Cohesive Powders Research Articles

Nanosilica to improve the flowability of fine limestone powders in thermochemical storage units

Fine powders are the cornerstone of new energy storage solutions to assist concentrated solar power plants. Though, their ability to behave like fluid can be seriously affected at high temperatures. This work investigates the use of nanosilica in fine limestone (calcium carbonate, CaCO3) powders to mitigate the promotion of cohesion forces at high temperatures. Experiments were conducted over limestone powder samples with particle sizes around 45μm. The analysis was performed monitoring the tensile yield strength as the samples were subjected to different temperatures and consolidation stresses while varying the nanosilica content up until 0.82wt%. Temperatures reached a maximum of 500°C(close to the Tamman temperature in limestone), whereas consolidation stresses were increased up to 2kPa. Results show that nanosilica coating is an efficient solution to inhibit the enhancement of powder cohesiveness at high temperatures and consolidations. A solution that offers better control to smooth the granular flow regimes in production environments.

Laboratory experiments with self-cohesive powders: Application to the morphology of regolith on small asteroids

A series of experiments have been conducted to study the failure behavior of columns and piles comprised of cohesive fine powders in 1 ​g as a proxy for those which might occur in the agglomerated asteroid structure composed of cm-m size pebbles and boulders in a microgravity environment. Initially symmetrical piles of fine powders, under gravitation or centrifugal stress, develop features similar to those observed on asteroids, such as slide planes and finer cohesive structures. Failure of cohesive columns of fine powders occurs by the nucleation and propagation of fracture planes. In some cases, forming steep cliffs, also reminiscent of features observed on asteroids. Correlation between observed column failure and numerical simulations has been demonstrated based on preliminary results. Microstructure and particle size distribution are shown to substantially determine the extent of cohesiveness. Enhanced cohesion was observed for specific ratios of larger particle intermixed with fine powders. We propose that the wide range of qualitative features and behaviors may reasonably represent those observed on asteroid features as the surface ages. This work has important implications for our understanding and preparation for future missions to NEOs.

Dynamic Behavior of Air Void during the Discharge of Cohesive Powder in a Hopper Using a Rubber Air Spring

An unstable discharge rate occurs during dry fine powder discharge from a hopper because of the significant two-phase solid/gas interactions that occur in powder flows. In addition, the air bubble phenomenon may occur in a silo during fine powder discharge. In this study, we conducted experiments using a semi-conical dual-structure hopper, and examined the effects on the hopper internal flow structure, cavity fluid pressure, pressure inside the airtight cavity section, and the powder discharge rate when changes are made in the position of the supplied air injection port and the solenoid valve open/close timing. From the experimental results, it was confirmed that an appropriate pressure supply port position exists, and the change in expansion/contraction of the flexible container due to air vibration is determined by the balance between the amount of air inserted and the amount of air discharged, and does not affect the presence or absence of powder so much. Furthermore, as the pressure value in the airtight void is directly related to the change in the expansion and contraction of the flexible container, the maximum amplitude value of the pressure in the airtight void can be kept high and constant at the time of opening and closing the solenoid valve.

Combined effect of particle size and surface cohesiveness on powder spreadability for additive manufacturing

The dependence of powder flowability on particle cohesion can often be captured by a dimensionless Bond number, however, whether the same is true for powder spreadability remains unclear. This paper thus presents a particle-scale study on the spreading of cohesive fine powders for additive manufacturing, focusing on the combined effect of particle size and surface cohesiveness. With an experimental validated model, detailed analysis revealed two competing mechanisms due to geometrical constraint and interparticle cohesion. For small particles that allows multiple particles across blade clearance, layer homogeneity is largely maintained before deteriorating sharply, while for large particles that forming a mono-layered packing structure, an optimum homogeneity can be obtained at a moderate level of particle cohesion. The efficacy of increasing blade clearance to improve layer quality was also evaluated. This study can help to identify a critical set of powder descriptors that defines spreading performance.

Effect of liquid addition on the bulk and flow properties of cohesive powders

In this work, the effect of water on the packing and flow properties of cohesive powders was experimentally investigated. Two cohesive powders with small particle sizes were studied with weight percentages of added water of 0, 10, 25, 40, and 50%. Particle properties including conditioned bulk density, porosity, compressibility, flow function coefficient, cohesion, etc., were tested using the Freeman FT4 Powder Rheometer and examined as a function of water content. Results show that water content has a significant effect on the packing and flow of the powders. With small amounts of water added, the particle beds became more cohesive, due to the formation of liquid bridges, and the bed volume increased roughly in proportion to the mass of water added leading to a fairly constant conditioned bulk density. When greater amounts of water were added to the particle beds, there was a substantial increase in conditioned bulk density. The bulk densities of both powders increased from around 0.3–0.55 g/ml when the added water was increased from 40% to 50%. As the water weight fraction increased, there was generally an increase in basic flow energy and cohesion, while the porosity and pressure drop generally decreased.

