A numerical technique for quantifying the key material properties that describe how a flocculated suspension behaves under constant pressure filtration is presented with unrestricted compressive yield stress and permeability models used to describe these material properties. Using an iterative procedure, the optimal parameters for these models are calculated as are pressure and solid fraction distribution profiles. Input parameters to the numerical analysis are flux and final cake height data obtained from batch filtration experiments, which are driven to steady-state. This technique is validated against piston driven filtration (odeometer) and centrifuge experiments for zirconia, soil particles, and yeast assemblages. The compressive yield stress calculated from filtration experiments agrees well with values obtained by odeometer and centrifuge studies for all particle systems studied at both the low and high solid pressure regions. Similarly, the calculated permeability agrees well with the measured permeability. (c) 2007 American Institute of Chemical Engineers

It is shown here that a grade distribution scheme commonly used to moderate peer assessments where self assessment is excluded is based on a false premise and will give an erroneous ranking in the situation where the best performer in a student group ranks the second best performer much higher than the other group members. An alternative to normalisation is proposed based on the idea that the rank order of peer grades should be preserved and should as far as possible be consistent between assessors. It is shown that the method correctly recovers the rank order of students within the group for all cases examined, while still eliminating biases that can result from differences in marking standards in the group. It is suggested that the approach could also be used to check for bias when self assessment is included.

The interparticle rolling friction model used by Zhou et al. [1] was adapted from a model for a viscoelastic sphere rolling on a hard plane. We show that the Zhou et al. model is incomplete because it does not predict rolling friction in all cases when rolling friction occurs. Hence, the conclusion drawn by those authors that their numerical results favour an angular velocity independent formulation for rolling friction is premature.

Several methods to measure the structures of coal aggregates are compared. Loose and compact coal aggregates were generated through flocculation of ultrafine coal particles (mean volume diameter of 12 mu m) under specific shearing conditions. Aggregate structure in terms of mass fractal dimension, D-f, was determined using various methods; namely 2D and 3D image analysis, interpretation of intensity patterns from small angle light scattering, changes in aggregation state through light obscuration, and settling behavior. In this study, the measured values of D-f ranged from 1.84-2.19 for coal aggregates with more open structures, and around 2.27-2.66 for the compact ones. All of these approaches could distinguish structural differences between aggregates, albeit with variation in D-f values estimated by the different techniques. The discrepancy in the absolute values for fractal dimension is due to the different physical properties measured by each approach, depending on the assumptions used to infer D-f from measurable parameters. In addition, image analysis and settling techniques are based on the examination of individual aggregates, such that a large number of data points are required to yield statistically representative estimations. Light scattering and obscuration measure the aggregates collectively to give average D-f values of the particulate systems; consequently ignoring any structural variation between the aggregates, and leaving possible small contaminations undetected (e.g. by dust particles or air bubbles). Appropriate utilization of a particular method is thus largely determined by system properties and required data quality.

In this study, both short-range and long-range structures of silica aggregates were studied by small-angle light scattering. The silica particles were aggregated by using different concentrations of KCl and MgCl2, with and without continuous shear. It was found from both small-angle light scattering and transmission electron microscopy (TEM) measurements that the aggregates had a compact short-range structure and a looser long-range structure. The floc sediments were studied by allowing the silica aggregates to settle under gravity, as well as having them consolidated by centrifugation. The results show that under gravity and lower centrifugation force (50×g), the short-range structure of silica aggregates affects the compacted sediment structure, while under higher centrifugation force (453×g), the sediment structure is independent of the short-range structure of the aggregates.

A novel approach to quantify aggregate bond strength in colloidal suspensions based on a correlation between aggregate properties (size, structure) and aggregation conditions accounting for particle and fluid properties (number concentration and size of primary particles, applied shear, viscosity and density of suspension) is presented. Examples utilising systems of latex, silica, and coal particles, aggregated with inert electrolyte, surfactant, or charged polymers, are given. The relative binding strength of aggregates could be evaluated as the ratio of forces between particles. For silica aggregates, these ratios were shown to be comparable to those estimated by measuring the adhesive forces through Atomic Force Microscopy (AFM). It was noted that the presence of minute levels of contaminant in the sample greatly hampers AFM measurements. Accordingly, the proposed analysis offers practical advantages, including the ability to quantify directly the comparative strength of particulate systems flocculated under different conditions through a set of straightforward tests.

