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 Main subject - mar/apr 2008
High Speed Drying
Silmar Teichert Peske
peske@ufpel.edu.br
Francisco Amaral Villela
The production of high quality seed in large scale requires the adoption of cutting edge technology. It is relatively easy to produce small amounts of high quality seeds, however, to produce enough supply to adequately cope with market demands requires that a set of technologies be available, being one of them the artificial drying. The easiest way is to simply wait for seeds to dry out while still in the cropping field and proceed to harvest, yet this procedure favors seed deterioration since once physiological maturity has been achieved the seed will literally remain stored in the field, so that the sooner the harvest, the better.
Studies on this subject have confirmed that corn seeds will take one to two months, while still on the field, to dry from 35% moisture content (physiological maturity) to 13%, that being the main reason why seed companies prefer to harvest corn while still in the cob with a moisture content of approximately 35% and then proceed to artificial drying.
Studies in soybean have shown that seeds demand two to three weeks to dry from 50% moisture (physiological maturity) to 13%, depending on the weather conditions. When seed that has dried below 20% is exposed to precipitation, its quality is reduced beyond the minimum quality standard.
Rice farmers have long known that they must proceed to harvest, whether grain or seed, when the moisture content ranges from 24% to 20% and then artificially dry the produce.
Seed is a live material that the wetter it remains the lower its storage potential will be. As a rule of thumb, it can be said that under conventional storage conditions a seed bearing 13% moisture can be stored through a period of 8 months. It remains obvious that seed harvested in excess of 13% moisture should be immediately dried, to which purpose artificial drying appears indispensable through any of its many ways.
This article will focus on the intermittent drying system, with a drying capacity above one percentage point (pp) per hour. This system consists basically in maintaining the seeds under continuous movement and exposes them to a hot airflow at regular time intervals.
Seeds can be considered hygroscopic material that will gain or lose moisture depending on the ambient air relative humidity (RH), a feature that has helped in dryer design to allow for seed moisture to be removed. The drying environment requires that moisture around the seed be driven far off, through air that is insufflated or sucked through the mass of seeds through a high-power fan. The more intense the airflow, the faster the drying process up to a certain limit imposed by the migration rate of moisture deep in the seed structure to the outer boundaries.
Drying speed
On a practical basis, drying speed can be considered in two different ways:
1. The moisture pp removed by hour;
2. Tons of dry seed per day (on the basis of reducing from 18% to 13% moisture).
This article will refer to drying speed in pp per hour, since it's the unit that best suits the intermittent drying system.
As mentioned previously, drying speed is essentially the function of two physical properties of the air, RH and the airflow that goes through the seeds. When considering RH, the easiest and most economical way to lower it is to heat the air, and as a rule of thumb, it can be taken that for every 1°C increase in air's temperature RH will decrease by 2 to 4%. Intermittent dryers operate at air temperatures between 60-70°C, at which RH falls below 10%, enhancing its capacity to remove moisture from the seed.
Under a situation in which the control of the drying operation is not appropriate, seed moisture will reach equilibrium with that of the surrounding air mass, which could lower it to dangerous levels, causing extreme desiccation. To avoid this, it is a common sense procedure to measure seed moisture at the dryer at regular time intervals (20-30min).
In general, the temperature of the heated air used for drying, reaches much higher values than those of the seed mass. Regarding the existence of several models of intermittent dryers it's a advisable, especially during the first drying sessions, to relate the drying air temperature to that of the seed mass. For security purposes, it is recommended that seed mass temperature doesn't exceed 40°C in the case of soybean seed, or 43°C in rice, wheat and corn during the final stages of the drying process, when normally seed moisture content is of around 14%. Seeds with lower moisture content can support higher temperature.
Another physical feature of the air influencing drying speed is the airflow passing through the seeds, which is established by the manufacturer regarding the standards for the type and model of dryer, typically over 60m3/min/ton of seed. One practical clue of the airflow strength is to check if it is able to literally push the seed upward during the final stages of the drying operation.
It is fairly common that even according to the manufacturer's specifications the resulting air flow remains low when traversing the seed mass, resulting in a low drying speed. The slower drying rate is the consequence of the hot air's low flow capacity to transport the moisture released by the seeds away from the drying environment. This happens during the process of heating the air from around 25°C - 30°C to 60°C - 70°C, employing ovens that are fueled by different energy sources to heat the air.
In Brazil wood is a common energy source, whereas gas has seen and important growth during the last years. The heating system needs to be designed so that the airflow is continuous and there are no strangling points. To avoid this kind of situation, intermittent dryers have incorporated to their heating systems a opening for the entrance of cool air, so that it can mix with the hot air.
It is normal for hot air to exit the oven at a temperature above 150°C and then mix with the cool air, so that upon entering the dryer the air temperature reaches 60°C - 70°C. Without the cool air opening, the oven must be extremely well designed in order not to strangle the airflow.
The intermittent type dryer must be capable of drying a minimum of three loads per day, from initial 20% moisture to 13%. To achieve this performance the moisture removal speed needs to be above 1pp/h and the time needed to load and discharge the dryer no longer than 40min, depending on the model. Regarding this specifications, an intermittent dryer with a static capacity of 25 tons will yield a minimum drying potential of 75 tons per day.
This kind of information on the drying capacity of the different types of intermittent dryers is very important, since any user will be able to instantly figure out the dryer's performance, which many times falls below three loads per day.
Follow are some experiences on low dryer performance and their causes.
Day and Night
Some years back we have received an intermittent dryer for research and practical teaching purposes. During its first year the dryer was used to dry basic seed from a breeding program and since there were no trained personnel to do the job, a market search was undertaken to contact experienced operators. The dryer started its operation with a load of rice by mid-afternoon, reaching an average drying speed of 1.5pp/h. By early evening two operators were appointed for the night shift; the next morning they eagerly reported that the load was about to be fully dried. Then, after a quick check it became clear that the drying speed had been extremely low. The operators were asked what had happened, to which they replied that it was only natural that by night drying would be slower than during the day.
Clearly, this is not so. During day time, the drying speed reached 1.5pp/h and what happened was that by the next morning the dryer was found shut off, due to load strangling along the feeding system. The next night two students were assigned the duty of supervising the shift and everything went normal, reaching a speed of 1.5pp/h. Moral of the story: day or night drying are exactly the same in relation to potential drying speed, since the latter depends on the dry air RH as well as the airflow. The basic difference is due to fuel consumption, which is higher at night, since ambient temperature is lower. Whereas during daytime temperature has to be risen from 25 - 30°C to 60 - 70°C, by night the lower level decreases to 20°C, which has to be heated to 60 - 70°C.
page 2 ->> High Speed Drying
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