SIZE matters

Rotterdam air isn't the sweetest in the world. As befits the combination of Europe's largest port and one of its primary chemical manufacturing complexes, the whiff of ozone from the sea struggles to compete with the distinctive odours of heavy shipping and the hydrocarbon tang of petrochemical plants and refineries.

Yet, when split into its component parts, the Rotterdam air becomes the central feature of a series of major projects at these refineries - and the plant that does the splitting is the largest of its kind in the world.

With a daily capacity of around 3000t of oxygen per day, Air Products' Air Separation Unit (ASU)-3 plant is almost twice the size of its nearest competitor, according to plant manager Huib Schreurs. Its efficient use of energy also makes it possibly the most environmentally friendly of its type, he claims, although this energy-hungry process lists electricity as its second most important raw material after the air.

If it weren't for environmental concerns, ASU-3 would never had been built. Its origins were in a request from Shell, a long-standing customer of Air Products at Rozenburg, for a large-scale supply of oxygen, which it needed for a project to improve its environmental performance.

Shell's concern stemmed from a project to revamp its nearby refinery at Pernis, to reduce emissions of sulphur and nitrogen oxides and to produce cleaner fuels. To do this, it intended to replace its catalytic cracking unit with a hydrocracker. The Pernis refinery processes crude that tends to contain large proportions of sulphur compounds, which contaminate and poison catalytic crackers. The hydrocracker reacts light oil fractions and naphtha with pure hydrogen, converting the hydrocarbons into ethylene, diesel and petrol, and automatically removes sulphur.

Hydrogen or oxygen?

The refinery already receives hydrogen from another Air Products plant, which produces syngas (carbon monoxide and hydrogen). But instead of expanding this facility, it opted to generate hydrogen on-site using a process requiring pure oxygen. This allowed the company to use one of its less marketable products, the heavy residues from crude oil distillation, explains Koos Sanderse, Shell's executive accounts manager.

The oxygen feeds into a series of gasifier units, where it mixes with the heavy residues. The mixture is subjected to intense heat, and forms syngas. The hydrogen is taken off to the hydrocracker, while the CO is burned through a turbine to produce steam and power.

ASU-3 will supply 1701t/day of 96 per cent pure oxygen to the Pernis refinery, at 73 barg through a dedicated pipeline, for 15 years. This is sufficient to keep the refinery's three gasifiers running. In return, Shell provides 25MW of electricity - enough power for ASU-3 to fulfil its contract to Shell. At full capacity, the plant consumes 60MW of power.

The conditions of the contract are stringent. Shell's refinery operates on very tight margins, and cannot afford unscheduled shutdowns. The company therefore demanded 99.99 per cent availability for its oxygen supply - which allows for just one unscheduled half-day shutdown during the entire 15-year run of the contract. Air Products' older air separation unit, ASU-1, provides back-up duty in the event of a failure on ASU-3.

The process used in the air separation plant is a conventional procedure involving refrigeration and distillation. The air is drawn into the plant at a rate of 450m3 per hour, passes through a filter to remove dust, and is pressurised to nine bar by four compressors - one axial, three radial. A molecular sieve then removes carbon dioxide, water and hydrocarbons. The plant contains two sieve units arranged in parallel; one on duty, the other being regenerated by a hot nitrogen-rich stream which strips away the impurities. The units switch over in an eight-hour cycle.

The clean, dry air then enters the main heat exchanger section, where the gases entering the plant are cooled by the product streams returning from the distillation columns. This, combined with the cooling effects of expansion units, liquifies the gases, reducing their temperature to -165 degreesC. The expansion takes place through turbines, which also generate power.

The next stage of the process, the separation itself, occurs in three distillation columns. Normally, these would be arranged one on top of another in a single tower, but the columns for ASU-3 were so large that they had to be placed side-by-side, and enclosed in the insulation-stuffed concrete cylinder that dominates the plant.

A pair of columns, one low and one high pressure and linked by a reboiler, provides the primary separation. These yield four products: gaseous nitrogen, pure to 4-5ppm; gaseous and liquid oxygen; and stream containing a mixture of gaseous nitrogen and argon. This is pumped to a third `sidearm' column, which produces virtually pure argon, bound primarily for the metals industry.

The deal between Shell and Air Products is reciprocal. Shell takes the oxygen from the plant, delivered through a dedicated pipeline at 73barg pressure. In return, the company supplies the power that ASU-3 needs to produce the oxygen to meet its contract - some 25MW of the plant's 60MW requirement at full capacity. The rest of the power - and the remaining capacity - will be accounted for by smaller contracts with local customers. There is currently no customer for the plant's nitrogen, so it is vented back to the atmosphere.

It's the sheer size of the project that marks out ASU-3, says Schreurs. Generally, air separation plants have a capacity of about 1500t/day of oxygen, and can be built off-site and delivered as complete units, only needing to be hooked up to utilities. But ASU-3 was so large that it had to be made and delivered in sections and erected on-site.

King of the road

The main column, weighing 250t, 174 feet long and 15 feet in diameter, was built at Air Products' own works in Wales and taken to Rozenburg by road and sea. It was the largest load that the company had ever handled, Schreurs notes; even though the column was transported horizontally, concrete had to be shaved off bridges along the road route to allow it to pass underneath. Schreurs believes that the ASU-3 column is the largest that could be moved in this way, which means that the plant is likely to be the largest single-train plant ever built.

The cost savings from such a large plant are `significant', says Schreurs. Advanced control techniques mean that manning levels are very light; on the night shift, only six operators run the plant. The large size allows very efficient use of power, with added help from the turbines connected to the gas expanders in the refrigeration unit.

The limiting factor on the capacity of future plants is the size of the separation columns, says Schreurs; ASU-3's may well be the largest that can be transported in this way. Moreover, he adds, it's likely that the gap between the savings generated by the size of the plant and the costs associated with on-site assembly would narrow if plants got any larger. `But what we really need at the moment is built operating experience to see what the unit delivers,' he comments. `It might already be a bigger plant than we thought it would be.'