Why Electrolyte Carryover Is a Hidden Threat in Alkaline Electrolysis
Alkaline water electrolysis is one of the most established routes to green hydrogen production. Cells operate with a concentrated potassium hydroxide (KOH) electrolyte — typically 25–30 wt% — at temperatures between 60 °C and 90 °C. Under these conditions, vigorous gas evolution at both electrodes creates a fine aerosol of KOH droplets that becomes entrained in the product hydrogen stream. This phenomenon, known as electrolyte carryover, is not a minor nuisance: it is a progressive, corrosive threat to every piece of equipment downstream of the electrolyser stack.
Left unaddressed, KOH mist attacks carbon steel pipework, causes stress corrosion cracking in certain stainless alloys, blocks pressure-relief valves, degrades compressor valve seats, and contaminates hydrogen storage vessels. In systems feeding fuel cells or industrial processes with strict purity requirements, even trace electrolyte contamination can render the hydrogen unusable. Yet many operators underestimate the problem — or attempt to solve it with off-the-shelf compressed-air filters that are wholly unsuited to caustic service.
This article explains the mechanisms behind KOH aerosol generation, why standard filter elements fail, and how a correctly specified two-stage filtration train using chemically resistant K-type elements provides reliable, long-service protection.
How KOH Aerosol Is Generated Inside the Electrolyser
Inside an alkaline electrolyser cell, hydrogen is evolved at the cathode and oxygen at the anode. The violent nucleation and detachment of gas bubbles from electrode surfaces creates a turbulent two-phase flow within the electrolyte. As bubbles burst at the gas–liquid interface, they eject tiny droplets of KOH solution into the gas space above. The droplet size distribution spans a wide range — from large droplets (>10 µm) that settle quickly, down to sub-micron aerosol particles that remain suspended almost indefinitely in the gas stream.
Several operating parameters intensify carryover:
- Higher current density — more vigorous bubble evolution, greater aerosol generation
- Elevated temperature — lower surface tension, smaller and more persistent droplets
- Higher gas velocity — increased entrainment of droplets from the liquid surface
- Foaming — caused by organic contamination or electrolyte degradation products
Modern high-performance electrolysers operating at elevated current densities to reduce capital cost per kilowatt are therefore particularly susceptible to carryover. The gas leaving the electrolyser stack typically passes through an internal gas–liquid separator, but this removes only the coarser droplets. Fine aerosol — the fraction most damaging to downstream equipment — passes straight through.
Why Standard Filter Elements Fail in KOH Service
The most common mistake engineers make when specifying filtration for alkaline electrolysis is to reach for a standard compressed-air coalescing filter. These elements are manufactured from borosilicate glass microfibre bonded with an epoxy or phenolic resin binder. In dry or mildly humid compressed-air service, this construction is excellent. In a 25–30 wt% KOH environment at 70–90 °C, it is catastrophically wrong.
Concentrated potassium hydroxide is strongly alkaline (pH >14) and attacks borosilicate glass through a process of network dissolution. The silica (SiO₂) component of the glass reacts with hydroxide ions to form soluble silicate species, progressively destroying the fibre structure. Simultaneously, many resin binders are hydrolysed by hot caustic solution, causing the element to delaminate and shed fibres into the gas stream. The result is rapid loss of filtration efficiency, structural collapse of the element, and downstream contamination — the exact opposite of the protection sought.
Housing materials present a parallel challenge. Standard aluminium filter housings are attacked by KOH even at moderate concentrations. Polycarbonate bowl guards dissolve. Only 316L stainless steel — or higher-grade alloys — provides adequate corrosion resistance in alkaline electrolysis service.
The K-Type Element: Engineered for Caustic Environments
R+F FilterElements offers its own range of K-type filter elements specifically developed for sour gas, caustic, and chemically aggressive service. Unlike standard elements, K-type elements use a chemically inert fibre matrix — free from borosilicate glass — combined with a PTFE-based binder system that resists both strong acids and strong alkalis. The result is an element that maintains its structural integrity and filtration efficiency throughout its service life in KOH environments.
