On-Farm Rice Drying and Storage

Air is typically being used as a tool for rice drying by forcing large quantities of unheated or heated air though the grain bulk. Air temperature and relative humidity (RH) are the two key characteristics that determine the drying rate and the grain final moisture content. For a given air temperature and relative humidity, rice will only loose moisture until a certain moisture and  eventually achieve a state of equilibrium with the environment. This property is called "Equilibrium Moisture Content” (EMC). Thus, temperature and relative humidity properties of the drying air determine rice moisture level. The EMC may be determined by measuring air temperature and relative humidity. A sling psychrometer is one of the tools used for measuring relative humidity, and is relatively inexpensive. The following table present the EMC of rice in equilibrium with air at various temperatures and relative humidity levels. To illustrate the use of the EMC table, assume that air at 70% relative humidity and 60oF temperature is being forced through rice kernels in the bulk, rice will not dry below 15.1%. If air under the same RH (70%) is heated, the EMC will be reduced.

Accordingly, rice may be dried without adding any heat to the drying air if the EMC is low enough as compared to rice moisture content. Careful monitoring of the EMC and management of drying times to optimize low values will provide the most economical drying. In many cases, particularly at night, the addition of heat is needed to condition the air to the correct its EMC.

 To determine the value of EMC in your area now, click the link Determination of the current temperature and relative humidity based on your zip code.

To learn more about Equilibrium Moisture Content, see the fact sheet titled: "Grain Drying Tools: Equilibrium Moisture Content Tables and Psychrometric Charts"

To determine the values of EMC for various grains, download the Excel sheet by clicking the link: "Equilibrium Moisture Content" 

• As mentioned earlier, air is the medium used to carry moisture away from the rice during drying and conditioning. The air is typically forced into the bottom section of a bin under a perforated floor supporting the rough rice using one or more fans. This open area is called the plenum. The fans are mounted to the plenum using a transition that allows even distribution of air flowing through the bed of rice. Even air distribution is critical to allow complete drying of the rough rice and avoiding “hot spots” where sections of rice do not dry properly resulting in spoilage. Rice producers should select the manufactured fan that best fits their drying needs. Over sizing, the fan leads to unnecessary energy consumption in the form of electricity from the fan motor and gas or electricity from the air heater. The greater the airflow rate the more energy required to heat the air to a specific temperature. Also, higher airflow rates through the rough rice result in higher pressure requirements for the fan resulting in higher purchase costs for the fan and less efficient operation. On the other hand, under sizing the fan size will cause too little airflow resulting in drying being too slow. Airflow rate through the rough rice and air temperature directly control the drying rate. The higher the airflow rate and temperature accelerate the drying rate and increase the cost. Proper dryer operation allows rice to reach a sufficiently low moisture content to prevent spoilage. Properly designed dryers optimize the balance between airflow rate and air temperature to dry rice to the proper moisture content to maximize quality while minimizing overall costs.

• Grain drying fans are classified as either axial-flow or centrifugal flow. Each type of classification could be used to optimize the airflow rate and minimize the energy consumption for maintaining rice quality. In both types, air is forced into the bin by the fan. Axial-flow fans move air parallel to the axis or impeller shaft. This type of fan is suitable for grains that create low static pressure, less than 4 inches of water which generally not the case of rice. The axial flow fans, however, could provide adequate airflow for aeration of already dried rice, which required much less airflow than when drying. Axial flow fans are also typically creates much more noise during operation than centrifugal fans, which should be considered when locating drying facilities near residences. The second type of grain drying fans is the centrifugal fan. In the centrifugal fans, air enters one end of the impeller parallel to the shaft and exits perpendicular to the shaft. Centrifugal fans used for grain drying and storage generally have backward-curved blades. They are usually the most efficient type of fans when static pressure is greater than 4 inches of water and are typically capable of generating much greater pressure than axial fans. Because of high resistance to air flow generated by rough rice, centrifugal fans are the ideal fans to use for drying operations, which generally require moving air flow  rates of 1 to 4 cfm/bu. Centrifugal fans also operate with less noise than axial fans.

  The actual amount of air needed to dry rough rice depends on its initial moisture content. The following table shows the minimum recommended airflow rate to dry rough rice at various levels of initial moisture contents. Fans sized specifically for corn or soybeans will not move enough air for rice placed in bins at the same grain bed depths due to the higher static pressure developed by rice.

