Page 132 - Design of Simple and Robust Process Plants

4.2 The Methodology of Process Synthesis 117

Unit A Unit B

Direct exchange
STEAM C.W.

Unit A Unit B
Exchange with back-up heater/cooler
STEAM

BFW
COND
Unit A Unit B

Exchange with utilities back-up

Fig. 4.28. Heat exchange options between process units.

Operability The points for consideration regarding operability are important, as
these may lead to additional provisions that require more capital. One questions to
be addressed is: Can we cope with sufficient operational flexibility from the consu-
mer, as well as the supplier, perspective? Operational scenarios to be considered are:

. Start-up
. Shut down
. Hot stand-by situation (idle operation)
. Regeneration
. Difference in operational capacities of consumer versus supplier plants
. Difference in operational efficiency compared to design (start-of-run or end-
of-run conditions of reactor systems).
The design must be able to cope with these variable conditions in order to permit
operational flexibility.

Controllability Next to the availability and variability, disturbances often also lead to
problems, and the integration of processes often leads to fluctuations during opera-
tion. These may be caused by process variations (disturbances) that are transferred
from one section to another, and/or the large response times that are inherently part
of integration. The disturbance sensitivity can be identified when static and dynamic
controllability studies are performed. Based on the controllability studies, the cont-
rol strategy eventually including process design modifications needs to be developed
to absorb/reject these disturbances. The design of control strategies is an efficient