Stirred-tank reactor internals:\nWhat is the main reason for installing baffles in a stirred tank, considering typical industrial operating speeds and common impellers?

Introduction / Context:
Baffles are vertical strips mounted on the vessel wall of stirred tanks. Their presence dramatically changes flow patterns by suppressing swirl and vortex formation, thereby increasing turbulence and improving macro- and micro-mixing. This question asks for the primary reason baffles are used in practice.

Given Data / Assumptions:

• Standard baffling is 4 baffles with width about 0.1 * tank diameter.
• Common impellers: Rushton (radial), pitched-blade (mixed), hydrofoil (axial).
• Operating speeds are sufficient to enter transitional/turbulent regimes.

Concept / Approach:
Without baffles, the liquid forms a central vortex and rotates as a solid body. This reduces relative velocity across the impeller, lowers turbulence, and degrades mixing and gas dispersion. Baffles interrupt tangential flow, convert it into radial/axial components, and promote top-to-bottom circulation—even with radial-flow turbines—resulting in better axial mixing and earlier onset of turbulent behavior at lower speeds.

Step-by-Step Solution:

Verification / Alternative check:
Mixing correlations (e.g., gas dispersion, just-suspended speed) are typically reported for baffled tanks; unbaffled data show poorer performance and strong vortexing.

Why Other Options Are Wrong:

• (a) alone and (b) alone are true but incomplete.
• Baffles prevent turbulence: opposite of reality; they enhance it.
• Eliminating sparging: unrelated; baffles do not supply oxygen.

Common Pitfalls:
Equating vortex presence with “good mixing”; a deep vortex actually reduces useful turbulence and gas dispersion efficiency.

Final Answer:
Both (a) and (b)