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Development of a cyclone rice dryer

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Bunyawanichakul, P (2006) Development of a cyclone rice dryer. PhD thesis, University of Tasmania.

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Abstract

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 °C 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°C 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.

Item Type: Thesis (PhD)
Keywords: Rice, Grain
Copyright Holders: The Author
Copyright Information:

Copyright 2006 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Available for library use only and copying in accordance with the Copyright Act 1968, as amended. Thesis (PhD)--University of Tasmania, 2008. Includes bibliographical references

Date Deposited: 25 Nov 2014 00:56
Last Modified: 11 Mar 2016 05:54
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