Gabrie M.H. Meesters

Effect of shear schedule on particle size, density, and structure during flocculation in stirred tanks

The effect of shear history on the evolution of the polystyrene—alum floc size, density, and structure is investigated by small-angle light scattering during cycled-shear and tapered-shear flocculation in a stirred tank using a Rushton impeller. First, various sampling schemes are experimentally evaluated. The floc structure is characterized by the mass fractal dimension, Df, and the relative floc density. During turbulent shear flocculation, small floc structures are shown to be more open (Df = 2.1) than larger floc structures (Df = 2.5) as a result of shear-induced restructuring during steady state attainment. Flocs produced by cycled-shear flocculation are grown at shear rate G-50 s−1 for 30 min, are fragmented at Gb = 100, 300, or 500 s−1 for one minute, and then are regrown at G = 50 s−1. This shear schedule decreases the floc size but compacts the floc structure. When flocs are produced by gradual reduction of the shear rate from G-300 to 50 s−1 (tapered-shear flocculation), smaller though equally dense flocs are produced compared with cycled-shear flocculation. The cycled-shear flocculation method produces the largest flocs with the highest potential for sedimentation when the fragmentation shear rate is Gb = 300 s−1.

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

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.

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.

Failure mechanism determination for industrial granules using a repeated compression test

This work describes the development of a particle compression test that allows direct and repeated application of the stress. The test is designed to quickly reduce the load on a granule during its incipient failure. By so doing, the breakage can be arrested and thus the process can be studied in detail. Experimental tests have been made on samples of industrial enzyme granules, which have a complex layered structure, and reproducible results have been obtained. The contribution of the various layers to the strength of the granule has been investigated, showing that the use of coating materials results in improved granule strength. The microstructure of the granule determines the failure mode of the granule. It is concluded that the failure mechanisms can be defined from tests on only a few granules as can assessment of the relative contribution of the layers and of the granule core to its strength. A measurement of the distribution of strength requires a larger, statistically representative, sample.

Measurement of Granule Attrition and Fatigue in a Vibrating Box

This paper describes a new test machine that has been designed to measure the strength of single particles in the size range of 102–103 μm. The device is a vibrating box that subjects each particle in the sample to a large number of impacts of known but variable strength. By tracking the size and shape of the particles as a function of the number of impacts, their strength characteristics against the mechanisms of fracture, fatigue and attrition can be differentiated. The number of particles tested in one sample is restricted in order to make any particle-particle interaction negligible but is sufficiently large that the distribution of these characteristics can be determined.

A (sub-)micron aerosol generator,using a high electric field

Abstract A aerosol generator, based on the phenomenon of the Taylor cone, is described. These liquid cones, oftencalled Taylor cones, are formed when a semi-conducting liquid flows slowly from a nozzle at a high electric field strength(V≈10 kV, E≈2·10 6 V/m). From the tip droplets are ejected at frequencies of log to 10 10 Hz, having diameters of about 1 μm. Smaller droplets can be produced using a mixture of a volatile and nonvolatile liquid (e.g. DOP and ethanol). The generator neutralises the highly charged droplets almost immediately after they are produced. The phenomenon of liquid cones shows many similarities with solid state needle corona discharges. Thecurrent voltage characteristic is similar, as is the gas discharge around the tip of the cone. The two first corona regions are shown to exist too.

A monodisperse-aerosol-generator, using the Taylor cone, for the production of 1 μm droplets

A new type of aerosol generator is described. The generator is based on the phenomenon of the Taylor cone, which appears when a high voltage (V≈10 kV, E≈106 V/m) is applied in between a nozzle, through which a liquid slowly flows, and a collecting plate. From the tip of this Taylor cone small uniformely sized droplets are ejected with diameter around 1 μm. When using a solution e.g. DOP in ethanol, very small droplets can be obtained by evaporating the solvent. Because of the large nozzle opening (0.2–2 mm), there will be little or no blockage. The great advantages over the established aerosol generators are the small droplet size of the primary droplets, the high production frequency (≈ 108 Hz) and the easy operation of the system with little or no clogging.

A population balance model for high shear granulation

In a previous paper, Hoornaert et al. ( Powder Technol. 96 (1998); 116-128) presented data from granulation experiments performed in a 50 L Lödige high shear mixer. In this study that same data was simulated with a population balance model. Based on an analysis of the experimental data, the granulation process was divided into three separate stages: nucleation, induction, and coalescence growth. These three stages were then simulated separately, with promising results. It is possible to derive a kernel that fit both the induction and the coalescence growth stage. Modeling the nucleation stage proved to be more challenging due to the complex mechanism of nucleus formation. From this work some recommendations are made for the improvement of this type of model.

Encapsulation of Enzymes and Peptides

A large part of formulated peptides and proteins, e.g., enzymes used as food ingredients, are formulated in a liquid form. Often, they are dissolved in water to which glycerol or sorbitol is added to reduce the water activity of the liquid, thus reducing the change of microbial growth. Still, there are reasons to formulate them in a solid form. Often, these reasons are stability, since a dry formulation is often much better than liquid formulations, and less transportation cost, since less mass is transported if one gets rid of the liquid; however, most of the times, the reason is that the product is mixed with a solid powder. Here, a liquid addition would lead to lump formation.