Cheaper cuts from a compact catalyst
15 Jan 2000
Volatile organic compound (VOC) emissions are one of the biggest bugbears for the chemical industry at the moment. Nearly all processes produce them, in widely varying quantities and compositions.
A team at Shell's research and technology centre in Amsterdam has been working on catalytic incineration of VOCs. The technique, which decomposes VOCs by passing them over a catalyst bed, scores over thermal incineration because of its lower temperature requirement. This, the team explains, means that fewer toxic compounds, like NOx and dioxins, are formed; fuel consumption is lower; and monitoring is easier.
There's always a trade-off, however, and here it's one of cost. Catalytic reactors are more expensive than thermal units, but the team now claims to have developed a process that combines the low fuel consumption and associated advantages of catalytic incineration with the low capital cost of thermal units.
The system, dubbed DeVOx, uses a lateral flow reactor, as shown in the diagram (bottom left). The catalyst is packed into narrow slabs with gauze walls, and the channels on either side of these slabs are open only at one end; alternately, the inlet and outlet. Gas flowing into the reactor therefore has to pass through a catalyst slab before it can leave. Because the slabs are narrow, the pressure drop across the reactor is very low, so only low-powered blowers are needed. Space velocity through the reactor is very high, however, (up to 10000 reactor volumes per hour) so the reactor can be very small and use only a small amount of catalyst. The team estimates that capital cost is about one-third less than other catalytic incinerators.
The DeVOx catalyst, based on a non-noble metal, is a proprietary secret. However, according to the researchers, it can convert 95 per cent of a toluene feed to CO2 and water at just 350oC, and can achieve 80 per cent conversion of methane at 550oC. Even more striking, its activity isn't compromised by the presence of H2S, a notorious catalyst poison. This makes it suitable for incinerating waste gases from refinery gas treatment units, says the team.
