Examples of System Boundaries

Figure 3.4.3 shows an example of setting system boundaries for a product baseline analysis of a bar soap system. Tallow is the major material in soap production, and its primary raw material source is the grain fed to cattle. The production of paper for packaging the soap is also included. The fate of both the soap and its packaging end the life cycle of this system. Minor input could include the energy required to fabricate the tires on the combine that plants and harvests the grain.

The following analysis compares the life cycles of bar soap made from tallow and liquid hand soap made from synthetic ingredients. Because the two products have different raw material sources (cattle and petroleum), the analysis begins with the raw material acquisition steps. Because the two products are packaged differently and have different formulas, the materials manufacture and packaging steps must be included. Consumer use and waste management options should also be examined because the different formulas can result in varying usage patterns. Thus, for this comparative analysis, an analyst would have to inventory the entire life cycle of the two products.

FIG. 3.4.3 Example system flow diagram for bar soap. (Reprinted from Keoleian, Menerey, and Curran, 1993.)

Again, the analyst must determine the basis of comparison between the systems. Because one soap is a solid and the other is a liquid, each with different densities and cleaning abilities per unit amount, comparing them on equal weights or volumes does not make sense. The key factor is how much of each is used in one hand-washing to provide an equal level of function or service.

A company comparing alternative processes for producing one petrochemical product may not need to consider the use and disposal of the product if the final composition is identical.

A company interested in using alternative material for its bottles while maintaining the same size and shape may not need filling the bottle as part of its inventory system. However, if the original bottles are compared to boxes of a different size and shape, the filling step must be included.

After the boundaries of each system are determined, a flow diagram as shown in Figure 3.4.3 can be developed to depict the system. Each system should be represented individually in the diagram, including production steps for ancillary input or output such as chemicals and packaging.

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