Agglomeration during spray drying: Air-borne clusters or breakage at the walls?

Particle agglomeration, wall deposition and resuspension are inherent to many industries and natural processes, and often inter-connected. This work looks into their relation in a confined particle laden swirling flow. It investigates how the size of detergent powder spray dried in a swirl counter-current tower responds to changes in the air flow. Four sets of sprays are investigated under varying combinations of air temperature and velocity that cause the same evaporation. The use of high air velocities accumulates more of the droplets and dry powder in the chamber swirling faster, but it leads to creation of a finer product. Particle-particle and particle-wall contacts are made more frequent and energetic but in turn the swirl troughs the solids to the wall where deposits constantly form and break. Past PIV and tracer studies revealed that the rates of deposition and resuspension are balanced; the data discussed here indicate that the dynamic nature of the deposits is a major contributor to particle formation. In contrast with the usual assumption, the product size seems driven not by inter-particle contacts in airborne state but the ability of the solids to gain kinetic energy and break up a collection of clusters layering on the wall. As a result, the dryer performance becomes driven by the dynamic of deposition and resuspension. This paper studies the efficiency of limiting operation strategies and shows that a low temperature design concept is better suited to control fouling phenomena and improve capacity and energy consumption

Agglomeration in counter-current spray drying towers. Part B: Interaction between multiple spraying levels

A new experimental method is developed here to investigate agglomeration in spray drying towers operating with multiple nozzles. It allows studying independently the contribution of each spray to the product and obtaining a valuable insight into the agglomeration processes. The paper studies a two level swirl counter-current dryer of detergent in a full-scale production system. It shows that operation with two nozzle levels increases the energy efficiency compared to the use of single sprays, but in turn promotes both agglomeration and elutriation of powder from the top of the dryer. The product size distribution becomes bi-modal and the composition and porosity of the product more heterogeneous due to the different thermal histories experienced by droplets from each spray. The method described here controls the air temperature and humidity nearby the nozzles to quantify the agglomerates resulting from particle contacts within each individual spray or from their interaction. Particle agglomeration is shown to be suppressed at the bottom of the dryer where the heat transfer rates are highest and promoted at the top spray, which originates a second coarse mode in the size distribution. Both levels do not operate independently; the powder elutriated upwards from the bottom nozzle is captured entirely by the top spray when it is centrally located. By isolating the independent impact of each nozzle in a dryer, the method provides powerful data to correlate the agglomeration behaviour with local process conditions, and so facilitate the development and validation of spray dryer models

Effect of surfactant on emulsification in microchannels

Drop formation in a microfluidic flow-focusing device (cross-junction) was studied in absence and presence of one of two ionic surfactants. Four different flow regimes: squeezing, dripping, jetting, and threading were observed in line with existing literature. The effect of surfactant on the transition between flow regimes was shown to depend upon the value of critical micelle concentration and correlates with dynamic surface tension. Drop length in the channel increased as the ratio of flow rate of dispersed to continuous phase, φ increased. For drops smaller than the channel width, the increase was slow, proportional to φ 0.1 , yet was much faster, proportional to φ for larger drops. In contradiction to the expected stabilisation of drops by surfactant, surfactant-laden drops larger than the channel height coalesced inside the channel at a higher rate than surfactant-free drops. It is proposed that the coalescence is caused by the electrostatic attraction due to surfactant redistribution under the high shear stresses near the wall of the channel

On mathematical approaches to modelling slender liquid jets with a curved trajectory

© 2018 Cambridge University Press. Slender liquid jets that have a curved trajectory have been examined in a range of papers using a method introduced in Wallwork et al. (Proc. IUTAM Symp. on Free-Surface Flows, 2000, Kluwer; J. Fluid Mech., vol. 459, 2002, pp. 43-65) and Decent et al. (J. Engng Maths, vol. 42, 2002, pp. 265-282), for jets that emerge from an orifice on the surface of a rotating cylindrical container, successfully comparing computational results to measurements arising from laboratory experiments. Wallwork et al. (2000, 2002) and Decent et al. (2002) based their analyses on the slenderness of the jet, and neglected the torsion of the centreline of the jet, which is valid since in most situations examined the torsion is zero or small. Shikhmurzaev & Sisoev (J. Fluid Mech., vol. 819, 2017, pp. 352-400) used differential geometry and incorporated the torsion. This paper shows that these two methods produce identical results at leading order when the torsion is zero or when the torsion is , in an asymptotic framework based upon the slenderness of the jet, and shows that the method of Wallwork et al. (2000, 2002) and Decent et al. (2002) is accurate for parameters corresponding to scenarios previously examined and also when the torsion is . It is shown that the method of Shikhmurzaev & Sisoev (2017) should be used when the torsion is asymptotically large or when the jet is not slender

The trajectory of slender curved liquid jets for small Rossby number

© The Author(s) 2018. Wallwork et al. (2002, The trajectory and stability of a spiralling liquid jet. Part 1. Inviscid theory. J. Fluid Mech., 459, 43-65) and Decent et al. (2002, Free jets spun from a prilling tower. J. Eng. Math., 42, 265-282) developed an asymptotic method for describing the trajectory and instability of slender curved liquid jets. Decent et al. (2018, On mathematical approaches to modelling slender liquid jets with a curved trajectory. J. FluidMech., 844, 905-916.) showed that this method is accurate for slender curved jets when the torsion of the centreline of the jet is small or O(1), but the asymptotic method may become invalid when the torsion is asymptotically large. This paper examines the torsion for a slender steady curved jet which emerges from an orifice on the outer surface of a rapidly rotating container. The torsion may become asymptotically large, close to the orifice when the Rossby number Rb " 1, which corresponds to especially high rotation rates. This paper examines this asymptotic limit in different scenarios and shows that the torsion may become asymptotically large inside a small inner region close to the orifice where the jet is not slender. Outer region equations which describe the slender jet are determined and the torsion is found not to be asymptotically large in the outer region; these equations can always be used to describe the jet even when the torsion is asymptotically large close to the orifice. It is in this outer region where travelling waves propagate down the jet and cause it to rupture in the unsteady formulation, and so the method developed by Wallwork et al. (2002, The trajectory and stability of a spiralling liquid jet. Part 1. Inviscid theory. J. Fluid Mech., 459, 43-65) and Decent et al. (2002, Free jets spun from a prilling tower. J. Eng. Math., 42, 265-282) can be used to accurately study the jet dynamics even when the torsion is asymptotically large at the orifice

Experimental studies on droplet formation in a flow-focusing microchannel in the presence of surfactants

The formation of an aqueous droplet in an organic continuous phase was studied experimentally inside a flow-focusing microchannel (190 μm × 195 μm: depth × width) in the presence of surfactants. A low viscosity silicone oil (0.0046 Pa s) was used as the continuous phase and a mixture of 48% w/w water and 52% w/w glycerol was the dispersed phase. Two ionic surfactants, C₁₂TAB (50 mM) and C₁₆TAB (5 mM) were added in the aqueous phase, at concentrations above the CMC values. Four regimes of drop formation were identified, namely squeezing, dripping, jetting and threading, whose boundaries changed when the surfactants were present. The drop formation process and the velocity profiles in both phases in the squeezing and dripping regimes were studied in more detail using a two-colour Particle Image Velocimetry technique. For all solutions studied, three distinct drop formation stages were identified, expansion, necking and pinch-off. The surfactant-laden solutions produced smaller drops. Considering the dynamic interfacial tension, rather than the equilibrium one, it was possible to explain differences in the drop formation between the two surfactant systems in the expansion stage. The forces acting on the forming drops were estimated and showed that the drag force overcomes the interfacial tension force at the transition between the expansion and necking stages. During this transition, the curvature of the neck changed while its thinning rate was increased. The transition from the necking to the pinch-off stage was signified by a flow reversal at the bottom part of the drop

A kinetic analysis methodology to elucidate the roles of metal, support and solvent for the hydrogenation of 4-phenyl-2-butanone over Pt/TiO<inf>2</inf>

The rate and, more importantly, selectivity (ketone vs aromatic ring) of the hydrogenation of 4-phenyl-2-butanone over a Pt/TiO₂ catalyst have been shown to vary with solvent. In this study, a fundamental kinetic model for this multi-phase reaction has been developed incorporating statistical analysis methods to strengthen the foundations of mechanistically sound kinetic models. A 2-site model was determined to be most appropriate, describing aromatic hydrogenation (postulated to be over a platinum site) and ketone hydrogenation (postulated to be at the platinum–titania interface). Solvent choice has little impact on the ketone hydrogenation rate constant but strongly impacts aromatic hydrogenation due to solvent-catalyst interaction. Reaction selectivity is also correlated to a fitted product adsorption constant parameter. The kinetic analysis method shown has demonstrated the role of solvents in influencing reactant adsorption and reaction selectivity.We acknowledge EPSRC for funding as part of the CASTech grant (EP/G011397/1) and the Department of Employment and Learning for a studentship (IM). NSB was funded by a PhD scholarship from the University of Birmingham. SKW was supported by an Engineering Doctorate Studentship in Formulation Engineering at the University of Birmingham sponsored by the EPSRC (EP/G036713/1) and Johnson Matthey.This is the final version of the article. It was first available from Elsevier via http://dx.doi.org/10.1016/j.jcat.2015.06.00

Effect of solvent on the hydrogenation of 4-phenyl-2-butanone over Pt based catalysts

In part I of this study, experimental data were presented for the hydrogenation of 4-phenyl-2-butanone with a 4% Pt/TiO2 catalyst where the reaction rate and selectivity (ketone vs. aromatic ring) varied with solvent. In this paper, a rigorous kinetic model is presented utilising these data, incorporating statistical analysis methods to strengthen the foundations of mechanistically sound kinetic models. A fundamental kinetic model for the system is presented and a 2-site model was determined to be most appropriate, describing aromatic hydrogenation (postulated to be over a platinum site) and ketone hydrogenation (at the platinum titania interface). Solvent choice has little impact on the ketone hydrogenation rate constant but strongly impacts aromatic hydrogenation due to solvent-catalyst interaction. Reaction selectivity is also correlated to a fitted product adsorption constant parameter. This kinetic analysis method is the first of its kind demonstrating the role of solvents in influencing reactant adsorption and reaction selectivity.We acknowledge EPSRC for funding as part of the CASTech grant (EP/G011397/1) and the Department of Employment and Learning for a studentship (IM). NSB was funded by a PhD scholarship from the University of Birmingham. SKW was supported by an Engineering Doctorate Studentship in Formulation Engineering at the University of Birmingham sponsored by the EPSRC (EP/G036713/1) and Johnson Matthey.This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.jcat.2015.06.00

Particle tracking for polydisperse sedimenting droplets in phase separation

When a binary fluid demixes under a slow temperature ramp, nucleation, coarsening and sedimentation of droplets lead to an oscillatory evolution of the phase separating system. The advection of the sedimenting droplets is found to be chaotic. The flow is driven by density differences between the two phases. Here, we show how image processing can be combined with particle tracking to resolve droplet size and velocity simultaneously. Droplets are used as tracer particles, and the sedimentation velocity is determined. Taking these effects into account, droplets with radii in the range of 4 -- 40 micrometers are detected and tracked. Based on this data we resolve the oscillations in the droplet size distribution which are coupled to the convective flow.Comment: 13 pages; 16 figures including 3 photographs and 3 false-color plot

Bulk advection and interfacial flows in the binary coalescence of surfactant-laden and surfactant-free drops

This work focuses on the study of bulk flows accompanying the coalescence of two aqueous drops, one containing surfactant and the other surfactant-free, in silicone oils of various viscosities. It is observed that the surfactant-free drop intrudes into the surfactant-laden drop in the form of a penetrating jet whose speed increases and average radius decreases with increasing outer phase viscosity. Mixing patterns within the coalescing drops are due to the force imbalance caused by capillary pressure difference and surfactant-induced Marangoni stresses. The driving force for mixing associated with the difference in interfacial tension between the drops is considerably stronger than that related to the drop size. The long timescale mixing of the drops is driven by rapid interior convection, and the subsequent, slow, diffusive process. Three-dimensional numerical simulations show excellent qualitative and quantitative agreement with the experimental results. The implications of our results to formulation strategies of complex microstructures in practical applications are also discussed