Page 150 - Design of Simple and Robust Process Plants

4.2 The Methodology of Process Synthesis 135
2. Dynamic analysis already starts parallel to all steps 2, and needs to be com-
pleted before the final selection of the process alternatives.

4.2.4.1 Static analysis
Static analysis (controllability step 1) requires the mass and energy balances as
input for the analysis. The objective is to determine the window where the process
can be operated at steady state and the degree of interaction in the process. This can
easily be determined by static simulation, as a change in input may appoint to an
unsteady situation when the system does not find a new steady state solution. This
is important input also for switchability and for operation optimization, as in both
situations the objective is to change set points and to bring the process to another
steady state conditions.
Process interaction analysis is performed with static simulations. Steady-state
gains of the flowsheet and interactions are determined based on the perturbations
of the input one at a time. The overall successive steps of the methodology are as
follows:
1. The flowsheet must be divided into subsystems, feed system, reactor section,
evaporation section, or distillation section, and any sensitive recycle system
(sensitive in the sense that it has a reasonable impact on reaction or separa-
tion). It is not necessary to select all processing units for an interaction anal-
ysis at controllability step 1 ± only those that are suspicious for interaction.
We know that a distillation column, an extraction, absorber and stripper as
single processing units are controllable. An interaction analysis within these
last units can be started, but this may be better postponed to controllability
step 2.
2. Determine the degree of freedom analysis and, as a result of this, the number
of independent variables.
3. For an interaction analysis, the input and output variables must be deter-
mined. The output variables are those variables that should be controlled as
product qualities, streams, pressures, levels, etc., and these are called the
ªcontrolled variablesº. The input variables are those independent variables
that affect the process and its outputs. The input variables are subdivided
into manipulated variables (such as valves) and disturbance variables. The
number of independent manipulated variables isin general equal to the
number of controlled variables. Now, the feasible loop combinations are
determined which will be part of the interaction analysis, for details see
Chapter 8.
4. Relative gain analysis starts with the determination of:
Process gain as seen by a given controller with all other loops open
Process gain as seen by a given controller with all other loops closed
The values are determined based on the perturbations of the input one at time.
The results of the analysis are shown in a relative gain array (RGA) essentially
a matrix which shows the interaction between input and outputs.