Questions
7–9 questions in major PSU papers
Difficulty
Medium-Hard
Importance
Core — never skip
Overview
Chemical Reaction Engineering (CRE) is the study of how chemical reactions occur and how to design reactors to optimize yield, selectivity, and conversion. It is a high-weightage core subject for all major PSU exams as it forms the technical foundation for process engineering and plant operations.
Reaction Kinetics and Rate Laws
Kinetics focuses on the rate at which reactants are converted into products and the dependence of these rates on temperature and concentration. Understanding the Arrhenius equation and order of reaction is critical for predicting reactor performance under varying conditions.
- Rate law: -rA = k[CA]^n
- Arrhenius Equation: k = A*exp(-E/RT)
- Zero, first, and second-order kinetics
- Effect of temperature on rate constant
- Elementary vs. non-elementary reactions
Ideal Reactors: Batch, CSTR, and PFR
These idealized models provide the mathematical framework for reactor sizing and performance analysis. PSUs frequently test the design equations derived from mass balances for these systems.
- Batch Reactor: t = integral(dXA / (-rA))
- CSTR: V/FA0 = XA / (-rA)
- PFR: V/FA0 = integral(dXA / (-rA))
- Comparison: PFR requires smaller volume for n > 0
- Damkohler Number (Da) for reactor performance
Non-Ideal Reactors and RTD
In real-world scenarios, flow patterns deviate from ideal behavior due to channeling, bypassing, or dead zones. Residence Time Distribution (RTD) analysis using E(t) and F(t) curves is the standard method for quantifying these deviations.
- E(t) curve: Exit Age Distribution
- F(t) curve: Cumulative distribution
- Mean residence time: tm = integral(t*E(t)dt)
- Segregation and Maximum Mixedness models
- Tanks-in-series model for PFR approximation
Heterogeneous Catalysis
Heterogeneous systems involve multiple phases and complex steps like adsorption, surface reaction, and desorption. These problems are common in advanced PSU technical papers and focus on the rate-limiting step.
- Langmuir-Hinshelwood kinetics
- Eley-Rideal mechanism
- Thiele Modulus (phi) for diffusion effects
- Effectiveness factor (eta)
- External and internal mass transfer resistance
Formula Sheet
Arrhenius: k = A*exp(-E/RT)
CSTR Design: V = FA0*XA / (-rA)
PFR Design: V = FA0 * integral(0 to XA) (dX / -rA)
Batch Design: t = CA0 * integral(0 to XA) (dX / -rA)
Mean Residence Time: tm = V/v0
Effectiveness Factor: eta = actual rate / rate at surface conditions
Thiele Modulus: phi = L * sqrt(k/Deff)
Exam Tip
Master the design equation graphs (1/-rA vs XA) as they allow you to visualize and solve complex reactor volume questions faster than algebraic derivation.
Common Mistakes
- Mixing up the design equations for CSTR and PFR during high-pressure exam scenarios.
- Ignoring the temperature sensitivity of the rate constant when solving non-isothermal problems.
- Miscalculating the order of reaction by assuming all reactions are elementary.
More Revision Notes
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