Page 365 - Design of Simple and Robust Process Plants

352 Chapter 9 Operation Optimization
9.3
General Operation Optimization ofContinuous Processes

9.3.1
Incentive

The objective is to operate a process plant on-going at its most economical condi-
tions. This can only be realized if operational conditions are evaluated and imple-
mented each time the surrounding conditions are changed. The following circum-
stantial conditions will impact on economical performance:

. Price changes of raw materials, products, energy.
. Feedstock changes.
. Production capacity.
. Product quality.
. Meteorological conditions (may restrict capacity and pressure operations).
. Process parameters as fouling, aging, catalyst quality.
. Transient operation.
The spin-off of OO can be considerable. The development of an OO package in
whatever form always drives a process unit to its constraints, or into the edges of its
constraints. This forces the developers and operational personnel to build up an
extensive knowledge of the process constraints. The knowledge of these constraints,
which limit the operational capacity, may trigger engineers for relative simple hard-
ware modifications to achieve cheap capacity increments.
OO is critical for the economic operation of the facility and its relation to other
plants and the overall business. The prices used for the raw materials, products and
energy are crucial for the optimal operation. The economics of OO are normally based
on incremental conditions. Due to the criticality of the issues mentioned, it is essen-
tial to involve an economic evaluator from the business to achieve agreement about:
± objective function;
± economic calculations as incorporated in the optimization routine; and
± prices and their regular updating.
Most of the circumstantial conditions change at a certain point in time, and remain
constant for a while (prices, product quality), while others are subject to continuous
change, for example meteorological conditions.

9.3.2
Continuous Processes

These are normally operated around a steady-state point. The optimization calcula-
tion procedure is based on a process model and represented in a control loop (Fig-
ure 9.1). The consequence is that every time conditions change, the steady-state sit-
uation has to be updated. The frequency of the changing conditions determines if
the operational adjustments will be implemented manually or automatically.