23-Chem-B6 Petroleum Refining and Petrochemicals · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: Open-book, 3 hours; five questions of equal value (10 marks each), and exactly five questions constitute a complete paper — all five printed questions are solved below. Most parts call for concise, qualitative essay answers built on the given flow sheets; Question III is a short characterization calculation.
Reference texts: Gary, Handwerk, Kaiser & Geddes, Petroleum Refining: Technology and Economics (5th ed., CRC Press) — refinery configuration, conversion and treating units, product cuts; Fahim, Al-Sahhaf & Elkilani, Fundamentals of Petroleum Refining (Elsevier) — crude characterization factors, hydrotreating, coking, catalytic cracking; Jones & Pujadó, Handbook of Petroleum Processing (Springer) — unit operating windows; supporting property data from Perry’s Chemical Engineers’ Handbook (9th ed.).
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.
[Figure not reproduced: Figure 1.1 — Simplified reproduction of the key units and interconnections in the exam block flow diagram (unit numbers as printed). CD = crude/atmospheric distillation (1), VD = vacuum distillation (2), thermal cracker/coker (3), naphtha HDS (4), gas-oil HDS (6), FCC . See the official exam paper.]
(a) Dashed lines. The dashed lines are recycle / internal utility streams rather than once-through product streams — principally the recycle hydrogen loop feeding the hydrotreaters and the light-ends / off-gas that is returned to the gas-processing section for hydrogen and fuel-gas recovery. They denote material that is generated inside the complex and routed back to another unit, as opposed to the solid lines that carry fresh feed forward or finished products out.
(b) Streams 34, 35 and 58. Reading their positions on the diagram: streams 34 and 35 leave the light-ends / gas-plant and gas-treating section at the top of the sheet and are light gaseous product streams — fuel gas and the LPG cut (C₃/C₄, propane and butane) sent to the LPG pool. Stream 58 is one of the finished liquid streams running to the product tank farm on the right-hand edge — a heavy liquid product (fuel-oil / heavy gas-oil pool). Because the individual stream tags cannot be identified with certainty from the flow sheet, the identification is given by section rather than by an exact component list.
(c) Unit 3 versus unit 9. Unit 3 is the thermal cracker/coker and unit 9 is the fluid catalytic cracker (FCC). Both break large molecules into lighter, more valuable ones, but the mechanism and conditions differ: the thermal unit cracks purely by heat (free-radical chemistry, ~480–500 °C, no catalyst) and makes gas, distillates and petroleum coke from the heaviest residue; the FCC cracks catalytically over a hot fluidised zeolite catalyst (~500–540 °C riser, very short contact time), is optimised to make high-octane gasoline and C₃/C₄ olefins, and continuously regenerates its coke-laden catalyst by burning off the coke. In short: thermal, non-catalytic, coke-making (unit 3) versus catalytic, gasoline-making, catalyst-regenerating (unit 9).
(d) Purpose of CD and VD. CD (the atmospheric crude-distillation unit) and VD (the vacuum-distillation unit) are the refinery’s primary physical separation steps: they split the crude into boiling-range cuts by fractional distillation without changing its molecules. CD takes desalted crude and separates it into gases, naphtha, kerosene, gas oils and an atmospheric residue; VD then takes that atmospheric residue and, under vacuum, recovers additional vacuum gas oil (feed for the FCC/hydrocracker) and leaves a vacuum residue for coking or asphalt.
(e) CD column versus VD column. The essential difference is operating pressure and its consequences. The CD column runs at roughly atmospheric pressure (slightly above) and moderate temperatures (feed ~350 °C); the VD column runs under a strong vacuum (~25–70 mmHg absolute). The vacuum lowers the effective boiling temperature so that the heavy residue can be vaporised and separated without thermally cracking it — cracking would occur if the same cuts were taken at atmospheric pressure and the required ~450 °C+ temperatures. As a result the VD column is physically much larger in diameter (low pressure ⇒ huge vapour volumetric flow), uses low-pressure-drop internals (structured packing rather than trays) and relies heavily on stripping steam, whereas the CD column is a conventional trayed atmospheric fractionator.
(f) Why VD product is hydrotreated (H-HDS) before the FCC. The vacuum gas oil from VD carries the sulphur, nitrogen and metals concentrated from the heavy end of the crude. Sending it first through the heavy-gas-oil hydrodesulphurizer (H-HDS) removes sulphur and nitrogen and saturates some aromatics, which (i) protects the FCC catalyst — basic nitrogen and metals are catalyst poisons that permanently deactivate the zeolite, (ii) keeps SO₃ out of the FCC regenerator flue gas so the unit meets emission limits, and (iii) improves FCC conversion and gasoline yield and lowers product sulphur so the gasoline meets specification. In short, hydrotreating conditions the feed so the catalytic cracker runs cleanly and the products are on-spec.
