Intensity Of Segregation Research Articles

Spreading and mixing during solutal convection in uniform porous media with application to geologic CO2 storage

Convective dissolution in saline aquifers is expected to positively impact subsurface storage of carbon dioxide (CO2) by accelerating its dissolution rate into reservoir brines. By largely focusing on the dissolution flux, previous studies lack a systematic evaluation of the mixing process following CO2 emplacement, including a quantitative analysis at conditions representative of subsurface traps (Rayleigh number, Ra≤1 000). Here, we investigate solutal convection numerically in a two-dimensional uniform porous medium in the regime Ra=100−10 000. The macroscopic evolution of the convective process is characterized by means of fundamental macroscopic measures of mixing that use the local spatial structure of the solute concentration field. It is shown that the intensity of segregation closely mimics the evolution of the in situ convective pattern arising from the stretching and merging of downwelling plumes. The spreading length and the dilution index both confirm that the mixing process accelerates over time (t) with a power law scaling (∝tα) that transitions from diffusive (α=0.5) to superdiffusive mixing (α≥1) irrespective of Ra. This transition time scales τon∝Ra−2 and is used as a measure of the onset time of convection. The dilution index indicates that the time needed to reach close-to-complete mixing reduces linearly with Ra. On the contrary, the non-dimensional mass flux, expressed in terms of the Sherwood number, Sh, reveals a natural logarithmic scaling for Ra≤2 500.

Correlations between segregation intensity and material properties such as particle sizes and adhesions and novel methods for assessment

Assessment of powder segregation is essential in most powder-based industries when homogeneity of the powder is a concern. This study focuses on exploring correlations between segregation possibilities of the powders with a wide size range and some common material properties such as particle size, size ratio and adhesion, so that a simple method for assessing segregation intensity of a powder just based on the material properties can be developed, although the current study is designed for surface rolling segregation only.In this study, eight blends were made from different grades of calcium carbonate, which were selected based on different particle size ranges and particle adhesions. Segregation of a sample was determined by measuring segregation index, which was defined as a ratio of fines concentration change between segregated samples at the top and bottom sections of a surface segregation tester and the averaged concentration. Particle size distributions of the blended samples were measured before segregation tests and then the segregation tests were completed for the same blends. Powder adhesion of the virgin blends was also examined and represented by measuring the “bond number” using a mechanical surface energy tester developed at the Wolfson Centre.The experimental results showed correlations between the segregation index and the material properties such as particle size, size ratio and bond numbers, and was fitted to multiple empirical functions. With the correlations determined, novel methods for assessing surface segregation have been developed; one method considers just the particle sizes and the other one considers both particle sizes and particle adhesion identified by the bond number.

Characterisation and improvement of the quality of mixing of recycled thermoplastic composites

A low-shear mixer was used to blend thermoplastic composite scrap material into a molten mixed dough, which was then compression moulded. This process is a key step in a novel recycling solution for thermoplastic composites. A study was carried out to characterise the quality of mixing (QoM) of the blended doughs to understand how to improve the QoM of mixed doughs towards further improvement and implementation of the recycling solution. In order to achieve this, the effect of mixing parameters and fibre length on the QoM were studied. This study used shredded C/PPS flakes, originating from consolidated laminate scrap. These flakes are about 20 mm in size, and contain woven fabric reinforcement, making them far different from regular pellets, and therefore more difficult to mix. The QoM was characterised by means of image analysis of a large set of cross-sectional microscopy images, based on which the scale and intensity of segregation of the fibre clusters were evaluated. Bundle size distribution was determined by applying Delaunay triangulations to cluster the fibre centres. These methods were found to be suitable for characterising the QoM of such doughs. Increasing the mixing time and mixing speed were identified as key ways to improve the mixing process. With the current mixing machine, it is also suggested not to use fibres longer than 15 mm on average in order to limit intra-dough variability. For doughs made of fibres longer than 15 mm, improvements on the mixing device could sufficiently increase the QoM.

On the Relationship between Poverty Segregation and Homelessness in the American City and Suburb

Although existing scholarship notes that homelessness thrives in concentrated poverty, models estimating the association between the intensity of residential poverty segregation and local homelessness rates across communities remain absent from the literature. To fill this gap, the author considers this relationship for 272 homelessness Continuums of Care covering urban and suburban spaces spanning 43 states and the District of Columbia. Models suggest that poverty segregation is positively associated with the expected homelessness rate of a Continuum of Care, a relationship that remains significant when controlling for a range of established drivers of the condition. The author discusses this finding within a framework qualifying residential poverty segregation as both a cause and a consequence of the local prevalence of economic disadvantage that predicts homelessness via its relationship with disadvantage and unique spatial effects.

Vibration induced segregation of single large particles

The vibration-induced segregation (e.g., rising of one large intruder - so called Brazil Nut Effect (BNE)) is studied by discrete element method. Vibration frequency and amplitude are two dominating factors in the occurrence of BNE and a phase diagram is constructed. For fixed vibration amplitude, segregation only occurs when vibration frequency is within a certain range. Larger vibration amplitude can expand the range of vibration frequency for BNE. Size ratio and the intruder shape are studied under certain vibration conditions. Larger size ratio can enlarge the segregation intensity. The shape of the intruder influences the segregation process by the intruder′s orientation. Standing-like initial orientation can increase the time required for the intruder to reach the top while lying-like initial orientation cannot significantly affect the vertical segregation.

Assessing Eulerian Indicators for Predicting Mixing in a Blinking Vortex System with Varying Degrees of Continuous Transition

A discontinuous change in sequential velocity fields is known to generate laminar flow mixing through the mechanism of streamline crossing. However, previous research has suggested that a small degree of continuous transition between velocity fields may not necessarily be detrimental. This study therefore used a modified blinking vortex system with varying degree of continuous transition to assess the precise effect that this continuous transition has on mixing performance. This system was studied for the parameters: blinking period, vortex spacing, and the fraction of time spent in transition. Continuous Eulerian indicators were computed to investigate their correspondence with Lagrangian-based metrics, such as Intensity of Segregation, under such conditions. The results showed that up to 30% transition time yielded improvements in mixing, most notably when vortex spacing was large, and this was consistent across different time periods. The mixing prediction by the Eulerian indicators, particularly mobility, showed good agreement with actual mixing quality, albeit not perfectly, suggesting room for refinement in these metrics. Overall, the findings imply that mixing systems, such as continuous pipe flow-based devices, which are designed assuming a discontinuous change in velocity fields, might benefit from the presence of a small degree of continuous transition between discrete states.

Impact of granular segregation on heat transfer in horizontal drums

In the production of biofuel from scrap tire pieces by pyrolysis in a rotary kiln, particle size segregation is believed to affect the temperature evolution and control within the reactor. To study the impact of granular segregation on heat transfer in rotary kilns, a thermal discrete element method-based model incorporating a thermal resistance network was developed for bidisperse particulate blends. It was validated using a system of glass and aluminum beads differing in size. The harmonic mean radius was found to best represent bidisperse contacts. Heat transfer in six different segregated bed structures in a wall-heated drum for two particle size ratios was studied. Granular segregation significantly influenced the temperature profiles in the beds, dragging down the temperature of the center of the bed, where a core of smaller particles had formed. This resulted in an inverse relationship between the effective heat transfer coefficient of the bed and the intensity of segregation. Within the segregated cases investigated, the temperature difference between the large and small particles could be up to 30 times higher than that of a well-mixed configuration.

