B. Scarlett

JET BREAK-UP IN ELECTROHYDRODYNAMIC ATOMIZATION IN THE CONE-JET MODE

The jet break-up mechanism has been investigated with a high-resolution camera . A model is presented, which is able to predict the droplet size, the velocity at jet break-u p, and the wavelength at jet break-up. A new theoretical derivation of the droplet size scaling will be given. It was found that the jet break-up mechanism depends on the ratio of the electric normal st ress over the surface tension stress. At a low value of this ratio, the jet breaks up due to varicose instabilities. The number of secondary droplets is much lower than the number of main droplets. With increasing flow rate, the current increases, the stress ratio increases, and the number of secondary dro plets and satellites increases. A threshold value of the stress ratio on the jet was found, above which the jet starts to whip. In order to reduce the number of secondary droplets, the current through the liquid cone should be reduced. It is shown, that viscosity, surface charge, and the acceleration of the jet, have to be taken into account in the jet break-up process. The main droplet diameter for varicose jet break-up scales with the flow rate as dd=Q0.48. When, the jet breaks up in the whipping regime, then the main droplet size scales as dd=Q0.33.

A Simple Model for the Evolution of the Characteristics of Aggregate Particles Undergoing Coagulation and Sintering

A simple model describing the evolution of particle morphology, size, and number concentration by coagulation and sintering is presented that neglects the spread of the polydispersity of aggregate and primary particles. The influence of irregular/fractal structure on the collision kernel is accounted for, from the free molecular to the continuum regime. The model predictions compare well to those of a detailed two-dimensional sectional model of nonspherical particle dynamics. After a theoretical evaluation of the main sintering mechanism, the proposed model was applied to laser synthesis of silicon in an aerosol reactor.

Growth regime map for liquid-bound granules: further development and experimental validation

An attempt was made to quantify the boundaries and validate the granule growth regime map for liquid-bound granules recently proposed by Iveson and Litster (AlChE J. 44 (1998) 1510). This regime map postulates that the type of granule growth behaviour is a function of only two dimensionless groups: the amount of granule deformation during collision (characterised by a Stokes deformation number, St(def)) and the maximum granule pore saturation, s(max). The results of experiments performed with a range of materials (glass ballotini, iron ore fines, copper chalcopyrite powder and a sodium sulphate and cellulose mixture) using both drum and high shear mixer granulators were examined. The drum granulation results gave good agreement with the proposed regime map. The boundary between crumb and steady growth occurs at St(def) of order 0.1 and the boundary between steady and induction growth occurs at St(def) of order 0.001. The nucleation only boundary occurs at pore saturations that increase from 70% to 80% with decreasing St(def). However, the high shear mixer results all had St(def) numbers which were too large. This is most likely to be because the chopper tip-speed is an over-estimate of the average impact velocity granules experience and possibly also due to the dynamic yield strength of the materials being significantly greater than the yield strengths measured at low strain rates. Hence, the map is only a useful tool for comparing the granulation behaviour of different materials in the same device. Until we have a better understanding of the flow patterns and impact velocities in granulators, it cannot be used to compare different types of equipment. Theoretical considerations also revealed that several of the regime boundaries are also functions of additional parameters not explicitly contained on the map, such as binder viscosity

Population balances for particulate processes—a volume approach

Abstract Population balance models have been used in chemical engineering since the 1960s and have evolved to become the most important tools for design and control of particulate processes. In this paper we show that the intrinsic particle parameter that determines changes in the process and should thus be included in the population balance is the particle volume. The basic population that is modeled should be the mass distribution, or the volume distribution if the density is constant. The population balance thus describes the change of the volume distribution of volume with time. Furthermore, we suggest that the “birth” and “death” terms that are often used to describe discrete events in particulate processes can almost always be replaced by a rate of change term. To design and control existing and future processes, a multi-dimensional population balance model is required. We propose a volume-based model in which the particle properties that are modeled are the volumes of solid, liquid, and air, respectively. In the most general case the model will consist of a properties vector and a distribution tensor. Depending on the complexity of the process, one or more of the properties may be omitted from the model. This is shown in three examples of increasing complexity: comminution, sintering, and granulation.

THE EVOLUTION OF ELECTROHYDRODYNAMIC SPRAYS PRODUCED IN THE CONE-JET MODE,A PHYSICAL MODEL

Abstract This study concerns the effects that occur in a spray produced by Electrohydrodynamic Atomization in the Cone-Jet mode. Droplet sizes and velocities were measured at various places in the spray. These results are compared to the dispersion calculated with a physical model. The model takes the drag force, electric forces, and the droplet inertia into account. The results show the electrical interactions between the charged droplets. They also show an acceleration of the gas in the center of the spray. A relation between droplet size and droplet charge in a spray could be established. The results of the model confirm that the mutual electric interaction of charged droplets, and differences in inertia, are the reason for the size segregation effect in the spray.

