Page 86 - Materials Chemistry, Second Edition

70 3 Life Cycle Inventory Analysis

Co-product A

Pre-product Intermediate
Unit process x
product

Co-product B

Figure 3.4 Branching due to several main outputs (multi-output process).

essential, not negligible ancillary materials and cannot be cut off. For clarity,
in this and the following illustrations all further inputs and outputs such as
energy, ancillary materials and emissions are omitted.
2. A unit process yields several usable products of which only one is further
processed within the product system (multi-output process). Besides an inter-
mediate product, which is necessary for the product assessment under study,
in Figure 3.4, two further products A and B are released for use in other
production chains. These are called co-products (see Section 2.2.2.2), because
the formation processes of the intermediate product as well as the products A
and B are necessarily coupled.

In systems analysis each unit process must be examined with respect too its
co-products. The data are needed either to allocate material and energy demand
as well as emissions to the intermediate products and co-products or to be able
to make an adequate system expansion (see Sections 2.2.2.2 and 3.3). Co-products
of the product system under examination are not integrated into the system flow
chart; they leave the system and can be presented outside its boundary (Figure 3.5,
case A). This is different in the case of system expansion; here co-products remain
within the system boundary, which can lead to very large systems (Figure 3.5 case
B), especially if such a system expansion has to be performed more than once in a
given product system.
Another possibility of branching in a product tree occurs if several processes are
considered as an output (Figure 3.6). This is true for the life cycle phase ‘disposal’ if
there are several ways of disposal or recycling. Closed-loop recycling (CLR) occurs
if waste from the production is re-inserted into the production; open-loop recycling
(OLR) occurs if waste is used in other production processes. As in the case of
co-products, a decision has to be made concerning the position of the system
boundary. The quantitative handling of recycling processes is discussed in Sections
3.3.3 and 3.3.5.
A real, although highly simplified, flow chart is depicted in Figure 3.7. It describes
the production of linear alkylbenzene sulfonate (LAS) (sodium-n-dodecyl benzene