Christophe Guy

Understanding gas-phase hydrodynamics in bubble columns : a convective model based on kinetic theory

Abstract Bubble column hydrodynamics exhibit a bubbly flow regime at low superficial gas velocity and a churn turbulent regime at higher superficial gas velocity, except in small diameter columns where slugging is observed. A convective model developed previously is compared to dynamic gas disengagement (DGD) data. Theoretical disengagement curves calculated from the convective model bubble velocity distribution (at steady state) compare well to DGD curves at the transition point. Our analysis of DGD curves using the convective model indicates that in the churn turbulent regime, gas hold-up consists of a superposition of large bubbles on a transition point bubble velocity distribution. A kinetic model for gas-phase hydrodynamics capable of describing both the bubbly and churn turbulent flow regimes is proposed. Absolute bubble velocity distributions are calculated based on an ideal bubble velocity distribution and rules of bubble-bubble interaction. A transition zone is predicted by the model. Overall gas hold-up is predicted in the bubbly and churn turbulent flow regimes and the fraction of gas hold-up in the form of large bubbles is determined in the churn turbulent flow regime. The transition zone is in agreement with the literature and the large bubble fraction in the churn turbulent flow regime is well predicted by the model.

Gas phase hydrodynamics of a gas-solid turbulent fluidized bed reactor

Transient gas mixing tests in a gas-solid fluidized bed are analyzed with a dispersive plug flow model from the bubbling to the turbulent regime, and with the two-phase model of van Deemter. It is shown that dispersion fluctuations can also be used besides pressure fluctuations or capacitance probes to characterize the transition between the bubbling and the turbulent regime. Furthermore, the gas Peclet number in the turbulent regime is correlated in terms of the operating parameters, the bed diameter and the particles and gas properties as: Pe=7.10−2 Ar0.32 δ−0.4 The cross-flow mass transfer coefficient α12 of the two-phase model of van Deemter in the bubbling and the turbulent regimes is correlated in terms of the height of a transfer unit as: Uα12=0.613Sc−0.37 The parameters from both correlations are coupled with the kinetics of an industrial catalyst to model the performance of a catalytic combustor.

Measurement of odor intensity by an electronic nose

ABSTRACT The possibility of using electronic noses (ENs) to measure odor intensity was investigated in this study. Two commercially available ENs, an Aromascan A32S with conducting polymer sensors and an Alpha M.O.S. Fox 3000 with metal oxide sensors, as well as an experimental EN made of Taguchi-type tin oxide sensors, were used in the experiments. Odor intensity measurement by sensory analysis and EN sensor response were obtained for samples of odorous compounds (n-butanol, CH3COCH3, and C2H5SH) and for binary mixtures of odorous compounds (n-butanol and CH3COCH3). Linear regression analysis and artificial neural networks (ANN) were used to establish a relationship between odor intensity and EN sensor responses. The results suggest that large differences in sensor response to samples of equivalent odor intensity exist and that sensitivity to odorous compounds varies according to the type of sensors. A linear relationship between odor intensity and averaged sensor response was found to be appropriate for the EN based on conducting polymer sensors with a correlation coefficient (r) of 0.94 between calculated and measured odor intensity. However, the linear regression approach was shown to be inadequate for both ENs, which included metal oxide-type sensors. Very strong correlation (r = 0.99) between measured odor intensity and calculated odor intensity using the ANN developed were obtained for both commercial ENs. A weaker correlation (r = 0.84) was found for the experimental instrument, suggesting an insufficient number of sensors and/or not enough diversity in sensor responses. The results demonstrated the ability of ENs to measure odor intensity associated with simple mixtures of odorous compounds and suggest that ANN are appropriate to model the relationship between odor intensity measurement and EN sensor response.

Natural gas combustion in a catalytic turbulent fluidized bed

Catalytic fluidized bed combustion of natural gas is shown to be an emerging technology capable of meeting all environmental constraints as far as nitrogen oxides and carbon monoxide are concerned. This technology uses powder catalysts in the turbulent flow regime where the gas-solid contact is optimal so as to maintain a high combustion efficiency. In fact, the catalytic combustion carried out in both the bubbling and the turbulent regimes at 450–500°C shows that the turbulent regime is more favorable. A single phase plug flow model with axial dispersion is shown to fit satisfactorily the data obtained at 500°C where the combustion efficiency is very good. A self-sustained combustion was achieved with a mixture of 4% methane at around 500°C with a complete conversion of methane and a zero emission of NOx and CO.

CATALYTIC COMBUSTION OF NATURAL GAS IN A FIXED BED REACTOR WITH FLOW REVERSAL

A new application of the fixed bed catalytic reactor with flow reversal for combustion of natural gas is investigated by mathematical modeling and computer simulation. Comparison between the results obtained for this new reactor and those for a classic catalytic fixed bed is made. Inexpensive perovskite type catalysts containing no noble metals were used. It is shown that an appropriate choice of operating parameters (concentration and temperature of input gas mixture, superficial gas velocity, size and shape of catalyst and inert material, volumetric ratio between catalyst and inert material in the bed) allows for a methane combustion at must lower temperatures in the reactor with flow reversal than in a classic catalytic reactor. Under such a low temperature combustion, no nitrogen oxides are produced.

