Lec5_Mixing of Solids

Lecture 5: Mixing of Solids

Overview

  • Focus on mixing techniques for cohesive and non-cohesive solids.

Solid and Liquid Mixing

Liquids

  • Low Viscosity Liquids

    • Intermingle components for uniform product formation.

    • Liquid blending generates flow currents that move unmixed material to mixing zones.

    • Resultant product is a homogeneous liquid phase.

    • Requires small samples to ensure composed similarity.

Solids

  • Cohesive and Free-flowing Solids

    • Equipment for liquid mixing is sometimes applicable for solids.

    • Lack of current generation increases energy requirements.

    • Comprises distinct individual particles, hence small samples may vary in composition.

    • Important properties include wettability, stiffness, and tackiness.

Types of Mixture

  • Attainability of Perfect Mixture

    • Perfect mixture is unattainable; the aim is to achieve a random mixture.

      • In this scenario, every particle has an equal probability of being found anywhere.

    • Segregating Mixtures show a higher likelihood of finding particles in specific regions.

    • Better mixtures use attractive forces between solid particles.

Examples

  • Coin toss exemplifies a random probability scenario (1 in 2 chances).

  • Dice usage indicates spatial distribution (lower half holds more relevant numbers).

Causes of Segregation and Demixing

Factors Leading to Segregation

  • Particle Size

    • Differences in size lead to easier separation.

  • Density of Particles

    • Density disparities are critical in gas fluidization, where size is less influential.

  • Importance of Maintaining Mixture Quality

    • Essential for proper packaging (e.g., detergents, drugs).

    • Ensures visual uniformity, high strength, and consistent burning rates.

Mechanisms of Mixing

Types of Mixing Mechanisms

  • Shear Mixing

    • Creates slip zones leading to particle interchange.

  • Diffusive Mixing

    • Rolling of particles on inclined surfaces.

  • Convective Mixing

    • Involves deliberate bulk movements of particle packets.

Assessing the Mixture

Scale of Scrutiny

  • Measures for Degrees of Mixing

    • Region of segregation size indicates mixing level.

    • Specific measurements might include size of detergent scoops or tablet material quantities.

    • Different products necessitate varied scrutiny scales.

Rules of Sampling

  • Sample should be taken while powder is in motion, with multiple increments to gather representative data.

Effect of Scale of Scrutiny

Variance with Samples

  • Sample sizes directly affect the perceived degree of mixing.

    • 36 samples yield variance of 0.25.

    • 9 samples yield variance of 0.04.

    • 4 samples yield variance of 0.02.

    • Higher quality mixtures show lower variance, indicating improved mixing.

Normal Distribution

Importance

  • Utilized to calculate mean and standard deviation, helping assess mixed-ness of solids.

Mixture Composition Measurement

Key Metrics

  • Mean Composition

    • Key for evaluating component concentrations.

    • Standard deviation informs about variance metrics in mixing operations.

Measure of Degree of Mixing

Mixing Indices

  • Ratio of achieved mixing to the maximum achievable.

    • An index of 0 indicates complete segregation.

Equipment for Mixing Solids

Classifications of Mixers

  • Bunker and Silo Mixers

  • Rotating Tumbling Mixers

  • Agitated Mixers

Mixer Versatility

  • Capable of addressing materials with changing properties and performing under varying conditions.

  • Operate continuously or in batch processes and can perform additional functions (e.g. drying, humidification).

Specific Mixer Types

Bunker and Silo Mixers

  • Operate continuously or batchwise; recirculation is essential.

  • Can perform functions like humidification and drying.

Gravity Silo Mixers

  • Mix individual solid layers through different flow patterns, employing various inserts like cones.

Pneumatic Mixers

  • Designed for fluidizable solids via bubble formation and air introduction.

Additional Equipment

  • Rotating Tumbling Mixers

    • Achieve mixing through particle flow over surfaces, appropriate for free-flowing solids.

  • Agitated Mixers

    • Utilize mechanical means (paddles, ribbon) for stronger agitation, suitable for various material types.

Dynamic Mixers

  • Fluidizing Paddle Mixer

    • Design includes paddles on twin shafts, crucial for strong convective mixing.

  • Screw Mixers

    • Characterized by screw mechanisms that cause local dispersive mixing.

High Shear Mixers

  • Suitable for fine powder production and generally employ high energy.

Conclusion

  • Mixing of solids encompasses multiple methodologies that vary based on material properties, equipment functionality, and performance outcomes.