Question 4 of 6: Overall Procedure for Chemical Reactor Design

Question 4: Overall Procedure for Chemical Reactor Design (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Reactor design proceeds from reaction data, through selection of type and conditions, to sizing and mechanical specification, iterating with the rest of the flowsheet because conversion and selectivity fix the separation and recycle loads downstream. A representative overall procedure is:

1. Collect the reaction data. Establish the stoichiometry of all main and side reactions, the thermodynamics (equilibrium conversion, heat of reaction), the kinetics (rate expression, activation energy, order), and, for catalytic systems, the catalyst activity, selectivity and deactivation behaviour.
2. Collect the physical-property data. Gather densities, viscosities, heat capacities, phase-equilibrium and transport properties of feeds, products and any diluent or solvent — needed for heat and mass balances and for transport/mixing calculations.
3. Choose the reactor type and operating mode. Decide batch vs. continuous and the ideal-reactor model (CSTR, PFR/tubular, fixed-bed, fluidised-bed, slurry, etc.) based on the phases present, catalyst form, required residence-time distribution, and the heat duty.
4. Select the operating conditions. Fix temperature, pressure, feed composition/ratio, diluent and target conversion to maximise yield and selectivity while respecting equilibrium, catalyst limits and safety — and decide the thermal mode (isothermal, adiabatic, or externally cooled/heated).
5. Establish the rate-controlling regime and design rate equation. Determine whether intrinsic kinetics, or heat/mass-transfer resistances, control, and adopt the appropriate rate expression for sizing.
6. Size the reactor. Apply the material and energy balances for the chosen reactor model to compute the volume (or catalyst mass) and residence time that deliver the specified conversion, yield and selectivity.
7. Determine the heat-transfer requirement. From the heat of reaction and the thermal mode, specify heating/cooling surface, coolant/heating medium and temperature control to keep the reactor within its safe and selective operating window (avoiding runaway or hot-spots).
8. Select materials of construction. Choose materials compatible with the reactants, products and conditions (corrosion, temperature, pressure), including catalyst supports and internals.
9. Carry out the preliminary mechanical design. Specify vessel geometry and thickness, internals (agitators, baffles, distributors, catalyst supports, heat-transfer surfaces) to an appropriate pressure-vessel code.
10. Cost, optimise and iterate with the flowsheet. Estimate capital and operating cost, optimise the design (e.g. conversion vs. recycle/separation cost trade-off), and revisit the choices above in the context of the whole process, since reactor conversion and selectivity set the downstream separation and recycle duties.