Light olefins, namely ethylene, propylene, and 1-butene, are essential building blocks of the chemical industry, yet their production remains almost entirely reliant on fossil feedstocks through steam cracking and fluid catalytic cracking. As the chemical sector seeks to reduce its carbon footprint, the direct conversion of captured CO₂ into light olefins via Fischer–Tropsch synthesis (CO₂-FT) has emerged as a promising Power-to-X pathway. Despite its potential, comprehensive process-level assessments integrating reactor modeling, process design, energy integration, and techno-economic analysis remain limited.

This thesis develops and evaluates two process configurations for the production of C₂–C₄ olefins from direct air-captured CO₂ and renewable hydrogen using Aspen Plus. The base configuration consists of a kinetic iron-based CO₂-FT reactor model integrated with a complete product recovery train comprising CO₂ removal and cryogenic separation to produce polymer-grade ethylene and propylene together with chemical-grade 1-butene. A second configuration extends the process by incorporating a downstream steam-cracking unit to convert the C₅–C₁₀ Fischer–Tropsch products into additional light olefins. Both configurations are evaluated within a consistent Power-to-X framework through process simulation, pinch-based heat integration, and techno-economic assessment.

The developed CO₂-FT reactor model operates at an optimal temperature of 350°C, a pressure of 15 bar, a H₂/CO₂ molar ratio of 3, and a chain-growth probability of α = 0.6, achieving a single-pass CO₂ conversion of 39%. The FT-only configuration produces 10.28 t/h of light olefins with a carbon efficiency of 43%, whereas integration of the steam-cracking section increases olefin production to 13.49 t/h and improves carbon efficiency to 56%, while reducing the specific CO₂ consumption from 7.38 to 5.70 tCO₂ per tonne of product. Following heat integration, the FT-only and FT-cracking configurations achieve overall fuel energy efficiencies of 73% and 58%, respectively, corresponding to light-olefin energy efficiencies of 31% and 40%.

The techno-economic assessment indicates that integrating steam cracking decreases the net light olefin production cost from €8.02/kg to €7.03/kg, representing a 12.3% reduction primarily through improved carbon utilization and increased product yield. In both configurations, renewable hydrogen dominates the operating expenditure, and sensitivity analysis identifies hydrogen price as the principal economic driver, substantially outweighing the influence of electricity and CO₂ prices. Although the calculated production costs remain significantly higher than those of conventional fossil-based olefin production, the results demonstrate that economic competitiveness is primarily constrained by hydrogen costs and reactor selectivity.

Overall, this work provides an integrated process design and evaluation of direct CO₂-to-olefins production, combining detailed reactor modelling, process simulation, heat integration, and techno-economic analysis within a unified framework. The results demonstrate that downstream steam-cracking integration significantly enhances carbon utilization and economic performance, while identifying hydrogen cost reduction and improved Fischer–Tropsch catalyst selectivity as the key priorities for the industrial deployment of defossilised olefin production.
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