c-11heated air drying

8/2/2019 C-11Heated Air Drying

1/10

Rice Quality Workshop 2003

1

High Temperature Rice Drying

Goals of high temperature drying

Rice should be dried with minimum quality loss and as quickly as possible. In hightemperature dryers this is done by:

1) starting drying within 8 hours of harvest if rice is greater than 24% moisture,

2) drying in several passes with limited amounts of moisture removal depending on rice

moisture,

3) temper rice for at least 2 hours after a drying pass, but not long enough to cause off-odor

development caused by microbial activity,

Continuous-flow dryers

Over 80% of California's rice is dried in continuous-flow, heated-air dryers. The most

common type is called a column dryer, Figure 1, where rice flows by gravity downward

between two screens, separated by 6 to 12 inches. Heated air flows horizontally through the

screens. (This dryer is sometimes called a cross-flow dryer because the air flows at a 90

degree angle to the flow of rice.) Metering rolls, at the bottom of the screens, control the rate

of rice flow through the dryer. Rice is removed from the metering rolls with screw

conveyors. Air is supplied at a rate of 2 to 4 cubic feet per minute-pound and heated to 130

to 165F depending on rice residence time in the dryer.

This type of dryer has several disadvantages compared with mixing-type dryers. Rice tends

to flow straight down between the screens, without much horizontal movement. This means

that the rice close to the hot air plenum is always exposed to the hottest air and dries more

than the rice next to the screen near the air exhaust. Also rice next to the screens flowsslower than the rice in the middle between the two screens. This leads to even more

variability in the amount of drying experienced by individual grains. Some dryer designs

include mixing sections to reduce moisture variability. Commercial operators have found

that finger-type mixers are very effective in minimizing kernel to kernel moisture variability.


2/10


2

Two other types of dryers are designed to mix the rice as it flows through the dryer. The

baffle dryer, Figure 2, causes the rice to flow in a zig-zag pattern and heated air flows

through openings between baffles. Distance between baffles is generally about 6 inches.

The LSU dryer, Figure 3, is a large rectangular solid with a series of inverted V-shapedtroughs running across the entire width of the dryer. The rice flows downward past the

troughs, but dose not fill them. The open space below each trough is used to distribute

heated air through the rice. Alternating layers of troughs are used for air-supply or air-

exhaust channels. Air takes the shortest path from an air supply duct through the grain to an

exhaust duct. In some designs the air supply troughs are oriented at a right angle to the air

exhaust troughs. Rice mixes as it flows around the air channels.

The LSU type dryer costs significantly more than an equivalent capacity column dryer

primarily because of the air pollution equipment needed to control particulate emissions.

Because of their higher cost few of these dryers have been installed in recent years. Most

rice drying operations in California use column dryers.

Rice is subject to lower milling yields if it is dried too quickly. Quality is maintained by

drying in several 20 to 30 minute long passes through a dryer. Between passes, rice is stored

in temporary holding bins. This is called tempering and allows moisture to equalize within

kernels. Most tempering is accomplished in about four hours although drying schedules

often dictate that the rice must be held for about 24 hours between passes.

A recirculating batch dryer, Figure 4, is the most widely used system in Asia. Paddy is

loaded into the tempering section and it slowly flows downward to the drying section. After

passing through the drying section it is returned to the tempering section and the process is

repeated until the batch of rice is dry. The units usually remove water a rate of 0.6 to 1.0

%/hr and have holding capacities of 800 kg to 20 tons.


3/10


3


4/10


4


5/10


5

Rice can be completely dried in a column dryer or dried to 16% to 18% and then finished

with unheated air in flat storage or perforated-floor grain bin. (See the chapter on bin drying

for operational details.) The two-stage system works only if flat storage has adequate airflow

for drying, which is much greater than needed for storage aeration. Late season drying in flatstorage may be slow because of low ambient air temperatures.

Before drying

Rice should be brought to the dryer as quickly as possible to prevent damage caused by

microbial (bacteria, yeasts, fungi, etc.) growth in wet grain. Wet grain held too long may

develop off-odors, off-flavor, yellow kernels, and may even become contaminated with

toxins. Damage susceptibility related to rice moisture. Rice less than 21% moisture can besafely held for 24 hours before drying begins, Figure 5. Rice greater than 23% should never

be held over night before unloading at the drying facility. Rice harvested at 25% moisture

should begin drying within 8 hours after harvest. The safe delay period is total time from

actual removal from the plant until the rice is exposed to drying air. Commercially, this

means that the drying facility must be located within in an hour or two of the field and

harvest and drying operations must be closely coordinated so that the rice starts drying

shortly after it is received. Wet rice should be aerated and cooled if it must be held for morethan 8 hours before drying. Rice should not be harvested at moistures greater than 25%. It is

so microbially active that it is difficult to start the drying process quickly enough to prevent

off-odor development. Rice at 25% moisture is also expensive to dry and will probably not

have any better head rice quality than it would have if it were harvested at 21% to 23%

moisture.

