Energy and EnviroFiciency
Electrical design considerations in ASUs
Eskom recently imposed a number of mandatory energy efficiency
measures which are required for any new electricity consumer. A
modern ASU will generally exceed the latest Eskom requirements as
simple analysis of the operating costs dictate that only the highest
efficiency equipment should be used.
The compressors used in ASU applications are high efficiency
units incorporating the latest 3D impeller aerodynamic designs. Com-
pressors will also be fitted with extended surface area intercoolers to
achieve as near isothermal compression as practically possible. Inlet
guide vane capacity control is universally applied to achieve the best
possible efficiency under part-load operation. It goes without saying
that the compressor drive motors are of high efficiency design.
The refrigeration requirements of the ASU are met by expanding
the process gases (ie air or nitrogen) in one or more process expan-
sion turbine/s. Conventional mechanical refrigeration systems simply
expand the refrigerant across a Joule-Thompson (throttling) valve to
generate low temperatures. ASUs require much lower temperatures
than can be obtained by Joule-Thompson expansion and therefore
utilise some form of expansion turbine as the primary cooling device.
These turbines simultaneously provide process cooling, and recover
process energy. The energy extracted from the process is typically
used to drive a booster compressor, or to drive a generator which
feeds electricity into the plant’s electrical system, and would provide
a portion of the plant’s electricity requirements. The generators used
for this application are commonly induction machines, which are
relatively simple to operate, and do not need accurate speed control
or synchronisation equipment.
ASUs will typically require a number of large MV drives for the
air and gas compressors, and numerous smaller LV drives for the
process cooling water and utilities systems. Variable speed drives,
although not generally suitable for the main compressors due to their
high power requirements, can help to reduce overall plant power.
They are typically employed on process pumps, and on cooling water
systems, where the system duty may vary significantly with ambient
conditions or plant loading, and power can be saved by turning these
systems down during certain periods.
Selection of compressor drive motors
The main air and gas compressor motors can vary in size from 1 MW
on a small ASU up to 50 MW on a large ASU. The main criteria in
selection of the drives are plant reliability and efficiency. Continuous
process industries such as air separation require that machinery be
able to operate for long periods with minimal maintenance inter-
vention. This dictates the selection of cage type induction motors,
or synchronous motors for the larger applications. Either of these
motors will provide the necessary reliability and efficiency. Slip ring
motors, despite their attractive starting torque-current characteristics,
are not suitable for ASU service because of their higher maintenance
requirements.
Synchronous motors generally provide a higher overall efficiency,
not least because they can be designed to operate at unity power fac-
tor, or even a slight leading power factor to correct for the lag effect
of the smaller drives on the plant. This reduces the overall current
T
ake note
A
bbreviations
ASU – Air Separation Unit
GTL – Gas To Liquid
HMI – Human Machine Interface
LV – Low Voltage
MV – Medium Voltage
NPV – Net Present Value
SCADA – Supervisory Control And Data Acquisition
TSA – Temperature Swing Adsorbtion
• Air separation is a continuous process in which atmospheric air, which
is a mixture of gases (mainly nitrogen, oxygen and argon) is separated
into these pure component gases.
• Air separation can be an energy-intensive process.
• Although air separation by cryogenic distillation is a relatively mature
technology, it is becoming far more energy efficient.
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July ‘13
Electricity+Control
