Flat-blade disc turbine (Rushton) versus pitched-blade turbine: which distinguishing feature most reliably differs between these impellers in standard practice?

Introduction / Context:
Bioreactor impellers strongly influence flow pattern and shear. The flat-blade disc turbine (FBDT, Rushton) is a radial-flow mixer, whereas the pitched-blade turbine (PBT) provides significant axial flow. This question asks for a robust distinguishing feature when comparing the standard versions used in practice.

Given Data / Assumptions:

• Standard Rushton: six flat vertical blades mounted on a central disc, mainly radial discharge.
• Standard PBT: four pitched blades (typically 45 degrees) without a full disc, mixed axial/radial flow.
• We compare common commercial versions rather than all possible variants.

Concept / Approach:
While geometry and flow direction differ, one reliable, easy-to-verify difference is blade count: Rushtons commonly have six flat blades; PBTs often have four pitched blades. Statements about blades projecting beyond the disc or universal air requirements are not generally correct across suppliers and operating conditions.

Step-by-Step Solution:
1) Identify canonical Rushton geometry → 6 flat blades on a disc.2) Identify canonical PBT geometry → 4 pitched blades, no solid disc.3) Recognize that this blade-count distinction is consistent across many standard designs.

Verification / Alternative check:
Vendor catalogs and mixing handbooks depict Rushtons with six blades and PBTs with four as the most common stock offerings; variations exist but are less typical.

Why Other Options Are Wrong:
Option A mischaracterizes geometry and curvature; Option B over-generalizes aeration needs and confounds impeller choice with gas–liquid mass transfer specifics; Option C inherits the inaccuracies of A and B; Option E is plainly false.

Common Pitfalls:
Assuming every supplier adheres to a single geometry; custom blade counts exist, but the 6-vs-4 distinction remains a practical rule of thumb.

Final Answer:
Number of blades used on the impeller (Rushton typically 6 flat blades; pitched-blade turbine often 4)