Conclusion
The different design choices made in the construction of an electrolyzer
greatly affect the mass transfer performance. Between the electrolyzers
found in literature, up to a factor 10 difference is observed (see
figure 8). This variation is the result of several different geometric
design choices in the electrolyzer. Using a 3D printed electrolyzer, we
were able to investigate the effect of some of these choices.
Depending on the type of inlet that was used, up to a factor 2.2
difference in Sherwood numbers was seen. Furthermore, an earlier
transition to the turbulent regime was found. The tube inlet already
produced turbulent flow at Re = 65, the conic and divider inlet
transitioned around Re = 300. The higher than expected mass transfer is
due to the sudden expansion of the inlet to the channel. In turbulent
flow, the correlation by Djati et al. predicts mass transfer fairly
well. [19] This correlation uses the ratio of cross-sectional area
of the inlet and channel as parameter to predict the magnitude of
expansion turbulence. In the conic inlet, the cross-sectional area
varies throughout the inlet and this was accounted for by using the
geometric mean cross-sectional area. The correlation deviated from the
experimental results by <18% for the tube inlet,
<8% for the divider inlet and <6% for the conic
inlet.
The addition of a calming section minimized these effects. With a
calming section of 550 mm, the type of inlet no longer seemed to affect
the rate of mass transfer. This length is over twice the predicted
hydrodynamic entrance length of 240 mm at Re = 1200 (based on eq. 5).
Therefore, it is reasonable to assume that the flow is fully developed
and that the inlets no longer matter. Despite this, our results did not
completely match the correlations for hydrodynamically developed flow
established by Ong. [12-13] This is likely the result of the
limitations of 3D printing, as this process can result in imperfections
in the printed parts that may disturb the flow.
Turbulence promotors generally lead to an enhancement of mass transfer.
The presence of a calming section significantly changed the enhancement
effect of the promoters. Without calming section, most of our promoters
resulted in comparable Sherwood numbers, whereas with an entrance length
a larger variance in performance was found. Inlet turbulence therefore
greatly influences the effect of a turbulence promoter.
The pressure drop was measured for the different configurations of inlet
length, inlet type and turbulence promoters. Overall, only very small
differences between each configuration were observed (in the order of
100 pascal). Though the difference is marginal, it appears that higher
pressure drops result in higher mass transfer rates.
Mass transfer in electrolyzers can be significantly enhanced by
turbulence promoters or turbulence causing inlets. The added pressure
drop for these is minimal, which implies that large performance
increases can be achieved for little extra pumping costs. However, due
diligence must be taken in extrapolating results from the lab-scale to
the industrial scale. Since the importance of the inlet effect
diminishes as the electrolyzer scales up, mass transfer may be slower
than expected from the lab-scale. As we have shown, a good inlet design
or a calming section can reduce inlet turbulence in smaller
electrolyzers, so that they are more representative of their larger
counterparts.