Final Purification of Synthesis Gas

The Selectoxo (Selective Catalytic Oxidation) Process reduces 1) the hydrogen consumption of the methanation system and 2) the inert gas content of the purified synthesis gas that is fed to the ammonia synthesis loop74. 

In the early 1960 s Engelhard developed and commercialized the Selectoxo catalyst and process for H2 plants. The heart of this technology is a highly selective catalyst, which oxidizes up to 10,000 ppm CO without significantly oxidizing the 70% H2 (dry) in the feed stream. CO levels were reduced to less than 5 ppm under steady state conditions (50°C, 10,000 h 1 space velocity and 200-400 psig). The Selectoxo catalyst contains 0.5% platinum (Pt) supported on y -alumina, /8 inch tablets promoted with a base metal oxide. 

The alumina is impregnated with salts of Pt and base metal oxide, dried, and then calcined202. 

The Selectoxo process provides good energy efficiency because it minimizes carbon monoxide slip (only about 0.03%), improved process flexibility, and increased productivity in revamps when compared to other oxidation options. The Selectoxo unit is capable of increasing a plant s capacity by 1.5-2.0%202. 

The Selectoxo unit can also help in a grass root plant by maintaining carbon dioxide/ammonia production ratios which is favorable for full conversion of ammonia to urea. The economics of this option are to be considered against the extra cost of carbon dioxide production by other means (either from the flue gas of the primary reformer or through back burning of extra synthesis gas)202. 

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Final purification of synthesis gas by removal of oxygen-containing impurities... 

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

This process for production of synthetic ammonia by catalytic steam reforming of natural gas is a relatively clean process and presents no unique environmental problems. To assess the environmental impacts of a modem ammonia plant on air, water, and soil, each step in the ammonia synthesis namely, desulfurization, reforming, shift conversion, carbon dioxide removal, final purification, ammonia synthesis, and refrigeration should be examined. The sources of pollutants need to be identified and matched with cost-effective solutions for minimization/elimination by using the best available pollution control measure. 

Final purification of the synthesis gas uses molecular sieve dryers. This enables the synthesis gas to be added to the synthesis loop at converter inlet instead of upstream of the ammonia separator. 

The oldest process for final purification of ammonia synthesis gas is the copper liquor wash, which was used in the world s first ammonia plant in Oppau, Germany in 1913 [321]. Descriptions of the process are given in [321-323]. The... 

Final purification by methanation followed by adjustment of synthesis gas composition (removal of excess nitrogen and part of the inerts) in a cryogenic unit (referred to as the Braun purifier). Gas drying upstream of the purifier. 

Conventional reforming with methanation as the final purification step produces a synthesis gas that contains inerts (CH4 and argon) in quantities that do not dissolve in the condensed ammonia. Most of the inerts are removed by taking a purge stream out of the synthesis loop. The size of this purge stream controls the level of inerts in the loop at about 10% to 15%. The purge gas is scrubbed with water to remove ammonia and then it can be used as fuel or sent to hydrogen recovery. 

After C02 removal, final purification includes methanation (8), gas drying (9), and cryogenic purification (10). The resulting pure synthesis gas is compressed in a single-case compressor and mixed with a recycle stream (11). The gas mixture is fed to the KAAP ammonia converter (12), which uses a ruthenium-based, high-activity ammonia synthesis catalyst. It provides high conversion at the relatively low pressure of 90... 

Recycle Operation. NH and CO, flashed from the separator (7) (90 per cent comes here) is fed to the countercurrent absorber (10), using urea nitrate solution to dissolve NH . The overhead gas from (10), mainly CCh, joins the NHs and 00 gas flashed from the concentrator (8). The gas mixture now flows to the 0 purification column (11), which removes the last traces of NH before the CO is recycled. NH solution from the bottom of the absorber (10) is desorbed in (12), and the absorbent is returned to the absorber (10). NH from the desorber (10) goes through a condenser to the NH column (13) for final purification before it is collected in a gas holder which feeds the NH liquefaction unit (3) in the synthesis section. The NH and CO in the liquid effluents from the CO column (11) and the NH column (13) are removed in the stripping column ahead of the absorber (10) and added to the gases entering (10). 

Capillary column gas chromatography (GC)/mass spectrometry (MS) has also been used to achieve more difficult separations and to perform the structural analysis of molecules, and laboratory automation technologies, including robotics, have become a powerful trend in both analytical chemistry and small molecule synthesis. On the other hand, liquid chromatography (LC)/MS is more suitable for biomedical applications than GC/MS because of the heat sensitivity exhibited by almost all biomolecules. More recent advances in protein studies have resulted from combining various mass spectrometers with a variety of LC methods, and improvements in the sensitivity of nuclear magnetic resonance spectroscopy (NMR) now allow direct connection of this powerful methodology with LC. Finally, the online purification of biomolecules by LC has been achieved with the development of chip electrophoresis (microfluidics). 

Selective H2S-removal processes do itot result in complete elimination of H2S and, if the treated gas is intoided for use as a domestic iiiel or in synthesis processes, a final purification stq> is required. The degree of H2S removal depends on several operating variables, but it aipears that elimination of about 90% of the H2S is the maximum that can be attained economically. Substantial amounts of hydrogen cyanide are also removed in the selective absorber. 

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