Page 166 - Sustainability in the Process Industry Integration and Optimization

Fu r t h e r A p p l i c a t i o n s o f P r o c e s s I n t e g r a t i o n 143

CO separation as well as a considerable amount of steam for the
2
stripper.
The synthesis of power-dominated energy systems is envisaged
with the aid of superstructure-based mathematical optimization.
The proposed superstructure (Figure 6.18) includes all possible
design options, and the optimization involves (1) the systematic
screening and evaluation of possible flowsheets and (2) assessing
economic trade-offs between capital costs and operating costs. As
usual, the optimization objective is to minimize the overall cost (i.e.,
capital and operating costs) while accounting for the model’s
constraints (Zheng, Kim, and Smith, 2008b). This task is typically
formulated as an MILP problem in which piecewise linearization is
used to capture the capital costs.

6.5.2 Low-Temperature Energy
The levels of power required for compression constitute a major
component of energy consumption when cryogenic cooling is applied
to process streams. Thus, the efficient use of such “cold energy”
contributes to the cost-effectiveness of low-temperature processes.
Heat Integration—in particular, one of its most powerful tools, the
Grand Composite Curve—has a long history of application to saving
energy in cryogenic plants (Linnhoff et al., 1982; Linnhoff and Dhole,
1992; Smith, 2005); see Chapter 4 for details.
Pure refrigerant systems cannot avoid some degree of
thermodynamic inefficiency, since otherwise the heat exchanger(s)
would exhibit large temperature differences and this would push
the system away from thermodynamic reversibility. However, if
mixed refrigerants are used then the refrigeration cycle’s structure is
simplified, considerably reducing the duty requirements for
compression. The advantage of mixed refrigerants is that they

BO-1 BO-N
CN.....C1
HG/HM GT-1 HRSG
VHP
ST DR/EG
HG/HM GT-N HRSG
HP
ST DR/EG
Process Stream
EM-1 PP-1
MP
ST DR/EG
EM-N PP-N LP
ST DR/EG
ExE CON
FIGURE 6.18 Superstructure for energy system used in a low-temperature
process (after Zheng, Kim, and Smith, 2008a).