Experimental and Theoretical Study of Agitated Contact Drying of Sewage Sludge under Partial Vacuum Conditions

Abstract

Agitated contact drying experiments were carried out in a cylindrical lab-scale paddle dryer to study the drying kinetics of sewage sludge under partial vacuum conditions. To explore the drying mechanisms, the penetration model for vacuum contact drying of agitated particulate materials, developed by Schlünder and Mollekopf, was applied to simulate the experimental results. The influence of the drying parameters (system pressure, heating temperature, stirrer speed, and dryer load) on the drying kinetics was studied both experimentally and theoretically. The results indicate that pressure and temperature significantly influence the drying rates of sewage sludge, whereas stirring speed and dryer load have no obvious effects on drying rates in the examined range. The experimental and simulated drying rates were in acceptable agreement as long as the temperature difference between dryer and sludge was small. A crust is likely to form on the dryer surface, however, when the temperature difference becomes large; this crust leads to markedly lower drying rates than the calculated ones. Furthermore, a comparison between partial vacuum contact drying and atmospheric contact drying is presented. The results indicate that the mass transfer resistance increases with rising drying potential in the examined experimental range.

Keywords:

• Crust formation
• Drying
• Paddle dryer
• Penetration model
• Sewage sludge

ACKNOWLEDGEMENTS

The authors are grateful to Important Project on Science and Technology of Zhejiang Province of China (2007C03003) for their financial support during this work. The authors are also grateful to Alfons Buekens, Chun wai Lee, and Dr. Zheng Peng for their kind assistance in improving the quality of the English text.

Notes

ND: not detected.