K-type elements are available in both coalescing and particulate configurations, covering the two distinct separation duties required in an alkaline electrolysis filtration train:
- K-type coalescing elements (RF-C-K series) — capture and coalesce liquid KOH aerosol droplets, draining them to a sump for periodic removal
- K-type particulate elements (RF-P-K series) — remove solid KOH particles and any residual fine aerosol not captured in the coalescing stage
Both types are available in the standard R+F element sizes (12032, 12057, 25064, 25178, 51230, 51476), allowing them to be fitted into the appropriate process gas filter housings from the RF-H-150 and RF-H-160 series.
Designing the Two-Stage Filtration Train
Effective KOH mist removal requires two complementary stages in series. A single-stage approach — whether coalescing or particulate alone — cannot reliably achieve the purity levels required for downstream equipment protection or hydrogen quality specifications.
Stage 1: Coalescing Separation
The first stage targets liquid KOH aerosol. Gas enters the coalescing element from the outside and flows inward through the fibre matrix. Fine droplets are captured by inertial impaction, interception, and diffusion mechanisms. As droplets accumulate on the fibres, they coalesce into larger drops that drain downward under gravity into the housing sump. The sump is fitted with an automatic float drain to remove collected liquid without interrupting gas flow.
For this duty, R+F recommends the RF-H-152 housing — a compact 316L stainless steel process gas filter rated to 100 bar — fitted with K-type coalescing elements. The RF-H-152 is available with FKM (Viton) O-ring seals, which provide excellent resistance to hot KOH solution and are mandatory for this application. NBR seals must not be used, as they swell and degrade rapidly in caustic service.
Stage 2: Final Particulate Removal
The second stage captures any residual solid KOH particles and sub-micron aerosol that passes through the coalescing stage. Solid particles arise from crystallisation of KOH on cooler surfaces within the pipework, or from dried aerosol droplets. A K-type particulate element rated to 99.99% efficiency at ≥0.3 µm provides the final barrier before the gas enters compression or storage.
For this stage, the RF-H-160 housing — rated to 250 bar in 316L stainless steel — is appropriate where higher operating pressures are encountered downstream of a booster compressor. Again, FKM seals are mandatory.
Complete Filtration Train Specification
| Parameter | Stage 1 — Coalescing | Stage 2 — Particulate |
|---|---|---|
| Housing model | RF-H-152 | RF-H-160 |
| Housing material | 316L stainless steel | 316L stainless steel |
| Max. operating pressure | 100 bar | 250 bar |
| Max. operating temperature | 200 °C (S-type) / 100 °C (standard) | 200 °C (S-type) / 100 °C (standard) |
| Element type | RF-C-K (K-type coalescing) | RF-P-K (K-type particulate) |
| Filtration efficiency | 99.99% ≥ 0.1 µm liquid | 99.99% ≥ 0.3 µm particulate |
| Seal material | FKM (Viton) — mandatory | FKM (Viton) — mandatory |
| Drain | Automatic float drain | Manual or timed auto-drain |
| Connections | ¼″ to 1½″ BSP/NPT | ¼″ to 1½″ BSP/NPT |
Positioning the Filtration Train in the Process
The filtration train should be installed immediately downstream of the electrolyser's internal gas–liquid separator and before any compression stage. This positioning is critical for two reasons. First, the gas at this point is at relatively low pressure (typically 1–30 bar depending on the electrolyser design), which means the aerosol droplets are larger and easier to coalesce. Second, protecting the compressor from KOH ingestion is paramount — compressor internals are extremely difficult and expensive to clean or replace once contaminated.
Where a booster compressor raises the hydrogen pressure to storage or pipeline pressure (typically 200–700 bar), a second particulate filter stage downstream of the compressor is advisable to capture any solid particles generated by the compression process itself. The RF-H-160 or RF-H-170 housings are appropriate for this high-pressure duty.
Operators should also consider the complete hydrogen filtration solution offered by R+F FilterElements, which covers the full pressure range from electrolyser outlet to high-pressure storage, including oxygen-side filtration with EPDM-O₂ sealed housings.
