Page 191 - Sustainability in the Process Industry Integration and Optimization

168 Cha p te r E i g h t

reaction network can be generated using the Maximal Structure
Generation (MSG) algorithm. From that, all feasible reaction networks
can be generated as solution structures. Historically, it has been
extremely difficult to construct an exact maximal reaction network.
The problem has become solvable only since the arrival of the
combinatorial approach based on P-graphs. Reaction network
synthesis is completely combinatorial in nature because all chemical
species participating in the reactions are defined discretely.
The application of PNS to Reaction Network Synthesis (RNS) is
illustrated by a chemical process for the manufacturing of vinyl
chloride (C H Cl). Because of its structural simplicity, the balanced
2 3
synthesis of vinyl chloride (C H Cl) leads to only a single complex
2 3
route containing one loop. It is a trivial example from the standpoint
of constructing the maximal reaction network and the feasible
reaction networks, but it will serve to illustrate the RNS methodology.
The balanced process aims to produce vinyl chloride (C H Cl) and
2 3
water (H O), where the former is the desired product and the latter is
2
the by-product, from the three starting reactants: ethylene (C H ),
2 4
chlorine (Cl ), and oxygen (O ). This process involves the following
2 2
three unit reactions: R , R , and R .
1 2 3
R : 2C H + 2Cl = 2C H Cl
1 2 4 2 2 4 2
R : 2C H + 4HCl + O = 2C H Cl + 2H O
2 2 4 2 2 4 2 2
R : 4C H Cl = 4C H Cl + 4HCl
3 2 4 2 2 3
The unit reactions yield the following overall reaction:
4C H + 2Cl + O = 4C H Cl + 2H O
2 4 2 2 2 3 2
Note that C H Cl is the final target and that C H , Cl , and O are the
2 3 2 4 2 2
starting reactants. From the perspective of materials (M), we have:
Starting reactants
Final products

Intermediates

M { CH , Cl , O , CH Cl, H O , CH Cl , HCl }
2 N 2 4 2
2
4
2
2
3
2
Raw materials Product By-product Intermediate materials
One or more of the feasible paths and valid vertices in the input
structure may disappear if some of the invalid vertices are eliminated.
Thus, the final maximal structure is composed (or reconstructed)
from the remaining skeleton of the input structure after the
elimination. This is accomplished step by step, linking alternately
the vertices of the M-type (for materials) to the vertices of the O-type
(for operating units) and vice versa. At each step, the vertices linked
are assessed in view of the appropriate axioms (see Section 7.2.2):
vertices of the M-type must satisfy axioms (S1), (S2), and (S5); and
vertices of the O-type must satisfy axioms (S3) and (S4). The execution
is initiated from the structure’s shallowest layer—that is, the final,
desired product end. The stepwise procedure for the composition is
illustrated in Figure 8.2.