Environmental (E) Factor
The first general metric for green chemistry remains one of the best. Roger Sheldon’s E-factor can be made as complex and thorough or as simple as required. Assumptions on solvent and other factors can be made or a total analysis can be performed.
The E-factor calculation is defined by the ratio of the mass of waste per unit of product:
E-factor = total waste (kg) / product (kg)
The metric is very simple to understand and to use. It highlights the waste produced in the process as opposed to the reaction, thus helping those who try to fulfil one of the twelve principles of green chemistry to avoid waste production. E-factors ignore recyclable factors such as recycled solvents and re-used catalysts, which obviously increases the accuracy but ignores the energy involved in the recovery (these are often included theoretically by assuming 90% solvent recovery). The main difficulty with E-factors is the need to define system boundaries before calculations can be made and these vary from scientist to scientist. This limitation is the main drawback of all green chemistry metrics with the exception of the extremely complex life cycle assessment. Sheldon took his publications one stage further and produced Table 1.
Table 1 E-Factors across the chemical industry
| Industry sector | Annual production (t) | E-factor | Waste produced (t) |
|---|---|---|---|
| Oil refining | 106-108 | Ca. 0.1 | 105-107 |
| Bulk chemicals | 104-106 | <1–5 | 104-5×106 |
| Fine chemicals | 102−104 | 5–50 | 5 × 102−5 × 105 |
| Pharmaceuticals | 10–103 | 25–100 | 2.5 × 102−105 |
Crucially, this metric is simple to apply industrially, as a production facility can measure how much material enters the site and how much leaves as product and waste, thereby directly giving an accurate global E-factor for the site. Table 1 shows that oil companies produce a lot less waste than pharmaceuticals as a percentage of material processed. This reflects the fact that the profit margins in the oil industry require them to minimise waste and find uses for products which would normally be discarded as waste. By contrast the pharmaceutical sector is more focussed on molecule manufacture and quality. The (currently) high profit margins within the sector mean that there is less concern about the comparatively large amounts of waste that are produced (especially considering the volumes used) although it has to be noted that, despite the percentage waste and E-factor being high, the pharmaceutical section produces much lower tonnage of waste than any other sector. This table encouraged a number of large pharmaceutical companies to commence “green” chemistry programs.
By incorporating yield, stoichiometry and solvent usage the E-factor is an excellent metric. Crucially, E-factors can be combined to assess multi-step reactions step by step or in one calculation.
Read more about this topic: Green Chemistry Metrics
Famous quotes containing the word factor:
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