Page 47 - Materials Chemistry, Second Edition

2.2 Scope 31

Cut-off rules prevent arbitrariness in the choice of system boundaries
Example : Analysing material input

Mass fraction Energy
(%) (%)
Raw material 1 73.8 12.0
Pre-product 2 54.7
3 23.3
Ancillary 4 1.2 0.9
material 5 0.1 0.1
6 0.1 < 0.1
7 1.7 0.6
8 1.4 0.7
9 0.2 2.7
10 19.8 4.5
11 1.7 0.4
12 < 0.1 < 0.1
13
Sum 100.0 99.9

Figure 2.2 Application of the cut-off criteria ‘mass’ and ‘energy’.

Mass percentage input
52.2%
1
23.7%
2
9.5%
3
4 7.4% Downstream
0.9% unit processes
5
0.9%
7 Production of
8 0.9% product X
9 0.9%
10 0.9%
11 0.9%
12 0.9%
0.9%
13
Figure 2.3 Application of cut-off criteria: the 5% rule.

13 inputs is presented. The first analysis shows that the mass ratios of the inputs
5–13 are below 1% each. However, the cumulative mass ratio adds up to 7.2%,
which would not be traced back to the raw materials in case cut-off criterion of
1% is applied. Therefore, the sole application of the 1% rule would result in large
asymmetries when in a second variant, for example, just 1.5% would be the overall
cut-off result.
In systems with high energy or mass throughput and simultaneous long life
time of the product, the cut-off of less important branches of the product tree,