Page 405 - Design of Simple and Robust Process Plants

392 Chapter 9 Operation Optimization
± The determination of model variables as; activation energy, frequency factors
or the reaction scheme for fundamental models, updating flow characteris-
tics as dispersion coefficients or mass/heat transfer coefficients.
± The selection of parameters for updating over (aging)time.
± Extension of the operational range of the validation; this is frequently done at
laboratory scale, but at plant scale the options are severely restricted.

9.6.7
Develop Process Model with Reactor Model, Including Optimizer: Step 6M

The process model in general is developed in an equation-based commercial simula-
tor. These simulators are provided with library models, physical property data banks
and equations, economic section and optimizers. To include the reactor models
these also have to be written equation-based. Based on the results of the feasibility
study, the detailed process flowsheet model needs to be developed and to cover the
selected DOFs and constraints. The factors which play a major role in the level of
detail are:
. Required optimization accuracy.
. Degrees of freedom as feed stocks, product slide.
. Constraints defined in the OO models versus the MBC.
It is advisable to make an overall process model (including the reactor model), but
not up to a large detail, as unit operation can be described in a simplified way. The
next step will be to replace the simplified unit models with more rigorous models,
for those sections which have a major contribution to the operational cost. The
units that will form a constraint are also candidate for a more detailed representa-
tion in the model, as far as the constraints are implemented in the optimization
model.
During the development of the process model, we might extend the sensitivity
analysis as performed during the feasibility stage regarding the impact of con-
straints and DOFs on the optimization results. This might result in additional con-
straints, DOFs and parameters.
The process model is built up unit by unit, and these unit models are verified
against process data. Parameters such as tray efficiencies, mass transfer coefficients,
furnace efficiencies, heat transfer coefficients are calculated and verified to match
model versus process unit. Parameters can be differentiated according to the way
they are handled:
. fixed at a constant value, determined during verification of the unit model;
. fixed, but capacity-dependent, as compressor efficiencies;
. updated during operation, particularly those that vary subject to fouling.
Updating might be done by back-calculation or by estimation of the parame-
ters. In the last case, the redundancy in measurements required for estima-
tion must be available.