Hydrogen pressure swing absorption

Within such a plant, depending on the pressure of the syngas, the separation can be performed by chemical absorption (usually with amine solvents) under lower pressure conditions or by physical absorption (e.g., with methanol) under higher pressure conditions (see also Chapter 6). Likewise, pressure-swing absorption can be employed. With the special properties of hydrogen, membrane separation processes could also be a very promising solution for the separation task. 

Fig. 7.14. The catalytic synthesis of hydrogen from natural gas using hydrodesulfurization, steam reforming, water gas shift, and pressure swing absorption (PSA).

The Haldor Topsoe Convection Reformer (HTCR) is a relatively small piece of equipment that combines the radiant and waste heat sections of the conventional reformer. It uses PSA (pressure swing absorption) to make 99.9 percent hydrogen purity. It is best for small and medium-sized hydrogen plants (500 to 10,000 Nm3/hr).75... 

There is a recent trend in refineries away from the first option and toward the second, because of the falling cost of hydrogen separation from gas mixtures via pressure-swing absorption. If the C02 from refinery production of hydrogen... 

Product purification. In a liquid absorption system, carbon dioxide is removed. The product gas undergoes a methanation step to remove the residual traces of carbon oxides. Recent SMR plants use a pressure swing absorption (PSA) unit instead, producing 99.99% pure hydrogen. 

Depending on the downstream process (H2, CO, oxosynthesis, ammonia, etc.), the product gas goes to the different unit operations as described in the section Overview. In the case of hydrogen production, this usually involves a shift reactor and a pressure-swing-absorption reactor (PSA) (Fig. 4). For CO production, the scheme involves a CO2 removal section and a low temperature separation unit (so-called cold box). CO2 is recycled to the reformer. If in this case, H2 product is also desired, a PSA unit may also be present (Fig. 5). 

As to a comparison of membrane gas separation technologies with such methods as pressure swing absorption (PSA) and low-temperature or cryogenic separations, the last-cited chapter in Polymeric Gas Separation Membranes must remain somewhat inconclusive, given the wide range of variables, parameters, and applications. Moreover, for the most part, the separations compared were confined to air and hydrogen-containing systems. 

After scrubbing the exiting acidic gases, that is, CO and traces of H S, under pressure by an alkaline aqueous solution containing either diethanolamine or sodium carbonate, all the carbon dioxide and impurities are removed below acceptable levels for the final purification of the hydrogen gas. Usually, the final purification utilizes pressure swing absorption (PSA). 

Industrially, the above reaction is conducted at atmospheric pressure and at 800-900 C with nickel or iron catalysts. The hydrogen-rich gas mixture (i.e., 75 vol.% and 25 vol.% N ) that contains only traces of unreacted ammonia can be used as-is or purified by pressure swing absorption, or gas permeation through Pd-Ag membrane. 

Purification of hydrogen by high temperature shift conversion and Pressure Swing Absorption. Optional carbon dioxide-removal located upstream of the PSA unit in case carbon dioxide is required. 

Production of pure hydrogen by tubular reforming high temperature shift conversion and purification by proprietary Pressure Swing Absorption technology. 

Methanation catalysts or, alternatively, pressure-swing absorption are used to purity synthesis gas and refineiy hydrogen throughout the world. The methanation reaction has also been applied in other interesting and useful processes although not yet on a veiy significant scale. 

The gas is then cooled to 30-50 °C and the carbon dioxide is removed by amine absorption or other processes. The remaining impurities - carbon monoxide, methane, nitrogen, argon - are removed in a final pressure-swing adsorption (PSA) step to yield >99.5% pure hydrogen. One of the main problems with this process is that the carbon dioxide is removed by the amine unit as a low-pressure gas. This gas must be compressed to 80 bar to be pipelined for sequestration. This compression step alone requires massive compressors and uses 4—5% of the total power output of the plant. The amine treatment step itself uses even more energy, so the total energy consumption is 15% of the power produced by the plant. 

Depending on the purity required, hydrogen can be further processed by cryogenic adsorption, solvent absorption, pressure swing adsorption (PSA), cryogenic absorption using liquid nitrogen, membrane separation, and metal hydride adsorption. 

---