequency (fB) and the terminal bubble rising velocity (UB). Only the dynamic bubble regime was considered in this work (ReOR = 150 - 1000 and We = 0.002 - 4). This study has clearly shown that the presence of surfactants affects the bubble generation phenomenon and thus the interfacial area (a) and the different mass transfer parameters, such as the volumetric mass transfer coefficient (kLa) and the liquid-side mass transfer coefficient (kL). Whatever the operating conditions, the new kLa determination method has provided good accuracy without assuming that the liquid phase is perfectly mixed as in the classical method. The surface coverage ratio (se) proves to be crucial for predicting the changes of kL in aqueous solutions with surfactants
A new direct technique for visualizing and measuring gas–liquid mass transfer around bubbles moving in a straight millimetric square channel
The present paper focuses on the local characterization of gas–liquid mass transfer in a straight millimetric square channel, as constituting the preliminary step required for performing gas–liquid reactions in such devices. For this purpose, a new colourimetric technique using an oxygen sensitive dye was developed. It was based on the reduction of a colourimetric indicator in presence of oxygen, this reduction being catalysed by sodium hydroxide and glucose. In this study, resazurin was selected as the colourimetric indicator as it offered various reduced forms, the colours of which ranged from colourless (without oxygen) to pink (when oxygen was present). Thus the mass transfer around bubbles flowing in a straight millimetric square channel could be visualized in space and time. Some pictures were recorded by a monochromatic CCD high speed camera and, after post-processing, the shape, size and velocity of the bubbles, and the grey-level maps around them were measured. A calculation method was also developed to determine the transferred oxygen fluxes around the bubbles and the associated liquid-side mass transfer coefficients. The results compared satisfactorily with global measurements made using oxygen microsensors (Roudet et al., 2011. Hydrodynamics and mass transfer in inertial gas–liquid flow regimes through straigth and meandering millimetric square channels. Chem. Eng. Sci. 66, 2974–2990). This study constitutes a striking example of how interesting a tool this new colourimetric method could be for investigating gas–liquid mass transfer in transparent fluids with a view to quick millireactor design
Study of different membrane spargers used in waste water treatment : characterisation and performance
In urban waste water treatment, a novel gas sparger based on flexible rubber membrane has
been used for the last ten years. The objective of this present work is to compare two flexible
membranes (the new membrane and the old membrane provided by ONDEO-DEGREMONT
group) used in waste water treatment. For this purpose, the different membrane properties
(hole diameter, pressure drop, critical pressure, deflection at the centerline and elasticity) have
been characterized. The bubble generation at the membranes with a single orifice and with
four orifices have been studied and their performances have been compared in terms of
interfacial area and power consumption. From the experimental and theoretical approach, the
new membrane is less elastic (or more rigid) than the old membrane. The bubble diameters
generated from the new membrane remain constant with the gas velocity through the orifice,
whereas they increase logarithmically for the old membrane. The inverse behaviours are
observed in terms of the bubble formation frequency. Moreover, the bubbles generated from
the new membrane have significantly larger sizes and lower formation frequencies than those
obtained with the old one. From these results, it can be noted that the new membrane has a
behaviour comparable to a rigid orifice. No coalescence phenomenon at the bubble formation
is observed from the new and the old membranes with four orifices. The interfacial area and the power consumption are evaluated and show slight differences between the interfacial area
provided by the old and the new membranes for one value of power consumption
Theoretical Prediction of Volumetric Mass Transfer Coefficient (kLa) for Designing an Aeration Tank
The objective of this present paper is to propose a new theoretical prediction method of the volumetric mass transfer coefficient (kLa) occurring in a gas-liquid contactor based on the dissociation of the liquid-side mass transfer coefficient (kL) and the interfacial area (a). The calculated results have been compared with those obtained with the experimental process in a small-scale bubble column. Tap water was used as liquid phase and an elastic membrane with a single orifice as gas sparger. Only the dynamic bubble regime was considered in this work (ReOR= 1501000 and We = 0.0024). This study has clearly shown that, whatever the operating conditions under test, the generated bubble diameters (dB), bubble frequency (fB) and their associated rising velocities (UB) were the important parameters in order to predict, not only the values of kLa, but also the values of a and of kL. Moreover, these obtained results could provide a better understanding of the parameters which influence the oxygen transfer mechanism in the aeration process. By using the correlations to estimate these bubble hydrodynamics (dB and UB), it diminishes times for measuring the associated mass transfer parameters and also their experimental complexities and errors
Optical methods to investigate the enhancement factor of an oxygen-sensitive colorimetric reaction using microreactors
Visualization of mass transfer is a powerful tool to improve understanding of local phenomenon. The use of an oxygen-sensitive dye (colorimetric technique1) has showed its relevancy for locally visualizing and characterizing gas-liquid mass transfer at different scales2,3. At present, the occurrence of a possible enhancement of the gas-liquid mass transfer by this reaction has not been yet demonstrated. This paper aims at filling this gap by evaluating the Hatta number Ha and the enhancement factor E associated with the oxygen colorimetric reaction when implementing in milli/micro channels. For that, as data on the kinetic of the colorimetric reaction are seldom in the literature, the reaction characteristic time was firstly estimated by carrying out experiments in a microchannel equipped with a micromixer. The diffusion coefficients of dihydroresorufin and O2 were then determined by implementing two original optical methods in a specific coflow microchannel device, coupled with theoretical modelling. The knowledge of these parameters enabled at last to demonstrate that no enhancement of the gas-liquid mass transfer by this colorimetric reaction existed. Complementary information about the reliability of the colorimetric technique to characterize the gas-liquid mass transfer in milli/micro systems was also give