Hadi Abou-Chakra

Three-dimensional particle shape descriptors for computer simulation of non-spherical particulate assemblies

Recent advances, accelerated by the application of computer technology, allow numerical characterization of particle shape in two dimensions. However, characterization of three-dimensional (3D) particle shapes is still largely an unresolved problem. Here, 3D pseudo-shape descriptors are proposed for characterising the particle shapes. The microscopic characteristics are recorded using a computer-based image analysis system to assess particle shape. The particle shapes are based on the complete coverage by a set of images of the particle projections in three mutually perpendicular directions. The circularity of particle projections in each plane was measured in turn and found to be suitable for particle shape estimation. Analogues of the resulting 3D pseudo-shapes are constructed for the purposes of computer simulation of non-spherical particulate assemblies. Preliminary results of the simulation work are illustrated.

Constructing an engineering model for moisture migration in bulk solids as a prelude to predicting moisture migration caking

The aim of this study was to examine one of the mechanisms behind moisture migration caking, where liquid solution bridges form between particles in a bulk solid system because of an increase in local relative humidity, and then solidify as the local relative humidity drops - the effect being increased as more cycles occur. The goal was to develop a one-dimensional model for the heat and mass transfer processes involved, based on established physics and the characteristic moisture sorption curve of the solid (in this case sugar). The model was verified using scaled-down equipment (a caking box) to simulate the caking in a big bag. The results of this study will assist in the prediction of caking produced in this way.

Microstructural blending of coal to enhance flowability

Abstract Coal-fired power stations for electricity generation are well known to suffer from chronic problems in coal handling and flow affecting the operation of pulverisation mills and combustors. The economic impact of coal handling facilities can be significant, resulting in total shutdown of the power generating plant in some extreme cases. In coal-fired power generation, optimal operation of the combustors requires the ash content of pulverised coal not exceed 20%. Excessive ash content would result in lower calorific value. Therefore, the overall profitability of the market coal tends to be strongly effected by the ash content of the raw coal. However, the flowability of coal tends to be strongly affected by the moisture content of the constituent particles, as well as the presence of high fines fraction. The experimental work described in this paper aims to illustrate how flowability can be enhanced by optimising particle size distribution and particle surface moisture in blends of raw and washed coals. The results of the flowability tests obtained with “microstructurally” blended samples indicate that to ensure flowability in process vessels, it is necessary to blend according to important microstructural criteria as well as minimising ash content. Microstructural blending is not included in current industrial practice, which only blends washed and raw coals to produce high calorific value and low ash content. The work presented here establishes methodology for microstructural blending to enhance bulk flowability.

Computational model for prediction of particle degradation during dilute-phase pneumatic conveying: modeling of dilute-phase pneumatic conveying

A complete model of particle impact degradation during dilute-phase pneumatic conveying is developed, which combines a degradation model, based on the experimental determination of breakage matrices, and a physical model of solids and gas flow in the pipeline. The solids flow in a straight pipe element is represented by a model consisting of two zones: a strand-type flow zone immediately downstream of a bend, followed by a fully suspended flow region after dispersion of the strand. The breakage matrices constructed from data on 90° angle single-impact tests are shown to give a good representation of the degradation occurring in a pipe bend of 90° angle. Numerical results are presented for degradation of granulated sugar in a large scale pneumatic conveyor.

Measuring the tensile strength of caked sugar produced from humidity cycling

Abstract The aim of this study was to develop moisture migration modelling in stored sugar due to the temperature cycling of the storage environment. This was done by forming cake of sugar in a gas-bearing tensile tester. The sugar and tester were enclosed in a controlled humidity system and the relative humidity was cycled between four hours at 70 per cent relative humidity and four hours at 20 per cent relative humidity. Liquid bridges formed between the sugar particles at high relative humidity, with the bridges hardening subsequently when the humidity was reduced. It was found that the relationship between the tensile strength and number of cycles could be approximated by the relationship σT = K √N and the agglomerate tensile strength was in the order of 150Pa after 32 cycles. This suggests a soft cake rather than hard cakes with a tensile strength of twice this order of magnitude, which are formed, for example, in salt. A value of 1300kPa was obtained for the parameter representing the average strength of the bridge material in a simplified model of monosized spheres linked by pendular bridges in a system of uniform packing.

