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

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

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

Corrigendum to "Hydrodynamic and mass transfer in inertial gas-liquid flow regimes through straight and meandering millimetric square channels" [Chem. Eng. Sci. 66 (2011) 2974-2990]

Heat-exchanger reactors are an important part of process intensification technology. For plate geometries, one solution for intensifying transfer and increasing residence times is to construct two-dimensional meandering channels. Supported by this scientific context, the present work aims at characterising gas-liquid mass transfer in the same square millimetric meandering channel, as in Anxionnaz (2009), this constituted the preliminary step required for performing exothermic gas-liquid reactions. Firstly, the gas-liquid hydrodynamics were characterised for a water/air system. When compared to a straight channel of identical compactness and sectional-area (2×2 mm²), the meandering channel induced (i) a delay in the transition from Taylor to annular-slug regimes, (ii) a rise of 10-20% in bubble lengths while conserving almost identical slug lengths, (iii) higher deformations of bubble nose and rear due to centrifugal forces (bends). Secondly, an original method for verifying the relevancy of the plug flow model and accurately determining kla was used (measurements of concentrations in dissolved oxygen along the channel length). For the Taylor flow regime, kla increased coherently when increasing jg, and the meandering geometry had a small influence. On the contrary, this effect was found no more negligible for the slug-annular flow regime. Whatever the channels, the NTUl remained low, thus showing that, even if millimetric channels allowed to intensify kla, a special attention should be paid for generating sufficient residence times. At identical compactness, the meandering channel was found to be the most competitive. Finally, results on gas-liquid interfacial areas and mass transfer coefficients were confronted and discussed with respect to the predictions issued from the model developed by Van Baten and Krishna (2004)

Hydrodynamic and mass transfer in inertial gas–liquid flow regimes through straight and meandering millimetric square channels

Heat-exchanger reactors are an important part of process intensification technology. For plate geometries, one solution for intensifying transfer and increasing residence times is to construct two-dimensional meandering channels. Supported by this scientific context, the present work aims at characterising gas–liquid mass transfer in the same square millimetric meandering channel, as in Anxionnaz (2009), this constituted the preliminary step required for performing exothermic gas–liquid reactions. Firstly, the gas–liquid hydrodynamics were characterised for a water/air system. When compared to a straight channel of identical compactness and sectional-area (2×2 mm2), the meandering channel induced (i) a delay in the transition from Taylor to annular-slug regimes, (ii) a rise of 10–20% in bubble lengths while conserving almost identical slug lengths, (iii) higher deformations of bubble nose and rear due to centrifugal forces (bends). Secondly, an original method for verifying the relevancy of the plug flow model and accurately determining kla was used (measurements of concentrations in dissolved oxygen along the channel length). For the Taylor flow regime, kla increased coherently when increasing jg, and the meandering geometry had a small influence. On the contrary, this effect was found no more negligible for the slug-annular flow regime. Whatever the channels, the NTUl remained low, thus showing that, even if millimetric channels allowed to intensify kla, a special attention should be paid for generating sufficient residence times. At identical compactness, the meandering channel was found to be the most competitive. Finally, results on gas–liquid interfacial areas and mass transfer coefficients were confronted and discussed with respect to the predictions issued from the model developed by Van Baten and Krishna (2004)

A consistent dimensional analysis of gas–liquid mass transfer in an aerated stirred tank containing purely viscous fluids with shear-thinning properties

This paper deals with gas–liquid mass transfer in an aerated stirred tank containing Newtonian or shearthinning fluids. The aim is to demonstrate that, for a given mixing system, an unique dimensionless correlation gathering all the mass transfer rates (150 kla measurements) can be obtained if and only if the variability of the rheological material parameters is correctly considered when implementing the theory of similarity. More particularly, it is clearly illustrated that a too gross simplification in the relevant list of the parameters characterizing the dependence of apparent viscosity with shear rates leads to pitfalls when building the PI-space set. This is then a striking example showing that a robust predictive correlation can be established when the non-constancy of fluid physical properties ceases to be neglected

Photochemical synthesis of a “cage” compound in a microreactor: Rigorous comparison with a batch photoreactor

An intramolecular [2 + 2] photocycloaddition is performed in a microphotoreactor (0.81 mL) built by winding FEP tubing around a commercially available Pyrex immersion well in which a medium pressure mercury lamp is inserted. A rigorous comparison with a batch photoreactor (225 mL) is proposed by means of a simple model coupling the reaction kinetics with the mass, momentum and radiative transfer equations. This serves as a basis to explain why the chemical conversion and the irradiation time are respectively increased and reduced in the microphotoreactor relative to those in the batch photoreactor. Through this simple model reaction, some criteria for transposing photochemical synthesis from a batch photoreactor to a continuous microphotoreactor are defined