Page 294 - Design of Simple and Robust Process Plants

280 Chapter 7 Optimization of an Integrated Complex of Process Plants
Evaluation is the most important step that already starts during the collection of the
input. It is not the intention here to provide specific data, but rather to emphasize
the important factors for evaluation.
. The flowsheet already shows the critical material suppliers, those that do not
have a storage facility like, hydrogen, fuel gas, power are subject to special
attention.
. The failure rates of the utility systems or fuel gas supplies set the basis for
total complex outages. Careful study can give surprises, such as the relevance
of a reliable water supply, and others. Electrical power is always of concern,
as the availability might be high, but a high frequency of short interruptions
leads to frequent load shedding (losses). Utilities that do not activate a load-
shedding facility and are essential for the majority of the plants should have
a high MTBF, in the order of 50 000±100 000 h.
. The failure rates of the plants provide a good insight into specific plant relia-
bility, and might ask for a detailed component analysis to evaluate the relia-
bility. The base case of the simulation shows the impact of different compo-
nents on the final product availability. These can be tabulated and prioritized
as function of down-time and number of outages, in the same way as is car-
ried out for plant reliability studies. The priority list may contain the follow-
ing elements: the plant's unreliability; storage tanks full and empty; utility
per item; and external feed interruptions. This provides a clear insight into
the main contributors.
Based on the availability data of the final product streams, the opportunity gap can
be quantified in money terms. These potential benefits are the bases for economic
evaluation of process alternatives for reliability improvements as; external supplies,
plant(s) reliability, storage capacities, load-shedding systems, redundant generation,
or back-up for direct feeds (feeds without storage).
Next to the evaluation of design alternatives to optimize the availability of an inte-
grated complex, this will also provide data to optimize the operation. The operational
set point for the storage tanks can be optimized.

7.8
Summary

. Site vulnerability is mostly approached by the application of policy rules,
which leads to over- or under-investment in logistic and utility provisions.
. Currently, reliability engineering techniques enable the simulation of a site
from import of the raw materials up to delivery of the final products. These
studies are applicable for new as well existing situations. Modification of the
site (and in particular of the utility systems) would justify upgrading of the
reliability study.
. The following facilities are a degree of freedom for vulnerability studies:
± Individual process reliability