Page 371 - Design of Simple and Robust Process Plants

358 Chapter 9 Operation Optimization
It was said earlier that the accuracy of a performance meter is essential for its
function. Such accuracy is determined by the mass balance reconciliation in which
three factors play a role:
1. Measurement accuracy
2. Degree of redundancy of the flowsheet
3. Process variability

9.4.1.1 Measurement accuracy
This is initially determined by the selected instrument. It is common practice to
measure the volumetric flow and to calculate the mass flow based on an assumed
density, but this already introduces a considerable error. Most streams are subject to
density variations caused by temperature or concentration, while the density is often
not measured together with the volumetric measurement. In addition, there is also
a volumetric measurement, such as an orifice, that is already density dependent. In
this application a mass flow meter is recommended, not only to avoid these implicit
errors, but also to create a high accuracy.

9.4.1.2 Degree ofredundancy
This is achieved by installation of additional measurements for reconciliation of the
flowsheet. An example of adding redundancy is shown in Figure 9.9. The addition
of more redundant nodes is illustrated, in the bottom part also the storage area is
included in the flowsheet. The accuracy of the individual streams can be calculated
for the selected flowsheet based on the installed instrument accuracy. and described
in the Appendix 9.1 (Heyen, 1994). This has been extended by Heyen et al. (1996) by
the introduction of the contribution of the variance of measurement k in the estima-
tion of the variance of reconciled state variable i. These contributions might be listed
in order of impact. Conclusions drawn in this leading article, about these sensitivity
analysis are:
. Which are the measurements that contribute significantly to the variance of
the validated results for a set of state variables?
. Which are the state variables whose variance is influenced significantly by
the accuracy of a given measurement?
. How is the value of a state variable influenced by the value and the standard
deviation of all measurements?
The sensitivity analysis has been implemented in Vali II software from Belsim.

9.4.1.3 Process variability
Depending on the application of the performance meter, the process dynamics play
a role in the performance measurement. There are different ways to limit the effect
of process variability:
± Integrate the measurement over a larger time interval.
± Include process hold-up variations in the model.
± Improve process stability.