Drying kinetics and performance evaluation of solar bubble parboiled rice dryers through computational and experimental studies

Detailed experimental and computational fluid dynamics (CFD) modeling were carried out on solar bubble parboiled rice dryer. The effects of parboiled rice layer density and mixing time on the drying time and moisture loss were investigated. Twelve thin-layer drying models were used to fit the experimental data. The results showed that the drying occurred in the falling rate period, and the Page and Diffusion Approach model were the most adequate fit. A suitable milling moisture content of 14% (w.b) was achieved within 10 to 33 h. In the CFD simulation process, the moisture content of parboiled rice decreased from 25 % to 17 % within 35-37.5 hours. The collector, dryer and overall efficiency of the solar bubble parboiled rice dryer were in the range of 69 % to 79 %, 24 % to 29 %, and 22 % to 28 %, respectively for thin layer density. The effective moisture diffusivity was in the range of 1.5×10-9 - 2.3×10-9 m2 s-1. The activation energy was 23.2 KJ/mol. The inlet air heating zone and drying zone temperatures were in the range of 32-47 oC and 36-52 oC, respectively. The CFD results indicate that the CFD model can predict accurately the airflow, heat, and mass transfer inside a solar bubble rice dryer.