Lecture 5

Physical Properties of Grains

• Geometrical Properties

• Engineering Properties

Why are Physical Characteristics Important?

• Optimize aeration or drying processes

• Affect grain flow properties as well as heat and mass transfer rates

• Can calculate thermal conductivity, diffusivity, surface and volume heat transfer coefficients, bulk density, resistance to airflow, internal friction, and breakage

• Each of these are directly or indirectly related to economics

Geometrical Properties

• Shape (roundness, sphericity, roundness, sphericity), volume, kernel density, bulk density.

Shape• Describes the object• Tracings compared with shapes listed on standard charts (Subjective).• Shape Description• Round Approaching a sphere• Oblate Flattened at the stem end and apex• Oblong Vertical diameter > horizontal diameter• Conic Tapered toward the apex• Ovate Egg-shaped• Obovate Inverted ovate• Elliptical Approaching an ellipsoid• TruncateHaving both ends squared or flattened• Regular Horizontal section approaches a circle • IrregularHorizontal cross section deviates from a circle

Roundness• Roundness measures sharpness of corners

Roundness=Ap/Ac

Where, Ap = largest projected area of object (kernel)

Ac = area of smallest circumscribing circleArea is obtained by projection or tracing.

Roundness ratio = r/R

Roundness = r/NR Where, r = radius of curvature of small circleR = Radius of maximum inscribed circleN = total number of corners

Sphericity: Measuring axial dimensions

• Trace outline of projected images of seeds.

• Maximum (a, b axes) and minimum (axes c) projected areas are traced.

• Sphericity = di/dc

• Where, di = diameter of largest inscribed circle

• dc = diameter of smallest circumscribed circle

Sphericity

Isoperimetric property of a sphere:

Assuming volume of the object is equal to 3

axial measurements of an ellipsoid (a, b, c) and the

diameter of the circumscribed sphere is the longest

axial measurement, sphericity is expressed as:

•Sphericity can also be defined in terms of volume of an object (kernel)

•Sphericity = de/dc

•Where de = diameter of a sphere of the same volume as the object

•dc = diameter of the smallest circumscribing sphere

Sphericity Continued

Calculating Volume (V) and Surface Area (S)

There are several standard formulas for different shapes. For example,

(Prolate spheroid)

(Oblate spheroid)

(Cone)

• Regression method

• V = A a1b1a2

b2a3b3

• ln V = lnA + b1 ln a1 + b2 ln a2 + b3 ln a3

Calculating Volume

Measuring Volume

• Grains are irregular objects

• Volume determined by water displacement method for large objects like fruits, potatoes, etc.

For smaller objects like small fruits and grains, a specific gravity balance can be used to measure volume, density, and specific gravity

Volume = (weight in air – weight in water) _____________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Weight density of water

Volume

Volume

Measuring Surface Area

• Important for heat and mass transfer through grains•Expressed as Square ft/Cubic ft

•One Method utilizes coating the grains with a single layer of metal powder (i.e. nickel) and measuring the change in weight. A control group, consisting of shapes of a known surface area and density (similar to grain being tested) is also coated. A factor representing the coating weight per unit surface area for the control group is used to calculate the surface area of the grain. You need to know the bulk volume to make the calculation work.

•A second method utilizes principles of flow through porous media. It is measured by air permeability though a packed bed of grains. This method measures the available surface area in a bed of granular materials.

Data obtained using the metal powder coating method

Porosity• Ratio of intergranular volume to total space occupied by

the grain (kernel volume plus the volume of intergranular space)

• A quantitative measure of kernel packing influenced by method of filling and period of storage

Grain Interporosity• Refers to pores within a kernel

• Can influence the variety differences as well as mechanical properties such as hardness, elasticity, plasticity, resistance to compression, and water movement in grain.

• Less than 103 Angstroms are called micropores

• More than 103 Angstroms are called macropores

• Based on a mercury penetration technique– Intrusion of mercury into a capillary is a function

of its diameter as given by a formula

• See reading assignment for more detail

Kernel Density vs. Bulk Density

• Kernel Density: the ratio of the kernel’s mass to its volume (excluding the intergranular spaces)

• Bulk Density: the ratio of the grain mass divided by the volume occupied by the grain kernels and the intergranular spaces– Varies with kernel density, spatial arrangement,

and moisture content

Moisture Content• Many methods

– Oven Drying: determined on a dry weight basis– Published in the ASAE Standards Book

Wheat: 10 g of wheat at 130oC for 19 hours

Corn: 15 g of corn at 103oC for 72 hours

Canola: 10 g of canola at 130oC for 4 hours

% moisture = 100 [(original weight – dried weight) / original weight]

Standards for:

Engineering Properties

• Angle of Repose

• Distribution of weight in a bin

• Hardness of grain

• Breakage of grain

• Stress cracking

• Grain dust

Angle of Repose

Influenced by• frictional forces generated by the grain flowing against itself

• distribution of weight throughout the grain mass

• moisture content of the grain

Could calculate how much grain will flow from a door in the side of a bin

Distribution of Weight

= Force

Magnitude of the force can be estimated using Janssen’s equation

Grain

Headspace

Grain Hardness

• Defined as resistance to being reduced in particle size

• Measured by NIR (near-infrared reflectance) or SKH (single kernel hardness tester)

Influences:– Susceptibility to insect attack (rice weevils but not lesser

grain borers)– Breakage during handling– Milling properties– Damage to starch produced during dry milling or grinding

Breakage

• % broken kernels are a determinant of grade

• Low moisture = more breakage

• Corn is more susceptible than other grains

Breakage occurs:

during harvest, handling, turning, or anytime the grain is moved

Stress Cracking

• A consideration when drying grain– Moist grain will swell and dry grain will shrink– If the rate of drying is too fast than the outside

dries and shrinks before the center of the kernel have shrunk resulting in high internal pressure to crack

– Result in planes of weakness that lead to breakage when moving the grain

Grain Dust

• Produced by forces generated during handling.

• Grain rubs against:– Itself– Bin walls– Equipment

• Can lead to dust explosions

The End

Your brain on engineering properties?