MINIATURE marvels
15 Jan 2000
It happened to the electrical industry in the 1950's, it's happening to the chemical industry now. Roger Brownlie reports on the latest developments in condensing process technology to microscopic scale.
Forget pipes, tanks, condensers and furnaces. Imagine a process world where there is no turbulence, no leakage and product is produced through a giant lattice of microscopic channels embedded in silicon, sealed with quartz. Corporations across the globe have invested in this new technology, billed to revolutionise speciality chemical, sampling and microclimate industries.
Developments in micro-fabrication techniques, based on silicon chip technology, are creating novel hardware for performing chemical processing at a miniaturised scale. Features can now be designed with resolutions approaching 1-10microns in microchemical components. At this scale with an intense density of channels, the distances for heat and mass transfer are minimised, giving rapid transport rates that result in very short residence times and high throughput per unit volume. In addition, dangerous chemicals are only exposed on a minuscule scale.
The microprocessors look like computer chips but physically they are more akin to living systems, with the efficient internal kinetics, energy recycling and mixing characteristics of veins, capillaries and cells.
In the macro world inertia is the dominant effect, but in the microscopic world tiny volumes have very little mass and thus low inertia. The Reynolds number is so low that all flows are laminar and this condition lends itself to highly efficient heat exchange reactions. On the molecular scale the liquid molecules are travelling in neat layers that only mix by gradual diffusion. But without turbulence, fluids do not mix. The paradox of the situation is: how do reagents react if they don't mix?
Using a scientific tradition of train analogies, David Bernard of Control Research Laboratories (CRL) in Middlesex explains that two immiscible laminar flows making contact are `like two trains entering a station. When the trains arrive the passengers can change trains. Then, the trains leave the station and continue on their original path - the reaction has occurred at the meniscus without disrupting the laminar flow'. The two newly formed chemicals are already separated - production has begun.
In this era of environmental insecurity the chemical industry is faced with demands to mitigate threats posed by biochemical weapons, nuclear pollution, and industrial pollution and inefficiency. Uncertainty regarding the contents of some radioactive waste tanks has focused research at Pacific Northwest National Laboratories (PNNL) in Richland, Washington and British Nuclear Fuels, to design bespoke separation systems for each type of waste. Robert Wegeng, chief engineer at PNNL hopes the units `might be small enough to be lowered into a tank through a riser. The result would be massive economic savings to the taxpayer in the order of billions of dollars'.
PNNL made an estimate based on the processing capability of a scaled-up system that would fit in 1ft3. For residence times within the chip of 100s, such a device could process up to 33Mgal/yr. The beauty of such units is their modularity. If the market demands more product, extra chips can be added on in parallel without altering the chemistry of the process.
One of PNNL's more successful devices is a miniature 350W chemical heat pump that weighs 0.65kg. The complete working unit was half the weight of an equivalent macroscale pump.However Wegeng cautioned that while the technology is demonstrated, reliability and cost effectiveness has not.
Microchannel technology, if successful, may shift the economies of mass production toward more distributive processing. Instead of producing bulk chemicals that must be stored, many chemicals may be manufactured in situ, leading to a devolved chemical industry.
