Transcript Paddy Drying Systems Overview - Home

Paddy Drying Systems
By:
M Gummert
J Rickman
Agricultural Engineering Unit
IRRI, 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
Low-Temperature
Drying
Heated-Air Drying
Flash Drying
Panicle drying
Batch dryer
Aeration
Mat drying
Re-circulating Batch
Dryer
In/store drying
Pavement drying
Continuous Flow Dryer
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
Recovery, %
88
86
84
82
80
78
76
74
0 1 2 3 4 5 6 7
Head rice
Milled rice
Drying time
Source: IRRI, (Gayanilo)
90
6.5
85
6.25
80
6
75
5.75
70
5.5
65
5.25
60
5
0 1 2 3 4 5 6 7
Location: Philippines
Layer thickness, cm
Intital M.C. = 24%
Final M.C. = 14%
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
Drying Time, h
8
7.5
7
6.5
6
5.5
5
4.5
4
3.5
3
Recovery, %
90
Drying Time, h
Sundrying and quality
Head rice
Milled rice
Drying time
Source: IRRI, (Gayanilo)
Location: Philippines
Mixing interval, h
Intital M.C. = 24%
Final M.C. = 14%
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.
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 5060ºC
– Cover during rain.
– Prevent contamination
– keep animals off the grain
Use tools to improve sundrying
Heated air drying
Components 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
Air
Grain
Dryer
Tempering
Section
Tempering bins
Drying
Section
In
Out
Fixed bed batch dryer
Re-circulating batch dryer
Continuous flow drying plant
Fixed bed batch dryer (1)
Flat bed dryer
• Key features
– Batch Capacity: 1-10t
– Drying time 6-8h
– Approximate prices: US$ 10002000
– 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
Heated-air drying
Low-Temperature Drying
25
MC, % w.b.
25
Drying
Zone
Top
Middle
20
Bottom
Avg.
15
10
0
5
10
15
Drying time, h
Drying air temp.:
43ºC
Air velocity:
0.15-0.25 m/s
Airflow rate per t grain:
>0.7 m³/s
Power requirement: 1.5-2.5kW/t grain
Layer depth:
< 40 cm
Drying time:
6-12 h
Initial MC:
up to 30%+
Advantages:
Simple management
Fast drying
Affordable
Low level of integration
Disadvantages:
3-4% moisture gradient in final product, requires
mixing or reduced layer depth
Reduction in milling yield
Danger of killing seeds
Wet
Grains
Drying
Zone
Dry
Grains
MC, % w.b.
30
20
Top
Middle
Bottom
15
10
0
48
96
144
192
240
Drying Time, h
Drying air temperature:
Δ T = 0-6 ºK
Air velocity:
0.1 m/s
Airflow rate per t grain:
>0.05-0.4 m³/s
Power requirement:
0.05-0.15 kW/t grain
Layer depth:
<2m
Drying time:
days to weeks
Initial MC:
18% ( 28%)
Advantages:
Very energy efficient
Bins can be filled at harvest rate
Maintains grain quality optimally
Drying in storage structures
Disadvantages:
Increased risk with poor power supplies
Requires bulk handling system (high level of integration in
postharvest system)
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
•
•
•
•
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%
Wet
– Long drying time
Grains
Drying
Zone
Dry
Grains
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