23-Chem-B6 Petroleum Refining and Petrochemicals · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 16-Chem-B6, Petroleum Refining and Petrochemicals — December 2019. 3 hours, OPEN BOOK (any non-communicating calculator permitted). Per the exam notes, FIVE (5) questions constitute a complete paper and each is of equal value (10 marks). The paper prints five questions; the last (the refining-process question on page 6) is mislabelled “IV” in the source but is the fifth question and is answered here as Question 5. All five questions are worked in full.
Reference texts: Gary, Handwerk & Kaiser, Petroleum Refining: Technology and Economics, 5th ed. (CRC, 2007); Fahim, Al-Sahhaf & Elkilani, Fundamentals of Petroleum Refining (Elsevier, 2010); J. G. Speight, The Chemistry and Technology of Petroleum, 5th ed.; M. R. Riazi, Characterization and Properties of Petroleum Fractions (ASTM MNL50, 2005); Felder & Rousseau, Elementary Principles of Chemical Processes, 4th ed. (material balances).
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.
(i) The pseudo-components A–E. Reading the cuts from light (low boiling) to heavy along the curve, the five straight-run pseudo-components of an atmospheric crude distillation are:
(ii) IBP and EP. The initial boiling point (IBP) is the temperature at which the first drop distils — the (non-zero) start of the curve at 0% distilled; the end point (EP) is the temperature at 100% distilled, the top of the curve (marked on the figure).
(iii) Cut points of product D. A cut point is the TBP temperature at a boundary between adjacent fractions. Product D occupies the interval from ≈60% to ≈80% distilled, so its lower cut point is the temperature at the C/D boundary (≈60%) and its upper cut point is the temperature at the D/E boundary (≈80%); these two temperatures define the boiling range of D (shown as the two green markers on the curve).
(i) Functional role. The ADU is the first and central separation step of the refinery: it physically fractionates desalted, pre-heated crude oil into straight-run cuts by boiling range (overhead gas + naphtha, kerosene, diesel/gas oil, and atmospheric residue). It makes no chemical change — it simply sorts the crude into feedstocks for the downstream conversion and treating units.
(ii) Typical operating temperature and pressure. Crude is heated in the fired heater to a flash-zone temperature of about 340–370 °C (kept below ≈370 °C to avoid thermal cracking of the crude), and the column runs at slightly above atmospheric pressure — roughly 1.2–2 atm (about 120–200 kPa, or ~5–20 psig) — with the tower top around 100–150 °C.
(iii) Feed and products. The feed (stream 1) is desalted crude oil. The products are, top to bottom: (2) overhead vapour — light gases (LPG) and light straight-run naphtha; (3) kerosene/jet side-cut (labelled C); (4) diesel / atmospheric gas oil side-cut (labelled D); and (5) atmospheric residue (the bottoms).
(iv) Destination and role of streams 2 and 5. Stream 2 (overhead naphtha + gas) goes to the overhead receiver/stabiliser, where gas and LPG are separated and the naphtha is routed to the naphtha hydrotreater and catalytic reformer / isomerisation units — its role is to supply the light, low-octane feed that is upgraded into reformate and gasoline blendstock. Stream 5 (atmospheric residue) is the heavy bottoms; it is fed to the vacuum distillation unit (VDU) for further separation under vacuum — its role is to recover additional gas oils (VGO for the FCC/hydrocracker) and to leave a vacuum residue for coking, visbreaking or asphalt.
