Drying of Starch Suspension in Spouted Bed with Inert Particles: Physical and Thermal Analysis of Product

Abstract

Drying in a spouted bed with inert particles promotes high heat and mass transfer rates due to the gas–solid contact, which in turn is successfully achieved by particles' cyclic movements. Because of its advantages and versatility, spouted bed drying of suspensions on inert particles is a potential alternative to flash and spray drying and has received attention in research and development in recent years. In this context, this article describes the drying process of cassava starch suspension (35% initial solids content) in a laboratory-scale cone-cylindrical spouted bed dryer with polypropylene particles. The setup consists of a cone-cylindrical spouted bed dryer (internal angle of 60°, 20-cm column diameter, and 5-cm inlet orifice diameter) with top feeding of suspension, which is sprayed by a double-fluid atomizer. The study focused on the fluid dynamics of polypropylene particles at inlet air temperatures of 60, 75, and 90°C and the effects of air temperature on dried starch properties, such as gelatinization temperature, pasting temperature, particle size distribution, crystalline profile, and final moisture content. The pressure drop fluid dynamic curves did not allow a proper analysis and assessment of minimum spout air flow when air was atomized in the bed. Therefore, the minimum spout air flow was determined by the visual observation of the fountain height in this case. Inlet air temperatures did not show a significant difference in the minimum spout air flow, but the bed presented dynamic instability with increasing inlet air temperature. The results showed that the spout fluid dynamic regime during drying was stable with air flow around 1.5 times the minimum spout air flow in the range of inlet air temperature studied. Drying air temperature influenced final moisture content of samples, which was lower than 13% for all conditions. However, physical properties of dried starch were not affected by process conditions.

Keywords:

• Cassava
• Fluid dynamics
• Gelatinization
• Spouted bed dryer
• Starch characterization

ACKNOWLEDGMENT

The authors are indebted to FAPESP for the scholarship and financial support through project nos. 06/06268-1 and 07/5555-1 and to CAPES for a scholarship associated with this work. The authors are also grateful to Dr. Patrícia Cecília Araújo-Farro for useful discussions about starch characterization techniques.

Notes

P_atomization = atomization pressure; W_sus = volumetric flow of suspention; C_sus = concentration of starch suspension; H_{fixed bed} = high of the bed; m_inert = mass of inert material; t_process = process time; T_air = temperature.

Average values within a column followed by same superscript letters are not significantly different (p > 0.05).

Average values within a row followed by same superscript numbers are not significantly different (p > 0.05).

Average values within a column followed by same superscript letters are not significantly different (p > 0.05).

Values are mean of duplicate determinations.