Question 2 of 5: Petroleum coke; hydrogen manufacture and purification

Question 2 — Petroleum coke; hydrogen manufacture and purification (10 marks)

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) Petroleum coke

(i) Three types. The morphology of delayed-coking coke depends on feed quality and drum conditions:

• Sponge coke — the common, porous, dull-black fuel-grade coke (looks like a sponge) made from ordinary vacuum residue; used as fuel or, when low in metals/sulphur, calcined to anode-grade coke.
• Needle coke — a premium, highly ordered, anisotropic coke with a needle-like grain structure, produced from clean, aromatic feeds (FCC decant/slurry oil, coal-tar pitch) under high recycle; it is the raw material for graphite electrodes.
• Shot coke — an undesirable coke that forms as hard spherical "shot" (BB-sized pellets) from heavy, high-asphaltene/high-sulphur residua at high severity; low value, difficult to handle.

(The fluid-bed processes give two further grades: fine-particle fluid coke and the gasified flexicoke.)

(ii) Five applications. (1) Anode-grade (calcined) coke for the carbon anodes of aluminium smelting; (2) graphite electrodes for electric-arc steelmaking (from needle coke); (3) solid fuel for power-station boilers and cement kilns (fuel-grade petcoke); (4) a recarburiser / carbon raiser in iron- and steel-making; (5) feedstock for TiO₂ production, calcium carbide and activated carbon (also gasification to syngas/hydrogen).

(iii) Three drawbacks. (1) High sulphur and heavy-metal (vanadium, nickel) content, so combustion releases SO₂ and metal-laden ash requiring flue-gas treatment; (2) high carbon intensity — it is almost pure carbon, so it emits more CO₂ per unit heat than other fuels and is increasingly restricted; (3) it is hard, dusty and low in volatiles, making it difficult to ignite and burn and a fugitive-dust/handling hazard.

(b) Hydrogen manufacture and purification

(i) Main production methods from hydrocarbons. The dominant route is steam reforming (SMR) of light hydrocarbons (natural gas/naphtha): $\text{CH}_4+\text{H}_2\text{O}\rightleftharpoons \text{CO}+3\text{H}_2$ (strongly endothermic, over a Ni catalyst at 800–900 °C), followed by the water-gas shift. Alternatives are partial oxidation (POX), $\text{CH}_4+\tfrac12\text{O}_2\rightarrow\text{CO}+2\text{H}_2$ (non-catalytic, for heavy feeds/residua), and autothermal reforming (ATR), which combines steam reforming and partial oxidation in one vessel. Hydrogen is also recovered as a by-product of catalytic reforming of naphtha, and made by residue/coke gasification.

(ii) Impurities formed. The raw reformer/POX gas ("syngas") contains carbon monoxide (CO), carbon dioxide (CO₂), unconverted methane and higher hydrocarbons, water vapour, nitrogen (if air is used), and — from any sulphur in the feed — hydrogen sulphide (H₂S) and traces of COS.

(iii) Purification. First the CO is largely converted to more hydrogen in the water-gas shift reactors ($\text{CO}+\text{H}_2\text{O}\rightleftharpoons \text{CO}_2+\text{H}_2$, high- then low-temperature). Bulk CO₂ is removed by amine absorption (MEA/MDEA) or hot potassium carbonate; any H₂S is taken out with the CO₂ or in a guard bed. Residual CO/CO₂ are then either converted to methane by methanation (for ammonia-plant hydrogen) or, in modern plants, the whole gas is polished in a pressure-swing adsorption (PSA) unit that delivers 99.9%+ hydrogen in one step (adsorbing CO₂, CH₄, CO, N₂ and water). Membrane and cryogenic separation are also used for hydrogen recovery from refinery off-gases.