Page 274 - Design of Simple and Robust Process Plants

260 Chapter 7 Optimization of an Integrated Complex of Process Plants
within a process plant was easy to handle with the traditional approach of ªlook and
seeº, as the systems were small and the number of contaminants limited. For site
integration, the system became complex due to the size and multiple contaminants,
and this required a more structural approach.
The major concern with mass flow integration is that direct contact is created
between different processes. In the case of heat integration, transfer of heat is
through a heat exchanger, most commonly with a utility inter-medium. Direct con-
tact may lead to process or safety problems which need to be carefully studied in
each case, before this type of integration is applied. A preliminary safety study for
these applications is mandatory. The use of lower-grade product streams with recy-
cling of the impurities back to the supplying process (as discussed in Section 7.5.1)
is also a form of mass flow integration. Also in that case, safety aspects with regard
to process interaction must be reviewed up-front.

7.5.2.2 Heat integration
Heat integration of a chemical complex and its energy system is generally organised
along the utility systems (Linnhoff and Dhole, 1993). The advantage is that by inte-
gration through utility systems a back-up becomes easily available, although it does
not exclude direct integration between processes (see Figure 4.28 in Chapter 4). In
general, that is applied for temperature level above 250 C and between 80±120 C,
and at refrigeration levels.
In the design of a site energy system, a complex has a considerable steam and
power demand; however, the steam to power ratio for different complexes is diverse.
Herein, it is assumed that the complex has its own co-generation system for power
and steam. On the other hand, the option to buy power and steam might be realistic.
In the case of a low steam to power ratio (as will be the case with electrolysis pro-
cesses), buying might be an attractive alternative, especially if a switchable contract
is an option. For the discussion about the co-generation system it is indifferent if
the unit is owned by an outside company or by the owner of the complex. There is a
trend to buy steam and power from public utility companies, as these often operate
at lower profit margins, as they have access to cheaper capital due to its lower finan-
cial risk.
The most energy-efficient way of energy generation with fuel is through a co-gen-
eration system (Figure 7.2). The design of such a system at a large complex is based
on a gas turbine, driving a power generator. The off gases go to a waste heat boiler,
generating high-pressure (HP) steam which, depending on the power/steam ratio,
has additional firing. The HP steam is fed to back-pressure or extraction steam tur-
bines which are sometimes equipped with a condensing turbine for operational rea-
sons. The steam turbines drive is either used for mechanical power for process
equipment such as large pumps, compressors or a power generator, but also smaller
redundant rotating equipment to avoid common cause failures. Any imbalance
shortage in the steam system is compensated by additional firing, and de-superheat-
ing let-down stations are provided. The connection between the different steam lev-
els is shown in Figure 7.3. The heat exchangers, condensate headers and condensate
flash drum are included. The number of steam levels depends on the specific