Jacques Bouillard

Hydrodynamic simulation of gas-solid flow in a riser using kinetic theory of granular flow

Abstract The dynamic behavior of gas–solids flow in a 6-m high riser was predicted using a transient two-dimensional (2D) hydrodynamic model based on the kinetic theory of granular flows. Instantaneous and local gas-particle velocity, void fraction and turbulent parameters were obtained. Predicted time-averaged particle concentrations and velocities reflect the classical core-annular flow structure in agreement with experimental measurements, in particular, with those reported by Miller and Gidaspow [AIChE J. 38 (1992) 1801]. Predicted instantaneous solids concentration frequencies compared well with the experimental data for various regions of the riser. Computed total granular temperature distributions in the riser qualitatively agree with experimental data. High thermal conductivities of fluidized powders (about 50 times that of the fluidizing gas) were estimated from the kinetic theory without adjusted parameters. Effects of initial conditions, inlet geometry, riser diameter and riser vertical inclination were assessed. Unexpected strong distortions of solids concentrations and vertical fluxes were predicted for small inclination angles of the order of 2°. Analysis of experimental data should, therefore, be carefully conducted to ensure that riser inclination is not too important over the length of the riser in order to eliminate potential artifacts due to this geometric parameter.

Study of hydrodynamic behaviour in bubble columns and external loop airlift reactors through analysis of pressure fluctuations

Recent methods of regime identification based on pressure fluctuations analysis have been applied both in a bubble column and an external loop airlift reactor with several spargers. Their ability to determine regime transition and to extract regime features is compared. A new method based on the auto-correlation function is proposed. This method is shown to be simple and efficient. It also provides quantitative information about the characteristic time and the axial dimension of the flow structure in the prevailing regime.

State-of-the-art review of erosion modeling in fluid/solids systems

Abstract Erosion in fluidized-bed combustors, commercial process units used to burn coal cleanly, has surfaced as a serious issue that may have adverse economic effects. The evidence suggests that the key to understanding this erosion is detailed knowledge of the coupled and complex phenomena of solids circulation and bubble motion. The FLUFIX computer code has been developed for this purpose. Computed hydrodynamic results compare well with experimental data (including the bubble frequency and size and the time-averaged porosity and pressure distributions) taken in a thin ‘two-dimensional’ rectangular fluidized beds containing a rectangular obstacle and a few-tube approximation of the International Energy Agency Grimethorpe tube bank ‘C1’ configuration. Six representative erosion models selected from the literature, comprising both single-particle and fluidized-bed models are critiqued. A methodology is described whereby the computed hydrodynamic results can be used with such erosion models. Previous attempts (none involving fluidized beds) to couple fluid mechanics and erosion models are reviewed. The energy dissipation models are developed, and are shown to generalize the so-called power dissipation model used to analyze slurry jet pump erosion. It is demonstrated, by explicitly introducing the force of the particle on the eroding material surface, that impaction and abrasive erosion mechanisms are basically the same. In doing so, it has been possible to unify the entire erosion literature developed for over a century. Linkage is made to two previously developed single-particle erosion models: Finnies and Neilson and Gilchrists. The implementation methodology, which can be applied to any erosion model, be it single-particle or fluidized bed, is summarized. The monolayer energy dissipation (MED) erosion model is developed. The erosion rates computed from the EROSION code are compared with each other and for the cold few-tube approximation of the IEA Grimethorpe tube bank ‘C1’ fluidized-bed experiment, and with other available erosion data literature to validate the calculations. The simplified closed form MED (SCFMED) erosion models and erosion guidelines are developed using semi-empirical correlations in order to allow quick engineering estimates of erosion. Alternative methodologies to couple hydrodynamics and erosion using the kinetic theory of granular flow and discrete element method (DEM) models are briefly reviewed. Finally, a critical review of the integrated experimental and computational fluid dynamics (CFD) pressurized fluidized-bed hydrodynamics and erosion research ongoing at Chalmers University is presented. This body of work has been influenced by the research at Argonne National Laboratory (ANL) and Illinois Institute of Technology (IIT) and reinforces the trends and conclusions reported in this review.

Liquid flow velocity measurements in stirred tanks by ultra-sound Doppler velocimetry

Abstract A new first-of-its kind technique based on ultra-sound pulsed-Doppler velocimetry (UPDV) is proposed to measure flow velocities in stirred tanks as a non-intrusive manner for laminar and turbulent regimes of Newtonian and Non-Newtonian (complex mixture) fluids. Two ultra-sound probes at 1 and 2 MHz were tested and the salient features of these probes were analyzed. Measured velocity profiles in a stirred tank compared favorably with those obtained by microimpeller (intrusive) method and by laser Doppler velocimetry (LDV non-intrusive) technique. The presence of bubbles in the liquid phase greatly hampers the reliability of the UPDV technique, but when this constraint is removed or minimized, the technique remains adequate for rapid spatial mapping of flow velocity in pure liquid and emulsion systems.

