Page 277 - Design of Simple and Robust Process Plants

7.5 The Optimization of an Integrated Complex 263
extensive evaluation of the site energy integration options and design of the energy
system with its levels.
The selection for the site steam levels will be made by evaluation of the site source
and site sink profiles, which are accumulated from the individual plant composite
curves (Figure 7.4) (Dhole and Linnhoff, 1992). As was conclude earlier, in existing
situations the energy levels have already been selected; in that case only expansions
are under discussion, while changing the existing levels is in general not economical
as the existing plants have already adapted their designs on the existing situation.
The most common solution is the selection of [none2] site steam levels with back-
up from utilities, applied in the temperature range of 120±250 C. The final result is
a steam site balance which can be used for heat engine (steam turbine) selection.
On occasion. it may be beneficial to have an exchange of heat between individual
plants (Ahmad and Hui, 1991). The exchange between plants mainly occurs through
different intermediates, depending on the temperature levels:

. at levels above 250 C with hot oil systems.
. between 80±120 C with hot water or local steam systems (exceptionally with
an organic medium).
. at temperatures below ambient, though the refrigerant.
The drawback is that back-up facilities needed to be available to guarantee indepen-
dent operation of both partner plants, in order to meet the site design philosophy.
The potential for cross-contamination of chemicals of the two plants encourages the
use of a harmless intermediate for both plants. These must prevent any potential
reactive chemical problem. The interaction between both plants due to heat
exchange should not be underestimated in a design. The opportunities for exchange
between plants are derived from the grand composite curves of the individual pro-
cesses. When a plant has energy available at a temperature level which is close to
the energy utilisation level of another plant, then the potential is there.
The selection of refrigeration levels is different from steam level selection, and is
discussed in detail by researchers as Linnhoff and Dhole, 1992; Dhole and Linnhoff,
1994.

7.5.2.3 Turbine drivers selection
When a process furnace is an option for a gas turbine with power generator, this
should be evaluated first as it has an interaction with the composite curves of the
plant. The process furnace option should not necessarily incorporate a back-up for a
failure of the gas turbine. Recall the plant design philosophy ± ªDesign for single
reliable components unless; economically or safety wise justifiedº (Koolen, 1998).
The potential of power (mechanical or electrical) generation by letting down
steam to lower levels over steam turbines has to be exploited, as is foreseen in the
co-generation scheme. The exploitation can in all practicality only be done once, so a
choice must be made between the different options. In steam turbine driver selec-
tion, an inventory is first made of the requirements of plants for steam turbine
drive. Large drivers are preferred, and especially those drivers which have a priority
from a safety perspective such as a back-up cooling water pump drive for emergency