Explosive limit

It's dark in the bunker, and cold. Tunnelled into a small, wooded mound, the walls are lined with blackened planks and studded with shards of twisted metal.

'We place the sample in here,' says the technician, indicating a clamp set three feet above the concrete floor. Surrounded on three sides by a thick steel shield, the clamp is at the focus of four powerful gas jets. The technician indicates the metal fragments buried in the ceiling. 'Sometimes, the tests are a little energetic.'

It's not what you'd expect for a chemical plant, but this is no ordinary chemical plant. Dynamit Nobel Special Chemistry, part of the Dynamic Synthesis group, is located at Schlebusch, just outside Leverkusen in Central Germany, and has been a centre for explosives expertise for nearly a century.

These days it uses and processes potentially explosive materials for the pharmaceutical, agorchemicals and fine chemicals industry, and hasn't made dynamite or other explosives for many years. Even so, a major part of the activities at the site involve determining how explosive the materials are. The bunker is an integral part of this process, housing the Koenen Test, which shows whether a substance will explode when heated.

Dynamit Nobel's diversification into the pharma business stems from its expertise in making nitroglycerine - originally as an explosive for mining, but since the 1970s made to treat angina and heart disorders. Making and handling explosive materials, with their very specific hazards and peculiarities, is highly specialised, so the company has used its knowledge to carve out a niche.

Hazardous chemicals are often called for in pharmaceutical manufacture. For example, many drugs contain heterocyclic rings in their molecular structure - similar to benzene rings, but with one or more carbon atoms substituted for nitrogen or other elements.

As Jürgen Haase, head of R&D at Schlebusch, explains, making heterocycles often requires the use of azides, which contain three multiple-bonded nitrogen atoms, or hydrazines, carcinogenic compounds which contain two double-bonded nitrogens. 'Sodium azide, NaN3, is comparable to sodium cyanide in toxicity,' Haase says.

'In addition, in almost all azide reactions you have to expect the generation of HN3, which is extremely sensitive to shock and friction, and detonates with a speed comparable to TNT.' Other molecules which are in daily use at Schlebusch include carbon disulphide and diborane compounds.

So what kind of facilities are needed to handle such aggressive substances? The main concern, as with all chemical plants, has to be safety, but this shows itself in ways quite different from most complexes. For a start, standing in the middle of Schlebusch, you'd be forgiven for thinking you weren't on a chemicals complex.

In place of the usual towering columns and steel pipework are a widely-scattered collection of unassuming buildings, surrounded by trees. The forestry isn't just for show: in the event of an explosion, the trees would help absorb and contain the blast.The facilities themselves are also designed with the worst in mind. Johannes Schlupp, head of cGMP production, explains that the design philosophy behind the plants has changed little since 1912.

'The need is to direct any blast that might occur, so you need some thick walls that will withstand the pressure, and some blowdown walls which will fall down intentionally.'

The most recent building on the site, the active pharmaceutical ingredients facility built in 1999, is designed so that the most risky operation - drying - occurs on the ground floor and has extra reinforcement. 'Drying is much more dangerous,' says Schlupp. 'Once the liquid solution starts to evaporate, the thermocouples are no longer immersed and they are not reliable.'

Another instantly noticeable design feature is the widespread use of glass within the plants. This is a safety feature, and is due to the formation of shock-sensitive sublimates during the recrystallisation of acidic substances.

'If the condenser and receiving vessels are made of glass, you can see if these sublimates are forming,' Schlupp says. 'If it happens, we rinse the equipment from the top down, to redissolve the sublimates and make the plant safe. If the equipment were made from carbon steel, we wouldn't be able to see the sublimate; it would build up to dangerous levels, and it's a more difficult material to clean.'

Despite these design features, however, the general design of reactors at Schlebusch is very similar to other fine chemical production plants. However, according to Stefan Löbbecke of the Fraunhofer Institute of Chemical Technology, new developments in microreactors could present some interesting alternatives.

Microreactors, where the reactions take place in channels a fraction of a millimetre across, are particularly suitable for reactions involving or producing explosive materials, Löbbecke says. The mixing characteristics are very good, so hot spots do not form. Retention times are small, reducing the opportunity for explosive conditions to develop. Moreover, the small hold-up characteristics of microreactors allows process conditions to be controlled precisely, enhancing the selectivity and yields of the reactions.

Among Löbbecke's experiments is the synthesis of nitroglycerine in silicon microreactors. These consist of a series of channels around 700um in diameter, arranged to maximise the mixing of the materials flowing inside them. The reaction rate is controlled thermally, by immersing the whole microreactor in a water bath.Nitroglycerine is made by nitrating glycerin with a mixture of concentrated sulphuric and nitric acids and oleum.

The product is highly sensitive to shock, friction and elevated temperature.

Conventionally, it is kept below 30 degrees C to avoid the risk of explosion. However, the microreactor method does not require the use of oleum, and allows safe handling of the reaction mixture at temperatures as high as 45 degrees C, says Löbbecke - and higher temperatures mean faster reactions. Moreover, if a microreactor becomes too hot and an explosion does occur, the damage caused is as small as the equipment.

Although the volume of individual microreactors is tiny, an array of them could provide industrial-scale capacity. 'Several hundred kilogrammes, up to several tonnes, is possible,' Löbbecke says.

And the technique need not be limited to nitrations - many different types of reaction could be tackled, as long as they occur only in the liquid and gas phase. 'It's a chance to work on the chemistry itself,' Löbbecke comments.