Page 149 - Design of Simple and Robust Process Plants

134 Chapter 4 Process Synthesis and Design Optimization
. The contradiction between process integration and process simplification.
Pinch analysis, being the bases for process integration, is focused on minimi-
zation of utilities, and this may lead to a wildly growing network of exchan-
gers. Process simplification will take advantage of exergy analysis as well as
pinch analysis, but will implement these in an order of priority for process
integration. The final answer must be found in the economics after the pro-
cess constraints are respected.

4.2.4
Controllability Analysis

Controllability analysis (Perkins, 1989; Skogestad and Postlethwaite, 1996; Seider et
al., 1999; Luyben et al., 1998) is an important technique to achieve robust control,
especially for simple and robust plants which were earlier defined as:

An optimal designed safe and reliable plant, operated hands-off at the most economical
conditions.

The requirement of hands-off operation is new in respect of conventional designs, as
it does not focus only on plant automation. Robust control includes the process being
able to track its set points and having good disturbance rejection capability over a
wide operating window. In other words, operators are no longer part of a control loop
± they become supervisors and determine the overall set-point for the operation.
The demand on control ± next to hands-off operation ± is higher than in the past
due to:
. High requirements on product quality.
. Elimination of intermediate storage as lot tanks, check tanks/hoppers and
minimization of storage, (implementation of JIP and TQC).
. High level of process integration.
. Switchability of process conditions for campaign operations (switchability is
defined as the ease with which the process can be moved from one stationary
point to another).
. Bases for closed loop optimization.
The consequence is that the process design needs to include controllability analysis
as part of the synthesis process to assure that adequate control is possible. It was
John Perkins, 1989, who specifically draw attention to the interaction between pro-
cess design and control (For details on controllability, see Chapter 8.) At this point,
only the overall methodology is discussed.
Controllability analysis is split into two parts:

1. Static analysis is executed when the first flowsheet selection is carried out, at
the end of synthesis step 1, and is used as input for synthesis step 2. Further
static analyses are performed during step 2, when the mass balances of the
alternatives for evaluation become available.