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Paddy Drying Systems By: M Gummert J Rickman Agricultural Engineering Unit IRRI, Los Baños , Philippines

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Rice Paddy Drying systems

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Page 1: Paddy Drying Systems

Paddy Drying Systems

By: M GummertJ Rickman Agricultural Engineering UnitIRRI, Los Baños , Philippines

Page 2: Paddy Drying Systems

Content

• Why drying• Drying and quality• Drying methods• Mechanical drying systems• Drying strategies

Page 3: Paddy Drying Systems

Why Drying?

• Rice is harvested at high moisture contents >20%

• Quality deterioration starts immediately after harvest

• The wetter the grain the faster the loss of quality

• Different MC for different purposes (see Table)

Purpose Required MC

Potential problems

2 - 3 weeks storage 14 - 18% Molds, discoloration, respiration loss

8 - 12 months storage

<= 13% Insect damage

> 1 year storage <= 9 % Loss of viability

Milling 14% Damaged grains, cracking

Page 4: Paddy Drying Systems

Improper drying, 1

• Heat build-up– from natural respiration– excellent growth conditions for molds and

insects• Mold development

– propagate diseases in the grain– may release toxins into the grain– proper drying and storage can reduce

propagation of molds• Insect infestation

– insects are always a problem in stored grain

– at lower moisture content insect activities are lower

– proper drying helps keeping insects at acceptable level (4 insects per kg)

Page 5: Paddy Drying Systems

Improper drying, 2

• Discoloration/Yellowing– heat build-up in the paddy grain

before drying– drastically reduces the market

value of rice• Loss of germination and vigor

– active respiration depletes the nutrition reserves

– molds and diseases can reduce the ability of the seed to germinate

– the lower the MC at the beginning of storage, the longer the seed remains viable

Page 6: Paddy Drying Systems

Improper drying, 3

• Loss of freshness/odor development– Heat build up -> musty odor in rice. – Reduces the market value of rice. – If from mycotoxin-producing fungi rice

might become unusable.• Reduced head rice yield

– moisture adsorption of individual dry grains with moisture contents below 16% - fissuring

mixing dry with wet grains Exposing dry grains to humid air

– Fissures cause cracking in milling process -> reduced the head rice recovery.

Page 7: Paddy Drying Systems

Equilibrium moisture content (EMC)

• Rice is hygroscopic

• Equilibrium moisture content (EMC)

– If grain is exposed to air for a longer time it will reach EMC

– Dry grain will adsorb water from humid air

– Wet grain will dry

• Grain properties– Moisture Content, MC– Temperature

• Air properties– Relative Humidity, RH– Temperature

Purpose Required MC

Potential problems

2 - 3 weeks storage 14 - 18% Molds, discoloration, respiration loss

8 - 12 months storage <= 13% Insect damage

> 1 year storage <= 9 % Loss of viability

Milling 14% Damaged grains, cracking

Page 8: Paddy Drying Systems

Drying methods

Drying

Traditional Methods (Sun drying)

Mechanical drying

Field Drying

Panicle drying

Mat drying

Heated-Air DryingLow-Temperature

Drying

Batch dryer

Re-circulating Batch Dryer

Continuous Flow Dryer

Flash Drying

Aeration

In/store drying

Pavement drying

Page 9: Paddy Drying Systems

Field drying

• Why field drying?– Waiting for the thresher– Manual threshing

• How?– Spreading the crop in the field– Stacking/piling

• Advantages– Can reduce MC by 1% per day

• Disadvantages– Rapid quality deterioration– Shattering– Losses to bird and rodents when spread in

the field– Heat build up and rapid quality deterioration

in piles– Re-wetting from straw in piles

It is impossible to produce good quality grains with field drying practices. Field drying should therefore be avoided.

Page 10: Paddy Drying Systems

Panicle drying

• Traditional method– harvested with a small knife (ani ani)– paddy grains that are still attached

to the panicles– for drying small amounts of paddy. – stored in farmers’ houses, e.g. under

the roof for protection from rodents.• Problems

– low capacity– grains inside the panicle dry slower

than the grains that are exposed directly to the sun.

• Tips– turning of the panicles improves the

drying process

Page 11: Paddy Drying Systems

Mat drying

• Paddy can be placed on nets, mats or plastic sheets (canvas).

• Advantages– Most hygienic method. – less contamination with

stones and other dirt– Easy collection in case of

sudden rainfalls. – Easy mixing

• Disadvantages of nets– danger of re-wetting of

the bottom grains from soil moisture

Page 12: Paddy Drying Systems

Pavement drying

• Better-off farmers, grain collectors, traders and millers use drying pavements

– specifically constructed for drying– multi purposes (basketball court)

• Advantages– high capacity / economics of scale– can be partially mechanized– tools for mixing and grain

collection– larger mills often use two or four

wheel tractors• Disadvantages

– capital requirements for the pavement

– pollution with stones and dirt

Page 13: Paddy Drying Systems

Sundrying and quality

Layer thickness• Spread the grains in thin layers,

ideally 2-4 cm. • Too thin layers -> heat up very

quickly• Too thick result in a large

moisture gradient

Mixing interval• mixing the grain is the most

important activity for maintaining good quality

• Turn or stir the grain at least once per hour, better every 30 minutes to achieve uniform MC.

