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) PurposeRequired MC Potential problems weeks storage %Molds, discoloration, respiration loss months storage<= 13%Insect damage > 1 year storage<= 9 %Loss of viability Milling14%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 PurposeRequired MC Potential problems weeks storage %Molds, discoloration, respiration loss months storage<= 13%Insect damage > 1 year storage<= 9 %Loss of viability Milling14%Damaged grains, cracking

Drying methods Drying Traditional Methods (Sun drying) Field Drying Panicle drying Mat drying Pavement drying Mechanical drying Heated-Air Drying Batch dryer Re-circulating Batch Dryer Continuous Flow Dryer Low-Temperature Drying Aeration In/store drying Flash 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 Mixing interval, h Recovery, % Drying Time, h Head rice Milled rice Drying time Source: IRRI, (Gayanilo) Location: Philippines Intital M.C. = 24% Final M.C. = 14% Layer thickness, cm Recovery, % Drying Time, 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 ºC – 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 Fixed bed batch dryerRe-circulating batch dryerContinuous flow drying plant Tempering Section Drying Section 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$ – 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$ – 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.: 43ºC Air velocity: m/s Airflow rate per t grain:>0.7 m³/s Power requirement: kW/t grain Layer depth: < 40 cm Drying time: 6-12 h Initial MC: up to 30%+ Drying Zone Dry Grains Wet Grains Heated-air drying Low-Temperature Drying Drying air temperature: Δ T = 0-6 ºK Air velocity:0.1 m/s Airflow rate per t grain:> m³/s Power requirement: kW/t grain Layer depth: < 2 m Drying time: days to weeks Initial MC: 18% ( 28%) 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 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) 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: minutes – Air temperatures: °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 Zone Dry 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%

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