Photocatalytic oxidation of volatile organic compounds using fluorescent visible light.

Abstract Photocatalytic oxidation (PCO) of volatile organic compounds (VOCs) is a highly attractive alternative technology for purification and deodorization of indoor air. The main objectives of this study were to demonstrate that a common fluorescent visible light (FVL) lamp can be used to effectively remove by PCO low concentrations of VOCs from slightly contaminated air and to provide some fundamental and technical details on the process. The target VOC was n-butanol, which is a standard reference odorant. Its PCO was studied under a long residence time in a 3.7-L cylindrical reactor with commercial titanium dioxide (TiO2) as the reference photocatalyst and using mostly FVL for illumination. For comparison only, a UV (black) light lamp was used. The gas-phase products were detected and quantified online by gas chromatography (GC). The effects of reactor residence time, of inlet concentration, and of the relative light intensity on the efficiency of the process were also evaluated. At a high n-butanol concentration (0.1 vol %), butanal and propanal were identified as the intermediate products of the process; ethanal appeared when the initial concentration was <850 ppmv. This indicates that PCO leading to CO2 and H2O is relatively slow and proceeds in a stepwise manner. Although the efficiency of the process with an FVL lamp was significantly lower than when using a UV black light, complete PCO of low concentrations was achieved for 100 ppmv. In a search for a material with photoactivation extended to higher wavelengths or increased photo-activity, several samples of transition metal- or silver ion-doped (2 atomic %) TiO2 as well as SrTi1xFexO3 (x = 0.1 and 0.15) perovskites were included in the study. None of these materials was more active than pure TiO2. The results of this study open new horizons in the area of indoor air quality (IAQ) control.

Two-phase model for a catalytic turbulent fluidized-bed reactor: Application to ethylene synthesis

A turbulent fluidized-bed (TFB) reactor for the ethylene synthesis by catalytic oxidation of natural gas was simulated employing a two-phase model, and the hydrodynamic structure of the TFB was characterized for the MgO catalyst particles. The overall gas phase distribution in bubbles and emulsion phases was estimated by using the probability distribution function of local voidage fluctuations in the bed. The mean voidage corresponding at the bubble phase increased with gas superficial velocity according to a first-order model, and the mean voidage in emulsion phase rose proportionally with gas superficial velocity. Moreover, the emulsion/bubble voidage ratio at the center of the bed was almost constant in the turbulent regime, and radial profiles of the phase distribution ratio were observed in the turbulent bed. The two-phase model developed predicted satisfactorily the experimental data and can be used to quantify the influence of homogeneous and catalytic reaction in the TFB for the oxidative coupling of methane.

Solids mixing in gas-liquid-solid fluidized beds : Experiments and modelling

Mixing in monosized particles and binary mixtures of solids in three-phase fluidized beds is investigated by means of a non-invasive Radioactive Particle Tracking technique (RPT). Pulses of particles at different column heights are constructed from the trajectory of a single radioactive tracer whose motion is tracked for several hours. For each pulse released, number distributions of particles in the axial direction are thus obtained for each instant of time from injections at different axial positions in the reactor. Using this information, axial mixing times for the solids are measured for the experimental conditions studied. A one-dimensional two-zone model based on the three-phase counter-current backmixing model used for gas-solid fluidization (Gwyn et al., 1970) is proposed and solved for these conditions to calculate solids axial number distributions and mixing times. Agreement between experimental and predicted results is satisfactory.

Characterization of bubble column hydrodynamics with local measurements

The objective of this work is to study the hydrodynamics of the liquid phase and its interaction with the gas-phase hydrodynamics. The local liquid flow is investigated by means of thermal pulse anemometry. That is, local residence time distributions and local velocity distributions are experimentally measured. The study is carried out in the liquid upflow region and through the homogenous and heterogeneous flow regimes. Liquid mixing is analysed separately on the basis of two different assumptions: as a dispersive mechanism or as a convective mechanism. Moreover, the liquid velocity distributions are compared with global absolute bubble velocity distributions.

Mean and Turbulent Particle Velocity in the Fully Developed Region of a Three-Phase Fluidized Bed

A thorough experimental description of the time-averaged solids flow in the fully developed region of a cylindrical gas-liquid-solid fluidized bed was provided by using a noninvasive radioactive particle tracking technique (RPT). The 3-D local instantaneous velocity components (radial, axial, azimuthal) of a single radioactive solid tracer, having properties identical to those of the solids in the bed, were measured noninvasively over extended time periods to establish the radial distributions in the fully developed region of axial and radial particle mean and turbulent velocities, shear stress, and axial and radial eddy diffusion coefficients. Glass beads and polyvinyl chloride particles of various shapes and sizes were water-fluidized singly or as binary mixtures in a 100 mm ID Plexiglas column over air and water superficial velocity ranges that spanned the dispersed bubble, coalesced bubble, and transitional flow regimes.