Residence time distribution: a tool to improve spray-drying control

Dairy powders are mainly obtained by spray drying, which is an effective process as it makes possible long-term storage at an ambient temperature. However, the control and design of this operation is still based on empirical knowledge. Improvement in product quality, which is governed by time/temperature history, thus involves greater understanding of the process via physico-chemical, thermodynamic and kinetic approaches. With regard to the latter, the residence time distribution (RTD) of the product provides valuable information about the product flow pattern in the dryer according to the operating conditions. The aim of this study was to determine the RTD of skim milk in a drying plant with different configurations, according to fine particle recycling (top of the chamber or internal fluid bed) and internal fluid bed thickness (4 to 16 cm). The RTD signal of the atomisation device was established first; then the RTD signals of the different spraydryer configurations were obtained by deconvolution of the experimental curves obtained and the RTD signal of the atomisation device, and modelled according to a combination of four reactor sets. The mean residence time of the product was only slightly modified by the dryer configuration (range 9.5 to 12 min). However, the results showed that a thicker internal fluid bed tends to increase mean residence time due to higher product retention, whereas top recycling of fine particles tends to decrease the mean residence time because of better agglomeration. RTD modelling provides additional information concerning the product flow rate fraction and the residence time distribution of each part of the dryer (chamber, cyclones and fluid bed), indicating that 60 to 80% of the powder passes through the cyclones, depending on the configuration. This study provides greater understanding of dryer operation, and allows further correlation between process parameters and biochemical changes (protein denaturation, Maillard reaction, etc.)