Page 75 - Materials Chemistry, Second Edition

LIFE CYCLE INVENTORY MODELING IN PRACTICE 57

3.4.2 Multi-Output Processes
Many processes produce more than one useful output. This makes it necessary
to use some method to divide or partition the process input and output flows
among the useful outputs. The preferred hierarchy for handling multi-output
processes is defined in ISO 14044 section 4.3.4.2 as

1. Avoid allocation where possible,
2. Where allocation cannot be avoided, allocate inputs and out-
puts among useful coproducts in a way that reflects physical
relationships,
3. Where physical relationship alone cannot be used, allocate based
on other relationships.

Two options are given for avoiding allocation. The first option is to further
subdivide the given process into subprocesses with inputs and outputs that
can be assigned to individual co-products. This approach can be used, for
example, when operating data on a manufacturing facility are provided as a
"black box," but individual co-products can be traced to separate processes
within the facility. In many cases, however, even at the most detailed subpro-
cess level, a single process produces multiple co-products.
The second option is to avoid allocation by expanding the system boundar-
1
ies . In this approach, all process burdens are assigned to the primary product
of interest, and credit is given for materials or services that are displaced by
the other co-products of the process. System expansion may not be a suitable
approach if the process being evaluated is the primary or only commercial
route for producing one or more of the co-products, so that there is not a basis
for displacement credit. A hydrocracker unit uses inputs of refined oil and
natural gas to produce outputs of ethylene, propylene, other hydrocarbons,
fuel gas, and heat. An energy credit can be applied for the fuel gas and heat
co-products used outside the system boundaries, but the remaining process
burdens must be allocated among the material co-products.
When allocation cannot be avoided, physical relationships such as mass or
energy are commonly used as the basis for allocating process flows among
useful outputs. The allocation should be related to the function of the prod-
ucts. Using the hydrocracker example, the ethylene, propylene, and other
co-products are used as material inputs for production of plastic resins and
other petrochemicals rather than as fuels, so burdens can be allocated among
the co-products based on their mass rather than on their energy value.
Economic value may also be used as a basis for allocation; however, this
approach should be used with caution, since fluctuations in price of co-products
can change the results and conclusions of an analysis, even if there has been no
change in the physical relationship between the co-products.

1
System expansion is described in EPA/600/R-06/060 Life-Cycle Assessment: Principles and
Practice. This approach is also referred to as the "avoided burden" approach.