Grain drying to maximize capacity, quality and profit
April 01, 1995
by Teresa Acklin
Part II of II analyzes drying techniques and modifications that can improve the efficiency and profitability of high temperature crossflow dryers.
BY DIRK E. MAIER
The two major disadvantages of conventional crossflow dryers are: (1) large gradients in moisture content and quality across the width of the column, and (2) low energy efficiencies.
In recent years numerous improvements have been implemented as standard design features or are available as options. However, non-uniformity of moisture, temperature and quality can never be completely eliminated using the crossflow design for grain drying.
Energy conservation and recovery
Recovering some of the discharged air from high-temperature crossflow dryers can reduce energy use by 20% to 30%, while reducing the drying capacity only slightly.
The amount of saturation of the exhaust air varies along the length of the column of a crossflow dryer. Near the grain inlet, the exhaust air is nearly saturated, and saturated exhaust air feels relatively cool. For example, air heated to 110°C, forced through the grain column, and exhausted at 90% humidity may be as cool as 16°C.
On the other hand, the air exhausted near the transition from the heating to the cooling section is relatively dry, with a lot of its drying potential remaining. The exhaust of this air wastes fuel and significantly reduces energy efficiency.
An energy recovery system aims at capturing as much of the useful energy as possible left in the exhaust air. Some dryers recover all or part of the cooling air and recirculate it into the burner; others recover and recirculate the cooling air and part of the discharged drying air from the lower sections of the drying zone. The actual amount of energy savings from exhaust air recovery depends on dryer airflow direction, final grain moisture content, moisture reduction in the dryer, weather conditions and the proportion of airflow that is recovered.
The additional costs of an energy reclaiming system have to be weighed against the potential savings. For example, drying 75 tonnes per hour of maize from 25% moisture to 15% requires the removal of approximately 9,000 kilograms of water.
At an energy efficiency of 7,000 kiloJoules per kg of water, drying would require about 270 Therms of natural gas per hour, assuming 232,600 kJ per Therm and a burner efficiency of 85%.
If the drying efficiency was improved to 3,500 kJ per kg of water, the fuel consumption would be cut by 50% to 135 Therms per hour. At U.S.$1.00 per Therm, a 50% improvement in energy efficiency would save U.S.$135 for every hour the dryer is operated.
If the dryer handled 25,000 tonnes of maize per year, about U.S.$45,000 could be saved in energy costs alone (in other words, the drying costs could be reduced by about U.S.$1.80 per tonne).
Reverse airflow (suction) cooling
One of the primary crossflow dryer design modifications uses reverse airflow, or suction, in the cooling section.
In this design, the fan and burner unit are typically mounted inside the dryer, drawing the cooling air through the grain in the cooling section and pushing it into the heated air plenum. In the heating section, the air is forced through the grain column and exhausted in the same manner as in the conventional crossflow dryer.
Suction cooling has been primarily implemented in upright tower dryers. The reverse airflow design has two primary advantages:
during grain cooling, part of the heat is reclaimed by prewarming the drying air before the burner, decreasing the specific energy consumption by as much as 30%;
ambient cooling air enters the grain column where the grain temperature is the lowest, reducing the cooling shock by minimizing the grain-air temperature difference and improving the moisture gradient and breakage susceptibility.
The significant reduction in the moisture gradient is attributed to the low drying air temperatures, which maintain the maximum maize kernel temperature near 60°C throughout the dryer.
Another modification of upright tower dryers is the addition of an enclosure of the drying section or the entire dryer. This decreases the wind effect, which minimizes overall heat loss, assures a more even temperature distribution throughout the dryer circumference and reduces condensation and caking on the outer screened column.
Blocking off part of the drying air escape near the bottom of the heating section allows for the recirculation of exhausted drying air back to the fan and burner in a positive pressure dryer and into the upper part of the cooling section in a suction system. This reduces the initial cooling shock and improves energy efficiency even further.
One of the disadvantages of reverse airflow cooling is the accumulation of more chaff and fines sucked into the plenum from the cooling columns. This requires more maintenance and frequent cleaning of the dryer.
A recent dryer design places the blowers at the side of the tower and allows a choice between both pressure and suction cooling for maximizing energy efficiency or minimizing trash build-up.
Reverse airflow (suction) drying
Another modification of conventional crossflow dryers is to reverse the airflow along the drying column. Although a rather expensive modification, this design has been shown to reduce the moisture gradient across the width of the column by up to 3.5 percentage points and improve energy efficiency by up to 29% compared with a conventional crossflow dryer.
The air from the top half of the drying section flows from the inside of the plenum through the column and is exhausted to the atmosphere, while the air for the second half of the drying section is introduced from the outside of the column. The inlet air for both drying stages is recycled air, which is mixed from the exhaust cooling air and the second drying stage air.
