Energy and EnviroFiciency
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demand of the ASU, and leads to lower network access and demand
charges. Synchronous motors do, however, carry a higher capital
cost, and in the South African context it is now unusual to see a syn-
chronous motor selected for a duty of less than 10 MW. The precise
cut-off point where synchronous motors become more economical
has shifted upwards in recent years partly due to the availability of
reasonably priced MV capacitors, which when used in conjunction
with induction motors can achieve similar overall operating costs at
a lower initial cost. There are many factors which influence the mo-
tor selection decision; such as the specific electricity tariff structures
applicable. It is also common to purchase a capital spare for a large
ASU compressor motor. This clearly favours the choice of induction
motors due to the duplication of the capital cost. The selection of the
compressor drive motors plays such an important role in an ASU’s
life cycle cost that it is necessary to evaluate the specific economics
of each case based on a life cycle cost analysis using the Net Present
Value (NPV) financial method.
Another consideration is the relatively high starting current
requirements for the ASU compressor drives. A careful analysis of
the supply system fault levels and starting current requirements is
required to determine the best starting method. Smaller drives are
preferably started direct on line, but for larger compressor drives
some form of soft starter is often required. The preferred starting
method is reduced voltage starting using a Korndorfer type auto-
transformer system. This system has proved to be the most reliable
and economical system for medium sized MV drives. Special purpose
thyristor-type soft start drives are employed where system fault levels
are too low to use an autotransformer. This is, however, the last resort
in motor starting methods due the high capital cost.
Other efficiency considerations
The ability to interrupt electrical demand during peak power has
become increasingly important due to the almost universal adoption
of ‘time of use’ electricity tariffs, such as Eskom’s Megaflex Tariff.
ASUs are not suited to stop-start operation because the time required
to start-up and reach stable product purities. Nevertheless, certain
ASU configurations do allow for at least partial load interruption by
shutting down the Nitrogen recycle circuit. This temporarily reduces
plant output but does not unduly disrupt the distillation system. Con-
siderable reductions in overall energy cost per kWh can be achieved
by such load reduction methods.
Current developments and trends
One area which is still showing considerable efficiency gains is the ap-
plication of advanced process control systems. These are essentially
supervisory systems which are installed at a level above the SCADA
or HMI system, and they seek to emulate the 'best operator' behaviour
for any set of operating circumstances. Advanced control can have a
significant effect on specific energy consumption (kWh/ton of product)
as the plant set-points are constantly adjusted and optimised to the
best possible operating point. Ramping between various operating
points can also be controlled in a more consistent manner than by
operator intervention. This has considerable benefits for ASU plants
as they are sensitive to external influences such as changing ambient
conditions, and changing product demands.
Process integration is another avenue which holds further prom-
ise for improving energy efficiency on large tonnage ASU plants
supplying oxygen to petrochemical or metallurgical processes.
Many of these petrochemical and metallurgical processes generate
significant amounts of waste heat which can be used to raise steam,
and hence to generate electricity, but is often more efficient from a
holistic point of view to directly drive the ASU compressors by steam
turbine. This removes the number of conversion losses in the energy
chain and results in a greater overall efficiency. There are operational
challenges with such process integration due to the resulting inter-
dependence of the systems. This can adversely affect reliability and
operability and the technical challenges revolve around these issues.
Nevertheless, such process integration is already being effectively
used in many processes such as ‘Gas to Liquid’ (GTL) production,
and methanol production.
Conclusion
Although air separation by cryogenic distillation is a relatively mature
technology considerable strides are being made in improving energy
efficiency. Because of the electrically intensive nature of the process
the selection of electrical and control equipment is a key ingredient
in this development. It is often stated that cryogenic air separation
is reaching a plateau in development, and various competing (non
cryogenic) technologies are indeed gaining ground, but their use is
currently still limited to smaller plants making a single product (ie the
production of only one gas; either oxygen or nitrogen). The alterna-
tive oxygen production technologies are also currently only suited
to lower purity applications. Air separation by cryogenic distillation
is therefore here to stay, and we can expect further incremental de-
velopments in efficiency as electricity costs escalate.
Robert Richardson holds a B.Sc. Degree in Mechanical Engi-
neering, and a M.Sc. Degree in Industrial Engineering from
the University of the Witwatersrand. He is also a registered
Professional Engineer in the Republic of South Africa. He has
over 28 years of diverse engineering and project management
experience, including 10 years spent in the petrochemical engineering
industry, and 18 years in the industrial gases industry. His experience
spans the full spectrum of technical activities including plant design,
construction, commissioning, as well as the operation and maintenance
of petrochemical and gas production plants. He currently holds the posi-
tion of general manager On-Sites for Air Products South Africa. This is an
executive level position with the responsibility for managing a portfolio
of projects under development, as well as Air Product’s fleet of ‘owned
and operated’ industrial gases production facilities.
Enquiries: Tel. 27 11 570 5152 or email
Electricity+Control
July ‘13
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