Wet-Season Paddy Drying: 40 t at 24%, One Undersized Dryer and a Seed Lot
On a 12 ha paddy farm in Isabela, harvest of a 40 t lot (wet weight) begins at 24% moisture, wet basis, and the miller will only accept 14%. The cooperative owns one recirculating batch dryer rated 6 t of wet paddy per batch, able to remove about 6 moisture points in 14 hours. Harvest arrives at about 8 t per day. The only fuel on hand is rice husk with a lower heating value of about 13 MJ/kg; the dryer's thermal efficiency is about 55%. Ambient conditions during the wet-season harvest are 28°C and 85–90% relative humidity. About 8 t of the lot is seed-grade and the buyer requires germination above 90%. Two days into harvest a fan bearing fails and the dryer is down for 36 hours, during which wet grain sits in sacks inside the shed. The cooperative asks whether to keep drying with one unit, buy a second dryer, or sell part of the crop wet at a discount.
1. Compute the mass of water to be removed and the rice husk needed for the whole 40 t lot. State any energy terms you have deliberately left out.
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Dry matter conservation. At 24% wb, dry matter = 40 t × 0.76 = 30.4 t. This does not change during drying.
Final mass. At 14% wb, final mass = 30.4 / 0.86 = 35.35 t.
Water removed. 40.00 − 35.35 = 4.65 t = 4,650 kg. A useful cross-check is the shortcut 40 × (24 − 14)/(100 − 14) = 4.65 t.
Energy. Latent heat plus the sensible heat of warming the grain is taken as about 2,500 kJ per kg of water evaporated: 4,650 × 2,500 = 11.6 GJ of useful heat.
Fuel energy. At 55% dryer thermal efficiency, fuel input = 11.6 / 0.55 = 21.1 GJ.
Husk. 21.1 GJ / 13 MJ/kg = 1,625 kg, roughly 1.6 t of husk - a convenient figure because the millstream that produced the paddy also produces the husk.
Deliberately omitted. Fan and drive electricity, heat lost up the stack and through the dryer shell beyond the 55% figure, heat needed to bring the dryer structure up to temperature each start, and the extra evaporation if the paddy is rewetted by rain during handling. In practice add 10–15% contingency, so plan on about 1.8 t of husk. Also note that if fuel were LPG at about 46 MJ/kg instead, the same lot would need roughly 460 kg.
2. The dryer takes 6 t per 14 h batch. Can one dryer keep up with 8 t per day of arriving wet paddy? Show the batch arithmetic and give a recommendation for the 40 t lot.
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Batch arithmetic for the lot. 40 t / 6 t per batch = 6.67, so 7 batches (the last is partial). At 14 h each: 7 × 14 = 98 h, about 4.1 days of continuous running if the dryer never stops.
Arrival rate. 8 t/day requires 8 / 6 = 1.33 batches per day = 1.33 × 14 = 18.7 h of dryer time per day. That leaves only 5.3 h per day for cool-down, discharge, cleaning, refilling and any breakdown.
Verdict. The dryer is structurally undersized: it needs 18.7 of every 24 h to keep pace, so the backlog grows during every delay. The 36 h outage alone added about 12 t of arrivals plus whatever was already queued, and that grain sat at 24% moisture in sacks at roughly 28–32°C - ideal conditions for heating and mold.
Options ranked. (1) Rent or borrow a second dryer or a continuous-flow unit for the season; the wet grain cannot wait. (2) If a second unit is impossible, arrange two-stage drying: take the lot down to about 18% in the existing dryer, temper, then finish - this lets the dryer turn more tonnes per day because less water is removed per pass. (3) Selling a portion wet at a discount is rational only for the table-grade paddy and only if the discount is smaller than the cost of delay; never sell the seed lot wet.
Priority order. Dry the oldest and wettest grain first, keep the queue aerated, and record the moisture of every sack so nothing is dried twice or left behind.
3. The seed-grade 8 t must retain more than 90% germination. At 28°C and 85–90% RH, can aeration alone bring paddy to 14%? Answer that, then set out the drying schedule for the seed lot and the actions to take after the 36-hour outage.
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Aeration alone: no. At 28°C and 85–90% RH the equilibrium moisture content of paddy is roughly 16–17% wb. Aeration can only move grain toward equilibrium, so ambient air at that humidity cannot pull paddy below about 16%; in fact aerating 14% grain in that weather would rewet it. Aeration in this weather is a holding and cooling tool, not a drying tool. Heated drying is required to reach 14%.
Seed-lot schedule. Limit grain temperature to about 40–43°C - seed embryos are killed above roughly 45°C, whereas table paddy can tolerate 50–55°C. Dry in two passes: 24% down to about 18% at a moderate temperature, temper in the bin for 8–12 h to equalize moisture inside the kernel, then 18% to 14% at a lower temperature. Tempering between passes is what protects germination and head rice; a single fast pass from 24% to 14% causes fissuring and a 10–15 point drop in head rice, and can cut germination below the buyer's 90% threshold.
After the outage. The sacks sat at 24% moisture for 36 h in warm conditions, so: (1) smell and hand-feel each lot for mustiness; (2) probe the centre of each sack for temperature - a rise of more than about 5°C above ambient means active heating; (3) run a rapid germination test on a sample from each sack before committing the seed lot; (4) dry these lots first, ahead of freshly harvested grain, because quality is already degrading; (5) if any lot has heated above about 40°C internally, divert it to the table-grade or feed stream rather than the seed contract. Record the outage on the lot so the buyer sees a documented history.