Experimental investigate the effect of liquid distribution state on size segregation of wet granular materials in rotating drums

In this paper, we investigated the effect of liquid distribution state on size-induced granular segregation in a thin rotating drum. An image-processing method and particle-tracking technology were employed to measure the average velocity, average granular temperature, dynamic angle of repose, and intensity of segregation. Both the liquid distribution state and the rotation speed had a significant influence on size-induced segregation of wet granular materials. The rupture and formation of liquid bridges could dissipate the kinetic energy of particles, restrict the particle movement, and mitigate the size segregation behavior. The intensity of segregation decreased non-monotonically with liquid content. Moreover, the dynamic repose angle did not increase monotonically with the increasing liquid content. The results indicated that the liquid distribution state plays an important role in determining the size-induced granular segregation behavior. The relation between the granular temperature and segregation behavior is also discussed in this study.

Impact of polydispersity on the flow dynamics in the riser of a circulating fluidized bed

Circulating fluidized bed (CFB) has been commonly operated with polydisperse particles with a wide particle size distribution (PSD), which significantly affects solid transportation and the resulting thermochemical performance. Dense gas-solid flows in a pilot-scale CFB riser are simulated via the multiphase particle-in-cell (MP-PIC) coupled with a heterogeneous drag model. Gas-solid hydrodynamics together with the particle-scale information with a wide PSD is discussed. The results show that in the riser, the non-uniformly spatial distribution of solid phase appears with the presence of a core-annulus structure and a dilute-upper dense-bottom profile. The wide PSD gives rise to the axial segregation phenomenon with the accumulation of large particles in the bottom region. Enlarging the superficial gas velocity and PSD width respectively decreases and increases the segregation intensity of solid phase. As compared with gas velocity, the PSD width performs limited impacts on solid concentration, particle-scale information along the axial direction, and solid residence time. The horizontal solid dispersion coefficient is at the scale of 10−3 m2/s, while the vertical dispersion is at the scale of 1 m2/s. For a wide PSD, particles with larger sizes have larger slip velocities but smaller dispersion coefficients in the riser.

Flow and mixing characteristics of a groove-embedded partitioned pipe mixer

We propose a groove-embedded partitioned pipe mixer (GPPM) and conduct an in-depth numerical study on the flow and mixing characteristics of the GPPM in the creeping flow regime. The GPPM is a variant of a previously reported mixer, the barrier-embedded partitioned pipe mixer (BPPM), and is designed to achieve better energy-efficient mixing compared to the BPPM. In this paper, we first introduce the working principle of the GPPM and its mixing protocols. Then, the flow system affected by mixing protocols and geometrical parameters of the GPPM is investigated using Poincare sections. As for mixing characteristics, the flux-weighted intensity of segregation is employed for quantitative mixing analysis. It turns out that a GPPM with a proper set of design parameters can indeed lead to a globally chaotic mixing. More importantly, the best GPPM showed better mixing in terms of energy consumption compared to its counterpart, the best BPPM.

An experimental study on free-surface rolling segregation and correlations with angle of repose and particle sphericity

Bulk solids commonly segregate when particles move on an inclined surface, often causing significant variations in content uniformity of particle properties such as size and species. Often called free surface rolling segregation, this is one of the most common segregation phenomena for powders and granules, where different sized and shaped particles become separated due to differential movement on the surface. Intensity of rolling segregation is widely understood to depend upon various factors including differences in particle size and shape as well as flow properties and densities. These influences have been studied and reported previously, but correlations between levels of rolling segregation and particle shape assessed by particle sphericity or sphericity ratio with consideration of angle of repose has not been investigated in detail.This is an experimental study of rolling segregation of one particulate material (calcium carbonate) with different particle size ranges; using a single material eliminates variations of influences not being studied, especially solid density. The influence of bulk and particle properties has been studied systematically, especially particle size distributions, angle of repose and particle sphericity. The segregation results show an increased trend with higher particle sphericity and especially sphericity ratio. Also, the results indicate higher levels of segregation with a low angle of repose, but little segregation when angle of repose is higher than 32 degrees. However, the authors caution that this angle of repose of 32° degrees should not be taken as a general indicator of a limitation of the susceptibility of blends to free-surface rolling segregation, for materials other than the one tested.

