### **Official Solution & Step-by-Step Derivation (GATE BT 2016 Q.31)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Bioprocess Engineering / Bioreactor Agitation (Impeller Selection for High Viscosity Fluids)
**2. Detailed Step-by-Step Analysis**:
- The choice of impeller in bioprocess reactors depends strictly on fluid rheology and viscosity:
1. **Propellers and Flat-blade Rushton turbines**: Small diameter ($D_i / T \approx 0.33$), operated at high rotational speeds; generate high shear and local turbulence. Highly effective for low-viscosity fluids (
lt; 10^3\text{ cP}$), but in high-viscosity fluids they produce a localized cavern of motion, leaving the bulk fluid unmixed.
2. **Paddles**: Moderate diameter ($D_i / T \approx 0.5 - 0.8$), operated at low speeds for intermediate viscosities up to $10^4\text{ cP}$.
3. **Helical ribbon and Anchor impellers**: Large diameter close to vessel wall ($D_i / T \approx 0.95 - 0.98$), operated at slow rotational speeds. The continuous spiral ribbon forces viscous material to flow axially along the vessel wall and down the core, ensuring positive bulk top-to-bottom displacement without dead zones.
- For extremely high viscosity broths (viscosity
gt; 10^5\text{ cP}$, e.g., fungal mycelia, xanthan gum, or heavy polymer solutions), the **helical ribbon impeller** is the most suitable mixing impeller.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: Marine propellers are axial impellers for low-viscosity fluids (
lt; 1000\text{ cP}$).
- **Option (B) [CORRECT]**: Helical ribbon impellers provide positive displacement and uniform bulk turnover for highly viscous non-Newtonian fluids (
gt; 10^5\text{ cP}$).
- **Option (C) [INCORRECT]**: Standard paddles cannot eliminate stagnant zones in highly viscous fluids.
- **Option (D) [INCORRECT]**: Flat blade turbines lose mixing efficacy in high-viscosity fluids, producing localized channeling.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **B**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).