This paper tests an approach to the estimation of relative particle bond strength based on the nondimensional floc and aggregation factors. The strength of flocs formed by aggregating nanosized silica particles with the addition of potassium chloride (KCl) or cationic surfactants, alkyltrimethylammonium bromide (mixture of CTAB, DTAB, and MTAB) was analyzed. The bonding force of the flocs formed in surfactant compared to that formed in the KCl system was estimated using the new dimensional analysis approach. This force ratio was then compared to that obtained by atomic force microscopy.

In this work, a dimensional analysis yields a correlation between floc properties, encompassing floc size and structure, and aggregation conditions that include fluid properties, applied shear, number and size of primary particles, and the estimated bonding force between particles. The measurement of aggregate properties was conducted using a small-angle light scattering technique, where information regarding the floc structure can be obtained from scattering intensity patterns of aggregates, indicating their degree of compactness. Experimental data obtained from current study, as well as from published results in the literature, show that the analysis is applicable for a range of particle sizes and shear fields. The non-dimensional factors offer a better appreciation of significant flocculation parameters, and provide a more convenient way of presenting data for various flocculation conditions.

This paper studies the short and long‐range structure of silica aggregates using the small angle light scattering technique. Silica particles were made to aggregate by the addition of MgCl2, with and without continuous shear. Two different short‐range structures were observed for different aggregation conditions. The small angle light scattering reveals two different floc structures at different length scales, a very compact floc at short length scale and a loose floc at large length scale. The sediments of these flocs were studied by allowing them to settle under gravity and consolidate at different centrifugal forces. The results show that the floc short‐range structure is important in governing the compaction behaviour of sediment.

The effect of shear on floc properties was observed through population balance to comprehend the mechanisms of flocculation, in particular the role of restructuring. Little fundamental attention has been given before to the shear influence responsible for creating compact aggregates, while the floc characteristics might differ in other conditions. It is crucial to understand how aggregates evolve to steady state, if their properties are to be ‘tailored’ to suit subsequent solid–liquid separation. From a previous experimental study (Langmuir 18(6) (2002) 1974), restructuring was observed to occur extensively in the flocculation of 380nm latex particles in couette-flow, and was proposed to be responsible for the decrease in floc size on their transition to equilibrium. On the other hand, flocs of larger primary particles (810nm) were more susceptible to breakage, with densification occurring as a result of fragmentation and re-aggregation. Denser flocs were found when structural deformation dominated, particularly in the initial stage of the process, while comparatively tenuous ones were observed when formation and breakage kinetics were the governing mechanisms. The distinct manners in which aggregates of different primary particle sizes evolved with time, were replicated with a population balance that incorporated the floc structural variation; verifying that restructuring indeed played a crucial role under certain flocculation conditions.

Latex particles with diameters of 60, 380, and 810 nm, respectively, were sheared in a controlled shear environment of circular couette flow after being destabilized by the addition of MgCl2. The evolution of aggregate size and structure, as well as a measure of aggregate mass, was monitored with use of a particle size analyzer (Coulter LS230) operating on the principle of small-angle light scattering. The aggregates of different primary particle sizes displayed distinct behavior in attaining steady state under similar shear conditions, notably at low to moderate shear rates (G less than or equal to 100 s(-1)). Restructuring of aggregate structure was favored over fragmentation for aggregates composed of 60- and 380-nm particles, whereas fragmentation and reaggregation were the main mechanism in governing the final floe size and structure for aggregates made up from 810-nm particles. Also presented in this study is a dimensional analysis that yields a correlation between a floe factor (consisting of floc size and structure) and an aggregation factor, which encompasses the fluid properties, applied shear, number concentration and size of primary particles, as well as the estimated bonding force between particles. This relationship provides a better appreciation of other significant aggregation parameters. apart from the shear level and aggregate size, which are often ignored in the more conventional manners of presenting data from flocculation processes.