Dust explosions in spherical vessels: The role of flame thickness in the validity of the ‘cube-root law’

Abstract A well known limitation of the ‘cube-root law’ is that it becomes invalid when the flame thickness is significant with respect to the vessel radius. In the literature, flame thicknesses in dust-air mixtures ranging from 15 to 80 cm have been reported, which exceed the radii of the 20 litre sphere and the 1 m3 vessel. Therefore, we have developed a model (the three-zone model) for the pressure evolution of confined dust explosions in spherical vessels which takes the flame thickness into account. The pressure-time curves that are generated with this model show a good resemblance with those measured in practice. It is shown by numerical simulations that the maximum rate of pressure rise can be normalized with respect to the vessel volume as well as to the flame thickness and that the ‘cube-root law’ becomes inaccurate for relative flame thicknesses exceeding 1%. Furthermore, the actual burning velocity and the flame thickness during real dust explosions can be obtained by fitting the model to the experimental pressure-time curve.

Generation of micron-sized droplets from the Taylor cone

The development of an aerosol generator is reported. It makes use of a strong electric field, in which a pendular droplet deforms into a conical shape. This cone shaped droplet is known as the Taylor cone. From the tip of the cone droplets are generated at a frequency of about 108–1010 Hz. The initially highly charged droplets repel each other thus preventing coalescence but can be neutralized by use of a strategically-placed needle of opposite polarity. In this study droplets of ethylene glycol were produced, whose modal diameter varied with increasing potential between 1.33 and 1.55 μm. Their standard deviation decreased from 0.3 to 0.2 μm. By using mixtures of DOP and ethanol, droplets of DOP as small as 0.08 μm were produced by evaporation of the ethanol. A calibration curve for the droplets produced before and after evaporation is given for this particular apparatus configuration. The Taylor cone exhibits current-voltage characteristics which are analogous to those of a corona discharge. Other phenomena characterizing corona discharge are also found.

New developments in particle characterization by laser diffraction: size and shape

Abstract Laser diffraction has become a popular technique in many fields for measuring particle size distributions (PSDs). It is not only one of the standard techniques for laboratory measurements but is also gaining importance for on-line process monitoring and process control. This article reports some developments of this technique while recognizing the potential that still exists. In this report both particle size and shape are taken into account. For optimum results, both effective sensing and analysis of the diffraction pattern are required. This is the goal of our present study with a special interest in on-line application. In this paper, a statistical approach — principal component analysis (PCA) — to the scattered light signals from a conventional detector array is shown to improve the sensitivity to a few large particles relative to the main size distribution. For determining particle shape, a wedge-type photo-detector for sensing the azimuthal light intensity has been applied together with the procedures of cross-correlation, sweep selection for single particle presence and Fourier analysis. Finally, a novel light scattering sensor is presented, which enables the measurement of both size and shape and which offers even more promising applications for on-line process control.

Agglomeration behaviour of powders in a Lödige mixer granulator

Abstract The agglomeration of a powder mixture which is commonly used to make granules containing enzyme was examined in a high shear mixer granulator of the Lodige type. The validity and extension of current granulation theory for practical high shear granulation was investigated. The effects of process variables such as the amount of binder liquid, choppe impact, binder viscosity and temperature on the granulation were in agreement with the theory. The onset of the different granulation mechanisms (nucleation, compaction, coalescence, and crushing and layering) was demonstrated.

Application of matrix‐assisted laser desorption/ionization to on‐line aerosol time‐of‐flight mass spectrometry

Matrix-assisted laser desorption/ionization (MALDI) mass spectra were obtained from single biological aerosol particles using an aerosol time-of-flight mass spectrometer (ATOFMS). The inlet to the ATOFMS was coupled with an evaporation/condensation flow cell that allowed the aerosol to be coated with matrix material as the sampled stream entered the spectrometer. Mass spectra were generated from aerosol composed either of gramicidin-S or erythromycin, two small biological molecules, or from aerosolised spores of Bacillus subtilis var niger. Three different matrices were used: 3-nitrobenzyl alcohol, picolinic acid and sinapinic acid. A spectrum of gramicidin-S was generated from approximately 250 attomoles of material using a molar ratio of 3-nitrobenzyl alcohol to analyte of approximately 20:1. A single peak, located at 1224 Da, was obtained from the bacterial spores. The washing liquid and extract solution from the spores were analyzed using electrospray mass spectrometry and subsequent MS/MS product ion experiments. This independent analysis suggests that the measured species represents part of the B. subtilis peptidoglycan. The on-line addition of matrix allows quasi-real-time chemical analysis of individual, aerodynamically sized particles, with an overall system residence time of less than 5 seconds. These results suggest that a MALDI-ATOFMS can provide nearly real-time identification of biological aerosols. Copyright 2000 John Wiley & Sons, Ltd.