60

65

70

75

80

85

90

0 1 2 3 4 5 6 7

Mixing interval, h

Rec

ove

ry, %

5

5.25

5.5

5.75

6

6.25

6.5

Dry

ing

Tim

e, h

Head rice

Milled rice

Drying time

Source: IRRI, (Gayanilo)

Location: Philippines

Intital M.C. = 24%

Final M.C. = 14%

74

76

78

80

82

84

86

88

90

0 1 2 3 4 5 6 7

Layer thickness, cm

Rec

ove

ry, %

3

3.5

4

4.5

55.5

6

6.5

7

7.5

8

Dry

ing

Tim

e, h

Head rice

Milled rice

Drying time

Source: IRRI, (Gayanilo)

Location: Philippines

Intital M.C. = 24%

Final M.C. = 14%

Page 14: Paddy Drying Systems

Tips for better sundrying

• Management– Layer depth of 4cm– Mixing every 30 minutes– Monitor moisture content– Monitor temperature

• Protection– Cover the grain when

temperature rise above 50-60ºC

– Cover during rain. – Prevent contamination– keep animals off the grain Use tools to improve sundrying

Page 15: Paddy Drying Systems

Heated air dryingComponents of a dryer

• Main components– Drying bin– Air distribution system– Fan– Air heater

• Optional– Conveyors

• Accessories– Moisture meter– Dust separator

Page 16: Paddy Drying Systems

Options for heated air drying

Fixed bed batch dryer Re-circulating batch dryer Continuous flow drying plant

Tempering Section

DryingSection

Dryer Tempering bins

In

Out

Air

Grain

Page 17: Paddy Drying Systems

Fixed bed batch dryer (1)Flat bed dryer

• Key features– Batch Capacity: 1-10t– Drying time 6-8h– Approximate prices: US$ 1000-

2000– Kerosene or rice hull fired

• Advantages– Simple and affordable

• Disadvantage– Labor intensive– Moisture gradient– Temperature control

Page 18: Paddy Drying Systems

Fixed bed batch dryer (2)Low cost batch dryer

• Key features– Batch Capacity: 0.2-2t– Drying time: 1-2 days– Approximate prices:

US$ 100-200– Wood, coal or rice hull fired

• Advantages– Simple and affordable– Very cheap– Uses local storage structures

• Disadvantage– Labor intensive– Moisture gradient– Temperature control

Page 19: Paddy Drying Systems

Fixed bed batch dryer (3)Reversible air flow dryer

• Key features– Same as flat bed dryer– Airflow is reversed after ½ to ¾

of drying time was completed• Advantages

– Minimized moisture gradient– Less labor intensive since

mixing is eliminated• Disadvantage

– Additional cost

Page 20: Paddy Drying Systems

Re-circulating batch dryer

• Key features– Batch Capacity: 4-12t– Drying time: 8 hours– Approximate prices:

US$ 8,000-10,000– Kerosene fired

• Advantages– Automatic operation– Produces excellent quality– Little floor area

• Disadvantage– Wear of conveying elements– Problems with very wet paddy

Page 21: Paddy Drying Systems

Mechanical drying Methods

Drying air temp.: 43ºCAir velocity: 0.15-0.25 m/sAirflow rate per t grain: >0.7 m³/sPower requirement: 1.5-2.5kW/t grainLayer depth: < 40 cmDrying time: 6-12 hInitial MC: up to 30%+

Drying Zone 10

15

20

25

30

0 5 10 15

Drying time, h

MC

, %

w.b

. Top

Middle

Bottom

Avg.

Drying ZoneDry Grains

Wet Grains

10

15

20

25

0 48 96 144 192 240

Drying Time, h

MC

, %

w.b

.

Top

Middle

Bottom

Heated-air drying Low-Temperature Drying

Drying air temperature: Δ T = 0-6 ºK Air velocity: 0.1 m/sAirflow rate per t grain: >0.05-0.4 m³/sPower requirement: 0.05-0.15 kW/t grain Layer depth: < 2 mDrying time: days to weeks Initial MC: 18% ( 28%)

Advantages:Simple managementFast dryingAffordableLow level of integrationDisadvantages:3-4% moisture gradient in final product, requires mixing or reduced layer depthReduction in milling yieldDanger of killing seeds

Advantages:Very energy efficientBins can be filled at harvest rateMaintains grain quality optimallyDrying in storage structuresDisadvantages:Increased risk with poor power suppliesRequires bulk handling system (high level of integration in postharvest system)Long drying time

Page 22: Paddy Drying Systems

Other drying systems

• First stage dryers– Fluidized bed dryer– Rotary drum dryer

• Low-temperature dryer (often second stage dryer)– In-store dryer– Aeration facilities

Page 23: Paddy Drying Systems

Flash dryer

• Principle– Grains are pre-dried quickly in a

fluidized bed– As a first-stage dryer in a two-stage

drying strategy• Key features

– Batch Capacity: 4-12t– Drying time: 10-15 minutes– Air temperatures: 110-120°C– Air velocity: 2.3 m/s

• Advantages– Very fast pre-drying– High capacity

• Disadvantage– For pre-drying to 18% MC only– High energy requirement

Fluidized bed dryer from Thailand

Page 24: Paddy Drying Systems

In-store dryer

Drying ZoneDry Grains

Wet Grains

• Principle– Slow EMC based drying with ambient air or

slightly pre-heated air• Key features

– Batch Capacity: 1… x.000 tons– Drying time: 4 days to 2 weeks– Air temperatures: ambient, 3-6°C above

ambient– Air velocity: 0.1 m/s

• Advantages– Produces very high quality– Low energy requirement– Drying in storage bin

• Disadvantage– High risk if MC is > 18%– Long drying time

Page 25: Paddy Drying Systems

Drying Strategies

• Decentralized on-farm drying– Requires quality incentive– Low utilization of equipment– Training and technical support service

• Centralized drying– Contractors (service providers)– Mills– Economics of scale

• Two-stage drying– Ideal process to produce best quality– Two machines are needed for one operation– First stage dryer dries only to 18%

Page 26: Paddy Drying Systems

Thank you