Overview of mass transfer correlations in electrolyzers
Numerous experimental studies with the aim of establishing mass transfer
correlations have been reported. [1-5,12-14,18-37] Table 4 and
figure 8 list a selection of these correlations. Generally, they are of
the form \(Sh=a\ Re^{b}Sc^{c}Le^{-d}\). For these, the Schmidt power
is typically set to 0.33 which is in accordance with the Chilton-Colburn
analogy. In rare cases, the relative length of the electrode\(Le\ =\ L/d_{H}\) is taken into consideration, denoted by a non-zero
d coefficient. Typically, this is done when a dependency on the
electrode length could be confirmed. However, most studies do not vary
the electrode length which could be a reason for not including this
parameter in their investigation.
[TABLE 4]
[FIGURE 8]
Generally, literature correlations show Reynolds powers between 0.6 and
0.8, which is an indication of a turbulent regime. The correlations
established by Picket et al. [13] for developed turbulent flow also
follow this trend: for short electrodes the power is 0.66, for long
electrodes it is 0.8. Interestingly, most electrolyzers appear turbulent
long before the expected transition point of Re = 2000. This could be
due to several reasons, such as surface roughness, turbulence caused by
the inlet or even imperfections in the cell itself.
The rate of mass transfer varies significantly depending on the
electrolyzer type that is used. In for instance the UA16.15 a rate of
mass transfer is found that is nearly 10 times higher than the
correlations established by Picket et al. In other electrolyzers,
increased mass transfer is also observed, though the effect is less
extreme. There are several possible explanations for this difference.
Firstly, most electrolyzers do not use calming sections and are
therefore not operating in developed flow conditions, unlike the one
used in the work of Pickett et al. [13] Secondly, significant
turbulence is generated by the inlets into the channel, which varies
between designs. Thirdly, the design of the channel could be of some
importance: square, rectangular and even circular parallel plates have
been used in literature. [23,28,35] Finally, within the same
electrolyzer variations in performance may exist as a result of small
differences in the way the cell is reassembled after maintenance. For
instance, with a filter-press design, the layers of gaskets spacers and
electrodes may be stacked in slight misalignment. This in turn would
result in more mass transfer than expected due to extra protrusions of
these misaligned components into the channel. In turbulent flow the
surface roughness is also of importance. [29]