Monsoon paddy drying when half the lot is seed and half is milling grain
A farmer in Golaghat has harvested 12 tonnes of Ranjit paddy at 24% moisture (wet basis) on a cloudy day. Rain is forecast for the next 36 hours, with 85% relative humidity and 27°C. He has a 2-tonne batch flat-bed dryer with a diesel burner and a maximum plenum temperature of 55°C. Half the lot is meant for seed and half for milling. The local mill will accept grain only at 14% moisture or below; above 18% it pays a heavy discount. Jute bags are available, but the only covered space is a bamboo platform in a ventilated shed. A neighbour's seed dryer can handle 1 tonne per batch at 43°C but is booked for the next 24 hours. Seed germination tested 92% before harvest. He must decide what to dry first, at what temperature, and whether to risk temporary storage.
1. Calculate the water that must be removed to bring the 12 t lot from 24% to 14% wet basis, and explain why the answer is not simply 10% of 12 t.
Read the worked answer
Use a dry-matter balance. At 24% wet basis, dry matter is 76%:
- Dry matter = 12 × 0.76 = 9.12 t.
- At 14% wet basis, dry matter is 86% of the final mass.
- Final mass = 9.12 / 0.86 = 10.60 t.
- Water removed = 12.00 − 10.60 = 1.40 t.
The shortcut of taking 10 percentage points of 12 t gives 1.20 t, which is wrong because the percentage is always expressed on a changing total mass. As water leaves, the denominator shrinks. The correct removal is 1.40 t, about 17% more than the shortcut. This matters for dryer capacity, fuel and time: if the dryer is rated at 0.25 t of water removal per hour, the job needs 5.6 hours of actual evaporation, not 4.8 hours.
2. Recommend a drying sequence and temperature for the seed and milling fractions, and state what to do with the grain during the 36-hour rain.
Read the worked answer
Seed must be protected first because high temperature and rapid drying kill germination. The safe sequence is:
- Move the seed fraction (6 t) under cover and spread it in a thin layer, 15–20 cm, on the ventilated bamboo platform. Turn it every 2 hours. Do not bag wet paddy; bagging traps heat and encourages mould.
- Use ambient-air aeration if a fan is available. At 24% moisture, 24–36 hours of temporary aeration is safer than immediate high-temperature drying.
- When the neighbour's dryer is free, dry seed at 43°C in two passes, with tempering between passes: 24% to about 18%, temper 4–6 hours, then 18% to 12–12.5%. Do not exceed 43°C plenum temperature. Test germination on a sample before committing the whole seed lot.
- Dry the milling fraction in the 2 t batch dryer at up to 55°C, but use a two-stage schedule rather than blasting to 14% in one pass. A reasonable target is 24% to 18% in the first pass, temper, then 18% to 14%.
During the 36-hour rain, priority is to keep grain dry and cool, not necessarily to dry it fast. Cover the platform, keep the layer shallow, and avoid heaping wet grain in bags. If the rain persists beyond 48 hours, start drying the milling fraction at 55°C and hold the seed fraction for the 43°C dryer.
3. If the dryer removes 0.25 t of water per hour at 55°C, estimate the drying time for the 6 t milling fraction from 24% to 14%, and state two quality risks of over-drying.
Read the worked answer
For 6 t at 24% wet basis:
- Dry matter = 6 × 0.76 = 4.56 t.
- Final mass at 14% = 4.56 / 0.86 = 5.30 t.
- Water to remove = 6.00 − 5.30 = 0.70 t.
- At 0.25 t/h, evaporation time = 0.70 / 0.25 = 2.8 h.
But the dryer is a 2 t batch unit, so the 6 t must be handled as three batches. Each 2 t batch at 24% contains 1.52 t dry matter and must finish at 1.52/0.86 = 1.77 t, removing 0.23 t of water, or about 0.93 h at 0.25 t/h. Three batches therefore need about 2.8 hours of drying plus filling, tempering and cooling time. Realistically plan 6–8 hours.
Two quality risks of over-drying:
- Stress cracks and reduced head rice yield when grain is dried below about 13% or cooled too fast. Cracks appear later during milling and turn into brokens.
- Weight loss and unnecessary fuel use; every extra percentage point of moisture removed is sold as water loss. Over-dried seed also loses viability.
Stop at 14% for milling and 12–12.5% for seed, verify with a calibrated moisture meter, and temper before cooling.