   Recommended Minimum Airflow Rates for Drying Rough Rice

  Initial Moisture Content (%)	Airflow Rate (cfm per bushel)
  11% to 13%	0.5
  13% to 15%	1.0 cfm/bu
  15% to 18%	2.0 cfm/bu
  18% to 20%	>3.0 cfm/bu

• To select and maintain your fan, please see the fact sheet titled "Selection, Performance and Maintenance of Grain Bin Fans"

Rice kernels contain dry matter and water. The dry matter translates to the actual value of rice. A base moisture content of 13.0% is typically used to price rice. Variations of the rice moisture content change the weight of the water in the it and its weight per standard bushel as shown in the following table.

When rice is delivered to the elevator at moisture content above its base standard, buyers apply a “shrink factor” to adjust the quantity for the excess moisture. This is because rice buyers will not pay the price of excess water. Applying the shrink factor approximates the equivalent number of bushels that would be in the load if rice were dried to the base moisture content. Conversely, some farmers often deliver wheat to the elevator at moisture levels below the base. There will be no compensation factor increasing the price of the rice in this case, so it is very important to avoid over drying of the rice to prevent lowering its the value.

The following example will provide an illustration of applying the shrink factor on rice. Suppose a farmer harvests a portion of a field and dries the rice to the market standard moisture content of 13.0% and the dried wheat weighs 100,000 pounds. This means that the weight of the water in the wheat is 0.130 x 100,000 = 13,000 pounds. The weight of the dry matter in the wheat is 100,000 – 13,000 = 87,000 pounds. At 45 lbs per bushel at standard base moisture content of 13.0%, the farmer will get credit for 100,000 pounds/45 pounds per bushel = 2,222 bushels of rice. 

Alternatively, suppose this same portion of the field is harvested and the rice is dried to 16% moisture content. The farmer would still harvest 87,000 lb of dry matter, but rice would now contain more water because of the higher moisture content. At 16% moisture content, the total weight of the water in this rice is now 16,571 pounds. The total weight of rice is 87,000 + 16,571= 103,571 pounds. As mentioned earlier, the elevator will not be willing to buy excess water. The elevator applies the shrink factor to adjust the quantity. From the previous table, the market standard bushel weight at 16% moisture content is 46.61 pounds per bushel. The total bushels calculated accounting for the greater moisture content is now 103,571 pounds divided by 46.61 pounds per bushel equals 2,222 bushels, which is the same as if the moisture content were 13.0%. The total price the farmer would receive remains the same. However, the farmer would also need to pay drying costs to the elevator to remove the extra moisture.

• In-Bin Drying

In-bin rice drying processes can utilize either natural air (unheated) or low temperature air (slightly heated usually less than 10 °F) to dry grain in bins (see photo below). The air is forced up through the grain with fans until the grain moisture content is sufficiently reduced. This is typically done in bins with a raised perforated floor to ensure even airflow, but can also be done using air ducts laid on the concrete bin floor prior to adding grain. Rough rice is among the grains that offer high resistance to airflow requiring a significant pressure output from the fans, which is represented as static pressure in inches of water. In-bin drying methods used for other crops such as corn and soybeans can be adapted to work properly for rice if some adjustments are made to compensate for rice high resistance to air flow. The simplest adjustment is to reduce rice depth in the bin to half that normally used for corn or soybeans. In addition, the installation of appropriately sized centrifugal fans may help deliver higher airflow rates under higher static pressures.Bin capacity measured in bushels of grain shown in Table 5 increases by increasing the bin diameter and/or the grain depth. For example, a grain bin with 28 ft diameter filled to a level height of 16 ft height can hold up to 7,882 bushels of rice. Increasing the grain depth increases the static pressure that the fan has to overcome to provide the same cfm/bu. For example, if air is delivered to a grain bin that holds 9,048 bushels creating static pressure of 8.00 inches of water, the airflow rate will not exceed 1.77 cfm/bu. The following table can be used to select rice depth for known moisture content and fan size. If high moisture rice is to be dried and stored in the same bin, extra care is advised. If the initial moisture content is 17% or more, use heat to dry the top layer to less than 17% before adding more rice. Generally, moderate airflow (2-5 cfm/bu) along with a temperature-rise less than 20°F are required. Stirring devices, re-circulators, or automatic unloading augers can be used to increase drying rate. After drying rice to 17%, use unheated air to dry it to about 15%. During this period, run the fan continuously to provide uniform drying and moisture distribution within the rice. Operate drying fans only during low humidity hours to finish drying to around 13.0%. This management scheme will minimize the amount of rice over drying in the bottom of the bin. It should be mentioned that excess heat could cause severe over drying. Table 7 shows the approximate degrees of heat needed to dry wheat to 12% moisture content.

