Development of a spouted bed dryer as part of a two-stage drying strategy for grain in Vietnam

• University of Agriculture and Forestry, Vietnam

In earlier research from 1994 to 1996 research teams of the University of NSW in Sydney and of the University of Agriculture and Forestry in Ho-Chi-Minh-City (with funding from ACIAR) collaborated to promote the concept of two-stage drying for grain in Vietnam. The project demonstrated that two-stage drying was of benefit to Vietnam's grain industry in terms of improvement of quality and low energy consumption.
Most of the research work was done on in-store drying, which was a new concept in Vietnam. However some of the designs related to high temperature drying could also be tested in Vietnam. This applied particularly to the fluidised bed dryer that had been developed by the Thai research team and rapidly commercialised, not only in Thailand but other countries including Indonesia, Philippines, Myanmar and Taiwan.
One of the main advantages of the fluidisation technique for grain drying is the short residence time in a dryer, allowing high throughputs of grain. However, difficulties in handling grain with non-uniform initial moisture content, as well as need for high pressure fans and thus higher energy consumption characterising fluidised bed dryers, indicated that a spouted bed dryer may be more appropriate as a first stage dryer under the conditions of Vietnam's grain processing sector. In order to benefit more from the advantages of fluidisation, spouted bed dryers became the target of new research.
As a result of these considerations, project member Nguyen Le Hung from the University of Agriculture and Forestry (UAF) obtained a John Allwright Fellowship to undertake his PhD in the Department of Food Science and Technology of the University of New South Wales He undertook studies on the use of a spouted bed dryer for industrial-scale drying of paddy.
The work in the laboratory showed that the spouted-bed concept seemed to meet expectations with regard to its potential to compete with a fluidised bed dryer. But the design proposed by Dr Hung still needed scaling-up and extensive field testing in conjunction with an in-store dryer. A small project would enable Dr Hung working with others to transfer the knowledge acquired during his stay at the University of NSW and also contribute to the consolidation of the work on two-stage drying performed under the earlier project.

Project objectives were to develop a technically and economically acceptable spouted bed dryer to integrate into the two-stage drying system for grain in Vietnam.

In the first instance the scientists developed computer-based drying models. Model development included a spouted bed drying (SBD) model within the two-stage drying system, a quality model related to the use of SBD, and an economics model resulting from the use of SBD. They also undertook a system analysis leading to a prediction of optimal drying requirements, in order to determine the design capacity of the dryer. They undertook model testing and validation with a prototype, modifying as required.

The scientists then attempted to design, construct and evaluate a commercial SBD in Vietnam.
They formulated the specifications for the design of the SBD and the in-store dryer, as well as the grain handling equipment associated with it. Their work program involved prototype construction and workshop testing, field testing under commercial conditions, assessment in comparison with conventional high temperature dryers, and promotional activities including training and demonstrations and liaison with manufacturers.

Based on the design and drying tests of the pilot-scale triangular spouted bed (PTSB), a hexagonal spouted bed (HSB) dryer was built to investigate drying performance in full scale. Drying tests of paddy in the HSB confirmed the tests done before in a PTSB. Similar drying kinetics of paddy were found in the HSB and the PTSB, thus the information from testing a PTSB is reliable for constructing a HSB. HSB dryers could function as a first stage treatment for high moisture content paddy, or as a complete dryer with multi-pass drying procedures to help reduce the drying time and preserve grain quality.
A lab-scale cylindrical spouted bed was built to study drying process of corn and coffee. Preliminary results showed that a spouted bed dryer could dry these commodities to moisture contents required for safe storage. With regard to the extension of use of in-store drying to corn, it was found that applying suitable layer loading, corn could be dried from 20% down to 16% in the rainy season.
The project in principle concluded at the end of June 2001 as per project agreement. Some of the project activities related to the use of two-stage drying system for corn were conducted through 2002 with the funds of UAF. A private producer of corn for animal feed showed interest in using a high temperature dryer to reduce the risk of aflatoxin build up in wet grain. A series of field experiments with the 1 tonne/h unit were conducted in order to determine the feasibility of this process. If successful, the fluidised bed dryer could be adopted industrially, as it is the case in Thailand.
Promotional activities continued through demonstration of the technique of fluidisation to potential users at local training workshops on drying. Also the topic was included in the Grain Storage Tutor of the AusAID-funded CARD program using a computer-assisted learning technique.
Further work was envisaged with regard to extension of the use of fluidised/spouted bed dryer to other crops than rice and corn. ACIAR funding was sought for a small project on drying of high value grain crops using these drying techniques.