Question 5 of 6: Alkylation selection and safety; hydrogen production and recovery

Question 5: Alkylation selection and safety; hydrogen production and recovery (20 marks — equal value)

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.

(a) Factors in selecting an alkylation process (3 marks)

The choice between the two commercial catalysts — sulphuric-acid (H₂SO₄) and hydrofluoric-acid (HF) alkylation — turns on: (1) the catalyst safety and environmental profile (HF toxicity and vapour-cloud hazard vs. H₂SO₄ handling and spent-acid disposal); (2) the feedstock (olefin type C₃–C₅ and isobutane availability) and required alkylate octane and yield; and (3) economics — acid consumption and regeneration, refrigeration/utilities, capital cost, and site/regulatory constraints.

(b) Why these factors matter (3 marks)

The catalyst decision dominates because it sets the plant’s hazard and cost structure: HF gives excellent octane and low acid consumption but, being volatile and acutely toxic, can form a dense ground-hugging vapour cloud on release, so it demands costly mitigation and faces mounting regulatory and community opposition. H₂SO₄ is far less volatile (lower acute risk) but is consumed in large quantities and must be trucked off-site for regeneration, and it needs refrigeration to hold the low reaction temperature. Feed quality and octane targets set the alkylate value and volume, while acid handling and refrigeration dominate operating cost — so these factors together decide which process is safe, permittable and economic at a given site.

(c) Main safety risks (3 marks)

The principal hazards are: toxic-acid release — especially HF, which is corrosive and acutely toxic and forms a heavier-than-air aerosol/vapour cloud; fire and explosion of the light, volatile hydrocarbons (C₃/C₄ olefins and isobutane) handled at pressure; corrosion and equipment failure from the acids leading to leaks; and chemical burns from acid contact (skin/eye), with HF additionally causing deep-tissue and systemic (hypocalcaemia) injury.

(d) Precautions (2 marks)

Precautions include rapid acid de-inventory and isolation systems with remotely operated valves; water-spray curtains and vapour-suppression (and additives that reduce HF aerosol formation) to knock down any release; robust leak detection, secondary containment and corrosion-monitoring programmes; pressure relief routed to a flare; adequate separation distance from occupied areas; and strict PPE, calcium-gluconate first aid for HF, and rigorous operator training and emergency response.

(e) Two hydrogen-production processes (6 marks)

1. Steam methane reforming (SMR). Natural gas (methane) reacts with steam over a nickel catalyst in fired tubes; the strongly endothermic reforming is followed by the exothermic water-gas shift:

Conditions: reformer ~800–900 °C and 15–30 bar with excess steam (steam/carbon ~3), then high- and low-temperature shift (~350 °C and ~200 °C), with final purification by PSA. This is the dominant on-purpose hydrogen source.

2. Catalytic naphtha reforming (hydrogen as by-product). Naphtha is reformed over Pt–Re / alumina catalyst to raise octane, and the dehydrogenation of naphthenes to aromatics liberates hydrogen, e.g.

Conditions: ~490–525 °C and 10–35 bar with hydrogen recycle; the net hydrogen make is a major refinery supply. (Partial oxidation / gasification of heavy residue, $C_nH_m + \tfrac{n}{2}O_2 \rightarrow nCO + \tfrac{m}{2}H_2$ at 1200–1500 °C, is a third route for hydrogen-deficient refineries.)

(f) Three processes to recover concentrated hydrogen (3 marks)

• Pressure-swing adsorption (PSA) — at high pressure the methane and heavier hydrocarbons are adsorbed on molecular-sieve/activated-carbon beds while high-purity (99%+) H₂ passes; the beds are regenerated by depressurisation in a cyclic multi-bed operation.
• Membrane separation — the small, fast hydrogen molecule permeates a polymeric membrane preferentially, giving an H₂-rich permeate and a hydrocarbon-rich retentate; simple and driven by partial-pressure difference.
• Cryogenic (partial-condensation) separation — the gas is cooled in a cold box so the methane and heavier hydrocarbons condense out, leaving a hydrogen-enriched vapour; favoured when hydrocarbon liquids are also to be recovered.