Underpinning mechanistic understanding of the segregation phenomena of pharmaceutical blends using a near-infrared (NIR) spectrometer embedded segregation tester

The segregation of an active pharmaceutical ingredient (API) within a powder blend is one of the major manufacturing obstacles in achieving content uniformity. Segregation can be due to differences in physicochemical properties of formulation components and/or perturbations experienced during secondary processing steps, such as granulation, fluidization, die-filling and compression. A near-infrared (NIR) spectrometer embedded segregation tester, which could mimic the external stimulations (vibration and fluidization) experienced by a blend in a manufacturing facility, was used to evaluate and predict blend segregation. Two different GlaxoSmithKline (GSK) product blends with variations in the API particle size and concentration were tested. Drug content was further measured at different locations along the powder bed by NIR to sketch the segregation profile and calculate the overall segregation intensity of each blend. The study indicated that the segregation potential was dependent on the particle sizes of API and excipients, as well as the type of stimulus applied (vibration vs fluidization). Drug concentration profiles obtained from this mode of analysis decoded the underlying segregation mechanisms (sieving, trajectory and air elutriation) easily. The employed NIR-based segregation tester proved to be a useful small-scale predictive tool to evaluate and rank the segregation risk of the studied pharmaceutical blends.

Flow regimes and mixing characteristics in non-aligned T-jets reactors

Flow regimes and mixing characteristics in non-aligned T-jets reactors were investigated by Planar Laser Induced Fluorescence (PLIF) at 30 ≤ Re ≤ 400. The misalignment of inlet jets in non-aligned T-jets reactors leads to a steady engulfment flow at low Reynolds numbers. With the increase of Re, the unsteady engulfment flows with different oscillation behaviors occur. Unlike the vortex-merging oscillation in reactors of ws/h = 0 and 0.5 (ws and h are the misalignment degree and height of inlet jets), an S-shaped self-excited oscillation is identified in reactor of ws/h = 1. Mixing in non-aligned T-jets reactors is quantified by time-averaged Intensity of Segregation (IOS) and the mixing mechanism is revealed. The mixing is significantly enhanced in non-aligned T-jets reactors at low Re due to the earlier emergence of the steady engulfment flow. Both the vortex-merging oscillation in reactors of ws/h = 0 and 0.5 and the S-shaped self-excited oscillation in reactor of ws/h = 1 contribute to the mixing enhancement.

Synthesis of carbon matrix with tunable carbide formation ability for reactive infiltration techniques

The reactive melt infiltration (RMI) is a cost- and time-efficient technique to produce ceramic matrix composites with low porosity. In the case of carbide-forming infiltrating agent, the carbon matrix should have sufficient open porosity, wettability, and controlled reactivity. A synthetic approach towards carbon matrices with tunable reactivity and mass transport properties is proposed. The approach is based on resol resin/ethylene glycol systems and variable conditions of polymerization-induced phase separation (PIPS). It is shown that increasing the polymerization rate in the PIPS step by raising the temperature or by modifying the resol resin, for example, by partial replacement of phenol with meta-cresol, results in higher segregation intensity and higher capillary pressure during RMI. The mobile pore-forming agent (ethylene glycol) results in the formation of a less defective carbon matrix after pyrolysis in comparison with corresponding thermosetting resol component. The PIPS step and partial replacement of phenol by meta-cresol increase the reactivity of carbon matrix towards carbide-forming infiltrating agent.

Simulation and experimental validation of the effect of superheat on macrosegregation in large-size steel ingots

A 3D model was employed to study the effect of melt initial superheat on the macrosegregation formation using FE modeling and experimentation methods. The casting process of three ingots with the initial melt superheats of 75 °C, 65 °C, and 55 °C were simulated. The three cases represented three variables encountered in industry during casting of large size ingots. For the above three studied cases, all other casting conditions were kept the same. Results showed that the variation of initial melt superheat gave rise to changes in temperature pattern, liquid flow field, solidification speed, and thermomechanical contraction. Under the combined actions of all these changes, lower superheat tended to alleviate the segregation intensity in the upper part of the ingot body, in the hot-top, and in the solute-rich bands between the ingot centerline and periphery. The beneficial effect of lower superheat on alleviation of segregation severity was confirmed by experimental chemical measurement results. The results were analyzed in terms of heat and mass transfer theories and allow for a better understanding of the underlying mechanisms responsible for the occurrence of macrosegregation in ingot casting process. The findings should be helpful for the casting process design of a given ingot of high value-added steels or other alloys.

