Abstract
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.
Highlights
Spray dryers are used to produce powders from the atomization of liquid feeds in dry hot air, where the volatile phases are removed
Understanding how and where agglomeration occurs is a challenge for many spray drying technologies [1,2] and the main pitfall of theoretical models [3,4,5] versus empirical approaches or the use of neural networks to try reproducing the complex net of relations between process conditions, drying rates [6] and deposition [7]
This paper studies the sources of agglomeration arising near the spray and how they respond to the position of the nozzle region NR in the chamber; Part B [46] studies how they are affected when two NR's are stacked
Summary
Spray dryers are used to produce powders from the atomization of liquid feeds in dry hot air, where the volatile phases are removed. In order to upgrade designs, reduce recirculation and better energy efficiency it is vital to understand where the most relevant contacts occur in a dryer and generate data to correlate the growth patterns to the configuration of sprays To this purpose, two independent investigations have been carried out: (a) Part A, given here, provides a comprehensive examination of detergent production: it compartmentalises a swirl drying tower revisiting the list of particle contacts proposed by Huntington [5] in light of the new insights, and reports a series of experiments that link the position of a spray to the performance of the dryer in terms of capacity, efficiency and product quality, and (b) Part B is reported separately [46] and quantifies the interactions established when multiple spraying levels are used
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