Modeling and simulation of the two-stage rice drying system in the Philippines

The general objective of this study is to develop a mathematical deep-bed drying simulation model to analyze and optimize the two-stage drying system in the Philippines. The specific objectives are: (1) to model the thin layer drying and rewetting behavior of rice; (2) to model the effect of drying and rewetting on the milling quality in terms of head rice yield; (3) to develop a deep-bed model for two-stage drying and validate its accuracy against the actual performance of dryers; and (4) to analyze and optimize by computer simulation the two-stage drying system performance in terms of capacity, energy consumption, grain quality and cost. Thin-layer equations that accurately describe the drying and rewetting behavior of Philippine rice are required in the development of a robust deep-bed rice drying simulation model. Using non-linear and stepwise regression procedures, equations based on the Page's modified exponential model were fitted over the experimental results. Results showed that the developed thin-layer equations closely predicted the thin-layer drying and rewetting behavior of rice. Head rice yield is the main quality index considered in assessing rice drying systems. Several equations have been already developed to model head rice yield behavior of rice but none has been successfully applied to deep-bed drying simulation studies. Using non-linear regression procedure, equations based on sigmoidally-shaped Gompertz model were fitted over the experimental results. Results showed that the developed equations more than adequately predicted the behavior of rice in terms of head rice yield. The successful modeling of grain drying in deep bed requires as accurate simulation model which is fast in terms of computational speed. A mathematical model was developed based on Thompson's near equilibrium assumptions. The model assumed thermal equilibrium during each time step but the rate of moisture transfer was governed by the characteristics thin-layer drying and rewetting equations. A computer program, written in Microsoft Visual Basic, was developed to run the simulation model. The accuracy of the drying simulation model was validated by comparing the prediction of the model against the results of dryer performance tests. The predicted final moisture content profiles, reductions in head rice yield and specific energy consumptions were compared against the dryer test results. Results showed that the developed model adequately predicted the performance of both dryers. Finally, the performance of the BPRE flash dryer and the in-store dryer, working together as a two-stage rice drying system, was analyzed and optimized by computer simulation in terms of capacity, grain quality and cost. Results showed that the flash dryer must be operated at 70-75 deg C drying air temperature and 500-700 kg/hr discharge rate, and the in-store dryer at 2.00 to 2.25 m grain depth to maximize the annual drying capacity and return on investment and minimize the drying cost