[ gas-liquid separation ] liquid filter separators are used for purifying ammonia and natural gas,¹¹ removing acid mist from sulphuric acid-absorbing towers and removing light organic oils from asphalt operations. 10
Green hydrogen production via electrolysis is a relatively clean process. In the case of PEM systems, ultrapure water is used as the electrolyte. For alkaline systems, lye( KOH) is used as the electrolyte. As this is corrosive, degradation of internals can be problematic if corrosion-resistant materials are not specified, but a lye filter is often used to protect against generated particulates. Therefore, the improved separation efficiency of filtration-separators makes them a better choice for green hydrogen processing. Removing as much bulk water as possible from the hydrogen stream in the gas-liquid separator helps improve the efficiency of the plant by reducing the size of downstream equipment, such as chillers, heat exchangers, and the hydrogen dryer.
Coalescing media
The process of gas-liquid coalescence occurs within the filter media. Two or more droplets
that have been intercepted by the fibres can overcome the energy of surface tension, make contact, and merge, forming larger droplets that eventually fall under gravity. Densely packed fibres have a higher separation force than loosely packed fibres. The wettability of the fibre material( hydrophilicity) also influences coalescence growth.¹²
The process of coalescence is shown in Fig. 4. Densely packed, smaller-diameter fibres are found at the inlet face of the filter element to better capture the smallest droplets. Sufficient residence time is then required for the droplets to encounter other droplets on the surface of the fibres. The larger droplets move further into the fibre bed as the force of adhesion is overcome by the drag force. To accommodate this the density of the fibrous material is reduced with filter depth by design. The aim is for the droplets to be large enough to fall and not re-entrain once the drainage layer has been reached.¹³
The effectiveness of liquid coalescence varies with pressure. At higher pressure, the interfacial
Fig. 4. Coalescence process inside a filter element ¹³
Hydrogen Tech World | Issue 22 | June 2025 35