[ gas-liquid separation ]
TotalVesselVolume L �
� 2
Dmin
4
The length and diameter should then be adjusted to give a L / D ratio of 3 to 5, depending upon the operating pressure( see Table 2).³
Gravity separators are simple vessels with no moving parts, ensuring minimal pressure drop and low wear. They can deal with large flows, are easy to control, and are a mature technology. Separation is enhanced by a larger difference in density between the liquid and gaseous phases, and by a reduction in the gas viscosity. 5 Therefore, relative to other common industrial gases, gravity separation for hydrogen applications should be more easily achieved. However, a large footprint is often required to give both adequate liquid residence time and the gas flow straightening necessary for droplet settling, 5 which could be difficult to accommodate with large flows of electrolyte. Horizontal separators are likely to be more common than vertical separators for smaller MW electrolyser systems, as these are often containerised solutions.
Additionally, the liquid residence time in the separator vessel should allow for the disengagement of gas bubbles from the liquid. A liquid residence time of 5 minutes is industry convention for the degassing of hydrogen from seawater subjected to electrolysis. 6 Minimising the recycle of gas bubbles to the stack inlet is of paramount importance for both safety and efficiency. Explosive recombination of H 2 and O 2 must be avoided at the stack inlet, and gas bubble coverage of electrode surfaces is known to increase ohmic resistance and overpotential, reducing the energy efficiency of the electrolyser. 7, 8 The internals of separator vessels can be designed to enhance gas bubble separation, thus reducing vessel footprint and cost.
Table 2. Recommended vessel length / diameter ratio versus |
operating pressure ³ |
Operating pressure( bar) |
L / D |
0 – 20 |
3 |
20 – 35 |
4 |
> 35 |
5 |
Mist extraction
The required separation performance of the gas-gravity section is dependent upon the mist extractor device chosen for the separator outlet. The gas-gravity section preconditions the flow and must remove all droplets of a target size, ensuring that the mist extractor can remove the remaining droplets. 9 The critical droplet size for gravity separation is usually 100 microns. 5 Empirical K-factors are also associated with mist extractor devices, as shown in Table 3, for determining the maximum allowable velocity before excessive re-entrainment of impinged droplets occurs. Note that these K-values should be de-rated with increasing pressure. For example, a factor of 0.75 should be applied at 1200 psi( g). 9
Vane or wave-plate demisters Vanes, also known as wave-plate demisters, achieve droplet separation by inertial collisions. The corrugated plates are typically arc, trapezoid or triangular in shape, and hook grooves can be applied to the bend points for improved separation efficiency( see Fig. 2). They offer a simple structure, operational stability, large processing capacity, and are resistant to blocking. 5 For vertical gas flows, a horizontal configuration is employed, whilst a horizontal gas flow requires a vertical arrangement. 10
Table 3. Typical K-factors for common mist extractor devices 9 |
Mist extractor device |
K( m / s) |
High-capacity wire mesh pad |
0.122 |
Standard wire mesh pad |
0.107 |
High-efficiency co-knit mesh pad |
0.076 |
Simple vane |
0.152 |
High-capacity pocketed vane |
0.250 – 0.351 |
2-inch axial flow cyclone |
0.244 – 0.305 |
32 Hydrogen Tech World | Issue 22 | June 2025