May-2024

Hydrogen recovery from refinery off-gas – Part 1: An overview

Using next-generation hydrogen separation membranes to recover unused hydrogen wherever possible.

Zach Foss
Divigas

Article Summary

Modern refineries have adapted to exploit heavier crude oils and meet ever more stringent environmental requirements for fuel specifications, increasing demand for hydrogen to meet the most severe hydrotreatment needs. Increased hydrotreatment severity has caused an increase in refinery off-gas (ROG) production in most refineries, which frequently exceeds the possibility of sending it to other units or burning it entirely as fuel gas.

The value of these ROG streams increases dramatically when the hydrogen component can be purified and recycled as a feed gas back to the hydrotreaters, reducing hydrogen purchases through merchant suppliers or hydrogen production from the refinery itself.

With an increased focus on carbon footprint and net zero goals by 2050, hydrogen production is a key focus area for reducing greenhouse gas emissions. Greater than 96% of hydrogen manufactured today is through steam methane reforming (SMR), where natural gas is heated in the presence of steam over a catalyst to produce hydrogen and carbon monoxide (CO). The CO can be further processed via a water gas shift (WGS) reaction to produce additional hydrogen as well as carbon dioxide (CO₂).

Once the hydrogen product is separated and purified from the CO₂ and other contaminants, most hydrogen plants currently release the CO₂ directly into the atmosphere. This type of hydrogen production is referred to as ‘grey’ hydrogen. Grey hydrogen production prices are highly variable depending on the cost of natural gas, but a good rule of thumb is a price of $1.50-2.00/kg.

To reduce the carbon impact of hydrogen production, many refiners look to move to blue hydrogen, SMR production paired with carbon capture, utilisation, and storage (CCUS) or green hydrogen produced via electrolyser driven by renewable electricity. These process changes have a significant impact on the price. Blue hydrogen is estimated to double hydrogen costs to $2.80-3.50/kg, while green hydrogen is a staggering five times price increase up to around $8/kg.

This cost increase of a crucial feed gas for refinery processes has several implications, one of which is the increased importance of recovering unused hydrogen wherever possible. Most of the hydrogen supply for refiners now comes from three locations: the refinery hydrogen plant, which is typically an SMR unit; the catalytic reformer (CR), which produces hydrogen as a byproduct of its reaction; and third-party-produced hydrogen via pipeline or other transportations methods. To reduce the need for hydrogen from those sources, refineries look to their ROG streams for economically recoverable hydrogen.

Gas separation options
Hydrogen separation membranes have been utilised in refinery operations since the 1980s but have remained niche in their application due to the limitations of legacy membrane technology. Divigas has developed the Divi-H membrane, a polymeric hollow-fibre membrane capable of separation and purification of hydrogen even in extreme environments. Unlike legacy technologies, Divi-H excels at separation of hydrogen from CO₂.

It can operate in environments rich in H₂S and acidic gases that would cause legacy membranes to fail and operate at temperatures up to 150°C, whereas prior technology was limited to 50-80°C. These improvements in the performance of polymeric hydrogen separation membranes allow for significant use-case expansion within refinery operations. Table 1 gives a summary of the different gas separation technologies available to refiners.

Part 1 of this article will preview sample ROG streams produced from several types of units for their potential separation and purification via Divi-H. The membrane demonstrates separation costs as low at $0.015/kg hydrogen separated, with returns on investment (ROIs) exceeding 2,400% over the life of the product when compared to grey hydrogen production. Part 2 in PTQ Q3 2024 will analyse its effectiveness in hydrotreaters and hydrocrackers.

Membrane technology advantages
There are several qualitative benefits to leveraging membrane technology for gas separation, of which not all will be considered in the following analysis:
1: Energy efficiency: Membrane separation requires significantly lower energy consumption compared to cryo or pressure swing adsorption (PSA) separation. The driving force for separation is provided purely by the feed gas pressure.
2: Compact and modular design: Membrane systems have a smaller footprint and are easily scalable, making them suitable for applications where space is limited or modular expansion may be required. This scalability allows for great flexibility for ROG stream applications, as the flow rates and composition of the ROG can change quickly and vary greatly.
3: Selectivity and design versatility: While cryogenic and PSA separation are limited in their hydrogen purity options, membrane systems can be designed for a massive range of desired purities, pressure drops, and recoveries. If the purity of the hydrogen product stream does not need to be at 99.95%+, the economics of membrane separation are difficult to beat.
4: Continuous operation: Membrane separation operates continuously without the need for intermittent cycles or regeneration steps, providing a steady supply of separated gases. The Divi-H membrane modules allow for ‘hot swapping’, where a fibre cartridge is replaced within a single module while the system continues operating.
5 Simplicity and ease of operation: Membrane systems are simple to operate and require minimal supervision. They do not involve complex processes like cryogenic cooling or adsorption-desorption cycles.
6: Fast start-up and shutdown: Membrane systems can quickly reach operational conditions, reducing downtime during start-up and shutdown.
7: Versatility: ROG streams can change compositions, flow rates, and pressures quickly. Membrane separation can handle a wide range of gas streams with varying compositions and flow rates.
8: Minimal maintenance requirements: Membrane systems have no moving parts, resulting in lower maintenance and operational costs.
9: Reduced environmental impact: Membrane separation avoids the need for cryogenic fluids or chemical adsorbents, minimising the release of harmful substances into the environment.
10: Cost-effectiveness: Membrane separation offers a cost-effective solution for gas separation due to lower capital and operational expenses, shorter project lead times, and reduced utility requirements.