Interdependence Between Segregation and Flow Properties of Bulk Solids

Abstract

This thesis presents the results of an ongoing investigation into the characteristics of segregation, which is critical for ensuring product homogeneity and process reliability. Additionally, moisture content certainly affects the segregation behaviour of powders, which may degrade the product quality and cause lot rejection. Therefore, understanding the segregation characteristics affected by changes in moisture content plays a vital role in achieving the desired goals in powder handling industries. In this research, a deeper understanding of powder segregation and flow behaviour was achieved by addressing a critical gap in the literature for Geldart Group A to B borderline powders, providing practical insights and predictive tools applicable to real-world scenarios. A key contribution is the introduction of a novel dimensionless parameter, the modified dimensionless cohesion number, which improves the prediction of segregation in complex powder fluidising systems. High-accuracy models were developed by incorporating important factors such as moisture, cohesion, flow function, and particle properties. Notably, the research also established clear empirical relationships between moisture levels and segregation behaviour, an area with minimal prior modelling despite its industrial significance. By highlighting the role of moisture management in maintaining product uniformity and preventing lot rejection, this work supports higher safety and quality standards. These models were validated across a diverse set of powders, including fly ash, pharmaceuticals, detergents, and semolina, demonstrating strong generalisability. The study further explores segregation challenges in pneumatic conveying systems. A 75:25 blend ratio of coarse-to fine ash was found optimal for dense phase flow. Criteria based on the bulk powder Froude number (<10) and coarse-to-fine ratio (>10) were proposed for reliable conveying. By highlighting the role of moisture management in maintaining product uniformity and preventing lot rejection, this work supports higher safety and quality standards. Based on the comprehension testing that includes the sifting segregation test, fluidisation test, particle and flow properties testing on various powders such as 6 different fly ash, sand, three different brands of detergent and semolina, the model for sifting and fluidisation segregation index has developed based on particle, flow and powder bed properties. The developed models can be used as a design tool based on particle, flow, and powder bed properties, which may assist in controlling the segregation issues by considering the required changes in designing and operating parameters. A study which aims at modelling the sifting and fluidisation segregation index for Geldart group A to B borderline materials, for which very little research has been carried out till date. In a separate set of experiments, physical and flow property tests were carried out on 8 pharmaceutical powders. The results obtained from heap analysis and shear cell testing have been compared. A relationship to represent the static angle of repose has been developed using a bulk powder-based Froude number and fine size. The experimental results and predictions for a range of bulk properties have shown that the values of static angle of repose decrease with an increase in the value of bulk powder Froude number, increase in particle shape factor and decrease in the median to fine ratio. Out of all the dimensionless parameter groupings, the particle shape factor strongly influences the static angle of repose, as indicated by its larger absolute value of the exponent in the power function format relationship. Sifting and fluidisation segregation characteristics were determined for 6 different fly ash samples (particle size ‘d(50)’ ranging from 68 to 141 µm) using standard testers. The results have shown that the coarser particles have a greater tendency to sifting segregation, and the finer powders respond more to fluidisation segregation. The angle of repose for the fine ash and coarse ash were 55° and 38˚, respectively, which indicated poor to good flowability conditions. The flow function test shows that all the samples were in an easy-flowing to a free-flowing zone. The angle of repose and material flow function have provided a good correlation with the sifting segregation index. In contrast, cohesion between particles, the ratio of free terminal velocities and diameters for coarse to fine particles have shown a good fit with fluidisation segregation indices. For both sifting and fluidisation segregation, the model correlation values are 0.91 and 0.94, indicating the predicted results are a good fit to the experimental data. Experiments were carried out using sand, three different brands of detergent and semolina powders. Additionally, data from six fly ash samples were taken for the purpose of modelling. While comparing the powder characteristics in the first and last samples in sifting segregation and top and bottom samples in fluidisation segregation, considerable differences in the bulk properties were found, indicating the occurrence of segregation in the case of Geldart group A to B borderline powders. A new model has been developed for the sifting segregation index using flow function, course-to-fine ratio, and shape factor, which resulted in 97% prediction accuracy. A novel dimensionless cohesion number has been developed as a ratio of inter-particle cohesion to the dynamic pressure of air along; this number has been used with minimum fluidisation velocity to model the fluidisation segregation index. The model has shown an 86 % fit to the experimental data. Another study aims to model the sifting and fluidisation segregation index for powders from Geldart group A to B borderline while considering the change in moisture content. Five different powders, such as sand, three different brands of detergent, and semolina powders, have been utilised for physical and flow properties and segregation tests at three different moisture content levels. The results revealed that with an increase in moisture content, there is a decrease in the value of the sifting and fluidisation segregation index. A new model has been developed for the change in sifting segregation index w.r.t change in moisture content based on flow function, course-to-fine ratio, and shape factor, providing a good fit of 91 % accuracy with the experimental data. By using the minimum fluidisation velocity of the particle and a novel dimensionless cohesion term, a model was developed for the change in fluidisation segregation index that showed an 85 % fit with the experimental data. In the initial study, the model presented in Chapter 4 was best described by the parameter "Median to Fine ratio (d50/d10)." To encompass the entire range of a powder, the parameter "Coarse to Fine ratio (d90/d10)" was found to be more relevant for the models discussed in Chapters 5 to 8. Finally, the difficulty of handling segregated powder samples having narrower particle size distribution compared to wider size distribution has been illustrated for a pneumatic conveying system. Based on a pilot plant study of conveying 10 blends of ash, 75% coarse ash and 25% fine ash provided the optimal blend for dense phase conveying. A new bulk powder Froude number term (based on loose poured bulk density) and coarse-to-fine ratio have been used to represent reliable conveying criteria. For reliable dense-phase conveying, the ash mixture should have a bulk powder Froude number < 10 and a coarse-to-fine ratio > 10. The significance of this study improves scientific understanding and predictive ability for segregation characteristics of Geldart group A and B borderline powders by integrating particle properties, flow characteristics, and moisture effects into highly accurate models. The introduction of novel parameters, including the modified dimensionless cohesion number and bulk powder Froude number, provides robust, generalisable tools applicable across diverse industrial contexts such as fly ash, pharmaceuticals, detergents, construction, and food powders. The validated models deliver practical design and operational criteria to reduce segregation, maintain product uniformity, reduce lot rejection, and ensure reliable pneumatic conveying, thereby improving process reliability and overall quality standards.