Pilot‐scale production and physicochemical characterisation of spray‐dried nanoparticulated whey protein powders

Spray‐dried whey protein isolate (WPI) powders were prepared at pilot‐scale from solutions without heat (WPIUH), heated (WPIH) or heated with calcium (WPIHCa), which were analysed and compared with a control sample (WPIC). WPIC, WPIUH, WPIH and WPIHCa solutions had whey protein denaturation levels of 0.0, 3.2, 64.4 and 74.4%, respectively. Computerised tomography scanning showed that 52.6, 84.0, 74.5 and 41.9% of WPIC, WPIUH, WPIH and WPIHCa powder particles had diameters of ≤30 µm. WPIHCa and WPIH powders were cohesive, while WPIC and WPIUH powders were easy flowing. Marked differences in microstructure were observed between WPIH and WPIHCa. There were no measured differences in wall friction, bulk density or colour.

A study of particle adhesion for cohesive powders using a novel mechanical surface energy tester

Powder cohesiveness has a strong correlation with particle adhesion, which is studied using a novel mechanical surface energy tester and a method to measure Bond numbers of the powders. The mechanical surface energy tester measures particle adhesion by detaching particles adhered to a substrate surface. When the substrate is dropped from a set of heights and stopped against a stopper, the particles are subject to a detached force, which in principle is equivalent to the particle adhesion force between the detached particles and the substrate. The detached particles are collected for further particle size analysis. The Bond number of the powders is calculated as a ratio of adhesion to gravity with the particle physical properties measured such as solid density and full-size distribution.In this study, particle adhesion forces for a wide range of sample powders were selected and investigated with powder tabletted substrates (same as the test powders), including Calcium Carbonate, Lactose, Microcrystalline Cellulose, Paracetamol, Ibuprofen and Titanium Dioxide for a wide range of material properties. Influences of substrate materials on the measurements are studied between the powder tabletted substrates and other standardised materials such as mild steel, glass, stainless steel and TIVAR. The study shows that the substrate material has little influence on the measurements of particle adhesion within a maximum variation of about 2.5%. This allows using different substrates for the measurement of Bond numbers. The adhesion forces measured are also compared to those calculated by other established methods, and some correlations have been found.

The cohesive behavior of granular solids at high temperature in solar energy storage

Solar technology has shown a keen interest in thermochemical storage to extend its operational timespan beyond a daily basis. Thermochemical solutions are operated by fine powders. Unlike regular granular media, fine powders become cohesive. However, despite the paramount importance of controlling powder flowability to integrate these solutions in solar technology, little research has focused on how powder cohesiveness shapes the flow regime through the storage circuit. This paper investigates two critical factors governing the granular flow through the storage route: temperature and consolidation. The experimental setup emulates the transition powders undergo from silos to reactors. Powders were consolidated before being fluidized at a given temperature, ranging from ambient to 500 °C. Tensile yield strength was then measured for different powders. The results exhibit the expected rising trend in the tensile yield strength of powders as temperature increases. But, interestingly, tensile yield strength skyrocketed when powders were subjected to higher consolidations. Backed by theoretical estimations based on mechanical models, the analysis unveils a cross effect between temperature and consolidation. A combined action that reinforces cohesion by promoting a plastic deformation; worsening, thus, powder flowability. In conclusion, consolidation introduces a multiplying effect on the powder cohesiveness as temperature increases, which represents a serious caveat to solar energy storage technology. To mitigate potential flowability issues, this work explores the use of nanoparticles of silica to layer limestone (calcium carbonate, CaCO3) particles. The nanosilica coatings turned out to be a very promising solution to preclude the enhancement of cohesion induced at high temperatures. Coated samples showed powder cohesiveness at high temperatures similar to the values obtained at room temperature. A solution that offers a simple and reliable alternative to smooth the flow regime in solid-based energy storage technologies at production environments.

