Page 332 - Design of Simple and Robust Process Plants

318 Chapter 8 Instrumentation, Automation of Operation and Control
The DI/DO ratio (hardware) is dominated by the amount of limit switches on valves
and motor drivers, and a ratio of 1 should be easily achievable (this is based on the
assumption that are not two DIs per DO).

8.3.5
Summary: Automation of Operation

The approach to a simple and robust plant is one of hands-off operation. The reason-
ing behind this is that human beings also have, next to their superior function in
nature, certain disadvantages. In the operation of process plants they require certain
characteristics that are not always available. The consistent performance of tasks is
one weak point; they also do not have a good memory, and if something is compli-
cated they are not always going to figure it out. This makes human beings less cap-
able for the operation of a simple and robust process plants. Such a plant leaves less
freedom for interpretation during operation, despite the drive for consistency and
high-quality production. Thus, the ultimate concept for these plants is one of total
automation, and in this respect the role of the process operator is changing to that of
process supervisor. Automation is based on an operational strategy which is
reflected in instrumentation selection and implementation operational procedures.
. Instrument selection is based on the following key elements: selection of
CVs and MVs for control, automation of operational actions and start-up and
shut-down trajectories, safeguarding, and observation.
. The operational strategy requires:
± A set of operational objectives which provide safety and environmentally
sound production, and total quality control with first-pass prime production.
± A layered structure of operational states which have to be passed in a defined
safe order.
± Defined transient operations to bring the process safely from one operational
state to the next.
± A set of detailed operational procedures.
. For the operational states, the difference is between reversible and irreversi-
ble unit operations.
± Reversible unit operations are units which can be operated in a stand-alone
mode.
± Irreversible units are started by supplying an appropriate feed and dischar-
ging the effluents to downstream units.
. Start-up of irreversible units for FPPP requires: conditioning of the unit,
mostly with recycle streams close to its operational conditions, and imple-
mentation of trajectory control on feeds and operational temperatures. The
design of the optimal trajectories for start-up might be based on experience,
but at best is based on dynamic simulations.
. An overall start-up procedure is developed for continuous process plants; this
methodology has been verified for three processes.