[ gas-liquid separation ]
Fig. 2. Wave plate demister profile representation for horizontal gas flow 10
In the flow passage of the plates, the gas flow undergoes curvilinear motion, causing droplets to hit the vane surface as they cannot make sharp turns due to their greater inertia. A liquid film accumulates, and the gas leaves freely. For lower gas velocities, increasing the velocity improves separation efficiency, but at the cost of higher pressure drop. At higher gas velocities, secondary re-entrainment of the formed liquid film can be a problem, causing a loss of demisting efficiency. The separation performance of droplets < 25 microns can be poor. In general, decreasing the plate spacing, adding more bends, and decreasing the bend wavelength can all improve performance. 5
Many end users of green hydrogen, particularly fuel cell applications, require very dry hydrogen with high purity(< 5 ppm water). The poorer removal of small droplets likely makes vane demisters ill-suited for green hydrogen flowsheets. Higher liquid carryover would place more demand upon the downstream hydrogen gas dryer, resulting in larger desiccant beds,
larger heating and cooling requirements, higher CAPEX and OPEX, and reduced energy efficiency.
Coalescence filtration
Alternatively, gas-liquid separation can be achieved using filtration media contained within a filter element. This process occurs via the following three mechanisms( see Fig. 3). These mechanisms apply to both droplet and particulate removal from a gas stream.
1. Inertial impaction – the gas changes direction as it flows through the mesh structure, whilst larger liquid droplets have sufficient momentum to continue on a straight path. These larger liquid droplets break away from the gas streamline and impact the filtration media. This mechanism is characterized by the Stokes number:
Where d p is droplet diameter, d b is the diameter of a monofilament, and K M is the Cunningham
Hydrogen Tech World | Issue 22 | June 2025 33