A review is presented of a number of techniques available for the characterisation of the structure of aggregates formed from suspensions of sub-micron particles. Amongst the experimental techniques that have been commonly used are scattering (light, X-ray or neutron), settling and imaging and these are the focus of this work. The theoretical basis for the application of fractal geometry to characterisation of flocs and aggregates is followed by a discussion of the strengths and limitations of the above techniques. Of the scattering techniques available, light scattering provides the greatest potential for use as a tool for structure characterisation even though interpretation of the scattered intensity pattern is complicated by the strong interaction of light and matter. Restructuring further complicates the analysis. Although settling has long been used to characterise particle behaviour, the absence of an accurate permeability model limits the technique as a means of determining the porosity of fractal aggregates. However, it can be argued that the determination of fractal dimension is relatively unaffected. The strength of image analysis lies in its ability to provide a great deal of information about particle morphology and the weaknesses lie in the difficulties with image processing and sample size as this is a particle counting technique. There are very few papers which compare the fractal dimension measured by more than one technique. Light scattering potentially provides a useful tool for checking settling results. However, further work is required to develop proper models for aggregate permeability and flow-through effects.

Small-angle static light scattering has been used to probe the evolution of aggregate size and structure in the shear-induced aggregation of latex particles. The size of aggregates obtained from the particle-sizing instrument (Coulter LS230) was compared with the size of those obtained with another approach utilizing the Guinier equation on the scattering data. Comparison of the two methods for studying the effects of mixing on the evolution of the aggregate size with time revealed similar trends. The aggregate structures were quantified in terms of their fractal dimensions on the grounds of the validity of Rayleigh-Gans-Debye scattering theory for the fractal aggregates. Analysis of the scattering patterns of aggregates verified that restructuring of the aggregates occurred as the aggregates were exposed to certain shear environments, resulting in a scale-dependent structure that could not be quantified by a fractal dimension. The effect of restructuring on aggregate size was particularly noticeable when the aggregates were exposed to average shear rates of 40 to 80 s(-1), whereas no significant restructuring occurred at lower shear rates. At 100 s(-1), the fragmentation of aggregates appeared to be more significant than aggregate compac-tion. Copyright 2001 Academic Press.

Simulations of diffusion-limited cluster-cluster aggregation (DLCA) with no restructuring, full restructuring, and partial restructuring have been performed. The scattering patterns produced from these aggregates have been simulated using the Rayleigh-Gans-Debye approximation. Pure DLCA aggregates produced a scattering pattern with the slope of the fractal region being about -1.8. In contrast, the slope of the fractal region of the scattering pattern for fully restructured aggregates was about -2.1, indicating an increase in fractal dimension. Partial restructuring at large length scales produced an upward turn in the scattering pattern at low qr(o), while at high qr(o) the fractal section of the pure DLCA aggregate was retained. This last result was expected and is consistent with the results and postulations of several other workers. This simulation shows that the type of scattering pattern often obtained from orthokinetic or sheared aggregation can be produced by restructuring of aggregates at large length scales. Copyright 2001 Academic Press.

The structure of aggregates of bidisperse particles was probed with small angle light scattering. Various mixtures of nearly spherical hematite particles with diameters of 70 and 216 nm were induced to aggregate by the addition of KCI. The fractal dimension of the resulting aggregates was unable to be determined because of effects from the aggregate edges and short range non-fractal behaviour. Significant restructuring of the aggregates was found to occur as a result of pumping.

Bridging the gap between academic learning and professional practice is a challenging but highly desirable outcome of the fi nal-year engineering learning experience. This case study describes the evolution and current practice of two components in the fourth-year chemical engineering and industrial chemistry course CEIC4120: Plant Management and Operation. These signifi cantly self-guided components, both of which aim to integrate and apply student knowledge in the context of future professional practice, are to successfully operate a chemical process in a large plant (pilot plant component) and to steer a fi ctitious company to success in a competitive market environment (management component). Although both components emphasise different spheres of practice - that is, chemical and engineering skills in pilot plant and marketing skills in management - they overlap signifi cantly in their learning outcomes and their approaches to student learning. The case study also highlights how the authors are attempting to shift the course from a teacher-centred to a student- centred framework.