Drying kinetics study of irregular fibril materials in a 'differential' laboratory rotary dryer: Case study for cut tobacco

摘要

Rotary dryers are commonly used in the modern large-scale tobacco drying industry that consumes huge amounts of energy. In fact, rotary dryers are commonly used in chemical industry in general. It is difficult to investigate the drying behavior at industrial scale. A "differential" laboratory rotary dryer was therefore designed and tested. The large diameter of the industrial dryer was preserved, but the width was a section of the industrial dryer. The drying characteristics of cut tobacco from top leaves and bottom leaves with initial moisture contents (22.5 +/- 1.0% on the wet basis) were studied in the "differential" dryer at air temperatures of 65, 85, 105, 125, and 145 degrees C, respectively. The results show that increasing drying temperature accelerated the drying process, whereas the surface temperatures of the cut tobacco samples stayed in the temperature range of 48-71 degrees C when their moisture contents were reduced to 12.0 +/- 1.0% (wb). This 12.0% (wb) was required by commercial operations. Uniquely, the drying kinetics was captured using the reaction engineering approach (REA). Although different settings were applied, the model can be used to describe all the data well. The unique relationship between the normalized activation energy and the moisture content is approximated which is independent of the drying air temperature and the tobacco origin. The different drying behaviors for the cut tobacco from top leaves and bottom leaves can be attributed to their different equilibrium isotherms. Through controlling the drying time as predicted by REA model, the outlet moisture contents of cut tobacco from top leaves dried at 95 degrees C/RH0.034/320s and 115 degrees C/RH0.017/250s were shown to be 12.3 and 11.8% (wb), with the relative deviations of 2.5 and 1.7%, respectively, and these were within the industrial permissible range.