Page 210 - Sustainability in the Process Industry Integration and Optimization

Co m b i n e d P r o c e s s I n t e g r a t i o n a n d O p t i m i z a t i o n 187

of appliances to function without unexpected problems or major
breakdowns. If these equipment units fail, the consequences can be
catastrophic: contamination, smog, acid rain, injury, loss of life,
production cutbacks, amassed garbage heaps, energy losses, and so
on. Catastrophic failures would also entail substantial added costs.
For these reasons, solid waste management is a matter of serious
concern, which in some cases (waste collection in Naples, Italy) has
even led to a change in government. The models that have been
developed to manage waste-producing processes are of two types:
optimization models deal with specific aspects of waste-related
problems; in contrast, integrated waste management models focus on
sustainability. The latter type can be subdivided into three main
subcategories: models based on cost–benefit analysis, models based
on life-cycle inventory, and multicriteria models (Morrissey and
Browne, 2004).
However, there is an element of uncertainty or risk associated
with most environmental decisions. Multicriteria techniques can
be extended to consider reliability issues along the entire waste
management chain and need not be limited to comparing the environ-
mental impacts of different waste treatment methods. As the
complexity of unit arrangements increases, risk assessment becomes
more complicated. Risk is a measure of the plant’s ability to carry
out its specific operating mission reliably. The expected return on
related investments is a function of the plant equipment’s capacity,
which is defined in terms of reliability, availability, durability, and
performance.
Reliability engineering in waste management addresses all
aspects of the waste life cycle, from its collection and treatment
processes and through the energy generation lifetime, including
maintenance support and availability. The concepts of reliability,
maintainability, and availability can be quantified with the aid of
reliability engineering principles and life data analysis (Kececioglu,
2002). A significant fraction of any system’s operating cost is due to
unplanned system stoppages for unscheduled repair of components
or the entire system. One method of mitigating the cost (and impact)
of such failures is to improve the system’s reliability and availability.
Of course, improvements in reliability that are made by the supplier
early in the equipment’s life cycle may well result in additional
development cost being passed on to the customer in the form of
higher equipment acquisition cost. However, this cost increase can be
more than offset by the operational cost reduction associated with
improved reliability and increased uptime, which also improve
productivity. Note that, in the context of waste management,
reliability, availability, and maintenance have specialized meanings.
Reliability is the probability that a system will perform satisfactorily
for at least a given period of time t when used under stated conditions
(Kuo and Zuo, 2003).