Polypropylene study states that carbon footprints vary massively depending on manufacturing routes

A cradle to plant gate study to determine the carbon footprint or environmental impact of polypropylene, one of the chemical industry’s largest volume products, has found that a diversity of manufacturing routes leads to a diversity of carbon footprints for the product that range by almost a factor of six, according to a new report from IHS, the information and analysis provider.

The IHS report, entitled ‘Polypropylene’s Carbon Footprint: Diverse, with Major Sensitivities — SRI Consulting Carbon Footprint Initiative’, examined the carbon footprint of polypropylene from all of its primary manufacturing routes. This represents the first public study to provide a footprint analysis for bio-polypropylene, which is not currently produced commercially, but is technically feasible.

‘We found that the different paths to propylene lead to a diversity of footprints driven primarily by the source of raw materials which includes crude oil, liquefied natural gas (LNG) or vegetable oil,’ said Russell Heinen, chemicals director at IHS, and one of the authors of the report. ‘These materials, rather than the actual process used to make the polymer, seem to be the key indicator for carbon footprint, and since they vary significantly, so did the resulting footprints. For a given process, the ratio between the highest and the lowest carbon footprints is nearly five to one.’

In sensitivity terms, he added, the ‘elephant in the room’ for polypropylene footprints are regional variations in crude oil, natural gas and LNG production. The per-ton footprint for crude oil production is three to four times greater in Nigeria, for example, than in the US The footprints of Russian gas-based LNGs are three to five times higher than in other parts of the world.

The study, Heinen said, sought to identify and clarify this diversity and variation of results, which he said is lacking in other public studies currently available. ‘Unfortunately, there is not a great deal of transparency in other footprint studies, so our clients asked us to shed some light on this issue by conducting a detailed study on polypropylene refining. In doing so, we were able to map the wide array of footprints for this critical product, but also discovered what appears to be a major error in another public study, so hopefully with this report we’ll eliminate some of the discrepancies that may exist on this issue.’

Understanding the carbon footprints and greater transparency are essential, Heinen said, since many producers are now being asked by their customers to account for the carbon footprint of their chemical products across the entire production and manufacturing chain. ‘This is a growing expectation, which is being driven largely by consumers, but also by large retailers, who seek to provide carbon footprint information to customers, so they in turn expect this type of transparent reporting from their suppliers, and the suppliers of their suppliers.’

A number of different routes - starting with raw materials including crude oil, natural gas liquids or even vegetable oil - are possible to get to propylene. Vegetable oil can be hydrotreated to generate ‘bio-propane,’ which is chemically identical to ‘fossil-based’ propane.

Most of the world’s propylene is made in refineries or in ethylene crackers, where it is typically considered a byproduct. ‘Increasingly,’ Heinen said, ‘we are seeing propylene processes intentionally being used to increase supplies, which in turn, increases the number of potential sources that need to be examined to understand the range of carbon footprints.’

Polypropylene is produced by polymerising propylene, sometimes with minor additions of ethylene. Produced at a rate of more than 50 million tpy, currently, polypropylene has a wide variety of applications: approximately one third of output goes into fibers, another third into injection molding and another 20% into films and sheets. Likewise, it has a widespread array of end uses including carpets and textiles, automotive and construction components, and packaging.

In the IHS study, analysts examined different methodologies for allocating the main sources of emissions: those driven by energy use in refining and those driven by hydrogen use in refining. Said Heinen, the latest methodologies for allocating carbon to refined products are separately allocating the impact of the hydrogen based on the amount of hydrotreating required. This raised the question of how hydrogen generation in steam crackers should be accounted for.

‘Although it is not conventional practice at this time,’ Heinen said, ‘when we looked at the data, we asked ourselves why no footprint credits are currently given to the steam cracking process for the hydrogen it produces. To do so would lower the carbon footprint of the steam-cracker considerably, by about one third to nearly one half.’

While the difference, he said, is watered down in polypropylene footprints, it is still meaningful, and compared to the base case for crude oil refined products that employ the steam cracker chain to get polypropylene, footprints with a hydrogen credit for the steam cracker are lower by as much as 10%.

Although it is made from vegetable oil, bio-propylene has a much higher production footprint than its fossil counterpart, which might be surprising to some. This is a combination of two factors: emissions in the cultivation of the vegetable oil, and a high consumption of hydrogen in converting the oil to bio-propane (and its primary product, hydro diesel). Bio-propane’s high footprint rolls through to bio-polypropylene, giving it a far higher footprint than ‘fossil-based’ polypropylene, the study said.

However, this is only cradle to plant gate analysis, Heinen warned. If both products (bio and fossil based polypropylene) are incinerated at the end of their lifetimes, then bio-polypropylene actually shows a 15% lower footprint than fossil-based polypropylenes. The difference, he said, is that incineration of the fossil-based polypropylene releases carbon into the atmosphere, whereas for the bio-polypropylene, incineration has a footprint of zero because it is presumed to be carbon neutral.

‘This distinction,’ added Heinen, ‘is why it was so critical that our study be transparent, so our clients have access to all the data and the analysis. Since, under different conditions, the same product could yield significantly different results.’

Read the article online at: https://www.hydrocarbonengineering.com/gas-processing/15062011/polypropylene_study_states_that_carbon_footprints_vary_massively/