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10.52 CHAPTER TEN

A cryogenic air separation system consumes about 260 to 340 kWh (936 to 1,224 MJ)
of energy per ton (1,016 kg) of oxygen produced, depending on the plant capacity, with
about 90% of it used by the main compressor. Because of its high power requirements,
high-efficiency compressors and drives are required to reduce the total operating cost.
Turndown of a cryogenic air separation system is limited primarily by the main air
compressor and the distillation column. The maximum turndown for a single main air
compressor is about 70% to 75% of the rated capacity. Below that limit, the compressor
will become unstable and must be vented. The turndown of a cryogenic system is also
limited by the distillation column to about 50%. Below 50% of system capacity, GOX
must be vented or LOX must be produced in order to maintain system stability.
The turndown range can be increased by using multiple trains. For a two-train system
with each train rated at 50% of capacity, the turndown can extend to 30% of total rated
capacity. However, because of high capital costs and operating complexity, the selection
of the number of trains must be based on process requirements rather than turndown range.
The response rate to demand changes of a cryogenic system is relatively slow, nor-
mally about 2% to 3% per minute. Therefore, from 100% to 70%, it will take about 10
to 15 min to reduce the production rate. If one train is shut down but maintained in a cold
condition (sometimes referred to as "bottled up") for less than 12 h, the "cold start-up"
time to reach full production is about 1 to 2 h. If the system is maintained in cold condi-
tion for more than 12 h, the cold start-up time will increase to 2 to 3 h. If the cryogenic
system is restarted from ambient temperature, LOX is required, and the "warm start-up"
time is about 2 to 3 days.
Providing redundancy of equipment to ensure uninterrupted operation is a common
practice for municipal agencies. Redundancy normally includes multiple trains of equal-
capacity equipment, with one or two more trains on standby. However, manufacturers in-
dicate that with a sound operation and maintenance program, a cryogenic plant can be on-
line over 98% of the time without a second train. They also indicate that a standby cold
box is not needed, because there are no rotating parts in this unit. However, this would
require adequate LOX backup storage in the event of a cold box problem. Redundancy
of rotating equipment and product storage tanks is desirable.
Noise levels from an air separation plant can be high unless special consideration is
given during the design to reduce the noise. Noise levels from the main air compressors
in general exceed 90 dBa (decibels a-weighted) at a distance of 3 ft (0.9 m). Therefore it
is common to house this equipment in a sound-insulated enclosure. A separate silencer in
addition to the inlet filter/silencer assembly may be needed to reduce the noise at the in-
let of the compressors. Silencers should be provided at the outlet of the compressor surge
vent line and the product and waste vent lines. Switching valves would generate a noise
level over 100 dBa at a distance of 3 ft (0.9 m) and should be provided with an enclo-
sure and silencer to reduce the noise.
Depending on the equipment size, sheet metal enclosures with an acoustic treatment
may be required for the booster compressors and gas product compressors. Most liquid
product pumps are small and should not cause concern about the noise level. The cool-
ing water pumps are usually centrifugal and should have noise levels below 85 dBa.
Depending on the size, the main air compressor may require a power supply of 480,
2,400, or 4,160 V. The remaining equipment normally requires either a 120- or 480-V
supply. The main air compressor is started unloaded and may take 5 to 15 min to reach
its operating status, depending on compressor size. If the standby unit is started before
the duty unit is shut down to maintain uninterrupted air supply, the power requirement
should include the standby unit.
The cryogenic plant requires large quantities of cooling water equal to 8 to 10 gpm
(30 to 38 L/min) per ton (907 kg) of oxygen production.