23-Chem-B6 Petroleum Refining and Petrochemicals · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 16-Chem-B6, Petroleum Refining and Petrochemicals — May 2019. 3 hours, OPEN BOOK exam (any non-communicating calculator permitted). Per the exam notes, FIVE (5) questions constitute a complete paper and only the first five as they appear in the answer book are marked; the paper as printed contains exactly five questions, all answered in full below. Questions are answered in essay format where required (clarity and organization are explicitly marked); the two calculation parts — Q1(b) blending density and Q5 distillation-sequence mass balance — follow the worked-solution format. All five questions are printed as “10 Marks”, consistent with the page-1 note that each question is of equal value.
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. (Sec. 13, Distillation). ASTM test methods cited by number for the property definitions.
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.
A hydrocracker is a high-pressure catalytic conversion unit that breaks heavy, high-boiling molecules into lighter ones in the presence of hydrogen. It operates at roughly 350–425 °C and a high hydrogen partial pressure of about 100–200 bar over a bifunctional catalyst — a hydrogenation metal (Ni–Mo, Ni–W, or noble metal) dispersed on an acidic support (amorphous silica-alumina or a zeolite). The acid sites crack the C–C bonds while the metal sites hydrogenate the fragments and saturate olefins, so the products leave fully saturated and largely free of sulphur and nitrogen. Feedstock: heavy vacuum gas oil (VGO), coker and FCC cycle oils, and other refractory heavy gas oils. Desired products: high-quality middle distillates — low-sulphur diesel and jet/kerosene — plus naphtha and some LPG. Because it adds hydrogen, hydrocracking gives high yields of clean, high-cetane distillate and is the natural complement to the carbon-rejecting FCC.
Alkylation combines isobutane with light olefins (propylene, butylenes and amylenes, typically from the FCC) over a strong liquid-acid catalyst to make alkylate, a mixture of highly branched, high-octane, low-vapour-pressure paraffins that is a premium gasoline blendstock. The representative reaction is isobutane + butene forming an isooctane:
$$\text{i-C}_4\text{H}_{10}+\text{C}_4\text{H}_8\;\xrightarrow{\;\text{H}_2\text{SO}_4\ \text{or}\ \text{HF}\;}\; \text{C}_8\text{H}_{18}\ (\text{2,2,4-trimethylpentane, RON}\approx 100).$$
Typical flow sheet: the olefin feed and a large excess of isobutane are contacted with the acid catalyst (sulphuric acid at about 4–15 °C, or hydrofluoric acid at about 25–40 °C) in a cooled reactor/contactor; the effluent flows to an acid settler where the dense acid phase separates and is recycled to the reactor; the hydrocarbon phase is neutralized/washed and sent to a deisobutanizer, which recovers unreacted isobutane overhead for recycle; a depropanizer rejects the propane, and the tower bottoms are the finished alkylate. Important inputs: light olefins, a large recycle of isobutane (high isobutane-to-olefin ratio suppresses undesirable olefin polymerization), and make-up acid catalyst; tight temperature control and vigorous mixing are essential to selectivity.
For a given separation (fixed feed, products and pressure), the number of theoretical stages $N$ and the reflux ratio $R=L/D$ trade off against one another along the curve below.
The two limiting cases anchor the curve. At total reflux ($R\to\infty$, $D\to 0$) the operating lines coincide with the $y=x$ diagonal and the separation needs the fewest stages, the minimum number of stages $N_{\min}$ (obtainable analytically from the Fenske equation). As reflux is reduced, the operating lines swing toward the equilibrium curve; at the minimum reflux ratio $R_{\min}$ they just touch it at a pinch point, and an infinite number of stages would be required. Every real design lies between these bounds. The optimum reflux is an economic balance: a lower $R$ saves reboiler/condenser duty and utilities (operating cost) but demands more stages (a taller, costlier column), while a higher $R$ does the reverse. The total annualized cost is minimized in the flat knee of the curve, conventionally at $R_{\text{opt}}\approx 1.1\text{--}1.3\,R_{\min}$.
Why it is needed: raw crude carries entrained brine containing inorganic chloride salts of sodium, calcium and magnesium, together with suspended solids and water. If these are not removed before the atmospheric distillation unit, the calcium and magnesium chlorides hydrolyze at furnace temperatures to release hydrochloric acid, which causes severe corrosion in the tower overhead and condensers; the salts and solids also foul preheat exchangers and furnace tubes and poison downstream catalysts. Desalting protects the whole crude train.
How it is performed: the crude is heated to about 120–150 °C and mixed with a few volume percent (typically 3–10 %) of wash water across a mixing valve, which disperses the water into fine droplets that dissolve the salts. A demulsifier is injected, and the water-in-oil emulsion is then broken in an electrostatic desalter vessel, where a high-voltage field polarizes and coalesces the brine droplets so they settle to the bottom and are drawn off as effluent brine. Desalted crude leaves the top of the vessel. Difficult (heavy) crudes are treated in two desalting stages in series to reach salt specifications of a few pounds of salt per thousand barrels.
