Development and validation of cluster and EMMS drag model

In this work the influences of solid viscosity and the way to scale-down traditional drag models on the predicted hydrodynamics of Geldart A particles in a lab-scale gas-solid bubbling fluidized bed are investigated. To evaluate the effects of drag models, the modified Gibilaro et al. drag model (constant correction factor) and the EMMS drag model (non-constant correction factor) are tested. And the influences of solid viscosity are assessed by considering the empirical model proposed by Gidaspow et al. (1997, Turbulence, Viscosity and Numerical Simulation of FCC Particles in CFB. Fluidization and Fluid-particle Systems, AIChE Annual Meeting, Los Angeles, 58–62) and the models based on kinetic theory of granular flow (KTGF) with or without frictional stress. The resulting hydrodynamics by incorporating the different combinations of the drag model and solid viscosity model into two-fluid model (TFM) simulations are compared with the experimental data of Zhu et al. (2008, Detailed Measurements of Flow Structure inside a Dense Gas-Solids Fluidized Bed.” Powder Technological 180:339–349). The simulation results show that the predicted hydrodynamics closely depends on the setting of solid viscosity. When solid viscosity is calculated from the empirical model of Gidaspow et al., both drag models can reasonably predict the radial solid concentration profiles and particle velocity profiles. When the KTGF viscosity model without frictional stress is adopted, the EMMS drag model significantly over-estimates the bed expansion, whereas the modified Gibilaro et al. drag model can still give acceptable radial solid concentration profiles but over-estimate particle upwards and downwards velocity. When KTGF viscosity model with frictional stress is chosen, both drag models predict the occurrence of slugging. At this time, the particle velocity profiles predicted by EMMS drag model are still in well agreement with the experimental data, but the bed expansion is under-estimated.

Multi-fluid modelling of hydrodynamics in a dual circulating fluidized bed

On the Interaction of Capillary Shapes with Solid Surfaces

An investigation on carbon dioxide sequestration in Indian coal seams

A Lattice Boltzmann Approach to Multi-Phase Surface Reactions with Heat Effects

Contractile floating barriers for confinement and recuperation of oil slicks

Experimental measurements of the particle pressure were obtained for a liquid fluidized bed and for a vertical gravity driven liquid-solid flow. The particle, or granular, pressure is defined as the extra pressure generated by the action of particles in a particulate multi-phase flow. Using a high-frequency-response pressure transducer, individual collisions of particles were collected and measured to obtain a time-averaged particle pressure. Results were obtained for a number of different particles and for two different test section diameters. Results show that the particle pressure experiences a maximum at intermediate concentrations, and that its magnitude is scaled with the particle density and the square of the terminal velocity of the particles. The particle pressure was found to be composed of two main contributions: one from pressure pulses generated by direct collisions of particles against the containing walls (direct component), and a second one from pressure pulses due to collisions between individual particles that are transmitted through the liquid (radiated component). The direct component of the particle pressure was studied by an analysis of particle collisions submerged in a liquid. A simple pendulum experiment provides controlled impacts in which measurements are made of the particle trajectories for different particles immersed in water. The velocity of the approaching particle is measured using a high speed digital camera; the magnitude of the collision is quantified using a high frequency response pressure transducer at the colliding surface. The measurements show that most of the particle deceleration occurs at less than half a particle diameter away from the wall. The measured collision pressure appears to increase with the impact velocity. Comparisons are drawn between the measured pressures and the predictions by Hertzian theory. A simple control-volume model is proposed to account for the effects of fluid inertia and viscosity. The pressure profile is estimated, and then integrated over the surface of the particle to obtain a force. The model predicts a critical Reynolds number at which the particle reaches the wall with zero velocity. Comparisons between the proposed model and the experimental measurements show qualitative agreement. Experiments involving binary collisions of particles were performed to investigate the radiated component of the particle pressure. This component results from the pressure front generated by the impulsive motion of a fluid resulting from a collision of particles in a liquid. When the two particles come into contact, the impulsive acceleration due to the elastic rebound produces a pressure pulse, which is transmitted through the fluid. A simple dual pendulum experiment was set up to generate controlled collisions. Measurements were obtained for a range of impact velocities, angles of incidence, and distances away from the wall for different pairs of particles. The magnitude of the impulse pressure appears to scale with the particle impact velocity and the density of the fluid. Based on the impulse pressure theory, a prediction for pressure generated due to the collision can be obtained. The model appears to agree well with the experimental measurements. The fluctuating component of the solid fraction was studied, as one of the sources of the particle pressure. The instantaneous cross-sectional averaged solid fraction was measured using an impedance meter. The root-mean square fluctuation of the solid fraction signal was measured in a liquid fluidized bed and a vertical gravity-driven flow, for different particle sizes and densities. Two types of fluctuations were identified: low-frequency large-scale fluctuations which dominate at high concentrations, and high-frequency small-scale fluctuations which are dominant at intermediate solid fractions. The effect of each type was isolated by filtering. When the large-scale fluctuations were present, the magnitude of the rms fluctuation was found to scale with particle diameter, but when eliminated the mean fluctuation appeared to scale with the particle mass instead.