Computational model for prediction of particle degradation during dilute-phase pneumatic conveying: the use of a laboratory-scale degradation tester for the determination of degradation propensity

The overall objective of this work is to develop a computational model of particle degradation during dilute-phase pneumatic conveying. A key feature of such a model is the prediction of particle breakage due to particle-wall collisions in pipeline bends. This paper presents a method for calculating particle impact degradation propensity under a range of particle velocities and particle sizes. It is based on interpolation on impact data obtained in a new laboratory-scale degradation tester. The method is tested and validated against experimental results for degradation at 90° impact angle of a full-size distribution sample of granulated sugar. In a subsequent work, the calculation of degradation propensity is coupled with a flow model of the solids and gas phases in the pipeline.

Application of simulation technologies in the analysis of granular material behaviour during transport and storage

Abstract Problems in the preservation of the quality of granular material products are complex and arise from a series of sources during transport and storage. In either designing a new plant or, more likely, analysing problems that give rise to product quality degradation in existing operations, practical measurement and simulation tools and technologies are required to support the process engineer. These technologies are required to help in both identifying the source of such problems and then designing them out. As part of a major research programme on quality in particulate manufacturing computational models have been developed for segregation in silos, degradation in pneumatic conveyors, and the development of caking during storage, which use where possible, micro-mechanical relationships to characterize the behaviour of granular materials. The objective of the work presented here is to demonstrate the use of these computational models of unit processes involved in the analysis of large-scale processes involving the handling of granular materials. This paper presents a set of simulations of a complete large-scale granular materials handling operation, involving the discharge of the materials from a silo, its transport through a dilute-phase pneumatic conveyor, and the material storage in a big bag under varying environmental temperature and humidity conditions. Conclusions are drawn on the capability of the computational models to represent key granular processes, including particle size segregation, degradation, and moisture migration caking.

Assessing the potential of a fine powder to segregate using laser diffraction and sieve particle size measuring techniques

Abstract This paper describes a research programme undertaken with a view to solving a serious industrial powder handling problem. The aim of this research was to rationalize three grades of an additive fine powder used in the manufacturing of a mixed product to one grade, with the aim of reducing or eliminating the potential risk of particle segregation within the product. The use of a segregation tester, specifically to quantify the propensity of a sample of bulk solid to segregate when poured on to a heap, was central to this research. Particular attention was paid to the particle size distribution curves of the final product within different areas of the segregation tester. Two different techniques for characterizing particle size were used in the investigation — one based on size separation using sieving analysis and the other based on the laser diffraction technique. These techniques yield different measures of particle size distribution, resulting in different conclusions as to the feasibility of particle size analysis being a useful indicator of the propensity of a powder to segregate.

Utilising Computational Fluid Dynamics (CFD) for the Modelling of Granular Material in Large-Scale Engineering Processes

In this paper, the framework is described for the modelling of granular material by employing Computational Fluid Dynamics (CFD). This is achieved through the use and implementation in the continuum theory of constitutive relations, which are derived in a granular dynamics framework and parametrise particle interactions that occur at the micro-scale level. The simulation of a process often met in bulk solids handling industrial plants involving granular matter, (i.e. filling of a flat-bottomed bin with a binary material mixture through pneumatic conveying-emptying of the bin in core flow modepneumatic conveying of the material coming out of a the bin) is presented. The results of the presented simulation demonstrate the capability of the numerical model to represent successfully key granular processes (i.e. segregation/ degradation), the prediction of which is of great importance in the process engineering industry.

Computational modelling of particle degradation in dilute phase pneumatic conveyors

The aim of this paper is to develop a mathematical model with the ability to predict particle degradation during dilute phase pneumatic conveying. A numerical procedure, based on a matrix representation of degradation processes, is presented to determine the particle impact degradation propensity from a small number of particle single impact tests carried out in a new designed laboratory scale degradation tester. A complete model of particle degradation during dilute phase pneumatic conveying is then described, where the calculation of degradation propensity is coupled with a flow model of the solids and gas phases in the pipeline. Numerical results are presented for degradation of granulated sugar in an industrial scale pneumatic conveyor.