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Dive into the research topics where Mehrdji Hemati is active.

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Featured researches published by Mehrdji Hemati.


Powder Technology | 2003

Fluidized bed coating and granulation: influence of process-related variables and physicochemical properties on the growth kinetics

Mehrdji Hemati; R. Cherif; Khashayar Saleh; V. Pont

Abstract This study, which deals with the coating and granulation of solid particles by aqueous solutions of polymers or inorganic salts, aims to understand the effect of: – process-related variables such as the excess gas velocity, atomizer location, liquid flow rate and concentration, and atomizing air flow rate, – physicochemical-related variables such as the viscosity of solutions, wettability of the granulating liquid on solid particle surfaces, initial particle mean size, and porosity of the particles on the agglomeration kinetics of solid particles in a fluidized bed. The results showed that for a given particle size, the fluidizing air velocity was the most important factor affecting the growth kinetics and the stability of the operation. An increase of the relative humidity, depending on the liquid flow rate as well as the air flow rate, favor agglomeration mechanism especially for values greater than 0.4. An increase in the particle initial size leads to an enhancement of the layering mechanism, especially for values greater than 300 μm. The effect of the interfacial tension is investigated by adding different concentrations of a non-ionic surfactant to the binding solution. The effect of the contact angle is then studied using non-hydrophobic, partly hydrophobic, or totally hydrophobic particles. The growth of agglomerates appears to be favoured when the interfacial tension increases and the contact angle decreases. The viscosity of the solution has less effect than the interfacial parameters. The results show that the dominant forces in the granulation process are the capillary forces.


Powder Technology | 2003

EXPERIMENTAL STUDY AND MODELING OF FLUIDIZED BED COATING AND AGGLOMERATION

Khashayar Saleh; Daniel Steinmetz; Mehrdji Hemati

This work deals with the fluidized bed coating and agglomeration of solid particles. The effect of particle size on coating criteria was investigated using sand particles as the coating support and aqueous solutions containing NaCl as coating liquid. The results showed that both growth rate and efficiency increase with decreasing the particle size. The growth was mainly governed by layering for particles larger than 200 μm, whereas for finer particles it occurred by agglomeration. As the particle size became less than 90 μm, the coating operation led to uncontrolled growth and bed quenching. However, the coating of the same particles was successfully achieved by adding some coarser particles. In addition, a mathematical model based on the population balance concept, taking into account the simultaneous growth by layering and agglomeration, was established to predict the time evolution of the particle size distribution. The comparison between experimental and calculated data permitted the establishment of a law for the size dependency of the agglomeration kernel.


Powder Technology | 2003

A model of surface renewal with application to the coating of pharmaceutical tablets in rotary drums

C Denis; Mehrdji Hemati; D Chulia; J.-Y Lanne; B Buisson; G Daste; F Elbaz

Abstract A model of surface renewal is developed in this application for the coating of pharmaceutical tablets in rotary drums. The theoretical description of the model is based on a population balance for the processed particles, resulting in a system of partial differential equations. The tablet bed is divided in two separated domains: – where particles are dried and mixed (bulk of the bed), – where particles are coated by spray atomization of an aqueous solution of polymer (feed zone). The feed zone is located close to the surface of the tablet cascade and its size is determined by the surface impact of the spray. The model is completely determined by three parameters: the size of the feed zone, the rate of coating solution and the turnover time of the tablets in the bed. Experiments have been carried out in a batch pilot rotary drum. The results indicate that the volume of the feed zone depends on the rotational speed and the drum filling. The turnover time essentially depends on the rotational speed in the range of the studied operational conditions. The model shows that the coating mass distribution can be roughly approximated by a normal distribution centered around the mean coating mass. The model predictions are in agreement with the experimental results obtained in an industrial batch drum.


Powder Technology | 2001

Influence of the physicochemical properties on the growth of solid particles by granulation in fluidized bed

V. Pont; Khashayar Saleh; Daniel Steinmetz; Mehrdji Hemati

This work aims at investigating the effects of the physicochemical properties, such as the viscosity and the wettability of a granulating liquid on solid particles surfaces, on the agglomeration kinetics of solid particles in a fluidized bed. A series of batch experiments are carried out in a fluidized bed granulator at 50°C with 2 kg of solid particles. The use of solutions of Sodium CarboxyMethylCellulose (CMC) as binder with different concentrations leads to the study of the viscosity of the solution. The effect of the surface tension is investigated by adding different concentrations of a nonionic surfactant to the binding solution. The effect of the contact angle is then studied by using nonhydrophobic or partly or totally hydrophobic particles. The growth of agglomerates appears to be favored when the interfacial tension increases and the contact angle decreases. The viscosity of the solution has less effect than the interfacial parameters. The results point out that the dominant forces in the granulation process are the capillary forces. When an aqueous solution is pulverized into a fluidized bed containing hydrophobic particles, most of the binder is elutriated and collected in the cyclone. This shows that the main mechanism is in this case spray drying. As this process occurs at low temperature, it is useful for drying thermosensitive solutions.


