Experimental investigation and modelling of consolidation and layering mechanisms in high-shear granulation

High-shear wet granulation is a particle size enlargement process widely used in industries such as the pharmaceutical, food and agricultural industries. Despite its predominance, knowledge on several of the key mechanisms of granulation is lacking, driving up the costs of the design of new processes, scale-up and control. For the rate mechanism of consolidation and layered growth, models can be found in literature, one of which has been validated for the case of growth in a static powder bed. However, these models remain to be experimentally validated for application in an actual granulator. This study is the first to develop a method to investigate consolidation and layered growth under such dynamic conditions, and presents a detailed investigation of the kinetics, as well as a model to describe them. Initially, a high-shear mixer with three-bladed impeller was used for the method development of the study of the kinetics of consolidation and layered growth. However, experiments quickly showed that a dedicated piece of equipment was needed in order to isolate consolidation and growth from the other granulation phenomena, breakage in particular. Therefore, a novel, consolidation-only granulator (COG) was designed. Using the COG, the growth kinetics of a variety of powder-binder systems was evaluated. The granule mass was found to increase linearly with the square root of time, until a critical-packing liquid volume fraction had been achieved. This behaviour corresponds with surface tension-driven growth models. However, breakage and attrition were found to be prevalent for long granulation times, making it impossible to determine both the critical-packing liquid volume fraction and the final granulation time. Additionally, an unexpected rapid increase in initial granule mass was observed. Remarkably, the overall granule porosity was found to be constant. Tomography revealed that the granules developed a core-shell structure, with the higher-density shell becoming increasingly thick during granulation and the core becoming less dense. A further study using a high-shear mixer with flat plate impeller was successfully performed to determine the critical-packing liquid volume fraction and final granulation time. Although the qualitative kinetic behaviour was found to match that predicted by the surface tension-driven growth model, the quantitative behaviour varied. Efforts to incorporate the observed core-shell structure into the existing model revealed that such an extension did not represent the observed behaviour. As such, no predictive expression was found for the critical-packing liquid volume fraction and final granulation time. However, these parameters can be obtained from experimental work. Finally, the initial rapid increase in granule was addressed, and it was deemed probable that this effect would not have a significant impact on in-situ nucleation in a granulator. Finally, the results from all the studies were combined to adjust the existing model and convert it into two different population balance models (PBMs). The first, a three-dimensional PBM, was simply proposed. The second, a one-dimensional PBM, was solved by discretisation and compared to the experimental results to evaluate its performance. It was found that the discretisation method showed some deviation from the experimental results, but that this error could be reduced by decreasing the bin width. This work has successfully identified the underlying kinetics of layered growth, elucidated the consolidation and growth behaviour of granules, and contributed to the modelling and design of granulation processes

Direct simulation Monte Carlo for new regimes in aggregation-fragmentation kinetics

We revisit two basic Direct Simulation Monte Carlo Methods to model aggregation kinetics and extend them for aggregation processes with collisional fragmentation (shattering). We test the performance and accuracy of the extended methods and compare their performance with efficient deterministic finite-difference method applied to the same model. We validate the stochastic methods on the test problems and apply them to verify the existence of oscillating regimes in the aggregation-fragmentation kinetics recently detected in deterministic simulations. We confirm the emergence of steady oscillations of densities in such systems and prove the stability of the oscillations with respect to fluctuations and noise.Comment: 19 pages, 2 figures, 4 table

ADAPTIVE NONLINEAR MODEL REDUCTION FOR FAST POWER SYSTEM SIMULATION

The dissertation proposes a new adaptive approach to power system model reduction for fast and accurate time-domain simulation. This new approach is a compromise between linear model reduction for faster simulation and nonlinear model reduction for better accuracy. During the simulation period, the approach adaptively switches among detailed and linearly or nonlinearly reduced models based on variations of the system state: it employs unreduced models for the fault-on period, uses weighted column norms of the admittance matrix to decide which functions are to be linearized in power system differential-algebraic equations for large changes of the state, and adopts a linearly reduced model for small changes of the state.Two versions of the adaptive model reduction approach are introduced. The first version uses traditional power system partitioning where the model reduction is applied to a defined large external area in a power system and the other area that is defined as the study area keeps full detailed models. The second version applies the adaptive model reduction to the whole system.Speed improvement techniques using parallelization are investigated. The first technique uses parallelism in space; it further divides the study area into subareas that can be simulated in parallel. The second technique uses parallelism in time; it integrates the adaptive model reduction into the coarse solver of the Parareal method.In addition, the dissertation proposes integration of tensor decomposition into the adaptive model reduction approach to further improve the speed and accuracy of simulation.All proposed approaches are validated by comprehensive case studies on the 140-bus 48-machine Northeast Power Coordinating Council system, 2383-bus 327-machine Polish system, and 5617-machine 70285-bus Eastern Interconnection system using different dynamic models