The topic of this PhD work is to search the critical concentration of starch based disintegrant applying percolation theory and F-CAD (Formulation-Computer Aided Design) in order to design a pharmaceutical tablet formulation for a low water soluble drug. Critical concentration of maize starch (MS) for a ternary mefenamic acid (MA) tablet formulation with respect to a minimum disintegration time is investigated. Additionally implemented application of F-CAD to compute the disintegration time of MA tablet formulation is presented. This topic is related to push forward the idea of Quality by Design (QbD) of FDA (Food and Drug Administration) / EMEA (European Medicines Agency) / PMDA (Pharmaceuticals and Medical Devices Agency) and the exploration of the design space according to ICH (International Conference of Harmonization) Q8.
\nThe results of this work shows that the application of percolation theory is not limited to binary tablet formulation. The critical concentration of MS described by the renormalized MS concentration, MS/(MS+MA) applying the renormalization technique is always equal 0.198 (dimensionless). Moreover the critical concentration of MS is optimized using the spline approximation with the dataNESIA software. It is leading to a minimum disintegration time at 0.206, dimensionless, renormalized, which is very close to the experimental value of 0.198. According to the percolation theory, a minimum disintegration time corresponds to the formation of a continuous water-conducting cluster through the entire tablet. The critical volume fraction of an ‘infinite cluster’ that water can diffuse through the entire MA tablets are calculated with taking into account for the geometrical considerations between MS and MA particles based on random close packed (RCP) spheres system. The critical volume fraction of MS is calculated by the multiplication of critical concentration of MS and the solid fraction of MA tablets; which is within the range of 0.16 ± 0.01 (v/v). It is concluded that the critical volume fraction for three dimensional lattices is equal to 0.16 ± 0.01 (v/v); which is useful for the calculation of the critical concentration of starch based disintegrant in order to design the pharmaceutical tablet formulation based on scientific approach proposed by ICH Q8 guidance.
\nIn addition, the disintegration behavior in the neighborhood of the percolation threshold is explained mathematically by the basic equation of the percolation theory, yielding a critical exponent q equal to 0.28 ± 0.06 (Quality of fit: r2 = 0.84). This value is close to the critical exponent for three dimensional lattices (q = 0.4). Thus, it is important, within a planned experimental design to optimize the disintegrant to take into account the percolation theory. However it has to be kept in mind that the determination of the percolation threshold and critical exponent does not give an answer about the absolute value of the disintegration time. Dissolution Simulation (DS) module, which is the one of F-CAD based on cellular automata algorithm is used to simulate the disintegration time of a MA tablet. Disintegration time of tablet is assumed as the time elapsed till the water is detected at the geometric center of the virtual tablet. Comparison of experimental disintegration time of MA tablet and computed specific time point for water to reach the geometric center of the tablet by using F-CAD software has been carried out and shown an acceptable correlation (Correlation coefficient: r = 0.81). The detailed evaluation of the data shows that there is still a need for optimization of F-CAD for the calculation of the disintegration time in order to achieve a similar or the same performance like in the prediction of the dissolution profile of a tablet formulation. It is concluded that F-CAD software is the only software so far, which is capable of computing the disintegration time of tablets. The software has a great potential to be improved and to be not only used for the safe prediction of the dissolution profile of a tablet formulation but also for a safe prediction of the disintegration time. Thus, such a software is one of the tools for the substitution of laboratory experiments for the purpose of the design and development of new pharmaceutical solid dosage forms. The replacement of expensive laboratory experiments by in-silico experiments is an important issue to reduce development costs and to comply with the requirements of ICH Q8 exploring the design space with response surface methodology. The results of this thesis show in addition that the application of percolation theory is a must in order to detect percolation thresholds. It is important to know the response surfaces close to the percolation threshold of sensitive tablet properties such as the disintegration time to get information about the robustness of the selected formulation. In this context one has to put the question forward if the application of percolation theory should be an integral part of the guidelines of ICH Q8 exploring the formulation design space.

