Air Products Cold Box

Air Products, SA’s largest industrial gas producer, is currently carrying out a multi million Rand expansion and refurbishment project at its Newcastle plant. As part of this project, it has imported and transported a fully assembled cold box for cryogenic air separation. The cold box is essentially a huge structural steel box containing fractional distillation columns and heat exchangers. It is the first and largest of its type ever to be brought into South Africa. Mechtech attended to the lifting in Newcastle and spoke to Corrie Topham Air Products Contractor, Harry van Lieshout Air Products Projects Manager and Sibusiso Sibisi Air Products Newcastle Plant Manager.

Air separation technology lifted in Newcastle

Air Products’ Newcastle plant has been providing a continuous supply of oxygen to the ArcelorMittal steel plant for the past 38 years – currently via three existing Air Separation Units (ASU’s) which produce Oxygen, Nitrogen and Argon gases. Oxygen is used extensively in the steelmaking process for its ability to produce high combustion temperatures, and for the removal of impurities such as Carbon and Sulphur – with over 1 000 tons being required every day to produce thousands of tons of finished steel at the Newcastle plant. Nitrogen and Argon are also used in the steelmaking process for inert blanketing, or for stirring of hot molten metals.

The new Air Separation Unit which is under construction (with it’s massive cold box) – is part of a contractual agreement between ArcelorMittal and Air Products to increase steel production as a result of the reline of blast furnace N5 and the upgrade of the sister plant – a component of ArcelorMittal SA’s capacity expansion programme to increase its local steel production. Output from Newcastle’s N5 plant is set to increase significantly.

Air Products’ Project Manager, Harry Van Lieshout, tells us that the goal is to make sure that ArcelorMittal has the additional oxygen capacity on-stream by December 08.

S’bu (Facility Manager) takes us to the existing gas plant and shows us a sample of liquid Nitrogen – a clear slowly bubbling liquid and only the frost around the rim of the flask indicates how cold it is.

He spills some onto the concrete, which dissipates instantly leaving no trace.

“These three existing ASU’s were commissioned in the 1970’s” Van Lieshout continues, “so they are undergoing an extensive refurbishment to ensure that they continue to perform reliably and efficiently,
although they will still be less efficient than a new ASU.

He explains further: “The Oxygen produced by the older units is produced at low pressures, and has to be compressed further to get it to the required pressure for steelmaking. The Oxygen compressors are
specialized and costly pieces of equipment, and use a considerable amount of energy. The new plant will produce the Oxygen at high pressure directly off the cold box, by vaporizing pumped liquid Oxygen
against a warm high pressure air stream. The whole process is more energy efficient.”

S’bu takes us into the plant control room -a relatively simple office with two flat screens showing plant data on a graphical interface. The liquid oxygen storage tank level is showing several hundreds of tons of product, just above half a day’s capacity. “Most of the oxygen is taken straight to the furnaces but we store significant amounts of liquid products for plant back-up”.

He then shows us the old control room, a room full of controls and meters. “When I started working for the company in 2005, they brought me in here and I nearly cried,” says Sibisi. Air Products has now upgraded the entire control system, and has standardised on the Siemens/Moore digital control system at all of its plants in the world in an effort to simplify training and to ensure that any technician from anywhere in the world can be sent to maintain any plant.

Van Lieshout takes us back over to the new plant site and tells us how it works. “There are 12 main gases in air but the three dominant ones are Nitrogen, at 78%, Argon at nearly 1% and around 21% Oxygen. This
unit is called an Air Separation Unit (ASU) because it separates the air into its three main components by fractional distillation of liquefied air. This is possible because of the different boiling points of the Oxygen, Nitrogen and Argon. The technology is similar to the way crude oil is separated into petrol, diesel, waxes, etc., except that they work at very low temperatures,” he explains.

He points towards a large motor/generator attached to some piping 10 metres away from the cold-box unit. “The air is subjected to five main processes namely filtration, compression, purification, cooling and distillation. The cooling is achieved by expanding a portion of the air from a high pressure to a low pressure in a turbo-expander. The pressure-drop causes the gas to release energy so it cools down. A similar thing happens when you release gas from an aerosol can, the gas gets cold as soon as it comes out because it expands. We recover the energy by generating electricity, although this is a small amount of energy relative to the plant’s consumption”, he explains.

