Analyse locale du transfert de matière associé à la formation de bulles générées par différents types d’orifices dans différentes phases liquides Newtoniennes : étude expérimentale et modélisation

De multiples procédés de transfert de matière sont basés sur la dispersion de bulles de gaz dans une phase liquide (contacteurs à bulles). L’objectif général de l’étude est de comprendre les paramètres qui conditionnent le transfert de matière gaz-liquide et donc de maîtriser les performances en condition réelle des contacteurs à bulles. Des outils expérimentaux de caractérisation (analyse d’image) ont été proposés, dans un premier temps, afin d’appréhender l’influence des caractéristiques des membranes flexibles sur l’efficacité du transfert de matière et sur le coût énergétique. L’influence de la physico-chimie (tensio-actifs et vin) a été ensuite étudiée et démontrée sur les paramètres clés que sont le diamètre et la forme de bulles générées, la fréquence de formation, la vitesse d’ascension, le coefficient de traînée et donc l’aire interfaciale d’échange (a). Une nouvelle méthode expérimentale d’obtention du coefficient volumique de transfert de matière (kLa) a été proposée. Couplée à la détermination de l’aire interfaciale (a), elle permet de dissocier les termes kL et a du kLa et donc d’obtenir le coefficient de transfert de matière local côté liquide (kL). Un modèle de prédiction original du coefficient kL basé sur les propriétés physiques et physico-chimiques a été proposé pour comprendre enfin les phénomènes physiques régissant ce paramètre en milieu complexe\ud \ud A large number of technical applications in water treatment, metallurgy, medicine, and a variety of chemical engineering processes are based on mass transfer unit operation involving a dispersion of the gas bubble into the liquid phase (gas bubble reactors). In this study, the objective is to understand the parameters which control the mass transfer efficiency and thus control the reactor performances used in the industrial operating condition.Firstly, by using the membrane gas sparger comparison techniques (image analysis), the influence of different membrane characteristics on the mass transfer efficiency and on the power consumption is observed. Then, the effect of physical chemistry (surfactant and wine) has been studied in term of the generated bubble sizes and forms, the bubble formation frequency, the terminal bubble rising velocity, the drag coefficient and thus the interfaciale area (a). Regarding to the study of mass transfer, the new experimental method for determining the local volumetric mass transfer coefficient (kLa) has been proposed. The liquid side mass transfer coefficient (kL) can be then deduced from the dissociation of kLa by a obtained experimentally. Finally, an original kL determination model based on the bubble physical properties and the physical chemistry of liquid phase has been proposed to provide a better understanding of this parameter

Separation of Emulsified Metalworking Fluid by Destabilization and Flotation

Metalworking fluids (MWFs) is one among the emulsions widely applied in various industries in machining process. Generally, MWFs consist of oil, emulsifiers, and additives, are used either in the forms of diluted and undiluted fluids. The spent metalworking fluids usually become a very stable emulsion, it requires an appropriate handling procedure. Two typical approaches for dealing with rejected MWFs are recovery and disposal, in which largely involve separation as the first essential step. This chapter presents the topics related to metalworking fluids, ranging from their types, composition, usages, lifecycle, and handling. Afterwards, processes for separating MWFs emulsion are presented, including chemical coagulation, flotation, and electrocoagulation-flotation for their background and results from experiments. Performance in separation, condition, and mechanisms of these three processes dealing with oily emulsion are shown. The understanding in the separation of MWFs by physico-chemical processes can benefit the selection of proper technology for handling of oil emulsion, either generated from machining industries or other activities such as household or petrochemical process

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

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

Prediction Model for the Treatment of Stabilized Oily Wastewater by Modified Induced Air Flotation (MIAF)

The objective of this study is to propose a prediction model for characterizing stabilized oily-wastewater that has been treated using Induced Air Flotation (IAF) and Modified Induced Air Flotation (MIAF), which is a combination of coagulation and flotation. Wastewater samples containing an anionic surfactant at the critical micelle concentration (CMC) were prepared as the stabilized oily-emulsion. The amount of oil as well as the amount removed was measured using COD. The study shows that the coagulant dosage (CAlum), interfacial area (a), and velocity gradient (G) are the important factors that affect flotation performance. Therefore, the proposed model is based on these parameters in terms of the CAlum and a/G ratio. The maximum removal efficiency and the reduction of the COD values can be predicted for any of the operating conditions. It was found that predicted oily wastewater efficiency and removal kinetic fit reasonably well with those obtained from experiments

Enhancement of Crossflow Ultrafiltration for the Treatment of Stabilized Oily Emulsions

