Energy cost of seed drying

In this work, the energy costs of drying corn, rice and wheat seeds between 3 drying options were compared. They consisted of 1) two-stage drying by using fluidised bed dryer (FBD) in the 1 st stage and in-store dryer (ISD) in the 2 nd stage, 2) single-stage drying by fixed bed dryer (FXD) and 3) two-stage drying by using FXD in the 1 st stage and ISD in the 2 nd stage. The drying conditions selected for comparison were proved to be safe for seed viability by the previous studies. The results showed that the drying options 2 and 3 consumed less energy than option 1. However, the benefits from lower energy cost must be weighed against some advantages of using FBD. Furthermore, it appeared that running the burners of FXD and ISD for warming up the ambient air during humid weather condition could shorten drying time significantly with a little higher energy cost. Keyword: Energy cost, Seed drying ___________________________________________________________________________


[2] MATERIALS AND METHODS
In this study, the energy costs of 3 seed drying options were estimated and compared.These options comprise with Option1) Two-stage drying with the 1 st stage drying in FBD to dry seed from 25%wb to 18%wb and then the 2 nd stage drying in ISD to dry seed from 18%wb to 14%wb.
Option2) Single-stage drying in FXD to dry seed from 25%wb to 14%wb.
Option3) Two-stage drying with the 1 st stage drying in FXD to dry seed from 25%wb to 18%wb and then the 2 nd stage drying in ISD to dry seed from 18%wb to 14%wb.
The drying air conditions that were used for energy cost estimation for the FBD are presented in Table 1.They were proved to be safe for seed viability. [4] The drying times and the sizes of ISD and fixed bed dryer were estimated by using the existing drying simulation program (named "ISDryer") that had been developed by Driscoll and Srzednicki at the Department of Food Science and Technology, UNSW.The latest edition of this simulation program is version 6.01 edited in June 2006.In the program, there is a number of weather data that are used as an input for the simulation process.In this study, the drying simulation program was run using two weather files representing the wettest and the driest months of the existing weather records as detailed in Table 2.

[2] RESULTS AND DISCUSSION
From the estimation, the sizes of FXD and ISD that were compatible with the drying capacity of FBD are summarised in Table 3.The calculation procedures can be seen in Jittanit's research. [4]results of cost analysis are shown in Table 4 to 9 (2 tables for seeds of each crop).In the tables, due to the fact that Thailand is one of the major producers of food grains in the world, the costs were presented in the Thai currency Baht.However, the costs shown in the tables for wheat were also calculated in Australian Dollar (AUD) and placed in the brackets because Australia is an important wheat exporter.
According to the results, it appears that option 2 and 3 resulted in lower energy cost than option 1.Moreover, it is worthwhile to use the burner when the weather is wet because the running of burner can shorten the drying time significantly with a little higher energy cost.As a consequence, the risk of infestation by microorganisms or insects in the seed lot caused by long time exposure to the high moisture air is reduced.
[11] So, although option 1 resulted in higher energy cost, it is worthwhile paying a higher energy cost if it can reduce the risk of downgrading the grains from seed to food grade.Generally, the advantages of FBD are uniformity of product caused by the good mixing and flexibility in terms of the minimum required amount of product.Besides, the FBD could be designed to be mobile type and due to its high drying rate, the FBD could be used directly in the field to dry seeds immediately after harvest and to maintain the quality of seeds under severe weather conditions.Therefore, the lower energy cost of option 2 and 3 must be weighed with the higher quality of the seeds in term of uniformity and lower risks of mould growth and chemical reactions due to fast drying in the 1 st stage provided by option 1.
Another issue is that the large amount of dried product required the storage silo.Two-stage drying system using an ISD as the 2 nd stage dryer might be attractive because ISD could be used as a storage silo.In contrast, a single-stage drying system using a FXD would require an additional investment for storage silo.

CONCLUSIONS
The energy cost analysis showed that single-stage drying in a FXD and two-stage drying by FXD and ISD using ambient air together with burner consumed less energy than two-stage drying using FBD and ISD.Nevertheless, the benefits from lower energy cost must be weighed against faster drying rates, flexibility, possibility of a portable design, and product uniformity offered by the FBD.
Eventually, the recommendation is that in the future, the research should be expanded to other kinds of seeds especially higher value crops such as soybean and peanut because the twostage drying system might be more attractive for drying these kinds of seeds.