A bubble curtain model applied in chlorate electrolysis

4.5 Bubble growth and coalescence
In the chlorate process hydrogen bubbles are formed. Whether these bubbles coalesce or not has great
influence on the system. It is therefore important to have a better understanding of what coalescence is
and how it can be inhibited. If two bubbles collide and coalescence occurs, the surface area of the new
bubble will be smaller than that of the two small bubbles together. The change of surface energy due to
coalescence is given as
∆E = σ gas−liquid ∆A (4.31)
Since the surface area of the newly formed bubble will be smaller than before, the energy change will
always be negative. The process of two coalescing bubbles will result in a lower state of energy. This
means that seen from hydrodynamic perspective, coalescence is expected to happen (see Figure 5).
However, coalescence in electrolyte solutions is not a spontaneous process as the negative change of
surface energy indicates. It has been shown that coalescence does not always occur in the electrode gap
of the chlorate cell.[1] Therefore, there must be an activation energy barrier for the coalescence (see
Figure 5) [8].
Figure 5: Thermodynamics predict that two bubble will have a lower state of energy if they coalesce. An energy
barrier must be overcome before coalescence occurs.
Before going deeper into reasons that might explain this unexpected behaviour, a short description of
coalescence is given. The process of coalescence can be divided in three consecutive steps [10]:
1) The first step is two bubbles that will approach each other which will result in the formation of a
thin liquid film between liquid and gas interfaces.
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