Question 6 of 6: Gasoline Properties, Catalyst Deactivation and the Acetylene Plant

Question 6: Gasoline Properties, Catalyst Deactivation and the Acetylene Plant (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) Reid vapour pressure, boiling range and antiknock

(i) Meaning. The Reid vapour pressure (RVP) is the vapour pressure of the gasoline measured in a standard bomb at 37.8 °C (100 °F); it characterises the front-end volatility. The boiling range is the span between the initial and final boiling points from the ASTM distillation curve, describing how the fuel vaporises from light to heavy ends. The antiknock characteristic is the fuel's resistance to auto-ignition/knock, quantified by the octane number (research RON and motor MON, often reported as the pump $(R{+}M)/2$).

(ii) Importance. RVP must be high enough for easy cold starting yet low enough to avoid vapour lock and to limit evaporative (VOC) emissions, so it is set seasonally. The boiling range governs cold-start, warm-up driveability and combustion completeness, and its tail affects deposits and dilution of the crankcase oil. The antiknock quality sets the highest compression ratio (hence efficiency and power) an engine can use without knocking, and is the property that historically drove reforming, isomerisation and alkylation in the refinery.

(b) Deactivation of reforming catalysts

Reforming catalysts (Pt or Pt–Re on chlorided alumina) lose activity mainly through coke deposition: the dehydrogenation and aromatisation reactions generate unsaturated, coke-forming precursors that lay down carbon on both the metal and the acid sites, physically blocking them. This is aggravated by low hydrogen partial pressure and high temperature, which is why reformers run under hydrogen. Secondary causes are poisoning by feed contaminants (sulphur and nitrogen poison the metal function; water and chloride imbalance upset the acid function; trace metals such as As, Pb permanently poison Pt) and thermal sintering of the platinum crystallites at high temperature, which reduces active metal area. Coke is removed by periodic oxidative regeneration (controlled burn-off) followed by re-chlorination and reduction.

(c) Acetylene plant material balance

Given. Pure CH₄ and pure O₂ react in a burner by reactions (1)–(3) above; water is knocked out in a condenser, leaving a dry gas of C₂H₂, H₂, CO, CO₂ and unreacted CH₄.

Find. (i) O₂/CH₄ molar feed ratio; (ii) pounds of water removed per 100 lb-mol dry gas leaving the condenser; (iii) overall carbon yield to C₂H₂.

Approach. Sum the oxygen demanded by reactions (1) and (2) for part (i); water is produced only by (1) and (2), so scale it to a 100 lb-mol dry-gas basis for part (ii); and compare the carbon in the acetylene to the carbon fed for part (iii).

1. (i) O₂/CH₄ ratio. Only reactions (1) and (2) consume oxygen: $2\times10 + 1.5\times20 = 20+30 = 50$ mol O₂ for 100 mol CH₄: $$\frac{O_2}{CH_4} = \frac{50}{100} = \boxed{0.50}.$$
2. Dry gas leaving the condenser. Everything except water: $C_2H_2\,30 + H_2\,90 + CO\,20 + CO_2\,10 + CH_4\,10 = 160$ mol (per 100 mol CH₄ fed). Water produced $=20+40=60$ mol.
3. (ii) Water removed per 100 lb-mol dry gas. Scale by $100/160$: $$n_{H_2O} = 60\times\frac{100}{160} = 37.5\ \text{lb-mol},\qquad m_{H_2O} = 37.5\times18.02 = \boxed{675.8\ \text{lb}}.$$
4. (iii) Carbon yield to acetylene. Acetylene contains $2\times30=60$ mol C; carbon fed is 100 mol (one C per CH₄): $$\text{yield} = \frac{60}{100} = \boxed{60\%}.$$ Carbon closes: $60\,(C_2H_2) + 20\,(CO) + 10\,(CO_2) + 10\,(CH_4) = 100$ mol C.