Mixing of highly viscous liquids and pastes in process vessels: which impeller choice from the list is generally preferred?

Introduction:
Very viscous fluids and pastes require low-speed, high-torque impellers that can scrape, sweep, and move material without excessive shear heating or motor overload. Selecting the wrong impeller results in poor turnover and stagnant zones.

Given Data / Assumptions:

• Target media: highly viscous liquids/pastes (high apparent viscosity, often non-Newtonian).
• Conventional baffled tanks may be used, but wall-sweeping is valuable.

Concept / Approach:
Multiple-blade paddles (and related anchor/helical ribbon mixers) provide bulk movement at low rpm and high torque, suitable for viscosities where propellers or turbines would simply churn locally without overall circulation. Turbines excel in moderate viscosities; propellers are for thin liquids with high flow numbers; gas-inducing designs address gas–liquid dispersion rather than paste handling.

Step-by-Step Solution:
Identify rheological demand: high torque, low speed.Map impeller types to viscosity windows.Choose multiple-blade paddles for pastes and very viscous fluids.

Verification / Alternative check:
Design charts (e.g., Metzner–Otto type correlations) show paddles/anchors/ribbons in the high-viscosity corner of selection maps, confirming suitability.

Why Other Options Are Wrong:

• Propellers: inadequate pumping in high viscosity; poor turnover.
• Turbines: better than propellers, but still targeted to moderate viscosities.
• Gas-inducing impeller: used for gas–liquid mass transfer, not for pastes.

Common Pitfalls:
Running high rpm to compensate for poor circulation—this only increases power draw and shear without improving bulk mixing in viscous media.

Final Answer:
Multiple-blade paddles (slow-speed, high-torque)