Mixing Equipment Overview

Mixing Equipment

• Solid/Solid Mixing
  • Fixed Shell Mixers: Impeller (e.g., ribbon, paddle) rotates inside a fixed shell, lifting, rotating, and distributing material randomly. Primarily convective and shear mixing for rapid coarse dispersion, with some diffusive element for thorough mixing. Common types include trough, ribbon, and paddle mixers.
  • Rotating Shell Mixers/Tumbler Mixers: Various shapes (cuboidal, drum, cone). Intermediate Bulk Containers (IBC) can be used as the mixing vessel. More complex shapes, like V-blenders, provide all-plane material movement for rapid mixing. Materials are typically layered in the vessel for optimal mixing flow. Metal baffles or rods are sometimes added to increase mixing and reduce dead space; high-speed intensifier bars (lump breakers), can be used in specialized applications. Used for free-flowing and granular materials. Can be used with trace amounts of one component.
  • Fluidized Bed Mixers: Powder bed fluidized by high-velocity air stream. Mixing, coating, and drying can all occur in the same unit. Mixing occurs rapidly (1-2 minutes). Problems may arise if bed turbulence is low causing segregation.

Liquid/Liquid Mixing

• Miscible Liquids: Mixing rate can be increased with agitation (impellers).
  • Impellers include paddle, propeller, and turbine types. Intense shear regions are produced by the impeller, which help in mixing. Importance is placed on avoiding unmixed areas (dead zones).
  • Three different flow types: radial (perpendicular to shaft), longitudinal (parallel to shaft), and tangential (circular path around shaft).
• Immiscible Liquids: Surface active agents are often needed to stabilize the dispersed phase in the continuous phase.  High shear rates are crucial to break up the dispersed phase. Axial flow is preferred.

Solid/Liquid Mixers

• Mixer type depends on the liquid-to-solid ratio and density difference.
• Low solid-to-liquid ratios and low density differences can use paddle mixers.
• Higher concentrations or viscosity of solids or higher density differences benefit from more intense mixing methods, such as planetary mixers, sigma blades, and Z-blades.

Paddle Mixers

• Large paddles rotating slowly (10-100 rpm).
• Primarily tangential flow. Baffles can help increase mixing.
• Several types: Simple, gate, stationary, and anchor paddle mixers.

Propeller Mixers

• Smaller than paddles.
• Used in a wide speed range.
• Primarily axial flow and some tangential flow.
• Vortex reduction by offset mounting, helpful for large-scale operations where the propeller is horizontally mounted on the side of the vessel.

Turbines

• Intermediate size between paddles and propellers.
• Effective over a range of viscosities.
• Produce radial and tangential flow.
• Baffles can reduce tangential flow.
• Can improve mixing.

Homogenizers

• Used for emulsifying immiscible liquids.
• Multi-stage process.
• Initial high-speed rotor mixes materials.
• Then centrifugal forces drive to the periphery of the workhead which creates mechanical shear.
• Materials are forced through a screen at high velocity and circulated back to mix.
• Rapid globule size reduction.
• Whole vessel contents are rapidly circulated through mixing.