3.1 Introduction
Engineering properties of agricultural grains are fundamental characteristics that govern the design, operation, and optimization of post-harvest technology (PHT) equipment. These properties influence material handling, processing efficiency, storage behavior, and quality preservation. Understanding physical, thermal, aerodynamic, and hydrodynamic properties enables engineers to develop effective equipment for cleaning, drying, conveying, milling, and storage operations.
3.2 Physical Properties
3.2.1 Size and Shape
Linear Dimensions: Grain dimensions are characterized by three principal axes—length (L), width (W), and thickness (T). These measurements determine screen aperture sizes for cleaning and grading operations.
Equivalent Diameter: For irregularly shaped grains, equivalent diameter provides a single representative dimension:
Sphericity (φ): Describes how closely a particle resembles a sphere:
High sphericity values (wheat: 0.65-0.75, peas: 0.85-0.95) indicate better flowability and more predictable behavior in handling equipment.
Cleaning & Grading: Screen perforation sizes are designed based on grain thickness. Indent cylinder separators utilize length differences to separate wheat from broken kernels.
Pneumatic Conveying: Sphericity affects particle orientation and drag in air streams, influencing conveying velocities required.
Storage Bin Design: Shape influences flow patterns and determines hopper angles to prevent bridging (typically 50-65° for most grains).
3.2.2 Mass Properties
Thousand Kernel Weight (TKW): Mass of 1000 grains, a critical quality indicator. Typical ranges: wheat (30-45 g), rice (20-30 g), chickpea (150-300 g).
True Density (ρt): Mass per unit volume of the grain material itself, excluding pores. Determined by liquid displacement or gas pycnometry. Typical values: 1.2-1.5 g/cm³ for most cereals.
Bulk Density (ρb): Mass per unit volume including inter-particle voids:
Typical values: wheat (760-800 kg/m³), rice (550-600 kg/m³), soybeans (720-770 kg/m³). Affected by moisture content, variety, and damage.
Porosity (ε): Fraction of bulk volume occupied by voids:
Ranges from 35-45% for most grains, critical for aeration and drying calculations.
Storage Capacity Planning: Bulk density determines bin capacity and structural load calculations. A 1000-tonne wheat silo requires approximately 1250 m³ volume.
Dryer Design: Porosity affects airflow resistance and drying rates. Higher porosity allows better air distribution but requires higher fan power.
Conveyor Selection: Bulk density determines belt loading, power requirements, and throughput calculations for belt and bucket elevators.
3.2.3 Mechanical Properties
Angle of Repose (θ): The angle of the surface of a pile of grain with the horizontal. Influenced by moisture, shape, and surface friction. Typical values: 25-35° for smooth grains, 35-45° for rough seeds.
Coefficient of Friction:
- Static friction (μs): Resistance to initial movement. Critical for hopper design.
- Dynamic friction (μk): Resistance during movement. Important for chute and conveyor design.
Values vary with surface material: steel (0.25-0.40), concrete (0.45-0.60), galvanized iron (0.35-0.50).
Hardness: Resistance to deformation under load, affecting milling energy and dehulling operations. Measured by penetrometer or compression tests.
Rupture Strength: Force required to crack or break a kernel, important for minimizing mechanical damage during handling.
Hopper Design: Hopper outlet size and wall angle are calculated using angle of repose and friction coefficients to ensure mass flow and prevent arching.
Conveyor Incline: Maximum incline angles for belt conveyors are limited by friction coefficients (typically 18-20° for grains).
Milling Operations: Hardness determines grinding energy requirements. Durum wheat (harder) requires 40-50% more energy than soft wheat.
3.2.4 Moisture Content Relationships
Moisture content (MC) profoundly affects all physical properties:
| Property | Effect of Increasing MC | Typical Change |
|---|---|---|
| Bulk Density | Decreases | 3-5% per 1% MC increase |
| Angle of Repose | Increases | Linear above 15% MC |
| Coefficient of Friction | Increases | 20-30% from 10% to 18% MC |
| Hardness | Decreases | Softens, more plastic behavior |
3.3 Thermal Properties
3.3.1 Specific Heat Capacity (Cp)
Heat energy required to raise temperature of unit mass by 1°C. Varies with moisture content:
Where Cp,dm is specific heat of dry matter (~1.3-1.5 kJ/kg·K) and Cp,water = 4.18 kJ/kg·K
Typical values at 12% MC: wheat (1.75 kJ/kg·K), rice (1.68 kJ/kg·K), soybeans (1.97 kJ/kg·K)
Dryer Heat Load Calculations: Total heat required includes heating grain, evaporating moisture, and compensating for losses. For 10 tonnes/hour wheat from 20% to 14% MC:
Where λ is latent heat of vaporization (~2450 kJ/kg at grain temperatures).
Tempering Operations: Heating calculations for conditioning wheat before milling require precise specific heat values for energy estimation.
3.3.2 Thermal Conductivity (k)
Rate of heat transfer through grain mass, typically low due to air voids. Values range from 0.10-0.20 W/m·K for most grains at 10-15% MC. Increases with moisture content and bulk density due to reduced air gaps.
Temperature also affects conductivity: approximately 0.001-0.002 W/m·K increase per °C.
Heat Sterilization: Low thermal conductivity causes slow heat penetration, requiring extended holding times for insect disinfestation (60°C for 15-20 minutes).
Storage Cooling: Slow heat dissipation in bulk storage requires active aeration to remove field heat. Without aeration, core temperatures may take weeks to equilibrate.
Dryer Design: Thin-layer drying preferred over deep beds due to poor heat penetration; layer depth typically limited to 15-30 cm in batch dryers.
3.3.3 Thermal Diffusivity (α)
Rate of temperature change in response to heat flow:
Typical values: 1.0-2.0 × 10-7 m²/s. Lower values indicate slower temperature equilibration in grain masses.
3.3.4 Equilibrium Relative Humidity (ERH) and Sorption Isotherms
At equilibrium, grain moisture content relates to surrounding air relative humidity. Described by sorption isotherms (sigmoid curves). Hysteresis exists—desorption curves differ from adsorption curves.
Modified Henderson Equation:
Where A, B, C are empirical constants specific to each grain type.
Safe Storage Conditions: Determines target storage RH. For 12-13% MC in wheat, maintain 60-65% RH at 25°C to prevent moisture gain.
Dryer Control: Outlet air RH must be below grain ERH to continue drying. In-bin drying systems use ambient air conditions matched to grain equilibrium requirements.
Aeration Strategy: Cooling aeration should consider moisture migration. Running aeration when ambient RH exceeds storage equilibrium causes moisture addition to surface layers.
3.4 Aerodynamic Properties
3.4.1 Terminal Velocity
Air velocity at which gravitational force equals drag force, causing particle suspension. Critical for pneumatic separation and conveying.
Terminal velocity (Vt) for a spherical particle:
Where CD is drag coefficient, g is gravity, D is diameter, ρp is particle density, ρa is air density.
| Grain/Material | Terminal Velocity (m/s) |
|---|---|
| Sound wheat grain | 8.5-10.0 |
| Shriveled wheat | 6.0-7.5 |
| Rice (paddy) | 7.0-8.5 |
| Wheat straw/chaff | 2.5-4.5 |
| Dust particles | 0.5-2.0 |
| Soybean | 10.0-12.5 |
| Light foreign matter | 1.5-5.0 |
Aspirators & Air Classifiers: Air velocity set between terminal velocities of desired grain and impurities. For wheat cleaning, air velocity of 5-7 m/s removes chaff and dust while retaining grain.
Pneumatic Conveying: Conveying velocity typically 1.5-2.5 times terminal velocity to ensure continuous transport. For wheat, use 15-20 m/s in horizontal pipes, 20-25 m/s in vertical lifts.
Cyclone Separators: Inlet velocity exceeds terminal velocity to create centrifugal separation. Cut-point particle size determined by cyclone geometry and operating velocity.
3.4.2 Drag Coefficient
Dimensionless parameter representing flow resistance. Depends on particle shape, orientation, Reynolds number, and surface roughness. Spherical particles: CD = 0.4-0.5 in turbulent flow. Irregular grains have higher values (0.5-0.8) due to increased frontal area and flow disruption.
3.4.3 Airflow Resistance
Pressure drop through grain bulk follows the Shedd equation:
Where ΔP is pressure drop, L is bed depth, Vs is superficial air velocity, and a, b are empirical constants depending on grain type, MC, and packing.
For wheat: b ≈ 1.7-1.9 (nearly quadratic relationship). Resistance increases exponentially with airflow rate and bed depth.
Aeration System Design: Fan selection requires pressure-flow calculations. For 5 m deep wheat at 0.1 m³/min/m³ airflow rate, static pressure requirement ≈ 500-700 Pa.
Dryer Fan Sizing: Cross-flow dryers with 20-30 cm grain depth require 1000-2000 Pa static pressure at 30-50 m³/min/m² airflow rates.
Bin Floor Design: Perforated floor open area typically 8-12% to minimize velocity through perforations while maintaining structural strength.
3.5 Hydrodynamic Properties
3.5.1 Water Absorption and Hydration
Hydration Capacity: Amount of water absorbed when immersed in excess water. Pulses show high capacity (80-120% of dry weight) compared to cereals (25-35%). Controlled by seed coat permeability and internal structure.
Hydration Rate: Time-dependent water uptake following first-order kinetics:
Where Mt is moisture at time t, M∞ is equilibrium moisture, k is rate constant.
Chickpea: 4-8 hours for full hydration; Wheat: 1-2 hours. Temperature significantly affects rate (Q10 ≈ 2-3).
