Hydrocarbon processing hydrogen purification

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. 

Small-scale reforming systems are relatively complex because they need fuel and air-feed systems, the reformer, a hydrogen purification system, and various cooling and water processing ancillary systems to make it all work. The systems also have to employ a specific hydrocarbon that is available at a reasonable cost at a customer s location. These systems probably work best for customers with hydrogen consumption rates in the 1500 scf/hr to 10,000 scf/hr range. They... 

Hydrogen must be chemically separated from some other material. Currently, natural gas is the most common feedstock, but coal is also used. Biomass could be used in the future. The full costs of the production, processing, and purification of these hydrocarbon feedstocks are included in this analysis. When these materials are used to produce hydrogen, the required energy is embedded in the feedstock. (See Chapter 8 and Appendix E for more details.)... 

Hydrogen purification—Linde Division of Union Carbide, Hydrocarbon Processing, p. 188 (November 1%9). 

Hydrogen is used in a large number of chemical processes, and may be used as a fuel itself or as a reactant in the production of synthetic fuels such as in the Fischer-Tropsch hydrocarbon synthesis process, for example. In applications where hydrogen purification is required, membranes can be used for hydrogen separation. Other hydrogen purification methods include pressure swing adsorption and cryogenic separation. 

The water gas shift (WGS) reaction is a key technology in the hydrogen purification processes of syngas obtained by steam reforming or partial oxidation of hydrocarbons ... 

Low-temperature processes rely on the different boiling points of the species contained in the treated syngas. These processes are often used to separate hydrogen from hydrocarbons in refinery off-gas streams with H2 content below 50%, but they have been supplanted by PSA in hydrogen purification from steam... 

Adsorption is a viable option for hydragoi sulfide removal whrai the amount of sulfur is very small and the gas contains heavier sulfur conpounds (such as mercqitans and carbon disulfide) that must also be removed. For adsorption to be the preferred process for carbon dioxide ranoval, there must be a high CO partial pressure in the feed, the need for a very low concentration of carbon dioxide in the product, and the presence of otho- gaseous impurities that can also be removed by the adsorbent Typical examples are the purification of hydrogen fiom steam-hydrocarbon reforming, (he purification of land-fill efftuoit gas, and the purification of ammonia synthesis gas. Adsorption processes are described in detail in Chapter 12. 

The PSA cycle makes use of the simple fact that the partial pressure of adsorbate in the gas phase can be reduced by lowering the total pressure. Pressure reduction can thus be used to regenerate adsorbent that has been loaded with adsorbate at an elevated pressure. Since it is not necessary to heat or cool the bed between or during the adsorption and desorption steps, very rapid cycling is possible. The process is now widely used for hydrogen purification, air separation, hydrocarbon separation, and air drying, and new applications are under development. 

Figure 4.1. A process for producing hydrogen by steam reforming of hydrocarbons (1) reforming furnace (2,3) purification section, (4) shift converter, (5) pressure swing adsorption.

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