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.
(a) Functional role. The atmospheric distillation unit (ADU, or crude unit) is the first major processing step and the heart of the refinery: it separates desalted crude oil physically, by boiling range, into a slate of straight-run cuts (gases, naphtha, kerosene, gas oils) and an atmospheric residue. It performs no chemical conversion — it simply sorts the crude’s molecules by volatility so downstream units receive properly cut feedstocks.
(b) Typical operating conditions. The crude is heated in a fired heater to a flash-zone temperature of about 340–370 °C (kept below ~370 °C to avoid thermal cracking and coking in the column), and the column operates at close to atmospheric pressure — roughly 1–2 atm (about 100–250 kPa, i.e. a slight positive pressure at the top). Stripping steam is injected at the bottom and to the side-strippers to lower hydrocarbon partial pressure and sharpen the cuts.
(c) Feed and products. The feed (stream 1) is desalted crude oil. The products, lightest to heaviest, are: overhead gas and light straight-run naphtha (stream 2); heavy naphtha / kerosene (jet) as an upper side-draw (stream 3); light and heavy gas oils (diesel / atmospheric gas oil) as lower side-draws (stream 4); and the atmospheric residue “long resid” from the bottom (stream 5).
(d) Destination and role of streams 2 and 5. Stream 2 (overhead): the column overhead vapour is condensed in the overhead system; the gas goes to the gas plant/light-ends recovery (fuel gas and LPG) and the light naphtha is sent to naphtha hydrotreating and reforming — part of the naphtha is also returned as top reflux to set the column’s temperature profile. Its role is to recover the lightest, most volatile product and provide reflux. Stream 5 (bottoms): the atmospheric residue is the feed to the vacuum distillation unit, where more gas oil is recovered under vacuum; its role is to carry the un-vaporised heavy end forward for further separation and conversion (it is never a finished product as-is).
(e) Process flow diagram. Desalted crude is pumped through preheat exchangers and a desalter, raised to flash-zone temperature in the fired heater, and fed to the trayed atmospheric column. Products are withdrawn as an overhead (condensed to gas + light naphtha, with reflux returned), several steam-stripped side-draws (kerosene, light gas oil, heavy gas oil), and a steam-stripped bottoms residue:
