Breakage of drop nucleated granules in a breakage only high shear mixer

Wet granule breakage is a significant mechanism, particularly in high shear mixer granulation. This paper presents a study of the wet breakage mechanism using a Breakage Only Granulator. Granules with varying powder and liquid binder properties were created using single drop nucleation. These granules were inserted in a Breakage Only Granulator, a high shear mixer granulator with non-granulating cohesive sand as the bulk medium. Two different impellers were used at impeller speeds of 500 and 750 rpm. An 11° beveled edge impeller was used to create both impact and shear in the granulator, and a flat plate impeller was used to minimize impact and maximize shear in the granulator. The fraction of granules which broke during the granulation process was used as a measure of granule breakage within the granulator. These results were compared with Stokes deformation numbers calculated using mean dynamic peak flow stresses measured in unconfined uni-axial compression tests. Results for the beveled edge impeller blade show increasing breakage with increasing Stokes deformation number. Significant breakage was observed at high Stokes deformation number. Increasing impeller speed increased the magnitude of breakage. The Stokes deformations number appears to be a reasonable predictor for granule breakage within the granulator. Results for the flat plate impeller show very little breakage at 500 rpm, and significant breakage for only one formulation at 750 rpm. This suggests that either impact is dominant over shear for breakage within the granulator, or that the two impeller designs give substantially different collision velocities in the granulator. The impeller speed, type and shape have a profound effect on granule breakage in high shear mixer granulators.     

Filtration and loading characteristics of granular bed filters

The purpose of this study was to investigate the filtration and loading characteristics of granular bed filters. Stainless steel holders (diameter 71.6 mm, height 70 mm) were fabricated to accommodate 500 g of zirconium oxide (ZrO₂) beads, as the packed media of granular bed. Monodisperse ZrO₂ granules (0.3, 0.8, 2 and 4 mm in diameter) were used to demonstrate the effect of the granule size and packing geometry on both pressure drop and aerosol penetration. From the filter quality perspective, the selection of the ‘best” filter is complicated. Assuming a low face velocity (e.g., 0.58 cm/s), large granule size is more cost-effective because of the higher filter quality factor. The phenomenon implies that the gain in filtration efficiency due to larger surface area (of small granules in the filter) did not compensate for the increase in air resistance. After the cake formation point, the dust cake on glass fiber filter became compressed. This dust cake compaction caused the pressure to drop precipitously and intermittently. In contrast, the rate of increase in pressure drop of the dust cake formed on the granular bed filters decreased with time probably due to the pinhole channels in the increasing mass load. The size and density of the pinholes are determined by the granule size, the face velocity and the size of the challenge aerosols.     

Influence of liquid binder dispersion on agglomeration in an intensive mixer

In industrial scale mixer granulation, liquid binder is usually sprayed onto the agitated powder bed by means of a nozzle in order to enhance the agglomeration process. The early stage of this process, where granule nuclei are formed and grow, is not well understood. As it is desirable to model the agglomeration state right from the beginning of the process for the purposes of control and modeling, this nucleation step is therefore an important field of interest.To investigate the influence of binder droplet size on the nucleation stage of the agglomeration process, experiments were carried out with lactose and water in an intensive mixer. Water was sprayed in to the mixer with different nozzles to vary the size of the produced droplets. As a comparison, water was also directly poured into the turning mixer. Samples of the produced granules were taken at specific time intervals and analysed for size and water content. As the experiments were focused on examining short granulation times, the first samples were taken after only half of the water was added.Particle size distribution and liquid distribution in the wet granule samples were analyzed. It was found, that the droplet size of the binder liquid has great influence on agglomerate size and binder distribution at short mixing times, with increasing time, the mechanical stresses acting in the mixer becomes more and more dominating in the process. Preliminary comparisons are also carried out with single drop penetration tests in an attempt to correlate drop size to penetration time and also to produced granule size.In conclusion this paper studies the effect of different drop size conditions and subsequent spray flux on the behaviour of the nucleation and the early stages of the agglomeration process. The context of these findings for agglomeration in an intensive mixer is examined.     

