Effect of the liquid layer on the impact behaviour of particles

During a spray granulation process the moisture loading in fluidized beds has a great influence on\ud the inter-particle collision properties and hence on the flow behaviour. To study the influence of the\ud liquid layer as well as granule impact velocity on the impact behaviour free-fall experiments were\ud performed. During these experiments the g-Al2O3 granules were dropped from a predefined height\ud onto a liquid layer on the flat steel wall and the velocity-time curves were obtained using highspeed\ud video recording. The height of the liquid layer was varied from 50 mm to 1 mm. Moreover,\ud the tests were performed at different velocities and viscosities of liquid layer in the range of 1-300\ud mPa∙s. Both distilled water and water solutions of hydroxypropyl methylcellulose with different\ud concentrations (3, 6, 10 mass-%) were used.\ud The obtained restitution coefficients were compared with the experiments performed without liquid\ud film on the surface. For a granule impacted on a liquid film on the wall, the increase of liquid\ud viscosity decreases the restitution coefficient and thickness of liquid layer at which the granule\ud sticks. In the examined velocity range, with decreasing impact velocity the restitution coefficient\ud greatly decreases. To explain the obtained effects the force and energy balances for a particle\ud impacted on a liquid layer on the wall were derived. Both contributions to energy absorption\ud (granule-liquid layer and granule-wall contacts) have been taken into consideratio

Using of spouted bed spray granulation process for fabricating of metal/ceramic-polymer composites

Naturally occurring structural materials are composites with very high filling degrees on hard constituent. In recent years structure and properties of biological materials have been studied in detail. But it has not been possible to reconstruct this structural design. In this contribution we present a process for the fabrication of very highly filled composite materials by using of the spouted bed spray granulation process. Spouted bed granulation offers many advantages for the design of composite materials. In this investigation fine particles (dP =10-50 µm) could be uniformly spouted, and optimal properties for further processing to bulk materials can be obtained by means of granulation. For this a hybrid spouted bed with horizontal gas inlets was designed, which has a small prismatic process chamber with adjustable inlets and a high conical-cylindrical relaxation zone. The thickness of the polymer layer is adjusted by a two-fluid nozzle. The adjustment of the thickness of polymer layer is very important to fabricate composites of adjustable filling degrees. After granulation the particles are assembled to a composite material by means of warm pressing. Additionally for achievement of very high packing densities bimodal particle size distributions are used. For this purpose, particles in nm-range are firstly suspended in polymer solution and the suspension is sprayed in the spouted bed on coarser particles. Granulated particles are assembled to composites and mechanical properties of these were analysed by 4-point bending-tests. We gratefully acknowledge financial support from the German Research Foundation (DFG) via the collaborative research center SFB986. Please click Additional Files below to see the full abstract

Compartmental residence time estimation in batch granulators using a colourimetric image analysis algorithm and Discrete Element Modelling

In this paper we present an experimental technique and a novel colourimetric image analysis algorithm to economically evaluate particle residence times within regions of batch granulators for use in compartmental population balance models. Residence times are extracted using a simple mixing model in conjunction with colourimetric data. The technique is applied to the mixing of wet coloured granules (binary and ternary systems) in a laboratory scale mixer. The resulting particle concentration evolutions were in qualitative agreement with those from the mixing model. It was seen that the algorithm was most stable in the case of the binary colour experiments. Lastly, simulations using the Discrete Element Method (DEM) were also performed to further validate the assumptions made in the analysis of the experimental results. Particle concentrations from the simulations showed the same trends as the experiment and highlighted the importance of particle size distributions on the DEM residence times

Flowsheet simulation of solids processes: Current status and future trends

Complex manufacturing processes are nowadays applied for production of various solid products. It is very common that for production of particles with desired properties several transformation steps like drying, milling, classification, granulation, etc. should be involved. This leads to the process structures consisting of different apparatuses or transformation substeps connected with material and energy balances. Consequently, development of new processes or optimization of already existing, as well as an optimal control, is a very challenging task, which can be partially solved using numerical modelling. For the simulation of modern production processes, the flowsheet calculations can be effectively used. Starting from the 80 s a lot of work focused on the flowsheet simulation of liquid-vapor systems has been done and as result various well-established systems exist today. With respect to the solid processes the intensive research has been started much later. In this contribution we present our view about a current role of flowsheet simulation for modeling of particulate materials and specify the open fields which can be covered in future research

Effect of the liquid layer on the impact behavior of particles

During a spray granulation process the moisture loading in fluidized beds has a great influence on the inter-particle collision properties and hence on the flow behaviour. To study the influence of the liquid layer as well as granule impact velocity on the impact behaviour free-fall experiments were performed. During these experiments the g-Al2O3 granules were dropped from a predefined height onto a liquid layer on the flat steel wall and the velocity-time curves were obtained using highspeed video recording. The height of the liquid layer was varied from 50 mm to 1 mm. Moreover, the tests were performed at different velocities and viscosities of liquid layer in the range of 1-300 mPa·s. Both distilled water and water solutions of hydroxypropyl methylcellulose with different concentrations (3, 6, 10 mass-%) were used. The obtained restitution coefficients were compared with the experiments performed without liquid film on the surface. For a granule impacted on a liquid film on the wall, the increase of liquid viscosity decreases the restitution coefficient and thickness of liquid layer at which the granule sticks. In the examined velocity range, with decreasing impact velocity the restitution coefficient greatly decreases. To explain the obtained effects the force and energy balances for a particle impacted on a liquid layer on the wall were derived. Both contributions to energy absorption (granule-liquid layer and granule-wall contacts) have been taken into consideratio

