23-Chem-B6 Petroleum Refining and Petrochemicals · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 16-Chem-B6, Petroleum Refining and Petrochemicals — May 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 (and the material balance in 2b) 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.; Smith, Van Ness & Abbott, Introduction to Chemical Engineering Thermodynamics, 8th ed. (VLE, Raoult/Henry); 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) 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.
(ii) Cut. A fraction of crude oil collected between two specified boiling temperatures (its initial and final boiling points) — e.g. the 93–193 °C naphtha cut. Cuts are the basic products of distillation-based separation.
(iii) Pour point. The lowest temperature (recorded in 3 °C steps) at which an oil will still flow under the standard cooling test; it reflects the wax content and sets low-temperature handling and pipeline limits.
(iv) Flash point. The lowest temperature at which the vapour above the oil forms a mixture with air that momentarily ignites ("flashes") on applying a test flame. It is a volatility- and safety-related specification governing storage and transport.
(v) RON (Research Octane Number). A measure of a gasoline's antiknock quality obtained in a standard CFR test engine run under the milder "research" conditions (low speed, low inlet temperature). Higher RON means greater resistance to autoignition/knock.
(vi) Naphtha. The light-to-medium distillate boiling roughly 30–200 °C, lying between the light ends and kerosene. Light naphtha feeds isomerisation and gasoline/petrochemical blending; heavy naphtha is the feed to catalytic reforming.
(vii) API gravity. A standard inverse-density scale for petroleum, $^{\circ}\text{API}=\dfrac{141.5}{\mathrm{SG}}-131.5$ (SG at 60/60 °F). Lighter oils have higher °API (e.g. the API 28 crude of Q2 has $\mathrm{SG}=0.887$).
(viii) Total Acid Number (TAN). The mass of potassium hydroxide, in milligrams, required to neutralise the organic (chiefly naphthenic) acids in one gram of oil (mg KOH/g). It gauges the corrosivity of a crude toward refinery metallurgy.
Finished motor gasoline is blended from several unfinished naphtha streams produced across the refinery. Three principal sources are: (1) straight-run naphtha distilled directly from the crude in the atmospheric column; (2) cat-cracked (FCC) gasoline from the fluid catalytic cracker, the largest gasoline contributor in a typical refinery; and (3) reformate from catalytic reforming of heavy naphtha. (Alkylate, isomerate, hydrocrackate and coker naphtha are further sources.)
Delayed coking is a severe, once-through thermal cracking process that upgrades the heaviest bottom-of-the-barrel stream — vacuum residue — into lighter distillate products and solid petroleum coke. The feed is pumped through a fired heater and raised very quickly to about 490–505 °C; the short tube residence time is deliberate, so that the cracking/condensation reactions are "delayed" until the hot stream reaches a large, insulated coke drum operating at low pressure. There the heavy free radicals polymerise and lay down solid coke, while the cracked vapours rise overhead to a fractionator that recovers gas, coker naphtha and light/heavy coker gas oils; a heavy recycle is returned to the heater. Coke fills the drum over a cycle (typically ~24 h); feed is then switched to a parallel drum and the full drum is steamed, cooled and cut out hydraulically, making the unit semi-continuous.
What it is used for and why. Delayed coking is a residue-destruction (carbon-rejection) process: it converts near-worthless vacuum residue into valuable naphtha and gas oils and a saleable solid coke, greatly increasing the refinery's liquid yield from a barrel of crude. It is favoured because it is the cheapest and most robust residue-upgrading route and tolerates feeds with very high Conradson-carbon and metals content that would rapidly poison a catalytic (hydro) process.
