(131d) Process Development for High Efficiency Cryogenic Air Separation Unit

Due to a rise in electricity prices of past years, there are high expectations for improving efficiency of cryogenic air separation units (ASUs), which generally consume a large amount of energy to produce oxygen, nitrogen and argon. On the other hand, a basic process for ASUs has improved little for the past hundred years. ASUs have advanced in regards to safety, operability, load followability, etc. owing to development or improvement of comprised equipment (e.g., air compressor, air purifier, heat-exchanger, rectification column) and many process modifications. However, the specific power consumption of an ASU has remained nearly flat in the past two decades, especially for producing high purity oxygen (typically higher than 99.5 mole %). Hence, in this work we have developed a state-of-the-art ASU process for high purity oxygen product capable of reducing power consumption to break through this situation.

A conventional ASU has employed a classical double column process, which comprises a high-pressure column, a low-pressure column and an argon column as a side-cut column connected to the low-pressure column, and the bottom of the low-pressure column is thermally integrated with the top of the high-pressure column by means of an indirect heat exchanger, so called a condenser/reboiler.

In contrast, the developed ASU process comprises a high-pressure column, a low-pressure column and an argon column, and these three columns are thermally integrated in series. Liquid oxygen at the bottom of the low-pressure column is vaporized by condensing argon vapor at the top of the argon column, and liquid oxygen at the bottom of the argon column is vaporized by condensing nitrogen vapor at the top of the high-pressure column. This process cycle makes it possible to improve efficiency of the low-pressure column, and it can be reduced the required heat exchange duty to generate ascending oxygen vapor in the low-pressure column. Therefore, the power consumption of the ASU can also be reduced. In our case study under some practical conditions, it has been confirmed that this new ASU process is effective for saving energy to produce high purity oxygen, nitrogen and argon.