Exploring the regime map for high-shear mixer granulation

The aim of this study was to investigate the applicability of the regime map approach proposed by Litster/Iveson and co-workers [S.M. Iveson, J.D. Litster, Growth regime map for liquid-bound granules, AIChE Journal 44 (1998) 1510–1518; S.M. Iveson, P.A.L. Wauters, S. Forrest, J.D. Litster, G.M.H. Meesters, B. Scarlett, Growth regime map for liquid-bound granules: further development and experimental validation, Powder Technology 117 (2001) 83–97] over the whole parameter range, for a given material and agglomeration method. Agglomeration behaviour in a high-shear mixer granulator was investigated and categorised using the evolution of granule size distribution (GSD). MCC 102 (Microcrystalline cellulose, Avicel 102) and aqueous PEG 6k (Polyethylene Glycol 6000) were employed as solid and liquid materials. Different operating conditions were applied by changing impeller speeds and L/S (liquid-to-solid) ratios (weight of liquid/weight of solid). 12 representative settings were selected and typical agglomeration behaviours were identified, forming a regime map for the system. The effect of impeller speed was found to depend on the L/S ratio, very little effect being seen at low L/S ratio (L/S = 85/150), but much more effect at higher binder ratios. In general, the effect of L/S ratio is of paramount importance in these systems and usually determines the growth behaviour.     

Tape casting of Y-TZP with low binder content

The rheology of Y-TZP slurries with a low content of an acrylic/styrene binder is studied. The optimum amount of dispersant was found to be 0.3 wt.% for the starting slurry. After a 5 wt.% of binder addition the dispersant amount needed to achieve a minimum viscosity increases to 0.5 wt.%. Tapes obtained from slurries with a solid content of 45 vol.% shows the higher green density of 51 th.%. Higher solid contents do not yield to better green densities due to the high viscosity.     

Binder liquid distribution during granulation process and its relationship to granule size distribution

The aim of this study is to characterize the impact of binder liquid distribution on granule properties during the wet granulation process. A new parameter, namely the binder liquid transfer coefficient, is used to characterize binder liquid distribution. The relationships between binder liquid distribution coefficient and granule size distribution are discussed. Granules are made of lactose alpha-monohydrate (97.5% w/w, d₅₀ = 31 μm) and polyvinylpyrrolidone (2.5% w/w, d₅₀ = 89 μm) and are manufactured in a Mi-Pro high shear mixer (Pro-C-epT, Belgium). Nigrosine is incorporated as a tracer in the binder liquid in order to detect its distribution in the granules during the process. The results show that the binder liquid is heterogeneously distributed at the beginning of the process whereas it tends to be evenly distributed in the powder during the process. The binder liquid transfer in granule classes obeys a first-order law and the binder liquid transfer coefficient appears to be related to operating conditions: high rotation speed, low liquid flow rate and low liquid viscosity favour the achievement of high liquid transfer coefficient. In addition, the higher the coefficient, the earlier the homogenization and the wider the granule mean diameters. Thus, granule size distribution can be controlled by the binder liquid distribution process. A binder liquid distribution mechanism is proposed, which makes it possible to discuss the influence of the operating parameters on the granule construction process.     

Comparison of spray freeze dried nanozirconia granules using ultrasonication and twin-fluid atomisation

Granulation of nanostructured 3 mol% yttria stabilised zirconia using spray freeze drying was investigated to achieve flowable and crushable granules for subsequent die pressing. Commercial nanosuspension consisting of ∼16 nm particles was concentrated to ∼55 wt% solids content via a patented technique, followed by spraying into liquid nitrogen using either a vibrating ultrasonic probe or a twin-fluid atomizer and freeze dried to yield spherical granules. Control of the granule size fractions was investigated by changing the amplitude and the feeding rate of the nanosuspension during ultrasonication, whilst the flow rate of compressed air used for spraying was varied during twin-fluid atomisation. Granules retaining good crushability for pressing were in a size range of 125–250 μm, which were achieved with ∼60 wt% yields using the atomisation route, whilst a maximum of 35 wt% of granules in this size range were produced in previous research using ultrasonication.     

Pressure–temperature–viscosity relationship for heavy petroleum fractions

This paper deals with the influence that both pressure and temperature exert on the viscosity of heavy petroleum fractions, such as bitumen of different penetration grades, in temperature and pressure ranges comprised between 60 °C and 160 °C and 0–400 bars, respectively. From the viscous flow tests carried out, it is apparent that bitumen behaves as a Newtonian liquid in the above-mentioned range of temperature and pressure. The temperature–pressure–viscosity relationship for bitumen of different penetration grades, mainly used for paving applications, can be modelled using a modified WLF model, the FMT model. This model includes different physical parameters, such as material compressibility and expansivity, which have been obtained from pressure–volume–temperature (PVT) measurements.     

