Page 39 - Sustainability in the Process Industry Integration and Optimization

16 Cha p te r T w o

Cost estimation usually has a significant impact on a project’s
predicted profitability. Taal and colleagues (2003) summarized the
common methods used for cost estimation of heat exchange
equipment and the sources of energy price projections. This paper
showed the importance of choosing the right cost estimation method
and a reliable source of energy price forecast when retrofit projects
are evaluated for viability. Both retrofit and grassroots design projects
require accurate cost estimates in the project’s early stages so that the
correct choices are made. There are several methods available, most
of which lead to FOB (Free On Board) cost estimates. As for operational
costs, the range of possible future energy prices may prove crucial
when the margins of a proposed design or retrofit project are
analyzed. This is especially true for a large project that has a long
payback period and consumes a lot of energy. Such projects are
typical of plants in the chemical, petrochemical, refinery, and
paper industries. Taal et al. (2003) include a brief review of oil and
natural gas price projections reported by centers of excellence in the
energy economics field. There are numerous forecasts, sometimes
contradictory, but general trends can be observed. Some general
insights in oil and gas price and production behavior are mentioned
and refer to more detailed sources.

2.6 Modifying the Pinch Idea for HEN Retrofit

Bochenek, Jezowski, and Jezowska (1998) compared the approaches
of optimization versus simulation for retrofitting flexible HENs.
This is an important work that should have generated additional
research. Zhu, Zanfir, and Klemeš (2000) proposed a heat transfer
enhancement methodology, for HEN retrofit design, from which HI
could benefit substantially. This approach is worthy of wider
implementation, especially in the context of retrofit studies.
Heat Exchanger Network retrofit is a special case of optimization.
In retrofit problems, one must accommodate an existing network
with existing heat exchangers that are already paid for. This
circumstance substantially alters the economics of the problem as
compared with a new design. One example of this approach to
retrofit was the paper of Tjoe and Linnhoff (1986), which suggested
identifying heat exchangers with cross-Pinch heat transfer and,
where appropriate, attempting to replace such heat exchangers with
others that do not transfer heat across the Pinch, thereby reducing
energy consumption.
However, an ideal new design would not account for the existing
HEN equipment and topological constraints. One method for
overcoming these drawbacks is the Network Pinch (Asante,
1996; Asante and Zhu, 1997), which uses continuous nonlinear
optimization to identify the bottlenecking heat exchangers within