Hydrogen purification-liquefaction

Figure 10. Block flow diagram for hydrogen purification-liquefaction complex ( ll)...

Low-temperature adsorption systems continue to find an increasing number of applications. For example, systems are used to remove the last traces of carbon dioxide and hydrocarbons in many air-separation plants. Adsorbents are also used in hydrogen liquefaction to remove oxygen, nitrogen, methane, and other trace impurities. They are also used in the purification of helium suitable for liquefaction (grade A) and for ultrapure helium (grade AAA, 99.999% purity). Adsorption at 35 K will, in fact, yield a helium with less than 2 ppb of neon, which is the only detectible impurity in helium after this treatment. 

The exhaust gases contain CH4, N2, H2O, tar, acidic and basic compounds (NH3, HCN, H2S) considered as impurities. Tar conversion has to be controlled to maximize the reliability of mechanical equipments and to assure the operation of the successive clean-up catalytic steps for final hydrogen separation and purification [64]. This step involves the utilization of additional steam and selective catalysts, affecting the overall efficiency of the process [65]. The operation with oxygen instead of air may improve the efficiency of the process but it suffers the costs associated with air liquefaction process, necessary for O2/N2 separation. 

The absorption in alkanolamine solutions (MEA, DEA, ADIP, DGA, etc.) is the commercially most important process for the removal of CO2 from synthesis gas for ammonia and methanol production, for the production of hydrogen, in natural gas purification, coal liquefaction, and the like. In the present example a gas containing 13.55 mole-% of CO2 is to be purified by absorption into an aqueous solution of 13.6 wt-% MEA. The column, filled with 0.05 m steel Pall rings, has a diameter of 1.05 m and is operating at a temperature of 315 K and a pressure of 14.3 bars. The inlet flow rates of gas and liquid are 497 kmol/h and 76.9 mVh, respectively. Determine the packed column height necessary to reduce the mole fraction of CO2 to 5 x 10 at the top of the column. 

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