(438e) Towards a Better Understanding of Mixing Characteristics of Thin Film Flow in Spinning Disk Reactors

The spinning disc reactor (SDR) is an intensified reactor technology which exploits the benefits of intense fluid dynamics in thin films formed by applying high centrifugal fields to a liquid flowing on the rotating disk surface. The thin films are highly sheared and their surface is covered by numerous unstable ripples which have been shown to promote enhanced heat and mass transfer rates [1,2]. Industrially important reactions such as polymerisation and crystallisation have been performed in the SDR, whereby significant advantages related to improved reaction rates and product properties have been demonstrated [3-5]. In such reaction processes, the mixing characteristics of the thin films are critically important in determining the yield and selectivity of the process as well as the properties of the product(s) formed, especially in fast reaction schemes. Both macro- and micromixing have to be considered for a full characterisation of mixing.

In the present work, a quantitative experimental characterisation of the micro- and macromixing behaviour of thin films formed on the surface of rotating disks is reported. Our studies have demonstrated that the micromixing efficiency of the thin films, quantified via the well-established iodide-iodide test reaction [6], is enhanced under conditions of high disk rotational speeds, high feed flowrates and on grooved disk surfaces. These parameters provide the conditions for the highest shear to prevail in relatively thin continuous films, with a large number of surface ripples for enhanced molecular diffusion. Using a grooved disk with low flowrates can reduce the micromixing efficiency due to rivulet flow resulting from insufficient wetting of the larger surface area. Grooved surfaces are therefore better suited to high throughputs. We have also observed that the feed injection system can have a dramatic influence on the micromixing in the SDR. Multipoint distributors are more effective than single point distributors at high feed flowrates, giving enhanced liquid distribution as one stream feeds into another. 

Residence time distribution (RTD) data gathered from online conductivity measurements of the thin films injected with a pulse of potassium chloride as tracer indicate that the higher the disk speed and the liquid flow rates and the lower the viscosity of the processing liquid, the closer conditions within the liquid film on the disk approximate to plug flow behaviour. These effects are attributed to greater degree of turbulence induced in the liquid film, whereby transverse mixing is promoted across the film thickness, resulting in a more uniform velocity profile at any given radial position. The centrifugal force directing the liquid radially to the disc periphery may also play a part in reducing radial dispersion. With flow on a grooved disk, radial dispersion is also observed to be lower than on a smooth disk under identical operating conditions. With the constantly changing topography enabled by the series of concentric grooves across the disk surface, repeated film detachment from and re-attachment to the surface occurs. This is accompanied by induced recirculation or vortices within the film, giving better transverse mixing and eliminating velocity gradients. Based on our experimental data, empirical models relating the Peclet number to the disk operating parameters have been developed for both the smooth and the grooved disks, enabling prediction of plug-flow behaviour for any set of conditions within the range tested.

References

1. A. Aoune, C. Ramshaw, Process intensification: heat and mass transfer characteristics of liquid films on rotating discs, Int. J. Heat Mass Tran., 42, 2543-2556 (1999).
2. J.R. Burns, R.J.J. Jachuck, Monitoring of CaCO3 production on a spinning disc reactor using conductivity measurements, AIChE J., 51, 1497-1507 (2005).
3. K.V.K. Boodhoo, W.A.E. Dunk, and R.J. Jachuck, Continuous photo-polymerisation in a novel thin film spinning disc reactor, in Photoinitiated Polymerisation, K. Belfield and J. Crivello (Eds.), ACS Symp. Ser. 847, American Chemical Society, Washington D.C., 2003.
4. K.V.K. Boodhoo, R.J. Jachuck, Process intensification: spinning disc reactor for styrene polymerization, Appl. Therm. Eng., 20, 1127-1146 (2000).
5. L.M. Cafiero, G. Baffi, A. Chianese, R.J. Jachuck, Process intensification: precipitation of barium sulphate using the spinning disc reactor, Ind. Eng. Chem. Res., 41(21), 5240- 5246 (2002).
6. Fournier, M.C., L. Falk, and J. Villermaux, A new parallel competing reaction system for assessing micromixing efficiency--Experimental approach. Chem. Eng. Sci., 51(22), 5053-5064 (1996).