The air that I BREATHE

BOC is about to launch its newest product - liquid air. Is it trying to sell snow to the eskimos? Stuart Nathan chills out.

It's freezing in here. I'm standing in a cross between a space-age wigwam and an igloo, made out of metallised bubble-wrap, and the chattering of my teeth is almost drowning out the explanation of how this new refrigeration system works. However, the constant hiss from the perforated metal bars above my head, and the clouds of fog around my feet betray the fact that I'm steadily being sprayed with a very cold gas. The odd thing is, I'm breathing comfortably.

This is odd because this sort of refrigeration system would normally be spewing out liquid nitrogen, which would quickly choke anyone inside the igloo. But the cascade that's keeping the atmosphere frigid is synthetic liquid air (SLA). It's claimed to be the first new cryogen to be launched since liquid helium, in 1974. And it represents a multi-million pound gamble for the venerable industrial gas company BOC.

`We'd spent 100 years working out new ways of separating out the components of air, and then we said that we wanted to put them back together and sell it,' says Steve Waldron, general manager of BOC Gases Europe's food marketing division. `People thought we were crazy.'

It wasn't just this seemingly nonsensical business proposition that prompted these allegations of insanity, either - the technical challenge appeared insurmountable. Making SLA involves mixing liquid nitrogen and oxygen - both extremely cold fluids with different boiling points, one of which presents major fire and explosion risks.

HAVE GAS, WILL TRAVEL

There was a sound business reason for looking into such a tricky proposition, explains Waldron: in-transit refrigeration. BOC supplies liquid nitrogen for `single-drop' refrigerated lorries - that is, those which unload their entire stock at one stop. The nitrogen, which is sprayed onto the load to cool it, must be allowed to dissipate when the lorry stops, so it must stand for several minutes with the doors open. But these sorts of deliveries - almost exclusively to large supermarkets - represent a small part of the in-transit sector. BOC wanted to get into `multi-drop deliveries', where the lorries make several unloading stops. These are currently cooled mechanically, with chemical refrigerants, or by eutectic plates (large hollow plastic slabs filled with an ethylene glycol mixture that is frozen to sub-zero temperatures). But these are inefficient, especially in warm weather. Cryogens are cheaper and cool faster, says Waldron; what was needed was a breathable one, so lorry drivers could stop and unload without delays.

The problems were considerable, says Waldron. It was decided to make the SLA by mixing liquid oxygen and nitrogen, rather than simply liquefying air, because the mixed version is a relatively simple two-component mixture whose proportions can be controlled, whereas liquefied air contains several components, whose proportions are fixed. This became even more important later in the project.

AIRING SOME PROPOSALS

Once BOC's laboratories had established that the two liquids would mix without forming layers, the researchers had to tackle the mixture's thermodynamic instability. Nitrogen boils at -196iC, oxygen at -186iC, so a `liquid air' mixture containing around 20 per cent oxygen will have a temperature around -190iC. This means that the nitrogen will constantly try to boil away, making the mixture richer in oxygen. Any substance that absorbed this liquid would become explosive.

The challenge, therefore, was to find a way of mixing, storing and transporting the gas without allowing the nitrogen to boil away, and while constantly monitoring the composition of the mixture.

Normal synthetic nitrogen storage vessels have pressure relief valves, but this wasn't an option for SLA - any gas vented would be nitrogen, which would increase the proportion of oxygen left in the tank. BOC designed a pressure relief system for the SLA tank which ensures that no product escapes - the headspace vapours are constantly condensed, mixed and returned to the tank.

Filling the tank also posed problems. As the liquefied gas is pumped into the tank, it begins to vaporise, leading to turbulence. This is the enemy, as it generates heat and encourages more evaporation. So when the gases are mixed, the warmer component - oxygen - is put into the tank first, then the liquid nitrogen `collapses' the headspace gases. The same technique is used when pumping the SLA from the mixing vessel into the tankers, explains Waldron.

To ensure that the drivers always knew what they were loading, special couplings were designed so that SLA tankers could not fill up from the nitrogen or oxygen tanks. And new sampling equipment ensured that the technicians could check the exact composition of the mixture as it is mixed and loaded into the tankers.

`It turned out that the sampling was the key to it all,' says Waldron. Normally, sampling liquefied gas mixtures is extremely difficult, as the sampler acts as a small distillation column; this means that the analyser reads off a constantly changing composition. BOC claims to have overcome this problem, but the sampling system is hedged around with patents and is currently still classified.

It was the sampling that led BOC to the ideal composition for SLA. Nitrogen-oxygen mixtures are breathable between 16.5 and 21 per cent oxygen, but BOC chose to give SLA 18.5 per cent oxygen. `We couldn't use exactly the same composition as real air,' Waldron explains. `If you did, there would be no way of telling whether the system was leaking - you couldn't tell the product apart from the outside air.'

The safeguards, monitoring and special training ensure that SLA is `110 per cent safe' in use, claims Deb Chatterji, BOC's managing director for technology - as long as it is fully vaporised. This means that it cannot be used for any application where the cryogen might condense and form a pool, such as liquid nitrogen freezing tunnels, used in the frozen food industry. This would create a situation where the nitrogen would boil off, he explains.

However, the breathability of SLA opens up a wide range of air-conditioning applications. Normally, such systems have to be designed to cope with peak demand, which might only occur for a few hot days per year. For most of the time, a massively-oversized refrigeration plant is sitting around doing nothing. A SLA system, consisting of spray-bars connected to a storage tank, is much cheaper, costing some £2000/1000ft3, and can provide `top-up' cooling when it is needed. This means that the mechanical air conditioner need only be designed for average demand.

A BREATH OF FRESH AIR

SLA can also be used to create an instant `cold store' - for example, to store food at large sporting events. Using the metallised bubble-wrap as walls and pumping the SLA in along spray-bars, an insulated cold-tent can be set up and cooled very quickly. SLA can chill an area at more than 2degC per minute - five to ten times faster than mechanical refrigeration. As the atmosphere remains breathable, people can still work in the tent. And because of the fast pull-down rate, the system can also be used to start up mechanical refrigeration systems, especially in hot weather.

Another possible use is in testing mechanical parts that will be exposed to wide temperature ranges, such as those in jet engines. These have to be tested at very low temperatures, and are currently chilled with liquid nitrogen. As this atmosphere will not support life, however, the actual testing must be done remotely. Using SLA, engineers can work on the components as they are being chilled.

BOC has invested over £1million in the project so far, and is about to embark on a full-scale market test in the UK before launching the product worldwide. A pilot plant at Thame in Oxfordshire is currently being upgraded from 75 to 100t/day, while another plant, several times this size, is being planned for a site in Northern England. Chatterji is too cautious to predict the value of this new cryogen, although he hopes it will run into hundreds of millions of pounds. `The potential applications are limited only by our imagination,' he says.

BOC GASES ENTER 260