Method and apparatus for maintaining product availability during a disturbance in an air separation unit

Abstract

A method for operating an air separation unit during an unexpected disturbance is provided. The method can include the steps of: determining that a process disturbance has occurred; starting-up a liquid back-up system that is configured to deliver a product gas at a desired product pressure; and introducing compressed air from an air accumulator into the air separation unit at a location that is downstream a main air compressor and upstream a cold box, wherein the compressed air is introduced in an amount that is effective for maintaining nominal operation of the air separation unit during the process disturbance and until the liquid back-up system is delivering the product gas at the desired product pressure.

Description

TECHNICAL FIELD[0001]The present invention generally relates to a method for providing an air gas stream in the event of a disturbance in an air separation unit. The method is particularly useful for maintaining product purity of air gas streams (e.g., nitrogen, oxygen, argon, etc. . . . ) during the transient phase from normal production until a liquid backup system can be brought online in the event of a trip of the plant or reduced flows of incoming air feed.BACKGROUND OF THE INVENTION[0002]A typical air separation unit (ASU) may include use of 1) Main Air Compression (MAC) to medium pressure ˜6 bar, 2) purification unit, 3) Booster Air Compression (BAC) to high pressure ˜50-60 bar, 4) main heat exchanger, and 5) medium pressure, low pressure and argon distillation columns for the production of oxygen, nitrogen, and argon. Various arrangements for the MAC include a single large machine (1×100%), two half-sized machines (2×50%), or combination of the MAC and BAC into a single mach...

AI Technical Summary

Benefits of technology

[0010]In one embodiment, this can be achieved by including a front-end air accumulator that will bleed the air into the cold box in such manner as to minimize the rate of change in airflow to the process in the event of a process disturbance. In certain embodiments, when a compressor trips resulting in a sudden loss of airflow, the valve from the front-end air accumulator opens followed by its gradual closing while the distillation columns are ramped down, thus producing reduced stable flow rate loads to the customer / pipeline without loss (or with a minimal loss) of purity and / or pressure.

[0011]In another embodiment, when a compressor trips resulting in a sudden loss of air flow, the valve from the accumulator opens while the backup system (typically liquid vaporized from storage) is started and ramped up. Alternatively, without an additional backup system and without multiple air compressors, the accumulator allows temporary production while downstream customer systems are shutdown.

Problems solved by technology

GOX backup systems are also available; however, they are less used than the LOX backup system for economic reasons.

Process upsets can greatly affect the operation of the distillation column, since the purity profile inside the column changes, which ultimately affects the final product purity.

One example of such a process upset is a sudden change in airflow to the cold box (e.g., trip of the 1×20% MAC), which disrupts the purity profiles in the various distillation columns followed by the loss of product purities.

A complete ASU trip or one of multiple MAC trips inevitably leads to either a complete outage of product or rapid reduction of production capacity.

As the product stream is likely being fed either directly to a downstream user or to a pipeline, a change in product purity and / or flow rate can have dire consequences for the downstream user.

As noted above, liquid backup systems have an inherent delay, and therefore, cannot be the only design used in instances where the product purity and pressure must be maintained at all times. In those instances, an additional costly online gas buffer is often required.

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