Page 44 - Sustainability in the process industry

P r o c e s s I n t e g r a t i o n 21

Petrochemicals: Fluid Catalytic Cracking Process
The FCC unit is a major process element in oil refineries, and
improvements in yield and efficiency have been attempted over the
years in response to various external drivers. Retrofitting the HEN
associated with an FCC usually leads to improved energy recovery
and thus to reduced energy use and/or increased throughput. In this
example, the HEN of the FCC process includes a main column and a
gas concentration section (Al Riyami, Klemeš, and Perry, 2001). The
particular FCC plant considered had 23 hot streams and 11 cold
streams, and the associated cost and economic data required for the
analysis were specified by the refinery owners. The task was to
analyze the existing process and then propose a HEN retrofit plan for
improving energy recovery. Incremental area efficiency was used for
the targeting stage of the retrofit design, which was carried out using
the Network Pinch method (Asante and Zhu, 1997) consisting of a
diagnosis stage and an optimization stage. In the retrofit, four heat
exchangers were added and one existing exchanger was removed.
The resultant design produced energy savings of 8.955 MW or 74
percent. This translated into a 27 percent decrease in the plant’s utility
bill for an annual savings of $2,388,600. The modified HEN required
an investment of $3,758,420, so the payback period was less than 19
months.
Energy Integration of a Hospital Complex
Herrera, Islas, and Arriola (2003) studied a hospital complex that
included an institute, a general hospital, a regional laundry center, a
sports center, and some other public buildings. The diesel fuel used
to generate the steam required for the complex amounted to
75 percent of its total energy consumption and 68 percent of its total
energy cost ($396,131 in 1999). The Pinch Analysis that was performed
estimated the minimum need for external heating at 388.64 kW.
Because the actual heating used was consuming 625.28 kW, there
existed a potential energy savings of 38 percent.
Sunflower Oil Production
As reported by Klemeš, Kimenov, and Nenov (1998), an oil production
process operated with a minimal temperature difference of 65°C at
the Process Pinch has been analyzed. The external heating required
by the system was provided by two types of hot utilities, and the
required external cooling was provided by two cold utilities. The
study resulted in increasing heat recovery and reducing the minimum
temperature difference to 8–14°C. This reduced the hot and cold utility
requirements and eliminated the need for water steam and cooling
water, which considerably simplified the overall design.
A Whisky Distillery
In a study by Smith and Linnhoff (1988), the authors found that steam
was being used below the Process Pinch; this resulted in unnecessarily