PSA Hydrogen Purification

Sumitomo-BF A PSA hydrogen purification process using a carbon molecular sieve as the selective adsorbent. Developed by Sumitomo, Japan. 

ZeoHte adsorbents play a dominant role in purifications owing to their ability to both adsorb large quantities of material and to achieve extremely low mole fractions of these adsorbed these compounds in product gas. Zeolites are the preferred adsorbent types for dehydration to low levels, purification and in several bulk separations. Zeolites also are employed in a significant portion of the PSA hydrogen purification market segment where they add value to bulk separations by achieving particularly high purity specifications. 

Hydrogen purification was the first large-scale application of PSA technology. The first commercial PSA hydrogen purification unit was installed in conjunction with a steam reformer, in Toronto around 1966. The standard five-step PSA cycle (with a co-current depressurization step) is used. Three or more pressure... 

In most hydrogen plants, a pressure-swing adsorption (PSA) system is used for hydrogen purification. In these plants, a major portion of reformer fuel is PSA offgas with a hydrocarbon stream for makeup fuel. 

Unlike PSA air separation the adsorbents used in hydrogen purification are not limited to zeolite molecular sieves. Garbons and silica gel are used in many PSA installations. Zeolites are used for obtaining certain critical specifications where the nature of the isotherms that they possess helps in recovery, achieving purity and minimizing bed size factors. 

It has been mentioned that purity and recovery are very significant drivers for the business of hydrogen purification. Today PSA hydrogen purifiers can deliver purities of several nines 99.9% purity while simultaneously recovering nearly 90% of the hydrogen found in the feed gas. 

The POLYBED PSA system replaces several of the units associated with hydrogen purification in a conventional plant — specifically, the low temperature shift converter, the C02 removal system and the methanator (see Figure 2). The tail gas from the PSA system is used as fuel to the reformer and it is supplemented by an external supply. However, the amount of fuel consumed is greatly reduced. Because some hydrogen... 

Despite these problems, PSA is often used for oxygen and hydrogen purification, and a recent runner-up Kirkpatrick Award for Union Carbide s Polybed hydrogen process (T ) attests to the fact that it is now both practical and economical to perform bulk separations on feed streams in excess of one million cubic feet per hour. 

The SELEXOL unit removes H2S and most of the CO2 in the syngas stream. The final hydrogen purification is accomplished in a downstream PSA unit. The purified hydrogen stream is combined with nitrogen from the air separation unit (ASU) and converted to ammonia. A unique CO2 purification scheme was developed to remove traces of H2S and COS from the CO2 that is fed to the UAN plant185. 

The Sumitomo-BF PSA process uses carbon molecular sieves (CMS) as the selective adsorbent, CMS has a higher capacity of adsorption than zeolites for methane and oxygen, and it is considered to be advantageous for hydrogen purification. If dirty raw gases are fed to this process, minor amounts of heavy hydrocarbon components such as aromatics are likely to cause deterioration of the adsorbents. To remove the heavy hydrocarbons, prefilter columns that contain activated carbon are placed upstream of the main CMS adsorbent beds4. 

For fuel processors directly integrated to a residential fuel cell H2 must be maximized and no pressure is available for PSA. In these cases a high- and low-temperature shift catalyst will be required. Hydrogen purification to reduce the CO to less than 10 ppm will be managed by preferential oxidation.33... 

After the sulfur is removed, most traditional ammonia processes have employed the purification steps that are shown in Figs. 22.4 and 22.6. However, ammonia plants have been built that use hydrogen purification via PSA, membrane separation with polymeric membranes, and cryogenic separation. PSA achieves the greatest product purities (about 99.999% pure hydrogen), but it is capital-intensive. Polymeric membranes are the least capital-intensive, but they achieve the lowest... 

Description The generic flowsheet consists of feed pre-treatment, prereforming (optional), steam-HC reforming, shift conversion and hydrogen purification by pressure swing adsorption (PSA). However, it is often tailored to satisfy specific requirements. 

The selection of adsorbents is critical for determining the overall separation performance of the above-described PSA processes for hydrogen purification. The separation of the impurities from hydrogen by the adsorbents used in these processes is generally based on their thermodynamic selectivities of adsorption over H2. Thus, the multicomponent adsorption equilibrium capacities and selectivities, the multi-component isosteric heats of adsorption, and the multicomponent equilibrium-controlled desorption characteristics of the feed gas impurities under the conditions of operation of the ad(de)sorption steps of the PSA processes are the key properties for the selection of the adsorbents. The adsorbents are generally chosen to have fast kinetics of adsorption. Nonetheless, the impact of improved mass transfer coefficients for adsorption cannot be ignored, especially for rapid PSA (RPSA) cycles. 

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