Separation of stabilized oil droplets was conducted via crossflow ultrafiltration (UF) in a laboratory scale. A plate-and-frame membrane module was operated with two commercial organic membranes: regenerated cellulose (RC) and polyethersulfone (PES). Cutting oil was used for preparing oil-in-water emulsions. Membrane fluxes were observed under varied oil concentrations and transmembrane pressures (TMP). It was found that UF provided oil rejection more than 97% for all operational cases. The optimal operating condition was found at the oil concentration less than 1 g/L and TMP of 2−3 bar. As predicted by Hermia’s model, the dominant fouling mechanism was the cake formation upon the membrane surface. The fouled membrane was effectively regenerated by the sequential cleaning of 0.5N-SDS, 0.1N-NaOH, and 0.01N-EDTA, respectively. The cleaned membrane was acquired with 96% flux recovery (FR) and 55% resistance removal (RR). Additionally, an integration of UF and pretreatments (i.e., chemical destabilization and coalescence) could improve flux decline of the membrane, while satisfactory discharge quality was achieved

Study of Flow Pattern in Jet Clarifier for Removal of Turbidity by Residence Time Distribution Approach

This study aims to determine the performance of the jet clarifier for turbidity removal and its mechanisms for proposing the optimal operating conditions and design criteria. The experiment were performed continuously using a pilot scale jet clarifier with the volume of 243 L. Effects of liquid flow rates, types of liquid phase, and sludge blanket heights on turbidity removal efficiency were investigated. Moreover, the residence time distribution (RTD) study was carried out to investigate the flow pattern. The results indicated that the jet clarifier can effectively reduce the water with the efficiency of 80% under the optimal condition. The RTD results suggested that the flow pattern in the jet clarifier corresponded to the design as the plug flow and mixed flow conditions were found in the coagulation and the flocculation/sedimentation zones, respectively. The presence of the sludge blanket can reduce the bypass and recirculated flows. Besides, the increase of flow rate resulted in the increase recirculation in the tank. It can be suggested that the jet clarifier can be used for removing turbidity in the water treatment. The hydrodynamic in the reactor, which relates to flow pattern in the reactor, is one among the important factors in a jet clarifier

Removal of Hydrocarbons from Drill Cuttings Using Flotation Enhanced Stirred Tank (FEST)

Treatment of drill cuttings (DC) by washing processes consumes a considerable volume of solvents, resulting in high chemical wastes and operating cost. To minimize the chemical use, this work aims to develop an integrated process of dissolved air flotation (DAF) and mechanical stirring, called Flotation Enhanced Stirred Tank (FEST), as a pretreatment process for DC washing, in which total petroleum hydrocarbon (TPH) was a major pollutant of concern. The performance of an individual DC treatment process (stirring and DAF) was firstly investigated to determine the optimal experimental range. Then, response surface methodology with central composite design was applied to optimize three operational factors (saturated pressure (Ps), mixing speed (Vm), and treatment time (t)) for the integrated process, having TPH removal efficiency as the response output. Effects of hydrodynamic condition in terms of a/G ratio on the TPH removal performance were also analyzed. The experimental results revealed that mixing speed and saturated pressure were the significant factors affecting the TPH removal efficiency. FEST could yield the maximum TPH removal of 47% under the Ps of 4 bars, Vm of 400 rpm, and t of 70 min, showing its better performance than a single process from which less than 40% TPH removal was achieved. Combining DAF with stirring resulted in more turbulence in the system and thus improving the contact between hydrocarbon and bubbles. Therefore, better TPH removal could be obtained from FEST at lower a/G ratios compared to DAF. Furthermore, using saline water as a treatment medium was also possible. Overall, FEST exhibited its potential as an environmentally friendly process for the pretreatment of DC

Study of Liquid Film Forming Apparatus (LFFA) Mechanisms in Terms of Oxygen Transfer and Bubble Hydrodynamic Parameters

Aeration system is extensively applied in aquaculture and waste water treatment. It provides oxygen for organism living and mixing while consumes colossal amounts of energy for operating. Hence, the improvement of aeration system is not only providing enough oxygen and mixing but also concerning to the energy saving. Liquid film forming apparatus (LFFA) is a simple equipment that itself does not consume any power. It can be installed in existing conventional aeration system without large-scale retrofitting. Laboratory scale experiment were performed in a 190-litre aeration tank. The different types of air diffuser providing different bubble aspects were installed at the bottom of the aeration tank as the conventional diffused aeration systems. The volumetric mass transfer coefficient (kLa) of the aeration systems with LFFA are higher than the conventional systems notably. The mechanism of oxygen transfer in LFFA system can be summarized into 4 patterns: 1) Conventional mechanism, 2) Bubble collection mechanism, 3) Bubble recirculation mechanism and 4) Bubble-Liquid Foam mechanism. Then, the interfacial area (a) is improved comparing with the conventional diffused aeration system. The LFFA system should be operated with small bubble diameter generation (< 3 mm). The kLa can be increase 11 - 37 % depending on generated bubble size. By determining the additional interfacial area (a+), the bubble collection phenomena, as well as, the proper superficial gas velocity (> 0.13 m/s) can be defined and provide a better understanding on oxygen transfer mechanism in LFFA system

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