 

Grain bin

 

Number of wheat bushels in grain bins

    Maximum safe drying depth for wheat with typical bin and fan combinations.

 

Approximate degrees of heat needed to dry wheat to 12% moisture content.

 

Batch and Continuous Flow Drying

High temperature batch or continuous flow dryers are usually used to dry large capacities of rice. These units typically have very high airflow rates, and they do not require supplemental heat for daytime drying when harvesting rice at 18-20% moisture range. If heat is used, the drying air temperature can be limited by cycling the burner on and off or by changing the gas burner orifices.

Listed below are recommended steps provided by researchers and grain processing experts that can be adapted for on-farm rice drying. Following these steps can greatly increase the chance of having one of the best rice-drying seasons for your farm, which in turn could lead to an increase in total farm gross profit. 

Tips for Rice Harvesting:

• It is recommended to harvest rice at 19 to 21% MC (wet basis) for long-grain cultivars and 22 to 24% MC for medium-grain cultivars for maximum head rice yield. 
• Clean the rice to be dried as deemed practicable by adjusting harvesting equipment settings to minimize the amount of foreign materials in the bulk grain.
• Determine the MC of incoming rice using a calibrated moisture meter instrument and avoid mixing rice lots of different MCs; it is possible that mixing of low and high MC kernels can cause fissuring of low MC kernels by rapid adsorption of moisture from high MC kernels.  Rice at low average MCs (e.g. 15% and below) standing in the field have significant number of kernels with MCs critically low that may develop fissures when exposed to high MC kernels. Fissured kernels usually break during milling and results in reduction of Head Rice Yield HRY.

 Tips for Rice Drying:

• It is best to place the rice harvested first in the drying bin at a depth of 6 to 12 feet. This depth is dependent on the initial MC of the bulk rice and the capacity of the fans.
• Level the rice uniformly across the entire drying bin at the depth selected. It is very important to level the rice in order to distribute the air pressure equally throughout the entire horizontal cross-section of the bin to obtain uniform airflow.
• Open the bin exit windows to allow exhaust air to readily exit from the drying bin.
• Turn on the fans as soon as the ducts or the perforated floor is covered with approximately 1 foot or more of rice.
• If possible, do not hold wet rice in a bin, truck, combine hopper or grain cart longer than 12 hours without moving air through the container to cool the rice.
• Measure the relative humidity and temperature of the ambient air to determine the maximum temperature setting of the heater.
• It is highly recommended to exercise extreme caution when drying air temperature exceeds 100oF.
• Dry high moisture rice in shallow batches until the rice MC is 15% or less. Then, deeper depths with lower airflow requirements are acceptable.
• Drying time per batch is determined by airflow rate, measured as cubic feet per minute (CFM) per hundredweight (cwt), temperature difference between air entering and leaving the rice, the MC of the ambient air, and the original MC of the rice.
• One way to reduce drying time is to increase airflow, but this may not be the most energy efficient manner of drying.
• Once the rice has reached 15% moisture, move it to another bin where the depth can be increased and the airflow per hundredweight (cwt) can be decreased. Drying can be continued until the MC reached 12.5 to 13%.  Expect the drying rate of low MC rice to be much slower than high MC rice.