Pass rice through a cleaner (scalper) to remove foreign material before drying. Straw can

collect in the dryer and block rice flow. Fines reduce airflow and concentrations of foreign

material may cause hot spots in storage. Some dryers are set up to clean the rice after every

pass through the dryer.


6/10


6

0

2

4

6

8

10

12

14

18 20 22 24 26

Moisture (%)

S a

f e s

t o r a g e

T i m e

( d a y s

)

8 hr16 hr

Figure 5. Safe delay period between harvest and the beginning of drying. Figure is based on

data from Champagne, et al., 2003 and Matsuo et al., 1995. Data were adjusted assumingthat commercial loads may have significant amounts of rice at moistures 2% higher than theload average.

Dryer operation

A dryer is controlled by setting air temperature and rice discharge rates with the goals of

operating at maximum drying capacity while maintaining milling yield.

Principles of dryer operation:

1. Milling yield is kept high by limiting the points of moisture removed per pass.Rice is increasingly subject to cracking with increasing difference between moisture in the

center versus the outside of the kernel. Tempering equalizes moisture, so the next pass

can safely remove additional moisture. Generally more points of moisture can be safely

removed when rice is wetter.

2. Capacity is greatest when using high air temperature and fast discharge rates.

Moisture removal is fastest at the beginning of drying, the first point of moisture is

released much faster than the second point in a drying pass.

In practice these principles tell an operator to:

1) use fastest possible discharge rates,

2) use highest possible drying air temperature and

3) regularly check to guarantee that milling yield is not unacceptably lowered.


7/10


7

Table 1 shows the value of operating dryers at high discharge rates and high air temperatures.

The first series of tests shows that doubling discharge rate and keeping constant air

temperature increases drying capacity by an average of almost 10%, increases head rice by

almost 4%, but requires two to three more passes. Increasing air temperature from 118 -119 F to 136 F and keeping a constant discharge rate causes a 40% increase in drying

capacity, reduced the number of passes, but reduced head rice by 1.1 to 3.4%. When air

temperature and discharge rate are both increased, drying capacity increased by an average of

almost 50%, head rice was slightly reduced, and passes were increased by one or two.

Table 1. Effect of different column dryer operating conditions on rice quality and dryer capacity. Wasserman, 1958a, 1958b.

TestDryer

1 Dischargerate

(cwt/hr)

Air temperature

(F)

Capacitygain2

Head ricegain

No. passes

Increasedischarge A 960 135 0 0 3rate 1240 136 5% 1.6% 4

1760 137 10% 5.2% 5

B 990 117 0 0 31910 118 8% 2% 6

Increase air temperature A 1260 119 0 0 4

1240 136 50% -1.1% 43

B 1910 118 0 0 61910 129 12% -2.9% 51910 136 26% -3.4% 4

Increase 4 A 1260 119 0 0 4discharge 1760 137 58% 2.5% 5and air temperature B 960 117 0 0 3

1910 136 37% -1.1 51 Dryer A is a LSU-type mixing dryer and dryer B is cross-flow column dryer.2 Capacity gain is based on pounds of water removed per hour.3 High temperature test removed more total moisture than low temperature test.4 Increasing discharge rate and temperature caused a 15% reduction in fuel use for dryer A anda 3% increase for dryer B.

Best operating conditions must be determined for each installation and temperature and

discharge rates may vary depending on variety and incoming moisture. Dryer operators need

to keep good records of incoming and final quality of each drying lot.


8/10


8

Dryer testing

Finding the optimum discharge rate and air temperature is done by measuring the milling

yield of rice dried under the following conditions:1. with the usual air temperature and discharge rate settings, and

2. operating the dryer at 130F and highest possible feed rate.

Based on these results adjust only air temperature to get an acceptable head and total loss in

the dryer. Many dryers are operated to produce losses of less than one point of head rice and

two points of total. Repeat the testing of the new settings for several runs to sure of the

results. Do not test dryers during periods of wet weather or dry North winds. Testing

should be repeated during the season and particularly for different rice types. Japanese

varieties may have different drying characteristics than our locally developed varieties.

