Mathematical model and energy utilization evaluation of a coaxial impinging stream drying system for parboiled paddy

Abstract

The impinging stream drying technique has a high capability for removing water in a very short period. Energy consumption is strongly impacted by several of this type of dryer’s operating parameters, i.e., air temperature, air velocity, particle feed rate, and air recycle percentage. A mathematical model would be useful for finding the optimum condition from just a small number of experiments. A mathematical model at a mesoscopic scale that couples heat and mass transfer between gas and solid particles and tracks moisture or temperature distribution inside a single particle was developed in this study; it was used to simulate the effects of air velocity (15–25 m/s), parboiled paddy feed rate (80–250 kgdry solid/h) and exhaust air recycle level (0–80%) on the specific energy consumption (SEC). The developed model was able to predict the average moisture content, average grain temperature, and SEC accurately. The simulated moisture distribution inside a grain kernel showed that this drying technique, which incorporated 4 min tempering between drying cycles, provided a nearly constant rate of drying even though the moisture content decreased to 0.22–0.25 d.b. (dry basis). The use of higher air temperatures noticeably reduced the total specific energy consumption (SECtotal). The air velocity and parboiled paddy feed rate had synergetic effects on the energy consumption. Drying at lower air velocities consumed lower energy when the parboiled paddy feed rate was lower than 170 kgdry solid/h. At higher parboiled paddy feed rates, ranging from 170–250 kgdry solid/h, however, the use of higher air velocities tended to save more total energy. At 80% of recycled exhaust air, the SECtotal was around 4 MJ/kgH2O, which saved 40–45% of SECtotal compared with the case of without exhaust air recycle.