Page 443 - Design of Simple and Robust Process Plants

430 Chapter 10 The Efficient Design and Continuous Improvement of High-quality Process Plants
(1999). The final result will be a preliminary mass and energy balance based on the
consolidated flow diagram.

Energy optimization
This is done based on the preliminary mass and energy balance. A pinch study is
started to determine the energy targets (heating and cooling). This must be done
within the identified constraints for heat exchange, such as operability, controllabil-
ity, and safety. Based on these target data, iterations will take place between flow-
sheet optimization and energy optimization. The final result will be a consolidated
mass and energy balance, the energy levels are selected and the major equipment
dimensions fixed, such as number of trays, column diameter, and number of com-
pressor stages. Next, the energy network can be optimized, resulting in the sizing of
the heat exchangers.
Timing and documentation: conceptual design optimization needs to be done dur-
ing process synthesis. Optimization of the consolidated flowsheet in interaction
with energy optimization are planned after the process synthesis, with the exception
of heat exchanger network optimization, which can be done during basic engineer-
ing. All the optimization work needs to be documented, especially with its objective
functions, its price sets, constraints and the degrees of freedom for the optimization
and its sensitivities.

10.3.2.5 Reliability modeling
This is a fairly young applied design technique excluding nuclear and aircraft
industries to improve process design and to quantify the availability of the pro-
cess, including its infrastructure (Henley and Kumamoto, 1981&1992), It can gener-
ate a ranking of the elements contributing to plant unavailability and its down time
due to mechanical failures.
The objective of reliability modeling is based on a quantitative technique; to calcu-
late plant availability, identification of unreliable components, and to provide a tool
to optimize the design by the evaluation of design alternatives. The design philoso-
phy, ªDesign for single components unless, economically or safety wise justifiedº,
requires reliability modeling to justify any deviation from single components (Koo-
len, 1998). The design options for consideration are in order of priority:
. Can we eliminate this component?
. In case of failure, does that necessarily end in a process stop, and in case it
does not, can a component failure be survived within its repair time?
. Can we replace this component with a more reliable one?
. Consider redundancy
The technique has been applied both extensively and successfully in the power
industry, as well as off-shore explorations (Shor and Griffin, 1991).
Timing: during the conceptual design and basic engineering phase.
Documentation: results have to be documented in a reliability report.