Proteins at Interfaces: Conformational Behavior and Wear

Crystal Nucleation and Polymorph Control: Self-­association, Template Nucleation, Liquid?Liquid, phase Separation

Experimental and CPFD study of gas–solid flow in a cold pilot calciner

The worst enemy of sustainable use of reservoirs is sedimentation. Often the main silting process is the result of settling down of suspended sediments. In shallow reservoirs the flow pattern and the sediment deposition processes are strongly influenced by the reservoir geometry. The trap efficiency of a shallow reservoir depends on the characteristics of the inflowing sediments and the retention time of the water in the reservoir, which in turn are controlled by the reservoir geometry. With the purpose of controlling the sedimentation in shallow reservoirs, the effects of the geometry on flow pattern and deposition processes were investigated with systematic physical experiments and numerical simulations. This allowed identifying ideal off-stream reservoir geometries, which can minimize or maximize the settlement of suspended sediments. The objective was also to gain deeper insight into the physical processes of sedimentation in shallow reservoirs governed by suspended sediments. The systematic experimental investigations were carried out in a 6m long, and 4m wide and 0.3m deep shallow basin. The influence of the shallow reservoir geometry was studied for the first time by varying the width, the length, and the expansion angle of the basin in the experiments for clear water and with suspended sediment. In total 11 different reservoirs geometries and 4 water depths were analyzed. During the tests several parameters were measured, as 2D surface velocities, 3D velocity profiles, thickness of deposited sediments, and sediment concentration at the inflow and outflow. Crushed walnut shells with a median grain size, d50, of 50 µm, and a density of 1500 kg/m3 were used to simulate the suspended sediments. After having reached a stable flow pattern with the clear water, velocity measurements were performed. In a second phase, the evolution of the flow and deposition patterns under suspended sediment inflow were investigated. Tests were carried out with durations from 1.5 hours up to 18 hours, in order to follow the morphological evolution. In order to investigate the efficiency of flushing, flushing operations at normal water level as well as with drawdown were examined after the sedimentation tests. Numerical simulations of the laboratory basin were performed and compared with experimental results. The purpose was to assess the sensitivity of the results on different flow and sediment parameters and different turbulence closure schemes. The experimental investigation of the flow and sediment behavior in axi-symmetric geometries with different shapes provides further information on the evolution of the flow pattern and the sediment deposition. Beside the expansion ratio and form ratio of the basin the flow regime was classified by the geometry shape factor SK and inlet Froude number Frin. The geometry shape factor, defined as a function of wetted perimeter, total reservoir surface area, aspect ratio and expansion density ratio, was used to compare and analyze the experimental results obtained from the different investigated basin geometries. The clear water experiments investigations revealed under what geometrical conditions the flow changes from symmetrical to asymmetrical behavior. The length of the basin has a strong influence on the change of the flow field. The water depth also significantly affects on the type of jet and vortex structure forming in the basin. On the other, hand the sediment deposition pattern was strongly influenced by the jet type and the flow behavior changed with increasing deposits. The prediction of sediment deposition is linked to the prediction of flow behavior. Furthermore it is very sensitive to the basin geometry and the boundary conditions of inflow and outflow. Some tests were performed until the sediment released at the outlet was equal to the sediments entering at inlet into the basin that means a quasi equilibrium was reached. Flushing at normal water level allows only a relatively small part of the deposited sediment to be evacuated. As expected, drawdown flushing is much more effective and a significant amount of sediment deposits could be flushed out of the basin. Regarding the flow pattern in shallow basins empirical relationships for the estimation of the reattachment length of gyres and the normalized residence time as a function of the geometry shape factor, SK, were established. Also empirical equations for the prediction of the jet flow type and velocity magnitude depending on the basin geometry could be formed. Finally empirical equations for the prediction of sedimentation index, silting ratio, trap efficiency and relative deposition thickness, as well as normalized residence time and flushing efficiency could be found. The numerical simulations revealed that the observed asymmetry in flow and deposition patterns can be explained by the sensitivity of the flow regarding geometry and boundary conditions. The influence of the length and width of the basin on the flow pattern can be predicted in good agreement with experiments by these simulations. Some recommendations are given for the design procedure of a new shallow reservoir in view of minimizing the sedimentation due to suspended sediment. The deposited sediment volume can be efficiently minimized by an optimal designed reservoir geometry.

