Paddy Drying Systems
By: M GummertJ Rickman Agricultural Engineering UnitIRRI, Los Baños , Philippines
Content
• Why drying• Drying and quality• Drying methods• Mechanical drying systems• Drying strategies
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
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)
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
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.
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
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
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.
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
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
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
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%
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
Heated air dryingComponents of a dryer
• Main components– Drying bin– Air distribution system– Fan– Air heater
• Optional– Conveyors
• Accessories– Moisture meter– Dust separator
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
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
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
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
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
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
Other drying systems
• First stage dryers– Fluidized bed dryer– Rotary drum dryer
• Low-temperature dryer (often second stage dryer)– In-store dryer– Aeration facilities
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
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
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%
Thank you