Baffling techniques

Although the process industries are reknowned as innovators, it's quite rare for equipment using an entirely new mode of action to appear on the scene. Mixing, for example, is a ubiquitous but well-established part of most processes - it's almost always done by stirring a tank, or by forcing liquids through shaped flow-paths.

Oscillatory flow mixing, however, offers a new method of mixing fluids, using moving parts to induce vortices in the fluids. It's their potential for process intensification, however, which is beginning to attract industrial interest, especially among speciality chemical and pharmaceutical manufacturers.

Oscillatory baffled reactors (OBR) are simple in terms of construction, but complex in terms of their operation. The reactors are cylindrical tubes, divided into sections by orifice plates. A series of rods link the orifice plates to each other, and allow them to be oscillated backwards and forwards - that is, in and against the direction of fluid flow - at a variety of frequencies. It's the effect these plates and their oscillation have on the behaviour of fluids inside the tube that gives the reactors their properties.

When fluids flow through an orifice plate, the edges of the orifice create turbulence within the flow. The key to the operation of an OBR lies in the size of the holes in the orifice plate and their position - typically, the holes with be half the diameter of the inside of the tube, and the distance between the plates will be one and a half times the inner diameter of the tube - and the frequency and amplitude of the oscillation.

Typically, the plates move between 1 and 100mm at a frequency of 0.5 to 15Hz.The combined effect is to set up a very ordered type of turbulence. Vortices form in the liquid, effectively turning the section of tube between each baffle into a continuous stirred tank. As the sections are linked by the holes in the orifice plates, successive sections improve the mixing characteristics of the entire apparatus; the more tanks there are, the better the mixing.

The advantages don't stop there. According to research at Cambridge University by Paul Stonestreet (who is now at GlaxoSmithKline), OBRs approximate to plug-flow reactors, and the residence time of the material in the reactor can be controlled.

Moreover, the mixing action gives better mass and heat transfer than other types of reactor. Research at Heriot-Watt University by Xiong-Wei Ni and colleagues has also demonstrated that the reactors have good particle suspension and a very low, uniform shear rate. This could prove crucial to the reactors' application in industry - it makes them particularly useful for processes involving stress-sensitive materials.

It's this which has attracted speciality chemicals firm James Robinson to secure a grant from the Department of Trade and Industry to investigate how the reactors might be commercialised. Based in Huddersfield and with manufacturing and research facilities at Dieburg in Germany and Vapi in India, JRL manufactures products such as hair dyes, as well as photochromic and fuel marking dyes, photographic chemicals and intermediates.

In general, says R&D director Stan Higgins, JRL's processes were carried out in continuous stirred tank reactors. Such reactors are extremely flexible in terms of reaction conditions and products, he says. OBRs seem to promise to be as flexible - but also offer the advantages of process intensification, the combination of two or more process steps within a single piece of equipment to improve the efficiency of the process as a whole. 'We believe that oscillatory flow mixing technology may be applicable to a wider range of reaction conditions than other process intensification techniques that have been developed to date,' he says.

Ni's reactors are already finding uses in the water industry, where they are used in the flocculation of materials. The JRL work will 'demonstratie the flexibility of the technology in an industrial environment where corrosive and reactive chemistry is the norm,' he says.

Ni's research, at Heriot-Watt's Centre of Oscillatory Baffled Reactor Applications (COBRA) has focused upon how droplets are formed, broken and reformed inside OBRs. It has also used computation fluid dynamics, using Fluent software, to investigate the formation of eddies and vortices inside the reactors and how they affect particle and droplet formation and behaviour. This has given the COBRA researchers insights into how to control the size of particles formed in the reactors. 'The research plan for the next few years is to expand our understanding of droplet formation to processes of flocculation, crystallisation and bioprocessing, and be able to model floc/crystal/bubble/cell formation on a theoretical basis.'

Studies of how droplets form are important for considering whether OBRs could be used for suspension polymerisation - the main route for the production of PVC, polystyrene, polymethylmethacrylate and polyacrylamide. In suspension polymerisation, the monomer is dispersed in a continuous aqueous phase, so an understanding of the influences on size of droplets and their formation mechanisms is vital if OBRs are to be used.

The researchers are using the full array of tools for looking inside the reactors (for laboratory work, to make it easier, they use glass reactors). As well as CFD, techniques such as digital particle image velocimetry, Coulter particle sizers, digital droplet imager and analysis software, high-speed cameras and laser-induced fluorescence are coming into play. For example, one research project uses high-speed digital video cameras to characterise bubbles in the reactor, which is important for processes involving gas-liquid contacting.

There are reams of details of bubble size distribution in stirred tanks and bubble columns, but because the trajectories of bubbles under oscillation is so complex, there is very little data for OBRs. This could be highly significant: Stonestreet's research suggested that the mass transfer between gases and liquids could be six times greater in an OBR than in a conventional bubble column. This, along with their low-shear characteristics, could also make them an attractive option for fermentation reactions.