Assessing powder flowability at low stresses using ball indentation method: Evaluation of constraint factor

Powders can exhibit different flow behaviour resulting from a combination of physical properties of the material and equipment design. Problems with powder flow are ubiquitous in process industry and become prominent when dealing with fine and cohesive powders. It is therefore of great importance to characterise the flowability of cohesive materials for better process control. Powder flowability is commonly assessed under relatively high preconsolidation loads using shear cell and uniaxial compression methods by which the unconfined yield strength (Y) is evaluated as a function of the applied load. However, these techniques are typically limited to applied stresses greater than 1 kPa and require a relatively large quantity of powder. To overcome these limitations, the recently developed Ball Indentation Method (BIM) is used in this work for assessing powder flow behaviour at low stress levels. The unconfined yield strength (Y) is inferred from the resistance to ball penetration into the surface of a powder bed, based on the method for measurement of hardness (H). This requires the flow resistance, represented by hardness, to be related to the unconfined yield strength by a proportionality factor termed the constraint factor, C, following the analogy with yield stress measurement in continuum solids, i.e. Y=H/C. The constraint factor for silanised glass ballotini, calcium carbonate, α-lactose monohydrate, Avicel and limestone is evaluated and reported here. It is shown that the unconfined yield strength inferred by this method correlates well with those from the uniaxial compression and shear cell measurement. The characterisation of the constraint factor makes it possible to use BIM for powder flowability testing at low stress levels and using a very small powder quantity. This is highly desirable for applications such as capsule filling, tableting and dry powder inhaler devices.

DEM modeling of fine powder convection in a continuous vibrating bed reactor

Continuous vibrating spatial particle ALD reactors were developed to achieve high powder throughput while minimizing reactor footprint. Unlike fluidized bed reactors, continuous vibrating spatial particle ALD reactors operate below fluidization, using linear vibration to convey particles through alternating regions of precursor gas. Fine powder convection in these vibrating bed reactors is still not well understood, so cohesive discrete-element-method (DEM) simulations were performed to investigate the solids flow behavior. Using a Fast Fourier Transform (FFT) algorithm, we constructed a sum-of-sines model for the reactor kinematics based on accelerometer data. Accelerometer results and DEM simulations revealed the role of high-frequency excitations and need for backsliding and sticking avoidance in horizontal conveyors at low-g accelerations. From these observations, we propose a novel sawtooth excitation to enable convection of cohesive fine powders at low flow velocities. The model results were compared to data from an in-house continuous vibrating spatial particle ALD reactor.

Fluidization of fine lactose for dry powder inhalation: A comparison of assisting methods

AbstractFluidization of cohesive pharmaceutical powders is difficult to achieve and typically requires the introduction of external forces. This study investigates the fluidization of the fine inhalation grade of lactose powders (size range from 0.1‐20 μm) that are specifically developed for dry powder inhalation (DPI) applications. The fluidization behaviour of fine lactose powders was evaluated under six conditions: without fluidization aids, with only vertical vibration (VFA), with only a downward‐pointing micro‐jet (MFA), with both vibration and pre‐mixing with coarse particles (VCFA), with both vibration and micro‐jet (VMFA), and with the combined assistance of vibration, micro‐jet, and addition of coarse particles (VMCFA). The enhancement of fluidization due to the use of different assistance methods is reflected by the increase of bed expansion and the decrease in both the minimum fluidization velocity and agglomerate formation. However, applying micro‐jet results in considerable powder losses due to the high fraction of fine particles stuck to the wall. Combining any two assisting methods leads to better fluidization than using a single approach. In particular, the combination of vibration and micro‐jet shows the best performance in improving fluidization. Further addition of coarse particles does not play a significant influence on promoting fluidization. Finally, the analysis of the forces acting on the lactose agglomerates shows the enhancement of separation forces by introducing the fluidization assistance, which leads to a decrease in agglomerate size.

Particle breakability assessment using an Aero S disperser

Milling is widely used in various industries to tailor the particle size distribution for desired attributes. The ability to predict milling behaviour by testing the breakability of a small quantity of material is of great interest. In this paper, a widely available aerodynamic dispersion method, i.e. the Aero S disperser of Malvern Mastersizer 3000 has been evaluated for this purpose. This device is commonly used for dispersion of fine and cohesive powders, as the particles are accelerated and impacted at a bend, but here its use for assessing particle breakability is explored. The fluid flow field is modelled using one-way coupled Computational Fluid Dynamics approach, as the particle concentration is low, following which the particle impact velocity is calculated by Lagrangian tracking and used in the analysis of particle breakage. Experimental work on the breakability is carried out using aspirin, paracetamol, sucrose and α–lactose monohydrate particles. The relative shift in the specific surface area is determined and together with the calculated particle impact velocity and physical properties, they are used to calculate the breakability index. A good agreement is obtained with the single particle impact testing and aerodynamic dispersion by Scirocco disperser, indicating the breakability could also be inferred from this method.