Soaking Operations: Pre-processing for dal milling, canning, or germination requires controlled hydration. Pulse processing typically uses 6-8 hour soaking at 20-25°C to facilitate dehulling.
Tempering in Milling: Wheat conditioning to 15-16% MC over 12-24 hours toughens bran for cleaner separation and softens endosperm for efficient reduction.
Parboiling: Paddy hydration (28-32% MC) before steaming improves milling quality by gelatinizing starch and strengthening kernels.
3.5.2 Floatation and Density Separation
Grains with true density differences can be separated in liquid media. Stokes' Law describes settling velocity in viscous fluids:
Where r is particle radius, ρf is fluid density, μ is dynamic viscosity.
Sound seeds sink while immature, damaged, or insect-infested seeds float in appropriate density solutions (salt or sugar solutions at 1.10-1.25 g/cm³).
Seed Upgrading: Density graders using air-water or air-fluidized beds separate by specific gravity. Gravity tables utilize differential settling on oscillating, inclined decks combined with upward airflow.
Quality Sorting: Brine flotation (15-20% salt solution) for separating immature peas or damaged grains in seed conditioning plants.
Destoners: Fluidized bed with controlled air velocity allows stones (ρ ≈ 2.5-2.8 g/cm³) to sink while grains (ρ ≈ 1.2-1.4 g/cm³) remain suspended and are conveyed away.
3.5.3 Wettability and Surface Properties
Contact Angle: Measure of wettability. Hydrophilic surfaces (beans, pulses): θ < 90°; Hydrophobic surfaces (waxy coatings on some seeds): θ > 90°. Affects washing efficiency and chemical treatment adherence.
Surface Tension Effects: Capillary forces influence moisture movement in bulk storage and water uptake during soaking. Surface-active agents reduce contact angle, accelerating hydration.
3.6 Integrated Applications in PHT Operations
3.6.1 Cleaning and Grading Systems
Multi-Stage Cleaning: Sequential use of property differences:
- Scalping/Scalper: Uses size (length/width) via screens with perforations 1.5-2× grain thickness
- Aspiration: Uses terminal velocity differences to remove light impurities
- Gravity Separation: Uses density differences on inclined oscillating decks
- Indent Cylinder: Uses length differences—short particles lift out in indents
- Color Sorting: Uses optical properties for final quality improvement
Wheat cleaning achieves 99.5%+ purity using this integrated approach, removing 2-5% foreign matter and damaged kernels.
3.6.2 Drying Operations
Continuous Flow Dryers: Design integrates multiple properties:
- Thermal properties: Determine heat input (specific heat, latent heat)
- Airflow resistance: Dictates column width and airflow rate (typically 20-40 m³/min per tonne/hour capacity)
- Angle of repose: Determines hopper angles for flow control
- Sorption isotherms: Guide air temperature/humidity targets
Modern mixed-flow dryers process 10-20 tonnes/hour wheat with 4-6% moisture removal, using 120-150°C inlet air temperatures and residence times of 30-60 minutes.
3.6.3 Storage Structure Design
Integrated Design Considerations:
- Structural loads: Bulk density determines wall pressures—wheat exerts ~600 kg/m² lateral pressure per meter depth
- Flow patterns: Angle of repose and friction determine hopper geometry—60° cone angles prevent bridging in wheat
- Aeration requirements: Porosity and airflow resistance guide duct spacing and fan sizing—typical 0.1 m³/min/m³ for cooling wheat bulk
- Thermal management: Low thermal diffusivity requires forced air circulation to prevent hot spots from respiration heat
3.6.4 Milling and Processing
Wheat Milling: Hardness determines roll gap settings (0.1-0.3 mm gradual reduction), energy consumption (30-60 kWh/tonne), and flour particle size distribution. Moisture content affects bran toughness—tempering to 15-16% MC optimal for clean bran/endosperm separation.
Dal Milling: Pulse hydration capacity guides soaking parameters. Surface properties affect oil adherence in dehulling (1-2% oil applied). Splitting efficiency depends on achieving 18-22% MC for most pulses.
Oil Expelling: Oilseed moisture (8-10% for most seeds), thermal preconditioning (90-110°C), and pressure application (30-50 MPa in screw presses) are optimized based on thermal and mechanical properties to maximize oil yield (85-90% extraction efficiency).
3.7 Conclusion
Engineering properties of cereals, pulses, and oilseeds form the scientific foundation for PHT equipment design and optimization. Physical properties govern handling and flow behavior, thermal properties dictate heating and cooling processes, aerodynamic properties enable separation and pneumatic transport, and hydrodynamic properties facilitate aqueous processing operations. Successful PHT systems integrate multiple properties in their design, recognizing interactions between properties and their dependence on moisture content and temperature. Understanding these fundamental characteristics enables engineers to design efficient, reliable equipment that minimizes losses, preserves quality, and operates economically across the diverse range of grain types encountered in post-harvest operations.
