Question 4 of 6: Alkylation Safety, Pour Point and a Distillation Split

Question 4: Alkylation Safety, Pour Point and a Distillation Split (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)(i) Main safety risks around an alkylation plant (4 marks)

Alkylation reacts light olefins with isobutane over a strong liquid-acid catalyst — either hydrofluoric acid (HF) or sulphuric acid (H₂SO₄) — while handling large inventories of volatile, flammable hydrocarbons under pressure. The dominant hazards are:

• HF toxicity. HF is volatile and extremely toxic; a leak forms a dense, ground-hugging aerosol/vapour cloud that drifts off-site, causing severe respiratory injury, deep penetrating burns and systemic hypocalcaemia — potentially fatal at low concentrations.
• Acid corrosivity/burns. Both HF and concentrated H₂SO₄ cause serious chemical burns and corrode equipment; spent-acid handling adds exposure risk.
• Fire and explosion. The isobutane/olefin inventories are flammable and held above their boiling points under pressure, so a loss of containment can give a flash fire or vapour-cloud explosion.

(a)(ii) Precautions (4 marks)

• Minimise inventory of acid and light hydrocarbon, with rapid emergency de-inventory to a shielded dump vessel on detection of a leak.
• Aerosol mitigation for HF units: water-spray curtains/monitors to knock down and dilute a release, plus (in newer units) reduced-volatility/modified HF additives.
• Detection and isolation: HF/hydrocarbon gas detectors, remotely operated emergency isolation valves, and pressure relief routed to flare.
• Materials & integrity: corrosion-resistant metallurgy, inspection/monitoring programs, secondary containment/bunding.
• People: acid PPE, calcium-gluconate first aid for HF, plant spacing/siting away from occupied areas, and thorough operator training and procedures.

(b) Pour point of a crude oil (2 marks)

The pour point is the lowest temperature at which a crude oil (or product) will still flow or pour under standardized test conditions; a few degrees below it the oil stops moving as dissolved wax crystallizes into a gel. It indicates low-temperature pumpability/handling and reflects the paraffin-wax content — important for pipeline transport and cold-climate storage.

(c) Distillation-column split

Given. A binary feed is separated into an A-rich overhead and a B-rich bottom.

Find. The distillate rate $D$ and bottoms rate $W$.

Approach. Two independent balances — total moles and component A — solve for the two unknowns.

1. Total and component-A balances. $F = D + W$ and $z_A F = x_{D,A} D + x_{W,A} W$:$$100 = D + W, \qquad 0.60(100) = 0.90\,D + 0.15\,W.$$
2. Solve. Substituting $W = 100 - D$: $60 = 0.90D + 0.15(100-D) = 0.75D + 15$, so$$D = \frac{60-15}{0.75} = \boxed{60\ \text{mol/h}}, \qquad W = 100 - 60 = \boxed{40\ \text{mol/h}}.$$
3. Check component B. B in $= 40$; B out $= 0.10(60) + 0.85(40) = 6 + 34 = 40$ mol/h — closes exactly.