Development of a cyclone rice dryer
Creators: name: Bunyawanichakul, P
Publisher: name: University Of Tasmania
Year: 2023

description: This thesis evaluates the suitability of a cyclone dryer for the drying of paddy (rice grain). The design aim is to reduce the moisture content of paddy from a fresh harvest level of 33% dry-basis to a more manageable level of 22% of dry basis, and to replace all classical methods of drying. The cyclone dryer consists of a cylindrical tower containing a series of inverted conical baffles with central orifices that divide the tower into chambers. The moist particulate matter is fed into a stream of hot, dry air which enters tangentially at the base of the tower, creating a rotating flow within the dryer. The central vortex and through-flow jet transport the particles upwards from chamber to chamber until discharged tangentially at the top. Recirculation of flow within the chambers lengthens the particle residence time Single-phase numerical calculations with the commercial RANS-based Computational Fluid Dynamics (CFD) code CFX 5.7 are used for flow field and pressure drop predictions. Experimental observations in a small scale laboratory model are used for validation. Useful descriptions of the axial and tangential velocity distributions are obtained, and the pressure drop across the cyclone dryer chamber is predicted to about 20% accuracy. Particle residence time in the laboratory model cyclone dryer is measured by the pulse tracer stimulus response technique. Observations using paddy grain and spherical silica gel particles show the mean residence time to vary quadratically with particle concentration. The residence time distribution (RTD) is explained well by a tank-in-series model. Numerical predictions of particle residence time obtained from one-way coupled particle transport modelling without particle dispersion using a Lagrangian/Eulerian approach produce RTDs differing significantly from the experimental observations. However, the trends of mean residence time variations in response to changes in inlet air velocity and number of cyclone chambers are correctly predicted. Single-pass drying tests with paddy grain demonstrate maximum moisture reductions of 2.6-6.5% dry-basis obtained at inlet air temperatures of 82-89 ¬¨‚àûC and paddy grain feed rate of 0.03 kg/s. The specific energy consumption (SPEC) varies between 7.5-20.5 MJ/kg of water evaporated, depending on the initial moisture content of paddy grain. Compared with fluidised bed and spouted bed paddy dryers employing 50-70% exhaust air recycling, the cyclone dryer gives a lower moisture reduction and a higher SPEC. This indicates that practical application of the model-scale cyclone dryer would require multipass operation with exhaust air recycling to be sufficiently economic. Multi-pass laboratory tests show three-pass drying in a four-chamber cyclone dryer with inlet air temperature of about 80¬¨‚àûC and 0.03 kg/s paddy grain feed rate to reduce the moisture content of paddy grain by about 11% dry-basis, with an SPEC of 13 MJ/kg of water evaporated. Recycling about 90% of the air would give a 70-75% reduction in SPEC compared to non-recycled air operation, and 3.5-4 MJ/kg water evaporated. This is comparable to fluidised bed paddy dryer operation for similar initial moisture content of the paddy grain. Numerical simulations of silica gel particle drying based on two-way coupled particle transport modelling with a Lagrangian/Eulerian approach are also reported. The simulations consistently overpredict the moisture and heat transfer observed experimentally using silica gel particle 3.25 mm in average diameter. As the mean particle residence time was underpredicted , it is concluded that the water evaporation model used here gives a much higher moisture transfer rate than that observed experimentally. Computational studies of the increase in residence time with geometric scale-up of the dryer indicate that a commercial scale unit small enough to be field portable could achieve the desired moisture reduction of paddy grain with single pass operation.