The influence of material and process parameters on powder spreading in additive manufacturing

Additive manufacturing (AM) or 3D printing is beginning to mature from a rapid prototyping to an industrial production technology. However, there are still a lot of fundamental questions that must be addressed in order to make this leap forward. There are many different AM technologies; here, we focus on laser powder bed fusion (LPBF).A key step in LPBF is the initial spreading of the powder layer before it is melted in a solid object, via interaction with a laser. Ideally the powder should be spread as a dense, uniform layer. However, developing a spreading process that can produce a consistent layer, across the wide range of powders used, is a challenge for LPBF manufactures. Therefore, we investigate the influence of materials and process parameters on layer quality.To perform this study we perform computing simulations using the discrete particle method (DPM), a.k.a. discrete element method. This allows us to define metrics to evaluate the powder layer quality, allowing direct comparisons of different tools and parameters. We emulated the effect of the complex particle shape and surface roughness via rolling resistance and interparticle sliding friction. Additionally we investigated the effect of particle cohesion and type of spreading tool.We found that all these factors have a major, albeit sometimes surprising influence on the powder layer quality. In particular, more irregular shaped particles, rougher particle surfaces and/or higher interfacial cohesion usually, but not always, lead to worse spreadability. In general, there is a trade-off between material and process parameters. For example, increasing the spreading speed decreases layer quality for non- and weakly cohesive powders, but improves it for strongly cohesive ones. On the other hand, using a counter-clockwise rotating roller as a spreading tool improves the powder layer quality compared to spreading with a blade. For both tools, a unique correlation between the quality criteria uniformity and mass fraction is reported allowing an easily measured experimental value to be related to the layer quality. Finally, we showed that size-segregation occurs during spreading and this effects is able to explain some of our results.

Local dynamics of pharmaceutical powder fluidization using high speed long distance microscopy and particle image velocimetry

The local dynamics of fluidized pharmaceutical carrier powders in a turbulent channel flow was studied using particle image velocimetry (PIV) and High-speed, long-distance microscopy (HS-LDM). Four different lactose powders which have been used as a drug carrier in dry powder inhalers were used in this study. These powders have median powder particle diameters ranging between 61 and 121 µm. Air flow velocities ranging between 13.3 m/s and 66.7 m/s were examined. In addition, the effect of grid blockage ratio (ranging from ~25% to ~40% of the area of channel cross-section) was also investigated. Results show that the high-speed, long-distance microscopy (HS-LDM) technique was able to capture the mean velocity of the particles, and the results corresponded well with the PIV measurements. Results from the high-speed, long-distance microscopy (HS-LDM) method also demonstrate that the span of particle velocity closely follows that of the particle size distribution both for cohesive and non-cohesive powders. This study contributes towards an improved understanding of pharmaceutical carrier dynamics in turbulent channel flows and demonstrates how advanced image processing can be used to capture local particle dynamics.

Sound-Assisted Fluidization for Temperature Swing Adsorption and Calcium Looping: A Review.

Fine/ultra-fine cohesive powders find application in different industrial and chemical sectors. For example, they are considered in the framework of the Carbon Capture and Storage (CCS), for the reduction of the carbon dioxide emissions to the atmosphere, and in the framework of the thermochemical energy storage (TCES) in concentrated solar power (CSP) plants. Therefore, developing of technologies able to handle/process big amounts of these materials is of great importance. In this context, the sound-assisted fluidized bed reactor (SAFB) designed and set-up in Naples represents a useful device to study the behavior of cohesive powders also in the framework of low and high temperature chemical processes, such as CO2 adsorption and Ca-looping. The present manuscript reviews the main results obtained so far using the SAFB. More specifically, the role played by the acoustic perturbation and its effect on the fluid dynamics of the system and on the performances/outcomes of the specific chemical processes are pointed out.

The mixing of cohesive and flowable powder materials using a common laboratory powder mixer

This study presented the homogeneity obtained when mixing cohesive and flowable powder materials using a laboratory powder mixer. The mixing process parameters studied were the mixing time and the mixer rotational speed (20 rpm, 40 rpm and 60 rpm) at the different ratios (95%: 5%, 50%: 50% and 5%: 95%) of the cohesive cocoa and flowable mannitol powder materials. The homogeneity sampled at the powder bed surface showed that only at the highest rotational speed of 60 rpm used in this work yield acceptable homogeneity at the two extremes of the powder mass ratios; 95%: 5% and 5%: 95% of mannitol: cocoa for some of the locations on the powder bed surface, especially near the wall of the mixer. Other combinations of the experimental conditions did not yield acceptable mixture homogeneity. These results showed the difficulties in obtaining a homogeneous powder mix when mixing cohesive powder materials, especially in academic teaching and research laboratories using a simple powder mixer apparatus.

