Factor and Atom Economy

Previous sections have outlined the general aspects of evaluation of sustainability, and the value and limits ofthe different methodologies. Even though not exhaustive of the state of the art in this broad topic, which is beyond the scope of this book, the discussion can provide the elements for an evaluation of the specific indexes and methodologies proposed for "green" (sustainable) chemistry. We also remark that some of the indicators and methodologies discussed in the previous sections do not adapt well to the specific case of assessing sustainability of chemical production, while others, in particular the eco-efficiency method, are well suited for the purpose. However, this method, and most of the indicators/methodologies we discuss in this and the following section, is based mainly on the estimation of the input/output process ofa chemical production and has limited considerations on how the process and product is put in relation with the whole eco- and societal-system. This is why it would be necessary to move to evaluation models that also consider these aspects, for example, to develop accounting models for chemical sustainability.

Two of the most used indicators to assess "green" chemistry and evaluate the potential environmental acceptability of chemical processes are the following [31-34]:

• Atom economy, originally proposed by Barry Trost of Stanford University [35], which is a measure of how much of the reactants remain in the final product in a chemical reaction; mathematically, the atom economy (AE) is the molecular weight (MW) of the desired product divided by the sum of the molecular weights of all substances produced:

MW desired product

It is a simple calculation based on the stoichiometry of the reaction, but does not account for solvents, reagents, reaction yield and reactant molar excess. Atom economy is one ofthe 12 principles of green chemistry [36]. The larger the number, the higher the percent of all reactants appearing in the product.

• The Efactor, introduced by Sheldon [31, 32], is defined as the mass ratio of waste to desired product:

_ Total waste (tons)

Product (tons)

Notably, the E-factor depends on the definition of waste, and may include process use only, or chemicals needed for scrubbing, for example. The E-factor can be split into different sub-categories: (i) organic waste and (ii) aqueous waste. The E-factor takes the chemical yield into account and includes reagents, solvents losses, all process aids and, in principle, even fuel (although this is often difficult to quantify and it is usually not included). An exception is water as solvent, which is generally not included in the E factor.

The smaller the number the closer to zero waste is being produced.

A review of the effect that the E Factor concept has had over the last 15 years on developments in the (fine) chemical industry and pharmaceutical industry with regard to waste minimization and to assessing its current status in the broader context of green chemistry and sustainability has been presented recently by Sheldon [37]. It was concluded that the E Factor concept played a major role in focusing the attention of the chemical industry worldwide, and particularly the pharmaceutical industry, on the problem of waste generation in chemicals manufacture.

One of the most cited examples regarding this topic and the magnitude of the waste problem in chemicals manufacture regards the typical E factors in various segments of the chemical industry (Table 5.2).

There are various other related metrics: (i) atom efficiency, (ii) effective mass yield, (iii) carbon efficiency and (iv) reaction mass efficiency:

• Atom efficiency: the percentage yield (molar flow of the desired product divided by the molar flow of the limiting reactant, taking into account the stoichiometry of the reaction) multiplied by the atom economy. It could be used to replace yield and AE. For example, AE could be 100% and yield 5%, making this a not very green reaction.

• Effective mass yield (EMY): [38]: the percentage of the mass of the desired product relative to the mass of all non-benign materials used in its synthesis. However, what is "benign" it is not defined.

Table 5.2 The E factor (kg-wastea per kg-product). Source: Sheldon et al. [34].

Industry segment

Product tonnageb

E factor

Oil refining

106-108

<0.1

Bulk chemicals

104-106

<1-5

Fine chemicals

102-104

5 ! 50

Pharmaceuticals

10-103

25 ! 100

aDefined as everything produced except the desired product (including all inorganic salts, solvent losses, etc.).

b Typically represents annual production volume of a product at one site (lower end of range) or aDefined as everything produced except the desired product (including all inorganic salts, solvent losses, etc.).

b Typically represents annual production volume of a product at one site (lower end of range) or worldwide (upper end of range).

• Carbon efficiency: [39]: the percentage of carbon in the reactants that remains in the final products. It takes into account the yield and stoichiometry. The advantage is that it is directly related to greenhouse gases.

• Reaction mass efficiency (RME): the percentage of the mass of the reactants that remains in the product. It takes into account the atom economy, yield and reactant stoichiometry.

There are more indicators (e.g., mass intensity, MI, and mass productivity) that belong to this general class of resource intensity indicators, for example, which quantify "greenness" of chemical processes and products in terms of effectiveness of mass and energy intensity.

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Responses

  • harris thomson
    What happen to atom economy if you use excess reactants?
    2 years ago
  • Kathleen Washington
    How does the calculations from atom economy and economy factor affect ..?
    11 months ago
  • Richard Ott
    How can efactor and atom economy be used in industries?
    11 months ago
  • kauko
    Which is more important, e factor or atom economy?
    9 months ago
  • Wilimar
    How to affect environmental factor and perctage atom economy for addition reaction?
    5 months ago
  • rosa
    Why would atom economy and reaction mass efficiency be low?
    5 months ago

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