Question 6 of 6: Pour Point, Antiknock Additives, Hydrogen Recovery and a Benzene-Hydrogenation Recycle

Question 6: Pour Point, Antiknock Additives, Hydrogen Recovery and a Benzene-Hydrogenation Recycle (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) Pour point (3 marks)

The pour point is the lowest temperature at which a crude oil or petroleum product will still flow (pour) under standardised gravity-cooling conditions (ASTM D97) — i.e. 3 °C above the temperature at which the sample ceases to move when the test jar is tilted. It marks the point where enough paraffin wax has crystallised (or the viscosity has risen enough) to gel the oil, and it governs low-temperature handling, pumping and pipeline transport.

(b) Reducing knock (3 marks)

Knock is suppressed by raising the fuel’s octane number — its resistance to auto-ignition — using antiknock components and additives. Historically this was tetraethyl lead (TEL), now phased out for health/catalyst reasons. Modern practice uses oxygenates (ethanol, formerly MTBE), high-octane refinery streams (reformate rich in aromatics, alkylate, isomerate), and small doses of organometallic additives (e.g. MMT in some markets). Engine/operating measures (retarding spark, richer mixture, lower compression) also reduce knock but at an efficiency cost.

(c) Two hydrogen-recovery processes (4 marks)

• Pressure-swing adsorption (PSA): the dilute gas is passed at high pressure through beds of molecular-sieve/activated-carbon adsorbents that retain CH₄ and heavier hydrocarbons while H₂ passes through; the beds are regenerated by depressurisation. PSA delivers very high-purity hydrogen (99%+) and is the refinery workhorse.
• Membrane separation: a polymeric (or metallic) membrane preferentially permeates the small, fast-diffusing H₂ molecule, producing a hydrogen-rich permeate and a hydrocarbon-rich retentate — compact and simple, giving good (though lower-purity) recovery. Cryogenic partial condensation is the third main route: cooling condenses the hydrocarbons and leaves a hydrogen-rich vapour, favoured when hydrocarbon liquids are also to be recovered.

(d) Benzene hydrogenation to cyclohexane with recycle

Given. $C_6H_6 + 3H_2 \rightarrow C_6H_{12}$; overall benzene conversion 85%; single-pass conversion 35%; 25% excess H₂ in the fresh feed; recycle is 30 mol% benzene / 70 mol% H₂. Basis: fresh benzene $= 100$ mol/h.

Find. the ratio of recycle to fresh feed, $R/F$.

Approach. The overall conversion fixes the benzene reacted (equal, at steady state, to the benzene reacted per pass); the single-pass conversion applies to the reactor-inlet benzene (fresh + recycle), which yields the recycle; the fresh feed is benzene plus its 25%-excess hydrogen.

1. Benzene reacted. At steady state all reacted benzene is consumed in the reactor, so benzene reacted per pass = overall reacted $= 0.85(100) = 85$ mol/h.
2. Recycle from the single-pass conversion. The single pass acts on the reactor-inlet benzene (fresh + recycle benzene $= 100 + 0.30R$):$$0.35(100 + 0.30R) = 85 \;\Rightarrow\; 100 + 0.30R = 242.9 \;\Rightarrow\; \boxed{R = 476\ \text{mol/h}}.$$
3. Fresh feed. Stoichiometric H₂ is $3\times100 = 300$; with 25% excess, fresh H₂ $= 375$, so$$F = 100\ (\text{benzene}) + 375\ (\text{H}_2) = 475\ \text{mol/h}.$$
4. Recycle-to-fresh-feed ratio.$$\frac{R}{F} = \frac{476}{475} = \boxed{1.00}.$$