Experimental analysis of the transient axial bulk bed transport in rotary kilns and the influence of the operational parameters

Rotary kilns are widely used in chemical and metallurgical industries, where the transient transport of granular solids plays an important role in process stability, heat and mass transfer, and kiln design. Although transient operating conditions frequently occur in industrial practice, quantitative experimental studies at technical scale remain limited. This paper presents an experimental and theoretical investigation of the transient behavior of a technical-scale rotary kiln (5 m in length and 0.4 m in diameter). A novel experimental method was developed to continuously monitor the transient process using digital imaging and video analysis. Quartz sand with a mean particle diameter of 0.2 mm was used as the granular material. The rotational speed and inclination angle of the kiln were systematically varied, and the discharge mass flow rate, bed depth profiles, and stabilization time were measured. The dynamic model applied in this study was based on a previously developed mechanistic framework originally validated for small laboratory-scale kilns. Good agreement was obtained between the model predictions and the experimental results at technical scale. The results confirm that the mechanistic description of axial bed transport remains valid under transient conditions and at increased scale. In addition, the combined use of the dynamic model and continuous experimental observations provides further insight into the influence of operating parameters on transient kiln behavior.