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 Main subject - mar/apr 2008
continuation: High Speed Drying
Oven
During their teaching tenure, professors are normally exposed to be challenged by students, in an exercise that is essential to knowledge. At times, the situation may become hard enough to demand a practical verification of what's at stake. When, during a lecture, it was mentioned that an intermittent dryer could deliver a three-load-per-day performance, the comment was contested in such a way that the only thing left was to pay a visit to a drying system that was delivering only one load per day. The location was some 300km away, and after a warm reception by the seed processor unit manager we went straight to business, i.e. the drying system working with rice.
The system was composed by six dryers with a capacity of 10 tons each, all under normal operating conditions at an air temperature of 70°C, which heated seeds to a maximum temperature of 42°C by the end of the drying session. This was a clear sign that the system was operating smoothly, without setbacks. After checking on the temperature, the ventilation system was inspected, the fans were checked and since they were originals provided by the manufacturer, the idea was that the problem was somewhere else.
Walking around the dryers it was verified that the temperature control was performed outside the dryer, on the air that came out of the oven, that is, there was no opening for cool air. This caused airflow strangling, since six dryers were supplied by only one oven, which made the whole process highly inefficient. It was suggested that a solution could come by means of a window opened to allow the entrance of cool air (which would increase the airflow to the dryer), which was accepted, yielding an increase in the drying speed to 1.3pp/h.
Some drying tips
Precautions
High-speed drying demands some special precautions, of which:
a) Over drying - intermittent drying is performed with low RH and high airflow; thus, seed moisture should be monitored in short time intervals (20 - 30 min) to avoid over drying. This would mean that seeds would achieve a final moisture content of 10%, instead of the normal 12% - 13%. Low seed moisture facilitates mechanical damage and over drying not only increases the time necessary for the process but reduces drying efficiency as well, among other inconveniences. It's worth nothing that, since intermittent drying implies that seeds are under continuous movement, there's no drying zone formation and the process results more uniform for all seeds.
b) Mechanical damage - since seeds are in continuous movement through the dryer, the risk of damage may be higher when the elevator is not set appropriately or when it is not of the recommended type. Upon entering the dryer the risk for mechanical damage is very low, since seeds have a relatively high moisture content. However, special precautions should be taken with the elevator and the least number of cycles through the dryer - elevator system, the better.
c) Over-speed drying - as important and efficient as fast drying can be, it should be remembered that seed moisture needs to be transferred from the core to the surface, so that it can then be removed and carried outside the dryer environment. If drying speed is very fast there's a risk for the formation of a wide moisture gradient between the core and the surface which could trigger internal damage, such as cracks within rice and corn seeds, that will definitely impair their quality. Hence, it is recommended that drying speed do not exceed 1.3pp/h for soybean and common bean and between 1.8pp/h for rice and wheat. Drying rice at more than 2pp/h will cause cracks within the seeds, delivering a low yield of whole grain.
Recommended seed temperature in relation to seed moisture for soybean seeds
Corn Ear
For drying to be efficient, air should be able to easily traverse the seed mass. Many discussions have been held as to why corn on the ear demands a longer time to dry than threshed corn, being that one m3 weighs 750kg, whereas the same volume of corn ear weighs a mere 450kg. What really happens is that air moves freely around the corn ears, however, it's an entirely different story when considering its movement around the seeds attached to the cob. If the airflow is not able to circle all seeds there will be no efficient drying, since moisture will not be entirely forced out of the drying environment. Incidentally, even when it holds higher moisture content in relation to the seeds, the cob is not responsible for the slower rate of cob corn drying, even when the technical advice is to harvest seed corn with moisture over 30% and proceed to dry when still on the cob.
In the regions where harvest time does not coincide with the rain season, seed companies often perform threshing with moisture around 20% and dry with the intermittent system, achieving drying speeds above 1pp/h as air moves easily through the seeds, when previously threshed.
The myth
That drying roasts the seed can be a common line among those within the sector. However, the truth is different and in general those who do not dry their seed produce show that either they work with low quantity of seeds or that the mature seeds remained for too long on the field. Drying guarantees quantity with quality.
The drying capacity
Let's consider the following situation: A dryer with a static capacity of 25 tons of soybean seeds per load, performing 3.5 loads per day through a harvest season of 40 days. By the end of the harvest season, the dryer will have processed a total of 140 loads of 25 tons each, totaling 3500 tons, which can reasonably be considered a fair amount of seeds.
A soybean seed grower delivering for a demand of 8000 tons (200,000 40kg-bags) will be processing approximately 11,000 tons in its seed processing unit, an estimate that involves considering a processing loss of 25%. Assuming that he will have to dry 60% of its produce (6,600 tons), he will have to count with at least two 25 ton static dryers (total of harvest season 7,000 tons) to cope with the drying demands. With these data, an estimate for the cost of drying for quantity and quality seed can be calculated; considering the drying system, a 10-year liquidation period, personnel and fuel, the cost will be below US$0.6 per 40kg bag. Other alternatives may be considered, especially in relation to the dryer's static capacity, but for the volume of seed used in the previous example, the 25 ton-dryer best serves the purpose. A 25ton load can be considered as a seed lot, which in turn will make it easier to log and keep the records accurately. The proportion of soybean seed production estimated to need drying may vary from region to region, however, 60% can be considered a reference value since it is advisable to have a capacity in excess of that calculated, as drying requirements will surely vary through the harvest season.
In an ever demanding environment for efficient processing, the drying system should be considered as a determinant of the volume of quality seed at a competitive price.
This article discussed how intermittent dryers operate and the advantages for drying at rates over 1pp/h or, in other words, how they are capable of handling a minimum of three loads per day.
Issues regarding the RH and the airflow were presented, as well as the precautions to be taken to achieve a high drying capacity without compromising seed quality.
Finally, the budget for dryers for a seed company that markets around 8,000 tons per year was estimated.
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