Cycle time in draft tube conical spouted bed dryer for sludge from paper industry

• Conical spouted beds with draft tube are suitable for drying paper sludge wastes.
• Draft tube improves the uniformity of bed moisture content of paper sludge wastes.
• Draft tube narrows the frequency distribution of cycle times of particles.
• The higher the entrainment zone, the smaller the average cycle time of particles.

De-watering of sludge wastes from pulp and paper industry has been studied using a conical spouted bed dryer. Influence of operating parameters such as the presence of a non-porous draft tube, the entrainment zone height, the draft tube diameter to gas inlet diameter ratio and waste moisture content on bed stability have been investigated for drying temperatures ranged from room temperature up to 140 °C. In addition, cycle time distribution of particles was determined for different experimental conditions. The drying time to reach given moisture content of the material decreased with increasing both gas inlet diameter and temperature. Although the use of the draft tube slightly raised drying time, it enhanced the uniformity of bed moisture content of treated wastes. The minimum gas spouting velocity required to reach the stable spouted bed regime was found to increase with both higher stagnant bed height and moisture content. The operation with a draft tube narrowed the frequency distribution of particle cycle times, the more the smaller the height of entrainment zone. Moreover, at the same experimental conditions, cycle times were longer when working with the draft tube. Therefore, the conical spouted bed dryer with a non-porous draft tube has been proven to be an effective pre-treatment for handling of sludge wastes from paper industry.