Paddy Drying Systems

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Paddy Drying Systems

• By
• M GummertJ Rickman
• Agricultural Engineering Unit
• IRRI, Los Baños , Philippines

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Content

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

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Why Drying?

• Rice is harvested at high moisture contents gt20
• Quality deterioration starts immediately after
  harvest
• The wetter the grain the faster the loss of
  quality
• Different MC for different purposes (see Table)

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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)

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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

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Improper drying, 3

• Loss of freshness/odor development
• Heat build up -gt 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 -gt
  reduced the head rice recovery.

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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

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Drying methods
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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.
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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

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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

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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

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Sundrying and quality

• Layer thickness
• Spread the grains in thin layers, ideally 2-4 cm.
  
• Too thin layers -gt 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.

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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
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Heated air dryingComponents of a dryer

• Main components
• Drying bin
• Air distribution system
• Fan
• Air heater
• Optional
• Conveyors
• Accessories
• Moisture meter
• Dust separator

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Options for heated air drying
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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

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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

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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

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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

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Mechanical drying Methods
Heated-air drying
Low-Temperature Drying

• Drying air temp. 43ºCAir velocity 0.15-0.25
  m/sAirflow rate per t grain gt0.7 m³/sPower
  requirement 1.5-2.5kW/t grainLayer depth lt 40
  cmDrying time 6-12 hInitial MC up to 30

Drying air temperature ? T 0-6 ºK Air
velocity 0.1 m/sAirflow rate per t
grain gt0.05-0.4 m³/sPower requirement 0.05-0.15
kW/t grain Layer depth lt 2 mDrying time
days to weeks Initial MC 18 ( 28)
Advantages Simple management Fast
drying Affordable Low level of integration Disadva
ntages 3-4 moisture gradient in final product,
requires mixing or reduced layer depth Reduction
in milling yield Danger of killing seeds
Advantages Very energy efficient Bins can be
filled at harvest rate Maintains grain quality
optimally Drying in storage structures Disadvantag
es Increased risk with poor power
supplies Requires bulk handling system (high
level of integration in postharvest system) Long
drying time
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Other drying systems

• First stage dryers
• Fluidized bed dryer
• Rotary drum dryer
• Low-temperature dryer (often second stage dryer)
• In-store dryer
• Aeration facilities

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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 110-120C
• 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
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In-store dryer

• 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, 3-6C 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 gt 18
• Long drying time

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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

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Thank you