Particle size reduction

Particle Size Reduction

  • Presented by Assoc. Prof. Dr. Rabiha Binti Hj. Sulaiman

Introduction to Size Reduction

  • Comminution: Breakdown of solid material through mechanical forces.

  • Purpose: Acquire desired constituent from composite structures (e.g., flour from wheat, juice from sugar cane).

Importance of Size Reduction

  • Specific size ranges needed for products (e.g., icing sugar, spices).

  • Decreased particle size increases surface area, aiding in:

    • Faster drying times for moist solids.

    • Enhanced extraction rates for solutes.

    • Reduced process times in cooking and blanching.

    • More effective mixing in formulations (e.g., soups, cake mixes).

Forces Used in Size Reduction

  • Three types of forces:

    1. Compressive

      • Used for coarse crushing (e.g., crushing rolls).

    2. Impact

      • General-purpose forces for various grinding operations (e.g., hammer mill).

    3. Shear/Attrition

      • Used for fine grinding of softer materials (e.g., disc attrition mill).

Crushing vs. Grinding

  • Crushing: Reducing coarse material to ~3 mm, mainly using compressive forces.

  • Grinding: Producing powdered material, more associated with attrition forces.

Types of Equipment

  1. Crushing Rolls

  2. Hammer Mill

  3. Disc Attrition Mill

Equipment Selection Factors

  • Hardness and abrasiveness of feed.

  • Mechanical structure of feed.

  • Moisture content.

  • Temperature sensitivity of feed.

Specialized Cutting Operations

  • Types include:

    • Slicing: Produces parallel slices (e.g., fruit).

    • Dicing: Cuts materials into cubes following slicing.

    • Shredding: Creates small fragments, often for dehydration.

    • Pulping: Transforms unacceptable fruits into paste for products like jam.

Energy Requirements in Size Reduction

  • Key laws:

    1. Kick's Law: Energy proportional to size reduction ratio (n=1).

      • Formula: E = K * ln (x1 /x2)

      • E is the energy required, K is a constant specific to the material, x1 is the initial particle size, and x2 is the final particle size.

    2. Rittinger's Law: Energy proportional to new surface area produced (n=2).

      • Formula: E = E = K [1/x2 - 1/x1 ]

      • E represents the energy required for size reduction, K is a constant unique to the material being processed, x2 is the final particle size, and x1 is the initial particle size.

      • So E = K[I/p – 1/f]

    3. Bond's Law: Intermediary approach, n=-3/2.

      • Formula: E = Ei(100/L2)½ [1 – (1/Q)½]

      • E is the energy required for size reduction, Ei is the initial energy, L is the particle size, and Q is the final size reduction ratio.

Calculating Energy Requirements

  • Energy (E) can be expressed as a function of particle size reduction using different equations.

  • Example given for milling operations illustrating energy computations under varying conditions.

Particle Size Determination Methods

  • Methods include:

    1. Direct Microscopy or SEM.

    2. Sieve Method (solid samples).

    3. Particle Size Analyzers (laser diffraction for solid and liquid samples).

Sieve Method

  • Measures particles of 50 to 1500 μm.

  • Results in weight distribution of particles of different sizes.

Mesh and Micron Comparison

  • Understanding the relationship between mesh size and micron.

  • Various conversions demonstrated with tables outlining different sizes and examples.

Practical Applications of Findings

  • Discussions on effects of PSD (particle size distribution) on food products, specifically gluten-free items (rice flour and maize flour applications).

  • Tools and methods for analyzing and improving product quality using particle size analysis.