The New Cold Box – The History So Far
Air Products’ Newcastle plant recently underwent a multi-million refurbishment and expansion project. The need for this arose because of additional oxygen requirements from the ArcelorMittal steel plant.
The Air Products Newcastle plant has been providing a continuous supply of oxygen to the ArcelorMittal steel plant for the past 40 years, through three existing air separation units (ASUs) commissioned in the 70’s. These ASUs produce, oxygen, nitrogen and argon gases. The steel-maker’s requirement was for a substantially increased oxygen supply to its No 5 blast furnace.
Oxygen is used extensively in the steel making process because of its ability to produce high combustion temperatures, and for the removal of impurities such as carbon and sulphur. Nitrogen and argon are also used in the steelmaking process for inert blanketing and stirring of hot molten metals.
A key component in the upgrade of the Newcastle facility is a 54.7 metre ‘cold box” or distillation column. This new piece of equipment primarily produces oxygen and nitrogen with argon as a secondary product. The cold box is essentially a very large structural steel box containing fractional distillation columns and heat exchangers.
The three existing ASUs have been refurbished for continued operation. The installation of the new cold box is part of a contractual agreement between ArcelorMittal and Air Products, to increase the steel production, as a result of the reline of blast furnace N5 and the upgrade of the sinter plant.
The older ASUs produced oxygen at low pressures. This gas then still had to undergo further compression to make it suitable for the steel-making process. This was an extremely costly process, not just in terms of energy, but also in its use of consumables and ancillary equipment.
The new cold box has the very latest in technology, Air Products has upgraded the entire control system, and has standardised on the Siemens/Moore digital control system at all of its plants. This is in an effort to simplify training and to ensure that Air Products technicians are able to maintain the company’s plant wherever these might be.
The Earth’s atmosphere is almost completely comprised of 12 gases. The three major ones are nitrogen, (78%), oxygen (21%) and argon (nearly 1%). An air separation unit separates air into its three main gases 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 process in which crude oil is separated into petrol, diesel, and waxes, except that air separation happens at very low temperatures.
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 gas is released from an aerosol can; the gas cools rapidly as it expands.
In the new cold box, the main air compressor is a three-stage turbo compressor driven by an 11kV induction motor. The heat exchangers are intercoolers, which remove the heat of compression from the air between the three stages. A portion of the air is compressed to a higher pressure in a separate booster air compressor. 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. The water is chilled by evaporative cooling using waste nitrogen gas from the distillation system. The absolute dryness of the nitrogen gives it huge evaporative potential, and therefore the ability to cool the water to as low as four degrees.
The scrubbed and cooled air is then further purified by passing it through a molecular sieve adsorption system especially 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.
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. The adsorption granules are continually regenerated and are reusable – air being 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, it is swapped with 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°C – which is its liquefaction temperature. 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 falling droplets of liquid air.
Nitrogen, having the lowest boiling point of the three gases, evaporates fairly readily 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 is 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.
The whole process requires ultra-low temperatures; so another key feature of cryogenic distillation is the need to have good insulation. Expanded perlite – a mineral substance that expands when exposed to heat is used. The entire cold box is filled with very fine granules of
perlite to insulate the process vessels and piping from ambient heat.
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 erected to lift the top of the cold box by 58 metres. 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 on the pre-prepared concrete base.
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With the cold box erect, the ancillary equipment, such as the pump box, the turbo expander box, the associated feed piping, platforms and ladders had to be installed. Finally, the electrical and all the instrument cable connections needed to be connected.
The new plant had its origins from various parts of the globe. The core cryogenic components were designed in the UK, while the ‘warm end’ components were designed in Shanghai, by Air Products Chinese office. The utilities and the civil engineering were done locally. Rob Richardson, who led the project team, had the unenviable task of marrying the efforts of the three geographically-remote teams.
When the time came to switch the new plant on, there were a few hiccups during commissioning. With high pressures, and high-speed rotating machinery, the teething troubles were fairly typical of this type of project. However, these were completely sorted out in the first three weeks.
Air Products was very pleased with the support that it got from its partner OEMS and, indeed, our overseas parent company.
To ensure the new plant delivered on its design promises, the commissioning of the plant included performance testing. Here, the plant successfully passed its design performance tests – in fact, slightly exceeding its nameplate capacity.
However, with the new plant, its energy efficiency is where it scores over other earlier air separation units (ASUs).
The efficiency of ASUs has continually improved over the years, this plant being comparable or better than anything else in the country at the moment. It is only one of two plants in the country that produces high-purity argon directly by cryogenic distillation, a process that doesn’t require hydrogen as a feedstock.
The other major advantage of this plant is the quality of its products.
The new column is very much at the high end in terms of air-separation plants. It produces high purity oxygen and ultra-high purity grade nitrogen and argon. The key to its quality output is the very sophisticated analytical equipment necessary to carry out the quality assurance needed. To measure both in-process quality and final product quality, this plant has 10 to 12 on-line analysers verifying product quality parameters at any given time. It is also equipped with advanced distributed control systems to give the necessary sophistication of control to achieve these quality standards.
Consistency is a feature of the highly automated plant. In a steady state operation, it runs itself under computer control. Operator intervention is generally needed only for start up and shut down.
The new ASU is the fourth at Newcastle. The three existing ASUs, which are still in good working order, date back over a period of 38 years.
The new plant is only part of the recent history of the Newcastle site. The new plant was vconstructed as part of a roll-over and an expansion of Air Products existing contract with Arcelor Mittal. Part of that contract involved a refurbishment of the three existing ASUs. These were extensively refurbished in an R40-million project. Part of this project was a new centralized control room, which now controls all four ASUs.
Air Products has increased its storage capacity at Newcastle as well as increasing the truck-loading facilities.
While the new plant is delivering gas to Arcelor Mittal, it is also supplying merchant products to Air Products’ other customers. The Newcastle site now manufactures liquid nitrogen, liquid oxygen and liquid argon, which is trucked to customers in the KwaZulu-Natal region specifically. In some cases, the argon is delivered as far afield as Mpumalanga. With the new plant, KwaZulu-Natal customers continue to be assured continuity of supply, while Air Products benefits from not having to transport liquid product from Gauteng.
Feedback from the client, so far, has been positive in that the project went well.
Looking to the future of the Newcastle site, with the level of infrastructural spend planned for the country, it is anticipated that Newcastle will be one of the better-placed steelworks in terms of demand.
The upgrade of Air Products Newcastle site and the expansion of its capacity is very much an investment for the future. It is a vote of confidence in the future of ArcelorMittal, Air Products and South Africa.
Published on August 13, 2009 in Company News, Spotlight |