Effect of geometry on engulfment flow regime in T-jet reactors

T-jet reactors have been widely used in micro/mini reactors for their simple geometries and rapid mixing. Following our previous report (Zhang et al., Chem. Eng. J., vol. 358, 2019, 1561–1573), the present study describes the flow regimes in T-jet reactors with different geometrical parameters at 50 ≤ Re ≤ 400 by using planar laser induced fluorescence (PLIF). The influences of the headspace height (H/h), the outlet aspect ratio (W/w), and the depth of the reactor (w/h) on the origination and evolution of steady and unsteady engulfment flow have been investigated in detail, where h is the inlet height, w is the reactor depth, W is the outlet width, and H is the headspace height. A comparison of the critical Reynolds number (Re) of steady engulfment flow between current work and previous investigations in the literature is presented. The mixing performance of various T-jet channels has also been analyzed using the Intensity of Segregation (IOS). Results show that, the intertwinement of two inlet streams is weakened as the headspace height increases, and the unsteady engulfment flow disappears for H/h ≥ 0.3. The outlet aspect ratio influence the evolution of engulfment flow in the chamber and the unsteady engulfment flow vanishes as W/w reaches 8. Besides, the engulfment of two inlet streams decreases and the unsteady engulfment flow disappears whether the reactor depth increases to w/h = 2 or decreases to w/h = 0.5.

Economic and environmental consequences of implementing an EU model for collecting and separating wastes system in Lebanon.

In this study, we examine the economic and environmental significance associated with the implementation of an EU waste-separated collection scheme in a developing context - Lebanon. Two scenarios, S1 and S2, representing different intensities of source segregation were analysed. In S1, the average source segregation intensity reached 25% and 13% for the Italian test area and Lebanese test area, respectively. In S2, source segregation intensity increased to 48% and 68% for the Italian and Lebanese test areas, respectively. Passing from S1 to S2 increased collection costs significantly, up to 44% with greater increases in the Italian test area where labour cost is higher. In both areas, environmental impacts decreased with greater source segregation intensity. Savings in the climate change impact and stratospheric ozone depletion potential were lower under the Lebanese test area in comparison with the Italian test area. In contrast, savings in freshwater eutrophication and acidification impact were lower for the Italian test area. The increase in the source segregation intensity resulted in maximum savings for the depletion of abiotic resources, 74% to 77% and 79% to 80% in a developing and developed context, respectively.

Mixing intensification under turbulent conditions in a high pressure microreactor

The turbulent mixing of two miscible fluids is investigated in a high pressure (HP) coflow microreactor operated at 100 bar. Ethanol and CO2 are selected as model solvents to mimic the final targeted application, i.e.: a supercritical antisolvent process at microscale (μSAS). We first demonstrate experimentally that turbulent mixing can be reached in a microchannel using HP microfluidics. A computational fluid dynamic (CFD) model, performed using direct numerical simulation (DNS) down to the Kolmogorov scale has been applied for the turbulent mixing simulations. The effects of the main operating parameters on the final mixing efficiency have been studied, namely: the temperature, the fluid flowrates, the microchannel dimensions and the capillary inner and outer diameters. According to a predefined intensity of segregation, the characteristic mixing times are determined and used for determining mixing efficiency. The ratio of the total mixing time to the diffusion time depends on the ratio of the kinetic energies (the outer fluid to the inner one). The obtained micromixing times have been related to the turbulent energy dissipation rate ∊, calculated directly from the velocity fluctuations. The mixing intensification is obtained with much lower characteristic mixing times in the microreactor (one order of magnitude) than previously reported. This fundamental study is an indispensable guidance for several processes, including the μSAS applications.

