Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 22.4.3 Heat exchange autothermal reformingDescriptionFrom a thermodynamic point of view, it is wasteful to use the high level heat of the secondaryreformer outlet gas and the primary reformer flue-gas, both at temperatures of around 1000 °C,just to raise steam. Recent developments aim to recycle this heat to the process itself, by usingthe heat content of the secondary reformer gas in a newly developed primary reformer (gasheated reformer, heat exchange reformer), thus eliminating the need for a fired reformerfurnace. Surplus air or oxygen-enriched air is required in the secondary reformer to meet theheat balance in this autothermal design.Heat for the reform reaction is supplied to the reforming tubes by hot process gas from thesecondary reformer. In a heat exchange primary reformer, excess air has to be supplied to thesecondary reformer to ensure a heat balance between these two steps, and this results in anoverstoichiometric amount of nitrogen in the gas. The high temperature shift reactor and the lowtemperature shift reactor are, in this technique, also replaced by a single isothermal mediumtemperature shift reactor using heat from the shift conversion for the saturation of process gaswith steam and recycling process condensates. A “pressure swing adsorption” (PSA) system isused to remove carbon dioxide and residual carbon monoxide and methane, to produce apurified synthesis gas. A cryogenic purification system may need to be incorporated in theprocess to remove the surplus nitrogen. A modified synthesis converter using an improvedcatalyst with a total resulting in a lower overall synthesis pressure further simplifies the process.Other configurations of the heat exchange primary reformer concept use alternative steps for theshift reaction and synthesis gas purification and ammonia synthesis compared to the above. Inone of these alternatives, only one third of the feed passes through the exchanger reformer withthe residual amount being sent directly to the secondary (autothermal) reformer, which usesenriched air (30 % O 2 ) instead of excess air. The ammonia synthesis uses a new rutheniumbasedcatalyst.Achieved environmental benefits• emissions to the air are reduced significantly by eliminating the flue-gas from the primaryreformer• NO x emissions may be reduced by 50 % or more, depending on the extent of auxiliarycombustion in the plant compared to conventional steam reforming.For achievable emission and consumption levels, see:• Table 2.6 for energy consumption• Table 2.7 for NO x emissions• and Table 2.8 and Table 2.9 for other levels.Cross-media effects• energy may need to be imported to drive mechanical equipment• higher total energy consumption in comparison with other steam reforming concepts.Operational dataNo specific information provided.62 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers