Research program in
electrochemical processes
Sweden has an important electrochemical industry. Eka Nobel
Elektrokemi AB, which has its head office in Stockholm and
production units spread throughout the country, is the
world's leading producer of, among other things,
environmentally friendly bleaching agents for use in the
paper-manufacturing industry. Zinc is produced by the
Norwegian company Norzink AS, of which Boliden Metall AB
owns 50%. These companies will be industrial partners in the
Centre of Excellence for the Fluid Mechanics of Industrial
Processes.
The electrochemical processes for the production of zinc and
bleaching agents, such as sodium chlorate, are in some
respects very similar. Electrolysis takes place in baths of
electrolyte in so-called electrolysers, in
which a number of electrodes are immersed, coupled either in
series or in parallel. Due to the reactions at the
electrodes the concentration field varies in space, with the
result that the electrolyte's weight (per unit
volume) will be locally either less than or greater than the
average weight in the bath. Consequently the electrolyte is
set in motion by the force of gravity, light electrolyte
rises while heavy electrolyte sinks. This motion is nearly
always turbulent. Furthermore, in the production of sodium
chlorate hydrogen gas is generated at the cathode and in the
zinc electro-winning process, oxygen gas is evolved at the
anode. Due to the friction between the bubbles and the
electrolyte, the upwards motion of the bubbles of gas causes
turbulent circulation of the electrolyte in the
reactor.
Many problems which are closely related to the fluid mechanical
phenomena mentioned above arise in the optimization of the
design of electrolysers. For instance, the exchange of mass
at the electrodes should be maximized, which requires a
rapid supply of undepleted electrolyte. However, high
velocities result in short residence times in the
electrolysers, which leads to a lot of electrolyte passing
through the electrolyser without being fully used. The
evolution of gas bubbles at the electrodes is often
exploited to drive the electrolyte through the electrolyser.
But a large volume fraction of bubbles increases the
electrical resistance of the electrolyte, which increases
the loss of energy. The consumption of energy is perhaps the
most critical problem in the electrochemical process
industry.
Among the long-term goals of this industry which require detailed
knowledge of, among other things, the fluid mechanics of
electrochemical processes for their achievement, it is
particularly worth mentioning;
The third goal is definitely the most important, but
also at the same time the most difficult to achieve since
all the different parts of the process interact with each
other. The first two goals are in fact essential for
achieving the third goal but can probably be closely
achieved using existing fluid mechanical methods. However,
the presence of turbulence makes it difficult to predict the
rate of mass transfer to the electrode surface which is
significant for, for example, determining the current
distribution and the current efficiency at the
electrodes.
At the Centre this sort of phenomena will be investigated
experimentally with non-intrusive velocity measurements
using Laser-Doppler Velocimetry. The turbulent concentration
fields are significantly more difficult to measure. However,
concentration measurements are possible at the electrode
surface itself using so called micro-electrodes. Theoretical
calculations will also be carried out using the L.E.S.
method which, as was mentioned above, will require a
considerable effort. The next two sections present the two
classes of problems which have to be studied using both
theoretical and experimental methods.
Turbulent convection between vertical electrodes Convection driven by bubbles Measurements of the size, volume fraction and motion of bubbles are
complicated. However, there are a number of favourable
factors in the applications which are considered here. The
volume fractions are not very large, which makes it less
difficult to identify individual bubbles. Furthermore, in
many cases, the bubbles are small, their velocities are not
very large and their motion only deviates slightly from a
straight line. This implies that statistical information can
be obtained using video technique and computerized image
processing. These factors will also facilitate mathematical
modelling and theoretical calculations. The theoretical
investigations will probably consist to a large extent of
advanced numerical analysis. Initially the studies will be
limited to laminar convection.
Plans for future projects within
FaxénLaboratoriet (FLA)
If the distance between the electrodes is of the order of 1 cm or
more and the height of the electrodes is greater than
approximately 1 dm then the motion of the electrolyte will
usually be turbulent. For the time being, the influence of
bubbles of, for example, hydrogen will be not be taken into
account. The turbulent motion will strongly increase the
convective transport of mass to the electrodes and will
dominate the weaker transport mechanisms of diffusion and
migration apart from in the immediate vicinity of the
electrodes. Simpler conventional computational methods for
turbulence in aerodynamic applications, such as, for
example, k-e and R.S.T. models, are not suitable in their
original forms for the sort of calculations considered here.
All the same, efforts will be made to modify these models
for electrochemical applications.
This is one of the most difficult problems considered in this
application. At the same time, this problem is one of the
most generally important areas of Chemical Engineering. This
is one of the areas in which a considerable effort will have
to be made. If any results are to be obtained at all then
only moderate demands can be placed on the precision of the
theory and the experiments.
electrochemical processes
In this branch of engineering, research is planned in a longer time
perspective into electrochemical manufacture of printed
cards for electrical circuits, called printed circuit cards
once the components have been mounted. In the technology of
copper refinement, the prediction of the formation of
nodules (dendrites) and of various blocking phenomena is
important. Further areas of interest are electrochemical
treatment of cancer and the development of efficient methods
for the production of hydrogen by electrolysis of water.
This last problem is important as a method of storing
energy. Fluid mechanics is fundamentally important in all
these studies.
Last modified:1999-04-17