Milling yield samples should be collected for rice entering the first pass and rice leaving the

last pass. Collect samples periodically as the lot is dried and then split the composite sample

to get an adequate amount for a milling test. Milling tests are usually done on rice at less

than 13.5% moisture. Incompletely dried samples must be dried on a sample dryer. The

dryer should have continuous air flow and 75F air temperature or fan should cycle on for 2

minutes with a 110F air temperature and off for 28 minutes.

Some dryer operators use higher air temperatures and lower discharge rates in the first pass

than in subsequent passes. This can cause serious quality loss in the driest rice in a lot. Base

dryer settings on the driest part of a lot rather than average moisture. Carefully test the losses

caused by high temperature, low discharge rate passes.

Tempering

After passing through a high temperature dryer, moisture within the kernel must be allowed

to equalize. Maximum head yield is obtained with four hours of tempering at 105F and with

6 hours of tempering at 75F. Tempering also increases the amount of moisture loss in


9/10


9

subsequent passes. At higher moistures above 18% to 20%, maximum moisture loss is

attained after 4 hours of tempering. At lower moistures, 12 or more hours of tempering may

be required for maximum moisture loss. Although, there is only a small increase in moisture

removal caused by 12 hours versus four hours of tempering.

Harvest rates are high enough during the peak of the season so that it is often necessary to

store rice in the temping bins for more than 24 hours. At high rice moistures, long tempering

times may cause quality loss. Aeration of tempering rice will cool it and allow it to be held

for longer periods before damage occurs. Approximately 1.8 pounds of aeration air per

pound of rice is needed for complete cooling. (Fifteen cubic feet of air weighs about one

pound and the air can be supplied over a range of times.) Aeration and complete cooling will

reduce rice moisture by about 1/2%. Unfortunately, most tempering bins are quite tall and

fan power is quite large for the airflow needed for reasonable rates of grain cooling. Cooling

rice before tempering is not recommended because it reduces milling quality.

Energy use and conservation

Energy cost for drying is about evenly divided between natural gas or propane and

electricity. Typical energy use is about 7 therms of natural gas and 43 kWh of electricity per

ton of dried rice. Fuel is used for air heating and electricity is used for operating and the

dryer fan and grain conveyors and elevators.

Fuel use can be minimized by following the recommendations for increasing dryer capacity

and increasing head rice yield. These recommendations reduce the total amount of total time

the rice is actually exposed to heated air in the dryer and therefore reduce fuel use. Although

operators may choose to also increase drying air temperature in addition to increasing the

number of drying passes and this will offset some of the fuel savings.

Air recirculation is a common method of reducing fuel use in dryers but is not often used in

rice dryers. Air exhausting from the dryer has a great deal of fine particles in it, that are a

fire hazard if these particles are exposed to the flame in direct-fired air heaters.


10/10


10

Fan energy use is related to the total amount of time the rice is exposed to drying air.

Increasing the number of drying passes will reduce this time but will increase electricity use

for moving rice into and out of the dryer.

References

1. Champagne, E. T., J. Thompson, K. L. Bett-Garber, R. Mutters, J. A. Miller 1 andE. Tan . 2003. Impact of storage of freshly harvested paddy rice on milled whiterice flavor. Journal of Food Chem.

2. Hosokawa, A. ed. 1995. Rice post-harvest technology. Japan ministry of ag. food &fisheries, Tokyo.

3. Kunze, O. R.,D. L. Calderwood. 1985. Rough rice drying. in Rice chemistry andtechnology, B. O. Juliano ed. Am. Assn. of Cereal Chemists, St. Paul, MN.

4. Matsuo, T. K., Kumazawa, R.,Ishii, K. Ishihara, H. Hirato. 1995. Science of the rice plant, vol. II. Food & Ag. Policy Res. Center, Tokyo.

5. Steffe, J. F., R. P. Singh. G.E. Miller. 1980. Harvest, Drying, and Storage of roughrice. In Rice production and utilization. B. S. Luh. AVI Pub Co. Inc. Westport CT.

6. Wasserman, T., R. E. Ferrel, V. F. Kaufman, G. S. Smith, E. B. Kester, J. G.Leathers. 1958. Improvements in commercial drying of western rice, part I mixingtype dryer. The Rice Journal, Apr-May.

7. Wasserman, T., R. E. Ferrel, V. F. Kaufman, G. S. Smith, E. B. Kester, J. G.Leathers. 1958. Improvements in commercial drying of western rice, part II non-mixing columnar-type dryer. The Rice Journal, Apr-May.

8. Wasserman, T., M. D. Miller, W. G. Golden. 1965. Heated air drying of Californiarice in column dryers. California Ag. Expt. Station leaflet 184.

c-11heated air drying

Documents