New mass flow limiting lines based on segregation pattern and magnitude

Mass flow, by definition, means all of the mass in a hopper will move when some material is discharged from the outlet. Mass flow has been the recommended means to solve segregation issues. This generally works under conditions where the segregation pattern is radial and when the process operation is nearly steady state flow into and out of the hopper. If the segregation pattern is either axial or radial, but the mode of operation is to fill followed by completely emptying the bin, then segregation can easily be an issue – even in mass flow designs. Segregation prevention is all about velocity control, and matching the velocity to the segregation pattern and mode of operation.Ideally, one would like to estimate segregation using easily measured flow properties, segregation potential tests, and a description of the process geometry. This paper looks at a method of predicting segregation problems using flow properties like wall friction angle and effective internal friction angles coupled with the radial stress theory to compute the expected velocity profiles in a variety of process geometries. Using slope stability models, one can also relate the cascade behavior of cohesive material down a pile or slope. Imposing radial stress velocity profiles and cascade velocity profiles in a bin or hopper, then using particle tracking techniques, one can compute the time at which any particle in any portion of the bin might exit the system. Furthermore, imposing a measured segregation profile on the material initially placed in the bin, and then tracking groups of particles with an assigned segregation concentration through the bin, can help determine when and how segregated material might exit the bin. Doing enough of these calculations for a range of wall friction angles and hopper slope values, the velocity profile can be related to the segregation intensity exiting the bin as a function of these two variables. Since these two variables also determine the propensity for mass flow, this analysis can relate mass flow to segregation tendency for material subject to a filling and discharge mode of operation. Selection of an optimal mass flow design would then depend on the segregation pattern and the desired segregation intensity the process can live with.Applying this methodology to a conical hopper suggests that a material prone to segregation will segregate significantly in a funnel flow design. This is to be expected. However, this work also indicates that mass flow bins, when filled and then emptied completely, may experience significant segregation if the hopper walls are designed with traditional mass flow limits in mind. This work suggests that mass flow hoppers must be considerably steeper to mitigate segregation effects for a fill-then-empty mode of operation.

Effect of Powder Shape and Size on Rheological, Thermal, and Segregation Properties of Low-Pressure Powder Injection Molding Feedstocks

Optimization of molding parameters at different filling stages requires that the characteristics of low-viscosity feedstocks must be properly known. In this study, the influence of the particle size (3, 7, and 12 μm) and powder morphology (gas- and water-atomized) was quantified using different feedstock formulations based on a 17-4 PH stainless steel powder, all combined with the same binder system. The specific heat capacity, viscosity, and segregation levels were assessed using a differential scanning calorimeter, a rotational rheometer, and a thermogravimetric analyzer, respectively. It was then shown that the feedstock prepared with the gas-atomized powder exhibits a higher specific heat capacity value when compared to a water-atomized powder (i.e., Cp varying from 0.6 to 0.3 J/g K, respectively). It was also shown that the feedstock viscosity profiles and the intensity of segregation both depend significantly on the powder shape and size used in the feedstock formulation. For short processing times (e.g., < 1 min spent in molten state), a feedstock formulated with coarse gas-atomized powder can be considered to be the best candidate for an injection process because its viscosity and segregation potential are relatively low (with a viscosity varying from 0.5 to 2 Pa s, and a volume fraction of powder close to 60 vol.%). Following this, the moldability index was used to predict that coarser water-atomized powder (i.e., 12 versus 7 μm) will not have a significant effect on the molding properties, and can be seen as a potential means of decreasing shrinkage during sintering without impacting the injection properties. For very long processing times, the magnitude of segregation remains insignificant for feedstock formulated with fine powder (i.e., a volume fraction of powder remaining close to 60 vol.%). However, this important gain (i.e., no segregation) comes with a significant increase in viscosity varying from 2 to 10 Pa s, resulting in a decrease in the molding properties.