Numerical Simulations of Hydrodynamic Behaviour in Spouted Beds

A hydrodynamic model of dense gas-solid flow was developed to predict the flow behaviour in spouted beds. Constitutive equations describing the particulate solids pressure, viscosity and elasticity moduli were implemented into a hydrodynamic simulation computer program. The resulting hydrodynamic simulations agree well with experimental measurements of time-averaged particle-and-gas velocity profiles, as well as with experimental solids concentration distributions obtained from three different articles issued from the fluidization literature.

Ignition and explosion of nanopowders: something new under the dust

This work deals with the study of ignition and explosion characteristics of nanoparticles. It has been carried out on various powders: zinc, aluminum, carbon blacks... Specific behaviours have been highlighted during the first phase of this project (Nanosafe 2). For instance, it has been demonstrated that there mainly exists two combustion regimes that are either kinetically controlled, for small size particles, or diffusion controlled, for large size particles (generally with diameters greater than 1 or 2 µm). It has been found that as the particle size decreases, minimum ignition temperature and minimum ignition energy decrease (even lower than 1 mJ), indicating higher potential inflammation and explosion risks for metallic nanopowders. Moreover, the presence of agglomerates in the nanopowders could modify their reactivity. Thus, the explosion severity of Al powders tends to increase as the specific surface area decreases, before reaching a peak for 1 µm particle size. These results are essential for industries producing or handling nanopowders in order to propose/design new and proper prevention and protection means. Nevertheless, the validity of the classical characterization tools with regard to nanopowders should be discussed. For example, the experimental laminar flame velocity of Al dusts has been compared to a theoretical one, determined by Huangs model, which assumes that the propagation of the flame is run mainly by conduction. It has shown a good agreement. However, under certain conditions, the Al flame propagation is expected to be mainly conducted by radiation. Two hypotheses can then be made. On the one hand, it can be assumed that the 20 L sphere probably disturbs the flame propagation and thermal mechanisms by absorbing radiation (wall quenching effect). On the other hand, it has been observed, thanks to the use of a high speed camera that the preheating zone is smaller for some nanopowders than for micro-particles (figure below). It could notably be explained by the fact that the flame radiation is absorbed by the cloud of unburnt Al nanopowders. Several other factors may have an impact on the explosion severity. If these points are correctly addressed, it will be possible to get more reliable ignition and explosion characteristics.

Development of a complete model for an air-lift reactor

An 1D hydrodynamic model has been developed for gas hold-up and liquid circulation velocity prediction in air-lift reactors. The model is based on momentum balance equations and has been adjusted to experimental data collected on a pilot plant reactor equipped with two types of gas distributors and using water and water/butanol as the liquid phase. Agreement between the hydrodynamic model and pilot experimental points is shown to be fairly good. Different techniques of signal analysis have also been applied to pressure fluctuations in order to extract information about flow regimes and regime transitions. A good knowledge of the flow pattern is essential to establish adequate hydrodynamic correlations. This model has also been combined with mass transfer and the kinetics of a chemical reaction to yield a complete model of the performance of a reactor.

A complete model for oxidation air-lift reactors

Abstract A complete model for prediction performance of an oxidation air-lift reactor has been developed. The model includes reaction kinetics, flow configuration, mass transfer and hydrodynamics. Liquid bulk and gas phases are modeled using a cell model. The mass transfer rates are computed from the film model. Hydrodynamics parameters are calculated with an adequate model based on balance equations and on experimental data. The model predicts the variations of concentrations and hydrodynamics parameters along the reactor and hence is able to provide a good description of the reactor.

Explosion risks from nanomaterials

Emerging nanomanufactured products are being incorporated in a variety of consumer products ranging from closer body contact products (i.e. cosmetics, sunscreens, toothpastes, pharmaceuticals, clothing) to more remote body-contact products (electronics, plastics, tires, automotive and aeronautical), hence posing potential health and environmental risks. The new field of nanosafety has emerged and needs to be explored now rather than after problems becomes so ubiquitous and difficult to treat that their trend become irreversible. Such endeavour necessitates a transdisciplinary approach. A commonly forgotten and/or misunderstood risk is that of explosion/detonation of nanopowders, due to their high specific active surface areas. Such risk is emphasized and illustrated with the present development of an appropriate risk analysis. For this particular risk, a review of characterization methods and their limitations with regard to nanopowders is presented and illustrated for a few organic and metallic nanopowders.

Nanoparticle risk management and cost evaluation: a general framework

Industrial production of nano-objects has been growing fast during the last decade and a wide range of products containing nanoparticles (NPs) is proposed to the public in various markets (automotive, electronics, textiles...). The issues encountered in monitoring the presence of nano-objects in any media cause a major difficulty for controlling the risk associated to the production stage. It is therefore very difficult to assess the efficiency of prevention and mitigation solutions, which potentially leads to overestimate the level of the protection barriers that are recommended. The extra costs in adding nano-objects to the process, especially that of nanosafety, must be estimated and optimized to ensure the competitiveness of the future production lines and associated products. The risk management and cost evaluation methods presented herein have been designed for application in a pilot production line of injection-moulded nanocomposites.