Page 216 - Sustainability in the process industry

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material and operating unit appears exactly once. In the composition
phase, the nodes are linked stepwise, layer by layer, starting from the
shallowest end (i.e., the final-product end) of the remaining input
structure. The algorithm proceeds by assessing whether any of the
linked nodes violates any of the axioms described in Chapter 7.
The structure generation is performed transparently, and the
maximum structure that results is the input for the Solution Structures
Generation (SSG) algorithm. If a material has to be produced, then
the SSG algorithm generates all possible ways for its production. For
example, if M1 can be produced by O2 or by O3 then the possibilities
include production by O2 alone or by O3 alone or by using both.
Once an operating unit is included, its input materials must be
produced as well, and so forth. Materials are selected in a specific
order. The parent–child relation between steps ensures that the
materials are selected by the process according to this order.
The SSG algorithm yields all the combinatorially feasible solution
structures of a given problem. Unfortunately, the number of feasible
structures at this stage is often too large to be enumerated explicitly.
Therefore, the Accelerated Branch-and-Bound (ABB) algorithm is
used to determine the optimal structure without generating all the
possible solutions. Input to this algorithm includes not only the
structural relationships between materials and operating units but
also such additional information as the costs of each raw material,
the fixed and proportional costs of operating units, and the constraints
(if any) on the quantity of materials and the capacity of operating
units.
9.2.2 S-Graph Studio
S-Graph Studio is a software package that enables the user to design
batch processes and to optimize them via various optimization
methods (S-Graph, 2009). The program also allows scheduling
problems to be defined using graphical tools. It has a modular
architecture, so different solvers can be used with the program.
S-Graph Studio uses the industry’s standard file format: BatchML (as
defined by the ISA-88 standard of the World Batch Forum), which is
used to exchange information between industry sites and plants as
well as for other purposes. The Excel file format can also be used for
both input and output. The software includes a solver that utilizes
the S-graph methodology developed at the University of Pannonia
(S-Graph, 2009).
One of the main goals of batch process optimization is to
minimize makespan—that is, finding the shortest time in which a
process can be completed using available resources. S-Graph Studio
can be used to define batch processes in terms of the tasks to be
performed, the available equipment units, and task completion times
(as a function of the equipment units used). This information is
necessary and sufficient for minimizing the makespan of a process.