Page 382 - Design of Simple and Robust Process Plants

9.5 Closed Loop Steady-state Optimization 369
Model Input data reconciliation is based on mass or mass and energy balance
reconciliation. These balances have been selected because they do require any valida-
tion while based on conservation laws. The overall mass balance of the system is
used to provide the simulation/optimization of the process with good input data.
Feed quantity and quality are particularly important. The accuracy of the reconcilia-
tion step improves if more redundant nodes are built into the flowsheet (Figure 9.9).
It might be beneficial for the increase in redundancy of the flow-sheet for the plant
mass balance to include the storage area. Due to its inventory measurement, an
additional redundancy is added by averaging the mass measurement over a time
period. Large-sized vessels in the process can also be equipped with a hold-up meas-
urement and taken into account for the mass balance reconciliation.
The accuracy for the reconciled data depends on: the accuracy of the individual
measurements; the redundancy built into the flowsheet; and process stability. The
method of mass balance reconciliation over the process is also applied to the perfor-
mance measurement, as discussed under performance meter (see Section 9.4).

Redundant
nodes one

Redundant
nodes three

L

L
Redundant
nodes seven
L
L

Fig. 9.9. Illustration of adding redundancy for flow measurements.

Data reconciliation technique (Britt and Luecke, 1973; Albers, 1999) This technique
is based on minimization of the deviations of the square of the difference between
the reconciled value of the measurement and the measured value. In formulae
form: