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.
(a) Main functional role. The alkylation unit reacts light C₃–C₅ olefins (propylene, butylenes, amylenes) with isobutane to make a branched, highly paraffinic liquid called alkylate — a premium gasoline blendstock combining high octane (RON ≈ 92–96) with low vapour pressure and zero olefins/aromatics.
(b) Typical operating conditions. Alkylation runs cold and at modest pressure so the hydrocarbons stay liquid. A sulphuric-acid unit operates at about 4–15 °C; a hydrofluoric-acid unit at about 25–40 °C. Pressure is only enough to keep the reactants and acid in the liquid phase, roughly 2–10 bar (higher on HF units to hold HF as liquid).
(c) Catalyst. A strong liquid acid: concentrated sulphuric acid (88–98 wt% H₂SO₄) or hydrofluoric acid (HF). These protonate the olefin to form the reactive carbocation.
(d) Effect of the operating variables. Contact time must be long enough to fully react the olefin with isobutane, but excessive residence promotes olefin self-polymerisation and acid-soluble-oil (tar) formation, which lowers octane and consumes acid. Reactor temperature should be low: cold operation favours the desired isobutane-alkylation over competing polymerisation, so alkylate octane rises as temperature falls (too cold, however, raises acid viscosity and hurts mixing). Catalyst activity (acid strength) must be kept high: strong acid maximises alkylate quality, whereas a weak/diluted acid shifts the chemistry toward polymerisation and can cause an acid runaway. A large excess of isobutane (high isobutane:olefin ratio) is maintained for the same reasons.
(e) Naming the streams. Feed 25 is the olefin feed (C₃–C₅ olefins, chiefly butylenes, from the FCC); feed 44 is isobutane (make-up plus a large recycle). The products are 45 = alkylate (the main C₇–C₈ branched-paraffin product), 46 = n-butane, and 47 = propane/LPG (saturated light ends rejected from the reactor effluent after fractionation).
(f) Destination and role of streams 45 and 47. Stream 45 (alkylate) goes to the gasoline blending pool, where its high octane and low volatility make it one of the most valuable blend components, especially for premium and reformulated grades. Stream 47 (propane/LPG) is routed to the saturated-gas / LPG treating and recovery section, sold as LPG (or used as fuel gas or petrochemical feed); being saturated it is not recycled to the reactor.