The flotation process is the most important process for the enrichment of valuable minerals. Although the process is now known for approximately 160 years, the amount of investigations on the micro processes is up to now still small. Technological developments in the recent years made optical measurement techniques such as Particle Image Velocimetry (PIV) and Shadowgraphy (SH) available for scientific research. Most of the investigations in the last decades had, however, a practical orientation aiming for an optimisation of process parameters in order to increase the economics. The number of investigations applying these optical methods to the flotation process is surprisingly low. Reasons for this are the high solids and bubble concentrations as well as the high turbulence during the flotation process. The aim of the present work is the investigation of the complex flow structures during the flotation process via PIV and SH. To facilitate the application of the optical methods, a reduced flotation system with lower bubble and solids concentrations is set up. For this purpose, a novel flotation column made from transparent poly(methyl methacrylate) is designed, which allows the formation of single bubble-particle heterocoagulates and simultaneously facilitates a full optical accessibility. Investigations are carried out both in the two-phase as well as the three- phase system. Investigations in the two-phase system focus on the analysis of the bubble behaviour in deionized water and in presence of surface active agents at various concentrations. The aim is to develop a relationship between the bubble characteristics and the induced liquid velocity around the rising bubble. A model three-phase system consisting of glass particles is the starting point for flotation related investigations. The influence of flotation process parameters, e.g. volumetric gas flow rate and particle size, on the flotation outcome in form of maximum recoveries Rmax and flotation kinetics with respect to order n of kinetics as well as flotation rate constant k is investigated. Furthermore, the hydrodynamic structure of the rising single bubble-particle heterocoagulates is revealed via PIV measurements. Afterwards, an industrial fluorite flotation system is investigated using a design of experiment in order to find the optimal process parameters to maximise Rmax, k, as well as the grade of the concentrate with respect to the calcium fluoride weight fraction w(CaF2). Lastly, a fluorescent fluorite mineral is tested for application in optical analysis. The aim is to distinguish valuable fluorescent fluorite mineral particles from gangue particles such as silicon dioxide and barite. The results show that both PIV and SH are applicable for the investigation of multiphase flows. The designed apparatus facilitates both, optical accessibility into the reduced flotation system as well as the possibility to determine important flotation parameters. An automated analysis procedure with a high accuracy has been developed to allow a fast analysis of the obtained image sequences. The reproducibility of consecutive bubbles has been found to be excellent, thus allowing a statistical investigation of different operational parameters on the process performance. The measurements in deionized water have shown a strong relationship between the volumetric gas flow rate and the induced liquid velocity. The surfactants changed the bubble characteristics significantly with increasing concentration towards a spherical form. Furthermore, the rising velocities decreased until a threshold has been reached. Although the rising velocities decreased, an increase in the induced liquid velocity as well as in the induced kinetic energy has been observed. Analysis of the bubble trajectories revealed an oscillating behaviour in horizontal x direction. With increasing surfactant concentration this oscillation frequency increases. Thus, the oscillation has been found to be responsible for this controversial relation. The investigation of the model flotation system has shown the influence of the different operational parameters on both maximum recovery and flotation kinetics. While the order of flotation is best described by the zeroth order model at a high initial solids concentration cp,0, a shift to first order kinetics has been found for lower cp,0. Optical investigations showed large differences in the bubble-particle heterocoagulate rising behaviour for particles with different wettability. The poor flotation performance of hydrophilic particles could be related to a high mobility of the particles on the bubble surface, meaning a poor attachment efficiency. Hydrophobic particles have been found to form larger bubble-particle clusters. The hydrodynamic characterization of bubble-particle heterocoagulates showed an increase of the mean liquid velocity compared to unloaded bubbles. A relation to the particle size has, however, not been found. To investigate the industrial fluorite flotation system with respect to w(CaF2) in the concentrate, a novel quantification method via Fourier Transform (FT) Raman spectroscopy has been developed and cross-validated. This method was then applied in the design of experiment. The optimum process parameters for maximisation of Rmax, k, and w(CaF2) have successfully been determined and correlated to further process characteristics, e.g. Zeta potential. It has been found that the pH value should be slightly alkaline, while both collector concentration and stirrer speed should be high. The volumetric gas flow rate has been found to be insignificant in the investigated parameter range. Lastly, the applicability of the fluorescent fluorite in optical investigations has been proven. A distinction between the fluorescence and reflections at the silicon dioxide surface has been presented under static conditions. Furthermore, first single bubble-particle heterocoagulates have been shown with fluorescent fluorite particles attached to the bubble surface. Thus, further optical investigations are promising.

