Agitator scale-up techniques part-1

"Welcome, curious minds!"

         Scaling up an agitation system from a pilot-scale reactor to a commercial production reactor is an important engineering challenge. A mixing system that performs successfully at pilot scale does not automatically guarantee the same performance at commercial scale.

         During scale-up, the objective is to maintain the critical mixing characteristics of the pilot process while increasing the vessel volume and equipment dimensions. Depending on the process requirements, different scale-up criteria may be selected, such as geometric similarity, constant tip speed, constant torque per unit volume, or constant power per unit volume.

         The appropriate scale-up criterion depends on the process objective. For example, a process that is highly sensitive to shear may require constant tip speed, while a process where overall mixing intensity is the primary concern may be scaled using constant power per unit volume.

        In this blog, we will discuss the major agitator scale-up techniques and demonstrate their application through a practical calculation for scaling a 50-litre pilot reactor to a 6,250-litre commercial reactor.

      Agitator scale-up is the process of designing a larger agitation system based on the performance of a smaller, successfully tested pilot-scale system.

The basic objective is, to reproduce the important mixing conditions of the pilot-scale process at commercial scale while maintaining acceptable power consumption, shear, circulation, and process performance.

       There is no single scale-up rule that is suitable for every application. The appropriate criteria depends on the process requirements, fluid properties, impeller type, and desired mixing outcome.

Some of the most commonly used agitator scale-up criteria are:

Keep geometric similarity (shape factor)

Maintain constant geometric aspect ratios between pilot and commercial scales.

          (L/D) _pilot = (L/ D) _commercial

          (Da /D) _pilot = (Da / D) _commercial

Keep constant tip speed

Equates blade outer edge velocity to keep fluid shear rates stable.

          (π x  Da x n) _pilot = (π x Da x n) _commercial

Keep constant torque per unit volume

Equalizes rotational forces relative to liquid volume.

         (n x  T / V) _pilot = (n x T / V) _commercial

Keep constant power per unit volume (P / V)

Ensures matching energy distribution intensity across the liquid mass.

           (Np x ρ x n³ x Da⁵ / V) _pilot = (Np x ρ x n³ x Da⁵ / V) _commercial

 

The required power input depends strongly on the process. The following values can be used as general preliminary guidelines:

Intensity	Applications	Power (KW/m3)
Low power	For light solids and low viscosity fluid	0.2
Moderate power	For suspending moderate density solids, gas dispersion and mild heat transfer	0.6
High power	For suspending heavy solids, gas dispersion	2
Very High power	Blending paste like solids	4

 

Let us solve a practicle example

A pilot-scale run successfully achieves the desired reaction under the following conditions,

Vessel Volume (V₁): 50 litres = 0.05 m³

Impeller Diameter (D₁): 0.2 m (pitched blade turbine)

Agitator Speed (N₁): 285 rpm = 4.75 rev/s

Fluid Density (ρ): 1,000 kg/m³ (water-like)

Fluid Viscosity (μ): 0.001 Pa⋅s

Impeller Power Number Np: 1.3 (constant for this turbulent geometry)

scale up a turbulent liquid mixing process from a 50-litre pilot vessel to a 6,250-litre at commercial scale reactor.

   We apply geometric similarity, meaning all linear dimension ratios remain constant between scales.

Volume scale factor (V₂/V₁) = 6250/50 =125

Linear scale factor (R)= 125^0.33 =5                  (as, Vα D³ )

Commercial impeller diameter would be (Da₂) = 0.2 x 5 = 1 mtr

Commercial scale reactor agitator with same Power per unit volume would be calculated as,

 (P α n³ x Da⁵  and V α Da³ at H = D . So, P/V α n³ Da² )

 Therefore, n₂/n₁ = (Da₂/Da₁)^2/3

                                 N₂ = 285 x (0.2/1)^2/3 =98 rpm

        To recheck power per unit volume similarity, for pilot scale reactor power requirement would be,

   P₁ =Np _x n₁³ x ρ * Da⁵ = 1.3 x (285/60)³ x 1000 x 0.2⁵ =44.6 watts

   P₁/V₁ = 44.6 /0.05 = 892 watt/m³

      For commercial scale reactor, power requirement would be,

P₂ = 1.3 x (98/60)³ x 1000 x 1⁵ =5664 watts

P₂/V₂ = 5664 /6.25 = 906 watt/m³

 

           Ultimately, the goal of scale-up is not simply to make the equipment larger—it is to ensure that the commercial reactor delivers the required process performance reliably, safely, and economically.

I hope this blog gives you a practical understanding of the fundamentals of agitator scale-up techniques and helps you in your process and equipment design work. 

Happy learning and happy mixing!

Your comments are highly appreciated.