Seal and Material Selection: A Critical Detail
Material compatibility in KOH service extends beyond the filter element itself. Every wetted component — housing body, end caps, drain valves, pressure gauges, and connecting pipework — must be evaluated for caustic resistance. The following guidance applies:
- Housing body: 316L stainless steel as a minimum; duplex stainless steel for higher-temperature or higher-concentration service
- O-ring seals: FKM (Viton) rated to 200 °C; PTFE encapsulated seals for extreme conditions
- Drain valves: 316L stainless steel body with PTFE-seated ball valves
- Pressure gauges: Glycerine-filled with 316L stainless steel wetted parts and diaphragm seal
- Pipework: 316L stainless steel throughout; avoid carbon steel, copper, and aluminium
NBR (nitrile) seals — the default in many standard filter housings — must never be used in KOH service. NBR swells significantly in concentrated alkali, leading to seal extrusion, leakage, and potential housing failure. Similarly, EPDM seals, whilst chemically resistant to KOH, are not recommended where the gas stream may contain traces of hydrocarbons or lubricants from the compression stage.
Monitoring Performance and Determining Change-Out Intervals
Unlike compressed-air filtration, where element change-out intervals are often set by elapsed time alone, alkaline electrolysis service demands a more active monitoring approach. KOH loading on the filter elements varies with electrolyser operating conditions, and a heavily loaded element can reach its pressure-drop limit well before a time-based interval would suggest.
R+F recommends fitting a differential pressure indicator across each filter stage. A rising differential pressure indicates element loading and approaching end-of-life. Typical change-out criteria:
- Differential pressure exceeds 0.5 bar across the coalescing stage
- Differential pressure exceeds 0.3 bar across the particulate stage
- Visible KOH crystallisation on external surfaces of the housing (indicating seal weeping)
- Scheduled maintenance interval — typically 4,000–6,000 operating hours in continuous service
When changing elements, operators should wear appropriate PPE — chemical-resistant gloves, eye protection, and face shield — as spent elements will be saturated with concentrated KOH solution. Spent elements should be disposed of in accordance with local regulations for caustic waste.
For guidance on sizing the correct housing and element combination for a specific electrolyser flow rate, R+F FilterElements provides an online sizing wizard that accounts for gas composition, pressure, temperature, and required outlet quality.
Common Mistakes and How to Avoid Them
Based on enquiries received by R+F FilterElements from operators of alkaline electrolysis systems, the following mistakes recur frequently:
- Using standard compressed-air filters: As discussed, borosilicate glass elements dissolve in KOH. Always specify K-type elements explicitly.
- Single-stage filtration only: A coalescing stage alone will not remove solid KOH particles; a particulate final stage is always required.
- Aluminium housings: Aluminium is attacked by KOH even at low concentrations. 316L stainless steel is the minimum acceptable material.
- Incorrect seal material: NBR seals fail rapidly. FKM or PTFE seals are mandatory.
- Installing filtration downstream of the compressor only: The compressor must be protected from KOH ingestion. Filtration must be installed before the compressor inlet.
- Ignoring the oxygen side: The oxygen stream from an alkaline electrolyser also carries KOH aerosol and requires equivalent filtration, with EPDM-O₂ compatible seals.
Summary: A Reliable KOH Mist Removal Strategy
Electrolyte carryover in alkaline electrolysis is a well-understood but frequently mishandled problem. The engineering solution is straightforward when the correct components are specified: a two-stage filtration train using chemically resistant K-type elements in 316L stainless steel housings with FKM seals, positioned immediately downstream of the electrolyser's internal separator and before any compression stage.
R+F FilterElements, a German-based filtration specialist applying European engineering standards, offers its own range of K-type coalescing and particulate elements — the RF-C-K and RF-P-K series — together with the RF-H-152 and RF-H-160 process gas housings, providing a complete, chemically compatible solution for alkaline electrolysis service. All housings are available with FKM seals as standard for this application.
For a detailed specification or to discuss your specific electrolyser configuration, contact R+F FilterElements at [email protected] or use the engineering sizing tool to generate a preliminary filter selection.
