Powder agglomeration during the spray-drying process: measurements of air properties

Instant food powders are usually produced by spray drying of formulated liquid and powder agglomeration, either into the drying chamber or in an external fluid bed. During spray drying, according to the liquid composition, the viscosity of the initial liquid drops increases more or less rapidly, until the drop surface reaches a rubbery state considered as sticky, before further drying. This sticky property may be used to produce agglomerates inside the chamber by adding dry solid particles, as fines. The present study deals with a methodology to identify regions where the drying drops are sticky in relation to drying conditions (inlet air temperature 144-159-174 ◦C, liquid flow rate 15 to 75 mL·min−1), using a co-current pilot spray dryer with rotary atomizer. Assuming that drying air properties’ (temperature and humidity) evolution is representative of drop drying (drying rate, amount of evaporated water), we realized a map of temperatures and relative humidity for drying air inside the drying chamber. Trials were performed with water (feasibility) and a maltodextrin aqueous solution (40% w/w, DE12) as a model product with sticky properties during drying. The results showed that drying behavior was different for water and aqueous maltodextrin solution. For both, in the tested conditions, the percentage of evaporated water was found to vary from 57 to 89% in the top part of the chamber, close to the atomizer. That represents a possible region where dry fines could be introduced for agglomeration. spray drying / agglomeration / stickiness 摘要 – 喷雾干燥过程中乳粉的凝结:空气性质的测定。在喷雾干燥生产速溶配方食品时, 在干燥室内或在流化床外部容易发生干粉的凝结。在喷雾干燥过程中, 根据液体的组成, 在 进一步干燥之前, 初始液滴的黏度会或多或少的增加, 直到液滴表面达到胶状, 也就是粘稠状 态。利用这种粘稠特性可以通过在干燥室中加入干燥的固体颗粒,也就是纯的固体微粒来生 产凝聚物。本研究以带有旋转式喷雾机的顺流喷雾干燥器 (中试规模) 为研究对象, 确定了 干燥液滴与干燥条件之间相对应的黏度区域 (内部空气的温度在 144-159-174 ◦C, 液体的流 速在 15 ∼ 75 mL·min−1)。假设干燥空气性质 (温度和湿度) 的变化代表了液滴的干燥 (干燥 速率, 水分的蒸发量), 就可以获得在干燥室内部干燥空气的温度和相对湿度的关系图。以水 和糊精水溶液 (40% w/w, DE12) 为模型研究了干燥过程中的黏度特性。试验结果表明, 水和 糊精水溶液的干燥行为不同。在试验条件下, 两种溶液在干燥室顶部, 接近喷雾器部分的水 分蒸发量在 57% ∼ 89%, 这个数据表明可以在这个区域内引入干燥的固体微粒使得被干燥 物质凝结。