Powder Technology | 1994

Hydrodynamique d'un séchoir à lit fluidisé a flottation: Détermination des vitesses caractéristiques de fluidisation de mélanges de maïs et de sable

M. Mourad; Mehrdji Hemati; C. Laguérie

Abstract This paper presents a study on the hydrodynamic behaviour of a fluidized bed dryer containing two kinds of solids (corn kernels and sand particles) with a size ratio of 25 and density ratio about 0.45. For corn-to-sand weight ratio above 0.14, the pressure drop vs gaz velocity curve of the system shows four regions, corresponding respectively to fixed bed, fully segregated, partially segregated and completely fluidized beds. This result is confirmed by the measure of the axial concentration profiles of corn in the bed. Transitions between the states occur for three characteristic velocities which have been identified: the initial fluidization velocity ( U fi ), the apparent fluidization velocity ( U fa ) and the complete fluidization velocity ( U fc ) of the binary system. U fi is practically independent of the corn-to-sand weight ratio, while U fa and U fc increase monotonically with this parameter. Correlations have been proposed for these velocities.


Powder Technology | 2001

Silicon Chemical Vapor Deposition (CVD) on microporous powders in a fluidized bed

S Kouadri-Mostefa; P Serp; Mehrdji Hemati; Brigitte Caussat

The fluidized bed Chemical Vapor Deposition (CVD) process constitutes today one of the most efficient techniques to modify the surface properties of dense (nonporous) powders. The success encountered in this field is the reason why this process has been extended to the treatment of microporous powders, for which there are very important industrial interests. The article presents an experimental analysis of the influence of the main operating parameters on the spatial localisation and the nanostructure of silicon deposits from silane on microporous powders. The first results obtained show that uniform deposits appear on all the surfaces of the pores in the form of discontinuous rafts. These results, which will be coupled with a theoretical predictive model, prove that the control at the nanometric scale of the organization of CVD materials on microporous powders today has become possible.


Powder Technology | 2001

Natural gas combustion in fluidised bed reactors between 600 and 850°C: experimental study and modelling of the freeboard

S. Dounit; Mehrdji Hemati; D. Steinmetz

Abstract In this paper, an experimental study of the natural gas–air mixture combustion in a fluidised bed containing sand particles with 350-μm mean diameter and operating at temperatures lower than the critical temperature (less than 850 °C) has been presented. A particular attention has been given to the freeboard zone where the main part of the reaction rate takes place at such temperatures. The experimental results obtained at different operating conditions has shown the essential role played by the projection zone in the global thermal efficiency of the reactor working between 600 and 800 °C. Parallel to this experimental study, a model of natural gas–air combustion taking in consideration the interaction between dense and dilute regions of the reactor has been proposed. The proposed model takes into account thermal exchange by conduction and radiation in both between gaseous and particular phases, and gas–particle suspension and reactor walls. The predictions of this model agree well with the experimental data.


Powder Technology | 1994

Caractéristiques au minimum de fluidisation et expansion des couches fluidisées de particules de la catégorie D de Geldart

K. Tannous; Mehrdji Hemati; C. Laguérie

Abstract This paper presents results of an experimental study concerning the hydrodynamic behaviour at incipient fluidization and expansion of fluidized beds of large particles of category D of Geldarts classification. The effect of the physical properties of the solids and of the column geometry was analysed particularly with air at room temperature and atmospheric pressure. Relationships have been derived to predict the gas velocity and the voidage at incipient fluidization and expansion of the beds. These correlations take into account all the literature data relative to this kind of particle.


Computing Letters | 2005

Close-packing transitions in clusters of Lennard-Jones spheres

Florent Calvo; Mohammed Benali; Vincent Gerbaud; Mehrdji Hemati

The structures of clusters of spherical and homogeneous particles are investigated using a combination of global optimization methods. The pair wise potential between particles is integrated exactly from elementary Lennard-Jones interactions, and the use of reduced units allows us to get insight into the effects of the particle diameter. As the diameter increases, the potential becomes very sharp, and the cluster structure generally changes from icosahedral (small radius) to close-packed cubic (large radius), possibly through intermediate decahedral shapes. The results are interpreted in terms of the effective range of the potential.


Drying Technology | 2016

Determination of PVC powder drying kinetics at particle scale: Experimental study and modeling

Antoine Aubin; Renaud Ansart; Mehrdji Hemati; Thierry Lasuye; Marc Branly

ABSTRACT An original experimental method is used to determine drying kinetic at particle scale. The particle scale kinetics was obtained by immersion of a small mass of wet polyvinyl chloride (PVC) particles (cake) in a batch dense fluidized bed containing inert hot particles (glass bead). The results are summarized here and prove clearly that the PVC drying is controlled by a competition between internal and external transfers. The drying kinetic was described by a particle scale model taking into account the convective–diffusive (mass transfer) and the convective–evaporative (heat transfer) phenomena. To validate this model with the experimental data, the experimental fluidized bed dryer is modeled following two different approaches: a perfect stirred reactor model and a 3D numerical simulation using the multiphase flow code NEPTUNE_CFD. The aim of this 3D simulation is to simulate the phenomena occurring, at local scale, in a dense fluidized bed dryer and to show the limitations of the perfect stirred reactor model.

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Daniel Steinmetz

Centre national de la recherche scientifique

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Gilles Flamant

Centre national de la recherche scientifique

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