Page 315 - Design of Simple and Robust Process Plants

8.3 Automation of Operation 301
transient operation, the likelihood of a breakthrough of component B is high unless
specific precautions are taken. This system was studied through dynamic simula-
tions and optimizations to search for operational transient conditions under which
FPPP could be realized. In this particular example, FPPP was defined as staying
below a certain specified level of component B in the reactor outlet during start-up.
The controlled variables (CVs) are:

. Mass flows of A and B, and through them, the production rate.
. Reactor inlet temperature.
. Outlet concentration of component B.
The manipulated variables (MVs) for the reactor are:
. Reactant mass flow B is controlled by a control valve in the feed line before
reactant A is added.
. Reactant mass flow A is controlled indirectly by manipulating the total reac-
tor outlet flow.
. Reactor inlet temperature is controlled by the manipulating the heat flow to
the reactor pre-heater.
. The manipulation of the trajectories of the three MVs over time.
For observation and safeguarding temperature measurements were installed over
the length of the reactor. For the study of the reactor start-up, a dynamic model reac-
tor model was developed by describing the main reaction, this for reasons of simpli-
fication.
A reactor start-up strategy was developed in four consecutive steps to start the
reactor up and to attain full operational conditions:
1. The reactant A is recycled over the entire process section, which includes part
of the recovery section.
2. The recovery section for component A is brought to operational conditions.
3. Reactor inlet temperature is driven to operational conditions h (z, 0) at which
reactant B can be fed safely in the reactor.
4. Supply of reactant B to start production. The condition achieved at the end of
step 3 is the state of the reactor at time 0.
For the start-up, the degrees of freedom explored within the operational strategy are:

. Initial reactor inlet temperature at the introduction of reactant B.
. The total reactor flow rate u i for start-up.
. The flow rate trajectory of reactant B.
. The trajectory of reactor inlet temperature m h, with as initial condition is h
(z,o), and the normal operating condition h (o, r).

The final reactor conditions at full operation are known from steady-state simula-
tions.
The system was described in dimensionless terms by using a reference value, the
symbols used being: