The role of wall deposition and re‐entrainment in swirl spray dryers

Francia, Victor; Martín, Luis; Bayly, Andrew E.; Simmons, Mark

doi:10.1002/aic.14767

Cited by 29 publications

(44 citation statements)

References 41 publications

(134 reference statements)

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“…a single cycle in steady state) where the walls have been previously cleaned. They correspond to the fouling study available in Francia et al (2015b).…”

Section: Particulate Wall Deposits and Roughness Heightmentioning

confidence: 99%

Influence of wall friction on flow regimes and scale-up of counter-current swirl spray dryers

Francia

Martín

Bayly

et al. 2015

Chemical Engineering Science

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We study the vortex flow in swirl dryers across full scale designs and conditions. Three air flow regimes are related to the effects of friction and design features. The characteristics of deposits are vital to explain the evolution of the vortex. Friction reduces the swirl drastically and it has a major impact on the turbulence. Flow regime prediction facilitates the use of new control and scale-up criteria. a b s t r a c tThe structure of the vortex flow in swirl spray dryers is investigated after having fouled the walls with deposits typical of detergent manufacture. The range of Re and swirl intensity Ω characteristic of industry are studied using three counter-current units of varying scale and design. The friction with the deposits increases the flow turbulence kinetic energy and causes a drastic attenuation of the swirl and as a result, the vortex breaks down in the chamber forming recirculation regions (i.e. areas of reverse flow). Three flow regimes (1) no recirculation, (2) central and (3) annular recirculation have been identified depending on the swirl intensity. New control and scale up strategies are proposed for swirl dryers based in predicting the decay and the flow regime using the unit geometry (i.e. initial swirl intensity Ω i ) and experimental decay rates function of the coverage and thickness of deposits. The impact in design and numerical modelling must be stressed. Adequate prediction of the swirl is vital to study fouling and recirculation, which surely play an important part in the dispersion and aggregation of the solid phase. Current models have no means to replicate these phenomena, and yet, in this case neglecting the deposits and assuming smooth walls would result in (a) over-prediction of swirl velocity up to 40 À 186% (b) under-prediction of turbulent kinetic energy up to 67 À85% and (c) failure to recognise recirculation areas.

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“…a single cycle in steady state) where the walls have been previously cleaned. They correspond to the fouling study available in Francia et al (2015b).…”

Section: Particulate Wall Deposits and Roughness Heightmentioning

confidence: 99%

Influence of wall friction on flow regimes and scale-up of counter-current swirl spray dryers

Francia

Martín

Bayly

et al. 2015

Chemical Engineering Science

Self Cite

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“…It is assumed that no deposition of material occurs upon the collision of droplets/particles with the wall. In reality, the droplets and wet particles are deposited on the wall and may be re-entrained back into the gas flow; these phenomena though extremely complex [55] need to be considered for a realistic simulation of the spray drying process.…”

Section: Discussionmentioning

confidence: 99%

“…In a recent experimental study [55] in the spray drying tower at the P&G Technical Centre, it has been observed that around 20% of the dried powder originated from the deposited material on the wall and most large agglomerates in the product are a direct consequence of wall deposition.…”

Section: Modelling Of Particle-wall Interactionmentioning

confidence: 99%

“…The collision of wet particles with the wall commonly results in deposition on the wall and the deposited particles undergo drying as well as agglomeration before being re-entrained by the hot gas [55] . The period of retention on the wall, size distribution and moisture content of the reentrained particles is difficult to predict as it depends on many factors including impacting particle moisture, size, existing deposit layer moisture, near-wall velocity and material properties.…”

Section: Modelling Of Particle-wall Interactionmentioning

confidence: 99%

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CFD modeling of a pilot-scale countercurrent spray drying tower for the manufacture of detergent powder

et al. 2016

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A steady-state, three-dimensional, multiphase CFD modelling of a pilot-plant counter-current spray drying tower is carried out to study the drying behavior of detergent slurry droplets. The software package ANSYS Fluent is employed to solve the heat, mass and momentum transfer between the hot gas and the polydispersed droplets/particles using the Eulerian-Lagrangian approach. The continuous phase turbulence is modelled using the differential Reynolds stress model. The drying kinetics is modelled using a single droplet drying model [1] which is incorporated into the CFD code using user-defined functions. Heat loss from the insulated tower wall to the surrounding is modelled by considering thermal resistances due to deposits on the inside surface, wall, insulation and outside convective film. For the particle-wall interaction, the restitution coefficient is specified as a constant value as well as a function of particle moisture content. It is found that the variation in the value of restitution coefficient with moisture causes significant changes in the velocity, temperature and moisture profiles of the gas as well as the particles. Overall, a reasonably good agreement is obtained between the measured and predicted powder temperature, moisture content and gas temperature at the bottom and top outlets of the tower; considering the complexity of the spray drying process, simplifying assumptions made in both the CFD and droplet drying models and the errors associated with the measurements.2

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“…A recent experimental study in a pilot-scale counter-current drying tower by Francia et al (2015b) on the deposition and re-entrainment of particles on and from the wall revealed that these two phenomena drastically increase the residence times. These can lead to thermal degradation and excessive agglomeration and thus the product quality is degraded.…”

Section: Introductionmentioning

confidence: 99%

Residence time distribution of glass ballotini in isothermal swirling flows in a counter-current spray drying tower

et al. 2017

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Abstract:The particle residence time in counter-current spray drying towers has a significant influence on the moisture content of the powder exiting the tower. Therefore, the reliability of predictions of residence time by numerical methods is highly desirable. A combined experimental and computational fluid dynamics investigation is reported for the prediction of the residence time distributions of glass beads with a narrow size range of 300-425 m in a counter-current tower with isothermal swirling flows of air. The particle-wall collision is taken into account using a rough-wall collision model. Overall, a reasonably good agreement is obtained between the measurements and predictions. Consideration of wall roughness results in greater axial dispersion of particles in the tower compared to a smooth wall assumption. The rough particle-wall collision is important for a reliable prediction of residence time distributions. In addition, analysis of the results infers that the clustering effect of particles on drag and particle-particle interactions are important and should be investigated in a future study.

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The role of wall deposition and re‐entrainment in swirl spray dryers

Cited by 29 publications

References 41 publications

Influence of wall friction on flow regimes and scale-up of counter-current swirl spray dryers

Influence of wall friction on flow regimes and scale-up of counter-current swirl spray dryers

CFD modeling of a pilot-scale countercurrent spray drying tower for the manufacture of detergent powder

Residence time distribution of glass ballotini in isothermal swirling flows in a counter-current spray drying tower

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