High-speed Drying

by Dana Holt
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Dryeration is a high-speed grain drying and cooling process that derives its name by combining the words "drying" and "aeration." It involves a specialized application of high-speed aeration combined with other specifically designed grain handling management practices that result in higher grain quality than grain dried using conventional column drying processes.

The basic process of dryeration is that hot grain is transferred from the dryer into two or more separate holding tanks or bins where it tempers, or "sweats," for several hours before being cooled. Depending on the type of grain and the normal drying temperature, hot grain is usually removed from the dryer from 1 to 3 percentage points of moisture higher than in normal drying, as the remaining moisture is removed during tempering and cooling processes in dryeration tanks.

Because these final 1 to 3 percentage points of moisture are the most difficult to remove during conventional drying, the dryer's capacity is usually increased 40% to 70% just by separate cooling, depending on hot grain temperatures and total moisture removed.

Conventional dryer capacity is increased by:

• Eliminating the cooling zone from continuous dryers, using the entire grain column for drying;

• Eliminating the cooling cycle from batch dryers;

• Stopping high temperature drying at higher moisture levels;

• Increasing drying temperatures by 10° to 15°C (18° to 27°F).

In continuous dryers, an additional burner is added to the cooling fan to heat the cooling zone, increasing the overall grain heating zone in the dryer from 25% to 35%. To return to conventional drying, the burner is shut off. In batch dryers, eliminating the cooling cycle decreases the overall conventional batch drying/cooling time by 20% to 40%.

Since the final, "bound" moisture dried from grain at the end of the drying cycle is the most difficult to remove in any grain product, removing the final 2 to 3 points of moisture often requires as much time as the initial 5 to 7 points. Removing this final moisture outside the dryer reduces the conventional drying effort by 25% to 40%, depending on the product and the total moisture that needs to be removed.

Because high temperature drying stops at higher moisture levels, dryer plenum temperatures can be increased by 10° to 15°C (18° to 27°F) without significantly decreasing the product quality. The increase in plenum heat level further increases the drying rate by 10% to 20%.

When these three dryeration functions — external cooling and added drying column, removing the hardest drying, and increased temperatures — are incorporated in the drying process, the drying capacity can be increased by 70% to 100%, or more.

At the same time, drying efficiency (cost per kilogram or bushel of grain dried) is reduced by 25% to 40% because the most difficult drying — the final 1.5% to 3% — is dried outside the dryer by efficient use of the residual heat through tempering and evaporative cooling in tempering/cooling tanks.

HOW DOES IT WORK? How can grain in the dryeration process cool in 12 hours at 1/2 cubic feet per minute per bushel or six hours at 1 cfm/bu when normal aeration takes 15 to 30 hours to cool dry grain at 1/2 to 1 cfm/bu?

How can dryeration fans remove 2 to 3 percentage points of moisture from hot grain on one cooling cycle, when only 0.25% to 0.5% moisture is removed during a normal aeration cooling cycle when dry grain is only 10-150C warmer than ambient air?

The secret to dryeration is the cooling air exhausts saturated at very high temperatures. During normal dry grain cooling, a relatively small amount of moisture is removed because the cooling air exhausts at or near warm grain temperatures, 30° to 35°C (85° to 95°F), partially saturated.

This high moisture removal can best be analyzed using a high temperature psychrometric chart that covers the range of temperatures involved in cooling 50°C to 65°C (120° to 150°F) grain with specific aeration airflow rates. In dryeration, cooling air exhausts at hot grain temperatures; for corn, typically 55° to 65°C (130° to 150°F). From dryeration tanks, saturated exhaust air at 50°C (140°F) carries about 0.15 kg moisture/kg dry air, compared to aeration with 60°F, 80% RH ambient air that exhausts at 90°F, 70% RH with 0.02 kg moisture/kg dry air. Even at 100% RH, 90°F air only contains about 0.03 kg/kg of moisture.

So, dryeration exhaust air carries 50 to 75 times as much moisture as aeration exhaust air from dry grain cooling. This high rate of moisture removal results in adiabatic cooling of grain as the cooling front advances, reducing the cooling time by half to two thirds of the time.

A 1966 study found that the theoretical limit of dryeration moisture removal was about 3.5 percentage points, but due to convection and conduction heat losses during transfer and tempering in non-insulated bins, 2 to 2.5 percentage points of moisture removal was typical during corn drying in commercial elevator or farm drying systems.

COOLING TANK DESIGN. Hot grain is transferred into one of two dryeration cooling tanks designed to hold all the hot grain from the dryer during a 24-hour drying period. To provide extra holding volume for late season drying when only 5 to 7 percentage points of total moisture is removed (20% to 22% dried to 15%), cooling tank capacity should be designed to hold 2.5 times the conventional drying system capacity with a 10 point grain drying moisture removal range, from 25 to 15% M.C. (wet basis).

For example, if a dryer normally dries 200 tonnes (8,000 bu) of corn per 24-hour cycle from 25% to 15%, each dryeration cooling tank should be designed to hold 500 tonnes (20,000 bu) when drying from 22% to 17% with the final target cooled grain moisture level at 15%.

Sizes and volumes for the tempering/cooling tanks are nominal dimensions. Two hopper bottom steel grain tanks that hold two and a half times the conventional mid-season drying capacity of the dryer are recommended. But, for very large grain dryers or multiple dryers at one elevator site, larger steel flat bottom grain bins with full drying floors and sweep unloaders may be a more economical choice.

However, use of hopper bottom bins with gravity unloading decreases mechanical equipment maintenance and the potential for mechanical failure. Because of daily unloading and cleanout, hopper tanks work best for use as dryeration cooling tanks. Depending on tank size, one or two aeration fans are used.

A perforated aeration duct is placed down the hopper slope starting near the top of the hopper with the open lower end stopping about 0.5 to 1 m (20 to 40 inches) from the discharge opening. Small aeration fans are mounted directly to the hopper. On larger tanks, bigger fans should be mounted on the concrete base with a transition duct connected to the aeration duct inside the hopper.

The type of blower depends on the type of grain being dried and static resistance. For corn, soybean or other coarse grain cooling, vane axial blowers will usually develop adequate static pressure. Centrifugal blowers are required for small grains.

For large hopper tanks used with high capacity commercial dryers, two or more blowers and aeration ducts may be installed. As an alternative, one large blower manifolded to two to four ducts inside the hopper may be used.

SYSTEM MANAGEMENT. After high temperature drying, hot corn is typically cooled for eight to 12 hours. This cooling process is followed by an hour or two for unloading. The entire process is designed to work in a 24-hour cycle, so if cooling requires 12 hours and unloading takes three hours, cooling should be started after nine hours of loading and tempering.

When drying grains besides corn, where drying temperatures are lower than 60°C and grain temperatures are not high, less moisture is removed due to reduced enthalpy (low drying energy differential) so cooling time with the same airflow rate is longer.

In order to operate within a 24-hour cycle, when drying grain at low drying temperatures where hot grain temperatures are lower, the cooling cycle can be started with only four to six hours of tempering on the first hot grain transferred into the tank.

Cooling air quickly heats to grain temperatures and reaches saturation in the lower part of the grain mass. Thus, even though cooling has started, additional hot grain transferred into the tank later in the day will temper adequately before the cooling front reaches the hot grain in the middle to upper levels of the cooling bin.

Compared to conventional drying, grain quality is higher from dryeration because heat stresses from high temperature drying are reduced. Tempering the hot grain allows internal moisture to move to the surface of the kernel, so kernels shrink internally and surface tension relaxes. By eliminating the sudden high speed cooling which causes kernel surfaces to contract and shrink, stress cracking and subsequent breakage during later handling is greatly reduced.

Tempering for four to 12 hours before the grain begins to cool allows kernel temperatures and internal moisture to partially equalize. Recommended dryeration airflow rates of 0.5 to 1.0 m3/min/tonne (0.5 to 1.0 cfm/bu) are five to 10 times higher than normal dry grain aeration rates of 0.1 m3/min/tonne (0.1 cfm/bu). But, dryeration airflow is still only about 1% to 2% of the cooling airflow rates used in the cooling sections of high capacity grain dryers.

After hot grain tempers or "sweats" in its own heat for four to 12 hours, allowing internal grain moisture to equalize within the kernel, dryeration airflow gradually removes the moisture from the kernel surface, cooling the grain in the cooling bin in six to 12 hours instead of 15 to 30 minutes in a batch or continuous flow grain dryer.

FACILITY OPERATION. The dryer-ation process typically operates as follows: Tank #1 is filled on Day 1. Tank #2 is filled on Day 2 while Tank #1 tempers, cools and is unloaded. By Day 3, Tank #1 has been emptied and is ready to be filled again, and the process continues on a daily, 24-hour cycle.

Thus, each tank fits a 24-hour cycle, with four to 12 hours of tempering, six to 12 hours of cooling, and the balance for unloading and preparing for the next refill. Once hot grain has tempered in the tank for four to six hours or more, cooling can begin while tank filling continues if the cooling rate is marginal — less than 1/2 cfm/bu.

For example, if the dryeration tanks are designed to hold 760 tonnes (30,000 bu) but the aeration fans are designed to deliver 0.5 m3/min/tonne (0.5 cfm/bu) at 80% of capacity, or 610 tonnes (24,000 bu), by late in the season with drier grain, when the tank is filled to capacity, the fans may only deliver 3/8 cfm/bu, and require 15 to 18 hours to cool the grain.

To complete cooling within the 24-hour period, cooling can be started after the initial 125 to 200 tonnes (5 to 8000 bu) are transferred into the tank. Hot grain at 60°C (140°F) added after that will temper as if the fans are not operating as the air passing through the upper levels of grain will be 60°C (140°F) and saturated, so no cooling will take place for several hours until the cooling front reaches that grain level, at say the 300-tonne (12,000 bu) level.

Hot grain can be progressively added while the cooling front advances several feet below the fresh grain fill level. As more grain is added, airflow resistance increases and the cooling front movement slows. So, hot grain will have an adequate tempering period before the cooling front gradually passes through.

Tempering allows the hot internal temperature to drive the excess moisture to the surface, so the kernel "sweats." Surface moisture is removed as the cooling front passes through the grain mass. The air passing through the cooling grain behind the cooling front is no longer at maximum grain temperature and is not saturated at that point.

Compared to normal drying, dryeration requires one additional handling step and two extra specially designed hopper tanks. The two tanks provide additional storage and working capacity for other storage or handling uses after the drying season. But, the results are dramatic in terms of added drying capacity, and grain quality is maintained at a higher level. So, an extra leg or transfer conveyor is required.

Dryeration has the theoretical potential to remove from 2 to 3.5 points of moisture outside the dryer. This potential is a function of the hot grain temperature, conductive and convective heat loss during transfer, and final temperature of the cooling air.

Corn is generally dried hotter than other grains. It has been shown that 3 points of moisture can be removed if the hot corn temperature is 60°C (140°F) — a typical temperature for 17% corn leaving the dryer when dried at 105°C (220°F) — does not lose heat by conduction, tempers and cools at the prescribed rate of 1/2 cubic feet per minute per bushel to 21°C (70°F).

If the corn is cooled to 10°C (50°F) instead of 21°C (70°F), about 3.75 points could be removed. However, convective heat losses occur and grain cools some in tanks without insulated sidewalls, so a general rule of thumb is that about 2 points of moisture removal is typical for drying outside the dryer. Crops dried at lower drying temperatures with lower temperatures going into tempering will have reduced dryeration capacity compared to hotter grain.

RESEARCH DATA RESULTS. Tempering and slow cooling improves corn quality by reducing multiple stress cracks that normally occur during the final stages of drying and the cooling shock in a grain dryer cooling section with cooling airflow rates of 50 to 100 m3/min/tonne.

When the pericarp (outer skin of corn kernels) cools rapidly, it shrinks faster than the mass of the kernel, causing high surface tension. Multiple stress cracks or fissures develop when the pericarp cannot withstand the shrinkage forces.

The bottom line is that dryeration is a powerful grain management tool. Drying capacity can be increased by over 50% to 75% without investing in a new grain dryer. The downside is the need for one additional handling circuit into and out of the additional dryeration cooling tanks. However, the additional hopper bottom tanks are useful at other times during the year to handle grain.

Few commercial or industrial processes provide as much efficiency with so much advantage and so few disadvantages as dryeration.