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The economic and environmental considerations often
lead to more stringent process design. In terms of plant
operation, this means the existence of recycle structures in
processing plants. The steady-state and dynamic
behaviors of the reactor/separator system differ
significantly from their individual unit counterparts. A
notable difference is the limited throughput handling
capability when the control structure (selection of
controlled and manipulated variables) is not appropriately
chosen. Since the extra work, resulted from load change,
is not evenly distributed among process units, this imbalance
leads to large deviations in some process variables. Analyses
are given to illustrate this effect and a control
structure is proposed to overcome this disturbance
rejection problem. Furthermore, a systematic tuning
procedure is also proposed to find the controller parameters
in plantwide control. A reactor/separator process is used
to illustrate effectiveness of the balanced control
structure and controller tunin g procedure. Simulation
results show that the balanced scheme can handle large load
changes while maintaining good dynamic performance.
The indirect feedforward/feedback (i-FF/FB) control is
constructed based on these two constant gain elements.
Steady-state characteristics, dynamic properties and
robustness issues of the i-FF/FB control are explored.
Consequently, a condition minimizing two unfavorable factors
(maximum peak height and maximum rate of change in outflow)
in level control is derived. This, in term, simplifies the
controller design to a single parameter tuning problem.
Therefore, a design procedure is proposed for the surge
ta nk level control. As for reactor level control, since
the set point and load responses can be designed
separately under the i-FF/FB configuration, a design procedure
is also proposed for this type of process. A surge tank
example and a plantwide reactor level control problem are
used to illustrate the performance of the proposed control
configuration. Results show that effective control can be
achieved using the indirect FF/FB control only two constant
gain feedback. In this work, a complex
process with recycles, Tennessee Eastman Challenge
Process, is studied to explore the operability for plants with
recycles. A simplified process model is constructed to provide
physical insight into the Eastman Plantwide process. The
results show that the composition distribution in the reactor
(or the recycle flow rate) plays an important role in
determining the optimal operating condition, i.e., achieving
maximum one-pass conversion. The trajectory of these optimal
operating points at different production rates is employed to
construct the optimal operating policy (OOP). From the OOP,
input multiplicity is observed for the plant with recycles.
Therefore, care has to be taken in operating plants with
recycles. The optimal operating policy also indicates an
inherent constraint on the production rate imposed by the
process, e.g., process constraint. However, the design of
control system may lead to an even more limited operability.
Moreover, the OOP can be used to evaluate the
appropriateness of the designed equipment. The results show
that the recycle compressor is not adequately designed and,
subsequently, leads to an even smaller operating range. A
design procedure is summarized to analyze the process,
control and equipment design aspects of recycle processes.
The results indicate that complex recycle processes can
be analyzed in a systematic way and, more importantly,
all these analyses are based on a rather simple process
model in a transparent manner. ( Keywords: plantwide
control; operability; multiple steady states; process
constraint; equipment constraint; level control; integrator
process; load estimation; feedforward control; two-degree-
of-freedom control. )
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