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

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

Size Distribution of Raindrops

For more than a century scientists have systematically studied the diameters of raindrops and discussed which type of drop-size distribution function is best able to describe natural rain. Currently, the size distributions of raindrops are fitted to the observations using different approximations, including the gamma, Weibull, lognormal, Marshall-Palmer's exponential equation, and other mathematical functions. Here we show from first principles of matter and energy conservation, without presuming a-priori any type of drop size dispersion process as in the previous research, that in the stationary state both the mass and number fractions of raindrop diameters follow a lognormal distribution law. From our theory, which accounts for energy fluctuations of drops caused by random disturbances from both the raindrops and the surrounding atmospheric air flow, we derive analytical equations for the mass and count median diameters and size distribution of raindrops. Furthermore, we develop a map for the graphical determination of the range of possible diameters and expected median diameters of raindrops at different rain intensities. Our results are in good agreement with experimental data published by different research groups at various ground locations around the globe for convective and stratiform rains for the rain intensities 0.4-40 mm/h and insignificant evaporation. We anticipate that, besides rain, our theoretical approach can find applications in adjacent areas such as in liquid atomization of sprays and aerosols, production of bubbles and particles, prediction of drop-size distributions of non-water rainfalls on other planets and in assessing Earth conditions in ancient times that might help to explain the "Faint Young Sun" paradox.Comment: 20 pages, 4 figures and Supplementary Informatio