Cryogenic Air Separation Process Analytics

The further treatment of primary and intermediate materials across industrial sectors such as chemicals and petrochemical, metals, semiconductor and the food sector often requires large amounts of oxygen and nitrogen.

As the main constituents of ambient air, along with Argon, the ability to extract oxygen and nitrogen for such use relies upon air separation technology in highly automated air separation plants.
Bob Lane, Business Manager – Process Analytics, Siemens Industry, looks at the process of cryogenic air separation and the associated process analysis demands and solutions to ensure optimal operating efficiencies.

Air separation

The composition of dry air is approximately by volume 78% nitrogen, 21% oxygen and 1% argon, plus small amounts of noble gases, carbon dioxide, traces of hydrocarbons and other impurities. Oxygen, nitrogen and argon are required as industrial gases in significant quantities and crucially with a high degree of purity. To meet this demand from the industrial world, processes are continually being developed to produce these important gases via the separation of air.

Whilst there are some differences in processes, all air separation plants operate one of two types of process technology, which are:

• Air separation is undertaken at very low temperatures to liquefy the air and produce the desired products by subsequent distillation – cryogenic process – based on differences in boiling points
• Air separation at higher pressure using absorption effects based on differences in specific properties of the gases (pressure swing absorption – PSA).

This article deals with cryogenic air separation.

All cryogenic air separation processes consist of a similar series of steps, whether they be large stand-alone plants or small compact units located directly on the end user’s site.

While variations will reflect the desired product mix and individual customer priorities, in all cases process analysers are an essential technology to control and optimise the process.

Cryogenic Air Separation

Cryogenic air separation processes use differences in boiling points of the components to separate air into the desired products.

The three gases represent about 99% of dry ambient air; numerous process configurations exist caused by the demand of particular gas products and product mixes at various levels of purity. Nonetheless, all cryogenic separation processes comprise similar stages which are set out in figures 1 and 2.

Filtering, compressing and purifying ambient air

Ambient air is sucked in through a filter and compressed to approximately 6 bar. Then by passing the air stream through a cooler and a mole sieve, contaminants including water vapour, carbon dioxide and hydrocarbons are removed from the process stream.

The compressed and purified air is then cooled in stages to very low temperature through heat exchange and refrigeration processes in the main heat exchanger, which is located in an insulated container named a cold box.

Heat exchange occurs in counter current against other streams such as product and waste nitrogen leaving the distillation columns.

Products are then warmed almost to ambient temperature while the process air is cooled close to liquefaction temperature. Final cooling is achieved by expanding the feed in an expansion engine

Rectification (Separation)

Separation of air into its components is performed in a two-column rectification system comprising a high pressure and low pressure column (see figure 3).

A liquid oxygen-rich crude feed is produced as bottom product of the high pressure column. 

In a counter-current system a gas stream rises up the column while a liquid mixture flows down. High boiling liquid oxygen is formed from the rising gas stream by condensation while, from the liquid, lower boiling nitrogen is evaporated.

As a result, gaseous nitrogen is formed at the top of the column while liquid oxygen is produced at the bottom. By evaporating oxygen from the bottom and feeding fresh nitrogen at the top, the process is continued until the desired product purity is achieved.

This continuous distillation process is called rectification. Pure nitrogen is finally removed from the overhead of the column.

The oxygen-rich crude feed from the bottom of the high-pressure column becomes feed to the low-pressure column for final separation into pure oxygen and an oxygen containing nitrogen fraction that is withdrawn from the top of the column. The oxygen is then withdrawn from the bottom as product.

Achievable purities depend on the amount of ambient air fed to the process and the number of separation trays of the columns. Argon is enriched in the middle part of the low pressure column. It can be withdrawn from this point and processed to pure argon in additional concentrating steps.

Analysis demands

Two of the key drivers in air separation plant operation are to reduce production costs, including energy consumption and to ensure product quality.

Process gas analysers are used throughout the process to generate the required data to underpin such operational efficiency goals.

There are binding specifications for the composition of the desired end industrial gases with grades of purity up to 99.999% and process gas analysis is used to verify that the final products comply with the given specifications.

Likewise, with energy costs dominating total production expenditure, they can only be kept under control through exact monitoring to ascertain whether the process is running as close as possible to its optimal operation conditions. This requires continuous and reliable process gas analysis of the process streams.

Various process analytic solutions are available depending on requirements. They can range from a single analyser to a complete solution that can encompass planning and engineering to installation, commissioning and maintenance.

The range and types of analysers required for the measurement points shown in figure 2 are as follows.