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Enhancing PSA Efficiency in Renewable Natural Gas (RNG) Purification

Pressure swing adsorption (PSA) units play a critical role in the purification and upgrading of biogas to renewable natural gas (RNG) and in the purification of hydrogen gas. Biogas, typically produced through anaerobic digestion of organic waste, contains a mixture of methane (CH4), carbon dioxide (CO2), water vapor, and trace contaminants. To meet pipeline-quality standards for RNG, the methane concentration must be significantly increased, which requires the removal of CO2 and other impurities. PSA technology achieves this by exploiting the different adsorption characteristics of gases under varying pressures. In a PSA system, gas is passed through adsorbent materials—often zeolites or activated carbon—that selectively capture CO₂ and other unwanted components at high pressure. When the pressure is subsequently lowered, the adsorbed gases are released, regenerating the adsorbent for reuse.

 

This cyclic process allows PSA units to operate continuously and efficiently, making them well-suited for decentralized RNG production facilities. Compared to other upgrading technologies like water scrubbing or membrane separation, PSA systems offer advantages in terms of energy efficiency, modularity, and the ability to achieve high methane purity levels. Additionally, PSA units can be tailored to handle varying biogas compositions, which is particularly valuable in agricultural or municipal waste applications where feedstock variability is common. As the demand for low-carbon fuels grows, PSA technology is increasingly being integrated into RNG projects to support decarbonization goals and enhance energy resilience.

 

A leading refinery in Thailand operating a Continuous Catalytic Reforming (CCR) unit and Pressure Swing Adsorption (PSA) system faced persistent challenges in hydrogen purification. The PSA unit, critical for hydrogen recovery, was experiencing reduced adsorbent life due to contamination from liquid hydrocarbons (C5+) carried over from the CCR unit.

 

  • Challenge

  • Solution

  • Results

 

The Pressure Swing Adsorption (PSA) system at a facility in Thailand faced persistent challenges in gas purification. The PSA unit, critical for gas recovery and purification, was experiencing reduced adsorbent life due to contamination from liquid hydrocarbons (C5+) carried over from an upstream processing step. Previously this issue led to an unforced shutdown for the site to replace the adsorbent material resulting in a significant cost to the facility.

 

After extensive on-site review and testing it was determined that the site’s existing knockout (KO) drum, equipped with a demister pad, was insufficient in removing liquid hydrocarbons from the gas stream. As a result, C5+ aerosols were consistently carried over into the PSA unit, leading to:

 

  • Shortened PSA adsorbent life
  • Increased maintenance and replacement costs
  • Reduced gas recovery efficiency – product yield

 

Sidestream testing at the PSA KO pot outlet revealed aerosol concentrations ranging from 3.0 to 7.0 ppmw, confirming the inadequacy of the current separation system.

 

To address this issue, Pall’s Thailand division proposed a high-efficiency liquid/gas coalescer system.  A liquid-gas coalescer removes liquid aerosols from a gas stream by capturing fine droplets on a fibrous or porous media, where they coalesce into larger droplets. These larger droplets are then drained away by gravity or separated mechanically, preventing them from reaching and damaging the PSA adsorbent material.

 

This advanced filtration system was designed to effectively remove C5+ aerosols from the hydrogen stream, thereby protecting the PSA adsorbents and improving overall system performance.

 

The coalescer was commissioned in July 2019, and a performance test run was conducted at the outlet of the coalescer vessel. The test confirmed:

 

  • No liquid detected in the effluent gas stream
  • Stable pressure drop across test elements
  • No signs of liquid accumulation on test elements or housing internals

Pall High Efficiency Liquid-Gas Coalescer Vessels Internals

 

Since implementation, the site has experienced significant improvements:

 

  • Complete elimination of liquid carryover to the PSA unit
  • Extended PSA adsorbent life
  • Improved gas purity and recovery
  • ROI achieved in less than one year

 

The commercial-scale coalescer has remained in continuous operation demonstrating sustained performance and reliability.

 

 

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