Study on dark-field imaging with a laboratory x-ray source: Random stress variation analysis based on x-ray grating interferometry.

The dark-field image (DFI) in a grating interferometer involves the small-angle scattering properties of a material. The microstructure of the material can be characterized by an analysis of the auto-correlation length and the DFI. The feasibility of a DFI in a laboratory x-ray source with grating interferometry has been reported, but a follow-up study is needed. In this study, the random stress distribution was measured in the laboratory environment as an applied study. SiO2 mono-spheres as a cohesive powder with a 0.5 µm particle size were used as the sample. The microstructural changes according to the stresses on the particles were observed by acquiring a DFI along the auto-correlation length. In x-rays, a random two-phase media model was first used to analyze the characteristics of cohesive powder. This study showed that the microstructure of materials and x-ray images could be analyzed in a laboratory environment.

A modular testbed for mechanized spreading of powder layers for additive manufacturing.

Powder bed additive manufacturing (AM) processes, including binder jetting (BJAM) and powder bed fusion (PBF), can manufacture complex three-dimensional components from a variety of materials. A fundamental understanding of the spreading of thin powder layers is essential to develop robust process parameters for powder bed AM and to assess the influence of powder feedstock characteristics on the subsequent process outcomes. Toward meeting these needs, this work presents the design, fabrication, and qualification of a testbed for modular, mechanized, multi-layer powder spreading. The testbed is designed to replicate the operating conditions of commercial AM equipment, yet features full control over motion parameters including the translation and rotation of a roller spreading tool and precision motion of a feed piston and the build platform. The powder spreading mechanism is interchangeable and therefore can be customized, including the capability for dispensing of fine, cohesive powders using a vibrating hopper. Validation of the resolution and accuracy of the machine and its subsystems, as well as the spreading of exemplary layers from a range of powder sizes typical of BJAM and PBF processes, are described. The precision engineered testbed can therefore enable the optimization of powder spreading parameters for AM and correlation to build process parameters in future work, as well as exploration of spreading of specialized powders for AM and other techniques.

Physicochemical and Morphological Properties of European Cranberrybush Powder Manufactured by Freeze Drying

ABSTRACT This study investigated the rehydration, water activity, pH, Brix, color, bulk density, tapped density, flowability, cohesiveness, wettability, solubility values, and microstructures of freeze-dried European cranberrybush powders. Additionally, experimental freeze-drying data were coupled to nine thin-layer models, and the logarithmic model was chosen to estimate the moisture ratio of European cranberrybush. The pH and Brix values were measured between 3.0–3.2 and 40.33–52.00 for different powder sizes, respectively. A significant difference was observed for the water activity values of different powders that were classified into different powder sizes (P < .05). Moreover, it was identified that all powders exhibited “low” cohesiveness and “fair” flowability. However, the results showed that the highest bulk density (0.440 g/ml), tapped density (0.563 g/ml), wettability (4386 s) and solubility (264 s) were obtained in the samples with a “>500 µm” powder size. According to the all scanning electron microscopy images, skeletal-like structures formed on the surfaces.

Effect of strain rate on powder flow behaviour using ball indentation method

There are a number of techniques available for characterising powder flow behaviour, most of which are carried out under quasi-static conditions. Quasi-static methods are commonly used to define the conditions for the flow initiation, but they are inadequate for characterising the shear strain rate sensitivity of powder flow. The number of devices available to capture powder behaviour under dynamic conditions is indeed very limited. The very few commercially available instruments all require a large sample, which is not desirable for early stages of product development. We have recently proposed the use of ball indentation for characterising flow resistance of cohesive powders. The technique can be applied at very low consolidation stress levels (much less than 1 kPa) and requires only a small sample quantity, typically a few mm3. Previous work showed a good correlation of flowability with established methods under quasi-static conditions. Therefore, an attempt is made to extend the method to dynamic conditions and investigate the sensitivity of the stresses on the shear strain rate for a range of powders, including glass ballotini, α-lactose monohydrate, calcium carbonate (Durcal) and limestone. The results show the existence of a threshold boundary above which the flow resistance experienced by the penetrating ball becomes dependent on the shear strain rate. This is in line with the trends reported in the literature. Therefore, the ball indentation technique has the potential to be used to assess powder flowability at high strain rates.