We ask about the compressors. Topham takes us into an acoustic enclosure with a square air-intake duct with 10 metres sides at one end. “This is the main air compressor,” he says, showing us a set of large green heat exchangers with spiral volutes attached to them. “The machine is a three stage Turbo Compressor driven by an 11kV induction motor”, Topham informs us. “The heat exchangers are intercoolers which remove the heat of compression from the air between the three stages.,” he explains. “A portion of the air is compressed to a higher pressure in a separate booster air compressor.,” he adds pointing to a similar line of connected plant machinery alongside. “The compression plant requires a significant amount of process cooling water which is generated in a cooling tower alongside the unit”.

“After compression, the air is scrubbed and cooled by direct contact with chilled water in a tall vertical contacting column”. Van Lieshout then explains were the chilled water comes from. “We get the water cold by evaporative cooling using waste Nitrogen gas from the distillation system – the absolute dryness of the Nitrogen gives it a huge evaporative potential, and therefore the ability to cool the water to as low as 4 degrees”, he continues. The scrubbed and cooled air is then further purified by passing it through a molecular sieve adsorbtion system”. He points to two very large cylindrical pressure vessels side by side. “These are both filled with molecular sieve granules which are specially formulated to absorb moisture, CO2 and other contaminants from the feed air. The granules act like a sponge and remove contaminants which would solidify and block the plant at cryogenic temperatures,” he says. “Another function of the purification stage is the removal of airborne hydrocarbons, which would create an explosion hazard if allowed to enter the distillation section of the plant,” he informs us.

We ask how long the granules last. “The granules are continually regenerated and are reusable,” responds Van Lieshout, “Air is purified in one vessel while the other is undergoing regeneration. The vessels are periodically switched from service to regeneration mode. Therefore as one vessel becomes contaminated we swap over to the other vessel which has been regenerated. Regeneration is done by passing a stream
of heated waste nitrogen through the granules.”

“After purification the air enters the cold box where it is cooled down to approximately minus 180 Degrees – which is it’s liquefaction temperature. At this point the air is introduced into the first of several distillation columns. A distillation column basically consists of a tall vertical vessel filled with sieve trays or structured packing. The packing has the function of bringing the liquid and vapour fractions into direct contact with each other, so that heat can be exchanged between the liquid and vapour fractions. Typically the rising vapours heat-up a falling droplets of liquid air. Nitrogen, having the lowest boiling point of the three gases, evaporates fairly readily and concentrates-up in the rising vapour stream until it is withdrawn at the top of the column. Oxygen is less volatile and tends to be condensed into the liquid phase by the falling liquid stream. It’s therefore withdrawn from the bottom of the distillation column.

Several stages of distillation are required to reach the required purity and a modern air separation plant will typically have three or more distillation columns. Argon has a boiling point between that of Nitrogen and Oxygen and is withdrawn as a side-stream part way up one of the distillation columns” he explains.

“The whole process is very, very cold so another key feature of cryogenic distillation is the need to have very good insulation,” continues Van Lieshout. “We use expanded perlite – perlite is a mineral substance which when exposed to heat expands somewhat like popcorn. We fill the entire cold box with very fine granules of perlite to insulate the process vessels and piping from ambient heat. ”

What makes it so much more efficient, we ask. “It’s a combination of factors. For example, compressors have become more efficient as a result of advancements in computing power, which have made it possible to design and build more aerodynamically efficient turbo machines. Also, improvements in structured packing materials used in the distillation columns have reduced the pressure required to drive the air through the process. All of these factors contribute to a more energy efficient plant. Energy recovery is also important,” he tells us, “and we look for opportunities to improve. The turbo expander, which produces the cold temperatures, for example, drives a generator that puts electricity back onto our electrical system,” he advises.

The cold box was finally lifted into place on Sunday September 14 by two Target cranes. A 750 ton lattice boom crane with a 28 metre radius was built up to lift the top-end by 58 metres, while a 440 ton extended boom, tail-end crane was used to hold the base off the ground while it was slid across and rotated into place. The whole process took three hours.

“Now we have to get it producing gas by mid to late December,” concludes Van Lieshout.

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