Question 3 of 5: The isomerisation unit (black-box analysis)

Question 3 — The isomerisation unit (black-box analysis) (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.

(a) Functional role. The isomerisation ("penex/isom") unit converts low-octane, straight-chain light paraffins — principally normal pentane (n-C₅) and normal hexane (n-C₆) in light straight-run naphtha — into their higher-octane branched isomers (iso-pentane, 2,2- and 2,3-dimethylbutane, methylpentanes). Its purpose is to upgrade the otherwise low-value light naphtha into a clean, high-octane, low-benzene gasoline blendstock.

(b) Operating temperature and pressure. Because the reaction is mildly exothermic and equilibrium-limited, it is run as cool as the catalyst allows. Chlorided-alumina platinum catalysts operate at a low ~120–180 °C; zeolitic (Pt/mordenite) catalysts at ~250–280 °C. Both run under a hydrogen atmosphere at ~1.5–3.5 MPa (about 15–35 bar).

(c) Catalyst. A bifunctional catalyst: platinum (the metal / hydrogenation function) dispersed on an acidic support — either chlorided alumina (very active, low-temperature, but moisture- and sulphur-sensitive and needs continuous chloride addition) or a zeolite (Pt/mordenite) (more robust and regenerable, but higher-temperature).

(d) Effect of contact time, temperature and catalyst activity.

• Reactor temperature. Isomerisation is exothermic and thermodynamically limited, so the equilibrium iso/normal ratio (and hence product octane) is highest at low temperature; raising the temperature speeds the kinetics but shifts equilibrium back toward the normal paraffins and promotes undesirable hydrocracking, losing C₅+ liquid yield. There is thus an optimum — low enough for good equilibrium, high enough for adequate rate.
• Contact time (space velocity). Longer contact time (lower LHSV) lets the reaction approach its equilibrium conversion; too short a time under-converts. Excessive contact time, however, gives cracking and side reactions time to occur, again lowering liquid yield.
• Catalyst activity. A more active catalyst reaches equilibrium at lower temperature and/or shorter contact time, which favours the branched product and minimises cracking. Activity is lost to coking and to feed poisons (water, sulphur, nitrogen), so feed driers and hydrotreating/guard beds protect the catalyst.

(e) Feed (43) and products (44). Stream 43 is the light straight-run naphtha (C₅/C₆ n-paraffins), typically after hydrotreating and drying. Stream 44 is the isomerate — a branched iso-paraffin product (iso-pentane, the dimethylbutanes and the methylpentanes) of substantially higher octane.

(f) Destination of stream 44. The isomerate goes to the gasoline blending pool, where its high octane and low sensitivity/benzene content make it a valuable light blendstock.