New technology revitalises fluid bed scrubbers

The Environmental Group Ltd (EGL), both under its own name and under that of its predecessors Horizon APC, Transfield RP/C Division and Ceilcote Australia, has been designing and supplying wet scrubbing systems for over 30 years in Australia.

These systems have been targeted primarily at gas absorption applications, but often also involved fine particle collection.

Mineral processing and waste incineration are two major areas where off gas streams typically contain both acid gases and fine (usually heavy metal) particles.

Historically the group has offered multi-stage, venturi and fixed bed packed tower combinations for applications of this type.

While effective, EGL is always on the lookout for technology improvements that will achieve the same or better result in a more efficient way. One such technology is fluid bed wet scrubbing.

In traditional air pollution control packed towers the packed bed is usually a static or fixed bed of randomly dumped plastic packing pieces (tellerettes, pall rings, intalox saddles and the like).

These are very efficient at mass transfer for gas absorption, but do have their limitations.

In particular they (a) do not provide high performance on fine particle removal and (b) can be prone to fouling and plugging in dirty applications.

Fluid bed scrubbers use a highly mobile, constantly moving bed again of randomly dumped plastic packing pieces, albeit of a totally different design to those used in fixed beds.

The size and shape of the packing pieces and the liquid and gas rates chosen are such that the bed is fluidised.

This provides highly turbulent, extremely efficient contact between the gas and the scrubbing liquid that results not only in high efficiency gas absorption, but also in high efficiency fine particle collection.

The constant motion also makes the bed virtually impervious to plugging, even in the most fouling of environments.

In short, one fluid bed packed tower does the job of a two stage venturi, fixed bed packed tower combination.

Why haven't fluid bed scrubbers been more widely used? Because initial fluid bed designs suffered from problems of channelling of gas through parts of the bed, from poor distribution of the bed and generally unreliable performance.

These problems related to the design of the packing. Early fluid bed scrubbers used plastic spheres, or flattened spheres as the packing medium. These were found to result in the above problems.

Designs have now been developed that use a unique low-density hollow shape that overcomes these problems.

Fluid Technologies Ltd of Britain has been at the forefront of these developments. Under the broad technology name Turboscrubber, Fluid Technologies has developed Turboid and Turbopak fluid bed packings.

Unlike spheres or flattened spheres, these new packings are non-symmetrical in shape, the key to their performance.

The ability to carry out both gas absorption and particle collection in a single unit has obvious advantages. Apart from capital cost, there are fewer items of equipment to operate and maintain.

The design is also substantially more energy efficient than a venturi/fixed bed packed tower combination for the same performance.

Also, as noted earlier, the technology is virtually impervious to fouling problems and can operate on high solids content slurries if need be.

The technology also lends itself to retrofitting to existing conventional packed towers. Turboscrubbers are typically smaller in diameter and bed depth than the equivalent conventional packed tower.

Thus it is easy to retrofit the technology to an existing system either to increase the throughput, increase the gas absorption and/or particle collection efficiency or to overcome problems with bed fouling.

The ability to retrofit TurboScrubber has also been demonstrated in the conversion of existing venturi scrubber entrainment separator vessels to fluid bed scrubbers at Suncor/SPP's oil shale plant at Gladstone, Queensland.

This particular system is used for the absorption of process SO2 and fine dust removal using a 5% w/w slurry.

Performance was not adequate with the venturis alone, hence the retrofitting of Turboscrubber to improve both gas absorption and particle collection performance.

The ability to retrofit the existing equipment with a technology that could both handle a slurry and provide the required performance increase resulted in a project cost a fraction of what would have applied for any other solution.