Pharmaceutical Engineering
Mixing Time Estimator
Estimates blend time in a stirred tank from the vessel and impeller geometry, and reports the impeller Reynolds number, flow regime and power number.
Estimated mixing time
Enter the tank and impeller diameters, the impeller speed, and the fluid properties.
Vessel and impeller
Geometry, speed and the properties of the batch being mixed.
Internal diameter of the vessel.
Usually D/3 to D/2.
Revolutions per second. 120 rpm = 2 rps.
Water at 20 °C is about 0.001 Pa·s.
Bulk density of the batch.
Sets the constant K in the correlation.
Impeller reference
Turbulent power numbers and what each impeller is for.
| Impeller | Np (turbulent) | Typical use |
|---|---|---|
| Rushton turbine | 5.0 | Gas dispersion |
| Pitched blade | 1.5 | Blending |
| Propeller | 0.3 | Low power blending |
| Anchor | 0.3 | High viscosity |
| Helical ribbon | 0.3 | Very viscous |
The impeller Reynolds number decides the flow regime:
Re = (ρ × N × d²) / μ
Below Re 10 the flow is laminar, from 10 to 10 000 transitional, and above that turbulent. The power number is estimated from Re against a Rushton turbine: Np = 70/Re in the laminar region, 70/√Re in the transitional region, and a constant 5.0 when turbulent.
The blend time uses a geometric correlation with a constant K set by the mixing duty:
t = K × (D/d)² × (1/N) × 60
K is 4 for blending, 6 for a chemical reaction, 8 for solid suspension and 10 for dispersion. Note the unit inconsistency flagged beside the result: the factor of 60 assumes N in rpm, while Re above assumes rps.
Common questions
Because of the ×60 in the correlation. With the speed entered in revolutions per second, the dimensionless form t = K(D/d)²/N gives a time 60 times shorter. The tool reports the original figure and states the discrepancy rather than silently changing it.
2 rps. Divide rpm by 60. The Reynolds number and the flow regime depend on getting this right.
Above that the flow is fully turbulent and Np becomes independent of Reynolds number — a constant for each impeller geometry (5.0 for a Rushton turbine). Below it, Np still falls with increasing Re.
In this correlation yes, because time scales with (D/d)². In practice a larger impeller also needs far more power — power scales with N³d⁵ — so the choice is a trade-off, not a free win.
Prepared by the PharmaWallah student team. Verify results with your textbook or instructor. Report an error · Editorial policy