Tips for Rice Storage:

• Once the rice is about 12.5 to 13.0% moisture through the entire depth of storage, fill the storage bin and level the grain surface.
• Stored bulk rice generates heat inside the bin.  It is best to aerate, to cool the  bulk rice  using ambient air for the next few weeks, but aeration should only be done when the ambient air humidity is  below 60% and the air temperature is cool (50 to 60oF).
• Do not operate fans when ambient temperature is below 32oF.
• Try to monitor the bulk rice in the bin periodically (once a week is ideal) for temperature or moisture content variation.
• Moisture migration occurs in bulk rice in the bin. This phenomenon is caused by improper control of temperature in the bin making moisture to move or migrate from one section of the grain mass to another.  When this happens, moisture accumulates and cause kernel deterioration or spoilage. Normally, the usual place that moisture migrates to is on the center of the top layer during winter. If there is any indication, that moisture or temperature is increasing in this area or other area turn on the fans to cool and/or dry moistened rice.
• It is very important not to allow any spoiled rice mixed with good rice.
• Periodic aeration may be necessary to counter extreme temperature changes during storage.

As mentioned in the previous sections, that grain drying could be accomplished by utilization of high-pressure fans without heat. Other drying equipment utilizes natural gas or propane for heat and electricity to power fans to move the heated air through the drying system. Both types of systems dry grain effectively (depending on weather conditions) they contribute to the energy bill. As a result, reducing the grain drying costs is the goal of all producers. Following are some tips that might help producers reducing the drying costs.

• Perform annual maintains on grain dryers. This can reduce the energy cost and increase the lifespan of equipment. Dirt, poor bearings lubrication, and inaccurately calibration of thermostats reduce the unit's efficiency.
• Some researchers estimated that proper maintenance of grain drying equipment could save 10% on the energy bill.
• Clean drying bins and make sure that the floors, columns, screens, fan housings, and fan blades are dirt free.
• Check the drain holes to make sure that they are open and not clogged with dirt.
• Tighten the belt drives to ensure they are at appropriate tension levels. Review the alignment of the pulleys.
• Lubricate all bearings. Check all mounting bolts and secure locking collars.
• Calibrate grain moisture sensors and thermostats. Normally compare them to a certified unit.
• Check the gas pressure regulators. You may need to call the Gas Company or liquid propane supplier to perform this check.
• Check the color of the burner flame for proper combustion. Blue color indicates complete combustion, while yellow color indicates poor combustion. 

 

Working with either moving and/or stationary grain requires special precautions and training. Therefore, it is necessary to keep in mind the following tips while working with rough rice as well as other grain:

• Do not inspect grain bin alone.Always request help from coworkers when entering a bin. Inspection requires at least one worker inside the bin with a safety harness and one outside to assist if needed. When entering a questionable bin, ask two workers to stay outside for assistance if needed. A safety rope should be attached to the worker who is entering the bin. The workers standing outside must be capable of pulling the person inside if emergency arises.
• Avoid entering into a grain bin or gravity unload vehicle when grain is flowing.It should be mentioned that several accidental deaths occurred during handling and unloading grain. Therefore, lock out the control circuit on automatic unloading equipment before entering or cleaning a bin or repairing conveyors. Flag the switch on manual equipment so someone else does not start it. Do not enter a bin unless you know the nature of previous grain removal, especially if any crusting is evident.
• Avoid walking on any surface crust.Crusted or bridged grain can collapse and could bury workers. Do not depend on a second person-on the bin roof, on the ground, or at some remote point-to start or stop equipment on your shouted instructions. If a grain bin is peaked close to the roof, be extremely cautious. Crawling between roof and peak can cave grain and block the exit.
• Wear appropriate masks when working around dusts.Always wear a respirator mask capable of filtering fine dust to work in obviously dusty-moldy grain. Never work in such conditions, even with protection, without a second person on safety standby. Exposure to and inhaling mold can cause severe allergic reactions.
• Be alert while working with out - of condition grain.Grain that has gone out of condition may contain molds, cavities, cave-ins, or crusting.
• Exercise caution while working with flowing grain.Flowing grain can trap and suffocate a worker in seconds. Moreover, the noise coming out of the equipment further blocks the shouts for assistance. Even with moderate flow rates of a 6" auger, a worker is trapped only 2-4 seconds after stepping into the cone of flowing grain. This worker would be totally submerged within 20 seconds at a grain flow rate of 1,000 bu/h.
• Electrocution from grain augers.Grain augurs are usually as long as 40 to 50 feet, and are used to place grains in bins. Sometime while moving the auger from one bin to other, there is a chance that the upper end of it touches overhead power cable. Caution should be exercised while maneuvering the grain augers. Or, in the first place consider installing underground power cables or make an arrangement to move the wires too close to metal bins.