Gamma distribution function to understand anaerobic digestion kinetics: Kinetic constants are not constant

The Gamma model is a novel approach to characterise the complex degradation dynamics taking place during anaerobic digestion. This three parameters model results from combining the first-order kinetic model and the Gamma distribution function. In contrast to conventional models, where the kinetic constant is considered invariant, the Gamma model allows analysing the variability of the kinetic constant using a probability density function. The kinetic constant of mono-digestion and co-digestion batch tests of different wastes were modelled using the Gamma model and two common first-order models: one-step one-fraction model and one-step two-fraction model. The Gamma distribution function approximates three distinct probability density functions, i.e. exponential, log-normal, and delta Dirac. Specifically, (i) cattle paunch and pig manure approximated a log-normal distribution; (ii) cattle manure and microalgae approximated an exponential distribution, and (iii) primary sludge and cellulose approximated a delta Dirac distribution. The Gamma model was able to characterise two distinct waste activated sludge, one approximated to a log-normal distribution and the other to an exponential distribution. The same cellulose was tested with two different inocula; in both tests, the Gamma distribution function approximated a delta Dirac function but with a different kinetic value. The potential and consistency of Gamma model were also evident when analysing pig manure and microalgae co-digestion batch tests since (i) the mean k of the co-digestion tests were within the values of the mono-digestion tests, and (ii) the profile of the density function transitioned from log-normal to exponential distribution as the percentage of microalgae in the mixture increased

Evaluation of hydroxyapatite crystallization in a batch reactor for the valorization of alkaline phosphate concentrates from wastewater treatment plants using calcium chloride

In this work, phosphorous recovery as hydroxyapatite (Ca5(PO4)3OH(s) = Hap) from alkaline phosphate concentrates (0.25–1 g P–PO43-/L) using calcium chloride (6 g/L) in a batch reactor was evaluated. Ca(II) solutions was continuously fed (0.1–0.3 mL/min) up to reaching a Ca/P ratio of ~1.67 (5/3) to promote Hap formation. Hap powders were characterized by structural form (using X-ray diffraction (XRD), laser light scattering (LS) and Fourier transform infrared spectroscopy (FTIR)); textural form (using Field Emission Scanning Electron Microscopy with Energy Dispersive System (FE-SEM/EDS) and Brunauer–Emmett–Teller (BET)) and thermally (using Thermogravimetric Analysis (TGA)/Differential Thermal Analysis (DTA)). When pH was kept constant in alkaline values (from 8 to 11.5), Hap precipitation efficiency was improved. At pH 11.5, higher phosphorous precipitation rate was registered compared to that obtained for pH 8 and 10, but lower degree of crystallinity was observed in the Hap powders. The increase of the total initial phosphate concentration lead to the formation of Hap powders with higher degree of crystallinity and crystal diameter, but also lower mean particle size. As Ca(II) dosing rate increased Hap precipitation rate was higher, and also the mean size and degree of crystallinity of the prepared particles increasedPostprint (author’s final draft

FeAl and NbAl3 intermetallic- HVOF coatings: structure and properties

Transition metal aluminides in their coating form are currently being explored in terms of resistance to oxidation and mechanical behavior. This interest in transition metal aluminides is mainly due to the fact that their high Al content makes them attractive for high-temperature applications. This is also a reason to study their resistance to wear; they may be suitable for use in applications that produce a lot of wear in aggressive environments, thus replacing established coating materials. In this study, the microstructure, microhardness, and wear and oxidation performance of FeAl and NbAl3 coatings produced by highvelocity oxy-fuel spraying are evaluated with two main aims: (i) to compare these two coating systems¿a commonly studied aluminide (FeAl) and, NbAl3, an aluminide whose deposition by thermal spraying has not been attempted to date¿and (ii) to analyze the relationship between their microstructure, composition and properties, and so clarify their wear and oxidation mechanisms. In the present study, the higher hardness of niobium aluminide coatings did not correlate with a higher wear resistance and, finally, although pesting phenomena (disintegration in oxidizing environments) were already known of in bulk niobium aluminides, here their behavior in the coating form is examined. It was shown that such accelerated oxidation was inevitable with respect to the better resistance of FeAl, but further improvements are foreseen by addition of alloying elements in that alloy

Feasibility of using low pressure cold gas spray for the spraying of thick ceramic hydroxyapatite coatings

This article deals with the production of thick ceramic hydroxyapatite coatings obtained by Low Pressure Cold Gas Spray (LPCGS) system. Several factors such as powder microstructure, surface roughness and cold gas spray system are here discussed in the build‐up process. The use of nanocrystalline powder composed by fine agglomerates and needle‐like shape microstructure allows the realignment and compaction of individual crystallites to form thick deposits. In addition, the activation of the substrate surface results convenient for the first impinging particles anchored properly. Then, layer by layer, particles can remain attached leading to coating build‐up. Additionally, the use of low shock pressure as well as constant feeding system provided by LPCGS system lead to homogeneous coatings in comparison with High Pressure Cold Gas Spray (HPCGS) system. The successful coating build‐up has been achieved not only by the use of an agglomerated feedstock powder, but also by previous surface treatment and the use of the low pressure system. The obtaining of HA components by LPCGS is promising within biomedical field. An improvement of component strength is also suggested by means of thermomechanical analysis of the powder. The performance of a post heat‐treatment leads to an increase in HA strength, as well as crystal size