Modeling and Simulation of Metallurgical Processes in Ironmaking and Steelmaking

In recent years, improving the sustainability of the steel industry and reducing its CO2 emissions has become a global focus. To achieve this goal, further process optimization in terms of energy and resource efficiency and the development of new processes and process routes are necessary. Modeling and simulation have established themselves as invaluable sources of information for otherwise unknown process parameters and as an alternative to plant trials that involves lower costs, risks, and time. Models also open up new possibilities for model-based control of metallurgical processes. This Special Issue focuses on recent advances in the modeling and simulation of unit processes in iron and steelmaking. It includes reviews on the fundamentals of modeling and simulation of metallurgical processes, as well as contributions from the areas of iron reduction/ironmaking, steelmaking via the primary and secondary route, and continuous casting

16th European Symposium on Comminution and Classification: book of extended abstracts

Extended abstracts from the 16th European Symposium on Comminution and Classification, ESCC 2019 held at the University of Leeds, 2-4 September 2019. Based on the abstracts received, the symposium was structured in the following themes: fundamentals of size reduction, innovations in milling and classification, nanomilling, mechano-chemistry and solid state transformations, pharmaceuticals and foods, attrition and wear, and related modelling. The notable number of abstracts received on modelling made it possible to divide them in sub-themes: mechanistic, population balance, discrete element and coupling with computational fluid dynamics

Particle Attrition in Circulating Fluidised Beds System

Particle attrition plays an important role throughout the cycles of a circulating fluidised bed (CFB) and a fluidised bed (FB) process, gradually depriving the bed inventory of valuable mass and changing the bed particle size distribution. The mass loss has to be compensated by a make-up stream. For economic and design purposes, attrition cannot be neglected. Although the particles may be efficient catalysts (or reactants), if the compensation for the lost material amounts to very high expenses, the whole process may become uneconomical. It is then clear that the choice of the solids material should take into account its attrition propensity. The main sources of attrition in fluidised bed systems are the jet region, the bubbling bed and the cyclone. It is common practice to predict particle attrition in industrial scale fluidised bed systems by the population balance method, but is it possible to link that prediction with the breakage propensity of a single particle? This work aims at developing a predictive tool for particle attrition in fluidised and circulating fluidised beds, by attempting to build a path line from the single particle breakage propensity to the attrition occurring in the process. Here, the reference industrial process is the Chemical Looping Combustion (CLC). The CLC is a circulating fluidised bed process under development and as such, the choice of a solids material is critical. A powder of crushed manganese oxide is a candidate material for the CLC process and is used here as test material, as well as its equilibrium equivalent. For simplicity, the two materials are referred to as F-CLC (fresh CLC particles) and E-CLC (equilibrium CLC particles), respectively. The single particle breakability of F-CLC and E-CLC is assessed by impact tests. The experimental results are then used to correlate the extent of breakage upon impact with the particle size and impact velocity, according to the theoretical model of chipping of Zhang and Ghadiri (2002). Further tests are carried out to unveil the effect of impact angle and number of impacts. The results suggest that E-CLC is highly more inclined to attrition than F-CLC. Moreover, the single particle breakage is found to correlate with the magnitude of the impact velocity and the sin of the angle of impact for both materials. Recalling the modelling approach of Ghadiri and co-workers, the single particle breakage model, as derived, and the model of surface wear of Archard and Charj (1953) are coupled with CFD-DEM (Computational Fluid Dynamic-Discrete Element Method) simulations to compute the attrition of F-CLC particles in a Stairmand cyclone. Moreover, the same cyclone is used to characterise attrition of F-CLC particles experimentally as a function of particle size, gas inlet velocity and solids loading. Remarkably, the outcomes of the two approaches are found to agree well. A correlation is eventually derived which expresses the extent of attrition in a cyclone as a function of the variables mentioned above. The analysis revealed that the main source of attrition in the cyclone is given by the particle-wall collisions at the opposite section of cyclone inlet, at any operating conditions. Particle-particle collisions and particle sliding against the wall become significant contributors of attrition at high and low solids loading, respectively. Attrition in the jet region is evaluated at room temperature as the steady state loss rate, using a semi-pilot scale fluidised bed equipped with a porous distributor and a central orifice of variable size. The results of the tests show that jet attrition of F-CLC and E-CLC can be described by two different correlations. The steady state attrition propensity of E-CLC is found to be higher than F-CLC, confirming the outcomes of the impact tests. The analysis on the fines collected on the filter reveals that they are mainly composed by very small particles of about 1 μm. The correlations of cyclone and jet attrition are implemented in a non-dimensional population balance model (PBM) that simulates attrition in a fluidised bed and a circulating fluidised bed. The latter is composed of a fluidised bed where the recycle of solids is provided by a cyclone. The PBM is validated for the fluidised bed configuration against the experimental PSD (Particle Size Distribution) of F-CLC particles after jet attrition in the fluidised bed. The PBM is eventually used to simulate hypothetical cases of a FB and CFB with low and high single particle breakability as well as low and high superficial velocities to assess the dynamic response of the system in terms of material loss, solids circulation rate, requirements for a make-up and PSD in different regions of the system. The simulations allowed to identify the presence of two subsequent regimes where the loss is firstly dictated by the pre-existing fines of the bed inventory and then by attrition. During the two regimes the mean particle size of the bed inventory increases and decreases, respectively. The PBM reveals that the circulation rate is strongly affected by attrition because of the accumulation of entrained particles which are large enough to be captured by the cyclone and recycled. The loss of material and the need for the make-up stream are found to increase using either larger superficial velocities and/or weaker particles