MChem Master of Chemistry Master of Chemistry: Industrial Chemistry 4 — Questions and Answers
Question 1: In the context of reactor design, what does the Damköhler number (Da) represent?
- Ratio of convective to diffusive mass transfer
- Ratio of reaction rate to mass transfer rate (or residence time) (Correct answer)
- Ratio of heat generated to heat removed
- Ratio of reactor volume to catalyst surface area
Correct answer: Ratio of reaction rate to mass transfer rate (or residence time)
The Damköhler number (Da = reaction rate / convective transport rate) indicates whether a system is reaction-limited (Da<<1) or transport-limited (Da>>1).
Question 2: What is 'atom economy' as defined by Paul Anastas in green chemistry principles?
- The cost savings achieved by recycling raw materials
- The percentage of atoms in reactants that are incorporated into the desired product (Correct answer)
- The energy efficiency of a chemical process per mole of product
- The fraction of carbon atoms converted without loss to CO2
Correct answer: The percentage of atoms in reactants that are incorporated into the desired product
Atom economy = (MW of desired product / sum of MW of all reactants) × 100%, measuring how efficiently atoms are utilized in a reaction.
Question 3: Which industrial separation technique is used to purify para-xylene from a mixed C8 aromatics stream using zeolite adsorbents?
- Extractive distillation with sulfolane
- Simulated moving bed (SMB) chromatographic separation (Correct answer)
- Cryogenic fractional distillation
- Membrane pervaporation
Correct answer: Simulated moving bed (SMB) chromatographic separation
The UOP Parex process uses a simulated moving bed with zeolite adsorbent (e.g., faujasite) to selectively adsorb p-xylene from C8 isomers.
Question 4: What is the purpose of a 'stripping column' in a gas sweetening unit that removes H2S and CO2 from natural gas?
- To dehydrate the gas with molecular sieves
- To regenerate the amine solvent by heating and releasing absorbed acid gases (Correct answer)
- To compress sour gas before treatment
- To measure H2S concentration using Claus chemistry
Correct answer: To regenerate the amine solvent by heating and releasing absorbed acid gases
The stripping column (regenerator) reverses absorption by heating the rich amine, driving off H2S and CO2, and regenerating lean amine for recycle.
Question 5: In a CSTR (continuous stirred tank reactor) operating at steady state, how does conversion relate to reactor volume compared to a PFR for a positive-order reaction?
- CSTR requires less volume than PFR for the same conversion
- CSTR requires more volume than PFR for the same conversion (Correct answer)
- CSTR and PFR always require identical volumes
- CSTR is more volume-efficient only for zero-order reactions
Correct answer: CSTR requires more volume than PFR for the same conversion
Because a CSTR operates at the exit (lowest) concentration, its rate is lower than average, requiring more volume than a PFR that exploits higher inlet concentrations.
Question 6: What is the Claus process used for in oil and gas refining?
- Desulfurization of diesel using hydrogen
- Recovery of elemental sulfur from H2S-containing gas streams (Correct answer)
- Conversion of heavy residues to lighter fuels
- Removal of mercaptans from LPG
Correct answer: Recovery of elemental sulfur from H2S-containing gas streams
The Claus process partially combusts H2S to SO2, then reacts SO2 with remaining H2S over alumina or titania catalysts to recover >95% of sulfur as elemental S.
Question 7: Which thermodynamic concept governs the maximum theoretical work that can be extracted from a chemical separation process?
- Carnot efficiency of the heat engine driving the process
- Gibbs free energy of mixing (minimum work of separation) (Correct answer)
- Hess's law applied to bond enthalpies
- Van der Waals equation of state for non-ideal mixtures
Correct answer: Gibbs free energy of mixing (minimum work of separation)
The minimum work of separation equals the Gibbs free energy of mixing (−ΔGmix), setting a thermodynamic lower bound on energy consumption for any separation.
MChem Master of Chemistry Master of Chemistry: Industrial Chemistry 4
In the context of reactor design, what does the Damköhler number (Da) represent?