Pressure Swing Adsorption in Hydrogen Purification

Pressure Swing Adsorption is a cyclic adsorption process that separates gases based on differences in their affinity for a solid adsorbent material. In hydrogen purification, the impurity components — typically CO₂, CO, CH₄, N₂ and water vapour — are preferentially adsorbed onto the adsorbent material at high pressure while hydrogen passes through as the product.

The adsorbent is subsequently regenerated by reducing the pressure, which releases the adsorbed impurities and restores the bed's capacity. This pressure swing cycle is repeated continuously, allowing a multi-vessel PSA system to produce a continuous hydrogen product stream.

A PSA system for hydrogen purification typically consists of multiple adsorber vessels operating in a staggered cycle sequence. While one or more vessels are in the adsorption step producing hydrogen, others are undergoing various stages of depressurisation, purge and repressurisation.

The number of vessels depends on the required product purity, recovery targets, feed gas composition and cycle timing. Multi-bed configurations with pressure equalisation steps are commonly used to maximise hydrogen recovery by transferring high-pressure gas between vessels at the end of the adsorption step before venting to purge.

The adsorbent bed typically contains multiple adsorbent layers, each selected to adsorb specific impurity components. Activated alumina or silica gel may be used to remove moisture in the inlet layer, followed by activated carbon for heavier hydrocarbons and CO₂, with zeolite molecular sieve used for CO, N₂ and residual methane.

The composition of the feed gas to a PSA system has a significant influence on the design — particularly the number of vessels, adsorbent volumes and achievable hydrogen recovery.

Feed streams with high impurity concentrations, or impurities that are difficult to adsorb such as nitrogen, typically require larger adsorbent inventories or more complex cycle configurations to achieve the same purity and recovery as cleaner feed streams.

Moisture in the feed stream must be managed carefully. Liquid water or high water vapour concentrations can adversely affect adsorbent performance. Feed gas pre-treatment — typically a knock-out drum and, where required, a guard dryer — is normally included upstream of the PSA unit.

PSA systems for hydrogen purification are capable of producing hydrogen at high purity levels suitable for refinery use, fuel cell applications and, with appropriate pre-treatment of the electrolyser output, green hydrogen grid injection.

Product purity and hydrogen recovery are related parameters — improving purity generally reduces recovery, and vice versa, for a given feed composition and system configuration. The design target for purity and recovery is established based on the downstream application requirements.

PSA systems are used for hydrogen purification across a range of industrial applications including: recovery of hydrogen from refinery off-gas streams, purification of SMR product gas, purification of electrolysis-derived hydrogen for green hydrogen applications, and hydrogen recovery in ammonia and methanol plant networks.

In refinery applications, PSA systems recover hydrogen from catalytic reformer off-gas, hydrocracker and hydrotreater purge streams and other hydrogen-containing off-gases, improving the efficiency of the refinery hydrogen network.

Howe Baker has engineering experience in the design, supply and commissioning of PSA systems for hydrogen purification across refinery, petrochemical and hydrogen production applications.

Howe Baker's engineering scope for PSA systems typically covers process design, equipment specification, mechanical design, control system design and commissioning support. Systems are designed to meet the specific feed composition, product purity and flow rate requirements of each project.