What is Agitation Index?

In stirred tanks (chemical and process industries), the agitation index (Ig) is a single number that tells how effectively an impeller (mixer) creates overall liquid movement inside the tank.

In simple words: Agitation index is a stirring score for a tank.

  • Higher Ig usually means stronger tank-wide circulation.
  • Lower Ig suggests weaker overall movement and more slow-moving zones.

Note: "Agitation index" can mean different things in other fields. Here, it strictly refers to industrial stirred vessels and impellers.

Key Terms (Quick)

  • Impeller/agitator: rotating mixer inside the tank (e.g., Rushton turbine, pitched blade, propeller, hydrofoil).
  • Stirred tank: mixing vessel, often with baffles to improve circulation.
  • Velocity field: liquid velocity measured at many points in the tank (via instruments or CFD).
  • Impeller tip velocity (Utip): speed at the impeller's outer edge.
  • Volume-weighted mean velocity (U*): average tank velocity weighted by the volume each measurement represents.

How Agitation Index is Calculated (Step-by-step)

  1. Measure liquid velocities at many points in the tank (commonly using LDV or PIV).
  2. Divide the tank into grid cells, where each measured velocity represents a small tank volume.
  3. Compute velocity magnitude for each cell (from available components).
  4. Calculate volume-weighted mean velocity (U*) for the whole tank.
  5. Compare U* with impeller tip velocity (Utip) and express it as a percentage → Ig (%).

Formulae (Simple and Standard)

1) Volume-weighted mean velocity

U * = ∑ ( V i j U i j ) ∑ V i j

Where:

  • U* = volume-weighted mean velocity (m/s)
  • Vij = volume of grid cell (i, j)
  • Uij = velocity magnitude at that cell (m/s)

2) Agitation index

I g = 100 × U * U tip [ % ]

3) Impeller tip velocity

U tip ≈ π N D

Where:

  • N = rotational speed (rev/s)
  • D = impeller diameter (m)

How Engineers Use Agitation Index

1) Compare impellers objectively

Ig lets engineers compare different impellers using one measurable score—useful when deciding between radial vs axial designs or different brands/models.

2) Improve tank configuration

Ig helps evaluate whether changes like:

  • impeller clearance (height from bottom),
  • adding a second impeller,
  • changing spacing in multi-impeller tanks,
  • actually increase overall circulation.

3) Support scale-up and troubleshooting

When moving from lab to plant scale, mixing can change due to geometry, power input, or viscosity. Ig provides a consistent way to compare "how much bulk motion" the tank achieves across setups.

4) Use with CFD (simulation)

If CFD provides a velocity field, engineers can compute U* the same way and convert it into Ig for design comparison.

What Affects Agitation Index?

Impeller type (radial vs axial)

  • Radial impellers (e.g., Rushton turbine) push fluid outward; often higher shear.
  • Axial impellers (e.g., pitched blade, hydrofoils) pump fluid up/down; often strong circulation loops.

Operating and design factors

Typically increases Ig

  • higher rpm (higher N)
  • larger impeller diameter (D)
  • effective baffles
  • better impeller positioning (reduces dead zones)

Typically decreases Ig

  • higher viscosity (weaker circulation)
  • no baffles (more swirl/vortex, less useful circulation)
  • poor impeller placement (stagnant regions)

Agitation Index vs Agitation Scales (Common Confusion)

  • Agitation index (Ig) in this article is an engineering measure based on fluid velocity in a tank (often a %).
  • Medical terms like:
    • Agitated Behavior Scale (ABS)
    • Richmond Agitation–Sedation Scale (RASS)
    are patient scoring systems, not mixing metrics.

Quick rule:

  • If it mentions impellers, baffles, rpm, LDV/PIV, πND → engineering Ig.
  • If it mentions ICU, sedation, patient behavior → medical scales.

Where It Matters in India (Practical Examples)

Agitation index is useful in many Indian industries where mixing quality affects consistency and yield:

  • Pharma & API reactors
  • Fermentation and bioreactors
  • Paints, adhesives, polymers
  • Wastewater treatment tanks
  • Food processing (syrups, dairy mixes, beverages)
  • Chemical engineering education (mixing science, LDV/PIV, CFD validation)

FAQs

What is agitation index in a stirred tank?

A score that shows how effectively the impeller creates overall liquid movement in the tank.

Is a higher agitation index always better?

Higher usually means stronger circulation, but the best level depends on the process (blending, suspension, gas dispersion, shear sensitivity).

How do you measure velocities for Ig?

Commonly by LDV or PIV, or by CFD (if validated).

Does viscosity affect agitation index?

Yes. Higher viscosity often reduces circulation and can lower Ig.

Is agitation index the same as mixing index?

Not always. Many "mixing index" terms measure concentration uniformity, while Ig here is velocity/circulation-based.

Summary

The agitation index (Ig) is a clear, practical way to express how well a stirred-tank impeller moves liquid throughout the vessel. It is calculated by taking a volume-weighted average liquid velocity and expressing it as a percentage of the impeller tip velocity. Engineers use it to compare impellers, optimise tank setups, support scale-up, and validate CFD—especially in industries like pharma, food, wastewater, and paints.