Page 195 - Sustainability in the Process Industry Integration and Optimization

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FIGURE 8.4 A solution network for the workfl ow synthesis problem.

of silage on farms. By starting from silage, two crucial steps are combined:
storage (thus enabling the downstream processes to operate continuously) and
conversion of the carbohydrates in green biomass to lactic acid. In addition,
silage production transforms many proteins into amino acids or peptides
(Povoden, 2002; Koschuh et al., 2004).
The synthesis method requires a comprehensive list of raw
materials, intermediates, and possible products. Note that transport
is treated like a processing step: it uses trucks (or tractors), together
with the raw materials (or partially processed juice or press cake) and
“available time,” in order to derive a realistic logistics pattern.
Consequently, there has to be a plant-specific intermediate material
flow that leaves this “process step.” The steps listed in Table 8.2 reflect
the necessary logistical handling (i.e., the “local” and “central”
converters for various materials) involved in the process.
Once the cost function (including investment and operating
costs) is defined, the synthesis yields the optimal solution to process
silage as a part of the maximal structure; see Figure 8.6 (black lines
show the optimal solution, while the other options are grayed).
One major advantage of process synthesis is that it allows the
designer to apply sensitivity analysis not only to the process itself but
also to the entire value chain. In this example, sensitivity analysis
reveals a remarkable stability of the central biorefinery structure