Paddy Drying Systems By M Gummert J Rickman

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 l mixing dry with wet grains l 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 Layer thickness, cm Head rice Milled rice Drying time Source: IRRI, (Gayanilo) Location: Philippines 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 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 Mixing interval, h Drying Time, h 8 7. 5 7 6. 5 6 5. 5 5 4 3. 5 3 Recovery, % 90 Drying Time, h Sundrying and quality Head rice Milled rice Drying time Source: IRRI, (Gayanilo) Location: Philippines 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 4 cm – 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 Drying Section Tempering bins In Out Fixed batch dryer Re-circulating batch dryer Continuous flow drying plant

Fixed batch dryer (1) Flat bed dryer • Key features – Batch Capacity: 1 -10 t – Drying time 6 -8 h – Approximate prices: US$ 10002000 – Kerosene or rice hull fired • Advantages – Simple and affordable • Disadvantage – Labor intensive – Moisture gradient – Temperature control

Fixed batch dryer (2) Low cost batch dryer • Key features – Batch Capacity: 0. 2 -2 t – 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 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 -12 t – 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 Wet Grains Drying Zone 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. 5 k. W/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 Drying Zone Dry Grains Drying air temperature: Air velocity: Airflow rate per t grain: Power requirement: Layer depth: Drying time: Initial MC: Δ T = 0 -6 ºK 0. 1 m/s >0. 05 -0. 4 m³/s 0. 05 -0. 15 k. W/t grain <2 m days to weeks 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 -12 t – 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%

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