Page 111 - Materials Chemistry, Second Edition

3.3 Allocation 95

Allocation usually starts with the process that exhibits the final product as output
(in Figure 3.11 unit process 2). The production of this unit process amounts to 6 kg
overall, with the following constituents: 3 kg of the final product (50%), 1 kg of the
co-product 2.1 (16.67%) and 2 kg of the co-product 2.2 (33.33%). All inputs (here:
energy, ancillary materials and intermediate products) and outputs (here: CO and
2
waste) are distributed (‘allocated’) according to the mass ratio of the product output.
The following loads are therefore allocated to 3 kg of the final product in process
2: 25 MJ (50%) energy input, 1 kg (50%) of ancillary material 2 input, 1 kg (50%)
CO output, 1.5 kg (50%) waste output, and 3.5 kg (50%) of intermediate products
2
(input). Only those loads of process 1 can be allocated to the final product that are
related to the production of 3.5 kg (50%) of the intermediate product input into
unit process 2.
In addition, co-product 1 is formed in unit process 1. The allocation per mass
in process 1 is primarily done exactly as described in process 2. All in all, the
total sum of 9 kg is produced with 7 kg (77.78%) intermediate product and 2 kg
co-product 1 (22.22%). 7 kg of intermediate product are therefore loaded with
7.78 kg raw material, 77.78 MJ energy, 0.78 kg of ancillary materials 1, 1.56 kg waste
and 3.11 kg CO .
2
However, according to the allocation per mass for unit process 2, only 50% of
the intermediate product can be allocated to the final product. Hence the loads in
unit process 1 must be allocated correspondingly. If the allocation in unit process
2 were not done, the co-products in unit process 2 would never be loaded with
consumptions and emissions of upstream processes. This would surely not be fair.
To 3 kg final product 50% of load from process 1 has to be allocated which has
to be added to the load from process 2. The energy, for example, results are
25 MJ∕3 kg final product from process 2
+38.89 MJ∕3 kg final product from process 1
= 63.89 MJ∕3kgfinalproduct
This corresponds to 21.3 MJ kg −1 of the final product.
An indication per (kg final product) or a reference to the reference flow that
results from the choice of the fU is common practice.
Another example of an allocation per mass is illustrated in Figure 3.7 (see Section
3.1.4). The mass flow of raw material and intermediate products per 1000 kg LAS
are shown in the flow chart according to the ECOSOL study. Inputs of the final
3
process ‘sulfonation and neutralisation’ are 127 kg of NaOH (3.18 × 10 mol). The
assessment of the production of 127 kg NaOH includes the ‘extraction of 99 kg NaCl’
3
(1.71 × 10 mol). Because of the reaction equation of chlorine alkali electrolysis,
however, the molar ratio 1 : 1 of NaOH:NaCl can be assumed:
–
2NaCl + 2H O (+e ) → 2NaOH + Cl + H 2
2
2
117g 36g 80g 71g 2g
52.3% 46.4%1.3%
(stoichiometric conversion with rounded mol masses).