Section 4: Bioprocess Engineering and Process Biotechnology
Bioreactor types, scale-up
Subtopic: Fundamentals of Bioreactor types, scale-up
Question Statement:
In an aerobic fermentation with air sparging, which of the following options can be
used to increase the volumetric mass transfer coefficient for oxygen transfer?
Options:
(A) Using pure oxygen in place of air
(B) Increasing the volumetric flow rate of air
(C) Reducing the bubble diameter
(D) Decreasing the agitation rate
Official Answer Key:
B;C
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2026 Q.46)**
**1. Governing Concept**:
- **Topic**: Bioprocess Engineering & Oxygen Transfer Enhancement ($k_L a$)
- Volumetric oxygen transfer rate depends on $k_L a$ and the driving force $(C^* - C_L)$:
$\text{OTR} = k_L a (C^* - C_L)$
- $k_L a$ can be increased by:
1. Increasing agitation speed / power input per unit volume ($P/V$), which shears large bubbles into smaller ones, increasing gas holdup ($\epsilon_G$) and specific interfacial area ($a$).
2. Increasing superficial gas velocity / aeration flow rate ($v_s$).
3. Enriching the sparged gas with pure oxygen (which increases $C^*$ without changing $k_L a$).
4. Increasing bioreactor headspace pressure ($P$), which increases equilibrium solubility $C^*$ via Henry's Law ($C^* = P_{O_2} / H$).
**2. Option Analysis**:
- **Option (A) [Decreasing impeller agitation speed] is INCORRECT**: Decreasing agitation speed decreases turbulent shear, produces larger bubbles with lower specific area $a$, and substantially reduces $k_L a$.
- **Option (B) [Increasing impeller agitation speed] is CORRECT**: Higher impeller speed increases turbulent energy dissipation rate, breaks gas bubbles into smaller droplets, elevates gas holdup, and enhances interfacial area $a$.
- **Option (C) [Enriching sparging air with pure oxygen] is CORRECT**: Increases the partial pressure of oxygen in the gas phase ($y_{O_2}$), raising saturation concentration $C^*$ and thereby significantly increasing the oxygen transfer rate driving force $(C^* - C_L)$.
- **Option (D) [Decreasing total bioreactor operating pressure] is INCORRECT**: Lowers the oxygen partial pressure and decreases $C^*$, hurting mass transfer.
**3. Final Answer**:
- **Correct Options**: **B, C**
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