23-Chem-B6 Petroleum Refining and Petrochemicals · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 16-Chem-B6, Petroleum Refining and Petrochemicals — December 2018. 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); Questions 1–3 require essay-format answers where clarity and organisation are marked, while Questions 4 and 5 are quantitative. This paper contains exactly five questions, so all five are answered here 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) Reid vapour pressure (RVP). The absolute vapour pressure of a volatile product (chiefly gasoline) measured at 100 °F (37.8 °C) in the standard ASTM D323 bomb at a 4:1 vapour/liquid ratio, reported in psi (or kPa). It is a front-end volatility specification that controls starting behaviour, vapour-lock tendency and evaporative emissions, and is adjusted seasonally by butane blending.
(ii) Antiknock. The resistance of a gasoline to spontaneous autoignition ("knock" or detonation) ahead of the spark flame front in a spark-ignition engine. It is quantified by octane number (RON, MON) and improved by branched paraffins, aromatics and oxygenates or, historically, by antiknock additives such as tetraethyl-lead (now MMT/ethanol).
(iii) Boiling range. The temperature interval between the initial boiling point (IBP) and the end (final) boiling point (EP/FBP) of a fraction, as measured by a distillation test (ASTM D86 / TBP). It defines a "cut" and correlates with volatility, flash point and end-use (e.g. naphtha ≈ 30–200 °C, kerosene ≈ 180–250 °C).
(iv) RON (Research Octane Number). A measure of a gasoline's antiknock quality obtained in a standard CFR test engine under the milder "research" conditions (600 rpm, low inlet-air temperature), scaled against iso-octane (=100) and n-heptane (=0). It reflects low-speed, part-throttle knock resistance; higher RON means greater resistance to knock.
(v) API gravity. A standard inverse-density scale for petroleum defined by $^{\circ}\text{API}=\dfrac{141.5}{\mathrm{SG}}-131.5$, with specific gravity measured at 60/60 °F. Lighter (more valuable) oils have higher °API; water is 10 °API.
(vi) TAN (Total Acid Number). The mass of potassium hydroxide, in milligrams, required to neutralise the organic (chiefly naphthenic) acids in one gram of oil (mg KOH/g, ASTM D664). It gauges the corrosivity of a crude toward refinery metallurgy; crudes above ≈0.5 mg KOH/g are considered high-TAN and require molybdenum-bearing alloys (316/317 stainless steel) in hot service above ≈220 °C, where naphthenic-acid corrosion is active.
(vii) Watson characterization factor (K). A dimensionless index of a crude/fraction's paraffinic character, $K=\dfrac{(T_B)^{1/3}}{\mathrm{SG}}$, where $T_B$ is the mean average boiling point in degrees Rankine and SG is the specific gravity at 60 °F. $K\approx 12.5\text{--}13$ indicates a highly paraffinic stock, $\approx 11$ naphthenic, and $\approx 10$ aromatic.
All four are catalytic reactions of an oil fraction with hydrogen; they differ in how much the molecule is changed and in severity.
Hydroprocessing is the broadest umbrella term — any refinery process in which a hydrocarbon stream is reacted with hydrogen over a catalyst. It therefore encompasses both hydrotreating (mild, mostly heteroatom removal) and hydrocracking (severe, molecular-weight reduction) as its two limbs. Typical envelope: 25–200 bar and 300–430 °C depending on the sub-process.
Hydrotreating (hydrofining) is the mild branch: hydrogen is used to saturate olefins and aromatics and to strip heteroatoms (S, N, O) and metals with little change in molecular weight or boiling range. It cleans feeds for downstream units and finishes products. Typical conditions: 30–70 bar, 300–400 °C over Co–Mo or Ni–Mo on alumina.
Hydrodesulfurization (HDS) is a specific hydrotreating reaction that targets sulphur: organosulphur compounds react with H₂ to form H₂S (removed downstream) and desulphurised hydrocarbon, e.g. $\text{R-SH}+\text{H}_2\rightarrow\text{RH}+\text{H}_2\text{S}$. Conditions overlap hydrotreating: 30–80 bar, 320–400 °C over Co–Mo/Al₂O₃ (Co–Mo is preferred for S removal; Ni–Mo for N removal and aromatic saturation).
Hydrocracking is the severe branch: it combines catalytic cracking with hydrogenation to break heavy gas oils and residua into lighter, saturated products (naphtha, kerosene, diesel) while simultaneously removing S and N. It runs hot and at high hydrogen partial pressure: 100–200 bar, 350–430 °C over a bifunctional catalyst (Ni–Mo or Ni–W metal for hydrogenation on an acidic zeolite/silica-alumina support for cracking).
