Agglomeration in counter-current spray drying towers. Part B: Interaction between multiple spraying levels

A new experimental method is developed here to investigate agglomeration in spray drying towers operating with multiple nozzles. It allows studying independently the contribution of each spray to the product and obtaining a valuable insight into the agglomeration processes. The paper studies a two level swirl counter-current dryer of detergent in a full-scale production system. It shows that operation with two nozzle levels increases the energy efficiency compared to the use of single sprays, but in turn promotes both agglomeration and elutriation of powder from the top of the dryer. The product size distribution becomes bi-modal and the composition and porosity of the product more heterogeneous due to the different thermal histories experienced by droplets from each spray. The method described here controls the air temperature and humidity nearby the nozzles to quantify the agglomerates resulting from particle contacts within each individual spray or from their interaction. Particle agglomeration is shown to be suppressed at the bottom of the dryer where the heat transfer rates are highest and promoted at the top spray, which originates a second coarse mode in the size distribution. Both levels do not operate independently; the powder elutriated upwards from the bottom nozzle is captured entirely by the top spray when it is centrally located. By isolating the independent impact of each nozzle in a dryer, the method provides powerful data to correlate the agglomeration behaviour with local process conditions, and so facilitate the development and validation of spray dryer models.