Page 40 - Sustainability in the process industry

P r o c e s s I n t e g r a t i o n 17

the existing network. The network bottleneck occurs at a heat
exchanger, which constrains the load shifting to gain further
improvement in heat recovery. At least one of the heat exchanger
sides exhibits the minimum allowed temperature difference between
the involved streams, and the corresponding point in the network’s
temperature-load plot is referred to as the Network Pinch. To
overcome this Network Pinch, the network structure must be
modified. Possible modifications include the relocation of an existing
heat exchanger, the addition of a new exchanger, or a change in the
stream splitting arrangement. To identify the most promising
modifications, the Network Pinch method uses MPR guided by
thermodynamic insights. Nonlinear optimization is employed to
evaluate the capital energy trade-offs and to produce a new optimal
structure. This approach allows retrofit to be carried out one step at
a time, leaving the designer in control to accept or reject suggested
modifications at each step.
There have been some successful practical applications of these
targeting and retrofit methodologies. Pleşu, Klemeš, and Georgescu
(1998) demonstrated the wide applicability of PI in the Romanian
oil refining and petrochemical industry. In one of the first
comprehensive retrofit case studies, Hassan, Klemeš, and Pleşu
(1999) presented a PI analysis and retrofit suggestions for a fluid
catalytic cracking (FCC) plant. Pinch Technology and its recent
extensions offer an effective and practical method for designing the
HEN for new and retrofit projects. Al-Riyami, Klemeš, and Perry
(2001) demonstrated a HI retrofit analysis for the HEN of an FCC
plant. Their study found significant room for improvement in the
heat recovery process, and the new network was designed using
the Network Pinch method.

2.7 Mass Exchange and Water Networks

Water is widely used in the processing industries as an important
raw material. It is also frequently used as a utility (e.g., steam or
cooling water) and as a mass transfer agent (e.g., for washing or
extraction). Large amounts of high-quality water are consumed in
many industries that face strict requirements for product quality and
the associated manufacturing safety issues. The processing industry
is characterized by complex design and operation of storage and
distribution systems for water management. Today’s industrial
processes and systems that use water are subjected to increasingly
stringent environmental regulations on the discharge of effluents.
Increases in population and its quality of life have led to increased
demand for freshwater. The rapid pace of these changes has made
more urgent the need for improved water management. Adopting
techniques to minimize both water consumption and wastewater