Safety and environmental assessment at the conceptual design stage: an automated index-based methodology and its application to a pyrolysis process

Traditional process design prioritises technical and economic objectives, while environmental and safety matters are often addressed after the main design is complete. This late integration limits opportunities for improvement and increases costs, as changes made in later stages are more costly and less effective. Safety, in particular, must take precedence among design objectives, as unsafe plants are not only hazardous but also detrimental to profitability due to potential production and capital losses from accidents. The challenge is that limited information is available at this stage, making safety evaluation less objective than economic assessments, which rely on established indicators. Traditional methods such as HAZOP, HAZID, and Life Cycle Assessment (LCA) require detailed data and are unsuitable for conceptual design. Over the past few decades, several metrics have been tailored for the conceptual design stage, both for environment and for safety, allowing decision-makers to rank alternatives even with data constraints. Among these, index-based metrics are most common, as they rely on mathematical models that produce numerical outputs, usually on a scale. Despite their advances, these metrics face several limitations, including a lack of standard scales (e.g., 0–10), subjectivity, step-based scoring, disconnection from process simulators, reliance on manual calculations, and incomplete coverage of hazards or environmental impacts. Automation and comprehensiveness remain major gaps in existing tools. Here we developed a new tool, fully automated, to evaluate safety and environmental performance with fixed-scale metrics that are clear and easy to interpret, similar to economic indices like ROI or IRR. Built in MATLAB, the tool integrates with the process simulator Aspen Plus to extract necessary data and compute indices automatically. When process conditions change, the indices are recalculated, providing immediate feedback on design choices. The safety framework is structured in three levels: the Stream Safety Index (SSI) forms the basis for the Unit Safety Index (USI), which aggregates into the Process Flowsheet Safety Index (PFSI). The environmental analysis is conceived as a natural extension of the safety assessment. It considers three environmental compartments—air, water, and soil—to capture different impacts of each chemical. This analysis focuses on streams and the overall process rather than units, as unit-level detail adds little value at this stage. The Environmental Stream Safety Index (ESSI) is calculated for each stream and aggregated to provide process-level results. The tool was tested on various processes, with a main case study on two natural gas pyrolysis alternatives developed in collaboration with Université de Sherbrooke (Quebec, Canada). This continuous process operates in situ at 950 °C and 1 atm, without a metal catalyst, converting natural gas into hydrogen and solid carbon as a potential low-emission solution for Canada’s oil extraction industry. By combining safety, environmental, and economic indices, the tool identified the best-performing alternative. Indices are calculated in 20–60 seconds depending on flowsheet complexity and computer performance, enabling rapid iteration. SSI, USI, PFSI, ESSI, and environmental flags offer a robust, efficient, and user-friendly approach to evaluating safety and environmental performance. The tool helps engineers select process alternatives without requiring deep expertise in each area and, when paired with economic indicators, supports a holistic approach to conceptual design. Its application to pyrolysis case studies demonstrates its ability to integrate sustainability and safety early in process development.