Particle-segregation in chutes, silos, conveyor belts and rotating drums

Particles of differing size or density often segregate in industrial flows such as chutes, silos, conveyor belts and rotating drums. This is the single biggest cause of material non-uniformity, which poses significant problems in handling and processing the grains, leading to plant downtime and product wastage. The most common form of segregation occurs in surface avalanches, which develop whenever a static granular material is tipped above its angle of repose. For example, pouring one's muesli into a bowl at breakfast! These avalanches are very efficient at sorting particles by size, with the large ones rising to the surface and the small ones percolating down to the base. The density of the grains may enhance or counteract this effect. When these flows come to rest a rich variety of particle size and density distributions develop in the deposit, sometimes with large regions of just one particle type. This naturally presents a major problem in processes that are supposed to be well-mixed. Understanding the segregation process and being able to model it effectively is the first step in being able to develop strategies to mitigate its effects. This proposal aims to use a powerful combination of small scale experiments, theory, continuum simulation and discrete element simulations (where the interactions of every single particle are modeled) to determine the functional dependence of the segregation rates on particle properties, as well as the applied shear-rate and pressure. The resulting mathematical model will then be applied to more complex flows, where there is mass transport between the the surface avalanche and the static, or slowly moving, grains beneath. This presents the project with its biggest challenge, because the rheology of granular materials is still very poorly understood, compared to fluids, which makes simulating the flow in a silo problematical. Over the past decade there has however been significant progress in the development of the so called mu(I)-rheology, which works over a large range of parameter space. Our aim is to regularize the model, by including additional physics, so that it can be applied in all regions of the flow and hence solve for the bulk velocity field. This will then allow the evolving particle-size and density distribution to be computed, so that we can understand in detail how pockets of just one particle type form. With our industrial partners we develop mitigation strategies, that use our knowledge of segregation to design clever chutes and silos that greatly reduce its effects.

There about 7 billion people in the world all of whom will use a large variety of powders and grains on a daily basis. There are many common examples in our kitchens, including flour, cereals, rice, maize, tea, coffee, sugar and salt. Washing powder is another common example, which billions of people use throughout the world every day, but very few people will think about how it is made or what problems are associated in making it. In fact the single most important cause of poor quality powders is particle segregation, which occurs during the manufacturing process as well as en route to a customer. The propensity of the grains to segregate as they are transported is a major cause of product wastage and huge financial loss for the powder processing industry. It also hampers product innovation, because each time a product formulation is changed the degree to which the grains will segregate is unknown and extensive testing has to occur in pilot plants before it can go live at full industrial scale. This directly impacts on the time required to bring new products on stream as well as the time to construct new plants, which may not work correctly first time. Despite the huge quantities of granular materials that are used throughout the world each year, our ability to predict their behaviour is still limited and often relies on discrete numerical simulations that have to resolve each individual particle. This works well for small numbers of grains, but can not be scaled up to the realistic number of particles processed in a single silo, let alone a complete plant. This proposal aims to build on the recent advances in the continuum modelling of segregation and of the bulk flow to develop predictive models for common unit processing operations such as chutes, silos, conveyor belts and rotating drums. Having detailed knowledge about how the grains actually segregate is the first step to being able to design processes, or materials, to mitigate the chronic effects of segregation. This has the potential to significantly reduce start-up times as well as product wastage, making it easier for the powder processing industry to make a profit as well as for them to reduce prices to the customer. The applications of this research are not limited to everyday consumer products, but have important application to a wide range of powder processing industries including the bulk chemical, pharmaceutical and mining industry, as well as to agriculture and many food stuffs.