rel="nofollow" href="#ulink_fd9255f6-a51f-52dc-914a-4ceeafe5f3a8">1.1).
Table 1.1. E-factors for various sectors. Adapted from [5].
| Industry sector | Product capacity, tonnes | E-factor |
|---|---|---|
| Oil refining | 106–108 | ∼0.1 |
| Bulk chemicals | 104–106 | <1–5 |
| Fine chemicals | 102–104 | 5–50 |
| Pharmaceuticals | 10–103 | 25 to >100 |
The main reasons for such variations appear to be related to the product value/profit margins, relevant legislations, cost of waste and market competition. The E-factor analysis of a process is simple and provides quick estimates of wastefulness, which can lead to waste minimisation campaigns. While E-factor is easy to use, it can provide misleading information in some cases. For example, consider the following reaction:
Reactant1+Reactant2⟶SolventAproduct+waste.(1.8)
In reaction (1.8), when calculating the E-factor, if water is the waste, it will be treated as any other waste, although water is not inherently toxic or hazardous. In order words, E-factor does not take into account the nature and the actual impact of the waste or by-products and can treat waste on equal grounds despite significantly different environmental impacts.
Environmental quotient (EQ) [5] has been introduced in order to address the weakness of E-factor. EQ is essentially a modified E-factor, which takes into account the environmental ‘unfriendliness’ quotient (Q) of the waste or by-products (equation (1.9)). Effective mass yield (EMY) is another similar metric (equation (1.10)), which also disregards benign substances used or produced. Therefore, both EQ and EMY help to distinguish between hazardous waste and non-hazardous waste. However, both metrics are susceptible to inconsistencies due to the vagueness around what is environmentally unfriendly or benign, leading to debate over what values to assign to individual substances.
EQ=E-factor×Q(1.9)
%EMY=100×(mass of products÷mass of non-benignmaterials used).(1.10)
Life cycle analysis (LCA), is an extensive way of assessing the environmental impact and sustainability of a given process or product. LCA analyses the entire life cycle of a product, from the extraction of raw materials all the way to the fate of the product. In the context of nanomaterials, this has been extensively reviewed elsewhere [7]. This comprehensive analysis can overcome various pitfalls associated with the other metrics introduced above. However, performing LCA is laborious and time consuming, and it requires a large amount of process- and product-related data to be available. One important advantage of LCA is that for an alternative process or product, LCA can help differentiate between pollution/waste prevention and shifting pollution. For example, consider the reaction (1.8) shown above, and assume that the solvent A is a hazardous solvent. In order to remove the need for solvent A, an alternative reaction is available using a different precursor (equation (1.11)), where solvent B is benign:
Reactant3⟶SolventBproduct+waste.(1.11)
On the face of this new reaction, it appears to be ‘green’ because the hazardous solvent has been replaced with a non-hazardous one. However, during LCA, one needs to consider how the new reactant is derived (reaction (1.11)). Solvent C (equation (1.12)) used to produce reactant 3 may be as hazardous as solvent A. It quickly becomes clear that the alternative (reaction (1.11)) is not as green as it appeared.
Reactant1⟶SolventCReactant3+waste.(1.12)
Essentially, although the need for solvent A has been removed, in order to produce the same product, solvent C is required, thus simply shifting the problem to the synthesis of the new precursor. In addition, LCA can take into account the energy required to produce the desired products.
Carbon footprint is another measure for assessing the environmental impact of a process or a product. It estimates the CO2 equivalent emissions of greenhouse gases caused by a process or associated with a product. Carbon footprint analysis can help decide between processes where, for example, one option can produce a high quality product but require very high amounts of energy, while an alternative method could be lean on energy requirements but may produce a lower quality product.
1.2 Ways to improve sustainability
Estimation of environmental impact using the metrics discussed above can help identify the problems with an existing process or product. This can then help device targeted plans to find alternatives. Environmental impact assessment can also highlight future challenges, such as the need for technical developments in order to improve product quality for greener processes such that they can be competitive.
There are many ways to improve process sustainability, and most of them are based around finding alternatives to either the solvents used, the reactants used and/or the process conditions (e.g. temperature or pressure) [1]. Avoiding the use of solvents altogether is an excellent example of improving greenness of a process. Other alternatives include switching to solvents that are non-volatile organic compounds. The use of supercritical fluids (e.g. CO2 and water) has also been reported as an alternative due to the ease of solvent separation and reuse. However, one should consider the energy required to operate processes under supercritical conditions (typically high temperatures and pressures; e.g. for water, the critical point is 374 °C and 220 bar). In the case of processes that require heating, alternative ways of providing energy, such as microwave or ultrasound, could be effective. These methods work on the principle of providing energy only to the desired location or chemicals without wasting energy by heating the entire medium or system.
Various ways of estimating the environmental impact, as discussed above, are powerful tools, however they require involvement and inputs from chemists and process engineers [7]. It is critically important to analyse how the modifications made to a section of a process can affect the entire process and perhaps beyond to the entire business. For example, environmental impact estimates cannot predict how changes in feedstock can affect the product value, profits and market competition due to feedstock availability. Answers to such questions can be obtained by performing techno-economic evaluation of a process [8].
1.3 Green chemistry and nanomaterials
