Subtopic: Fundamentals of Biomolecules, enzyme kinetics
Question Statement:
For an ideal plug flow reactor, which of the following statements is/are true?
Options:
(A) It can be approximated by a large number of ideal continuous stirred tank reactors
connected in series
(B) It has significant back mixing
(C) It has significant axial mixing
(D) It has a uniform velocity profile across any cross section
Official Answer Key:
A;D
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2026 Q.49)**
**1. Governing Concept**:
- **Topic**: Chemical Reaction Engineering & Plug Flow Reactor (PFR) Dynamics
- An **ideal Plug Flow Reactor (PFR)** is characterized by:
1. **Zero axial mixing** (no back-mixing along the flow direction, Peclet number $Pe \to \infty$).
2. **Complete radial mixing** (fluid properties, concentration, and temperature are completely uniform across any cross-section perpendicular to flow).
3. **Continuous fluid flow**: Every fluid element has identical residence time equal to space time ($\tau = V / v_0$).
4. Concentration of reactants decreases continuously along the axial length of the reactor from inlet to exit.
**2. Option Analysis**:
- **Option (A) [There is no axial mixing of fluid elements] is CORRECT**: The defining characteristic of plug flow is that no fluid element mixes with any element ahead of or behind it.
- **Option (B) [Fluid elements have a broad distribution of residence times] is INCORRECT**: In an ideal PFR, all fluid elements spend the exact same time inside the reactor; the residence time distribution is an ideal Dirac delta function $\delta(t - \tau)$.
- **Option (C) [Concentration of reactant is uniform throughout the reactor] is INCORRECT**: That describes an ideal CSTR (continuous stirred tank reactor), whereas in a PFR concentration changes continuously along reactor length.
- **Option (D) [Fluid properties are completely uniform in the radial direction] is CORRECT**: Infinite radial dispersion is assumed across each circular cross-section.
**3. Final Answer**:
- **Correct Options**: **A, D**
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