Entrained flow pyrolysis and gasification of selected biomass – an experimental and modeling study

The aim of this study is to investigate about the complex phenomenology associated with the interaction of a particle-laden turbulent flow with the slag-covered wall of an entrained-flow gasifier. Recent observations, indeed, highlighted that this phenomenology can have an impact on the global gasifier performance greater than that expected from previous analyses. The design of new generation of entrained-flow coal gasifiers aims at favoring ash migration/deposition onto the reactor walls, whence the molten ash (slag) flows and is eventually drained separately at the bottom of the gasifier. In terms of efficiency, the oxidation of the volatile compounds released around the particles depends upon its mixing with the fresh oxidant mixture. Therefore combustion efficiency is influenced by the spatial distribution of the particle phase, with an homogeneous distributions favoring a better mixing. From the observation that a significant number of coal particles can spent most of the time in the gasifier close to the slag layer, where usually their concentration largely increase, leads to the need to understand the effective conditions experienced before complete conversion. An experimental evidence of a picture for the fate of coal particles has been recently assessed by analyzing the chemical composition of samples of coarse slag and slag fines generated in the ELCOGAS entrained-flow gasifier located in Puertollano, Ciudad Real (Spain). Quantitative SEM-EDX analysis of the coarse slag revealed the presence of small marks with a significant carbon content as high as 48.8%-54.2%. This fact can be explained by assuming the entrapment of not fully burned coal particles into the slag. The results of the SEM analysis performed on whole slag fines particles showed that the carbon content was larger than the value obtained from the inspection of coarse slag particles. This is particularly evident for porous particles where C-content ranged between 82.3% and 86.5%. A considerable amount of unreacted coal is therefore entrapped into the slag matrix. From this observations emerges that a certain level of spatial non homogeneity of the solid phase distribution exists. In a recently published study by Montagnaro and Salatino (2010), these data have been interpreted by assuming that different regimes of particles-slag interaction can occur: either char entrapment inside the melt or carbon-coverage of the slag may occur, depending on properties like char density, particle diameter and impact velocity, slag viscosity, interfacial particle-slag tension. Occurrence of char entrapment prevents further progress of combustion/gasification. On the contrary, if char particles reaching the wall adhere to the slag layer's surface without being fully engulfed, the progress of combustion/gasification is still permitted. The observed high rate of coal conversion can actually be explained only if this second regime establishes on the slag surface. The addressed considerations highlights the technological need to build up methods for the prediction of the mechanism particles clustering and segregation in condition representative of coal particle flying and converting into a gasifier. Actually a comprehensive numerical simulation of the whole range of spatial and temporal chemical and turbulent time scales involved in a full scale gasifier, is still unfeasible due to the high computational cost: the scales of turbulence involved in the gasification processes range from sub-micron scale up to the integral scale of a gasifier reactor chamber (of the order of tens of meters). To overcome this difficulty, the approach proposed in this study is based on the development of a multilevel approach.. In