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.