23-Chem-B6 Petroleum Refining and Petrochemicals · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-Chem-B6, Petroleum Refining and Petrochemicals — December 2016. 3 hours, OPEN BOOK (any non-communicating calculator permitted). Per the exam notes, FIVE questions constitute a complete paper — Question 1 is multiple-choice and Questions II–V require essay-format answers of equal value (10 marks each); clarity and organisation are explicitly marked. This paper contains exactly five questions, so all five are answered here in full. The exam is almost entirely qualitative; the only computed quantities are the two standard crude-characterisation relations used to justify the multiple-choice answers.
Reference texts: Gary, Handwerk & Kaiser, Petroleum Refining: Technology and Economics, 5th ed.; Fahim, Al-Sahhaf & Elkilani, Fundamentals of Petroleum Refining; J. G. Speight, The Chemistry and Technology of Petroleum, 5th ed.; Perry's Chemical Engineers' Handbook, 9th ed. (generic distillation / equipment methods).
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.
Answer key with reasoning. The answers are collected in the table below; the two characterisation relations that decide items 1, 2 and 5 are worked out immediately after.
| # | Answer | Reason (one line) |
|---|---|---|
| 1 | (b) decreases | Sour crudes are generally heavier/denser; higher SG ⇒ lower °API. |
| 2 | (b) Gasoline | Lightest, least-dense cut of the four ⇒ highest °API. |
| 3 | (b) decreases | Larger, more aromatic/naphthenic molecules are H-poor ⇒ lower H/C. |
| 4 | (d) Water | For a pure component the pour point coincides with the freezing point. |
| 5 | (a) highly paraffinic | Watson K > 12.5 is paraffinic; K ≈ 15 is at the paraffinic extreme. |
| 6 | (a) removing sulfur | HDS reacts organosulfur with H₂ over Co–Mo/Ni–Mo to strip H₂S. |
| 7 | (a) knocking tendency | Octane number = resistance to autoignition / knock. |
| 8 | (a) Naphtha | Lightest, lowest-MW cut ⇒ lowest viscosity. |
| 9 | (a) Asphaltenes > Resin > Olefin | Asphaltenes are heaviest, resins intermediate, olefins lightest. |
| 10 | (a) larger-diameter | At vacuum the vapour volume is larger, so a bigger cross-section keeps velocities comparable. |
Two standard relations underpin the property answers, and it is worth working them explicitly.
Given. A light crude of specific gravity $SG = 0.825$ (60/60 °F) for the gravity questions, and a kerosene-range stock of mean boiling point $T_B = 200\,{}^\circ\text{C}$ with $SG = 0.79$ for the characterisation-factor question.
Find. The API gravity (items 1–2) and the Watson characterisation factor (item 5), and the class of oil each implies.
The API gravity fixes items 1 and 2:
$$API=\frac{141.5}{SG}-131.5=\frac{141.5}{0.825}-131.5=\boxed{40.0\ ^\circ\text{API}}$$which is a light crude ($>31.1\,{}^\circ$API); sour, heavy crudes fall to much lower °API, so gravity decreases as sulphur content rises (item 1), and among cuts the lightest (gasoline) carries the highest °API (item 2). The Watson (UOP) characterisation factor fixes item 5. For a kerosene-range stock of mean boiling point $T_B = 200\,{}^\circ\text{C} = 852\,{}^\circ\text{R}$ and $SG = 0.79$:
$$K_W=\frac{(T_B)^{1/3}}{SG}=\frac{(852)^{1/3}}{0.79}=\boxed{12.0}$$Naphthenic stocks sit near $K \approx 11$–$12.5$ and aromatic near $10$–$11$; a Watson factor of $15$ therefore lies well above the paraffinic threshold of $12.5$, so the oil is highly paraffinic (item 5).
