(521b) Determination of Residence Time Distribution in Rotary Calciner

Determination of Residence Time Distribution in Rotary Calciner

Yijie Gao, Alberto M. Cuitino, Benjamin J. Glasser, Fernando J. Muzzio, Marianthi G. Ierapetritou

Dept. of Chemical and Biochemical Engineering, Rutgers University, Piscataway, NJ 08854

Jean W. Beeckman, Natalie A. Fassbender, William G. Borghard

Process Research Laboratories, ExxonMobil Research & Engineering Co., Annandale, NJ 08801

Continuous calcination is a widely used process in large scale catalyst manufacturing. Many works have been done recently that target on the estimation of material flow in the rotary drum, which is the first critical step on the quality control of the rotary calcination process. However, reliable prediction of material flow is not available yet¹. In this study, we investigated the influence of operation and material properties on the RTD in a rotary calciner. In order to accurately predict the residence time distribution (RTD) of different types of catalyst particles, the Saeman’s model² was used for the prediction of the bed depth distribution and the mean residence time (MRT) whereas the Taylor’s dispersion model^{3, 4}was applied to estimate the MRT and the axial dispersion coefficient through RTD fitting process.

The two important indices for continuous calcination are: (1) the residence time of the particles inside the calciner, and (2) the characteristic time of calcination. Since this unit operation requires that particles stay for a minimum residence time inside the system to guarantee complete calcination, long residence time and limited axial dispersion is preferred. This work results in three very important findings. Firstly, for the investigated catalyst particles, we found that large particle repose angle, small incline angle, and descending rotary speed led to long MRT and limited axial dispersion of particles. As a result, we suggest using slender particles with higher angle of repose in continuous calcination process. Secondly, based on the experimental observations, a reliable method was developed to provide reasonable RTD estimations for different types of catalyst particles, which can be conveniently used in the quality control and performance improvement of practical rotary calcination process. Finally, since the calciner performance is determined by the competition between the residence time and the characteristic time of calcination in the cross-sectional directions, a detailed study on the latter aspect is necessary before a thorough guidance of operation and material selection can be proposed. Therefore, the cross-sectional heat transfer and calcination process in a rotary cylinder is suggested for further investigation⁵.

Reference

1.         Liu XY, Zhang J, Specht E, Shi YC, Herz F. Analytical solution for the axial solid transport in rotary kilns. Chemical Engineering Science. 2009; 64(2): 428-431.

2.         Saeman WC. Passage of solids through rotary kilns: factors affecting time of passage. Chemical Engineering Progress. 1951; 47: 508-514.

3.         Sudah OS, Chester AW, Kowalski JA, Beeckman JW, Muzzio FJ. Quantitative characterization of mixing processes in rotary calciners. Powder Technology. 2002; 126(2): 166-173.

4.         Gao Y, Vanarase A, Muzzio F, Ierapetritou M. Characterizing continuous powder mixing using residence time distribution. Chemical Engineering Science. 2011; 66(3): 417-425.

5.         Chaudhuri B, Muzzio FJ, Tomassone MS. Modeling of heat transfer in granular flow in rotating vessels. Chemical Engineering Science. 2006; 61(19): 6348-6360.