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Ask the experts: polybed PSA

Hydrocarbon Engineering,

Pressure Swing Adsorption (PSA) units are the industry standard worldwide for purifying synthesis gas in hydrogen plants and extracting hydrogen from valuable off-gases in refining process units. The hydrogen purity is typically more than 99.9% with the low concentrations of carbon oxides (CO and CO2) required for the downstream processing units.

As the industry’s PSA expert, with 50 years of hydrogen purification technology innovation, Honeywell UOP receives lots of questions on how to maximise the technology’s efficiency and effectiveness. Below are some of the frequently asked questions we receive. Don’t see your question below? Ask a question here.

Q: A PSA unit is designed to produce 99.9%+ H2 with 10 ppmv CO. Due to upset conditions upstream of the PSA, the CO has increased in the feed resulting in higher levels in the product. What action should be taken to return to design H2 purity levels?

A: The adsorption time should be reduced to improve H2 product purity. This will load fewer impurities on the beds during the adsorption step and allow H2 purity to return to design. This can be implemented by manually reducing the adsorption time or reducing the capacity factor in automatic.

Q: What is the impact of operation at higher than design tail gas pressures? The PSA is designed for operation at 5 psig exiting the surge tank, but temporary operation at 10 psig will be needed.

A: PSA tail gas pressure is directly related to the unit’s performance. At higher pressures, the unit’s capacity and hydrogen recovery will be reduced. In order to maintain product purity, it will be necessary to reduce the adsorption time.

Q: What is the effect of liquids entrained in the PSA feed gas on the process and adsorbents?

A: The PSA is a vapour phase separation and must be free of entrained liquids. A separator upstream of the PSA should knock out liquids before being carried over into the PSA. Liquids will damage the adsorbents and reduce the unit’s capacity for design impurities. The damage is typically irreversible. Regular monitoring of the performance will allow for remediation before there are significant losses.

Q: How does a PSA work?

A: The main components of a PSA installation are the adsorbers, the valve, instrumentation and piping skid, the control system and the surge tank. A process block diagram follows (Figure 1).

Feed gas enters the PSA unit at high pressure; the product is delivered with minimal pressure drop through the unit while the tail gas is typically rejected at low pressure. A PSA unit can be designed to accommodate a wide range of operating pressures, from low pressure (< 100 psig) up to units approaching 1000 psig. Tail gas pressures are typically 5 psig to maximise hydrogen recovery, but can also be 75 psig or greater to match the plant’s fuel header.

Watch a video on how PSA technology works

Figure 1. H2 Purity 99.9 - 99.999%. H2 Recovery - 95%. H2 Feed pressure 90-1000 psig. H2 Product pressure 80-990 psig.

The driving force for the separation is the partial pressure of the impurities. The principle is simple: at high pressure, impurities are adsorbed from the feed gas, resulting in a high purity gas product. By stopping the feed and “swinging” the adsorber to low pressure, the impurities are desorbed, exiting in the tail gas. Although the PSA process is a batch process, it uses multiple adsorbers operating in a staggered sequence, so that the unit operates as a continuous process at the battery limits.

Selection of the adsorbents is dependent on the feed conditions and required product specification. PSA units can handle a wide range of feed impurities. With multiple layers of adsorbents within a vessel, the PSA can achieve extremely high hydrogen purities (up to 99.9999% with 1 ppmv impurities). Depending on the feed impurities, the adsorbent mix may contain any or all of molecular sieves, activated carbons, silica gels and activated aluminas.

The main operating parameter to be adjusted for operation of a PSA unit is its adsorption time. As the adsorber vessels are sized to remove a fixed amount of impurities, adjustment of this adsorption time is required when running at rates other than design. This will ensure the production of on spec product gas while maximizing recovery of the feed hydrogen.

PSA units are also designed with alternate modes of operation, or switchovers, to allow maintenance while keeping the unit on stream at design feed rates. In the unlikely event of an instrument failure, the switchover mode allows the unit to continue operation while a repair is safely made.

For the latest on UOP’s hydrogen purification solutions, technical papers, customer cases and more, visit Honeywell UOP online.

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