Evolution of particle properties during spray drying in relation with stickiness and agglomeration control

Spray drying consists in atomizing a solution into liquid drops in a hot air flow to get dry solid particles after solvent evaporation. The convective drying at the drop surface leads to a very fast evolution of temperature and water content due to initial high differences of temperature and water vapour pressure between the drop surface and the drying air. During drying, the drop surface viscosity is increasing due to potentially amorphous polymers reaching a rubbery state. The drop surface is becoming sticky with consequences on wall deposit. This sticky behaviour which appears in the range of 10 to 30 °C above the glass transition temperature Tg, may be utilized in a positive way for agglomeration of drying particles with dry powders, either recycled fines or new dry powder, to improve instant properties.The evolution of water content of drops along drying, is deduced from measurements of air temperature and relative humidity, at different places in the dryer and used to predict the drying and sticky behaviour of two maltodextrin solutions (DE12 and DE21) with different Tg. The studied parameters in a co-current spray dryer were the inlet air temperature (144, 174, 200 °C) and flow rate (80 and 110 kg h^− 1), the liquid flow rate (1.8, 3.6 and 5.4 kg h^− 1) and the rotation speed of the wheel atomizer. The results on particle water content combined with the evolution of Tg showed that particles are sticky close to the atomiser for the two maltodextrins, and also along the chamber for maltodextrin DE21 due to its lower Tg. The introduction of dry particles at different places in the chamber allowed validating the method to control agglomeration.