Page 413 - Design of Simple and Robust Process Plants

400 Chapter 9 Operation Optimization
These systems are subject to frequent changes due to demand, but even
more due to the supply of external power at different price sets, like during
peak hours and with switch-off contracts. At the same time, several power
units might be available, often at so-called ªhot standbyº conditions as back-
up. These systems described in a MINLP environment are provided with an
optimizer and will include options to influence plant capacities for demand.
The optimizations often run off-line and have a continuous advisory function
to operation. The optimization in general receives on-line updated informa-
tion about units availability and conditions; if required, parameters might
also be calculated/estimated. As the site vulnerability might be heavily
affected by this operation, procedures for back-up are included in the soft-
ware. The mixed integer problem is solved by running all available scenarios
and advising the optimal configuration and capacity set-points for the individ-
ual units within the defined constraints.
. Dynamic optimization is currently applied only to a very limited extent, for
several reasons:
± Dynamic simulation is currently restricted, and the simulations are mainly
applied around a steady-state point for control design and operational studies.
The simulations of start-up and shut-downs are subject to a large number of
discontinuities which are still difficult to describe in a fundamental way. Dis-
continuities such as phase changes, filling and emptying of vessels and also
trays in columns, reverse and alternating flows, absorption followed by de-
sorption for regeneration, but also in correlations which describe a physical
phenomena over large operational ranges.
± Estimation and prediction of process parameters during operation depend on
the technique, but also require a high accuracy of the dynamic model.
± Simultaneous data reconciliation and parameter estimation in a dynamic
environment is commercial not yet available.

Current applications are the optimization of transient trajectories during grade
changes in a continuous process, and the optimization of batch cycle times.

Summary project methodology for OO This section provides an overview of project
methodology with its activities that will lead to the successful implementation of an
OO project. An overview of the overall approach is given in a project methodology
flowsheet, see Table 9.2. The essence of each step will be discussed.

. A feasibility study should give a detailed overview of the potentials of an OO
project. It will give a technical evaluation as well as an economic overview
based on a cost±benefit analysis. The study needs to include time schedule,
manpower planning, and an overview of factors which might have an impact
on site or company standardization.
. Scope definition must be based on the results of the feasibility study, and
clearly define the objectives of the project.