a first level, the motion of particles representing classes of partially converted coal in a 3-dimensional representation of the gasifier is modeled with a Computational Fluid Dynamic (CFD) approach.. Turbulence of the flow field is described adopting the Reynolds Averaged Navier Stokes (RANS) approach, while particle motion is resolved with a Lagrangian Particle Tracking (LPT) approach. The use of the RANS method for the gas phase coupled with the LPT for the solid phase in this analysis is twofold. Firstly it has been used to address the behavior of coarse and fine coal particles trajectories when subjected to a swirl motion which induced a turbulent field. This model, while avoiding the great complexity and computational effort required by comprehensive numerical CFD models of gasifiers already proposed in the literature, is sufficient to characterize the range of conditions, in terms of momentum possessed and direction, that the different particles show when approaching the gasifier walls. The second aspect concerns the identification of regions where different mechanisms for the coal clustering becomes foreseeable: distinct regions close to the wall have been identified: finer particles could be mainly responsible of particle layering near the solid walls as they, after their first impinging on the wall, assumes a pathway parallel to the wall; in contrast, larger particles continue to bounce over the walls along the whole length of the gasifier. The identification of these two different regions and the characterization of particle classes representative of partially burned coal particles, was the basis for the proper setup of numerical simulations based on a Large Eddy Simulation (LES) approach in two completely different configurations. This level aims at a detailed investigation of the mechanisms of slag-particle interaction. The first is a plane particle-laden channel flow, that well represents the main features of the gasifier regions where particles move parallel to the wall. The second is a periodic particle laden curved channel flow, that best represent regions close to the wall but dominated by the external swirling flow. For these two configurations the particle interaction with the slag has been treated as a rebound on a not perfectly elastic wall. A parametric study has been conducted obtaining results for different particle sizes (representing different particle inertia) and different momentum restitution in the particle-wall impact. Numerical multiphase simulations are based on the Eulerian-Lagrangian approach implemented in the OpenFOAM CFD framework. Both RANS and LES turbulence models are implemented for the gas phase. The equations of particles motion were solved via a Lagrangian particle tracking algorithm with the TrackToFace method. Simulations were performed involving a number of particles from 10^5 to 10^6, a level of detail that allowed to obtain a clear picture of the multiphase flow behavior responsible for char deposition phenomena. Numerical simulation results with the LES approach do confirm the establishment of a region near the wall slag layer (the dense-dispersed phase leading to the formation of the slag fines), in which particles impacting the slag accumulate to an extent depending on the system fluid-dynamics and on parameters such as particles Stokes number and restitution coefficient. However, particle concentration near the wall in all the simulated cases does not appear perfectly steady not evenly spatially distributed. Interestingly, the segregation of char particles near the wall is more evident for the curved channel flow geometry and is enhanced for coarser particles, making evident the role played by the effective impact with the slag not recovered by the simpler models adopted in the RANS simulations.