Case Study: Paddy Drying and Rice Processing Optimisation

This case study evaluates the grain moisture removal process in paddy drying and rice production, based on observations from a visit to a large rice mill in Garmati, Natore, Bangladesh (380/500/1000 tpd capacity) on January 13-15, 2016. The analysis focuses on steam drying techniques, referencing a 32-ton/batch dryer, and compares various drying methods to optimise husk consumption and drying efficiency.

Review: February 2, 2016 – Steam Drying Analysis

Paddy Moisture and Drying Process

• Initial Conditions: Paddy typically has 22-26% moisture after soaking (Haldia Technical Institute reports up to 39%) from an initial godown stock moisture of 14-16%. Soaking occurs in tanks for 3-4 hours.
• Drying Requirement: For de-shelling, moisture is reduced to 5-6%, resulting in approximately 14% moisture removal. The process also removes 8% bran and 22-23% husk by weight.
• Moisture Removal Calculation:
  • For a 32-ton batch, 14% moisture removal equates to: [32,000 \{kg} x 0.14 = 4,480 \ kg of moisture]
  • Hourly rate over 6 hours: (4,480 \ 6 = 747 \ kg/hr).
• Latent Heat for Evaporation:
  • At 30°C: Latent heat difference (hg-hfg) = 36 kJ/kg.
  • For 747 kg/hr: (36 \ 747 = 26,892 \ kJ/hr).
  • Standard latent heat of water at STP: 2,261 kJ/kg (Clark’s Table, p. 63).
• Steam Heat Transfer:
  • Steam from return header at 160°C, 4.5 ksc, with enthalpy (h1 = 2,760 \ kJ/kg).
  • After cooling to 50°C, the enthalpy (h2 = 2584 \ kJ/kg ).
  • Heat transfer per kg of steam: (2760 – 2584 = 176 \ kJ/kg ).
  • Steam requirement: (26892 \176 = 153, \Kg/hr ).
  • Considering 73% efficiency (85% steam-to-air, 85% air-to-paddy): (153 \ 0.73 = 210 \ kg/hr).
  • At 43% efficiency (comparable to a supercritical boiler): (153 \ 0.43 = 355 \ kg/hr).
• Drying Passes:
  • Each pass takes approximately 45 minutes, with 6-7 passes required (Haldia Technical Institute suggests faster moisture removal from 37% to 23%, slowing from 23% to 19%).
  • Total steam for 7 passes: (7 x 355 = 2,485 \ kg/hr).
  • OEM-reported steam consumption: ~2,200 kg/hr.

Normal Rice Production

• Using 12 ksc steam, enthalpy ( h1 = 2,787 \ kJ/kg).
• Heat transfer: (2,787 – 2,584 = 202 \ kJ/kg).
• Theoretical steam requirement: (26,892 \ 202 = 133 \ kg/hr), but actual consumption is ~2,200 kg/hr, indicating higher temperature steam or extraction from the TG stage/main header.

Review: February 3, 2016, 9:33 AM – Soaking and Drying Insights

• Haldia Institute Findings:
  • Soaking paddy in cold water (25-30°C) raises moisture to 37%.
  • Drying at 54°C reduces moisture to 19% for milling.
  • Sun drying to 14% moisture is optimal for rice quality.
  • Practical moisture removal: 37% to 19% = 18%, validating the 14% moisture reduction assumption used earlier.

Review: February 3, 2016, 11:40 AM – Comparative Analysis

• LSU Dryer vs. Steam Dryer:
  • Comparison of the LSU dryer (hot air) and the saturated/superheated steam dryer.
  • Husk consumption analysis: Pressel’s Boiler vs. Thermax shows a 1:1.31 ratio, indicating significant husk savings with Thermax, potentially allowing boiler refurbishment.
• Energy Consumption (Reference: Zhang Jingiang & Wu Woosen):
  • Paddy with 30% initial moisture, dried to 14%:
    • Energy consumption: 10.5 kWh/100 kg paddy.
    • Drying time: 88% of conventional.
    • Drying cost: 64% of conventional.
    • Energy calculation: (10.5 x 3,600 = 36,000\ kJ/hr).
  • Equivalent steam at 160°C, 4.5 ksc (176 kJ/kg): (36,000 \ 176 = 205\ kg/hr) for 100 kg paddy.
  • For a 32-ton batch over 6 hours: ( (32 \ 6) x 10 x 205 = 10,933 \ kg/hr).
  • For 6 passes: (10,933 \ 6 = 1,822 \ kg/hr).

Grain Drying Capacity Calculation

Basic Conditions

1. Objective: Minimise husk usage for economical operation.
2. System: Cyclonic rice husk burner and LSU dryer.
3. Material:
   • Paddy: 1,000 tons (28 days).
   • Corn: 100 tons (10 days).
4. Hot Air Usage: LSU dryer for final drying, pre-drying in storage bins.

Calorific Calculation

Paddy Drying (LSU Dryer, Continuous Flow)

• Conditions:
  • Air volume: 500 m³/min.
  • Drying air temperature: 80°C.
  • Ambient temperature: 30°C, 60% RH.
  • Dryer efficiency: 85%.
• Psychrometric Chart:
  • Ambient (30°C, 60% RH): Enthalpy (h1 = 71\ kJ/kg dry air).
  • Drying air (80°C): Enthalpy ( h2 = 125\ kJ/kg dry air), specific volume = 1.025 m³/kg.
  • Required heat: ( (500 \ 1.025) x (125 – 71) \ 60 = 439 \ kW).
• Alternative Formula ((q = M \cdot cp \cdot (T2 – T1))):
  • (q = (500 \ 1.025) x 1.0 x (80 – 30) \0.85 \60 = 478\ kW).

