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.
(1) Name of the process. This is delayed coking — a thermal (non-catalytic) cracking process, recognisable from the paired coke drums, the furnace, the switch valves and the solid-coke product.
(2) Feed. The feed is the heaviest bottom-of-the-barrel stream: vacuum residue (vacuum-tower bottoms), sometimes together with other heavy residua such as atmospheric residue, FCC slurry or tars. It is the low-value, high-boiling, high-carbon-residue material that cannot be distilled further.
(3) Main purpose. The purpose is residue upgrading: to thermally crack the vacuum residue into lighter, more valuable liquid and gas products (coker gas, naphtha, light and heavy gas oil that feed downstream hydrotreating/FCC) while rejecting the excess carbon and metals as solid petroleum coke. It converts a near-worthless residue into distillate feedstocks and marketable coke.
(4) Reaction time, temperature and pressure in the coke drums. The furnace heats the feed to about 485–505 °C (~500 °C) and the coke drums operate at low pressure, roughly 1–4 bar g (about 15–40 psig). The cracking/coking is deliberately “delayed” until the fluid reaches the drum, where it is held for a long residence time — a full drum fill cycle of about 24 hours (roughly 12 hours filling with coke, then 12 hours to steam-out, cool, decoke and re-warm the drum).
(5) Why two coke drums. Coke removal is a batch operation, but the furnace and fractionator must run continuously. Using two (or more) drums in a swing arrangement lets one drum stay on-line filling with coke while the other is taken off-line to be steamed out, quenched with water, cooled, opened and hydraulically cut/decoked, then re-heated and returned to service. Switching feed between the drums thus turns an inherently batch coking step into a continuous process and keeps the furnace flowing (never allowing coke to form in the furnace tubes).