Dispersion of Cu2O particles in aqueous suspensions containing 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt

Effect of 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt's addition upon Cu₂O dispersion properties is studied to clarify dispersant's role in colloidal properties change and its underlying stabilization mechanism, and to quantify processing conditions for the oxide. Dispersing effectiveness was studied through adsorption, rheological and electrophoretic measurements, using as-received and surface charge modified Cu₂O particles. Maximum solid loading attained without dispersant was 73 wt.% (31 vol.%), with corresponding viscosity of 152.5 ± 7.3 Pa s. Addition of dispersant resulted in viscosity between 21.0 Pa s and 5.4 Pa s. No isoelectric point was found for as-received particles’ suspensions nor for dispersed suspensions, with particles presenting negative surface charge in all studied pH range, from pH 4 to 10. Adsorption of the organic molecule caused an absolute downshift of 8–25 mV of the electrophoresis curve. Dispersant/Cu₂O interaction was assessed through FTIR analysis. Attained results suggest that, at the natural suspensions pH, dispersant-modified Cu₂O suspensions are stabilized through inner-sphere complexation mechanism, resulting in high dispersion ability.     

Mapping of regimes for the key processes in wet granulation: Foam vs. spray

The evaluation of foam and spray granulation mechanisms and their performances in achieving uniform liquid distribution in a high‐shear mixer‐granulator is presented. A regime map is presented to describe the granulation mechanisms for the foam and spray systems. Foam and spray granulation are shown to successfully create granules of well‐distributed moisture at the end of wet massing despite there was a deviation from the theoretical moisture content at the end of binder addition. In the wetting and nucleation regime, spray granulation involves drop penetration nucleation outside of the drop‐controlled regime, whereas foam granulation operates favorably in the mechanical dispersion regime. For foam granulation, mechanical dispersion produces more uniform granule‐size distributions below the overwetting limit. Spray granulation exhibits steady granule growth, whereas foam granulation shows induction granule growth followed by rapid granule growth. The regime map provides a basis to customize formulations and compare the different foam and spray granulation mechanisms. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2328–2338, 2013     

Simulation of supercritical water–hydrocarbon mixing in a cylindrical tee at intermediate Reynolds number: Impact of temperature difference between streams

The objective of this work is to study the impact of the temperature difference between the streams on the flow dynamics and mixing of supercritical water (SCW) and a model hydrocarbon (n-decane), under fully miscible conditions, in a small-scale cylindrical tee mixer (pipe ID = 2.4 mm), at an intermediate inlet Reynolds number of 500 using 3-D CFD simulations. When the water and n-decane streams enter the mixer at inlet temperatures of 800 K and 700 K respectively (ΔT = 100 K), the flow remains laminar and the variations of density and viscosity with temperature do not have a significant impact on the flow and mixing dynamics. However, when the water inlet temperature is 1000 K (ΔT = 300 K), the water–HC shear layer becomes unstable close to x = 5D downstream of the mixing joint followed by shear-layer rollup and transition to turbulence. This leads to significant enhancement in the mixing rate. However, in a simulation of SCW n-decane mixing with the same inlet conditions but with the physical properties held fixed at the inlet values (no variation with temperature), the shear layer remains stable and steady state is reached. It was found that, the large variation of temperature of 300 K within the mixing layer leads to an increase in the local fluid density and a decrease in the local fluid viscosity within the mixing layer attributed mainly to the cooling of water and the heating of n-decane respectively. These physical property variations result in an increase in the local Reynolds number within the shear layer rendering it unstable to perturbations in the flow. Thus, the variations in mixture density and viscosity with temperature under near-critical conditions were found to have a significant impact on the flow and mixing dynamics in the tee mixer.     

Rheology and sedimentation velocity of alkaline suspensions of hematite particles at elevated temperature

This study aims to characterize the sedimentation velocity and the rheology of suspensions of hematite particles suspended in strongly alkaline media at 100 and 110 °C, as done for an alternative electrochemical process in development for iron production by direct electrode reduction of hematite. Considering the medium used in the process, i.e. 12% (v/v) suspension of hematite particles in 50% sodium hydroxide aqueous, the sedimentation velocity of hematite particle at 110 °C is 0.010 mm/s, which is very slow because the average size of the solid particles is around 10 μm and the significant collisions and interactions occuring between the particles in the concentrated suspension. Two geometries were used to characterize the rheological behavior of the apparent viscosity of the suspension of 12% (v/v) (i.e. 33 wt%) at 100 °C: a conventional Couette geometry and a helical ribbon mixer. The suspension was found shear thinning in the range of shear rate studied. The rheological behavior of the suspension can be described by a power-law model. The apparent viscosity of the hematite suspension estimated at a shear rate between 0.5 and 10 s⁻¹ is between 100 and 20 mPa s for the two geometries. The apparent viscosity calculated from the terminal velocity of 10 μm particles is of the same order of magnitude of the results obtained with the two rheometer configurations. The effect of the particle concentration on the sedimentation velocity and viscosity of the hematite suspensions was also studied.     

Wet granule breakage in a breakage only high-hear mixer: Effect of formulation properties on breakage behaviour

Wet granule breakage can occur in the granulation process, particularly in granulators with high agitation forces, such as high-shear mixers. In this paper, the granule breakage is studied in a breakage only high-shear mixer. Granule pellets made from different formulations with precisely controlled porosity and binder saturation were placed in a high-shear mixer in which the bulk medium is a non-granulating cohesive sand mixture. After subjecting the pellets to different mixing time in the granulator, the numbers of whole pellets without breakage are counted and taken as a measure of granule breakage. The experimental results showed that binder saturation, binder viscosity and surface tension as well as the primary powder size have significant influence on granule breakage behaviour. It is postulated that granule breakage is closely related to the granule yield strength, which can be calculated from a simple equation which includes both the capillary and viscous force of the liquid bridges in the granule. The Stokes deformation number calculated from the impact velocity and the granule dynamic strength gives a good prediction of whether the granule of certain formulation will break or not. The model is completely based on the physical properties of the formulations such as binder viscosity, surface tension, binder saturation, granule porosity and particle size as well as particle shape.     

Crystallization behavior and sintering of cordierite synthesized by an aqueous sol–gel route

The purpose of the research was to investigate crystallization behavior and sintering of cordierite synthesized by a low-price aqueous sol–gel route starting from silicic acid and magnesium and aluminum salts. Viscous sintering of the gel occurred in the temperature range of 800–850 °C, followed by μ-cordierite crystallization at about 900 °C, which proves the homogeneity of the gel. Decreasing of μ-cordierite crystallinity in a wide temperature range prior to commencing of α-cordierite crystallization at about 1200 °C indicates reconstructive type of μ- → α-cordierite transformation. The transformation was fully completed at 1350 °C. The value of the Avrami parameter indicates that μ-cordierite crystallization was controlled by surface or interface nucleation, which implies that viscous sintering occurred in the primary gel particles, which leads to shrinkage, and thereafter nucleation occurred on the surface or interface of the particles. The overall activation energy of μ-cordierite crystallization was 382.0 kJ/mol. The sinterability of the powder obtained by calcination at 1300 °C, where well-crystallized α-cordierite was formed, was better than that of the powder obtained by calcination at 850 °C, where the most intensive shrinkage occurred before the onset of crystallization of μ-cordierite.     

Processing of silica foam using steam heating and its characterization

Silica foams with 50–86 vol.% porosity have been developed through steam-heating route using slurries containing ovalbumin as binder, as well as sucrose and colloidal silica as additives. On steam-heating, only 1 h is required for drying of as-cast foams, as the cell-walls being restrained from shrinking by intra-cellular gas pressure, and simultaneously strengthened by ovalbumin protein coagulation, show minimum damage. Scanning electron microscopic studies of sintered foams have shown near-spherical pores with size distribution having mean of ≈250–300 μm. These pores appear interconnected through finer pores of ≈15–25 μm size along their walls. Solid loading, binder and sucrose concentrations of slurries for optimum viscosity have been obtained through rheological studies to tailor desirable pore content and size distributions in the sintered foams. Young's moduli and compressive strengths are found to be in the ranges of 14.4–544 MPa and 0.3–8.6 MPa, respectively for sintered foams having ≈55–90 vol.% porosity content.     

