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									Mixing technology



Fluid mixing for chemicals
processing
Michelle Cotterill of Imperial Innovations describes a novel technology for fluid mixing


E
        xperts in fluid mechanics at Imperial College London
        have created new fluid mixing technology for the global
        chemicals industry. Fractal flow control (FFC) grids could
provide significant economic benefits over current industrial
mixers, including power savings, optimal use of space and less
downtime. They can also be tailored to meet the specific needs
of different users.
   The concept ‘fractal’ is used to describe geometric shapes in
which each fractal or shape that makes it up can be split into
parts, each of which in turn is a reduced-size copy of the whole.
Professor Christos Vassilicos from Imperial’s department of
aeronautics initiated the use of these shapes to control turbu-
lence in mixing and flow.
   His research team used wind tunnel experiments to under-
stand the turbulence dynamics of fluids flowing through fractal
grid structures. These wind tunnel tests were the first to exam-
ine the turbulence generated by such grids and showed that
they could be used to control and manage the transition from
laminar to turbulent flow.
   Tests were also carried out in a water flume and the scaling
and decay of turbulence generated was investigated down-
stream of different two-dimensional fractal grids. The research       Commercially available top-   as pH control, additive mixing, the dilution of acids and bases,
team found that slight alterations in the grid’s dimensions relat-    performance industrial        chlorination, oxidation and aqueous dispersions.
                                                                      mixer compared with FFC
ed directly to the turbulence intensity generated by the fluid’s      grid
impact with the grid, as well as on the pressure drop across it.                                    Advantages of FFC grids
   Therefore it was possible to set the levels of turbulence inten-                                 The mixing and drag performances of the FFC grids were com-
sity and pressure drop independently. The grids could be                                            pared with those of a commercially available top-performance
designed to generate high turbulence intensities with low pres-                                     industrial mixer (referred to henceforth as the current standard
sure drops, creating energy-efficient mixers.                                                       of mixing (CSM)). Professor Vassilicos and his team conducted
   FFC grids are fractal-patterned meshes which generate a                                          experiments using a dye attenuation technique to make synop-
bespoke turbulence to control fluid flow and reduce friction                                        tic and non-intrusive measurements of the concentration field
drag. They are designed to be immersed in any fluid flow, for                                       downstream in a water pipeline. A wind tunnel was used to
example in pipelines, tunnels and tanks, for controlled mixing.                                     make drag measurements.
   The grids provide a smart and elegant alternative to current                                        Initial findings showed that the FFC grids had several advan-
static in-line mixers and can be used for mixing fluids or a fluid                                  tages over the CSM in terms of homogeneity, energy efficien-
and a gas to create a homogenous end product. They can be                                           cy, physical footprint and mixing control, and they can be used
used in numerous applications in the chemicals industry such          Fractal flow control grid     in combination with other mixing technologies.




46                                                                                                              September 2008 Speciality Chemicals Magazine

                                                                      www.specchemonline.com
                                                                                                                                  Mixing technology

                                                                                                       For the first time, the FFC grids provide the possibility of con-
                                                                                                    trolling pressure drop and turbulence intensity independently.
                                                                        PEFG2
                                                                        CSM                         They can be modified to become very efficient mixers by mak-
                     1
                                                                                                    ing the pressure drop low, by means of lowering the blockage
                                                                                                    ratio while simultaneously making turbulence intensities high
                    0.8
                                                                                                    by increasing the thickness ratio between successive fractal iter-
                                                                                                    ations.
              CoV




