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23-Chem-B6 Petroleum Refining and Petrochemicals · December 2016

Question 5 of 5: Sulphuric-acid alkylation flow sheet

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

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 5 (V) — Sulphuric-acid alkylation flow sheet (10 marks)

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.

[Figure not reproduced: Sulphuric-acid alkylation redrawn from the exam flow sheet: refrigerated contactor reactor with acid settler/recycle, depropanizer, deisobutanizer (iC₄ recycle) and debutanizer. See the official exam paper.]

  1. Why sulphuric acid: H2SO4 is the liquid strong-acid catalyst. It protonates the light olefin to form a carbocation (e.g. the tert-butyl cation), which then alkylates isobutane to produce highly branched, high-octane C7–C8 alkylate. The acid supplies the protons that drive the isobutane + olefin reaction at low temperature.
  2. Reactor type: a cooled, intensely stirred contactor / cascade emulsion reactor (e.g. a Stratco-type contactor or a cascade autorefrigeration reactor) operating at low temperature (~4–15 °C) with a fine acid–hydrocarbon emulsion — effectively a well-mixed (CSTR-like) contactor.
  3. Why a compressor: the reactor runs under autorefrigeration — light hydrocarbon (isobutane/propane) is allowed to vaporise, absorbing the exothermic heat of reaction; the compressor takes that vapour, so it can be condensed and returned. It circulates the refrigerant vapour of the refrigeration loop.
  4. Refrigeration / condenser job: remove the heat of the exothermic alkylation reaction and hold the reactor cold. Low temperature is essential — it maximises alkylate octane and minimises undesirable olefin polymerisation and acid consumption/runaway. The compressed vapour is condensed and the isobutane recycled to the reactor, while a slipstream goes to the depropanizer to reject propane that would otherwise accumulate.
  5. Deisobutanizer after the debutanizer? Re-ordering distillation columns violates no thermodynamic law in principle — both splits are ordinary relative-volatility separations — but here it defeats the design and is therefore not done. The deisobutanizer exists to recover unreacted isobutane and recycle it, keeping the high isobutane-to-olefin ratio the reaction needs. If you debutanize first, all C4 (both iso- and n-butane) leaves together in the overhead and the alkylate bottoms contain essentially no isobutane — so a deisobutanizer placed after the debutanizer would have nothing left to separate from the alkylate. The separation sequence must remove the more-volatile isobutane (b.p. −11.7 °C) before n-butane (−0.5 °C) so it can be recycled; hence the deisobutanizer must precede the debutanizer, and the boss's swap is rejected.
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