Corn Drying (LSU Heater)

• Conditions:
  • Air volume: 500 m³/min.
  • Drying air temperature: 100°C.
  • Ambient temperature: 30°C, 60% RH.
  • Dryer efficiency: 85%.
• Psychrometric Chart:
  • Ambient: Enthalpy (h1 = 71, \ kJ/kg dry air).
  • Drying air (100°C): Enthalpy ( h2 = 148\ kJ/kg dry air), specific volume = 1.08 m³/kg.
  • Required heat: ( (500 \ 1.08) x (148 – 71) \ 60 = 594 \ kW).
• Alternative Formula:
  • (q = (500 \ 1.08) x 1.0 x (100 – 30) \ 0.85 \ 60 = 635 \ kW).

Husk Combustion and Consumption

• Husk Calorific Value: 15,500 kJ/kg, efficiency 85%.
• Required Husk: (594 \ (15,500 x 0.85 x 3,600) = 162 \ kg/hr).
• Husk Burner Selection:
  • Type: Cyclonic rice husk burner.
  • Capacity: 200 kg/hr (required: 184 kg/hr).
  • Efficiency: 80%.
  • Effective calorie: ((200xx 15,500 x 0.8) \ 3,600 = 689 kW).
  • Capacity ratios: (689 \ 439 = 1.44) (paddy), (689 \ 478 = 1.09) (alternative).
• Husk Consumption:
  • Paddy: (0.2 \ ton/hr x 24 x 28 = 134.4 \tons).
  • Corn: (0.2 , \ ton/hr x 24 x 10 = 48 \tons).
  • Total: 182.4 tons.
• Husk Tank Capacity:
  • Daily consumption: (0.2 x 24 = 4.8 \ tons).
  • 7-day capacity: (4.8 x 7 = 33.6 \ tons).
  • Tank volume: (33.6 \ 0.12 = 280 \ m³).

Husk Furnace Design

• Combustion Rate: 70 kg husk/m² hr with 60% excess air.
• Design Features:
  • A mixing chamber for combustion products and ambient air to achieve the desired temperature.
  • Arresting fly ash and sparks to prevent entry into the drying chamber.
  • Mechanism to redirect flue gases to the chimney or drying chamber.
  • Smokeless combustion to maintain efficiency and grain quality.
  • Simple maintenance and portability.
• Furnace Specifications:
  • Box-type furnace, supplying 1,680 m³/hr (1,000 cfm) at 70-120°C.
  • Equipped with a 45° inclined grate (0.5 m²) and revolving grate for ash disposal.
  • Husk feed rate: 11 kg/hr, maintaining 1,680 m³/hr at 100°C.
  • Flue gas composition: 3% CO2, 16% O2, 0% CO, rest nitrogen.
• Husk Composition:
  • Water: 13.23%, Ash: 18.18%, Carbon: 29.50%, Hydrogen: 5.44%, Nitrogen: 0.46%, Oxygen: 33.19%.
  • Dry ash-free basis: Carbon: 43.01%, Hydrogen: 7.93%, Nitrogen: 0.67%, Oxygen: 48.39%.
• Air Requirement:
  • Theoretical oxygen: 1.2324 kg/kg husk (Carbon: 0.7867, Hydrogen: 0.4352, Nitrogen: 0.0105).
  • Air (23% O2 by weight): (1.2324 \ 0.23 = 5.36 \ kg air/kg husk).
  • For 87 kg/hr husk: (87 x 5.36 = 466 \ kg/hr dry air = 452.5 \ m³/hr).
  • Practical excess air: 100-200% for complete combustion.

Comparative Husk Consumption

LSU Method

• Paddy Processing: 1,000 tons over 28 days = 35.72 tons/day.
• Husk Consumption: 4.8 tons/day = 0.1343 tons/ton paddy.
• For 500 tpd Plant: (500 \ 0.1343 = 67.15 \ tons/day).
• Economic Analysis:
  • Cost: BDT 5.5 crore for 380 tpd, scaled to 7.3 crore for 500 tpd.
  • Husk sale: 67,000 kg/day at BDT 5.5/kg for 220 days = BDT 8.1 crore gain.
• Quality: Rice quality impact unknown.

Steam Dryer (32-ton Batch, 500 tpd Plant)

• Husk Production: 22% husk/ton paddy = 110 tons + 25 tons/day (2-3 trucks) = 135 tons/day.
• Husk Consumption: (135 \ (32 x 8) = 0.527 \ ton/ton paddy).
• Steam Generation:
  • Pressel’s Boiler: 1 kg of husk produces 4.2 kg of steam.
  • 18,000 kg/hr steam requires: (18,000 \ 4.2 x 24 = 102.857 \ tons/day).
  • Thermax vs. Pressel’s: 135:103 = 1:1.32, indicating husk savings.

Conclusion

The analysis highlights the efficiency of steam drying for paddy processing, with a 32-ton/batch dryer requiring ~2,200 kg/hr steam, aligning with OEM specifications. The Thermax boiler offers significant husk savings compared to Pressel’s, potentially enabling cost-effective boiler refurbishment. The LSU dryer, while effective, consumes less husk (0.1343 ton/ton paddy) but lacks rice quality data. The cyclonic husk burner and furnace design ensure efficient combustion and minimal environmental impact, supporting economical and sustainable rice milling operations.

Author

Bhaskar Sen
Consultant 

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