                    0.6
                                                                                                       Fractal square grids for which all parameters are equal except
                                                                                                    for the thickness ratio are shown in Figure 2. These three grids
                    0.4
                                                                                                    generate very different turbulence intensities even though the
                                                                                                    differences between them may be almost imperceptible to the
                    0.2
                                                                                                    eye, which demonstrates how very sensitively the FFC grids can
                                                                                                    control mixing performance.
                     0                                                                                 It would also be possible to experiment with further design
                          0   10   20   30    40     50     60    70   80    90    100
                                                   x (cm)                                           parameters beside the ‘I’ square and cross patterns tested to
 Notes: PEFG2 = Imperial College FFC grid
                                                                                                    date, such as those which determine the distance from the grid
    The Coefficient of Variance (CoVχ) values for the FFC grids        Figure 1 - CoVχ when dye     where the turbulence intensity peaks. Many other patterns
were smaller than those for the CSM when the dye contami-              source is upstream of grid   could also be used for the design of fractal grids. There are
nant is released upstream, indicating better homogeneity. The                                       therefore endless possibilities and potential opportunities for
results (Figure 1) show that the FFC grids mix as well as the                                       effective mixing engineering and design to be explored.
CSM in the far field and better in the near field. In terms of spa-                                    In terms of combining mixing technologies, using a FFC
tial distribution of mixing, the FFC grids kept the mixing activi-                                  grid and the CSM combined in the pipeline gave the most
ty away from the walls of the pipeline, whereas the CSM directs                                     impressive results in the tests. Mixing was several times better
liquid at the walls.                                                                                than the combination of CSM and CSM or CSM and FFC grids
    Although the mixing systems had comparable homogeneity,                                         individually.
the CSM achieves this by sending parts of the flow towards the                                         This combination presents the very tangible possibility of set-
walls of the pipeline in the near field. This may be damaging in                                    ting an entirely new standard of high quality mixing with excep-
particular applications where the substances to be mixed are                                        tionally low CoVχ values, homogeneous concentration maps
highly corrosive or continuously and/or recurrently hot.                                            and profiles, as well as little diversion onto the walls, at a very
    The FFC grids mix by generating high levels of turbulence                                       short distance downstream from the mixing elements
for relatively low levels of blockage without braiding different
parts of the flow together. The high levels of turbulence gener-                                    Development status
ated mean that the grids can mix effectively at short stream-                                       FFC grids in their current shape and form could potentially be
wise distances, keeping the substances to be mixed clear from                                       competitive mixers by comparison with a commercially avail-
the walls, whereas the CSM achieves most of its good mixing                                         able top-performance industrial mixer, and could transform the
far downstream. Using FFC grids could thus prevent unneces-                                         mixing of fluids over a broad range of mixing applications. The
sary damage to the pipeline, decreasing operational downtime                                        technology is generic and could be adapted as required, with
and maintenance and extending lifetime.                                For more information,        different optimisations for different functions.
                                                                       please contact:
    When looking at energy efficiency, the Imperial College                                             FFC grids offer process engineers an array of choices in fluid
                                                                       Ross Manning
team found that mixing effectiveness is a function of both mix-                                     flow mixing, where blockage ratio, plant floor space, mixing
                                                                       Technology Transfer
ing quality and the necessary power input to achieve it. The           Executive                    intensity and mixer lifetime and/or maintenance are all con-
normalised drag on FFC grids ranged between being lower and            Imperial Innovations         straints. The experiments conducted at Imperial suggest that
comparable to that on the CSM, implying possibilities for              E-mail: ross.manning@        the grids could increase both mixing efficiency and controllabil-
reductions in power loss.                                              imperial.ac.uk               ity over a wide variety of flow parameters. They are also simple,
    Where many mixers are used in a pipeline, the economy in           Website: www.                cost-effective and easy to install.
power input brought by replacing them with fractal grids would         imperialinnovations.co.uk        Professor Vassilicos is building on existing work that estab-
be even greater. These efficiency improvements offer major                                          lished the mixing properties of the fractal grids. He is currently
savings to the chemicals industry, which is said to be the sec-                                     running a proof of concept project development programme to
ond largest industrial consumer of energy world-wide.                  Figure 2 - Fractal square    test the technology in different operations and work is ongoing.
                                                                       grids for which all
    The physical footprint of an FFC grid is much smaller than                                      Patents on the technology have been filed by Imperial
                                                                       parameters are equal
that of the CSM, which has a complex three-dimensional con-            except for the thickness     Innovations, the technology commercialisation company based
figuration, is made up of baffles and sits along the length of a       ratio                        at Imperial College London.
pipeline. FFC grids are more compact and thinner, and can
therefore save valuable space.
    Looking at mixing control, varying standards of mixing are
required for different industrial applications and the flexible FFC
grids can be designed to fit predefined requirements and spec-
ified mixing properties. Small adjustments to a fractal grid have
very considerable effects on turbulence and drag.
    The experiments confirmed the roles for mixing effectiveness
of two of the main design parameters of the FFC grids: the
thickness ratio and the blockage ratio which can be changed
independently. The turbulence intensity generated by a FFC
grid increases with thickness ratio and measurements suggest
that the CoVχ values also increase functions of thickness ratio.
The drag increases non-linearly with the blockage ratio.


Speciality Chemicals Magazine September 2008                                                                                                                        47

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