What Is the True Cost of Producing Propylene from Methanol? The Role of Externalities

The demand for olefins has increased steadily in recent years, with a propylene demand around 100 million tons per year and an expected annual growth of 3-4%. Most propylene is presently produced via steam cracking of naphtha, but on-purpose processes based on selective propane dehydrogenation or utilizing methanol as an intermediate are also being investigated and deployed. The coal-to-propylene route in particular has gained wide interest in China. This paper presents an assessment of such emerging propylene production routes from methanol by combining detailed process simulation with life-cycle assessment and monetization of the environmental impacts. Though presenting a competitive direct production cost, the coal-to-propylene route has by far the highest total monetized cost after accounting for the human health and ecosystem quality externalities. As for the natural-gas-to-propylene route, it has about double the total monetized cost of conventional steam cracking of naphtha or propane dehydrogenation because of high human health and resource depletion externalities. These results provide a clear indication that both the coal-to-propylene and natural-gas-to-propylene routes are unsustainable. They also highlight the importance of accounting for negative externalities in assessing the techno-economic performance of industrial processes as it can radically change the outcome of the analysis.