Page 458 - Design of Simple and Robust Process Plants

11.7 Instrumentation, Operation/Automation and Control 445
evaluated, to avoid over- or under- redundancy in designs, as a result of design poli-
cies.
Site selection has a very high and long-term impact on the operational cost. An
objective site selection study is at least as important as technology selection.
Process selection within a product chain(s) and its level of process integration
create a large benefit in logistic cost. Process integration can generate additional sav-
ings by integration of lower purity (less than commercial) intermediate product
streams by avoiding/minimizing expensive separation steps.
Optimization of storage capacity for product chains must be based on reliability
studies over the whole supply chain, including raw material and product transport
system reliability.
Site vulnerability studies based on reliability calculations are to be performed for
integrated complexes of process plants. Cost±benefit analysis can be performed of
investments in logistic and utility or other back-up systems versus product availabil-
ity to obtain a well-balanced, reliable, integrated site.

11.7
Instrumentation, Operation/Automation and Control

A simple and robust process plant is designed for total automation and hands-off
control. This places extra emphasis on the design of instrumentation, automation
and control.
Application of reliable and robust in-situ measurements with short response
times is essential, as this is the basis for minimal instrumentation.
Total automation for hands-off operations puts specific requirements on the
operational strategy, control strategy, safeguarding, and process observability.
The operational concept asks for development of an operational strategy to enable
smooth start-up and shut-down, including interlocking with defined process states
and transient pathways.
The concept of first-pass prime production (FPPP) requires preconditioning of all
irreversible units, and the development of a detailed start-up procedure, in case of
reactors based on dynamic rate models.
Alerting and alarming strategies are required which, next to its alarming func-
tion, in particular should address: alerting of operators to keep them involved, and
the prevention of alarm showers to maintain an overview in an emergency situation.
Safeguarding of automated processes requires close observation of the critical
process sections; this is effected by shadowing the operation with dynamic simula-
tions. Deviation from normal operation can be detected at an early stage and used
for timely process intervention. Critical process sections are, in particular, exother-
mal reactions and reactions with gas generation.
Observability demands specific measures for operation regarding process presen-
tation, alerting, alarming, and control design.
Control design is a layered design consisting of: Basic process control layer
(BPCL), including emergency control; Advanced control or model-based control