Grain column inverting
Although reverse airflow dryers reduce the overall moisture differential in the grain column, they do not eliminate it.
To decrease the moisture gradient significantly in the drying section, a grain exchanger should be installed about halfway down the drying column of an upright tower dryer. This turns the overheated, overdried grain from the air inlet side to the air outlet side of the column, while the underheated, underdried grain is turned from the exhaust air side to the air inlet side.
For maximum quality, the installation of multiple grain exchangers should be considered. Grain exchangers mix the grain somewhat while turning it. They also reduce the specific energy consumption slightly because the drying air has to heat the underheated-underdried grain while lowering the temperature of the overheated-overdried grain now at the outside of the column. Upright stacked dryers achieve mixing of the grain in the transition between stages but do not accomplish actual inverting of the grain column.
Differential crossflow drying
A further improvement in crossflow drying is the differential grain flow dryer, which consists of tapered columns, dual variable-speed discharge augers and a tempering hopper separating the first and second drying zones.
The exhaust air from the second drying zone and from the cooling section are mixed and recycled. Due to the tapered column design and the dual discharge augers, the grain on the air inlet side of the columns moves more rapidly through the drying sections than the cooler grain near the air exhaust side of the drying zones. Thus, the moisture gradient across the width of the columns is significantly reduced. Additionally, the drying process is interrupted for 0.5 to 1 hour by tempering the grain between two drying stages.
Tempering between stages
When grain is tempered between drying stages, the grain is not subjected to any air treatment for an extended period of time. Thus, the temperature and moisture gradients that develop while moisture is evaporating from individual grain kernels are diminished, or equalized before drying or cooling of the grain is resumed.
The tempering process limits stress cracking and subsequently reduces the breakage susceptibility of the grain. Tempering between successive passes of drying has been used for years in the rice drying industry, where the prevention of fissures is of primary concern.
Tempering has also been used successfully in on-farm drying systems that transfer hot grain into a bin where it is slowly cooled. Not only is drying efficiency improved, but stress cracking is diminished.
Incorporating a tempering section into a high-temperature, high-capacity crossflow dryer is especially beneficial between the drying and cooling stage. Research has identified fast cooling as a primary cause of stress crack development. Thus, tempering before cooling in a continuous-flow dryer, or tempering followed by slow cooling in a bin will significantly reduce stress cracks.
Although a tempering section requires an increased overall dryer height, less than 30 centimeters of drying section is needed to make up for 30 cm of tempering. Incorporating a tempering zone along a drying section modifies a single-stage into a multi-stage dryer.
Segregating initial moistures
A major challenge for any dryer design is to adjust its drying speed quickly and accurately to reflect the inevitable variation of the moisture content of incoming grain.
Drying speed can be optimized using automatic dryer controllers. However, segregating grain into multiple wet holding bins according to different moisture content ranges will improve the performance of even the best dryer controller, with the added benefit of better energy efficiency and grain quality.
For example, some elevator operators have segregated wet grain into above and below 20% moisture categories, and into ranges of 18% and below, 18% to 22% and 22% and above. These actions have yielded more uniformity during the drying process.
Preheating the wet holding bin
Preheating of grain occurs to a limited extent in the wet holding hopper above most dryers. But preheating grain in the wet holding bin by using waste heat from the dryer and boosting its energy content with additional burners has been shown to increase the drying capacity of existing dryers by 15% to 20%. Preheating can reduce overall fuel costs and improve grain quality; however, these benefits have not been thoroughly quantified.
Crossflow drying and slow cooling
High-capacity, high-temperature cross- flow dryers are generally set up at elevators to rapidly cool the dried grain before transferring it to the storage structure. However, employing delayed cooling methods can reduce fuel costs, increase drying capacity, reduce stress cracks and brittleness and provide more operational flexibility.
Delayed cooling usually involves transferring hot grain (grain kernel temperatures of 38°C to 60°C) from the high-temperature dryer to separate cooling bins. The most widely used methods are known as dryeration, in-bin cooling and combination high-and-low temperature drying.
Although transferring the hot grain and cooling it slowly has been used as an on-farm technique for many years, it is rarely used by commercial elevators because of several significant limitations:
existing equipment and storage structures are not set up to handle the significant water condensation from the large volumes of grain transferred hot during cold weather;
existing storage structures are not equipped with large enough fans to provide the necessary airflow rates to complete cooling within the allowable safe time limits;
flat storage structures are never suitable for cooling hot grain. Airflow from aeration ducts will not flow uniformly through the grain pile, and significant condensation will occur on building roofs;
large storage tanks are generally not equipped with fully perforated floors. Non-uniform airflow and thus cooling would be a significant concern;
upright concrete silos are generally too high and would require too much fan horsepower to achieve the necessary cooling rates.
Before giving serious consideration to implementing slow cooling methods at commercial elevators, the potential gains in fuel savings and quality improvements need to be compared against the capital equipment costs. However, for utilization with high-value crops these investments may be justifiable.
A high-temperature, high-capacity dryer represents a significant capital investment that must be paid back primarily from drying revenues. The decision between long-term profitability and initial investment must be weighed carefully.
Although a short payback period of two to four years is desirable, future events such as stricter environmental regulations could turn what initially appears to be a long payback period into a much shorter one.
Cost factors when determining operating expenses include service life and availability, fuel, expansion feasibility, reliability, labor, taxes, insurance and maintenance. Service life and availability tie in closely with maintenance and reliability.
A short drying season means any breakdown or delayed repair can have a significant effect on drying business. An undersized dryer or one that frequently breaks down will turn customers to competing elevators. Once lost, drying business is not recoverable during a current season, and possibly not even in a future season.
Future growth of an elevator operation should allow for expansion of the drying facility either by adding a second dryer in parallel, or by adding additional stages to an existing system.
Annual operating costs can be calculated from the total amount of grain dried and fuel used, the average incoming moisture content, receiving volume and weather conditions. To improve accuracy, separate meters should be installed to monitor fuel and electricity consumption of the dryer itself.
Additionally, peak receiving volume should be monitored for future planning purposes. Operational data suggest that the higher the receiving volume, the lower the cost per tonne per point of moisture removed. The cut-off after which no significant reduction in operating costs appears to occur is about 25,400 tonnes per year.
Elevator operators need to consider that they not only compete with other elevators for drying business, but also with on-farm systems. If farmers can dry their crops cheaper, they will likely build or expand their own facilities.
Three areas are expected to affect significantly the drying business of grain elevators in the future: regulations, grain quality and dryer design.
Environmental regulations are one example. The United States and other countries are implementing rules setting specific limits on particulate matter (PM) emissions from drying and handling operations. Although current emission controls focus on PM, grain handling and processing facilities in the future also will need to address pollution from noise, odor and combustion products.
In the future, energy efficiency is expected to be regulated, as well. Efficiency standards already have been implemented in some other industries, such as the average fleet fuel rating for automobiles. Drying systems that do not maximize energy efficiency will be unacceptable in the future.
Also, safety standards will continue to be tightened, affecting maintenance and operation of drying and handling systems.
Stricter end user demands will continue to increase the importance of grain quality throughout the entire grain production and processing chain. The handling of grain according to specific intrinsic characteristics such as oil, protein and starch content may become the norm, and bulk commodity maize as it is known now may become a “specialty” grain.
This will require separate handling of possibly smaller batches and a grain flow through drying and storage facilities that will avoid destruction, contamination and blending of high-quality grains. Already, processors separate grain according to color (white and yellow food maize), and starch content (waxy maize). Large integrated livestock feeders are expected to follow this example, basing separation on nutrient content of feed grains.
For such a system to develop effectively, end users will have to pay premiums for grains handled to meet these higher quality standards. Grain handlers will need to pass these incentives on to producers because it is there that the identity separation is first initiated.
In the future, dryers will be designed, built and operated differently as well. “Smart” processing systems will automate many manual tasks and make it easier for operators to track performance of their dryers more closely.
Temperatures, moisture contents, grain quality and grain flow will be automatically controlled, and records including weather conditions and fuel consumption will be linked directly with the company data base. Critical to this automation will be the development of reliable sensors that withstand the harsh agricultural environment.
There will also likely be a demand for the use of alternative fuel sources for drying. Previous biomass burner designs will be improved, and the demand for ethanol-based fuels for drying will be answered. In overly strict clean air districts, the only feasible option may be electric grain dryers operating in conjunction with a new generation of super efficient dust, noise and odor separation equipment.
The bottom line for dryer design will be demand leading the supply. In other words, regulations and the quality demands of elevators and processors will force dryer companies to manufacture highly efficient, cost-effective and innovative new dryers.
Finally, in the short-term, many of the currently available options for crossflow dryers such as turning devices, tempering zones, moisture-based controllers, full heat drying and multiple stages will become standard features. Additionally, use of slow cooling applications and mixed and concurrent flow dryers will increase.
Dirk E. Maier is assistant professor and extension agricultural engineer at Purdue University, West Lafayette, Indiana, U.S. He specializes in post harvest engineering, including grain and feed handling, drying, storage and processing. This article is adapted from his 1994 presentation to the Grain Elevator and Processing Society.