Agglomeration in counter-current spray drying towers. Part A: Particle growth and the effect of nozzle height

Agglomeration of particles and droplets is critical to the operation of spray dryers, however it remains relatively unexplored. This paper studies the effect of the nozzle height on product properties, wall deposits and dryer conditions in a counter-current spray drying tower of detergent with a swirling air flow. The process efficiency is driven by changes in particle agglomeration. To interpret the results and facilitate the study of swirl towers, it is useful to subdivide these units according to the sources of growth in (a) spray region(s), (b) concentrated near-wall region(s) and (c) wall deposits. The particles formed are very heterogeneous and show a size-dependent composition. In this case, particle properties are driven by the separation of solid and liquid phases during atomization and the formation of a heterogeneous set of droplets. Agglomeration serves to homogenise the product and create a distinct source of porosity. The capacity and energy consumption of the dryer are also determined by the evolution of the particle size, as fine powder is elutriated from the tower top and coarse particles are removed from the product. When the nozzle is moved to lower positions in the tower the increased temperature near the spray suppresses agglomeration, however the residence time is shortened and ultimately it leads to creation of wet, coarse granules. An optimum location is found high enough to maintain the drying efficiency but sufficiently far from the top exit to minimise the loss of fine particles. In this way, a capacity ratio (i.e. product vs spray dried powder) C > 90% can be obtained and energy efficiency maximised

Residence Time Distribution of Glass Ballotini in Isothermal Swirling Flows in a Counter-Current Spray Drying Tower

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

Particle aggregation in large counter-current spray drying towers: Nozzle configuration, vortex momentum and temperature

This work investigates particle growth in a counter-current swirl detergent dryer, operating with a single nozzle, at a range of nozzle heights, air drying temperatures, TA, and superficial air velocities, UA, which were selected to enhance or inhibit particle aggregation in the dryer. The growth kinetics are discussed paying special attention to the impact of the cycle of deposition and re-entrainment of material from the wall deposits. All cases lead to substantial aggregation and mono-modal product size distributions. The operation at low UA and high TA, (i.e. low momentum) does not inhibit growth as one would expect from a lower particle concentration and faster heat and mass transfer, conditions which would lead to less particle collisions resulting in growth. In contrast, generation of aggregated particles > 850 μm is promoted, suggesting that a change in the erosion behavior of particles from the wall due to a reduction in energy of particle impacts. As a result of lower stresses, erosion is suppressed and clusters remain at the wall for longer, what allows them to sinter and be re-entrained at larger sizes. In contrast, increasing the momentum of the continuous phase by operation at low TA and high UA inhibits particle growth, particularly in the production of the largest sizes > 850 μm. In this case the rate and energy of impacts to the wall increases, this leads to higher disruptive stresses on the wall deposits, thus, reducing the size of the clusters re-entrained. In summary, this work describes aggregation mechanisms in swirl detergent dryers operated with single nozzles, suggesting that, contrary to expectations, wearing of deposits rather than air-borne contacts may be a key contributor to the enhancement or inhibition of growth

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

The 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 Ωi) 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 40-186% (b) under-prediction of turbulent kinetic energy up to 67-85% and (c) failure to recognise recirculation areas

CFD Simulation of a Counter-current Spray Drying Tower with Stochastic Treatment of Particle-wall Collision

In this study, a steady state, three-dimensional, multiphase CFD modeling of a pilot-plant counter-current spray drying tower is carried out to study the drying of detergent slurry and to predict spray-dried detergent powder characteristics. The coupling between the two phases is achieved using the Eulerian-Lagrangian approach. The continuous phase turbulence is modeled using the Reynolds stress transport model. The droplet drying kinetics is studied using a semi-empirical droplet/particle drying model. Emphasis is given on the modeling of particle-wall interaction by considering only the rebound effect and specifying the coefficient of restitution as a function of impact angle with wall surface roughness taken into account using a stochastic approach, as well as a function of moisture content. This influences the post-wall collision trajectories of particles, residence time distribution and the overall exchange of heat and mass transfer. The model predictions agree well with the measured outlet values of powder average temperature, moisture content and exhaust air temperature considering the complexity of the process and the measurements accuracy

An experimental investigation of the swirling flow in a tall-form counter current spray dryer

This work studies the air flow in a large swirl counter-current dryer using sonic anemometry. Air velocity and turbulence fields are reported at isothermal conditions and in the absence of particles. In a tall-form unit the structure of the flow is largely influenced by the design of the exit. A contraction originates a central jet and suppresses the formation of recirculation zones despite the vortex acquires a high swirl intensity Ω (i.e. 1<Ω<2). Access to a full scale tower has permitted to: (a) identify asymmetries owed to the design of inlet and exhaust ducts, (b) present the first detailed turbulence data in production units, characterized by a highly anisotropic field and the axial decay of the turbulence kinetic energy, (c) study the flow stability, identifying the precession of the vortex core and oscillations at a constant Strouhal number and (d) study the impact that a rough wall has in the strength of the swirl. This work presents the first clear evidence of significant friction in spray dryers. The swirl intensity Ω decays exponentially in the dryer at a rate between 0.08 and 0.09, much higher than expected in pipe flow and independent of Re in the range 105-2.2{dot operator}105. Production dryers have a large characteristic wall roughness due the presence of deposits, which explains the stronger friction and the discrepancies found in the past between data at full scale or clean laboratory or pilot scale units. It is essential to address this phenomenon in current numerical models, which are validated on laboratory or pilot scale facilities and ignore the role of deposits, thus causing an overprediction of the tangential velocity above 30-40%

Use of sonic anemometry for the study of confined swirling flows in large industrial units

This work explores the methodology and errors involved in using a commercial sonic anemometer to study confined industrial swirling air flows, such as those in large cyclones or dryers in the order of hundreds of m³. Common sources of uncertainty in time-of-flight techniques and multiple-path anemometry are evaluated and corrections and methodology guidelines are proposed to deal with issues typical of full scale measurement. In particular, this paper focuses on quantifying the error associated with the disruption of the local flow caused by a HS − 50 horizontal sonic anemometer under a range of turbulence characteristic of industrial swirl towers. Under the guidelines proposed and the conditions studied here, the presence of the instrument originates a measurement error <1 − 4 % in velocity, <1 − 3 ° in direction and < 7 − 31 % in turbulent kinetic energy for an isothermal flow in the absence of solids. These ranges are above traditional uses of sonic anemometry in meteorology due to the limitations inherent to industrial units, but remain within reasonable margins for engineering applications. Laser diagnostic methods are widely used in laboratory and pilot scale cyclones or dryers but are rarely applicable to large production scales. In this context, the data collected with sonic anemometers render much lower resolution but appear in agreement with historical Particle Image Velocimetry. Methods such as the one proposed here can be a useful alternative to improve the level of detail of fluid dynamic studies in industrial units, which are often qualitative or with a limited validation