Page 76 - Materials Chemistry, Second Edition

L1644_C02.fm Page 53 Tuesday, October 21, 2003 3:07 PM

IE 1 IE n

IP 1 P 1
SYSTEM

IP n P n
W 1 W n

FIGURE 2.6 Representation of a generic product system.

vectors with negative elements corresponding to the pollutants they contain. The
environmental load of the input and waste flows must be distributed among the
products of the process.
In this way, the LCI or the balance of environmental loads of the product system
under study is carried out similar to a material balance. Thus, in the case of the
whole and complex plant, this can be divided into its units or subsystems, and the
system of equations obtained for each of them is solved in order to calculate the
eco-vectors for every intermediate or final product. The solution of the equation’s
systems allows detailed knowledge of the origin of the pollution, which can be
assigned to each product of a plant.
The balances are carried out in a similar way for discontinuous processes, only
changing the basis of the computation. For example, instead of considering a pol-
lutant rate, the calculations are carried out in mass of pollutant per mass of obtained
product. An illustration of a generic discontinuous system taken with n inputs of
raw materials and energy and n outputs of products and waste releases is presented
in Figure 2.6.
The algorithm resulting from the global balance of EL is given as follows:

n
∑ Pi⋅ ( v m,pi ) = ∑ ( IPi⋅v m,IPi ) + ∑ ( IEi⋅v e,IEi ) − ∑ ( W ⋅v m,Wi ) (2.5)
n
n
n
i
i=1 i=1 i=1 i=1
where:
IP = mass inputs
i
IE = energy inputs
i
P = outputs (products and by-products)
i
W = wastes
i
v m,e = mass and energy eco-vectors of the flows
The only unknowns in Expression 2.5 are the eco-vectors associated with the
products. If only one product is assumed, the correspondent eco-vector will be
calculated by:

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