Rheological behavior and thermal properties of pitch/poly(vinyl chloride) blends

The effect of adding poly(vinyl chloride) (PVC) and coke filler on the rheological behavior and thermal properties of a coal tar pitch was investigated with a view to developing an appropriate viscoelastic binder for the injection molding of graphite components. Dynamic mechanical analysis revealed that the pitch formed compatible blends with PVC featuring a single glass transition temperature (Tg) intermediate to the two parent Tg’s. Adding PVC to the pitch increased melt viscosity substantially and resulted in strong shear thinning behavior at high PVC addition levels. Adding coke powder as filler increased the melt viscosity even further and enhanced shear thinning trends. Pyrolysis conducted in a nitrogen atmosphere revealed interactions between the PVC and pitch degradation pathways: the blends underwent significant thermal decomposition at lower temperatures but showed enhanced carbon yields at high temperatures. Pyrolytic carbon yield at 1000 °C was further improved by a heat treatment (temperature scanned to 400 °C) in air or oxygen. However, carbon yield decreased with addition of PVC. In addition, the degree of ordering attained following a 1 h heat treatment at 2400 °C also decreased with increasing PVC content.     

Coagulation of bentonite suspension by polyelectrolytes or ferric chloride: Floc breakage and reformation

Coagulation process usually involves different hydrodynamic conditions, in particular when it is followed by a filtration step. In this study, coagulation performance was investigated under a wide range of shear stress. Floc behaviour was followed in-line by laser granulometry to determine size distribution and structure. Synthetic suspension of bentonite in tap water was used as a reference for mineral solids in surface water. Three cationic polymers (polyamine based and polyDADMAC) and ferric chloride were tested using different coagulation reactor geometries. Jar-test indicated coagulation performance under mild hydrodynamic conditions and Taylor–Couette reactors were used to create shear stresses up to 8 Pa. Flocs formed with ferric chloride are not able to grow under middle shear stress like 1.5 Pa. On the contrary, polyelectrolytes lead to large flocs, dense (D_f = 2.6) and resistant to shear stress. A qualitative comparison of floc resistance to shear depending on hydrodynamic conditions and coagulant type is given through the calculation of the strength factor. Fractal dimension measurements indicate a mechanism of particle erosion when flocs are subjected to a higher shear stress in Taylor–Couette reactor. Floc re-growth is also investigated, and breakage appears to be non-reversible regardless of coagulant and conditions experimented.     

A ceramic–carbon hybrid as a high-temperature structural monolith and reinforcing filler and binder for carbon/carbon composites

A new ceramic–carbon nanostructured hybrid (86 vol.% ceramics, 14 vol.% carbon) formed from organoclay during pyrolysis is reported. It functions as a reinforcing filler and a binder for carbon/carbon (C/C) composites. Alone, it can also serve as a high-temperature structural monolith. During pyrolysis, the ordered montmorillonite clay (d₀₀₁ 31.5 Å) is transformed to mullite, cristobalite and disordered clay, allowing the clay part of the organoclay to serve as both binder and reinforcement. The organic part serves as a binder. Thus, a unidirectional C/C composite (50 vol.% fibers, 33 vol.% carbon matrix, 5 vol.% hybrid and 12% porosity) exhibiting flexural strength 290 MPa, modulus 55 GPa and toughness 2.9 MPa is obtained by 1000 °C 21-MPa hot-press pyrolysis in the presence of mesophase pitch powder, which serves as an additional binder, without densification after the pyrolysis. With the hybrid incorporation, the fiber content decreases from 53 to 50 vol.%, but the flexural strength and modulus are increased by 46% and 14% respectively, relative to the composite without the hybrid but with densification. Hot pressing the organoclay alone forms a black monolithic sheet with high thermal stability, electrical resistivity 6 × 10⁶ Ω cm, flexural strength 180 MPa, modulus 69 GPa, but low ductility.     

Welding of yttrium-doped zirconia granules by electric current activated sintering (ECAS): Protrusion formation as a possible intermediate step in the consolidation mechanism

The basic mechanisms of the current activated/assisted sintering (ECAS) of yttrium-doped zirconia's currently is a controversial issue. To gather more experimental data, we performed two-electrode-experiments (2EE) on staked spherical granules of cubic yttria-stabilised zirconia (YSZ) of diameters ranging from 10 to 100 μm. The observations were focused on the initial welding stage, under currents smaller than 4 A cm^?2. The impedance spectroscopy appears to be a sensitive technique to detect the very early welding stage. FEG-SEM analysis reveals protrusion formations at the granule surfaces which are probable intermediate microstructures in the neck formation.