Chemical Reaction Engineering Tutoring

Reaction engineering is where chemical engineers learn to size the vessel that the chemistry happens in. Underneath, it is one repeated idea. Write the mole balance, add the rate law, use stoichiometry to put everything in terms of conversion, combine, and solve. Students lose points by skipping a step, usually the stoichiometry, or by picking the wrong design equation for the reactor. STEM Tutor 360 drills that sequence on batch, CSTR, and PFR problems until it holds up under exam pressure.

Sessions cover rate laws and the Arrhenius equation, conversion and Levenspiel plots, reactors in series, multiple reactions and selectivity, adiabatic and heat-exchange reactors, residence time distributions, and the basics of catalysis. We follow your text, such as Fogler or Levenspiel, and your professor's exams. The calculus is light. The bookkeeping is not, and that is where most of the time goes.

A typical reactor design problem, worked

A liquid-phase, first-order reaction A to B has a rate constant $k = 0.25\ \text{min}^{-1}$. The feed is 10 L/min, and the goal is 80% conversion. Compare the volume of an ideal PFR with an ideal CSTR.

PFR: $$V = \dfrac{v_0}{k} \ln\dfrac{1}{1 - X} = \dfrac{10}{0.25} \times \ln 5 = 40 \times 1.609 = 64.4\ \text{L}$$ CSTR: $$V = \dfrac{v_0 X}{k(1 - X)} = \dfrac{10 \times 0.80}{0.25 \times 0.20} = 160\ \text{L}$$

The CSTR needs about 2.5 times the volume. All of its reaction happens at the exit concentration, which is the lowest in the vessel, so the rate is slow everywhere. The PFR sees high concentrations and fast rates near the inlet.

Now try two equal CSTRs in series. Each tank must satisfy $(1 + k\tau)^{2} = \dfrac{1}{1 - X} = 5$, so $k\tau = 1.236$ and $\tau = 4.94\ \text{min}$. Each tank is 49.4 L, for a total of 98.9 L. Adding tanks pulls the total volume toward the PFR value.

Where students usually get stuck

• Skipping stoichiometry on gas-phase reactions, where a change in moles changes the volumetric flow rate and the concentration.
• Using the CSTR equation where the PFR belongs, or the reverse. In a CSTR the rate is evaluated at the exit. In a PFR you integrate along the reactor.
• Rate constant units that do not match the order, such as $\text{min}^{-1}$ against $\text{L/(mol min)}$.
• In selectivity problems, maximizing conversion when the goal is the yield of the desired product.
• In non-isothermal problems, the sign of the heat of reaction, and forgetting that both the rate constant and the equilibrium constant change with temperature.