Page 195 - Design of Simple and Robust Process Plants

180 Chapter 5 Process Simplification and Intensification Techniques
5.6.1.3 Prevention of waste streams
The prevention of waste streams results in a need for fewer facilities to recover or
degrade the waste that otherwise would reach the environment. This is not limited
to continuous streams, but includes discontinuous streams and even releases from
emergency systems. All such streams need careful evaluation to prevent their forma-
tion, before recovery systems are considered. An example is the prevention of emer-
gency relief on ammonia storage described in Chapter 3 (see Figure 3.6). Here,
instrumental protection took over safeguard of the tank from a pressure relief sys-
tem, though this required a different design strategy. The pressure relief system pro-
tects against any overpressure, and is sized according to the largest load, but for the
instrumental protection system there is no relief device, which means that the sys-
tem must prevent any potential over-pressure at a required level of reliability.
A central header system for collection of the emergency relief devices of process
systems is something which needs to be re-considered. Such a collection system
results in a need for large recovery or destruction units. Another approach is to
align relief devices with an internal process vessel, which can largely avoid or mini-
mize any need for external relief. The same approach must be applied for draining
systems, as any drain into a central system often makes recovery for process re-use
more difficult. The re-routing of drains for internal re-use is worth studying, and in
such a case the drains of one particular process section might be collected and recov-
ered in the same section.

5.6.1.4 Adiabatic operation of reactors
The adiabatic operation of reactors has a wider impact than does the reaction section
alone. On comparing the three cooled CSTRs with the adiabatic CSTR and plugflow
reactor of Figure 4.7 in Chapter 4, it is clear that there is a large difference which is
not limited to a low cost reaction section. For the overall process, there is an impact
on the selectivity, and the conversion energy drops due to usage of the heat of reac-
tion; this results in a smaller steam system, while the role of the cooling water sys-
tem, together with its make-up water, is reduced. The same applies to the example
of the nitration reactor (Figure 4.8 in Chapter 4).
The advantages of adiabatic reactor designs therefore have a high priority in the
ranking for simple designs (see Section 5.7.1).

5.6.1.5 Direct coupling of separations
Several examples of direct coupled applications were discussed earlier in Section
5.3.2.4, the divided wall column being the most generic application. The example of
extractive distillation combined with a rectifier (see Figure 5.11) is achieving wider
application.
Searching the process for opportunities in this respect led to a number of applica-
tions (see Figure 5.12), but in this case two distillations are direct coupled, thus
combining a separation before and after a reactor. This type of application is encoun-
tered around a reactor system when the feed contains lights that are to be removed,
when the conversion in the reactor does not run to completion, and when the reac-
tor products are heavier than the unconverted feed. A similar concept can be applied