Selectoxo process

The Selectoxo process (Engelhard) reduces the hydrogen consumption of the methanation system, as well as the inert gas content of the purified synthesis gas fed to the synthesis loop. After low-temperature shift conversion, the cooled raw gas is mixed with the stoichiometric quantity of air or oxygen needed to convert the carbon monoxide to carbon dioxide. The mixture is then passed through a precious-metal catalyst at 40-135 °C to accomplish this selective oxidation [740]-[743], The carbon dioxide formed by the Selectoxo reaction adds only slightly to the load on the downstream carbon dioxide absorption system. 

Methanolation [744], [745] has been proposed for partially replacing methanation. It converts the residual carbon oxides to methanol, preferably at higher pressure in an intermediate stage of synthesis gas compression. Methanol is removed from the gas by water scrubbing. The methanol may be recycled to the steam reformer feed or recovered as product. As full conversion of the carbon oxides is not achieved, a clean up methanation unit must follow the methanolation section. 

It is energetically advantageous to add the purified synthesis gas at a point in the synthesis loop where it can flow directly to the synthesis converter (see Section 4.5.1). For this reason water and traces of carbon dioxide must be removed from the makeup gas downstream of methanation. This is accomplished by passing the makeup gas through molecular sieve adsorbers, which can be positioned on the suction side or in an intermediate-pressure stage of the synthesis gas compressor. 

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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. 

In order to further reduce the carbon monoxide concentration in the synthesis gas, a selective oxidation unit has in some cases been installed after the low temperature shift unit. After the gas has been cooled and condensate removed, a small amount of air is introduced, and the gas is passed to a reactor containing a precious metal catalyst. Carbon monoxide reacts with oxygen, and excess oxygen is removed by reaction with hydrogen. The process is known as the Selectoxo process [252-254]. Use of the process in a revamp project is described in [255]. 

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. 

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