Prediction of Powder Stickiness along Spray Drying Process in Relation to Agglomeration

Abstract

The spray drying process consists of a fast convective drying of liquid droplets by hot air. Initially, the water activity (a_w) of a drop is close to 1. During drying, the drop surface a_w decreases while viscosity increases until reaching a sticky rubbery state before further drying. This can be observed for products such as carbohydrates, leading to particles sticking on walls (product losses) or to adhesion between particles leading to agglomeration. In this study, particle stickiness was investigated in a cocurrent pilot spray dryer by measuring drying air properties (temperature and relative humidity) at different positions. This allowed describing the evolution of temperature and mean water content of the drying drops. Two model products (maltodextrin DE12 and DE21) were spray dried varying process parameters liquid flow rate (1.8, 3.6, and 5.4 kg/h), air temperature (144°, 174°, and 200°C), airflow rate (80–110 kg/h), and rotary atomizer speed (22,500–30,000 rpm). The two products exhibit different drying behaviors in relation to their affinity towards water (sorption isotherms) and glass transition temperature evolution with a_w (stickiness). Depending on drying conditions and product, the drop stickiness was observed very rapidly, close to the atomizer, or later, along the chamber. This approach can be used to identify conditions and positions corresponding to sticky particles.

Keywords:

• agglomeration
• glass transition
• spray drying
• stickiness

This work was financially supported by the EU 6th Framework Programme through the Marie Curie Research Training Network “Biopowders” (MRTN-CT-2004-512247).

Notes

Air temperature, relative humidity (RH), and water content (Y) at inlet and positions A and B; calculated particle water content (X) at positions A and B and final powder properties.

∗Not calculated.

Air temperature, relative humidity (RH), and water content (Y) at inlet and positions A and B; calculated particle water content (X) at positions A and B and final powder properties. T_IN 144°C; atomizer velocity 25,000 rpm.

Inlet air 144°C – 110 kg/h, air wet bulb temperature 40°C.