Polymerisation in refining chemistry Which statement correctly describes the essence and purpose of refinery polymerisation processes applied to cracked gas streams?

Introduction / Context:
Early gasoline upgrading relied on converting light, unsaturated components from cracking units into higher-value gasoline-range hydrocarbons. Polymerisation and alkylation are two classic routes, often confused in exams because both increase molecular size but by different mechanisms and feed combinations.

Given Data / Assumptions:

• Feed focus: light olefins in cracked gases (e.g., propene, butenes).
• Objective: produce gasoline-range hydrocarbons with improved octane.
• Comparison with alkylation (olefin + iso-paraffin) is implied.

Concept / Approach:
Polymerisation (acid-catalysed or on solid acids) couples olefin molecules with each other to form higher olefins (dimer, trimer, etc.), which after hydrogenation yield high-octane blending stocks called polymer gasoline. In contrast, alkylation reacts an olefin with an iso-paraffin (commonly isobutane) to form a high-octane iso-paraffin such as isooctane.

Step-by-Step Solution:

Verification / Alternative check:
Process flow descriptions separate polymerisation (olefin + olefin) from alkylation (olefin + isoparaffin), preventing confusion about i-octane formation.

Why Other Options Are Wrong:

• Produces i-octane from i-butane and butene: That is alkylation, not polymerisation.
• Aromatics combine with each other: Not the refinery polymerisation of interest for gasoline making.
• Produces lube base stocks: Lube production uses dewaxing, hydrocracking, solvent refining; polymerisation is not the primary route.

Common Pitfalls:
Confusing the similar goals (octane uplift) of polymerisation and alkylation; always check the reacting families.

Final Answer:
Causes olefins to combine with each other to form higher olefins/gasoline-range material