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

Question 4 of 5: Catalytic reforming 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 4 (IV) — Catalytic reforming 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: Catalytic reformer redrawn from the exam flow sheet: three reactors with interstage reheat furnaces, recycle-hydrogen compressor, separator, and the stabilizer (column 3). See the official exam paper.]

  1. Name / purpose: catalytic reforming (the fixed-bed semi-regenerative type shown). It upgrades low-octane heavy naphtha into high-octane reformate by converting paraffins and naphthenes into aromatics and isoparaffins; it is the refinery's main octane and aromatics (BTX) source and a major by-product hydrogen supplier.
  2. Feed source: heavy straight-run naphtha (C6–C10) from the atmospheric crude unit, first hydrotreated in a naphtha hydrotreater to remove S, N and metals that poison the platinum catalyst.
  3. Products: main product = high-octane reformate (aromatics-rich gasoline blendstock); significant by-product = hydrogen (consumed elsewhere in hydrotreating/hydrocracking); lesser by-products = light ends / LPG (C3, C4 gas).
  4. Number of reactors: three in series (occasionally 3–4), each preceded by a fired reheat furnace.
  5. Compressor: the recycle-hydrogen compressor returns H2-rich gas to the reactor feed to maintain a high hydrogen partial pressure, which suppresses coke formation and keeps the catalyst active (and controls hydrocracking).
  6. Last column (no. 3): the stabilizer / debutanizer — it strips dissolved hydrogen and C1–C4 light ends from the liquid to bring the reformate to its vapour-pressure (RVP) specification; overhead = gas + C3/C4, bottoms = stabilised reformate.
  7. Regeneration: the bifunctional Pt(–Re)/chlorided-alumina catalyst deactivates by coke; it is regenerated by a controlled low-oxygen coke burn, followed by oxychlorination to redisperse the platinum and restore the alumina chloride, then reduction in hydrogen.
  8. Keeping it continuous: use a CCR (continuous catalyst regeneration) reformer — catalyst flows slowly (moving bed) through stacked reactors to a separate regenerator and back continuously, so the unit never shuts down. (Alternatively a cyclic unit with a spare "swing" reactor that is isolated and regenerated while the others stay on-line.)
  9. Why one furnace is wrong: the dominant reforming reaction — dehydrogenation of naphthenes to aromatics — is strongly endothermic and very fast, so a large temperature drop (~30–70 °C) occurs across each reactor. With a single furnace before the first reactor only, the stream would cool sharply after the first bed and the reactions would quench, collapsing conversion and octane in the later reactors. That is precisely why interstage furnaces reheat the stream before each reactor. One furnace would sacrifice most of the octane uplift — a convincing techno-economic argument for the boss.
  10. Other alternates: for octane — isomerization of C5/C6 light naphtha, alkylation, FCC gasoline, and oxygenate blending (ethanol/MTBE); for the reformer itself — semi-regenerative, cyclic, or CCR configurations; for aromatics specifically — a dedicated aromatics complex.